Hibbitts Group Publications

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2019

35. M. Allen^, A. Hoffman^, T.W. Liu, D. Hibbitts*, T. Schwartz*
“Highly Selective Cross-Etherification of 5-Hydroxymethylfurfural with Ethanol”
Submitted, (2019).

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”
Submitted, (2019).

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”
Submitted, (2019).

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)”
Submitted, (2019).

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&nash;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.

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, In Press (2019).

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, In Press (2019).

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 Ni33-C5H10O) to Ni35-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 ┬Ámol 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.