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.