Photoelectrochemistry and its associated technologies show unique potential to facilitate the large-scale production of solar fuels - those energy-rich chemicals obtained through conversion processes driven by solar energy, mimicking the photosynthetic process of green plants. The critical component of photoelectrochemical devices designed for this purpose is the semiconductor photoelectrode, which must be optically absorptive, chemically stable, and possess the required electronic band alignment with respect to the redox couple of the electrolyte to drive the relevant electrochemical reactions. After many decades of investigation, the primary technological obstacle remains the development of photoelectrode structures capable of efficient and stable conversion of light with visible frequencies, which is abundant in the solar spectrum. Metal oxides represent one of the few material classes that can be made photoactive and remain stable to perform the required functions. The unique range of functional properties of oxides, and especially the oxides of transition metals, relates to their associated diversity of cation oxidation states, cation electronic configurations, and crystal structures.

In this dissertation, the use of metal oxide films, nanomaterials, and heterostructures in photoelectrodes enabling the solar-driven oxidation of water and generation of hydrogen fuel is examined. A range of transition- and post-transition-metal oxide material systems and nanoscale architectures is presented. The first chapters present results related to electrodes based on alpha-phase iron(III) oxide, a promising visible-light-active material widely investigated for this application. Studies of porous films fabricated by physical vapor deposition reveal the importance of structural quality, as determined by the deposition substrate temperature, on photoelectrochemical performance. Heterostructures with nanoscale feature dimensionality are explored and reviewed in a later chapter, which describes the methodologies to combine the unique and complimentary functional properties of dissimilar oxides to optimize the water photo-oxidation process Experimental results based on an iron(III) oxide-tungsten(VI) oxide system show enhancements associated with the heterostructure, which may indicate the presence of unexpected minority carrier dynamics, as observed additionally by ultrafast transient absorption spectroscopy.

Next, a new conceptual framework for the design of solar water oxidation photoelectrodes based on the spatially inhomogeneous doping of wide-bandgap metal oxide nanostructures is introduced and experimentally verified. It is found that optical absorption and electronic conduction can be decoupled and optimized by spatially segregating the functional impurity species that facilitate their associated physical processes. In the final chapters of this dissertation the electronic structures of key oxide-oxide interfaces, relevant to the operation of efficient photoanodes, are examined using synchrotron-based soft x-ray spectroscopy. These studies indicate that the interfacial regions of electrodes possess distinct electronic structures, which deviate in terms of orbital character and occupancy from those of their constituent bulk oxides. These observations inform methodology to address certain operational deficiencies associated with the use of metal oxides for solar energy conversion applications.

The dramatic increase in supply of natural gas in the U.S. presents a unique opportunity to develop new catalytic processes that utilize light hydrocarbons for chemical manufacturing and fuel synthesis. Conventional catalytic processes utilize extreme conditions requiring high energy input for the initial C-H bond activation and multiple-step processes for the conversion of activated compounds into higher-value products. Direct conversion routes, on the other hand, have been investigated and have shown promising results toward more-efficient utilization of natural gas resources. In this dissertation, direct conversion routes are classified into three different categories: liquid-phase thermal catalysis, electrocatalysis, and gas-phase thermal catalysis. Investigations involving each of these conversion routes are presented, adding new fundamental information on light alkane upgrading to contribute to the existing literature.First, liquid-phase thermal catalysis was studied by utilizing unsupported AuPd nanoparticles and H2O2 as an oxidant in water. Numerous C2 oxygenates, including ethyl hydroperoxide/ethanol, acetaldehyde, and acetic acid were generated with maximum-observed oxygenate yield 7707 μmol gAuPd-1 h-1 from ethane at 1 bar and 21 °C. In addition, continuous supply of low H2O2 concentrations to a semi-batch reactor over 50 h showed that on-site generated H2O2 can serve as an oxygenate-selective oxidant of ethane. Given that, electrochemical approaches could serve as the basis to continuously and sustainably generate the H2O2 fed into the chemical reactor to partially oxidize light hydrocarbons. In fact, utilizing air and water under electochemical potential can simply produce H2O2. Chapter 3.2 introduces an electrocatalyst that is engineered from electrochemical deposition of Pd ions during O2 reduction. This catalyst showed an extreme activity towards selective production of H2O2 which is determined as the second-highest partial kinetic current density for H2O2 production in acidic media reported in the known literature. The second direct conversion system, electrocatalysis, was also investigated to provide fundamental knowledge about how to effectively activate and functionalize the C-H bond. For this purpose, a prototypical electrocatalyst surface, polycrystalline Pt, was employed to assess the interaction of it with methane and water. Experiments performed by anodic stripping voltammetry techniques coupled with operando surface-enhanced infrared absorption spectroscopy (SEIRAS) indicated that the sites with lower Pt coordination number (defects or step-edges) are potentially responsible for initial C-H bond activation and its decomposition to surface-bond C-H-O or C-O fragments. For the last direct conversion route, oxidative coupling of methane (OCM) was considered as the model system and proposed to be investigated by near-surface optical and mass spectrometry techniques. These diagnostic techniques were initially gauged for the partial oxidation of methanol over a silver catalyst to demonstrate the value of the near-surface gas-phase measurements. The success in mapping out the coupling between the gas-phase and surface in a heterogeneous reaction encouraged us to work on the identical system with different metal surfaces. Chapter 4 mainly exhibits the studies on conversion of methanol near atmospheric pressure using molecular beam mass spectrometry on Pd film surfaces. Methoxymethanol, a rarely observed intermediate, was detected, and provided as supporting evidence to previously proposed reaction networks. Later, in order to shift the direction to OCM reaction, the near surface techniques were modified and optimized with the knowledge gathered during methanol oxidation experiments. Preliminary experiments were conducted on OCM and initial results were included in the Appendix of Chapter 4.3.5. More promising results including the radical detection are expected to be obtained in order to elucidate the complex reaction network taking place on the surface and near-surface. Overall this thesis extensively covers the direct conversion methods for the utilization of light hydrocarbons and provides the detailed experimental procedures in the context of the comprehensive literature.

Global climate change is a major motivation for the mitigation of greenhouse gas emissions. The most significant greenhouse gases contributing to US emissions, reported by the EPA, are carbon dioxide, methane, and nitrous oxide – all products of oil production and combustion. Catalytic co-conversion of these gases via CO2- or N2O-assisted methane coupling to high-value commodity chemicals, such as ethylene, has been demonstrated to be a highly selective process over some metal oxide catalysts. The research in this dissertation focused on identifying physical and electronic properties of selective metal oxide catalysts to develop structure-activity relationships between this class of catalysts and this class of oxidative coupling reactions. To study this reaction system, a series of CaO/ZnO catalysts were developed as a platform to study the mechanistic cooperation of binary metal oxide catalyst systems. Calcium oxide, a highly basic metal oxide, was deposited on zinc oxide, a reducible oxide. CaO/ZnO binary metal oxide catalyst is comprised of cheaply abundant materials and has been demonstrated to be highly selective toward C2 products during CO2-assisted methane coupling (CO2-OCM). A series of Ca/ZnO catalysts with varying Ca composition were characterized by microscopy (TEM), X-ray spectroscopies (L-edge XANES, XPS), and CO2 adsorption infrared spectroscopy-temperature programmed desorption (IR-TPD). Catalysts with less than 2 mol% Ca contained highly disperse Ca sites that had lower Lewis basicity compared to bulk CaO. The CO2-OCM performance of these catalysts with low-Ca-loading exhibited a strong dependence on Ca loading, where minor additions of Ca drastically increased C2 product selectivity. These results coupled with further catalytic tests report the medium strength basicity of the interface between dispersed Ca and ZnO present in low-Ca-loading catalysts is optimal for C2 product selectivity. X-ray absorption spectroscopy (XAS) and complementary theoretical simulations characterized the extent of Ca dispersion in the low-Ca-loading Ca/ZnO catalysts. For catalysts with less than 2 mol% Ca, the Ca most probably exists as linear one-dimensional and planar two-dimensional CaO clusters roughly 7 to 26 Å in length. A pre-edge feature of the XANES spectra unique to the low-Ca-loading catalysts was attributed to the presence of some under-coordinated Ca surface atoms by analysis of the local densities of states. The N2O-OCM performance of these catalysts was evaluated. The presence of the CaO clusters and under-coordinated surface atoms corresponded to higher C2-4 product selectivities than over high-Ca-loading catalysts. These Ca sites are highly dispersed on ZnO, creating many selective Ca/ZnO interfacial sites, which can lead to enhanced methane coupling performance. Additional experiments comparing kinetic and mechanistic information across various oxidants during methane coupling reveal a strong effect of oxidant partial pressure on reactivity. Co-feeding oxidants at varying partial pressures should be further explored as a route of optimizing product yields. To optimize this system for ethylene production, catalytic oxidant-assisted ethane dehydrogenation should also be further investigated. Preliminary results suggest that Ca/ZnO can effectively catalyze ethane dehydrogenation on the catalyst surface during CO2-OCM. Various morphologies of ZnO were tested with Ca impregnation for their CO2-OCM performance. Surface-area-normalized C2 product yields did show preliminary morphology-dependence, where rod-like structures with a dominant (100) facet had poorer product yields than a commercial ZnO.

When supported metal catalysts contain metal components existing at or near atomic dispersion, the support surface largely controls the properties of the highly dispersed metal species by determining the local chemical bonding environments of the metals. The framework structures of zeolites afford unique bonding environments for supported metals which can result in catalysts having unusual properties. The research described in this dissertation was aimed at identifying potential advantages of using zeolites as supports for catalysts containing chromium or platinum ‒ two industrially applied catalyst metals whose intra-zeolite chemistries are not fully resolved in the literature. Chromium was dispersed on HZSM-5. Samples were characterized using X-ray absorption near edge structure (XANES) and infrared (IR) spectroscopies and evaluated for ethane dehydrogenation. At low chromium loadings, chromium was located at zeolite aluminum sites and Cr/HZSM-5 samples displayed stable ethane dehydrogenation activity with time on stream. Higher chromium loadings resulted in catalysts with higher dehydrogenation activity per chromium atom but that deactivated quickly, and this was correlated to higher fractions of electron-rich or multinuclear chromium present in these samples. The results represent an attempt to assess the potential for catalytic application of Cr/ZSM-5, taking into account the speciation of chromium among various anchoring sites on the zeolite surface. Platinum was dispersed onto HZSM-5 and characterized using X-ray absorption and IR spectroscopies. During exposure of Pt/ZSM-5 to high-temperature, oxidizing conditions, Pt2+ ions were stabilized at six-membered rings in the zeolite that contained paired-aluminum sites. This interpretation was informed by a theory-guided analysis of X-ray absorption fine structure spectroscopy (EXAFS) data. These Pt2+ ions formed highly uniform platinum gem-dicarbonyls, and the steps leading toward formation of platinum clusters were monitored through the evolution of IR spectra during exposure of platinum carbonyls to reducing conditions. Platinum clusters in HZSM-5 were redispersed into Pt2+ cations under high-temperature, oxidizing conditions, with the Pt2+ cations returning to paired-aluminum, six-membered ring sites. Similar platinum gem-dicarbonyl complexes formed in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y), demonstrating the generality of the chemistry across zeolite frameworks. The findings connect catalyst structural properties to critical performance outcomes for an industrially-relevant catalyst material system. Chromium was dispersed onto a series of MFI zeolites with various support compositions. The catalysts were characterized by IR or X-ray absorption spectroscopies and evaluated for ethane dehydrogenation with or without CO2. The copresence of Cr2+ and Cr3+ in siliceous or borosilicate MFI zeolites was correlated with significant enhancements in the rates ethane dehydrogenation when CO2 was added to the reaction mixture. The aluminosilicate MFI zeolite, in contrast, stabilized chromium in the +2 oxidation state during reaction, resulting in a catalyst that exhibited low rates of CO2 reduction to CO by H2 and no enhancement of ethane dehydrogenation by CO2. The mechanistic role of CO2 is discussed in the context of ethane dehydrogenation with Cr/MFI catalysts. Additional experiments characterizing Cr/zeolite or Pt/zeolite samples provided insights into the local environments of the supported metals. Platinum carbonyl complexes in HZSM-5 and Y zeolite were characterized by XAS in order to complement the results of IR spectroscopy. Similar chromium or platinum species existed in zeolites of identical framework structure but different heteroatom identity. Supported chromium or platinum species similar to those found in HZSM-5 were found to exist in zeolites other than HZSM-5.