UC Davis

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.