Surface Adsorption Studies of CO2 and H2 on Defected Indium Oxide: Insights into Gas-Phase Heterogeneous Photocatalysis

The conversion of CO2 into fuels or chemical feedstocks is valuable for not only mitigating the rising atmospheric concentration of CO2, but also for utilizing CO2 as a globally available, abundant, and renewable resource. Indium oxide defected with oxygen vacancies and hydroxides, In2O3-x(OH)y, is a catalyst material that facilitates the light-assisted, gas-phase heterogeneous reduction of CO2 via the reverse water gas shift (RWGS) reaction, CO2 + H2 -> CO + H2O. The extent of defects in In2O3-x(OH)y correlates strongly with the rate of CO2 conversion as well as the CO2 adsorption capacity. Prior to this work, the specific interactions between the RWGS reactants, CO2 and H2, and the In2O3-x(OH)y surface were largely unknown. The work herein explores the In2O3-x(OH)y surface chemistry through carefully designed in situ thermogravimetric analysis and infrared spectroscopy techniques to identify and investigate surface-adsorbed CO2 and H2 species. CO2 adsorbs as a complex mixture of carbonates, bicarbonates, carboxylates, and linear CO2 species, which adsorb at readily available and specific surface sites, forming a partial monolayer with varying adsorption strengths. H2 easily reduces the In2O3-x(OH)y surface, resulting in a broad and intense infrared absorbance peak due to the delocalization of conduction band electrons, the intensity of which decreases with an increase in defects, agreeing with our understanding that defects promote charge trapping at the surface to be used in chemical reactions. The coadsorption of CO2 and H2 results in a strongly adsorbed formate species, providing indirect evidence for H2 heterolytic dissociation, in addition to possibly identifying formate as an intermediate for the RWGS reaction or a bystander that instead leads to methanol synthesis. Through detailed CO2 and H2 adsorption studies, this thesis has provided us with valuable insight into the In2O3-x(OH)y surface and its complex chemistry â a critical contribution to the development of a viable CO2 utilization technology.Ph.D.2019-11-06 00:00:0