RuO2 domains supported on SnO2, ZrO2, TiO2, Al2O3, and SiO2 catalyze the oxidative conversion of methanol to formaldehyde, methylformate, and dimethoxymethane with unprecedented rates and high combined selectivity (>99 percent) and yield at low temperatures (300-400 K). Supports influence turnover rates and the ability of RuO2 domains to undergo redox cycles required for oxidation turnovers. Oxidative dehydrogenation turnover rates and rates of stoichiometric reduction of RuO2 in H2 increased in parallel when RuO2 domains were dispersed on more reducible supports. These support effects, the kinetic effects of CH3OH and O2 on reaction rates, and the observed kinetic isotope effects with CH3OD and CD3OD reactants are consistent with a sequence of elementary steps involving kinetically relevant H-abstraction from adsorbed methoxide species using lattice oxygen atoms and with methoxide formation in quasi-equilibrated CH3OH dissociation on nearly stoichiometric RuO2 surfaces. Anaerobic transient experiments confirmed that CH3OH oxidation to HCHO requires lattice oxygen atoms and that selectivities are not influenced by the presence of O2. Residence time effects on selectivity indicate that secondary HCHO-CH3OH acetalization reactions lead to hemiacetal or methoxymethanol intermediates that convert to dimethoxymethane in reactions with CH3OH on support acid sites or dehydrogenate to form methylformate on RuO2 and support redox sites. These conclusions are consistent with the tendency of Al2O3 and SiO2 supports to favor dimethoxymethane formation, while SnO2, ZrO2, and TiO2 preferentially form methylformate. These support effects on secondary reactions were confirmed by measured CH3OH oxidation rates and selectivities on physical mixtures of supported RuO2 catalysts and pure supports. Ethanol also reacts on supported RuO2 domains to form predominately acetaldehyde and diethoxyethane at 300-400 K. The bifunctional nature of these reaction pathways and the remarkable ability of RuO2-based catalysts to oxidize CH3OH to HCHO at unprecedent low temperatures introduce significant opportunities for new routes to complex oxygenates, including some containing C-C bonds, using metanol or ethanol as intermediates derived from natural gas or biomass.

Fundamental mechanistic details regarding C-O bond activation and C-C bond formation remain unknown in catalytic CO hydrogenation to heavy hydrocarbons (Fischer-Tropsch synthesis, FTS). This study combines infrared spectroscopic and density functional theory methods (DFT) to first identify relevant surface CO coverages during FTS; reaction energy profiles are then calculated using DFT to determine the most facile path for C-O bond activation on Ru cluster surfaces. Kinetic responses of CO conversion rates and product selectivities to changes in H₂ and CO pressures are measured in a packed-bed reactor at differential conversions on supported Ru catalysts to develop kinetic rate equations for FTS and corresponding sets of elementary reaction

steps consistent with such equations. The effects of Ru cluster size on CO turnover rates, CO adsorption equilibrium constants, and CO conversion activation energies are also investigated to identify the Ru coordination environment in which C-O bonds are activated during FTS. C-C bond formation paths are then probed by measuring the effects of H₂ and CO pressures on chain termination parameters, which provide a direct

comparison between chain termination and chain propagation rates for all hydrocarbon products. DFT simulations of chain termination and chain propagation reactions as a function of carbon number are also performed to elucidate relevant C-C bond formation paths and to explain the apparent difficulty in forming the initial C-C bond during FTS. Calculated CO adsorption energies decrease with increasing CO coverage on flat extended Ru(0001) surfaces and on 201-atom Ru cluster surfaces(Ru₂₀₁) due to repulsive intermolecular CO-CO interactions; such repulsive interactions are alleviated on convex

Ru₂₀₁ cluster surfaces, which can also accommodate CO coverages above a stoichiometric monolayer by forming geminal di-carbonyl species (also identified by infrared

spectroscopy of CO adsorbed on Ru/SiO₂ catalysts) at under-coordinated corner and edge Ru atoms. DFT-derived activation barriers for H-assisted CO activation paths on Ru

atoms in high-coordination environments are much smaller than such barriers for direct CO bond cleavage, suggesting that CO species can be activated via H-assistance on low-index

surfaces. Direct CO activation barriers on under-coordinated corner and step-edge sites are larger than those on high-coordination sites because of unfavorable interactions between di-carbonyl species and vicinal C-O activation transition states. DFT results

also show that C-O bond activation during FTS is irreversible, and any direct CO dissociation path, as a result, is inconsistent with the reported effects of H₂ and COpressures on FTS rates.

Kinetic responses of CO conversion turnover rates and oxygen rejection selectivities (in the form of H₂O/CO₂ formation rate ratios) are consistent with CO activation occurring predominantly via H-assisted paths on Ru catalysts, irrespective of Ru cluster size. CO adsorption equilibrium constants and CO conversion activation

energies, measured during FTS reactions, are the same for both small and large Ru clusters, suggesting that CO activation occurs in similar environments on both particle sizes. Turnover rates are smaller on small (~ 1 nm) Ru clusters than rates on large (~ 7

nm) Ru clusters when rates are normalized by the total number of exposed Ru atoms. Such differences in turnover rates become much smaller, however, when rates are normalized by the number of exposed Ru atoms in low-index (111) planes; CO activation

reactions, as a result, likely occur on Ru atoms in high-coordination environments on both small and large Ru clusters.

Increases in H₂ pressures increases CH₄ selectivities and decreases C₅₊

selectivities because chain termination is proportional to the concentration of surface hydrogen atoms (H*); chain growth reactions are favored at high CO pressures where termination is suppressed. Chain termination parameters for C₁ species (to CH₄) are much larger than such parameters for C₂₊ species; these parameters depend exponentially on the difference in chain termination and chain propagation barriers for each carbon number. DFT-derived activation barriers for termination reactions (via hydrogenation of

surface alkyl species) are small and independent of chain length because such reactions are exothermic and proceed via early transition states; chain propagation reactions (via alkyl migration to vicinal CO* species), however, proceed through late transition states

where surface-alkyl bonds are broken completely. Surface methyl (CH₃*) species are bound to surfaces much more strongly than ethyl (C₂H₅*) and propyl (C₃H₇*) species; C₁

propagation barriers, as a result, are much larger than such barriers for C₂₊ hydrocarbons, a result that is consistent with measured chain termination parameters on supported Ru catalysts. A steady-state treatment of reactive intermediates shows that CO conversion rate equations must include terms for CO consumption via CO activation as well as for

consumption via chain growth with CO* monomers. This study unites experiment and theory to investigate fundamental C-O bond

activation and C-C bond formation reactions on relevant catalyst surfaces during FTS. Kinetic and selectivity measurements remain vital in the understanding of elementary steps involved in bond making and bond breaking events in FTS reactions; theoretical

tools can be used to investigate elementary reactions under conditions inaccessible to experimental techniques. Theoretical investigations of FTS reactions must be performed on cluster surfaces at high CO* coverages that prevail during relevant conditions because CO* coverages affect thermodynamic and kinetic parameters included in apparent activation energies. High CO* coverages also weaken Ru-CO bonds, consistent with

quasi-equilibrated CO adsorption-desorption processes during FTS. This study is an important example of how theoretical calculations performed on relevant surfaces at relevant coverages are an invaluable compliment to experimental studies of metal-catalyzed chemical reactions.

Thermodynamic constraints in alkane dehydrogenation and dehydrocyclization can be mitigated by the scavenging of H2 through reactions with O2 and CO2, an approach that promises significant increases in product yields but requires a detailed mechanistic understanding of H2-scavenging reactions to deploy them effectively and minimize undesired side reactions. The simplicity of the molecular rearrangements involved in these reactions and the ubiquitous nature of such rearrangements in catalysis also make them an attractive choice to probe fundamental concepts in surface catalysis. In this study, H2-O2 and H2-CO2 reactions are investigated using a combination of kinetic, isotopic, and spectroscopic measurements to address the kinetically relevant steps and site requirements for these reactions on transition metal nanoparticles. The resulting mechanistic inferences are benchmarked against theoretical assessments using models of active surfaces with relevant adsorbate coverages. The experimental and theoretical evidence presented herein indicate that O-O bond activation steps are kinetically-relevant in H2-O2 reactions on Pt and O atom removal and C-O bond activation steps are kinetically-relevant in H2-CO2 reactions that form CO and CH4 on Ru nanoparticles. O-O and C-O bonds are activated both through direct reactions with ensembles of uncovered surface metal atoms and through H-assisted pathways that form H-containing intermediates prior to bond activation. Significant effects of adsorbate coverages (H atoms in H2-O2 reactions; CO in H2-CO2 reactions) on the energies of the bound species and transition states that mediate O-O and C-O bond activation were observed, rendering traditional Langmuirian treatments an incomplete description of the reaction dynamics. Such conclusions underscore the importance of describing reaction energetics using theoretical models containing adsorbates at the appropriate coverages prevalent during steady-state catalysis.

H2 oxidation reaction rates on Pt nanoparticles at moderate temperatures (548-673 K) and suprastoichiometric H2/O2 ratios (above two) are much lower than H2-D2 isotopic exchange rates, indicative of quasi-equilibrated dissociative H2 adsorption steps to form H adatoms (H*). This equilibration, combined with kinetic trends that indicate negligible coverages of O2-derived species at steady state, allows H* coverages during H2-O2 reactions to be rigorously determined from H2 chemisorption isotherms independently measured at relevant reaction temperatures. The dependence of H2 oxidation rates on H2 and O2 pressures indicate that turnover rates reflect the reactive collision probabilities of gaseous O2 with catalytic surfaces, occurring via two kinetically-relevant pathways: (i) dissociation of O2 on bare Pt sites (*) to form two adsorbed O atoms and (ii) reaction of O2 with vicinal H*-H* pairs to form weakly-adsorbed hydrogen peroxide (*HOOH*) intermediates, which rapidly decompose to form H2O. The measured activation barriers for each of these two pathways are consistent with the barriers calculated using density functional theory (DFT) assessments on Pt(111) surfaces with varying H* coverages. The relative contributions of each pathway to the overall reaction rate depend on the relative surface coverages of H* and * and on the free energy barriers for each pathway; these two routes show similar enthalpic barriers and differ only in terms of the entropic penalties incurred upon forming the transition states from their respective precursors.

CO2-H2 reactions on Ru nanoparticles form CO and hydrocarbons via interlinked steps on catalytic surfaces. Kinetic trends and infrared spectra measured during CO2-H2 and CO-H2 catalysis on Ru nanoparticles are remarkably similar, thus providing compelling evidence for the inextricable mechanistic connections between CO2 and CO methanation reactions; after CO2 activation steps, these reactions proceed via an identical sequence of elementary steps occurring on Ru surfaces that contain significant coverages of adsorbed CO (CO*). During CO2-H2 reactions, activation of the first C-O bond in CO2 occurs via quasi-equilibrated steps, as evidenced by fast C16O2-C18O2 isotopic exchange rates and by the kinetic trends observed for CO2-H2 reactions; these CO2 activation steps form bound CO and O species (CO* and O*). The first H-addition step to O* is kinetically-relevant in CO2-consuming reactions and is followed by a more facile second H-addition step to form H2O. The activation of the second C-O bond in CO2 (now present in CO*) occurs via a H-assisted pathway involving the sequential addition of two H adatoms, consistent with the observed kinetic trends and previous theoretical assessments, as also shown for CO-H2 reactions on Ru, Co, and Fe catalysts. CO* species readily desorb and re-adsorb during CO-H2 and CO2-H2 reactions, leading to CO(g) pressures that are equilibrated with bound CO* species. In addressing the analysis of CO2 hydrogenation reactivity and selectivity data, the use of integral reactor methods is essential to derive accurate mechanistic interpretations, because of the ubiquitous strong inhibition by CO and the equilibrated nature of CO adsorption-desorption steps. Such conclusions underscore the importance of a detailed understanding of mechanistic features before any attempts at developing structure-function relations that seek to connect intrinsic catalytic properties with measured turnover rates and selectivities.

These mechanistic conclusions for H2-O2 and H2-CO2 reactions allow their use in tandem with C3-C4 alkane dehydrogenation reactions so as to scavenge the H2 formed as a byproduct of dehydrogenation, thus shifting equilibrium yields. H2-O2 reactions provide a stronger thermodynamic driving force (Gibbs free energy change of reaction: -215 kJ per mole H2, 573 K) than CO2-H2 reactions (-15 kJ per mole H2 for methanation, 18 kJ per mole H2 for reverse water-gas shift; 573 K), thus removing dehydrogenation thermodynamic constraints when O2 is concurrently introduced at the required stoichiometric H2/O2 ratios. The coupling of dehydrogenation with H2 oxidation reactions in practice requires that H2/O2 ratios be maintained at suprastoichiometric levels to prevent large yield losses due to parasitic hydrocarbon oxidation. However, even when O2 introduction is carefully staged such that sufficient H2/O2 ratios are maintained throughout the reactor bed, undesired hydrocarbon combustion reaction cannot be eliminated entirely. Encapsulation of the Pt metal function (for H2-O2 reactions) within small-pore zeolites was thus explored in order to exploit the sieving properties of these materials to increase the selectivities of H2-O2 reactions in the presence of C3-C4 hydrocarbons. The larger size of isobutane (0.49 nm kinetic diameter) and propane (0.43 nm) and of their respective alkene products relative to H2 (0.29 nm) and O2 (0.35 nm) allow LTA structures (0.4-0.5 nm apertures) to hinder access by the hydrocarbon molecules to the intracrystalline Pt function. In doing so, these materials lead to up to 30-fold increases in selectivity for H2 oxidation over alkane oxidation compared to the unencapsulated metal function (Pt supported on amorphous SiO2).

The challenges associated with the coupling of selective H2 oxidation catalyzed by Pt/LTA and alkane dehydrogenation catalyzed by Pt/SiO2 were further explored. Selectivity enhancements, defined as the ratio of the H2 oxidation selectivity measured on Pt/LTA to the H2 oxidation selectivity measured on Pt/SiO2, that are conferred by Pt encapsulation are diminished as O2 conversions increase, with selectivity enhancements approaching unity as O2 pressures approach zero. Such effects, resulting from the attenuation of intracrystalline concentration gradients of alkanes as O2 pressures decrease, require operation at sufficiently high O2 pressures in order to maintain significant intracrystalline hydrocarbon gradients. Selectivity enhancements of encapsulated Pt also depend on prevalent alkene concentrations; alkenes are much more reactive than alkanes, and alkene oxidation was detected even on Pt-free zeolites (LTA, FAU, MFI), amorphous SiO2, and other nominally inert support surfaces. Such observations reflect the formation of carbonaceous deposits, when catalytic surfaces are exposed to alkenes, that react with O2 and may impose additional transport restrictions. These findings highlight the challenges in the application of seemingly straightforward size-exclusion concepts to predict rates and selectivities on metals encapsulated within 8-MR zeolites without a concomitant analysis of the relevant dynamics of reaction and transport processes.

The selective encapsulation of metal clusters within zeolites can be used to prepare clusters that are uniform in diameter and to protect them against sintering and contact with feed impurities, while concurrently allowing active sites to select reactants based on their molecular size, thus conferring enzyme-like specificity to chemical catalysis. The apertures in small and medium-pore zeolites preclude the use of post-synthetic protocols to encapsulate the relevant metal precursors because cationic or anionic precursors with their charge-balancing double layer and gaseous complexes cannot diffuse through their windows or channels. We have developed general strategies to encapsulate metal clusters within small-pore zeolites by using metal precursors stabilized by ammonia or organic amine ligands, which stabilize metal precursors against their premature precipitation at the high temperature and pH conditions required for the hydrothermal synthesis of the target zeolite structures and favor interactions between metal precursors and incipient aluminosilicate nuclei during the self-assembly of microporous frameworks. When synthesis temperatures were higher than 400 K, available ligands were unable to prevent the premature precipitation of the metal precursors. In such cases, encapsulation was achieved instead via interzeolite transformations after successfully encapsulating metal precursors or clusters via post-synthesis exchange or ligand protection into parent zeolites and subsequently converting them into the target structures while retaining the encapsulated clusters or precursors. Such strategies led to the successful selective encapsulation of a wide range of metal clusters (Pt, Pd, Ru, Rh, Ir, Re, and Ag) within small-pore (SOD (sodalite), LTA (Linde type A (zeolite A)), GIS (gismondine), and ANA (analcime)) and medium-pore (MFI (ZSM-5)) zeolites. These protocols provide novel and diverse mechanism-based strategies for the design of catalysts with protected active sites.

We have demonstrated the selectivity of the encapsulation processes by combining transmission electron microscopy and chemisorptive titrations with rigorous catalytic assessments of the ability of these materials to catalyze reactions of small molecules, which can access the intracrystalline voids, but not of larger molecules that cannot access the metal clusters within such voids. The selective confinement of clusters also prevented their contact with sulfur compounds (e.g., thiophene and H2S), thus allowing reactions to occur at conditions that otherwise render unconfined clusters unreactive.

We have also developed synthetic protocols and guiding principles, inspired by mechanistic considerations, for the synthesis of zeolites via interzeolite transformations without the use of organic structure-directing agents (OSDA). More specifically, we have synthesized high-silica MFI (ZSM-5), CHA (chabazite), STF (SSZ-35) and MTW (ZSM-12) zeolites from FAU (faujasite) or BEA (beta) parent materials. Structures with lower framework densities (FAU or BEA) were successfully transformed into thermodynamically-favored, more stable structures with higher framework densities (MFI, CHA, STF, and MTW); to date, target materials with higher Si/Al ratios (Si/Al >10) have not been synthesized via interzeolite transformations without the aid of the OSDA species used to discover these zeolite structures and deemed essential up until now for their successful synthesis. Overcoming kinetic hurdles in such transformations required either the presence of common composite building units (CBU) between parent and target structures or, in their absence, the introduction of small amount of seeds of the daughter structures.

The NaOH/SiO2 ratio, H2O/SiO2 ratio and Al content in reagents are used to enforce synchronization between the swelling and local restructuring within parent zeolite domains with the spalling of fragments or building units from seeds of the target structure. The pseudomorphic nature of these seed-mediated transformations, which conserve the volume occupied by the parent crystals and lead to similar size and crystal shape in products, reflect incipient nucleation of target structures occurring at the outer regions of the parent domains and lead to the formation of mesoporosity as a natural consequence of the space-conserving nature of these structural changes and of the higher density of the daughter frameworks. The synthesis mechanism and the guidelines developed enable us to enforce conditions required for the formation of zeolites that previously required OSDA species for their synthesis, thus expanding to a significant extent the diversity of zeolite frameworks that are accessible via these synthesis protocols and providing potential savings in the time and cost involved in the synthesis of some of these zeolite structures.

The mechanistic origins of bifunctional synergies between metal nanoclusters and insulating oxide materials, present as a support or volumetric diluent, for catalytic hydrogenation reactions remains controversial despite decades of research. These bifunctional routes are often attributed to reaction pathways mediated by hydrogen spillover whereby H atoms migrate from metal surfaces, where they form from dissociated H2, to oxide surfaces and react with unsaturated molecules; however, such routes are unlikely to occur with insulating oxides which do not interact favorably with H atoms except at crystallographic defects. Herein, alternative explanations for such effects are investigated by exploring the mechanistic underpinnings of toluene hydrogenation on Pt surfaces and extending these insights to bifunctional reactions with γ-Al2O3 as a co-catalyst as an example of such phenomena.Turnover rates of methylcyclohexane formation from toluene and H2 reactants were measured on SiO2-supported Pt nanoparticles (3.6 mean Pt diameter) across a wide range of temperatures (333-533 K) and reactant pressures (20-80 kPa H2, 0.3-3 kPa toluene). Gaseous pressures of 1-methylcyclohexene and 4-methylcyclohexene intermediates are also reported. These methylcyclohexene species form as reactive intermediates during kinetically-relevant toluene hydrogenation routes; their gas-phase concentrations reflect the pseudo-steady state balance of their rates of formation (from toluene-H2 reactants) and consumption (by reaction with H2 to form methylcyclohexane) at lower temperatures (393-493 K), and their approach to thermodynamic equilibrium with toluene and H2 reactants at higher temperatures (493-533 K). These insights are leveraged to reduce the mechanistic complexity of toluene hydrogenation into two half reactions allowing for the kinetically-relevant steps and most abundant surface intermediates to be identified through kinetic modeling coupled with density functional theory calculations that probe binding energies and geometries of surface intermediates. Methylcyclohexane formation is shown to be kinetically limited primarily by the addition of the second H atom to toluene at low temperatures (333-493 K) and by the addition of the second H atom to methylcyclohexene intermediates at higher temperatures (493-533 K). These kinetic interpretations along with density functional theory calculations also show that the Pt surfaces are saturated with molecularly bound forms of toluene at all temperatures considered. Such a shift in the kinetically relevant steps towards later H additions along the toluene hydrogenation reaction coordinate as the temperature increases occurs because each H-addition step incurs a large entropic penalty due to the loss of highly entropic translation degrees of freedom of H2 molecules. The toluene-derived intermediates that participate in these later H-addition reactions are formed exothermically from stoichiometric amounts of toluene and H2, resulting in a negative apparent activation enthalpy when their hydrogenation becomes rate limiting. These behaviors account for the decrease in methylcyclohexane formation rates with increasing temperature that was observed at higher temperatures (>453 K). The ubiquitous presence of gaseous methylcyclohexene intermediates during toluene-H2 reactions on Pt surfaces informs plausible bifunctional reactions on Pt/SiO2+γ-Al2O3 mixtures. Two novel mechanistic interpretations are proposed and compared to relevant experimental and theoretical observations. One route involves the formation of partially hydrogenated toluene-derived intermediates (THn*) from toluene-H2 reactions on Pt surfaces. These species can desorb and hydrogenate at nearby γ-Al2O3 surfaces, without requiring atomic contact between Pt and γ-Al2O3, thus circumventing the monofunctional bottlenecks at Pt surfaces and increasing methylcyclohexane formation rates. Another route involves the formation of THn* species that undergo slow H-addition reactions to form methylcyclohexane, but which compete effectively for binding sites on crowded Pt surfaces with the predominant reactive intermediates involved in monofunctional routes. These stranded, less reactive THn* species can desorb to give very low equilibrium concentrations of their gaseous analogs; their migration to and hydrogenation at γ-Al2O3 surfaces present beyond atomic distances leads to their scavenging from the fluid phase, thus decreasing their equilibrium surface coverages and increasing the binding spaces available for the competitive reactions of intermediates that mediate monofunctional hydrogenation turnovers at Pt surfaces. Toluene hydrogenation was carried out over Pt/SiO2+γ-Al2O3 mixtures with varying separation between the Pt/SiO2 and γ-Al2O3 domains (via variation in the aggregate domain size of the respective catalytic components contained in physical mixtures). Methylcyclohexane formation rates were largest for Pt/SiO2+γ-Al2O3 mixtures with the smallest separation (and therefore greatest site proximity) and decreased asymptotically towards the rate observed on Al2O3-free Pt/SiO2 as the domain sizes of the respective components increased. Such trends resemble characteristic features of bifunctional reactions mediated by mobile and highly reactive gaseous intermediates. 1,3-Cyclohexadiene and 4-methylcyclohexene reactions with H2 were carried out over Pt-free γ-Al2O3 surfaces (30-300 Pa hydrocarbon, 0-90 kPa H2, 393 K). These unsaturated molecules are shown to hydrogenate predominantly via reaction with molecular H2 at rates that are catalytically significant in the context of Al2O3-catalyzed rates in bifunctional Pt/SiO2+γ-Al2O3 mixtures necessary to account for rate enhancements. Reaction-transport modelling based on the proposed bifunctional mechanisms and measured reactivity trends for independent Pt/SiO2 and γ-Al2O3 functions show that bifunctional synergies are best attributed to γ-Al2O3-mediated scavenging of methylcyclohexadiene inhibitors at low temperatures (393 K), prevalent only on Pt/SiO2 catalysts with small Pt clusters (0.7 nm mean diameter). At higher temperatures (493-533 K), in which methylcyclohexene hydrogenation becomes rate-limiting on Pt surfaces, an additive methylcyclohexene hydrogenation route at nearby γ-Al2O3 is likely to become more prevalent. These results thus provide novel mechanistic interpretations of bifunctional hydrogenation catalyzed by Pt/SiO2+γ-Al2O3 mixtures involving mobile, highly reactive gaseous molecular shuttles rather than H-atom spillover. Insights gained from this work advance the understanding of bifunctional synergies between metals and their nearby supports or diluent materials in composite catalytic materials.

The encapsulation of metal nanoparticles within zeolitic voids of molecular dimensions protects metal surfaces from contact by reactant or poison species that are too large to enter framework apertures, and confers sinter-stability to confined metal domains via intervening channels that prevent cluster coalescence. Such channels can also stabilize specific transition states or retain undesired products until they fragment into smaller molecules capable of egress by diffusion. These size-selective properties are governed, in each instance, by the size of the microporous channels and voids in a specific framework. The encapsulation of noble metal (e.g., Au, Pd, Pt) clusters within small-pore (8-member ring (8-MR) apertures) and medium-pore (10-MR) zeolites, however, often cannot be achieved through established post-synthetic exchange or impregnation techniques, because solvated metal cations may be too large to enter the apertures of these zeolites. Base metal cations (e.g., Ni2+, Co2+, Fe2+), though generally small enough to enter even small-pore zeolites, tend to form highly refractory complexes when ion-exchanged, thus precluding their conversion into catalytically active metal or metal oxide particles by reductive or oxidative treatments. We have developed synthetic strategies and guiding principles for the successful preparation of Au and bimetallic (AuPd, AuPt, PdPt) nanoparticles within the microporous voids of zeolite or zeotype materials with medium (MFI, TS-1) and small (LTA) sized pores. Synthetic techniques have also been devised for the encapsulation of base metal oxide (NiO, Co3O4, Fe2O3) nanoparticles within large (FAU; 12-MR), medium (MFI), and small-pore (LTA) zeolites.

The encapsulation of Au and bimetallic clusters within LTA and MFI was achieved via hydrothermal self-assembly of crystalline frameworks around ligated metal cation precursors (Au3+, Pd2+, Pt2+). Bifunctional ligands containing a thiol moiety are used to coordinate with and protect metal cations, thus preventing their premature precipitation or reduction in alkaline synthesis gels at elevated temperature. Such ligands also contain alkoxysilane moieties that form covalent linkages with nucleating silicate oligomers to enforce metal uptake into crystallizing frameworks. The controlled deprotection of ligated cations with sequential oxidative and reductive treatments forms encapsulated clusters that are small (<2 nm), uniform in size, and sinter-stable at temperatures as high as 870 K. The simple incorporation of two ligated metal precursors into zeolite synthesis gels leads to the encapsulation of bimetallic clusters that are uniform in size and composition.

The encapsulation of Au clusters within zeotype frameworks via hydrothermal assembly around ligated Au cation precursors poses a greater challenge for TS-1 than MFI or LTA, because the high temperature typically required for the crystallization of TS-1 (448 K; LTA: 373 K; MFI: 393 K) strongly promotes the deprotection of even ligand-protected Au precursors. TS-1 was successfully crystallized at much milder conditions than previously reported in order to prevent decomposition (or reaction with alkanols formed during hydrolysis of Ti or Si precursors) of Au-ligand complexes in the synthesis gels without adversely affecting Ti incorporation into the framework or sample crystallinity. The changes made to the established synthesis procedures for TS-1 include much lower crystallization temperatures (393 K vs. 448 K) compensated by longer crystallization times (120 h vs 48 h). Post-synthetic oxidative treatment procedures were developed in order to remove ligand species and organic structure-directing agents from the crystallized Au/TS-1 solids while minimizing the generation of local exotherms; these procedures led to the formation of small (~3.8 nm in diameter) Au nanoparticles embedded within TS-1 crystals.

Techniques developed for the encapsulation of base metals (Ni, Co, Fe) differ from those used for noble metals in (i) the types of ligands used for metal cation stabilization in synthesis gels (thiol ligands for noble metals; bidentate amine ligands for base metals), (ii) the precise order of reagent addition to zeolite synthesis gels needed to ensure ligand attachment and the prevention of metal precipitation, and (iii) the post-synthetic treatments that are most effective at removing ligand residues and subsequently forming encapsulated nanoparticles. It is critically important to prevent the attachment of base metal cations to zeolite exchange sites during both the zeolite assembly process and post-synthetic treatments, because the conditions required to convert such exchanged cations into clusters (>1100 K in H2) are so prohibitively extreme as to cause degradation of the host zeolite frameworks. Base metals were successfully encapsulated by adding Ni2+, Co2+, or Fe2+ cations protected by chelating amine ligands to LTA, MFI, or FAU synthesis gels. Such ligands protect metal cations from precipitation as metal hydroxides or oxides in the alkaline gels and form siloxane bridges with nucleating aluminosilicate oligomers, thereby enforcing metal uptake into zeolite crystallites during framework assembly. The amine ligands also sterically preclude the direct attachment of base metal cations to zeolite exchange sites, thus allowing the conversion of occluded base metal species into metal or metal oxide clusters at conditions that do not incur damage to the zeolite frameworks. Oxidative treatment of the crystallized zeolites removes ligand species occluded within the zeolite pores and forms encapsulated metal oxide clusters.

The selectivity of metal encapsulation within zeotype crystals was quantified by examining rates of probe reactions involving small molecules (e.g., ethanol, O2) on metal-zeotype samples exposed to bulky organosulfur compounds (e.g., dibenzothiophene) that are too large to enter framework apertures. Such organosulfur compounds selectively titrate and deactivate extracrystalline metal surfaces, leading to probe reaction rates that exclusively or predominantly reflect turnover events occurring on encapsulated metal surfaces. These experiments indicate that >95% of active metal surfaces reside within framework crystals for each metal-zeotype sample, confirming the efficacy of the encapsulation methods and demonstrating the size-selective properties conferred by confinement.

The synthesis strategies presented in this work, which demonstrate the requirements for successful syntheses, provide guiding principles for the selective encapsulation of metal and oxide nanoparticles within a wide variety of crystalline environments through simple one-step hydrothermal assembly routes. Such strategies provide versatile and broadly applicable alternatives to the limited or system-specific schemes for nanoparticle encapsulation that have been developed previously. These encapsulated clusters have potential uses in diverse applications that exploit the catalytic benefits conferred by metal encapsulation within microporous solids, including stability against sintering during reactions, reactant or product shape selectivity, and the protection of catalytic surfaces from contact with poisons that block their active sites.

Steam methane reforming (CH4-H2O) is the most widely used process for the production of H2 and is also a key reaction in the production of a wide variety of value-added chemicals and products from natural gas resources. Ni-based materials are the most commonly used catalysts in these processes but suffer from the undesired formation of carbonaceous deposits, most often in the form of filaments that can lead not only to deactivation but also catalyst disintegration. Low CH4/H2O feed ratios are frequently used to mitigate such carbon formation. These strategies reflect the wider effects and consequences of carbon thermodynamic activities on the catalyst surface during steady-state catalysis. The dynamics of carbon deposition during CH4-H2O and CH4-CO2 reactions (800-1000 K) on Ni-based catalysts and kinetic treatments show that carbon activities and thus carbon formation rates are uniquely determined by a ratio of pressures ψ ((CH4)(H2)/(H2O)) for a given temperature. These carbon activities provide the thermodynamic driving force for the diffusion of carbon through Ni nanoparticles and subsequent formation of filaments. The diameters of carbon filaments are correlated with the diameters of the Ni particles (5-11 nm). Smaller filaments are less stable and exhibit higher thermodynamic carbon activities, rendering their formation more difficult and decreasing thermodynamic driving force for their formation, thus decreasing also rates of carbon deposition. The conclusions from these carbon formation studies inform subsequent strategies for encapsulating metal (Pt) clusters in microporous materials.

The encapsulation of metals in zeolites provides many advantages over conventional metal-oxide supported metal nanoparticles, including the retention of small nanoparticles even at high temperatures. Such microporous materials are frequently used in catalytic applications to selectively sieve molecules based on their size and shape and can therefore be used to selectively impose intracrystalline concentration gradients, leading to changes in reactant ratios within the channels of molecular dimensions. These properties can be exploited for CH4-H2O reactions, where the diffusivity of smaller H2O molecules is expected to be much greater than for CH4 in pores of molecular dimensions. The extrapolation of diffusion data to reaction temperatures, however, can involve significant error. Furthermore, the measurement of H2O and CH4 can (R^2/De) at reaction temperatures by conventional transient uptake measurements is not feasible because of the extremely short timescales of these processes and low uptakes at such temperatures. The measurement of diffusion time constants (R^2/De) at reaction temperatures is therefore accomplished here by analyzing effective reaction rates, specifically isotopic exchange rates, in mass transport limited systems using reaction-diffusion models.

The extraction of diffusion time constants (R^2/De) from reaction-diffusion models can only be rigorously performed with accurate kinetic models. The kinetics of H2O-D2 and H2-D2 exchange and their mechanistic interpretations are therefore addressed here to allow for such calculations of diffusion time constants. H2-D2 isotopic exchange rates (5-80 kPa H2, 5-80 kPa D2; 383 K; H2/D2 = 0.0625-16) on Pt-based catalysts show monotonic increases in rate with H2 and D2 pressures, in contrast with the kinetics expected from the commonly cited recombinative desorption (H* + D*) pathway. Such recombination events only become significant at temperatures greater than 700 K and at low combined pressures (<10 kPa) of H2 and D2. H2-D2 exchange is instead shown to proceed via the reaction of H2 with D* and D2 with H* in a single-site mechanism that does not exhibit a kinetic isotope effect. These exchange events are shown, using theoretical calculations, to occur via the dissociative adsorption of H2 (or D2) at entropically-favorable vacancies that arise from fluctuations in mobile H*-adlayers. These reaction pathways circumvent desorption/recombination reactions; such exchange reactions therefore cannot be used to demonstrate reversibility of H2 adsorption on catalytic surfaces at conditions of practice for hydrogenation reactions, as is commonly practiced. The kinetics of H2O-D2 isotopic exchange reactions (473 K, 5-80 kPa D2, 2.5-40 kPa H2O) are consistent with the reaction between D2 and molecularly adsorbed H2O*. This reaction pathway remains the predominant pathway for temperatures below 900 K, at which point the dissociation of H2O (to form OH* and H*) and recombination of OH* and D* likely becomes the primary pathway for exchange. These kinetic studies also show that H2O irreversibly titrates Pt surface sites by the formation of OH* species that can only be fully removed by reductive treatments at temperatures greater than 700 K.

The kinetics of exchange inform the rigorous calculation of diffusion time constants from isotopic exchange events in mass transport limited materials. CH4-D2 (5-35 kPa CH4, 5-30 kPa D2) H2O-D2 (5-30 kPa H2O, 5-30 D2), and H2-D2 (10 kPa H2, 10 kPa D2) isotopic exchange rates are measured here on Pt/SiO2, Pt/γ-Al2O3, Pt/Na-LTA, and Pt/Ca-LTA samples at temperatures (573-900 K) relevant for CH4-H2O reforming reactions. Effectiveness factors for CH4-D2 and H2O-D2 exchange on Pt/Na-LTA and Pt/Ca-LTA are used in classical reaction-diffusion models to calculate diffusion time constants for CH4 and H2O. H2-D2 exchange reactions did not exhibit mass transport limitations in these materials. CH4 exhibited significantly larger diffusion time constants (by factors of more than 102) than H2O in both Pt/Na-LTA and Pt/Ca-LTA throughout these temperatures (573-900 K), indicating that H2O diffuses more readily in these materials, as required for elevated intracrystalline H2O/CH4 ratios during CH4-H2O reactions. The measured CH4 and H2O diffusion time constants are used in the interpretation of deactivation behaviors during CH4-H2O reactions at 873 K on Pt/γ-Al2O3, Pt/Na-LTA, and Pt/Ca-LTA samples. Deactivation rates on Pt/γ-Al2O3 were linearly dependent on a ψ ((CH4)(H2)/(H2O)), as expected from carbon formation studies. Deactivation rates on Pt/Na-LTA and Pt/Ca-LTA were undetectable for ψ ((CH4)(H2)/(H2O)) values below 10 (including stoichiometric CH4/H2O ratios) and exhibited deactivation rates that were 3-8 times slower than on Pt/γ-Al2O3 for ψ values between 10 and 40. These improvements reflect the high H2O/CH4 ratios within the zeolite pores, as indicated by extensive numerical models. Simulations of deactivation behavior are also used to provide additional insight into further optimizing these materials. The strategies for exploiting differences in diffusivity in zeolites, demonstrated here for CH4 reforming reactions, are generally applicable to reactions where the selective access for smaller reactants is desirable.

Bimetallic nanoparticles can lead to catalysts with improved turnover rates and selectivities, but many synthetic protocols, such as impregnation or precipitation, typically form particles of non-uniform size and composition. Colloidal methods may be able to improve their uniformity, but often require reagents that poison catalytic surfaces (ex. S, B, P). Such compositional non-uniformity and ubiquitous impurities have prevented rigorous conclusions about the consequences of alloying on reactivity and selectivity. Herein, we describe a sequential galvanic displacement-reduction (GDR) colloidal synthesis method using precursors containing only C, H, O, and N, that leads to bimetallic AuPt and PtPd nanoparticles narrowly distributed in size and composition.

Au3+ or Pt4+ precursors were added to monometallic Pt or Pd clusters, respectively, whose surface atoms are thermodynamically driven to reduce and deposit the solvated cations onto cluster surfaces due to the lower reduction potentials (E0) of the seed metal relative to the precursor cations (E0Au > E0Pt > E0Pd); oxidized Pt and Pd surface atoms subsequently return to cluster surfaces upon reduction by the solvent, a reductant (ethanol or ethylene glycol, respectively). Such methods have been previously used to synthesize AuPd clusters from Pd seed clusters.

TEM micrographs confirm that initial seed cluster sizes increase monotonically with increasing Au3+ or Pt4+ content, with final bimetallic cluster dispersity values near unity indicating a narrow size distribution. UV visible spectroscopy of AuPt cluster suspensions show no plasmon resonance features characteristic of Au nano-sized surfaces, indicating the presence of Pt atoms at bimetallic surfaces, as expected for GDR processes. Elemental analysis by EDS confirmed the formation of strictly bimetallic particles with the mean composition of the synthesis mixture.

The GDR model requires that bimetallic growth be proportional to the initial seed surface area, with the number of precursor atoms deposited per surface metal atom of the seed constant and independent of seed metal size. Elemental analysis using EDS supports this hypothesis for thermodynamically favorable alloys such as PtPd and AuPd, but not for AuPt, an unfavorable alloy. These differences appear to reflect the segregation of metals within AuPt clusters during synthesis, placing the metal with the lower surface energy, Au, at cluster surfaces, and decreasing the availability of Pt0 surface atoms for GDR. Consequently, autocatalytic Au3+ reduction on Au0 sites becomes a competitive Au3+ reduction pathway during the synthesis of AuPt clusters.

Polymers such as polyvinylpyrrolidone (PVP)—which bind to metal surfaces during synthesis via charge-transfer interactions—were required in colloidal suspensions to prevent particle agglomeration in solution, but must be removed prior to catalysis. We show that after depositing clusters on SiO2, PVP can be removed from particle surfaces by post-synthetic treatments at mild temperatures (≤ 423 K) in reductants such as H2 and/or EtOH without significant particle agglomerations. Reductants compete with the polymer at the metal surface, thus breaking the polymer-metal bond. The absence of surface residues was confirmed by the similar cluster sizes derived from O2 chemisorption and TEM measurements. Larger cluster sizes and surfaces that chemisorb oxygen more weakly—such as Pt relative to Pd—were found to facilitate the removal of PVP from metal particles due to weaker metal-polymer bonds.

The model catalytic materials prepared in this study are of both fundamental and practical interest to probe the effects of alloying. Using AuPd and AuPt, we investigate the consequences of alloying with Au on the reactivity of catalyst surfaces saturated with either chemisorbed CO* (CO oxidation) or O* (H2 oxidation) that bind strongly to Pt and Pd surfaces and inhibit rates. Singleton Pt-CO* bond energies, reflected in vibrational CO* stretches, were decoupled from dipole-dipole coupling effects using isotopic dilution methods, and were shown to decrease with increasing catalyst Au content. Despite lower CO* binding energies, CO oxidation turnover rates (normalized per metal surface atom) on AuPt catalysts decreased with increasing Au content. These results show that CO oxidation rates depend weakly on CO* binding energy—consistent with the reported structed insensivity of this reaction—and that Au acts primarily as an inert diluent of the active Pt ensembles required for catalysis.

In contrast, H2 oxidation turnover rates (normalized per metal surface atom) on AuPt and AuPd catalysts increase with increasing Au content (up to 11 % at. Au content on AuPt and up to 67 % at. Au content on AuPd), indicating that the reactivity of O* saturated surfaces is more sensitive to changes in adsorbate binding energy than surfaces saturated in CO*, consisted with the reported structure sensitivity of reactions on O* saturated surfaces. Reconstruction of CO* adlayers is facile due to highly mobile CO* molecules, thus allowing CO* adlayers to access configurations that help mitigate strong CO* binding and introduce vacancies. O* adlayers, meanwhile, are more strongly bound to Pt and Pd metal surfaces and less mobile. H2 oxidation rates thus depend more strongly on adsorbate binding energy than CO oxidation rates.