Conversion of carbon dioxide, an abundant greenhouse gas, into useful fuels can help solve issues associated with both energy and the environment. Experiments have successfully shown activity for CO2 conversion to products like CO, methane, and methanol using Cu/TiO2 photocatalysts. The oxidation state, morphology, and size of Cu can all significantly affect the photocatalytic activity. How this catalyst works and how it could be improved is an area of much research. We studied this catalyst using density functional theory (DFT) to obtain atomic level insights in the CO2 reduction process on the catalyst surface. A key activation step in CO2 reduction is the formation of CO2 anion species with a bent structure. Highly dispersed small clusters of Cu were synthesized in our previous work on TiO2 surface using a redox reaction between Cu2+ and Sn2+[1]. We modeled small Cux (x=1-4) clusters on a TiO2 anatase surface. Our results show that Cu is able to activate CO2 into a bent configuration that can be further reduced. Charge analysis indicates that CO2 does indeed become negatively charged in a bent configuration, but not in a linear adsorption mode. We analyzed charge on Cu to assign its oxidation state, as well as calculating adsorbed CO vibrational modes, a common experimental method to assign oxidation state of supported metals. The Cu dimer especially stabilized the bent CO2 through strong C-Cu bonds. Our results however show that formation of the Cu dimers on the anatase surface is kinetically and thermodynamically unfavorable. A synthesis method that could create Cu dimers could lead to very active photocatalysts, and may be a promising direction for experimentalists to pursue. Beyond Cu, we have also modeled other metal atoms on the anatase surface to explore possible active catalysts for CO2 photoreduction. Our results identify how metal clusters on TiO2 surfaces may activate CO2, and such knowledge is crucial towards refining and designing better CO2 conversion catalysts.

Conversion of CO2 into useful fuels using Cux/TiO2 photocatalysts

254th National Meeting (2017)

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