In this dissertation, we have employed a multi-faceted approach, combing well-controlled synthesis techniques, a variety of characterization techniques and Density Functional Theory calculations to optimize important heterogenous catalytic and photocatalytic process. The efforts were to investigate the supports effects on atomically dispersed metal electronic state, metal structure and pulsed illumination on methanol carbonylation, ethylene hydroformylation and methanol decomposition. In the first work, we successfully synthesize atomically dispersed Rh on a various of supports for ethylene hydroformylation. We utilized a combination of a series of characterization techniques and density functional theory calculations to illustrate that supports influence on the Rh electronic states resulting in different reaction barriers. This work demonstrates that the role of supports on atomically dispersed metal for ethylene hydroformylation and provides insights into how the electronic state of atomically dispersed metals can be controlled by the supports to drive important catalytic hydroformylation process. In the second part of this work, we research how the metal structure influences on methanol carbonylation reactivity and selectivity. We hypothesized that a pair-site catalyst consisting of a Lewis acid site and an atomically dispersed Rh site could more selectively drive methanol carbonylation than Rh cluster through a mechanism where CO coordinated to Rh and methanol dissociated on the acid site to enable CO insertion at the interface. Through qualitative and quantitative characterization techniques including CO probe molecule FTIR and NH3-TPD along with rigorous kinetic measurements of methanol carbonylation reactivity, it was shown that atomically dispersed Rh species selectively promoted the AA formation, while Rh clusters unselectively decomposed methanol into CO/CO2. Further, decreasing the concentration of support acid sites reduced the rate of dimethyl ether (DME, the primary byproduct) formation. This work demonstrates how atomically dispersed Rh, support engineering, and active site pairing can be used to control selectivity in methanol carbonylation process. Additionally, effort was placed on more deeply understanding the role of Lewis acid sites in this process and developing approaches that minimize byproduct DME formation. We hypothesized that the formation of isolated Lewis’s acid sites on an inert oxide support could decrease the rate of DME formation, by minimizing the possibility of surface dimerization reactions. It is well known that isolated Rhenium oxide (ReOx) species can be formed on oxide supports and that they exhibit strong acidic sites. The structures of ReOx were characterized by using UV-vis spectroscopy, STEM, and XAS. Reactivity measurements surprisingly showed that atomically dispersed ReO4 species on SiO2 showed high selectivity (more than 93%) and rates of AA formation, while ReOx clusters were primarily selective for DME formation. With optimization of atomically dispersed ReOx species weight loading, and total catalyst loading in the reactor, methanol conversions > 60% with > 93% selectivity to AA were observed with stable performance over 60 hours. This is the best AA yield in a single pass heterogeneous catalytic system operating in a halide free process with industrially reasonable CO:methanol feed ratios that we are aware of. This work introduces a new class of promising heterogeneous catalysts based on atomically dispersed ReO4 on inert supports for alcohol carbonylation. We further modified the ReOx/SiO2 catalyst with Rh, forming atomically dispersed Rh-ReOx pair sites, which further promoted AA production rates 10-fold and AA selectivity to >96%, resulting in volumetric AA production rates comparable to homogeneous processes. At last, photon illumination was utilized to promote reaction rate and selectivity through transient charge transfer to adsorbed species. In this project, we demonstrate that illumination of 2 nm diameter Pt nanoparticle catalysts with pulsed visible light enhances time-averaged rates of H2 production via methanol decomposition compared with static illumination. Our results suggest that using light pulses to dynamically control the energetics of elementary steps on catalytic surfaces may enable higher activity or selectivity than is possible with static illumination or dictated by linear free energy scaling relations.

In this dissertation, we have utilized a multi-faceted approach, combining carefully controlled synthesis techniques, qualitative and quantitative surface characterization, as well as rigorous reaction kinetics techniques to elucidate key insights for heterogeneous catalytic processes involving Cu catalysts. Specific focus was placed on developing a comprehensive analysis of the current state-of-the-art Cu surface characterization technique as well as studying the impact of Cu-support interfacial effects on the reaction activity and mechanism of ethanol dehydrogenation.

For Cu surface characterization, the widely accepted assumption that N2O titration of Cu at mild temperatures only oxidizes metallic Cu to Cu+ lead to significant overestimation of Cu particle dispersion due to the over-oxidation of Cu sites to Cu2+. As expected, with increasing oxidation temperature, the extent of over-oxidation (relative concentration of Cu2+ sites) increased. This trend was most prominent on smaller Cu particles which were composed mainly of under-coordinated sites. On the other hand, the larger particles, which were more difficult to oxidize required larger oxidation temperatures compared to smaller particles to induce complete titration of the Cu surface. This highlights the difficulty in predicting an optimal N2O oxidation temperature, as it is Cu particle size dependent. In response to this, a novel technique was proposed, in which NO chemisorption is used to exclusively probe the quantity of Cu2+ sites generated after oxidation, as a means of correcting the initial dispersion estimates.

For the second part of this dissertation, it was demonstrated that metal oxide supports with strong Lewis acid sites promoted ethanol dehydrogenation activity over Cu by not only modifying interfacial Cu sites, but also by potentially facilitating the initiation of the reaction mechanism. The results in this work provide a potential bridge to explain the discrepancy in literature about the rate limiting step for ethanol dehydrogenation, but also elucidated the role of support acid sites, specifically at the metal-support interface, in promoting reaction rates. This study may work as a model system for carefully using reaction kinetics to determine the reaction active site, rate limiting step, and corresponding mechanism for future studies of small nanoparticles loaded on active metal oxide supports.

Heterogeneous catalysts are vital for the industrial production of fuels, chemicals, and commodities that continue to increase standards of living across the globe. By facilitating chemical reactions with accelerated rates and promoting high selectivity to desired products, catalysts ensure efficient utilization of raw materials with minimal energy inputs. Heterogeneous catalysts often consist of an active transition metal phase dispersed across a thermally stable high surface area support, such as metal oxides. Understanding how the structure of metal species correlates with catalytic properties is an essential element of catalyst research that builds both fundamental intuition and informs the practical design of next-generation catalyst materials. In order to develop these structure-function relationships, accurate characterization of the active catalyst species under reaction conditions is crucial.In this dissertation we explore how catalyst structures change under reaction conditions and correlate these structural changes to changes in catalytic performance. Observing how these materials evolve requires characterization at a variety of length scales that range from angstrom length bonds to long range ordering on the order of microns. Using a combination of tools including spectroscopic and microscopic characterization techniques, theoretical electronic structure calculations, and kinetic analysis through reactivity measurements we can gain detailed insights into the roles these structures play during chemical conversion processes. We employ this suite of tools to investigate a variety of different catalyst systems. Chapters 3 and 4 focus on characterization of dilute alloy catalysts that consist of noble coinage metal (e.g., Cu, Ag, Au) nanoparticles doped with low concentrations (e.g., < 1:100 molar ratio) of transition metals (e.g., Pt, Pd, Ni, etc.). In chapter 3 we reveal how adsorbate induced surface segregation can cause Pt atoms in AgPt alloys to migrate to the surface and form a variety of structures. Using CO oxidation as a probe reaction, we demonstrate how surface segregation can have a pronounced effect on catalytic activity. We further show that the local coordination environment of dopant Pt atoms can significantly impact their electronic structure, and potentially offer a handle to further tune catalyst properties. In chapter 4 we explore longer range restructuring in the form of sintering for a similar CuPt dilute alloy catalyst. Sintering of metal nanoparticles results in deactivation through the loss of surface area over time, and coinage metal nanoparticles are particularly susceptible to sintering due to their low cohesive energy and high mobility. Both reactivity measurements of high-pressure methanol synthesis and characterization show that dilute doping of Cu nanoparticles with Pt can significantly inhibit sintering, resulting in small nanoparticles that are stable even under high temperature conditions. Evidence from a combination of experimental and theoretical techniques suggest that Pt atoms locally inhibit the motion of undercoordinated Cu atoms on the surface of nanoparticles, minimizing the detatchment of these undercoordinated atoms that leads to sintering through an Ostwald ripening mechanism. The surface segregation effects identified in chapter 3 and the sintering resistant properties observed in chapter 4 are general characteristics for dilute alloy nanoparticle catalysts and electronic structure calculations provide predictions for systems that are likely to behave in a similar manner. In chapter 5 we investigate how bulk iron catalyst particles evolve during the conversion of CO into solid carbon and CO2 via the Boudouard reaction. We find that particles that initially start with micron size dimensions are fragmented under reaction conditions to produce particles on the order of ~50 nm. At low temperatures the reaction proceeds at steady state yielding extended carbon filaments several microns in length. Conversely at high temperatures, rapid reaction rates result in graphitic carbon overlayers that quickly deactivate the catalyst. The combination of characterization and reactivity reveal an underlying principle for these catalyst systems: there is balance between controlled carbon nucleation and catalyst longevity that must be understood for the design of hydrocarbon conversion processes. In chapter 6 we take a step back from specific catalysts and reactions and examine a common tool for catalyst characterization under reaction conditions: X-ray absorption spectroscopy. Using modelling techniques to analyze extended X-ray absorption fine structure (EXAFS) spectra, we quantitatively determined limits of detection for identifying catalyst structures using EXAFS when a sample contains a mixture of different active site structures. We found that often times minority species may go completely undetected using EXAFS analysis, but demonstrate scenarios where X-ray absorption near edges spectroscopy (XANES) can provide acute sensitivity to small changes in catalyst structures. Regardless, our analysis emphasizes the need for a variety of complimentary techniques to adequately characterize catalyst systems – an approach employed for all studies throughout this dissertation.

At a time when global warming has led to more aggressive regulations on emissions control around the world, the demand for automotive pollution-mitigating catalytic converters has grown. The three-way catalyst (TWC) in catalytic converters, composed of platinum group metal active sites, has played a critical role in automotive pollution mitigation. Specifically, rhodium (Rh) is crucial in TWCs to reduce nitrogen oxide (NOx) emissions produced by burning hydrocarbon fuel. Rh is a distinct catalyst in its ability to reduce NO at high rates with a high selectivity towards N2 while remaining robust.Despite extensive investigation, NO reduction mechanisms under automotive conditions remain unclear. Particularly, it is unknown which sites are responsible for the unselective reduction of NO to NH3 under light-off conditions, or in CO-rich feeds. This is important for further optimization of TWCs as NH3 is currently an unregulated emission from automobiles, despite it being a strong greenhouse gas. While Rh is highly selective towards N2 at steady-state conditions, the current understanding in literature rarely incorporates realistic automotive operating conditions where large concentrations of CO and H2O are present. Additionally, most studies have been performed over nanoparticle surfaces at higher weight loadings of Rh, >1%, despite the fact that the actual Rh weight loading in the TWC is lower than ~0.3%, where a mix of nanoparticles, small clusters, and atomically dispersed species are likely to be present. In this work, we focused on identifying the distinct properties and reactivity of atomically dispersed Rh active sites on oxide supports for NO reduction under automotive conditions. We developed and optimized approaches to synthesize atomically dispersed Rh catalysts on various oxide supports using principles of strong electrostatic adsorption. Probe molecule Fourier-transform infrared spectroscopy (FTIR) was utilized in correlation with temperature-programmed desorption, aberration-corrected scanning transmission electron microscopy, and Density Functional Theory calculations to provide insights into the structure and local coordination of the Rh species. Probe molecule FTIR was particularly useful in characterizing the distribution of Rh species in a sample as probe molecules bond with different stoichiometries and vibrational stretching frequencies on different Rh species ranging from atomically dispersed to nanoparticles. Using a series of well-characterized samples with various Rh structures, we observed via light off and kinetic measurements that atomically dispersed Rh active sites result in distinct NO reduction reactivity as compared to Rh nanoparticles. Specifically, under dry environments, NO is reduced by CO at higher rates on Rh nanoparticles as compared to atomically dispersed Rh species. However, at low NO conversion, Rh nanoparticles more selectively produce N2O, while atomically dispersed Rh species favor N2. Interestingly, the addition of H2O to the feed significantly promotes the NO consumption rate for atomically dispersed Rh active sites and results in 100% selectivity to NH3 at relevant operating temperatures. Alternatively, Rh clusters are less reactive than atomically dispersed species in the presence of H2O, and the reaction primarily leads to N2 formation. Thus, Rh structure has strongly affected NO reduction chemistry in catalytic converter conditions, both in terms of NO consumption rate and selectivity. Our results suggest that TWC formulations and emission control strategies should be targeted to avoid the formation of atomically dispersed Rh species to mitigate low-temperature NH3 formation. More broadly, the results highlight the potential role (sometimes negative) of atomically dispersed metal species in industrial catalytic processes.

Over the last 50 years, the reduction of automotive emissions has been a critical component of environmental legislation. The catalytic converter is one of the tools used by automotive engineers to reduce emissions: it facilitates the conversion of residual fuels and CO to CO2 and the reduction of NOx to N2 over the ‘three-way catalyst’ (TWC) containing a mix of precious metals, such as Pt, Rh, or Pd supported on high surface area γ-Al2O3. NOx reduction is accomplished by Rh, and while it achieves nearly complete conversion to N2 at steady state operating conditions, harmful emissions such as NH3¬ and N2O are produced during startup.

Previous studies identified a mix of Rh particles and atoms in typical TWC formulations, and others found that Rh atoms are responsible for NH3 production in the presence of NO, CO, and H2O (which is present in automotive exhaust). We aim to study aspects of the mechanism of NO reduction over atomically dispersed Rh/γ-Al2O3 catalysts to inform the design of catalysts and control systems that limit the production of harmful emissions. We focus on the dry reduction of NO by CO (no H2O), as this will serve as the base case for future studies including water and employ a combination of in situ, temperature programmed, and cryogenic infrared (IR) spectroscopy with UV photolysis in our study.

First, we found that the stable Rh dicarbonyl, Rh(CO)¬2, is the most abundant Rh species during the reaction in 0.5 kPa CO and 0.1 kPa NO (5000 ppm and 1000 ppm, respectively) at 205 °C, which models dry reaction conditions during startup of the catalytic converter. The abundance of Rh(CO)¬2 suggests that reactions involving this species initiate the catalytic cycle. Experimental and theoretical study of the influence of surface *OH (* denoting species adsorbed on γ-Al2O3 or Rh) on Rh(CO)¬2 found that high local *OH density on the support (in excess of ~10 *OH nm–2) near Rh(CO)¬2 lowers *CO vibrational frequency and energy barriers to *CO desorption while stabilizing a monocarbonyl intermediate, Rh(CO). At reaction conditions, it is likely that Rh(CO)¬2 in *OH dense regions of γ-Al2O3 are the most active species, and that Rh(CO)¬2 is directly coordinated to a *OH ligand shared with the support. After *CO desorbs from Rh(CO)¬2, Rh was initially assumed to interact with gaseous NO to form a mixed Rh–(NO)x–CO intermediate during NO reduction, but this could not be verified in situ due to the presumed rapid reactivity of this intermediate and the abundance of Rh(CO)¬2 species. Therefore, we used photolysis to induce *CO desorption from Rh(CO)2 at cryogenic temperature in the presence of NO to form and kinetically trap what we believe is Rh(CO)(NO)2, which was highly reactive below –75 °C. The assignment of this species and its speculated participation in the reaction mechanism is supported by previous studies of NO reduction by CO by homogeneous Rh catalysts and by density functional theory (DFT) calculations presented in this work. Finally, we extended the boundary of Rh interactions with support species by identifying reactions between Rh(CO)¬2 and NO-derived species on γ-Al2O3 at cryogenic temperature which consume Rh(CO)¬2 and oxidize Rh.

In this work, we establish the kinetic relevance of Rh(CO)¬2 consumption to the NO-CO reaction over atomically dispersed Rh/γ-Al2O3 and identify three processes that could be involved: *CO desorption followed by formation of Rh(CO)(NO)2 and subsequent reaction, reaction of Rh(CO)¬2 with NO-derived surface species on γ-Al2O3, and a Rh redox cycle. We also demonstrate the feasibility of UV photolysis for the generation and observation of reactive intermediates. Finally, we propose further experiments to determine which of the three processes are relevant to NO reduction over atomically dispersed Rh/γ-Al2O3 at TWC conditions.

The present work employs a variety of approaches and techniques to explore the photocatalytic watersplitting phenomena of a well-known particulate photocatalyst—Gallium Zinc Oxynitride (GaNZnO) from two aspects: reaction kinetics and charge transfer phenomena.

Cocatalyst loading onto n-type semiconductors has been proven to be a highly effective approach to promote the activity of catalysts in photocatalytic processes. Watersplitting by particulate photocatalyst (PPC) systems, as a phenomenon, are usually studied via a macroscopic approach developed for Photoelectrochemical Cell (PEC) systems. However, the macroscopic nature of the PEC systems may have overlooked the subtle while intrinsic aspects of the PPC systems due to challenges in characterization techniques and biased views in describing such microscopic systems. Thus, a fundamental re-investigation of watersplitting process by cocatalyst-loaded PPC systems via exploring rate-limiting reaction kinetics and charge transfer phenomena could provide more insights for the study and the improvement of PPC systems in watersplitting.

The watersplitting kinetics of GaNZnO PPC systems were explored through overall watersplitting (OWS) and half-reaction experiments. The photoelectron tracing experiment was developed specifically to clarify the charge transfer direction on GaNZnO surface. In addition, various characterization techniques, such as: UV-VIS, XRD, HRTEM, Dark-field STEM and EDS were applied to gain more knowledge of the PPC system before and after reactions. From the kinetic experiments, we observed unusual rate trends which contradict or deviate from results predicted by PEC models. Further investigation indicates that those unusual behaviors were explained by an opposite charge transfer direction previously proposed by PEC models.

In this work we utilized quantum chemical calculations coupled with numerical and analytical models to predict macroscopic observables associated with catalytic and photocatalytic processes on transition metal surfaces. All the predicted macroscopic observables were validated based on experimental measurements. The theoretical models developed here provide atomic scale insights into the mechanisms of catalytic and photocatalytic processes and suggest areas for future research in the design of novel catalysts.

In the first part of this dissertation, we focused on understanding characteristics that control performance of late transition metals for the catalytic reduction of CO2 by H2. By coupling Density Functional Theory (DFT) calculations with mean-field microkinetic models we found that on Ru-based catalysts CHO* dissociation to CH* and O* is the rate determining step (RDS) for CH4 formation, while CO* desorption is the RDS for CO production. The affinity of late transition metals for O* adsorption was identified as an effective predictor of selectivity between CO and CH4 production on late transition metals, which matches well with previous experimental observations. We extended this study by developing an analytical approach, called the “scaled degree of rate control” (S-DoRC), to systematically identify rate and selectivity determining steps for reactions on late transition metals.

In the second part of this dissertation we coupled ΔSCF-DFT calculations with a Hamiltonian-based inelastic scattering model to understand the detailed mechanisms of photocatalysis on metal surfaces. We focused on understanding experimentally observed behaviors of photon-driven CO and NO desorption and O diffusion reactions on the Pt(111) surface. The developed models that accurately predicted previous wavelength-dependent and time-resolved measurements and suggested approaches to control selectivity in photon driven reactions on metal surfaces.

In the last part of this dissertation we coupled extensive DFT calculations with Wulff constructions to understand the process of CO-induced reconstruction of Pt nanoparticle catalysts, which is critical to catalytic converters. By correlating atom-resolved imaging via in-situ scanning transmission electron microscopy (STEM), with in-situ quantitative, site-specific infrared (IR) spectroscopy and DFT based Wulff-constructions we demonstrated that at high CO coverage, Pt nanoparticles undergo a facet selective reconstruction where (100) facets reconstruct to stepped vicinal surfaces, while (111) facets are stable. This is the first example of an atomic scale and quantitative view of adsorbate induced metal nanoparticle surface reconstruction at realistic conditions.

US interest in biomass as an alternative energy source continues to increase in demand with growing political, economic and environment concerns with fossil fuels. One potential route for converting cellulosic biomass to fuels is through the dehydration of fructose to produce 5-hydroxymethylfurfural (HMF), followed by the catalytic conversion of HMF to renewable fuels, such as dimethylfuran (DMF). Economically viable production of HMF from fructose requires high yields. The formation of HMF has been thought to occur through the acid-catalyzed dehydration of the furanose form of fructose, while other fructose tautomers likely lead to unwanted side products. The tautomeric composition of fructose is highly dependent on the reaction temperature and solvent system; thus, HMF production is likely sensitive to these factors. In order to develop detailed insights into the factors that control HMF yields, we executed extensive kinetic analysis using a variety of solvent systems, reaction conditions, acid types and acid concentrations to determine the mechanism and selectivity controlling factors in fructose dehydration to HMF by homogeneous acid catalysts. Through these studies we were able to strongly support the hypothesis that selectivity in fructose conversion to HMF is controlled by the tautomeric distribution. Furthermore, we found that dimethyl sulfoxide (DMSO) solutions produce the highest HMF selectivity, (up to 85%) and fastest conversion rates of any solvent system; however, DMSO is highly energy intensive to separate. To overcome separation costs, we aimed to develop novel heterogeneous catalysts which mimic the desirable solvation effects of DMSO in low boiling, aqueous co-solvent mixtures. We grafted DMSO-like functional groups near acidic sites on the surface of supports generate local conditions which mimic bulk solutions of DMSO, which increased HMF selectivity by ~30%. To synthesize these catalysts, the surface of silica was first functionalized by (3-mercaptopropyl)-trimethoxysilane. The grafted thiols were then converted into 1, 3-methyl propyl sulfoxides to mimic DMSO at the surface and characterized by RAMAN spectroscopy. A techno-economic analysis was performed on the conversion of fructose to dimethylfuran (DMF) by heterogeneous catalysts in aqueous tetrahydrofuran solutions, using Aspen Plus. We report a minimum selling price (MSP) of DMF of 1.74 $/kg, with fructose feedstock, DMF yield, catalyst cost, and water treatment being the main contributors to the MSP.

The design of heterogeneous catalysts typically emphasizes modifications of the electronic state of the active metal to control catalytic reactivity. Changes in the electronic structure of active metals often is correlated with changes in the enthalpies of adsorbed species and enthalpic reaction barriers, but inherent limitations exist on the activity and selectivity of a catalyst that cannot be overcome by these modifications alone. Less often, non-covalent interactions are considered in modifying catalysts. These interactions typically limit the degrees of freedom that reactants, transition states, and products can exhibit, often resulting in modified entropies of adsorbed species and entropic barriers. Even less commonly, the mobility of the active site is relevant to catalytic activity.Here, we present a system in which atomically dispersed Rh/Al2O3 catalysts are modified with straight chain alkylphosphonic acid self-assembled monolayers, such that the mobility of atomically dispersed Rh is altered. Careful kinetic measurements of CO desorption from Rh(CO)2, and ethylene hydroformylation evidence that Rh(CO)2 is mobile on unmodified Rh/Al2O3, which results in entropic barriers for reactions that is lowered when the mobility of Rh(CO)2 is decreased by the phosphonic acid self-assembled monolayers. This results in improved activity and selectivity for ethylene hydroformylation with negligible changes to the enthalpic barriers. Further, the extent by which Rh(CO)2 mobility is decreased was modified by changing the length of alkylphosphonic acid tails, in which longer tailed phosphonic acids vii formed more rigid self-assembled monolayers due to intermolecular attractions, resulting in lower Rh(CO)2 mobility. Despite the unique reactivity of atomically dispersed catalysts, the low active metal loadings often results in low catalyst productivity. By modifying existing synthetic protocols, changing support morphology, and using gentle and dispersive pretreatments, the Rh loading was increased by up to 80x (0.25% Rh to 20% Rh) while maintaining atomic dispersion. Additionally, phosphonic acids were used to stabilize atomically dispersed species at elevated pressures and temperatures. Overall, these studies establish the relevancy of active site mobility in atomically dispersed heterogeneous catalysts, methodologies by which to modify active site mobility, and methods to increase the loading and stability of atomically dispersed catalysts.