MFI zeolites contain distinct confining pore environments, either smaller (~0.55 nm) channels or larger (~0.70 nm) intersections. The substitution of trivalent heteroatoms generates Brønsted acid sites of varying acid strength, which together with their location within different pore environments, dictates the stability of transition states via electrostatic (ion-pair) and van der Waals interactions. Here, we report that structure-directing agents (SDAs) that control Al³⁺ substitutional patterns during MFI crystallization analogously position other trivalent heteroatoms (Ga³⁺, Fe³⁺, B³⁺), altering the distribution of acid sites between channels and intersections, as probed by low-temperature (403 K) toluene methylation kinetics. Heteroatom-substituted MFI crystallized with tetrapropylammonium result in low selectivity towards p-xylene (<30%), while those crystallized using (co-)SDAs containing peripheral hydrogen-bonding groups (e.g., ethylenediamine and tetrapropylammonium) show high p-xylene selectivity (~80%). These selectivities are consistent with DFT-calculated Gibbs free energy barriers for intersection-dominant and channel-dominant active site distributions, respectively. Transition states to form each xylene isomer are similar in size and charge; thus, their rate constants decrease with acid strength similarly, causing isomer selectivity to depend strongly on confinement but not on acid strength. These generalizable synthetic strategies enable independently controlling acid site strength and location in MFI zeolites and, in turn, catalytic rates and selectivities.

The oxidative conversion of propane to propylene presents a promising route to energy-efficient olefin production, but current catalysts fail to attain yields above 30% without concurrent direct dehydrogenation, which diminishes the value of proposition. Here, we report a one-step, isothermal process that achieves ~60% propylene yield with near-quantitative hydrogen removal by coupling a propane dehydrogenation (PDH) catalyst with a selective hydrogen combustion (SHC) material within a chemical looping (CL) framework. The PDH catalyst consists of quasi-atomically dispersed bimetallic PtSn clusters embedded in a K-MFI zeolite host that maintain high activity and selectivity over multiple reaction cycles, without requiring conventional hydrogen pre-activation—a critical feature when integrated with the SHC component. The SHC material (LaMnO₃ perovskite) exhibits high stability, large oxygen storage capacity, rapid hydrogen combustion kinetics, and strong selectivity against hydrocarbon oxidation. Together, the tandem CL-DH+SHC system achieves ~78% propane conversion, ~81% propylene selectivity, and ~90% hydrogen removal efficiency—tripling the yield achieved by state-of-the-art ODH catalysts. This strategy overcomes a long-standing tradeoff between propylene yield and hydrogen conversion by combining optimized DH and SHC components within a CL configuration that eliminates the need for energy-intensive air separation units, advancing, thus, a viable route to single-pass, high-yield oxidative olefin production.

Advancing methods to control and characterize the local cation arrangements on mixed metal oxide surfaces can afford opportunities for designing catalysts with tunable chemical properties. Here, we show that Ir_xRu_1−xO₂(110) solid solutions with variable near-surface cation arrangements can be synthesized by annealing IrO₂/RuO₂ layered structures under mild oxidizing conditions in ultrahigh vacuum. Temperature-programmed desorption (TPD) of adsorbed N₂ and atomic O provides a quantitative probe of the Ir and Ru populations in cus binding sites (M_cus) and their nearest-neighbor subsurface sites (M_subcus), due to the strong, highly localized influence of these cations on N₂ and O binding. The resulting M_cus/M_subcus configurations remain stable up to at least 650 K, the temperature range relevant for catalytic studies. Heating IrO₂/RuO₂ layered structures to ~800 K induces rapid Ru–Ir intermixing and drives Ru enrichment in the near-surface region. TPD measurements show that Ru strongly favors incorporation into cus-sites during our heating protocol and begins to occupy subcus-sites only after the Ru_cus population approaches saturation. After extended heating, the surface is dominated by Ru_cus over Ru_subcus motifs (Ru/Ru₂) characteristic of pure RuO₂(110), with small populations of Ru/Ir₂ and Ru/IrRu. Density functional theory (DFT) predicts binding-site distributions for O-terminated surfaces that closely match those determined from TPD. DFT reveals that enhanced O binding on Ru_cus relative to Ir_cus, along with stabilization by neighboring Ir_subcus atoms, drives Ir–Ru interchange to selectively form Ru/Ir₂ motifs and promote Ru occupation of subcus sites after cus sites are filled. Together, these results provide new insight into the synthesis and characterization of mixed Ir_xRu_1−xO₂(110) surfaces and demonstrate that controlling near-surface cation arrangements offers a promising route to tuning the chemical properties of mixed-oxide catalysts.

Hydrogen adsorption is important for understanding coverage effects for various chemical reactions and for characterizing the size of nanoparticles. Recent literature suggests that the often-assumed saturation coverage of unity may not be reasonable, at least for small nanoparticles. This includes our prior study which showed that Ir and Pt nanoparticles, which primarily bind H adsorbates in atop binding modes, can saturate at H/M_surf ratios well above 1, depending on the metal and particle size. This work examines H adsorption on small nanoparticles of Ru, Rh, Pd, and Os metals that preferentially bind H in 3-fold sites. We therefore investigate new adlayer structures to understand how undercoordinated atoms influence these H configurations. Calculations on 201-atom nanoparticles suggest H* (at low coverages) binds most favorably to undercoordinated bridge sites for all six metals of interest. As suggested by the slab model calculations, 3-fold sites are also among the most stable sites for Ru, Rh, and Pd particles, while atop sites are more stable for Os, Ir and Pt. These trends in single H* binding behavior do not significantly change with particle size. Saturation adlayers for 4d metals (Ru, Rh, and Pd) are dominated by H* bound to fcc 3-fold sites and undercoordinated bridge sites. With saturation coverages of 1.38, 1.48, and 1.28 ML respectively, Rh has the highest saturation coverage of the 4d metals, reflecting a non-monotonic periodic trend. For the 5d metals (Os, Ir, Pt), saturated adlayers are dominated by the same undercoordinated bridge sites, but also generally favor atop-bound H* instead of 3-fold except Pt. Their saturation coverages are 1.87, 1.93, and 1.57 ML, respectively. Saturation coverage increases when moving from a 4d to a 5d metal, and the Group 9 metals (Rh and Ir) exhibit the highest saturation in their respective series. Our results show that H* binding energy is largely independent of particle size for a given site type. Instead, changing the particle size primarily alters the distribution of available binding sites (corner, edge, and terrace). Therefore, the adlayer model developed for the 201-atom particle can be used to reliably estimate saturation coverages, allowing for extrapolation to both smaller and larger nanoparticles.

Toluene methylation on Brønsted acidic aluminosilicates at low temperatures (<433 K) and conversions (<1%) show initial rates and selectivities dictated by the stabilities of transition states confined within voids of molecular dimensions, but deactivation leads to decreases in rates and shifts in selectivity according to mechanisms that remain incompletely understood. Here, we elucidate the predominant mechanism by which MFI zeolites deactivate and provide insights into the identity and location of deactivating species. Gas-phase aromatic product distributions formed on aluminosilicates of varying pore size (MFI, BEA, FAU, MCM-41) during toluene methylation (403 K, 4 kPa toluene, 66 kPa dimethyl ether) indicate that methylbenzenes larger than xylenes and 1,2,4-trimethylbenzene form, but are unable to egress from medium-pore (~0.55 nm diameter) MFI zeolites. Co-feeding a selective poison of extracrystalline H+ sites (e.g., 2,6-di-tert-butylpyridine) reveals that intracrystalline H+ sites in MFI zeolites dominate observed deactivation behavior; as a result, deactivation-induced changes to xylene selectivity reflect disproportionate decreases in the numbers of intracrystalline H+ sites that remain active, relative to extracrystalline H+ sites. Post-reaction thermal treatments (up to 853 K) of deactivated MFI zeolites result in the concomitant formation of C6–10 monocyclic aromatic hydrocarbons and C2–3 aliphatic hydrocarbons, and are able to fully regenerate MFI zeolites. These results implicate bulky polymethylbenzenes (on average, C11) as predominant deactivating species that cannot egress from crystallites, and only dealkylate at higher temperatures. These findings provide clarity on how zeolites deactivate and engender changes in selectivity during low-temperature toluene methylation, and provide guidance for zeolite catalyst design and regeneration.

The identities and distribution of active site motifs on bimetallic nanoparticle surfaces depend upon their composition and size, and these factors impact rates and selectivities of surface catalyzed reactions. We combine steady-state rate measurements of O₂-H₂ reactions to form H₂O₂ and H₂O, ex situ spectroscopy, and density functional theory (DFT) calculations to elucidate the structure-performance relationships of silica-supported PdχAu catalysts (1.8 and 11.8 nm diameters, where χ describes the Pd mole fraction). Smaller (1.8 nm) PdχAu nanoparticles display greater H₂O₂ selectivities and rates relative to larger (11.8 nm) particles regardless of bulk composition, which suggests the number and coordination of exposed Pd atoms dictates these trends. Ex situ infrared spectra of adsorbed CO (CO*) and mixed ¹²CO*–¹³CO* adlayers combined with DFT-calculated structures and vibrational frequencies reveal greater Pd isolation on smaller nanoparticles. This is evidenced by weaker dipole coupling interactions between CO* species, likely due to differences in particle curvature and greater fractions of surface sites with stable binding upon nanoparticles of smaller diameters (but equivalent bulk composition). Molecular interpretations of measured apparent activation enthalpies further confirm Pd active site structure differences with nanoparticle size given the same Pd content, reflecting changes in H₂O₂ selectivities. H₂O₂ selectivities remain largely constant with reactant pressures on 11.8 nm PdχAu catalysts but vary significantly with H₂ to O₂ pressure ratios on 1.8 nm PdχAu catalysts. These differences show the coordination of Pd atoms in smaller nanoparticles respond more sensitively to reactant coverages, and high O₂ chemical potentials stabilize Pd atoms at the surfaces. Collectively, these results demonstrate how nanoparticle size affects the location, size, and spatial distributions of Pd ensembles, and nanoparticle surface restructuring in response to changes in reaction conditions—ultimately governing intrinsic kinetics and selectivity to H₂O₂.

Designing Mo-zeolite catalysts that retain structural stability over many reaction-regeneration cycles is a challenge in developing practical methane dehydroaromatization (DHA) routes to produce dihydrogen, ethene, and aromatics. Mo-MFI zeolites typically used for methane DHA irreversibly deactivate during regeneration due to framework dealumination and structural degradation. Here, we synthesize Mo supported on small-pore zeolites (e.g., CHA, AEI, RTH), which are more durable than MFI under hydrothermal aging conditions (823 K, >2 kPa H₂O). Methane DHA forms H₂ and ethene at equivalent rates on Mo-CHA and Mo-MFI. Aromatics formation rates (per Mo) are intracrystalline-diffusion limited in CHA, but increase with decreasing CHA crystallite size. Initial DHA rates (per Mo) remain invariant on Mo-CHA after successive reaction-regeneration cycles (>10), supported by high-resolution TEM and quantitative site characterization data showing minimal site or structural degradation on Mo-CHA, in sharp contrast to the systematic decrease in rates and structural degradation observed on Mo-MFI. This work reports materials that can be fully regenerable under potential industrial conditions and provides structure-function relations between catalysts properties and DHA rates, selectivity, and long-term stability.

MFI zeolites contain ten-membered ring channels (~0.55 nm diameter) that intersect to form larger voids (~0.70 nm diameter). The distribution of framework Al sites dictates the void environments that confine active H⁺ sites. MFI synthesized with pentaerythritol (PET) and Na⁺ as structure-directing agents are interrogated using kinetically controlled toluene methylation rates and xylene isomer selectivities. High para-xylene selectivity and low toluene methylation rates indicate that MFI-PET samples contain H⁺ sites predominantly in smaller channels. As-synthesized samples contain more framework Al than Na⁺, indicating that some charge compensation is provided by protonated PET-solvent complexes that bias Al siting towards smaller channels.

Mg-vanadate (Mg_xV₂O_x+5) catalysts are promising materials for selectively converting alkanes into desired alkenes during the oxidative dehydrogenation (ODH) reactions. Here, we employ density functional theory (DFT+U) calculations, to explore Ni, Cu, and Zn doped substitutionally into Mg positions in Mg-vanadate surfaces for the initial C–H activation of ethane, chosen here as a simple probe reactant. Cation exchange energies to replace the Mg²⁺ with a dopant are not strongly sensitive to changes in the Mg-V ratio, but exchange was more favorable into less-coordinated Mg-sites (i.e., those near surfaces). We examined the reducibility of 54 distinct O atoms on 12 distinct Mg-vanadate catalysts by calculating hydrogen addition energies (HAE), methyl-addition energies (MAE), and ethane C–H activation barriers. Our results show that these dopants can make O atoms more reactive or less reactive on the surface, depending on whether the dopants are reduced, or V is reduced (as in the undoped surface). Zn is rarely reduced in our calculations (~8% of cases), and as such Zn has a minor impact, on average, on the HAE, MAE, and C–H activation barriers. Cu, in contrast, reduces ~80% of the time, resulting in large increases in surface reducibility (decreases in the values of HAE, MAE, and C–H activation barriers). Ni is less likely than Cu to be reduced, and more likely than Zn, resulting in intermediate behavior. When these dopants do not reduce, dopants can still have smaller impacts on the reactivity of nearby O atoms, but the average shifts in reducibility are negligible, suggesting that the impact of their exchange is less predictable in situations that they do not become reduction centers. These results give insights into the role of altering or mixing divalent cations in metal-vanadate catalysts.

Rutile oxides offer a promising platform for creating and studying mixed metal oxide surfaces for catalytic applications. RuO₂ and IrO₂ have been used for the electrochemical oxygen evolution reaction and alkane oxidation reactions, and they have similar lattices which promotes the synthesis of layered structures (e.g., IrO₂/RuO₂) and IrO₂-RuO₂ solid solutions. Their low-energy surfaces, the (110) plane, expose rows of coordinatively unsaturated metal atoms (M_cus) which act as binding sites during catalysis. Here, we generated single-layer IrO₂ films on RuO₂(110), single-layer RuO₂ films on IrO₂(110), and their thick-film counterparts, and studied their binding properties using molecular N₂ and dissociative O₂ adsorption. Temperature programmed desorption measurements and complementary density functional theory calculations and electronic structure analysis show that the binding properties of M_cus sites are strongly influenced by the identity of cations in the subsurface beneath the binding site (M_subcus), such that M_subcus identity influences binding as much as the identity of the M_cus binding site itself. Our analysis shows that these strong effects of the subsurface cations originate from localization of the σ system involved in adsorbate binding, and the resultant accumulation of charge on the subsurface O atom beneath the M_cus site upon adsorbate binding. Replacing Ru with Ir at the subsurface sites stabilizes this charge and strengthens the binding of both N₂ and O with the M_cus site. We discuss a computational alchemy model that can explain both the site and ligand effects of chemical binding on IrO₂/RuO₂ heterostructures and is extendable to other mixed metal oxide and metal alloy surfaces.

Hydrogenolysis can upcycle inert polymers, such as polyolefins, to generate value from plastic waste. Here, we present a fundamental study investigating the mechanisms governing C–C hydrogenolysis of branch points within alkanes on Ir, Pt, and Ru surfaces using density functional theory (DFT). Previous work has shown that activation of unsubstituted C–C bonds occurs through the dehydrogenation of the C–C bond to form a bound alkyne, followed by a kinetically relevant C–C activation and the hydrogenation of the cleaved intermediates to form smaller alkane products. Substituted bonds, in contrast, involve the dehydrogenation of the C–C bond being cleaved, as well as other C atoms near the reacting center. This leads to the counterintuitive observation, that reactions of unsubstituted C–C bonds (having more H to lose) are less inhibited by H₂ than reactions of substituted C–C bonds (having less H atoms to lose). These prior studies of branched alkane activation, however, focused on Ir catalysts and on methyl-substituted alkanes and cycloalkanes, such that the impact of catalyst identity or of long branches (i.e., not methyl-substituted alkanes) on hydrogenolysis mechanisms is largely unexplored. Here, we consider isobutane activation mechanisms on Ir, Ru, and Pt catalysts, and use these results to predict how a larger branched alkane, 3-ethylpentane, would react, as that molecule is more reminiscent of the branches in polyethylene. DFT-estimated free energy barriers and turnover rates indicate that hydrogenolysis activity and rate inhibition from hydrogen pressure follow a general trend with catalysts following a reactivity trend of Ru > Ir > Pt, where Ru is the most active and most inhibited by H₂, with Pt being the least reactive and least inhibited by H₂ pressure. By categorizing the isobutane-derived transition states based on whether they are ‘extendable’ to larger compounds, we predict how 3-ethylpentane would react, and demonstrate why, on Ru, isobutane hydrogenolysis measurements are unlikely to be informative about the mechanisms that activate branches present in polyethylene or polypropylene molecules. This study lays a foundation for a better mechanistic understanding of how branch points activate in alkanes, relevant to polymer upcycling via hydrogenolysis.

The consequences of Brønsted acid site location in MFI zeolites were investigated for propene oligomerization. Adsorbates in MFI may reside in smaller channels (~5.5 Å diam.) or larger channel intersections (~7 Å diam.), with tighter confinement expected to enhance both stabilizing dispersive interactions and destabilizing steric constraints.  MFI samples synthesized using tetrapropylammonium (TPA⁺) as the structure directing agent have higher fractions of acid sites near intersections, while samples synthesized with 1,4-diazabicyclo[2.2.2]octane (DABCO) or ethylenediamine (EDA) have higher fractions of acid sites within channels. Measured propene dimerization rates (per H⁺, 315 kPa C₃H₆, 503 K, 1.6 H⁺/u.c.) are ~9 times higher on MFI-DABCO/MFI-EDA than MFI-TPA. Because propene oligomerization is transport limited in MFI at these conditions, experimentally measured rates are proportional to both effective kinetic (k_eff) and diffusion (D_e) constants. DFT was therefore used to investigate the kinetic influences of void environment in isolation of transport effects. Adsorption free energies for C₃ and C₆ alkenes and dimerization free energy barriers were calculated at all twelve T-sites present in the MFI framework and all accessible O-sites around each T-site. C₃ preferentially adsorbs as H-bonded propene with similar energies in all void environments, while C₆ alkenes are destabilized by 10–56 kJ mol⁻¹ and dimerization transition states are destabilized by 29–102 kJ mol⁻¹, on average, in the channels relative to intersections. The stability of C₃ and C₆ alkenes and dimerization transition states is largely governed by steric penalties arising from distortion of the MFI framework that outweigh stronger dispersive interactions with decreasing void size, even for species as small as C₃. Given that DFT predicts  values are lower at acid sites in smaller channel voids of MFI, higher measured rates on MFI samples synthesized using DABCO or EDA must reflect less severe diffusion restrictions and, in turn, higher values.

AuPd is a miscible metal alloy that is often used in catalysis. Supported AuPd catalysts, at high Au/Pd ratios, form single-atom alloys (SAAs) that have been shown to enhance rates and/or selectivities for many catalytic reactions, including (de)hydrogenations, hydrogenolysis, and C–C and C–O coupling reactions. While many computational studies have examined the stability of AuPd structures (the arrangement of atoms within the miscible alloy), most focused on generic alloys rather than SAAs and those that have closely investigated SAAs focused on single crystal surfaces. In this work, we use density functional theory (DFT) to calculate exchange energies (swapping an Au atom with a Pd atom) in a 201-atom truncated octahedral nanoparticle model with a focus on particles with high Au/Pd ratios. We calculate these exchange energies as a function of Pd location within the nanoparticle, the number of Pd atoms neighboring and near those exchange sites, and the total Pd content in the nanoparticle. These DFT-calculated exchange energies are also used to inform simple physics-based models (in contrast to cluster expansion or neural network models) that show good agreement with DFT-calculated values with relatively few regressed parameters. These models are then implemented into Monte Carlo (MC) simulations to predict the nanoparticle structure as a function of composition and temperature. The results show that Pd prefers to be in the subsurface of nanoparticles and that Pd prefers to be isolated from itself within Au. Both observations agree well with prior experimental and computational studies of single-crystal systems. We also show that the overall composition of the nanoparticle influences exchange energies by changing the electronic properties (e.g., Fermi level) of the system, which is relevant as Pd has one fewer valence electron than Au. MC simulations show that, in a vacuum, Pd begins to populate the surface of these ∼2 nm nanoparticles at around 20 mol % Pd (at 298 K) and that the number of Pd surface monomers, desired for SAA applications, goes through a maximum near 40 mol % Pd. As the temperature increases, Pd is more prevalent at the surface, but the influence of temperature is relatively muted. While AuPd structures are known to change in the presence of reactive gases (e.g., CO or O₂), these studies characterize the baseline thermodynamic arrangements that can be used to understand surface restructuring during catalyst characterization and reaction studies.

Enrique Iglesia is an internationally recognized leader in the field of heterogeneous catalysis. His trademark approach places a premium on kinetic and mechanistic descriptions of catalytic sequences, complemented by synthetic methods to prepare catalytic centers uniform in composition and by computational chemistry methods to adjudicate among competing hypotheses, with the aim of describing the function of catalytic active sites at the level of elementary steps in reaction mechanisms. Enrique began his independent career in industry, spending 11 years at the Exxon Corporate Research Laboratories. In 1993, he moved to academia to become a full professor in the Department of Chemical and Biomolecular Engineering at the University of California, Berkeley where he founded the Berkeley incarnation of the Laboratory for the Science and Applications of Catalysis (LSAC). In that time, he has coauthored >350 publications (with an h-index of >120) and >50 patents and has advised ∼30 Ph.D. students and ∼100 postdoctoral and visiting scholars, more than 30 of whom continue his legacy of teaching and scholarship in their own academic appointments around the world. Enrique is a member of the National Academy of Engineering, the American Academy of Arts and Sciences, the National Academy of Inventors, and the Spanish Royal Academy of Sciences, and has received numerous awards from the American Chemical Society, the American Institute of Chemical Engineers, and the Catalysis Societies of North America and Europe. In this Account, we discuss major research themes that have underpinned Enrique’s career, using examples that illustrate how his research has led to significant conceptual advances in our understanding of reactions facilitated by metals and metal oxides, of the consequences of acid strength and confinement in microporous solids, of the relevance of describing surfaces under realistic coverages for catalysis, and in disentangling the chemistry of active sites that mediate catalysis from the specific influences of the environments within which reactions proceed. These insights have allowed for the development of more precise and unifying descriptions of chemical reactivity and selectivity, including field-defining mechanistic interpretations and practical developments in the conversion of C₁ molecules, acid–base and redox catalysis, hydrocarbon and oxygenate chain growth chemistries, NO_x abatement, and Fischer–Tropsch synthesis across supported metals, carbides, and oxides. His work unifies some of the most enduring concepts in physical, organic, solid-state, and theoretical chemistry into surface catalysis, through deep knowledge about the fundamentals of catalysis and their translation into practical solutions. His research group chooses to study catalytic systems that are technologically relevant and often occur in complex environments, irrespective of whether such topics are in vogue at the time of inquiry, guided by an approach that seeks fundamental knowledge that can be recycled, one day, to develop solutions to problems that we cannot yet envision from our current vantage point. This constancy of purpose, intense focus, and dedication has allowed Enrique to become a leader in science and of scientists, including those who once trained under his guidance and continue to learn alongside him and to receive mentorship, in work and in life, well beyond their time in LSAC.

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.

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/mol_C. 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.

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*:Pd_surf 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*.

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 Mg_xV₂O_x+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.

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.

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 (ΔE_NH3), 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, ΔE_NH3 probes a combination of acid strength and confinement because a gas-phase species enters the zeolite pores. Ensemble average ΔE_NH3 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 ΔE_NH3 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.

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–C_nH_2n+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.

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.

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.

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.

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*/Pt_s), 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*/Pt_s). 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.

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.

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.

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.

Rh active sites are critical for NO_x 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 Rh_surf 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.

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 (ΔG_a҂ 55 kJ mol⁻¹) and CHA (ΔG_a҂҂ 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, ΔG_a҂ 83 kJ mol⁻¹) and (sec-butyl in CHA, ΔG_a҂ 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.

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, ReS_x, 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.

Pd and AgO_x phases grown on single crystal silver (Ag(111)) act cooperatively to oxidize adsorbed CO. A AgO_x single-layer on Ag(111) exhibits limited reaction with CO adsorbed at 100 K. However, Pd clusters deposited onto a single layer of AgO_x promote the reaction by attracting O-atoms from the surrounding AgO_x 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 AgO_x layer. Smaller Pd bi-layer clusters induce oxygen transfer from the AgO_x surface more efficiently, as they optimally incorporate oxygen from both the AgO_x beneath them and the surrounding AgO_x surface. Enhanced stabilization of the oxygen causes the edges of the Pd clusters to become enriched in oxygen and further promotes reduction of the AgO_x. 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 (AgO_x) and clarify mechanisms of oxygen interchange between co-existing phases that may have broad applicablility.

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.

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.

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.

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.

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.

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 CH₃XH_n 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 CH₃XH_n 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.

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.

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.