2024
55. Sopuruchukwu Ezenwa, Hansel Montalvo-Castro, Alexander J. Hoffman, Huston Locht, Jordan Attebery, Deng-Yang Jan, Michael Schmithorst, Bradley Chmelka, David Hibbitts*, and Rajamani Gounder*
“Synthetic Placement of Active Sites in MFI Zeolites for Selective Toluene Methylation to para-Xylene”
Journal of the American Chemical Society, 146 (2024) 10666–10678.
“Synthetic Placement of Active Sites in MFI Zeolites for Selective Toluene Methylation to para-Xylene”
Journal of the American Chemical Society, 146 (2024) 10666–10678.
Abstract
Brønsted acidic zeolites are ubiquitous catalysts in fuel and chemical production. Broadening the catalytic diversity of a given zeolite requires strategies
to manipulate the acid site placement at framework positions within distinct microporous locations. Here, we combine experiment and theory to elucidate how
intermolecular interactions between organic structure-directing agents (OSDAs) and framework Al centers influence the placement of H+ sites in distinct void
environments of MFI zeolites and demonstrate the catalytic consequences of active site location on kinetically controlled (403 K) toluene methylation to xylene
regioisomers. Kinetic measurements, interpreted using mechanism-derived rate expressions and transition state theory, alongside density functional theory (DFT)
calculations show that larger intersection environments similarly stabilize all three xylene isomer transition states without altering well-established aromatic
substitution patterns (ortho/para/meta ∼ 60%:30%:10%), while smaller channel environments preferentially
destabilize transition states that form bulkier ortho- and meta-isomers, thereby resulting in high intrinsic para-xylene selectivity (∼80%). DFT calculations
reveal that the flexibility of nonconventional OSDAs (e.g., 1,4-diazabicyclo[2.2.2]octane) to reorient within MFI intersections and their ability to hydrogen-bond
to form protonated complexes favor the placement of Al in smaller channel environments compared to conventional quaternary OSDAs (e.g., tetra-n-propylammonium).
These molecular-level insights establish a mechanistic link between OSDA structure, active site placement, and transition state stability in MFI zeolites and
provide active site design strategies that are orthogonal to crystallite design approaches harnessing complex reaction-diffusion phenomena to enhance regioisomer
selectivity in the industrial production of valuable polymer precursors.
54. Jinwoong Nam, Charanyadevi Ramasamy, Daniel E. Raser, Gustavo L. Barbosa Couto, Lydia Thies, David Hibbitts, and Fuat E. Celik*
“Predicting the Enthalpy of Hydrocarbon Radicals Adsorbed on Pt(111) Using Molecular Fingerprints and Machine Learning”
Journal of Physical Chemistry C, 128 (2024) 5030–5043.
“Predicting the Enthalpy of Hydrocarbon Radicals Adsorbed on Pt(111) Using Molecular Fingerprints and Machine Learning”
Journal of Physical Chemistry C, 128 (2024) 5030–5043.
Abstract
The reliable prediction of properties for the adsorbates, including their enthalpy, has been a long-standing challenge as a first key step in studying surface
reactions. It is especially difficult when large adsorbates are involved as the interactions between the adsorbates and surface atoms are complex. Here, we
developed machine learning (ML) models for the prediction of the formation enthalpy of various C2 to C6 hydrocarbon adsorbates on the
Pt(111) surface based on 384 density functional theory calculations. Focusing on larger and more intricate adsorbates, two-thirds of the total species were
C6 species. Four molecular descriptors that represent the valency and bonding of individual carbons within the adsorbates were generated without
intensive computation. They were subsequently used as the features of the ML models with three linear and four nonlinear algorithms. The models were developed
with 30 different samplings of train/test sets, and their results were statistically analyzed to ensure the performance of the models. Nonlinear models, especially
kernel ridge regression and extreme gradient boosting, outperformed linear models with lower absolute errors. The top two accurate models, based on these
algorithms, also displayed remarkable robustness in predicting various species. Employing ensemble average voting with these two models, we achieved the lowest
mean absolute error of 0.94 kcal/molC. Finally, ML was used to estimate the formation enthalpy of 3115 hydrocarbon adsorbates on Pt(111), highlighting
the promise of these methods to study more complicated reaction networks.
53. Nikki Kragt, Jalal Tavana, Mohammed Al-Gharrawi, Clayton Wheeler, David Hibbitts*, Thomas J. Schwartz*
“Kinetics and Reaction Mechanism of Pd-Catalyzed Chlorobenzene Hydrogenolysis”
Journal of Catalysis, 432 (2024) 115435.
“Kinetics and Reaction Mechanism of Pd-Catalyzed Chlorobenzene Hydrogenolysis”
Journal of Catalysis, 432 (2024) 115435.
Abstract
Chlorinated organic compounds are ubiquitous chemical intermediates, final products, and waste streams found in multiple industries, and these compounds
must often be dechlorinated prior to disposal to avoid negative consequences for environmental and human health. In this work, we employ kinetic experiments
and density functional theory (DFT) to examine Pd-catalyzed chlorobenzene hydrogenolysis, which is highly selective for the formation of benzene (> 95%).
Kinetic data (353 K, 97–101 kPa H2, 0.2–1.1 kPa C6H2Cl, 0.5—1 kPa HCl) on 5 wt% Pd/C catalysts revealed rates that are first-order,
half-order, and inverse first-order with respect to PhCl, H2, and HCl, respectively. These kinetic trends are consistent with PhCl dechlorination as the
kinetically relevant step occurring at a single site on surfaces saturated in Cl adatoms (Cl*) in equilibrium with H2 and HCl, consistent with previous
reports in the literature. Similarly, theory predicts that PhCl dechlorination is the rate-determining step and that it does not require C-H activation of
the phenyl ring. However, theory also predicts an apparent twosite requirement for PhCl dechlorination (i.e., to form Ph* and Cl* co-adsorbates), which
contradicts the measured kinetic data. Here, we resolve this inconsistency by calculating adsorption free energies of Cl*, which suggest it saturates Pd
surfaces at a Cl*:Pdsurf ratio of ca. 0.3–0.4, lower than the often-suggested ratio of unity. This low Cl* coverage enables Ph* (and other species) to
bind interstitially among the Cl* adatoms without requiring Cl* desorption. This non-competitive binding of Ph* among Cl* adatoms can be described as a
two-site mechanism that agrees with the kinetic, isotopic, and theoretical data from this and prior work, and this concept of site competition is consistent
with reports of other chemistries occurring on surfaces covered with strongly adsorbing species such as Cl* and S*.
52. Hansel Montalvo-Castro, Álvaro Loaiza-Orduz, Randall J Meyer, Craig Plaisance, David Hibbitts*
“Electronic and Geometric Features Controlling the Reactivity of Mg-vanadate and V2O5 Surfaces toward the Initial C–H Activation of C1–C3 Alkanes – a DFT+U Study”
Submitted
“Electronic and Geometric Features Controlling the Reactivity of Mg-vanadate and V2O5 Surfaces toward the Initial C–H Activation of C1–C3 Alkanes – a DFT+U Study”
Submitted
Abstract
This work employs density functional theory (DFT+U) calculations to explore initial C–H activations in C1–C3 alkanes on V2O5,
MgV2O6 (meta-vanadate), Mg2V2O7 (pyro-vanadate), and Mg3V2O8 (ortho-vanadate) surfaces. These
materials are selective catalysts for the oxidative dehydrogenation (ODH) of alkanes into alkenes, which offers practical and thermodynamic advantages
over non-oxidative alkane dehydrogenation. These Mg-vanadate catalysts, however, have not been characterized by theory. In this work, we explore fourteen
low-energy surfaces of MgxV2Ox+5 (x = 0–3) exposing 64
distinct O atoms (reaction sites). C–H activation barriers are largest on Mg3V2O8, lower and
similar for Mg2V2O7 and MgV2O6, and lowest for V2O5 surfaces; these predicted
trends are consistent with measured ODH reactivity in earlier studies. Barriers are lowest (on average) when alkanes react with O atoms
bound to a single V atom, with bridging O atoms having slightly higher barriers, and three-fold O atoms having the largest activation barriers. However,
there is scattering within each subset indicating that factors beyond O-atom coordination have a significant role in the barriers. Vacancy formation
energies (VFE) and the O 2p band energies were found to be weak descriptors of surface O reactivity for alkane activation barriers. Hydrogen addition
energy (HAE) and methyl addition energy (MAE) values, in contrast, were found to correlate well with alkane activation barriers. MAE, however, outperforms
HAE correlations because of the tendency of H* to form H-bonds with nearby surface O atoms, and those H-bonds are absent in C–H activation transition
states causing scatter in the correlation of barriers with HAE. Constrained-orbital DFT methods were used to establish a theoretical thermochemical cycle
that decouples surface reduction by CH3* into three components: surface distortion, orbital localization, and bond formation. These results
give insights into how Mg:V ratios, surface structure (O-atom coordination), and reducibility (HAE, MAE) impact the reactivity of vanadium-based metal
oxides toward alkane activation.
2023
51. Conor Waldt, Hansel Montalvo-Castro, Abdulrahman Almithn, Álvaro Loaiza-Orduz, Craig Plaisance, David Hibbitts*
“Role of Phosphorous in Transition Metal Phosphides for Selective Hydrogenolysis of Hindered C–O Bonds”
Journal of Catalysis, 421 (2023) 403–418.
“Role of Phosphorous in Transition Metal Phosphides for Selective Hydrogenolysis of Hindered C–O Bonds”
Journal of Catalysis, 421 (2023) 403–418.
Abstract
C–O hydrogenolysis can upgrade biomass to higher value chemicals, but often requires the selective activation of sterically
hindered C–O bonds. Previous work examining C–O hydrogenolysis of methyltetrahydrofuran (MTHF), a model biomass-derived
molecule, has shown that Ni2P and Ni12P5 show higher selectivities toward activation of the
hindered (3C–O) bond over the unhindered (2C–O) bond compared to pure Ni catalysts. These measured
selectivity differences—favoring 3C–O activations for materials with higher P content—were consistent
with calculated free energy barriers for the 2C–O and 3C–O activation pathways using density
functional theory (DFT). However, the role of P in causing this shift in selectivity is still unknown. In this work we use DFT to
study other transition metal phosphides (Co2P, Pd2P, Rh2P, Fe2P, and Ru2P)
and contrast them to their pure metal counterparts to determine if the role of P in Ni2P materials is consistent across
other transition metals. To do this, we constructed theoretical models of these other transition metal phosphides that were
isostructural to the Ni2P(001) surface. In comparing the phosphide materials to their pure metal counterparts, we saw a
nearly ubiquitous shift in selectivity towards hindered C–O activation. However, the magnitudes of these shifts were
significantly varied, with only Ni2P and Pd2P predicted to show high selectivity toward 3C–O
activation. Periodic trends and charge analysis suggest that the varied selectivity shifts (comparing metal-phosphide to pure metal)
can be rationalized based on the electronegativity of the metal and the resultant charge-transfer between P and the nearby metal
atoms, which typically results in metals with a positive partial charge showing greater 3C–O selectivity. These
results help to deconvolute the electronic and geometric impacts of P incorporation into transition metal catalysts and identify
new catalysts for selective C–O activation at hindered C-atoms.
50. Haliey Balcom^, Alexander J. Hoffman^, Huston Locht, David Hibbitts*
“Brønsted Acid Strength Does Not Change for Bulk and External Sites of MFI”
ACS Catalysis, 13 (2023) 4470–4487.
“Brønsted Acid Strength Does Not Change for Bulk and External Sites of MFI”
ACS Catalysis, 13 (2023) 4470–4487.
Abstract
Zeolites with different crystal habits can exhibit different catalytic behavior, particularly if they have high external
surface areas and short diffusion paths. Here, we study the acid strength of sites that form on the surfaces of MFI
zeolites using density functional theory (DFT) and compare them to sites within the crystal. We determine acid strength
by calculating heterolytic and homolytic O–H bond cleavage energies (deprotonation energy, DPE and dehydrogenation
energy, DHE, respectively), and the adsorption energy of ammonia (ΔENH3), a common base titrant.
These metrics indicate that most sites on the outer (010) surfaces of MFI have similar acid strength based on ensemble
averaged DHE values (457–483 kJ mol−1) to their bulk counterparts (457–476 kJ mol−1).
Ensemble average DPE values on surface sites (1640–1704 kJ mol−1) are similar to those in the bulk
(1644–1664 kJ mol−1) when spurious interactions of anionic charges across periodic boundaries are corrected
using a novel extrapolation technique. Four Al positions on the outer zeolite surface can yield Lewis acidic Al if the Si prior to
substitution formed a terminal silanol group (SiOH). H2O bound to these Lewis sites are much weaker Brønsted acids
than other sites, with ensemble average DPE values of 1684–1704 kJ mol−1. Unlike DPE and DHE,
ΔENH3 probes a combination of acid strength and confinement because a gas-phase species enters the zeolite
pores. Ensemble average ΔENH3 values indicate that most sites bind NH3 as strongly on the surface
(−160 to −113 kJ mol−1) as bulk sites (−160 to −133 kJ mol−1),
despite the reduced confinement on surface sites. These ΔENH3 change very little on the MFI surface
because most surface sites are within pockets that retain enough of the zeolite pore to solvate the NH4+
cations that form when NH3 binds. Some sites, however, bind NH3 more weakly if the solvating pore is absent
or if NH3 binds and deprotonates the H2O on a Lewis acidic Al. Finally, H2O on these Lewis sites
can desorb and NH3 can bind, providing a comparison between the Lewis and Brønsted acid strength of these sites.
NH3 binds at least 11 kJ mol−1 more strongly to Lewis acidic Al than any other Brønsted acid site
evaluated in this work. Together, these data suggest that fully coordinated Brønsted acidic Al sites on the outside of
zeolite crystals possess similar acid strength to those within the crystal, and that many surface sites effectively confine an
NH4+ cation. Such partial confinement on external sites indicates that NH3 adsorption experiments
are ineffective for broadly distinguishing between internal and external sites in MFI materials.
49. Hansel Montalvo-Castro, Mykela DeLuca, Lauren Kilburn, and David Hibbitts*
“Mechanisms and Kinetics of the Dehydrogenation of C6-C8 Cycloalkanes, Cycloalkenes, and Cyclodienes to Aromatics in H-MFI Zeolite Framework”
ACS Catalysis, 13 (2023) 99–112.
“Mechanisms and Kinetics of the Dehydrogenation of C6-C8 Cycloalkanes, Cycloalkenes, and Cyclodienes to Aromatics in H-MFI Zeolite Framework”
ACS Catalysis, 13 (2023) 99–112.
Abstract
This work employs periodic density functional theory (DFT) to elucidate dehydrogenation mechanisms
of C6–C8 cycloalkanes, cycloalkenes, and cyclodienes into aromatics during methanol-to-olefin (MTO)
chemistry in H-MFI zeolites. Aromatic compounds act as co-catalysts that predominantly form ethylene
over propylene products and lead to site-blocking polyaromatic compounds, thus understanding the
formation of aromatic compounds during MTO is critical to understanding its selectivity and stability.
Dehydrogenation reactions occur via sequential hydride transfer (forming a cyclic carbocation) followed
by deprotonation. The rate controlling hydride transfer reactions were investigated with surface-bound
protons (Z–H) and alkyls (Z–CnH2n+1) as hydride accepting species. Hydride transfers via proton-mediated
routes occur with intrinsic free energy barriers (623 K) of 215 kJ mol−1 for C6H12, 164 kJ mol−1 for
C6H10, and 141 kJ mol−1 for C6H8 whereas methyl-mediated counterparts occur with intrinsic free barriers
of 101 kJ mol−1, 95 kJ mol−1, and 81 kJ mol−1 respectively. Transition states for hydride transfer
reactions prefer channel intersections within MFI networks and their activation barriers suggest that
methyl-mediated routes are favored over proton-mediated counterparts during MTO. Methyl-substituents on
hydrocarbon rings generally increase activation barriers for hydride transfers that occur proximal to
the –CH3, partly because of steric hindrances between the hydride acceptor and ring-bound –CH3. Rapid
double-bond isomerization within cyclohexenes and cyclohexadienes allows hydride transfer reactions to
occur away from sterically hindering methyl substituents in methylated and dimethylated C6 ring hydrocarbons.
The impact of carbocation substitution in hydride accepting species was explored by contrasting barriers of
methyl, ethyl, 2-propyl, and tert-butyl (Z–C4H9) surface-bound alkyls. Intrinsic activation barriers
decrease with increasing substitution of the alkyl hydride acceptor, consistent with those alkyls forming
more stable carbocations. However, when accounting for steric hindrances associated with the co-adsorption
of hydrocarbon rings near surface-bound alkyls, effective free barriers are larger for more-substituted
alkyls. Taking these effective barriers into account, we predict that methyl-mediated hydride transfer
reactions are responsible for the aromatization of hydrocarbon rings during MTO, leading to the CH4 formed
during MTO reactions as the aromatic pool is enriched.
2022
48. Alexander J. Hoffman^, Chithra Asokan^, Nicholas Gadinas, Emily Schroeder, Gregory Zakem, Steven V. Nystrom, Andrew “Bean” Getsoian, Phillip Christopher*, David Hibbitts*
“Experimental and theoretical characterization of Rh single-atoms supported on γ-Al2O3 with varying hydroxyl content during NO reduction by CO”
ACS Catalysis, 12 (2022) 11697–11715.
“Experimental and theoretical characterization of Rh single-atoms supported on γ-Al2O3 with varying hydroxyl content during NO reduction by CO”
ACS Catalysis, 12 (2022) 11697–11715.
Abstract
Oxide-supported Rh catalysts are important components of commercial three-way catalysts for
pollution abatement. Despite their universal application, many mysteries remain about the active
structure of Rh on oxide supports as these materials, even after aging, which often contain a
mixture of nanoparticles and single-atom Rh species on the same support. Probe molecule Fourier
transform infrared spectroscopy (FTIR) in this work shows that atomically dispersed Rh prefer to
strongly bind CO when exposed to NO and CO mixtures and that light-off of NO reduction occurs
at similar temperatures to CO desorption, suggesting the first and rate-determining step in NO-CO
reactions may be the desorption of CO from single-atom Rh dicarbonyl complexes, Rh(CO)2. Two
sets of symmetric and asymmetric stretching frequencies associated with distinct types Rh(CO)2
species are observed in FTIR spectra at 2084/2010 and 2094/2020 cm−1. During temperature ramps,
the latter pair of bands at 2094/2020 cm−1 converts to the 2084/2010 cm−1 bands at 463 K before
the all symmetric and asymmetric bands disappear at 573 K. Bands then appear in the range of
1985–1975 cm−1 associated with Rh(CO) species upon the disappearance of the 2084/2010 cm−1 bands,
suggesting that CO desorbs sequentially from Rh(CO)2 by forming Rh monocarbonyl intermediates.
Combined DFT and FTIR experiments suggest that local OH coverage on the γ-Al2O3 surface distinguishes
the two Rh(CO)2 species: the higher frequency species resides on a less hydroxylated region and
migrates to a more hydroxylated region at higher temperature, causing the CO vibrational frequency
to decrease by ~10 cm−1. CO desorption occurs from this Rh(CO)2 structure with high local OH
coverage, consistent with the DFT predicted trend of CO binding energies. Because of the coincidence
of CO desorption with light-off of NO reduction, local support hydroxylation of atomically dispersed
Rh1/γ-Al2O3 catalysts like affects both Rh structure after CO desorption and the kinetics of NO
reduction, studies of which are enabled by the Rh(CO)2 model developed here.
47. Elizabeth E. Bickel, Alexander J. Hoffman, Songhyun Lee, Hannah E. Snider, Claire T. Nimlos, Natalie K. Zamiechowski, David D. Hibbitts, and Rajamani Gounder*
“Altering the Arrangement of Framework Al Atoms in MEL Zeolites Using Mixtures of Tetrabutylammonium and Sodium Structure-Directing Agents”
Chemistry of Materials, 34 (2022) 6835–6852.
“Altering the Arrangement of Framework Al Atoms in MEL Zeolites Using Mixtures of Tetrabutylammonium and Sodium Structure-Directing Agents”
Chemistry of Materials, 34 (2022) 6835–6852.
Abstract
The arrangement of Al sites in zeolite frameworks influences the structure and speciation of Brønsted acidic
hydroxyl groups and of metal cations and complexes that behave as active sites in acid and redox catalysis,
but synthetic approaches to systematically alter Al arrangement have yet to be developed for many zeolite
topologies. Herein, we report the synthesis of MEL zeolites with varied Al content (Si/Al = 35–118) using
tetrabutylammonium (TBA+; TBA+/Si = 0.3) as the organic structure directing agent (SDA), and with fixed Al
content (Si/Al ~ 50) using mixtures of inorganic (Na+) and organic (TBA+) SDAs of different charge density
((NaM+/TBA+)gel = 0–5, (Na++TBA+)/Si = 0.3). MEL zeolites crystallized using TBA+ as the sole SDA contained
one TBA+ per channel intersection (4 TBA+ per 96 T-site unit cell (u.c.)), with varying bulk compositions
(Si/Al > 23) consistent with charge density mismatch theory. Aqueous-phase ion exchange conditions to use
Co+ as a selective titrant of proximal Al sites in MEL were determined and validated by a cation site balance
on Co-MEL zeolites. MEL crystallized from TBA+ alone contained finite fractions of Co2+-titratable Al-Al pairs
that increased (2*Co2+/Al = 0.2–0.4) with total Al content (Si/Al = 35–118), as also observed for MFI crystallized
from tetrapropylammonium (TPA+) alone. MEL crystallized from mixtures of Na+ and TBA+ contained fractions of
Co2+-titratable Al-Al pairs that decreased (2*Co2+/Al = 0.22–0.10) with increasing occluded Na+ content
(0.0–2.4 Na+/u.c.). Analysis of occluded OSDA and inorganic SDA content in MEL samples reveal evidence for
competitive occlusion of Na+ and TBA+. DFT estimated energies reveal that Na+ co-occlusion with organic SDAs is
less likely in MEL than MFI frameworks. These findings, together with our prior results on MFI and CHA frameworks,
indicate that site-isolated Al arrangements generally tend to form when monovalent inorganic and organic SDAs
compete for occupancy within void and ring spaces of zeolite frameworks.
46. R.G. Rao, R. Blume, M. Greiner, A. Knop-Gericke, R. Schlögl, T. W. Hansen, K. Dreyer, D. D. Hibbitts, and J. Tessonnier*
“Oxygen-Doped Carbon Supports Modulate the Hydrogenation Activity of Palladium Nanoparticles through Electronic Metal−Support Interactions”
ACS Catalysis, 12 (2022) 7344–7356.
“Oxygen-Doped Carbon Supports Modulate the Hydrogenation Activity of Palladium Nanoparticles through Electronic Metal−Support Interactions”
ACS Catalysis, 12 (2022) 7344–7356.
Abstract
In heterogeneous catalysis, synergies between the metal active phase and the oxide support can enhance the catalytic function
through electronic metal-support interactions (EMSI). Such effects are unknown for carbonaceous scaffolds and carbon is often
selected as a support due to its expected inertness. Here, we report that conventional carbon supports do present EMSI effects
that alter the intrinsic rate of palladium atoms near the interface by up to two orders of magnitude compared to Pd atoms at
the apex of 5 nm nanoparticles. We also demonstrate that oxygen-containing functional groups, which are ubiquitous on carbon
surfaces, are responsible for these EMSI. Controlling the scaffold’s surface chemistry allows to tune its work function from
5.1 to 4.7 eV and, hereby, the intensity of the charge redistribution at the metal-carbon interface and the catalytic activity
of the corresponding metal atoms.
45. S. Leung, D. Hibbitts, M. Garcia-Dieguez, and E. Iglesia*
“Binding and Exchange Reactions of Hydrogen Isotopes on Surfaces of Dispersed Pt Nanoparticles”
Journal of Physical Chemistry C, 126 (2022), 3923–3938.
“Binding and Exchange Reactions of Hydrogen Isotopes on Surfaces of Dispersed Pt Nanoparticles”
Journal of Physical Chemistry C, 126 (2022), 3923–3938.
Abstract
H2-D2 isotopic equilibration is often used to assess the involvement and reversibility of H2 dissociative chemisorption
steps during hydrogenation surface catalysis under the premise that exchange turnovers require recombinative desorption of adatoms (H*, D*).
Experimental and theoretical evidence reported here show instead that exchange occurs via non-competitive adsorption mechanisms that circumvent
recombinative desorption steps at temperatures of catalytic relevance (< 700 K) and that H2 and D2 adsorption enthalpies are similar, leading to
equilibrium isotope effects near unity (0.7–1.0) on dispersed Pt nanoparticles. The kinetic effects of H2 and D2 pressures on exchange rates at
300–700 K are accurately described by elementary steps involving the non-competitive adsorption of H2 (or D2) and its reaction with D* (or H*),
mediated via local displacements of adatoms to alternate binding sites in H*/D* adlayers, but without requiring their intervening endothermic
recombinative desorption. Recombination (H*+D*) routes become prevalent only above 700 K, leading to different kinetic trends with H2/D2 pressures
and to higher activation barriers (78 vs. 32 kJ mol−1) than for the single-site non-competitive adsorption routes that prevail at lower temperatures.
Such conclusions are consistent with density functional theory (DFT) calculations on 4×4 Pt(111) models at saturation coverages (1 H*/Pts),
which show that barriers for single-site metallocycle mechanisms (205 kJ mol−1) are much larger than for routes involving H2 or D2 dissociation
on “spaces” created by displacements of D*/H* adatoms (55 kJ mol−1) and the concomitant local supersaturation of the Pt surface. Such pathways
weaken the binding of H* (or D*) at such locations, thus enabling the facile desorption and exchange of H2 and D2 at conditions that would otherwise
preclude the measured rates of exchange events. Pt201 nanoparticle models allow greater adlayer relaxation and supramonolayer H* coverages and lead
to even lower DFT-derived barriers for non-competitive adsorption routes (27 kJ mol−1 at 1.44 H*/Pts). Recombination routes exhibit higher barriers
and become the predominant exchange pathways above 700 K; for reactions requiring such temperatures, exchange rates can provide a reliable assessment
of the reversibility of dihydrogen dissociation events.
44. M. DeLuca and D. Hibbitts*
“Predicting diffusion barriers and diffusivities of C6–C12 methylbenzenes in MFI zeolites”
Microporous and Mesoporous Materials, 333 (2022), 111705.
“Predicting diffusion barriers and diffusivities of C6–C12 methylbenzenes in MFI zeolites”
Microporous and Mesoporous Materials, 333 (2022), 111705.
Abstract
Mass transport plays an important role in zeolite catalyzed reactions and catalyst deactivation, yet experimental measurement of mass transport,
particularly slow diffusion processes (e.g., of bulky aromatics), is limited because of time scale restraints. Here, we use density functional
theory to overcome these limitations and calculate diffusion barriers of benzene and all twelve C7–C12 methylbenzenes through the straight and
sinusoidal channels of silicalite-1 (MFI framework). Straight and sinusoidal diffusion barriers are well-predicted by a critical diameter
describing the minimum width of the molecule, where benzene, toluene, and para-xylene (all 6.6 Å) diffuse through both channels with barriers
200 kJ mol−1 lower than those of pentamethylbenzene (8.2 Å). The MFI framework distorts to accommodate species with larger critical diameters
and this distortion correlates to activation barriers; benzene diffusion via the straight channel causes a 0.38 Å distortion while
pentamethylbenzene causes a 3.64 Å distortion. Diffusing through the straight channel of MFI is always more facile than via the sinusoidal channel,
by an average of 39 kJ mol−1 because the tortuosity of the sinusoidal channels forces the framework to distort, on average, 3.5 Å more for a given
aromatic than straight channel diffusion. We show that DFT-calculated straight channel diffusion activation barriers agree well with those reported
by frequency response experiments and can be used to calculate self-diffusivities of molecules, with appropriate entropy corrections. Examining
all aromatics provides insights to the role of molecule size, channel tortuosity, and entropy during intracrystalline diffusion to provide a
reference point for the species that can likely reasonably diffuse through both channels (e.g., benzene, toluene, xylenes, durene), through
straight channels only (e.g., 1,2,3-trimethylbenzene), or simply are ‘stuck’ within intersections (e.g., pentamethylbenzene) in MFI.
43. E. Iglesia* and D. Hibbitts
“The Fischer-Tropsch synthesis: A few enduring mechanistic conundrums revisited”
Journal of Catalysis, 405 (2022) 614–625. Perspective
“The Fischer-Tropsch synthesis: A few enduring mechanistic conundrums revisited”
Journal of Catalysis, 405 (2022) 614–625. Perspective
First Paragraph
Few catalytic processes have garnered attention in practice as persistently as the Fischer-Tropsch synthesis (FTS); even fewer have
led to the passionate mechanistic polemics that have characterized its storied history. Not many reactions have generated such diverse
mechanistic proposals or faced the task of discerning among them from data often corrupted by ubiquitous gradients in temperature and
concentrations or by confounding matters of gas-liquid-solid interfaces. None have done so amidst active deployment in practice or
through cycles of attention that fluctuate with remarkable regularity, as gas conversion technologies have responded to cyclical
geopolitical disruptions, shifts in market demand, or unexpected discoveries of gas reservoirs, often remote from markets. These
waves of interest have stranded mechanistic debris; each cycle has faced the inconvenient truths, enduring inconsistencies, and
mechanistic proposals of those before.
2021
42.G. Marsden, P. Kostetskyy, R. Sekiya, A. Hoffman, S. Lee, R. Gounder, D. Hibbitts, and L. Broadbelt*
“Quantifying effects of active site proximity on rates of methanol dehydration to dimethyl ether over CHA zeolites through microkinetic modeling”
ACS Materials Au, 2 (2021), 163–175.
“Quantifying effects of active site proximity on rates of methanol dehydration to dimethyl ether over CHA zeolites through microkinetic modeling”
ACS Materials Au, 2 (2021), 163–175.
Abstract
Control of the spatial proximity of Brønsted acid sites within the zeolite framework can result in
materials with properties that are distinct from materials synthesized through conventional crystallization
methods or available from commercial sources. Recent experimental evidence has shown that turnover rates
of different acid-catalyzed reactions increase with the fraction of proximal sites in chabazite (CHA) zeolites.
The catalytic conversion of oxygenates is an important research area, and the dehydration of methanol to
dimethyl ether (DME) is a well-studied reaction as part of methanol-to-olefins (MTO) chemistry catalyzed
by solid acids. Published experimental data have shown that DME formation rates (per acid site) increase
systematically with the fraction of proximal acid sites in the six-membered ring of CHA. Here, we probe
the effect of acid site proximity in CHA on methanol dehydration rates using electronic structure
calculations and microkinetic modeling to identify the primary causes of this chemistry and their
relationship to the local structure of the catalyst at the nanoscale. We report a density functional theory
(DFT)-parametrized microkinetic model of methanol dehydration to DME, catalyzed by acidic CHA zeolite
with direct comparison to experimental data. Effects of proximal acid sites on reaction rates were captured
quantitatively for a range of operating conditions and catalyst compositions, with a focus on total paired
acid site concentration and reactant clustering to form higher nuclearity complexes. Next-nearest neighbor
paired acid sites were identified as promoting the formation of methanol trimer clusters rather than
inhibiting tetramer or pentamer clusters, resulting in large increases in rate for dimethyl ether production
due to the lower energy barriers present in the concerted methanol trimer reaction pathway. The model
framework developed in this study can be extended to other zeolite materials and reaction chemistries
toward the goal of rational design and development of next-generation catalytic materials and chemical
processes.
41. A.J. Hoffman, C. Asokan, N. Gadinas, P. Kravchenko, A. Getsoian, P. Christopher*, D. Hibbitts*
“Theoretical and experimental characterization of adsorbed CO and NO on γ-Al2O3-supported Rh nanoparticles”
Journal of Physical Chemistry C, 125 (2021) 19733–19755.
“Theoretical and experimental characterization of adsorbed CO and NO on γ-Al2O3-supported Rh nanoparticles”
Journal of Physical Chemistry C, 125 (2021) 19733–19755.
Abstract
Rh active sites are critical for NOx reduction in automotive three-way catalysts. Low Rh loadings used in
industrial catalysts lead to a mixture of small nanoparticles and single-atom Rh species. This active site heterogeneity
complicates the interpretation of characterization and reactivity, making the development of structure-function relationships
challenging. Density functional theory (DFT) investigations of Rh catalysts often employ flat, periodic surfaces, which
lack the curvature of oxide-supported Rh nanoparticle surfaces, raising questions about the validity of periodic surface
model systems. Here, we combine DFT with probe molecule FTIR spectroscopy and HRSTEM of supported Rh catalysts synthesized
to insure against the in situ formation of single-atom Rh species to compare periodic and nanoparticle DFT models
for describing the interaction of CO and NO with supported Rh nanoparticles. We focus on comparing the behavior of Rh(111)
to 201-atom cubo-octahedral Rh nanoparticles (Rh201; 1.7 nm diameter) model systems to explain the behavior of CO*
and NO* bound to Rh nanoparticles with an average particle diameter of ~2.6 nm. Our DFT calculations indicate that CO*
occupies a mix of three-fold and atop modes on Rh(111), saturating at 0.56 ML CO* (473 K, 1 bar), while CO* saturates
Rh201 near 1 ML. Similarly, NO* binds to three-fold sites and saturates the Rh(111) surface at 0.67 ML but saturates
the Rh201 particle surface at 1.38 ML, indicating more NO* binds than there are Rhsurf atoms. Moreover,
the adlayers on the Rh201 particle contain predominantly atop-bound CO*, with bridge CO* possible on particle edges,
and predominantly three-fold NO* with bridge- and atop-bound NO* bound to edges and corners. These binding modes and higher
coverages are made possible by the curvature of these nanoparticles and by the expansion of surface metal-metal bonds—neither
of which can occur on Rh(111)—which together permit the adlayer to laterally relax, reducing internal strain. FTIR
data for CO* on 10 wt% Rh/γ-Al2O3 shows predominantly atop binding modes (2067 cm−1) with small broad
peaks near bridge (1955 cm−1) and three-fold (1865 cm−1) regions. Meanwhile, NO* FTIR also shows a mixture of atop (1820 cm−1)
and three-fold (1685 cm−1) NO* features, with similar features observed at reaction conditions (5 mbar NO, 1 mbar CO, 478 K),
indicating that NO* dominates Rh surfaces during catalysis. Frequency calculations on these adlayers of Rh201 particles
yield dominant frequencies that more closely resemble those observed in FTIR spectra and demonstrate how coverage and dipole-dipole
coupling affect vibrational frequencies with surface curvature. Taken together, these results indicate that Rh surface curvature
alters the structure and spectral characteristics of NO* and CO* for Rh nanoparticles of ~2.6 nm diameter, which must be
accurately reflected in DFT models.
40. L. Kilburn^, M. DeLuca^, A.J. Hoffman, S. Patel, D. Hibbitts*
“Comparing Alkene Disproportionation and Formaldehyde-mediated Diene Formation Routes in Methanol-to-Olefins Catalysis in MFI and CHA”
Journal of Catalysis, 400 (2021) 124–139.
“Comparing Alkene Disproportionation and Formaldehyde-mediated Diene Formation Routes in Methanol-to-Olefins Catalysis in MFI and CHA”
Journal of Catalysis, 400 (2021) 124–139.
Abstract
Methanol-to-Olefins (MTO) processes, during which alkene and aromatic species autocatalyze reactions forming
light olefins, occur in Brønsted acid zeolites. Previous work has shown that formaldehyde and diene species
participate in both catalyst initiation and deactivation; however, the exact mechanisms through which these
species are formed during MTO is unknown. Here, we provide a mechanistic understanding of diene formation
during MTO using DFT to investigate two diene formation pathways–alkene disproportionation and CH2O-mediated–in H-ZSM-5 (MFI) and HSSZ-13 (CHA) zeolites. The rate-determining step of both pathways is a
hydride transfer between a surface-bound alkyl and C4H8 (alkene disproportionation) or CH3OH (CH2O-mediated). The rate controlling hydride transfer reactions were further investigated with C1–C4 surface-bound
alkyls, which are likely present during MTO processes. Alkyl--CH3OH hydride transfers follow carbocation
The relative rates of tert-butyl--CH3OH hydride transfer to form CH2O exceeds that of methyl--CH3OH hydride
stability trends with tert-butyl being the most reactive in MFI (ΔGa҂ 55 kJ mol−1) and CHA (ΔGa҂҂ 104 kJ mol−1).
transfer at C4:C1 alkyl ratios as low as 10−10 in MFI and 10−6 in CHA. Alkyl--C4H8 hydride transfers proceed most
favorably via secondary alkyls (propyl in MFI, ΔGa҂ 83 kJ mol−1) and (sec-butyl in CHA, ΔGa҂ 96 kJ mol−1),
rather than tertiary, as steric repulsion between tert-butyl and C4H8 mitigate enhanced carbocation stabilities
making these transition states unfavorable in both frameworks. Propyl--C4H8 (in MFI) and sec-butyl--C4H8 (in
CHA) hydride transfers to form butadiene occur over methyl--C4H8 when their relative surface concentrations are
10−10 and 10−5 times that of methyl, respectively. We observe that, once formed, C2+ surface alkyls are better
hydride acceptors than CH3–Z, and this work suggests that secondary and tertiary surface alkyls are primarily
responsible for forming dienes through direct and indirect (via CH2O) routes. This has crucial implications for
understanding deactivation, as prior studies assume CH2O is formed by reactions with surface-bound methyl,
formed by the reactant CH3OH, instead of larger surface-bound alkyls, formed by alkene products.
39. E. Yik, D. Hibbitts, H. Wang, and E. Iglesia*
“Hydrogenation and C-S bond activation pathways in thiophene and tetrahydrothiophene reactions on sulfur-passivated surfaces of Ru, Pt, and Re nanoparticles”
Applied Catalysis B: Environmental , 291 (2021) 119797.
“Hydrogenation and C-S bond activation pathways in thiophene and tetrahydrothiophene reactions on sulfur-passivated surfaces of Ru, Pt, and Re nanoparticles”
Applied Catalysis B: Environmental , 291 (2021) 119797.
Abstract
Thiophene-H2 reactions proceed via sulfur removal and hydrogenation
routes on dispersed metal nanoparticles that become decorated by refractory S-adlayers
during catalysis. The identity and kinetic relevance of the required elementary steps
are described here based on rates measured at S-chemical potentials set by H2S/H2 ratios
similar to those prevalent during practical catalysis on Re, ReSx, Ru, and Pt catalysts.
The free energies of formation of S adatoms from H2S/H2 reactants are large and negative
at low coverages (< −50 kJ mol−1 on Pt(111) and < −150 kJ mol−1 on Re(0001) and Ru(0001)),
but strong repulsion among adatoms prevents full saturation, leading to sub-monolayer
coverages and to passivated surfaces that expose uncovered interstices. The residual
interstitial spaces (*) within adlayers of unreactive S-atoms (S′) bind intermediates
and transition states reversibly, thus allowing these passivated surfaces to carry out
catalytic turnovers. These interstices are larger on Pt than on Ru or Re surfaces, leading
to concomitantly higher turnover rates for thiophene hydrogenation and desulfurization
routes. The identity and kinetic relevance of elementary steps for desulfurization (to
form C4 hydrocarbons) and hydrogenation (to form tetrahydrothiophene; THT) are similar
among these catalysts; they involve kinetically-relevant H-addition steps of thiophene-
derived intermediates that cleave their C-S bonds or “over-hydrogenate” to THT in one
surface sojourn. THT undergoes C-S bond cleavage in secondary reactions that correct this
over-hydrogenation to form the more unsaturated species that cleave C-S bonds. THT/C4
product ratios are insensitive to H2S/H2 ratios and thiophene pressure, even though active
interstitial spaces are covered by kinetically-detectable S* and bound thiophene; therefore,
primary and secondary reactions must occur on the same active ensembles. The observed
increase in THT/C4 ratios with H2 pressure shows that THT formation transition states
involve a larger number of H-atoms than for C-S cleavage. C-S bonds cleave in species with
partial unsaturation (H-contents between those in THT and thiophene), in processes that are
reminiscent of the requirement to add or remove H-atoms from C and O atoms in cleaving C-C
and C-O bonds in alkanes and alkanols, as well as C-O bonds in CO hydrogenation reactions.
The rate and selectivity data and the mechanistic conclusions described here show that
thiophene hydrogenation and desulfurization routes require similar binding interstices
within refractory S-adlayers and that metal-sulfur bond energies act as indirect descriptors
of reactivity because they determine the number, size, and binding properties of the exposed
weakly-binding ensembles that stabilize the relevant transition states and enable catalytic
turnovers.
2020
38. V. Mehar^, A. Almithn^, T. Egle, M.-H. Yu, C. O'Connor, M. Karatok, R. Madix, D. Hibbitts, and J. Weaver*
“Oxophilicity Drives Oxygen Transfer at a Palladium-Silver Interface for Increased CO Oxidation Activity”
ACS Catalysis, 10 (2020) 13878–13889.
“Oxophilicity Drives Oxygen Transfer at a Palladium-Silver Interface for Increased CO Oxidation Activity”
ACS Catalysis, 10 (2020) 13878–13889.
Abstract
Pd and AgOx phases grown on single crystal
silver (Ag(111)) act cooperatively to oxidize adsorbed CO. A AgOx
single-layer on Ag(111) exhibits limited reaction with CO adsorbed at 100 K.
However, Pd clusters deposited onto a single layer of AgOx promote the
reaction by attracting O-atoms from the surrounding AgOx to
the Pd where they efficiently react with adsorbed CO. The CO2
yield increases to a maximum with increasing Pd coverage to a bilayer
of metallic Pd present at ~2 monolayer of surface coverage, and CO
consumes all of the oxygen initially present in the underlying AgOx
layer. Smaller Pd bi-layer clusters induce oxygen transfer from the
AgOx surface more efficiently, as they optimally incorporate
oxygen from both the AgOx beneath them and the surrounding
AgOx surface. Enhanced stabilization of the oxygen causes
the edges of the Pd clusters to become enriched in oxygen and further
promotes reduction of the AgOx. CO coverages are high on
Pd domains and dominate the terrace regions of Pd particles, while
oxygen predominantly covers the edges of Pd clusters. These results
demonstrate that oxygen transport across metal/metal-oxide interfaces
can be highly efficient when the oxygen chemical potential is lower for
the metal phase (Pd) than for the metal-oxide substrate (AgOx)
and clarify mechanisms of oxygen interchange between co-existing phases
that may have broad applicablility.
37. C. T. Nimlos^, A. J. Hoffman^, Y. G. Hur, B. J. Lee, J. R. Di Iorio, D. Hibbitts*, and R. Gounder*
“Experimental and Theoretical Assessments of Aluminum Proximity in MFI Zeolites and its Alteration by Organic and Inorganic Structure-Directing Agents”
Chemistry of Materials, 32 (2020) 9277–9298.
“Experimental and Theoretical Assessments of Aluminum Proximity in MFI Zeolites and its Alteration by Organic and Inorganic Structure-Directing Agents”
Chemistry of Materials, 32 (2020) 9277–9298.
Abstract
The molecular structure and cationic charge density
of organic and inorganic structure-directing agents (SDAs) influences
the siting and arrangement of Al substituted in zeolite frameworks.
Yet, developing such synthesis-structure relations for MFI zeolites
is difficult because of the complexities inherent to its low symmetry
framework (12 unique tetrahedral sites), which generates a large
combinatorial space of Al-Al site pairs to exhaustively model by density
functional theory (DFT) and quantify by experiment. Here, we develop an
experimental protocol to reproducibly quantify Co2+-titratable Al-Al
site pairs in MFI, with saturation uptakes validated by corroborating
spectroscopic and site balance data. DFT calculations
performed using a 96 T-site MFI unit cell containing either an isolated Al
site (all 96 configurations) or various Al-Al site pairs (1773 out of
13,680 total configurations), charge-balanced by one or two TPA+ respectively,
reveal the dominant influence of electrostatic interactions between the
cationic N of TPA+ and the anionic lattice charge on Al siting energies.
Along with DFT calculations of Co2+ siting energies at Al-Al site pairs,
theory predicts that two TPA2+ cations in adjacent channel intersections
can form many Co2+-titratable Al-Al site pair configurations, providing
atomic descriptions that rationalize why MFI crystallized using only TPA+
contains a finite number of Al-Al site pairs. Using TPA+ and Na+ as co-SDAs,
while varying the Na+/TPA+ ratio (0–5) at constant SDA/Al ratio (TPA++Na+)/Al = 30)
in the synthesis medium, crystallized MFI with similar bulk Al content
(Si/Al ~ 50) but varying fractions of Al in pairs (0.12–0.44). Separate
syntheses using charge-neutral organic SDAs pentaerythritol or a mixture
of 1,4-diazabicyclo[2.2.2]octane and methylamine, together with Na+ to
compensate framework Al, crystallized MFI at similar bulk Al content
(Si/Al ~ 50), but with lower fractions of Al in pairs (>0.14). Among MFI
samples crystallized with an organic SDA and Na+ as a co-SDA, the number
of Al-Al site pairs formed increased generally with the co-occluded Na+
content on the solid zeolite, which is a synthesis-structure relation that
resembles our prior observations on CHA zeolites. These results provide a
microscopic model to define and quantify Al-Al site pairs in MFI, using a
combined experimental and theoretical approach that can be adapted to do
so for other framework topologies. These findings highlight how such Al
siting models can be exercised to interrogate zeolite materials to develop
synthetic strategies that exploit using mixtures of organic and inorganic
SDAs to crystallize zeolites with varying framework Al arrangements and,
in turn, with different catalytic and adsorption properties.
36. A.J. Hoffman, J.S. Bates, J.R. Di Iorio, S.V. Nystrom, C.T. Nimlos, R. Gounder*, D. Hibbitts*
“Rigid Arrangements of Ionic Charge in Zeolite Frameworks Conferred by Specific Al Distributions Preferentially Stabilize Alkanol Dehydration Transition States”
Angewandte Chemie Int. Ed., 59 (2020) 18686–18694.
“Rigid Arrangements of Ionic Charge in Zeolite Frameworks Conferred by Specific Al Distributions Preferentially Stabilize Alkanol Dehydration Transition States”
Angewandte Chemie Int. Ed., 59 (2020) 18686–18694.
Abstract
Zeolite reactivity depends on the solvating environments of
microporous voids and the proximity of Brønsted acid sites within
them. Turnover rates for methanol and ethanol dehydration increase
with the fraction of proximal sites in six-membered rings of chabazite
zeolites. DFT shows that activation barriers vary widely with the
number and arrangement of Al but cannot be described only by Al–Al
distance or density. Al substitution into zeolites introduces localized
framework anionic charges and certain Al distributions yield rigid
arrangements of charge that stabilize cationic intermediates and
transition states via H-bonding, decreasing barriers. This is a key
feature of acid catalysis in zeolite solvents, which lack the isotropy of
liquid solvents. The sensitivity of polar transition states to specific
arrangements of charge in their solvating environments and the
positioning of such charges in zeolite lattices herald rich catalytic
diversity among zeolites of varying Al arrangement.
35. M. Allen^, A. Hoffman^, T.W. Liu, M. Webber, D. Hibbitts*, T. Schwartz*
“Highly Selective Cross-Etherification of 5-Hydroxymethylfurfural with Ethanol”
ACS Catalysis, 10 (2020) 6771–6785.
“Highly Selective Cross-Etherification of 5-Hydroxymethylfurfural with Ethanol”
ACS Catalysis, 10 (2020) 6771–6785.
Abstract
Etherification of 5-hydroxymethylfurfural (HMF) and ethanol to ethoxymethylfurfural (EMF)
is an important reaction for producing diesel blendstocks from biomass resources. This aqueous-phase etherification
reaction occurs with higher
selectivity on H-BEA (94%) than on Amberlyst-15 (54%), H-FAU (64%), H-MFI (80%), and H-MOR (63%) at high
conversions (>60%). This selectivity towards EMF is not driven by transport limitations or Bronsted acid site
concentrations; instead, a kinetic preference for etherification on BEA leads to such high yields. Non-idealities
associated with the liquid-phase reaction are addressed by using UNIFAC-derived activities rather than concentrations,
and reaction kinetics measurements indicate that the reaction proceeds on a surface saturated by ethanol and with rates
that are first-order with respect to HMF activity and negative-order with respect to ethanol activity. Gas-phase
density functional theory calculations are unified with aqueous liquid-phase kinetics measurements by using partial
pressures of a hypothetical gas phase calculated from UNIFAC-derived activities. DFT calculations indicate that
etherification occurs in a single concerted step&emdash;rather than through
a two-step sequential pathway&emdash;in which HMF is protonated and dehydrated to form a relatively stable
methoxyfurfural carbocation (in contrast to ethanol dehydration). This methoxyfurfural carbocation is stabilized
by resonance in contrast to ethyl carbocations formed from ethanol protonation and dehydration, and it also leads
to the high selectivity for cross-etherification (to EMF) over self-etherification of either HMF or ethanol. This rigorous
mechanistic investigation iuses gas-phase DFT calculations to offer insights into aqueous-phase catalysis, thereby
elucidating an important reaction in biomass upgrading.
34. P. Kravchenko, V. Krishnan, and D. Hibbitts*
“Mechanism and Effects of Coverage and Particle Morphology on Rh and Pt Catalyzed NO-H2 Reactions”
Journal of Physical Chemistry C, 124 (2020) 13291–13303.
“Mechanism and Effects of Coverage and Particle Morphology on Rh and Pt Catalyzed NO-H2 Reactions”
Journal of Physical Chemistry C, 124 (2020) 13291–13303.
Abstract
Three-way catalysts, which typically include Rh, are used to treat automotive
exhaust and reduce NO with a combination of CO and H2, although few kinetic and theoretical
investigations have studied NO-H2 reactions on Rh. Here, we examine NO activation, which is
believed to control the rate of NO consumption, through direct, NO-assisted, and H2-assisted
dissociation pathways on NO*-covered Rh (111) surfaces and Rh nanoparticle models using DFT and
contrast these results with previously reported data on Pt (111) surfaces. Saturation
coverage—determined by incrementally adsorbing NO*—were determined to be 5/9 ML NO* on Pt (111),
6/9 ML on Rh (111), and 1.38 ML on a 201-atom Rh nanoparticle (~2 nm). Free energies of activation
and reaction were interpreted through maximum rate analyses over a wide range of NO and H2 pressures
to determine NO activation mechanisms. Rates are inhibited by NO at all relevant NO pressures and to
similar extents on all catalyst models. On Pt (111) surfaces, NO is activated through NOH* formation
and dissociation (to N* and OH*) at low H2 pressures (> 0.5 bar), and through HNOH* (to HN* and OH*)
at high H2 pressures (>0.5 bar), resulting in a shift in the H2 dependency from half-order to
first-order. NO is activated through NOH* formation and dissociation on Rh(111) at all relevant
H2 pressures, with all other pathways being >1000 times slower. NO activation occurs with similar
rates through either NOH* or HNO* on Rh particles at 1.38 ML NO*, indicating that these high
coverages can shift mechanistic preferences. Predicted NO consumption rates are half-order
in H2 on Rh particles and surfaces and are similar to one another, despite shifts in the mechanism;
these rates on Rh are 106-times slower than Pt, consistent with prior reports that demonstrate
equal turnover rates for Pt at 60 °C occur for Rh at 200 °C. This work demonstrates that
strong N=O bonds activate through bimolecular (H2-assisted) pathways and that particle models
of catalysts enable high coverages of strongly bound species and the modeling of high coverage catalysis.
33. M. DeLuca, C. Janes, and D. Hibbitts*
“Contrasting Arene, Alkene, Diene, and Formaldehyde Hydrogenation in H-ZSM-5, H-SSZ-13, and H-SAPO-34 Zeolite Frameworks during MTO”
ACS Catalysis, 10 (2020) 4593–4607.
“Contrasting Arene, Alkene, Diene, and Formaldehyde Hydrogenation in H-ZSM-5, H-SSZ-13, and H-SAPO-34 Zeolite Frameworks during MTO”
ACS Catalysis, 10 (2020) 4593–4607.
Abstract
Co-feeding H2 at high pressures increases zeolite catalyst lifetimes
during methanol-to-hydrocarbon (MTH) reactions while maintaining high alkene-to-alkane ratios;
however, the mechanisms and species hydrogenated by H2 co-feeds to prevent catalyst
deactivation remain unknown. This study uses periodic density functional theory (DFT) to
examine hydrogenation mechanisms of MTH product C2–C4 alkenes, as well as species related
to the deactivation of MTH catalysts such as C4 and C6 dienes, benzene, and formaldehyde in
H-MFI and H-CHA zeolite catalysts. Results show that dienes and formaldehyde are selectively
hydrogenated in both frameworks at MTH conditions because their hydrogenation transition states
proceed via allylic and oxocarbenium cations which are more stable than alkylcarbenium ions
which mediate alkene hydrogenation. Diene hydrogenation is further stabilized by protonation
and hydridation at α,δ positioned C-atoms to form 2-butene from butadiene and 3-hexene from
hexadiene as primary hydrogenation products. This α,&delta-hydrogenation directly leads to selective
hydrogenation of dienes; pathways which hydrogenate dienes at the α,β-position (e.g., forming
1-butene from butadiene) proceed with barriers 20 kJ mol-1 higher than α,&delta-hydrogenation and
with barriers nearly equivalent to butene hydrogenation, despite α,β-hydrogenation of butadiene
also occurring through allylic carbocations. Hydrogenation of formaldehyde, a diene precursor,
occurs with barriers that are within 15 kJ mol-1 of diene hydrogenation barriers, indicating that
it may also contribute to increasing catalyst lifetimes by preventing diene formation. Benzene,
in contrast to dienes and formaldehyde, is hydrogenated with higher barriers than C2–C4 alkenes
despite proceeding via stable benzenium cations because of the thermodynamic instability of the
product which has lost aromaticity. Carbocation stabilities predict the relative rates of alkene
hydrogenation and in some cases shed insights into the hydrogenation of benzene, dienes, and
formaldehyde, but cation stabilities alone cannot account for the poor hydrogenation of benzene
or the facile hydrogenation of dienes, boosted by stabilization conferred by a,d-hydrogenation.
This work suggests that the main mechanisms of catalyst lifetime improvement with high H2
co-feeds is reduction of diene concentrations through both their selective hydrogenation and
hydrogenation of their precursors to prevent formation of deactivating polyaromatic species.
32. A. Almithn and D. Hibbitts*
“Impact of Metal and Heteroatom Identities in the Hydrogenolysis of C–X Bonds (X = C, N, O, S, and Cl)”
ACS Catalysis, 10 (2020) 5086–5100.
Abstract
Hydrogenolysis of complex heteroatom-containing organic molecules
plays a large role in upgrading fossil- and biomass-based fuel and chemical feedstocks,
such as hydrodeoxygenation and desulfurization. Here, we present a fundamental study
contrasting the cleavage of C–X bonds in ethane, methylamine, methanol, methanethiol,
and chloromethane on Group 8-11 transition metals (Ru, Os, Rh, Ir, Ni, Pd, Pt, Cu, Ag,
and Au) using density functional theory (DFT). Previous kinetic and DFT studies have shown
that hydrogenolysis of unsubstituted C–C bonds in alkanes occur via unsaturated
intermediates (e.g., *CHCH* for ethane) after a series of quasi-equilibrated dehydrogenation
steps weaken the C–C bond by creating C–metal bonds. However, the effects of
the substituent group in CH3XHn on the required degree of
unsaturation to cleave the C–X have not been systematically studied and is critical
to understanding heteroatom removal. DFT-predicted free energy barriers indicate that the
carbon atom in C–X generally cleaves after the removal of 2 H atoms (to form CH*) on
Group 8-10 metals regardless of the identity of the metal or the heteroatom. Group 11 metals
(coinage metals: Cu, Ag, and Au) generally cleave the C–X bond in the most H-saturated
intermediates with barriers close to thermal activation of C–X in gaseous
CH3XHn molecules. The N-leaving group in C–N cleavage depends on
the metal identity as it can leave fully dehydrogenated (as N*) on Group 8 metals and fully
hydrogenated (as NH2*) on Group 9–11 metals. Although O and S are both Group 16 elements
C–S bonds always cleave to form S* (losing one H) while C–O bonds generally cleave
to form OH* (without preceding H removal). Cl does not have H atoms to be removed before
C–Cl cleavage in CH3Cl and thus the C atom sacrifices an additional H atom
to weaken the C–Cl bond on Group 8 metals. This study provides fundamental insights
into H2-based upgrading of complex organic molecules.
2019
31. J. Di Iorio, A. Hoffman, C. Nimlos, S. Nystrom, D. Hibbitts*, and R. Gounder*
“Mechanistic Origins of the High-Pressure Inhibition of Methanol Dehydration Rates in Small-Pore Acidic Zeolites.”
Journal of Catalysis, 380 (2019) 161–177.
Abstract
Turnover rates of Bronsted acid-catalyzed methanol dehydration to dimethyl ether
become inhibited at high methanol pressures (>10 kPa, 415 K) on small-pore zeolites (CHA, AEI, LTA, LEV),
irrespective of the distribution of framework Al and their attendant H+ sites, but not on medium-pore
or large-pore zeolites. High-pressure kinetic inhibition occurs concomitantly with the stabilization
of higher-order methanol clusters (e.g., trimers, tetramers) observed experimentally from physisorption
of liquid-like methanol and the appearance of vibrational modes for methanol clusters in IR spectra,
consistent with the attenuation of such inhibition at higher temperatures (>450 K) that result in
decreased methanol coverage. DFT-predicted methanol coverage phase diagrams confirm that higher-order
methanol clusters form in pressure and temperature ranges corresponding to the onset of kinetic
inhibition observed experimentally, and that higher-order methanol clusters are reactive but that excess
methanol ultimately increases apparent barriers to form kinetically relevant dimethyl ether elimination
transition states and thus inhibit turnover rates. This combined experimental and theoretical
investigation provides precise mechanistic interpretation of the high-pressure inhibition of methanol
dehydration turnover rates on small-pore Bronsted acid zeolites. This rigorous analysis enables the
development of kinetic models to account for the diverse structures of methanol precursors that dehydrate
to form dimethyl ether, and methods to assess the prevalence of higher-order clusters that serve as
reactive and inhibitory intermediates within small-pore zeolites during methanol conversion routes.
30. P. Kravchenko, C. Plaisance, and D. Hibbitts*,
“A New Computational Interface for Catalysis.”
Pre-print, (2019).
Abstract
Advances in computational resource availability and atomic understanding bring new challenges to
studying chemical reactions in sufficient detail—particularly in research of catalytic systems. Here,
we present a new interface for density functional theory (DFT) studies—the computational catalysis
interface (CCI). CCI simplifies DFT studies and enables multi-step and high-throughput studies with
additional automation. Calculations can be set up using descriptive language, allowing users with a
wide range of backgrounds to rapidly begin performing DFT calculations. CCI is designed to operate
from a centralized data server connected to all accessible compute servers with automated file management
between machines. Multi-step calculations with a gradual increase in accuracy increase CPU-efficiency by
up to an order of magnitude over single-step calculations. Structure manipulation tools allow users to
add or remove adsorbates from catalysts, modify chemical fragments, catalyst composition, and the location
of reactions on catalytic surfaces. Specific tools have been developed for modeling metal surfaces and
clusters as well as a suite of tools for tackling the configurational complexity of reactions occurring
within zeolite micropores. These structure manipulation tools also allow users to generate structures
from previously converged calculations by systematically altering the catalyst, reactants, and/or environment
(i.e., adding solvent or altering co-adsorbate coverages), which can significantly accelerate rigorous DFT
studies. The calculations are automatically monitored and emails are sent to the user when they begin to
diverge, fail, or finish. These emails can be customized to contain relevant information and links to a
web-based graphical user interface (GUI) that provides a 3-D visualization of structures and movies for
convergence trajectories, reaction pathways, and vibrational modes. This web-GUI also allows users to
start calculations directly using context-specific commands. CCI also facilitates dissemination of data
and interfaces with the POV-Ray utility to generate high-resolution publication- and presentation-quality
images. These tools are described here in detail with several examples of reactions and catalysts to show
how CCI significantly expedites computational catalysis studies.
29. M. DeLuca and D. Hibbitts*,
“Prediction of C6–C12 Interconversion Rates Using Novel Zeolite-specific Kinetic Monte Carlo Simulation Methods.”
Pre-print, (2019).
Abstract
This study introduces a novel kinetic Monte Carlo (KMC) simulation package which models H-ZSM-5
crystals across experimentally relevant time and length scales to understand the role of transport
during arene interconversion reactions (~100 reactions). This small subset of the methanol-to-hydrocarbon
(MTH) network was previously modeled using periodic, dispersion-corrected density functional theory
(DFT) to determine activation barriers and reaction energies for these KMC methods. Transport of arene
molecules through the straight and sinusoidal channels of MFI was modeled as site-hopping and the
DFT-calculated barriers are incorporated into the KMC model to account for mass-transport limitations.
Barriers of different arene molecules trend well with their effective radii, and species with a smaller
effective radii diffusive more readily. A previously published maximum rate analysis of arene
interconversion pathways—previously validated by experimental data—is compared to a diffusion-free
KMC model to confirm the accuracy of this KMC package. The temperature and pressure dependencies of
rates obtained from KMC agree well with those of maximum rate analysis on the diffusion-free model,
demonstrating that KMC effectively predicts rates as well as maximum rate analysis methods commonly
used in kinetic applications of DFT. Arene interconversion pathways were also analyzed on KMC models
incorporating diffusion to and from interior crystal sites. These simulations suggest that large species,
such as hexamethylbenzene, become trapped at 10–20% of sites, thus causing site deactivation by limiting
diffusion through MFI channels and lowering overall rates of product formation. Benzene diffusion barriers
are artificially varied from 20–200 kJ mol−1 and rates of benzene methylation decrease by 4-fold with diffusion
barriers greater than 80 kJ mol−1; this suggests that species with
diffusion barriers greater than 80 kJ mol−1
(such as penta- and hexamethylbenzene) will likely become trapped at interior sites and ultimately cause catalyst
deactivation. This study serves as a proof-of-concept for a novel KMC package that expedites kinetic analysis of
complex reaction pathways and introduces mass-transport limitations which are not commonly accounted for in kinetic
DFT studies. This KMC package can predict the behavior of diffusion-limited species, such as penta- and
hexamethylbenzene, and the mechanisms by which they are formed and eventually lead to catalyst deactivation.
28. M. Witzke, A. Almithn, C. Coonrod, M. Triezenberg, D. Hibbitts*, and D. Flaherty*,
“In situ Spectroscopic Methods for Isolating Reactive Intermediate Structures during Hydrogenolysis Reactions.”
Journal of the American Chemical Society, 141 (2019) 16671–16684.
Abstract
Identifying individual reactive intermediates within the 'zoo' of organometallic species that form
on catalytic surfaces during reactions is a long-standing challenge in heterogeneous catalysis.
Here, we identify distinct reactive intermediates, all of which exist at low-coverages, that lead
to distinguishable reaction pathways during hydrogenolysis of 2-methyltetrahydrofuran (MTHF) on Ni,
Ni12P5, and Ni2P catalysts by combining advanced spectroscopic methods with quantum chemical
calculations. Each of these reactive complexes cleave specific C–O bonds and give rise to unique
products and exhibit different apparent activation barriers for ring opening. The spectral
features of the reactive intermediates are extracted by collecting in situ infrared spectra while
sinusoidally modulating H2 pressure during MTHF hydrogenolysis and applying phase sensitive
detection (PSD), which suppresses features of inactive surface species. The combined spectra of
all reactive species are deconvoluted using singular value decomposition techniques that yield
spectra and changes in surface coverages for each set of kinetically differentiable species.
These deconvoluted spectra are consistent with predicted spectral features for the reactive
surface intermediates implicated by detailed kinetic measurements and DFT calculations. Notably,
these methods give direct evidence for several anticipated differences in the coordination and
composition of reactive MTHF-derived species. Compositions of the most abundant reactive
intermediate (MARI) on Ni, Ni12P5, and Ni2P nanoparticles during C–O bond rupture of MTHF are
identical; however, the MARI changes orientation from Ni3(μ3-C5H10O) to
Ni3(η5-C5H10O) (i.e.,
lies more parallel with the catalyst surface) with increasing phosphorus content. The shift
in binding configuration with phosphorus content suggests the decrease in steric hindrance to
rupture the 3C–O bond is the fundamental cause for increased selectivity towards 3C–O bond rupture.
Previous kinetic measurements and DFT calculations indicate that C–O bond rupture occurs on Ni
ensembles on Ni, Ni12P5, and Ni2P catalysts; however, the addition of more electronegative
phosphorus atoms that withdraw a small charge from Ni ensembles results in differences in
binding configuration, activation enthalpy and selectivity. The results from this in situ
spectroscopic methodology support previous proposals that manipulation of the electronic
structure of metal ensembles by the introduction of phosphorus provides strategies to design
catalysts for selective cleavage of hindered C–X bonds and demonstrate the utility of this
approach for identifying individual reactive species within the 'zoo'.
27. M. DeLuca, P. Kravchenko, A. Hoffman, and D. Hibbitts*,
“Mechanism and Kinetics of Methylating C6–C12 Methylbenzenes with Methanol and DME in H-MFI Zeolites.”
ACS Catalysis, 9 (2019) 6444–6460.
Front Cover Article!
Abstract
This study uses periodic density functional theory (DFT) to determine the reaction mechanism and
effects of reactant size for all 20 arene (C6–C12)
methylation reactions using CH3OH and CH3OCH3
as methylating agents in H-MFI zeolites. Reactant, product, and transition state structures were
manually generated, optimized, and then systematically reoriented and reoptimized to sufficiently
sample the potential energy surface and thus identify global minima and the most stable transition
states which interconnect them. These systematic reorientations decreased energies by up to 50 kJ
mol−1, demonstrating their necessity when analyzing reaction pathways or adsorptive properties of
zeolites. Benzene-DME methylation occurs via sequential pathways, consistent with prior reports,
but is limited by surface methylation which is stabilized by co-adsorbed benzene via novel cooperativity
between the channels and intersections within MFI. These co-adsorbate assisted surface methylations
generally prevail over unassisted routes. Calculated free energy barriers and reaction energies
suggest that both the sequential and concerted methylation mechanisms can generally occur, depending
on the methylating agent and methylbenzene being reacted—there is no consensus mechanism for these
homologous reactions. Intrinsic methylation barriers for step-wise reactions of benzene to
hexamethylbenzene remain between 75–137 kJ mol−1 at conditions relevant to methanol-to-hydrocarbon
(MTH) reactions where such arene species act as co-catalysts. Intrinsic methylation barriers are
similar between CH3OH and CH3OCH3 suggesting that both species are equally capable of interconverting
between methylbenzene species. Additionally, these methylation barriers do not systematically
increase as the number of methyl-substituents on the arene increases and the formation of higher
methylated arenes is thermodynamically favorable. These barriers are significantly lower than
those associated with alkene formation during the aromatic cycle, suggesting that aromatic species
formed during MTH reactions either egress from the catalyst—depending on that zeolite?s pore
structure—or become trapped as extensively-substituted C10–C12 species which can either isomerize
to form olefins or ultimately create polyaromatic species that deactivate MTH catalysts.
26. A. Hoffman, M. DeLuca, and D. Hibbitts*,
“Restructuring of MFI Framework Zeolite Models and their Associated Artifacts in Density Functional Theory Calculations.”
Journal of Physical Chemistry C, 123 (2019) 6572–6585.
Editor's Choice!
Abstract
This study compares and evaluates multiple orthorhombic silicalite MFI framework structures using periodic density functionals theory (DFT)
calculations implemented with a wide range of exchange-correlation functionals and dispersion-correction schemes. Optimization of the structure
available from the International Zeolite Association (IZA) yields only metastable forms, which restructure to arrangements 18 to 156 kJ mol−1
lower in energy (55 kJ mol−1 on average) through annealing and adsorption/desorption processes without altering their connectivity.
These restructuring events can occur unintentionally during DFT studies of adsorptive and catalytic properties, leading to very large artifacts
in DFT-predicted adsorption, reaction, and activation energies. Pre-annealing the IZA structure prevents restructuring and these artifacts,
but forms MFI structures which do not conform to the Pnma spacegroup symmetry and have significantly perturbed sinusoidal and straight
channel geometries. These issues persist across a wide range of exchange-correlation functionals, including common choices such as PBE and BEEF,
and dispersion correction schemes such as the D3 method. Direct optimization of structures generated from the work of van Koningsveld et al.
and Olson et al., in contrast, yield structures that are extremely similar across all functionals, restructure less often during annealing,
and have smaller energy shifts when they do restructure (5 kJ mol−1, on average). Optimizing the unit cell parameters of these structures
without constraining atoms or unit cell shape also yielded more stable structures, though often with unit cell parameters that did not closely
match structures found experimentally. Annealing of other commonly studied zeolites (BEA, CHA, and LTA) did not yield structures with energy
decreases or structural changes as significant as those for MFI. This study thus illuminates a potential source of significant error for
DFT studies of MFI and provides evidence-based solutions for a variety of DFT methods.
25. A. Almithn and D. Hibbitts*,
“Comparing Rate and Mechanism of Ethane Hydrogenolysis on Transition Metal Catalysts.”
Journal of Physical Chemistry C, 123 (2019) 5421–5432.
Abstract
The effects of metal catalyst identity on ethane hydrogenolysis rates and mechanism were examined using
density function theory (DFT) for Group 8–11 metals (Ru, Os, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au).
Previously measured turnover rates on Ru, Rh, and Ir clusters show H2-pressure dependence of
[H2]−3, consistent with C–C bond activation in *CHCH* intermediates in reactions
that require two H* (chemisorbed H) to desorb from the H*-covered surfaces that prevail at these
hydrogenolysis conditions. Previous DFT calculations on Ir catalysts have shown that C–C bonds
in alkanes are weakened by forming C-metal bonds through quasi-equilibrated dehydrogenation steps
during ethane hydrogenolysis, and these steps form *CHCH* intermediates which undergo a kinetically-relevant
C–C bond cleavage step. Here, DFT-calculated free energy barriers show that *CH–CH* bond
activation is also more favorable than all C–C bond activations in other intermediates on Group
8–10 metals by > 34 kJ mol−1 with the exception of Pd where *CHCH* and CH3CH*
activate with similar activation free energies (242 and 253 kJ mol−1, respectively, 593 K).
The relative free energy barriers between *CH–CH* bond cleavage and C–C bond cleavage in
more saturated intermediates decrease as one moves from left to right in the periodic table until
*CH3–CH2* bond cleavage becomes more favorable on Group 11 coinage metals
(Cu, Ag, and Au). Such predicted trends is consistent with measure turnover rates that decrease as
Ru > Rh > Ir > Pt and show H2-pressure dependence of ~[H2]−3
(λ = 3) for Ru, Rh, and Ir clusters and [H2]−2.3 (λ = 2.3) for Pt clusters.
The decrease in the measured λ value for Pt, however, is caused by a decrease in the number of
desorbed H* atoms from the surface (γ = 0–1) rather than a change in the mechanism as shown
here using a H*-covered Pt119 half-particle model. The lower H*-coverage on Pt compared to
other metals and the lateral relaxation of the adlayer in curved nanoparticle models, as reported
previously, allow *CH–CH* bond cleavage to occur at lower number of vacant sites on Pt.
24. M. Garcia-Dieguez, D. Hibbitts*, and E. Iglesia*,
“Hydrogen Chemisorption Isotherms on Pt Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited.”
Journal of Physical Chemistry C, 123, (2019), 8447–8462.
Abstract
Density functional theory (DFT) and dihydrogen chemisorption uptakes at temperatures relevant to catalysis
are used to determine and interpret adsorption enthalpies and entropies over a broad range of chemisorbed
hydrogen (H*) coverages (0.1 ML to saturation) on Pt nanoparticles (1.6, 3.0, and 9.1 nm mean diameters)
and Pt (111) surfaces. Heats of adsorption decrease by 30 kJ mol−1 H2 as
coverage increases from nearly bare (0.1 ML) to saturated (~1 ML) surfaces, because of the preferential
saturation of low-coordination surface atoms by H* at low coverages. Such surface non-uniformity also
leads to stronger binding on small Pt particles at all coverages (e.g., 47 kJ mol−1 on
1.6 nm, 40 kJ mol−1 on 3.0 nm, and 37 kJ mol−1 on 9.1 nm particles
all at 0.5 ML), because their surfaces expose a larger fraction of low-coordination atoms. As H* approaches
monolayer coverages, H*–H* repulsion leads to a sharp decrease in binding energies on all Pt
particles. Measured H* entropies decrease with increasing H* coverage and decreasing particle size,
but their values (35–65 J mol−1 K−1) are much larger than for
immobile H* species at all coverages and particle sizes. Two-dimensional gas models in which mobile H*
adsorbates move rapidly across uniform (ideal), excluded-area, or DFT-predicted non-uniform potential
energy surfaces all give H* entropies ≥ 30 J mol−1 K−1,
qualitatively consistent with measured values. Larger H* entropies are predicted for uniform (ideal)
PES while smaller H* entropies are predicted using non-uniform PES. DFT-derived barriers for H*
diffusion between hcp and fcc three-fold sites on Pt (111) surface are about 6 kJ mol−1,
a value similar to thermal energies (kT) at temperatures of catalytic relevance, consistent with fast
diffusion in the timescale of adsorption-desorption events and isotherm measurements. The
non-uniformity of Pt surfaces, repulsion among co-adsorbates, and high adsorbate mobility starkly
contrast with the requirements for Langmuirian descriptions of binding and reactions at surfaces,
despite the ubiquitous use and success in practice of the resulting equations in describing adsorption
isotherms and reaction rates on surfaces. Such fortuitous agreement is a likely consequence of the
versatility of their functional form, taken together with the limited range in pressure and
temperature in adsorption and kinetic measurements. The adsorption and kinetic constants derived from
such data, however, would differ significantly from theoretical estimates that rigorously account
for surface coordination, adsorbate mobility, and co-adsorbate repulsion.
2018
23. M. Cordon, J. Harris, J. Vega-Vila, J. Bates, S. Kaur, M. Gupta, M. Witzke, E. Wegener, J. Miller, D. Flaherty, D. Hibbitts, R. Gounder*,
“The Dominant Role of Entropy in Stabilizing Sugar Isomerization Transition States within Hydrophobic Zeolite Pores.”
Journal of the American Chemical Society, 140 (2018) 14244–14266.
Abstract
Lewis acid sites in zeolites catalyze aqueous-phase sugar isomerization at higher turnover rates when confined within hydrophobic rather than
within hydrophilic micropores; however, relative contributions of competitive water adsorption at active sites and preferential stabilization
of isomerization transition states have remained unclear. Here, we employ a suite of experimental and theoretical techniques to elucidate the
effects of coadsorbed water on glucose isomerization reaction coordinate free energy landscapes. Transmission IR spectra provide evidence that
water forms extended hydrogen-bonding networks within hydrophilic but not hydrophobic micropores of Beta zeolites. Aqueous-phase glucose
isomerization turnover rates measured on Ti-Beta zeolites transition from first-order to zero-order dependence on glucose thermodynamic activity,
as Lewis acidic Ti sites transition from water-covered to glucose-covered, consistent with intermediates identified from modulation excitation
spectroscopy during in situ attenuated total reflectance IR experiments. First-order and zero-order isomerization rate constants are systematically
higher (by 3–12x, 368–383 K) when Ti sites are confined within hydrophobic micropores. Apparent activation enthalpies and entropies reveal
that glucose and water competitive adsorption at Ti sites depend weakly on confining environment polarity, while Gibbs free energies of hydride-shift
isomerization transition states are lower when confined within hydrophobic micropores. DFT calculations suggest that interactions between
intraporous water and isomerization transition states increase effective transition state sizes through second-shell solvation spheres, reducing
primary solvation sphere flexibility. These findings clarify the effects of hydrophobic pockets on the stability of coadsorbed water and
isomerization transition states and suggest design strategies that modify micropore polarity to influence turnover rates in liquid water.
22. S. Nystrom^, A. Hoffman^, and D. Hibbitts*,
“Tuning Brønsted Acid Strength by Altering Site Proximity in CHA Framework Zeolites.”
ACS Catalysis, 8 (2018) 7842–7860.
Abstract
This study examines how Brønsted acid
strengths—as predicted by dispersion-corrected periodic
DFT calculations of deprotonation energy (DPE), dehydrogenation energy (DHE), and NH3 binding energy (NH3
BE)—are affected by site proximity in proton-form zeolites
and how adsorbates on one acid site alter the strength of
nearby acids. Protons can bind to four distinct O atoms around
the single crystallographically unique T-site of CHA, and all
such locations were examined as bare and NH3-occupied sites.
Protons prefer to bind to O1 atoms and orient within the plane
of six-membered-ring (6MR) structures of CHA. NH4+ cations
show a strong preference for binding in 8MR windows; 6MR
structures are too small to solvate them. These preferences
govern proximity effects on acid strength, studied here by probing the strength of a Brønsted acid site while a second site is
placed in 23 locations separated by 1–3 T-sites. Placing a second acid in the 6MR of CHA decreased DPE and NH3 BE values
for the first site by >10 kJ mol−1 because the proton of the second site stabilized the deprotonated site across the 6MR. Acid
site-pairs across 8MR structures interact very little when the second acid is bare, as residual protons do not prefer to orient
within 8MR. One location of the second acid stabilized the adsorbed proton without stabilizing the deprotonated state, resulting
in a significantly weaker acid. All of these effects are altered when the second site is instead occupied by an adsorbed NH3,
which acts as a proxy for strongly bound reactive intermediates and cationic transition states. The strength of the first site is
significantly weakened (DPE and NH3 BE increases of >20 kJ mol−1) when a second site is NH3-occupied and placed in the
6MR because such structures are too small to effectively solvate NH4+ cations. Acid sites are strengthened, however, when
second sites are NH3-occupied and placed across 8MR windows, because they are appropriately sized to solvate the NH4+
cations that simultaneously interact with both deprotonated sites. The alteration of acid strength by acid site proximity therefore
depends on the specific arrangement (not merely Al–Al distances), the structural motifs present (such as 6MR structures which
allow protons, but not NH4+, to stabilize proximal conjugate base anions), and the status of proximal sites as vacant or occupied,
which determines the distances over which cationic-anionic stabilizations of deprotonated sites can take place.
21. M. Witzke, A. Almithn, C. Coonrod, D. Hibbitts*, and D. Flaherty*,
“Mechanisms and Active Sites for C-O Bond Rupture within 2-Methyltetrahydrofuran over Nickel Phosphide Catalysts.”
ACS Catalysis, 8 (2018) 7141–7157.
Abstract
Nickel phosphide catalysts (Ni12P5 and Ni2P) preferentially cleave
sterically hindered 3C–O bonds over unhindered 2C–O bonds, and
Ni2P is up to 50 times more selective toward 3C–O bond cleavage than Ni.
Here, we combine kinetic measurements, in situ infrared spectroscopy, and density functional theory
(DFT) calculations to describe the mechanism for C–O bond rupture over Ni, Ni12P5,
and Ni2P catalysts. Steady-state rate measurements and DFT calculations of C–O
bond rupture within 2-methyltetrahydrofuran (MTHF) show that quasi-equilibrated MTHF adsorption
and dehydrogenation steps precede kinetically relevant C–O bond rupture at these conditions
(1–50 kPa MTHF; 0.1–6 MPa H2; 543 K). Rates for 3C–O and
2C–O bond rupture are inhibited by H2, and the ratio of these rates
increases with [H2]1/2, suggesting that the composition of the reactive
intermediates for 3C–O and 2C–O rupture differs by one H atom.
Site-blocking CO*, NH3*, and H* inhibit rates without altering the ratio of
3C–O to 2C–O bond rupture, indicating that these C–O bond
rupture precursors and transition states bind to identical active sites. DFT-based predictions
suggest that these sites are exposed ensembles of 3 Ni atoms on Ni(111) and Ni2P(001)
and 4 Ni atoms on Ni12P5(001) and that the incorporation of P disrupts
extended Ni ensembles and alters the reactivity of the Ni. Increasing the phosphorus to nickel
ratio (P:Ni) decreases measured and DFT-predicted activation enthalpies (ΔH‡,
473–583 K) for 3C–O bond rupture relative to that of 2C–O
bond rupture. Selectivity differences between specific C–O bonds within MTHF reflect
differences in the H content of reactive intermediates, activation enthalpy barriers, and
P:Ni of Ni, Ni12P5, and Ni2P nanoparticles.
20. A. Almithn and D. Hibbitts*,
“Effects of Catalyst Model and High Adsorbate Coverages in ab initio Studies of Alkane Hydrogenolysis.”
ACS Catalysis, 8 (2018) 6375–6387.
Abstract
Bare, low-index periodic surface models are typically used to examine metal-catalyzed reactions in
density functional theory (DFT) studies, and these most closely resemble low-pressure surface science
reactions and catalyzed reactions that occur on large terraces that prevail on large (>5 nm) supported
nanoparticles. Many catalytic reactions, however, occur near conditions at which catalytic surfaces are
saturated by one or more adsorbed intermediates, leading to strong coadsorbate interactions and surface
reconstruction leading to increased curvature. Alkane hydrogenolysis is such a reaction and has been
extensively studied using DFT—often on bare metal surfaces—with the assumption that omitted
coadsorbed hydrogen atoms (H*) do not significantly alter the relative activation barriers and with ad
hoc assumptions about the site requirements for relevant reactions. Here, we use ethane hydrogenolysis
on H*-covered Ir catalysts (using a periodic surface model and a nanoparticle model) as a probe reaction
to examine coadsorbate interactions and to demonstrate the rigorous determination of site requirements.
The kinetically relevant transition state [*CH–CH*]‡ is larger than the 0–3
coadsorbed H* atoms it replaces, such that the reaction has a positive activation area (a concept analogous
to activation volume in homogeneous reactions) and thus repels coadsorbed H* atoms when fewer than four
H* vacancies are created. This induced strain cannot be relieved on the periodic surface models, resulting
in large effective free energy barriers and predictions that four vacant sites are required (γ = 4).
These barriers and site requirements lead to turnover rates that are 4 orders of magnitude lower than
measured rates and incorrect H2-pressure dependencies. Furthermore, varying the unit cell
size of the Ir(111) surface dramatically alters the calculated reaction energetics, indicating that
relevant transition states destabilize one another over long distances through the H* adlayer. Curved
H*-covered Ir hemispherical particle models (119 atoms), however, stabilize transition states at a
lower number of vacant sites (γ = 2) through lateral relaxation of the adlayer, resulting in
correct predictions of H2-pressure dependencies and quantitative agreement between
calculated and measured rates.
19. A. Almithn and D. Hibbitts*,
“Supra-Monolayer Coverages on Small Metal Clusters and Their Effects on H2 Chemisorption Particle Size Estimates.”
AIChE Journal, 64 (2018) 3109–3120. Invited.
Abstract
H2 chemisorption measurements are used to estimate the size of supported metal particles,
often using a hydrogen-to-surface-metal stoichiometry of unity. This technique is most useful for small
particles whose sizes are difficult to estimate through electron microscopy or X-ray diffraction.
Undercoordinated metal atoms at the edges and corners of particles, however, make up large fractions of
small metal clusters, and can accommodate multiple hydrogen atoms leading to coverages which exceed 1 ML
(supra-monolayer). Density functional theory was used to calculate hydrogen adsorption energies on Pt and
Ir particles (38–586 atoms, 0.8–2.4 nm) at high coverages (≤3.63 ML). Calculated
differential binding energies confirm that Pt and Ir (111) single-crystal surfaces saturate at 1 ML;
however, Pt and Ir clusters saturate at supra-monolayer coverages as large as 2.9 ML. Correlations between
particle size and saturation coverage are provided that improve particle size estimates from H2
chemisorption for Pt-group metals.
2017
18. J. Liu, D. Hibbitts*, and E. Iglesia*,
“Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces.”
Journal of the American Chemical Society, 139 (2017) 11789–11802.
Abstract
High CO* coverages lead to rates much higher than Langmuirian treatments predict because co-adsorbate
interactions destabilize relevant transition states less than their bound precursors. This is shown here
by kinetic and spectroscopic data—interpreted by rate equations modified for thermodynamically nonideal
surfaces—and by DFT treatments of CO-covered Ru clusters and lattice models that mimic adlayer
densification. At conditions (0.01–1 kPa CO; 500–600 K) which create low CO* coverages
(0.3–0.8 ML from in situ infrared spectra), turnover rates are accurately described by Langmuirian
models. Infrared bands indicate that adlayers nearly saturate and then gradually densify as pressure increases
above 1 kPa CO, and rates become increasingly larger than those predicted from Langmuir treatments (15-fold at
25 kPa and 70-fold at 1 MPa CO). These strong rate enhancements are described here by adapting formalisms for
reactions in nonideal and nearly incompressible media (liquids, ultrahigh-pressure gases) to handle the strong
co-adsorbate interactions within the nearly incompressible CO* adlayer. These approaches show that rates are
enhanced by densifying CO* adlayers because CO hydrogenation has a negative activation area (calculated by DFT),
analogous to how increasing pressure enhances rates for liquid-phase reactions with negative activation volumes.
Without these co-adsorbate effects and the negative activation area of CO activation, Fischer–Tropsch
synthesis would not occur at practical rates. These findings and conceptual frameworks accurately treat dense
surface adlayers and are relevant in the general treatment of surface catalysis as it is typically practiced
at conditions leading to saturation coverages of reactants or products.
17. R. Rao, R. Blume, T, Hansen, E. Fuentes, K. Dreyer, S. Moldovan, O. Ersen, D. Hibbitts, Y. Chabal, R. Schlogl, and J. Tessonnier*,
“Interfacial charge distributions in carbon-supported palladium catalysts.”
Nature Communications, 8 (2017) 340:1–10.
Abstract
Controlling the charge transfer between a semiconducting catalyst carrier and the supported transition metal
active phase represents an elite strategy for fine turning the electronic structure of the catalytic centers,
hence their activity and selectivity. These phenomena have been theoretically and experimentally elucidated for
oxide supports but remain poorly understood for carbons due to their complex nanoscale structure. Here, we combine
advanced spectroscopy and microscopy on model Pd/C samples to decouple the electronic and surface chemistry
effects on catalytic performance. Our investigations reveal trends between the charge distribution at the
palladium-carbon interface and the metal's selectivity for hydrogenation of multifunctional chemicals. These
electronic effects are strong enough to affect the performance of large (~5 nm) Pd particles. Our results also
demonstrate how simple thermal treatments can be used to tune the interfacial charge distribution, hereby
providing a strategy to rationally design carbon-supported catalysts.
16. M. Neurock*, Z. Tao, A. Chemburkar, D. Hibbitts, and E. Iglesia,
“Theoretical Insights into the Sites and Mechanisms for Base Catalyzed Esterification and Aldol Condensation Reactions over Cu.”
Faraday Discussions, 197 (2017) 59–86. Invited.
Abstract
Condensation and esterification are important catalytic routes in the conversion of polyols and oxygenates
derived from biomass to fuels and chemical intermediates. Previous experimental studies show that alkanal,
alkanol and hydrogen mixtures equilibrate over Cu/SiO2 and form surface alkoxides and alkanals
that subsequently promote condensation and esterification reactions. First-principle density functional
theory (DFT) calculations were carried out herein to elucidate the elementary paths and the corresponding
energetics for the interconversion of propanal + H2 to propanol and the subsequent C–C and
C–O bond formation paths involved in aldol condensation and esterification of these mixtures over
model Cu surfaces. Propanal and hydrogen readily equilibrate with propanol via C–H and O–H
addition steps to form surface propoxide intermediates and equilibrated propanal/propanol mixtures. Surface
propoxides readily form via low energy paths involving a hydrogen addition to the electrophilic carbon
center of the carbonyl of propanal or via a proton transfer from an adsorbed propanol to a vicinal propanal.
The resulting propoxide withdraws electron density from the surface and behaves as a base catalyzing the
activation of propanal and subsequent esterification and condensation reactions. These basic propoxides
can readily abstract the acidic Cα–H of propanal to produce the
CH3CH(−)CH2O* enolate, thus initiating aldol condensation. The enolate can
subsequently react with a second adsorbed propanal to form a C–C bond and a β-alkoxide alkanal
intermediate. The basic surface propoxide that forms on Cu can also
attack the carbonyl of a surface propanal to form propyl propionate. Theoretical results indicate that the rates for both
aldol condensation and esterification are controlled by reactions between surface propoxide and propanal
intermediates. In the condensation reaction, the alkoxide abstracts the weakly acidic hydrogen of the
Cα–H of the adsorbed alkanal to form the surface enolate whereas in the esterification
reaction the alkoxide nucleophilically attacks the carbonyl group of a vicinal bound alkanal. As both
condensation and esterification involve reactions between the same two species in the rate-limiting step,
they result in the same rate expression which is consistent with experimental results. The theoretical
results indicate that the barriers between condensation and esterification are within 3 kJ mol−1
of one another with esterification being slightly more favored. Experimental results also report small
differences in the activation barriers but suggest that condensation is slightly preferred.
2016
15. D. Hibbitts, D. Flaherty, and E. Iglesia*,
“Effects of Chain Length on the Mechanism and Rates of Metal-Catalyzed Hydrogenolysis of n-Alkanes.”
Journal of Physical Chemistry C, 120 (2016) 8125–8138.
Abstract
C–C cleavage in C2–C10 n-alkanes involves quasi-equilibrated C–H
activation steps to form dehydrogenated intermediates on surfaces saturated with H atoms. These reactions
are inhibited by H2 to similar extents for C–C bonds of similar substitution in all acyclic
and cyclic alkanes and, thus, show similar kinetic dependences on H2 pressure. Yet, turnover rates
depend sensitively on chain length because of differences in activation enthalpies (ΔH‡)
and entropies (ΔS‡) whose mechanistic origins remain unclear. Density functional theory
(DFT) estimates of ΔH‡ and ΔG‡ for C–C cleavage via >150
plausible elementary steps for propane and n-butane reactants on Ir show that hydrogenolysis occurs via
α,β-bound RC*–C*R'‡ transition states (R = H, CxH2x+1)
in which two H atoms are removed from each C*. Calculated ΔH‡ values decrease with
increasing alkane chain length (C2–C8), consistent with experiment, because attractive
van der Waals interactions with surfaces preferentially stabilize larger transition states. A concomitant increase
in ΔS‡, evident from experiments, is not captured by periodic DFT methods, which treat
low-frequency vibrational modes inaccurately, but statistical mechanics treatments describe such effects well
for RC*–C*R‡ species, as previously reported. These findings, together with parallel
studies of the cleavage of more substituted C–C bonds in branched and cyclic alkanes, account for the
reasons that chain length and substitution influence ΔH‡ and ΔS‡
values and the dependence of rates on H2 pressure and consequently explain differences in
hydrogenolysis reactivities and selectivities across all alkanes.
14. D. Hibbitts, E. Dybeck, T. Lawlor, M. Neurock*, and E. Iglesia*,
“Preferential Activation of Carbon Monoxide near Hydrocarbon Chains during Fischer-Tropsch Synthesis.”
Journal of Catalysis, 337 (2016) 91–101.
Abstract
We report here theoretical evidence for an enhancement in CO activation to form C1 monomers at locations
near growing hydrocarbon chains as a result of their ability to disrupt the dense monolayers of chemisorbed CO*
present during Fischer–Tropsch synthesis (FTS). These previously unrecognized routes become favored at the
high CO* coverages that prevail on curved cluster surfaces at conditions of FTS practice and account for the
rapid growth of chains, which requires a source of vicinal monomers. CO activation initially requires a vacant site
(and consequently CO* desorption) and proceeds via CO* reactions with H* to form hydroxymethylene (CH*OH*), which
then dissociates to form OH* and CH*; CHx* species can subsequently act as monomers and insert into
chains, a process denoted as the 'carbene' mechanism. These CH*, and their larger alkylidyne
(CnH2n−1*) homologs, disrupt the dense CO* adlayers and in doing so allow the facile
formation of vicinal CH*OH* intermediates that mediate CO activation, without requiring, in this case, CO* desorption.
This causes CO* activation effective enthalpy and free energy barriers to be ~100 and ~15 kJ mol−1 lower,
respectively, near growing chains than within unperturbed monolayers. These effects are observed near alkylidyne
(CnH2n−1*) but not alkylidene (CnH2n*) or alkyl
(CnH2n+1*) chains. These phenomena cause monomers to form preferentially near growing
alkylidyne chains, instead of forming at undisrupted regions of CO* monolayers, causing chain growth
(via CHx*-insertion) to occur much more rapidly than chain initiation, a requirement to form long chains.
Such routes resolve the seemingly contradictory proposals that CHx* species act as monomers (instead of
CO*) and chain initiators, but their formation and diffusion on dense CO* adlayers must occur much faster than
chain initiation for such chains to grow fast and reach large average lengths. Chains disrupt surrounding molecules
in the adlayer, causing faster monomer formation precisely at locations where they can readily react with growing
chains. This work illustrates how interactions between transition states and co-adsorbates can dramatically affect
predicted rates and selectivities at the high coverages relevant to practical catalysis.
13. D. Hibbitts and M. Neurock*,
“Promotional Effects of Chemisorbed Oxygen and Hydroxide in the Activation of C–H and O–H Bonds on Transition Metal Surfaces.”
Surface Science, 650 (2016) 210–220.
Abstract
Electronegative coadsorbates such as atomic oxygen (O*) and hydroxide (OH*) can act as Brønsted bases when bound
to Group 11 as well as particular Group 8–10 metal surfaces and aid in the activation of X–H bonds. First-principle
density functional theory calculations were carried out to systematically explore the reactivity of the C–H bonds of
methane and surface methyl intermediates as well as the O–H bond of methanol directly and with the assistance of coadsorbed
O* and OH* intermediates over Group 11 (Cu, Ag, and Au) and Group 8–10 transition metal (Ru, Rh, Pd, Os, Ir, and Pt)
surfaces. C–H as well as O–H (X–H) bond activation over the metal proceeds via a classic oxidative addition type
mechanism involving the insertion of the metal center into the X–H bond. O* and OH* assist X–H
activation over particular Group 11 and Group 8–10 metal surfaces via a α-bond metathesis type
mechanism involving the oxidative addition of the X–H bond to the metal along with a reductive deprotonation
of the acidic X–H bond over the M–O* or M–OH* site pair. The O*- and OH*-assisted C–H
activation paths are energetically preferred over the direct metal catalyzed C–H scission for all Group 11 metals
(Cu, Ag, and Au) with barriers that are 0.4–1.5 eV lower than those for the unassisted routes.
The activation of the O–H bond of methanol is significantly
promoted by O* as well as OH* intermediates over both the Group 11 metals (Cu, Ag, and Au) as well as on all Group
8–10 metals studied (Ru, Rh, Pd, Os, Ir, and Pt). The higher degree of
O*- and OH*-promotion in activating methanol over that in methane and methyl is due to the stronger interaction between
the basic O* and OH* sites and the acidic proton in the O–H bond of methanol versus the non-acidic H in the C–H
bond of methane. A detailed analysis of the binding energies and the charges for on different metal surfaces
indicates that the marked differences in the properties and reactivity of O* and OH* between the Group 11 and Group 8–10
metals is due to the increased negative charge on the O-atoms bound to Group 11 metals. The promotional
effects of O* and OH* are consistent with a proton-coupled electron transfer and the cooperative role of the metal-O* or
metal-OH* pair in carrying out the oxidative addition and reductive deprotonation of the acidic C–H and O–H bonds.
Ultimately, the ability of O* or OH* to act as a Brønsted base depends upon its charge, its binding energy on the
metal surface, and the acidity of the H-atom being abstracted.
12. D. Hibbitts, D. Flaherty, and E. Iglesia*,
“Role of Branching on the Rate and Mechanism of C–C Cleavage in Alkanes on Metal Surfaces.”
ACS Catalysis, 6 (2016) 469–482.
Abstract
The kinetic relevance and rates of elementary steps involved in C–C bond hydrogenolysis for isobutane, neopentane,
and 2,3-dimethylbutane reactants were systematically probed using activation enthalpies and free energies derived from
density functional theory. Previous studies showed that C–C cleavage in alkanes occurs via unsaturated species
formed in fast quasi-equilibrated C–H activation steps, leading to rates that decrease with increasing H2
pressure, because of a concomitant decrease in the concentration of the relevant transition states. This study, together
with previous findings for n-alkanes, provides a general mechanistic construct for the analysis and prediction of
C–C hydrogenolysis rates on metals. C–C cleavage in alkanes is preceded by the loss of two H atoms and the
formation of two C-metal (C–M) bonds for each 1C and 2C atom involved in the C–C bond.
Metal atoms transfer electrons into the 1C and 2C atoms as C–C bonds cleave and additional
C–M bonds form. 3C and 4C atoms of isobutane, neopentane, and 2,3-dimethylbutane, however,
do not lose H atoms before C–C cleavage, and thus, transition states cannot bind the 3C and 4C
atoms in the C–C bond being cleaved to surface metal atoms. C–H activation occurs instead at 1C atoms vicinal
to the C–C bond, which lose all H atoms and form three C–M bonds. These transition states involve electron
transfer into the metal surface, leading to a net positive charge at the 3C and 4C atoms; these
atoms exhibit sp2 geometry and resemble carbenium ions at the C–C cleavage transition state, in which
they are not bound to the metal surface. These mechanistic features accurately describe measured H2 effects,
activation enthalpies, and entropies, and furthermore, they provide the molecular details required to understand and
predict the effects of temperature on hydrogenolysis rates and on the location of C–C bond cleavage within a given
alkane reagent. The result shown and the conclusions reached are supported by rigorous theoretical assessments for
C–C cleavage within about 200 intermediates on Ir surfaces, and the results appear to be applicable to other metals
(Rh, Ru, and Pt), which show kinetic behavior similar to Ir.
2015
11. E. Gurbuz, D. Hibbitts, and E. Iglesia*,
“Kinetic and Mechanistic Assessment of Alkanol/Alkanal Decarbonylation and Deoxygenation Pathways on Metal Catalysts.”
Journal of the American Chemical Society, 137 (2015) 11984–11995.
Abstract
This study combines theory and experiment to determine the kinetically relevant steps and site requirements for
deoxygenation of alkanols and alkanals. These reactants deoxygenate predominantly via decarbonylation (C–C
cleavage) instead of C–O hydrogenolysis on Ir, Pt, and Ru, leading to strong inhibition effects by chemisorbed
CO (CO*). C–C cleavage occurs via unsaturated species formed in sequential quasi-equilibrated dehydrogenation
steps, which replace C–H with C-metal bonds, resulting in strong inhibition by H2, also observed in
alkane hydrogenolysis. C–C cleavage occurs in oxygenates only at locations vicinal to the C=O group in RCCO*
intermediates, because such adjacency weakens C–C bonds, which also leads to much lower activation enthalpies
for oxygenates than hydrocarbons. C–O hydrogenolysis rates are independent of H2 pressure and limited
by H*-assisted C–O cleavage in RCHOH* intermediates on surfaces with significant coverages of CO* formed in
decarbonylation events. The ratio of C–O hydrogenolysis to decarbonylation rates increased almost 100-fold as
the Ir cluster size increased from 0.7 to 7 nm; these trends reflect C–O hydrogenolysis reactions favored on
terrace sites, while C–C hydrogenolysis prefers sites with lower coordination, because of the relative size
of their transition states and the crowded nature of CO*-covered surfaces. C–O hydrogenolysis becomes the preferred
deoxygenation route on Cu-based catalysts, thus avoiding CO inhibition effects. The relative rates of C–O and
C–C cleavage on these metals depend on their relative ability to bind C atoms, because C–C cleavage transitions
states require an additional M–C attachment.
10. D. Hibbitts and E. Iglesia*,
“The Prevalence of Bimolecular Routes in the Activation of Diatomic Molecules with Strong Chemical Bonds (O2, NO, CO, N2) on Catalytic Surfaces.”
Accounts of Chemical Research, 48 (2015) 1254–1262.
Abstract
Dissociation of the strong bonds in O2, NO, CO, and N2 often involves large activation barriers
on low-index planes of metal particles used as catalysts. These kinetic hurdles reflect the noble nature of some metals
(O2 activation on Au), the high coverages of co-reactants (O2 activation during CO oxidation on Pt),
or the strength of the chemical bonds (NO on Pt, CO and N2 on Ru). High barriers for direct dissociations from
density functional theory (DFT) have led to a consensus that "defects", consisting of low-coordination exposed atoms, are
required to cleave such bonds, as calculated by theory and experiments for model surfaces at low coverages. Such sites,
however, bind intermediates strongly, rendering them unreactive at the high coverages prevalent during catalysis. Such site
requirements are also at odds with turnover rates that often depend weakly on cluster size or are actually higher on larger
clusters, even though defects, such as corners and edges, are most abundant on small clusters. This Account illustrates how
these apparent inconsistencies are resolved through activations of strong bonds assisted by co-adsorbates on crowded low-index
surfaces.
2014
9. D. Hibbitts, R. Jimenez, M. Yoshimura, B. Weiss, and E. Iglesia*,
“Catalytic NO activation and NO‑H2 Reaction Pathways.”
Journal of Catalysis, 319 (2014) 95–109.
Abstract
Kinetic and isotopic data on Pt clusters and activation free energy barriers from density functional theory (DFT) on
Pt(1 1 1) are used to assess the elementary steps involved in NO–H2 reactions. Pt clusters 1–10 nm
in diameter gave similar turnover rates, indicating that these elementary steps are insensitive to surface-atom coordination.
N–O cleavage occurs after sequential addition of two chemisorbed H-atoms (H*) to NO* which are quasi-equilibrated with
H2 and NO co-reactants. The first step is equilibrated and forms HNO*, while the second addition is irreversible
and forms *HNOH*; this latter step limits NO–H2 rates and forms OH* and NH* intermediates that undergo fast
reactions to give H2O, N2O, NH3, and N2. These conclusions are consistent with (i)
measured normal H/D kinetic isotope effects; (ii) rates proportional to H2 pressure, but reaching constant values
at higher pressures; (iii) fast H2–D2 equilibration during catalysis; and (iv) DFT-derived activation
barriers. These data and calculations, taken together, rule out N–O cleavage via NO* reactions with another NO* (forming
O* and N2O) or with vicinal vacancies (forming N* and O*), which have much higher barriers than H*-assisted routes.
The cleavage of N–O bonds via *HNOH* intermediates is reminiscent of C–O cleavage in CO–H2
reactions (via *HCOH*) and of O–O cleavage in O2–H2 reactions (via OOH* or *HOOH*).
H*-addition weakens the multiple bonds in NO, CO, and O2 and allows coordination of each atom to metal surfaces;
as a result, dissociation occurs via such assisted routes at all surface coverages relevant in the practice of catalysis.
8. D. Flaherty, D. Hibbitts, and E. Iglesia*,
“Metal-Catalyzed C–C Bond Cleavage in Alkanes: Effects of Methyl Substitution on Transition State Structures and Stability.”
Journal of the American Chemical Society, 136 (2014) 9664–9676.
Abstract
Methyl substituents at C–C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic
C2–C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C–C cleavage transition states form via
equilibrated dehydrogenation steps that replace several C–H bonds with C-metal bonds, desorb H atoms (H*) from saturated
surfaces, and form λ H2(g) molecules. Activation enthalpies (ΔH‡) and entropies
(ΔS‡) and λ values for 3C–xC cleavage are larger than for
2C–2C or 2C–1C bonds, irrespective of the composition of metal clusters
or the cyclic/acyclic structure of the reactants. 3C–xC bonds cleave through α,β,γ-
or α,β,γ,δ-bound transition states, as indicated by the agreement between measured activation entropies and
those estimated for such structures using statistical mechanics. In contrast, less substituted C–C bonds involve
α,β-bound species with each C atom bound to several surface atoms. These α,β configurations weaken C–C
bonds through back-donation to antibonding orbitals, but such configurations cannot form with 3C atoms, which have one
C–H bond and thus can form only one C–M bond. 3C–xC cleavage involves attachment of other
C atoms, which requires endothermic C–H activation and H* desorption steps that lead to larger ΔH‡
values but also larger ΔS‡ values (by forming more H2(g)) than for 2C–2C
and 2C–1C bonds, irrespective of alkane size (C2–C8) or cyclic/acyclic
structure. These data and their mechanistic interpretation indicate that low temperatures and high H2 pressures favor
cleavage of less substituted C–C bonds and form more highly branched products from cyclic and acyclic alkanes. Such
interpretations and catalytic consequences of substitution seem also relevant to C–X cleavage (X = S, N, O) in
desulfurization, denitrogenation, and deoxygenation reactions.
7. D. Hibbitts, Q. Tan, and M. Neurock*,
“Acid Strength and Bifunctional Catalytic Behavior of Alloys Comprised of Noble Metals and Oxophilic Metal Promoters.”
Journal of Catalysis, 315 (2014) 48–58.
Abstract
The promotion of metal catalysts with partially oxidized oxophilic MOx species, such as ReOx-promoted
Rh, has been demonstrated to produce Brønsted acid sites that can promote hydrogenolysis of oxygenate intermediates
such as those found in biomass-derived species. A wide variety of alloy compositions and structures are examined in this
work to investigate strongly acidic promoters by using DFT-calculated deprotonation energies (DPE) as a measure of acid
strength. Sites with the highest acid strength had DPE less than 1100 kJ mol−1, similar to DPE values of
heteropolyacids or acid-containing zeolites, and were found on alloys composed of an oxophilic metal (such as Re or W) with
a noble metal (such as Rh or Pt). NH3 adsorbs more strongly to sites with increasing acid strength and the
activation barriers for acid-catalyzed ring opening of a furan ring decrease with increasing acid strength, which was also
shown to be stronger for OH acid sites bound to multiple oxophilic metal atoms in a three-fold configuration rather than
OH sites adsorbed in an atop configuration on one oxophilic metal, indicating that small MOx clusters may yield
sites with the highest acid strength.
6. D. Flaherty^, D. Hibbitts^, E. Gurbuz, and E. Iglesia*,
“Theoretical and Kinetic Assessment of the Mechanism of Ethane Hydrogenolysis on Metal Surfaces Saturated with Chemisorbed Hydrogen.”
Journal of Catalysis, 311 (2014) 350–356.
Abstract
Ethane hydrogenolysis involves C–C bond rupture in unsaturated species in quasi-equilibrium with gaseous
reactants and H2 on metal clusters, because C–C bonds weaken as C-atoms replace hydrogen with exposed
metal atoms from catalyst surfaces. The nature and reactivity of such adsorbed species are probed here using kinetic
data and density functional theory (DFT) for the case of Ir surfaces, but with conclusions that appear to be general
to hydrogenolysis on noble metals. On surfaces saturated with chemisorbed H-atoms (H*), theory and experiments indicate
that C–C cleavage occurs predominantly via an α,β-bound *CHCH* species that forms via sequential
dehydrogenation of adsorbed ethane; all other intermediates cleave C–C bonds at much lower rates (<107-fold).
Measured activation energies (213 kJ mol−1) and free energies (130 kJ mol−1) reflect the combined
values for quasi-equilibrated steps that desorb H*, adsorb C2H6, form C2-intermediates
by dehydrogenation, and form the transition state from *CHCH* species. DFT-derived activation energies
(218 kJ mol−1) and free energies estimated from these values and statistical mechanics treatments
of reaction and activation entropies (137 kJ mol−1) are in excellent agreement with measured values. The
removal of four H-atoms in forming the kinetically-relevant *CHCH* intermediates, taken together with measured effects
of H2 pressure on hydrogenolysis rates, show that 2–3 H* must be removed to bind this intermediate
and the transition state, as expected from the structure of the proposed adsorbed species and H* adsorption stoichiometries
on Ir surface atoms that vary slightly with surface coordination on the non-uniform surfaces of metal clusters. Theory
and experiments combine here to provide mechanistic insights inaccessible to direct observation and provide compelling
evidence for reaction pathways long considered to be plausible for hydrogenolysis on noble metals. The extent of
unsaturation in the single relevant intermediate and its C–C cleavage rates will depend on the identity of the
metal, but the elementary steps and their kinetic relevance appear to be a general feature of metal-catalyzed hydrogenolysis.
2013
5. D. Hibbitts, B. Loveless, M. Neurock, and E. Iglesia*,
“Mechanistic Role of Water on the Rate and Selectivity of Fischer-Tropsch Synthesis on Ruthenium Catalysts.”
Angewandte Chemie, 52 (2013) 12273–12278.
Abstract
Water increases Fischer–Tropsch synthesis (FTS) rates on Ru through H-shuttling processes. Chemisorbed hydrogen
(H*) transfers its electron to the metal and protonates the O-atom of CO* to form COH*, which subsequently hydrogenates
to *HCOH* in the kinetically relevant step. H2O also increases the chain length of FTS products by mediating
the H-transfer steps during reactions of alkyl groups with CO* to form longer-chain alkylidynes and OH*.
4. D. Hibbitts and M. Neurock*,
“Influence of Oxygen and pH on the Selective Oxidation of Ethanol on Pd catalysts.”
Journal of Catalysis, 299 (2013) 261–271.
Abstract
The selective oxidation of ethanol on supported Pd is catalytically promoted by the presence of hydroxide species on the
Pd surface as well as in solution. These hydroxide intermediates act as Brønsted bases which readily abstract protons
from the hydroxyl groups of adsorbed or solution-phase alcohols. The C1–H bond of the resulting alkoxide is
subsequently activated on the metal surface via hydride elimination to form acetaldehyde. Surface and solution-phase hydroxide
intermediates can also readily react with the acetaldehyde via nucleophilic addition to form a germinal diol intermediate,
which subsequently undergoes a second C1–H; bond activation on Pd to form acetic acid. The role of O2
is to remove the electrons produced in the oxidation reaction via the oxygen reduction reaction over Pd. The reduction reaction
also regenerates the hydroxide intermediates and removes adsorbed hydrogen that is produced during the oxidation.
2012
3. B. Braunchweig, D. Hibbitts, M. Neurock, and A. Wieckowski*,
“Electrocatalysis: a Fuel Cell and Surface Science Perspective.”
Catalysis Today, 202 (2013) 197–209.
Abstract
In this report, we discuss some of the advances in surface science and theory that have enabled a more detailed understanding
of the mechanisms that govern the electrocatalysis. More specifically, we examine in detail the electrooxidation of C1
and C2 alcohol molecules in both acidic and basic media. A combination of detailed in situ spectroscopic measurements
along with density functional theory calculations have helped to establish the mechanisms that control the reaction paths and the
influence of acidic and alkaline media. We discuss some of the synergies and differences between electrocatalysis and aqueous phase
heterogeneous catalysis. Such analyses begin to establish a common language and framework by which to compare as well as advance both fields.
2011
2. M. Chia, Y. Pagan-Torres, D. Hibbitts, Q. Tan, H. Pham, A. Datye, M. Neurock, R. Davis, and J. Dumesic*,
“Selective Hydrogenolysis of Polyols and Cyclic Ethers over Bi-Functional Surface Sites on Rhodium-Rhenium Catalysts.”
Journal of the American Chemical Society, 133 (2011) 12675–12689.
Abstract
A ReOx-promoted Rh/C catalyst is shown to be selective in the hydrogenolysis of secondary C–O bonds for a broad range of cyclic
ethers and polyols, these being important classes of compounds in biomass-derived feedstocks. Experimentally observed reactivity trends,
NH3 temperature-programmed desorption (TPD) profiles, and results from theoretical calculations based on density functional
theory (DFT) are consistent with the hypothesis of a bifunctional catalyst that facilitates selective hydrogenolysis of C–O bonds
by acid-catalyzed ring-opening and dehydration reactions coupled with metal-catalyzed hydrogenation. The presence of surface acid sites
on 4 wt % Rh–ReOx/C (1:0.5) was confirmed by NH3 TPD, and the estimated acid site density and standard enthalpy of
NH3 adsorption were 40 µmol g−1 and −100 kJ mol−1, respectively. Results from DFT calculations suggest
that hydroxyl groups on rhenium atoms associated with rhodium are acidic, due to the strong binding of oxygen atoms by rhenium, and these
groups are likely responsible for proton donation leading to the formation of carbenium ion transition states. Accordingly, the observed
reactivity trends are consistent with the stabilization of resulting carbenium ion structures that form upon ring-opening or dehydration.
The presence of hydroxyl groups that reside α to carbon in the C–O bond undergoing scission can form oxocarbenium ion intermediates
that significantly stabilize the resulting transition states. The mechanistic insights from this work may be extended to provide a general
description of a new class of bifunctional heterogeneous catalysts, based on the combination of a highly reducible metal with an oxophilic
metal, for the selective C–O hydrogenolysis of biomass-derived feedstocks.
2010
1. B. Zope, D. Hibbitts, M. Neurock, and R. Davis*,
“Reactivity of the Gold-Water Interface during Selective Oxidation Catalysis.”
Science, 330 (2010) 74–78.
Abstract
The selective oxidation of alcohols in aqueous phase over supported metal catalysts is facilitated by high-pH conditions. We have studied the
mechanism of ethanol and glycerol oxidation to acids over various supported gold and platinum catalysts. Labeling experiments with
18O2 and H218O demonstrate that oxygen atoms originating from hydroxide ions instead of
molecular oxygen are incorporated into the alcohol during the oxidation reaction. Density functional theory calculations suggest that
the reaction path involves both solution-mediated and metal-catalyzed elementary steps. Molecular oxygen is proposed to participate
in the catalytic cycle not by dissociation to atomic oxygen but by regenerating hydroxide ions formed via the catalytic decomposition of a peroxide intermediate.