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Elucidating the role of adsorbed states of hydrogen, water and carbon dioxide over TiO2 and Pd/TiO2

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2020
Yılmaz, Begü
Artificial photosynthesis studies aim to utilize solar energy for conversion of CO2 into valuable organics using H2O; however, developments in the area are far from realistic level applications due to the low production rates. In order to understand the mechanism, CO2 and H2O adsorption isotherms over TiO2 P25 were investigated by using a volumetric chemisorption technique with and without an illumination source. Adsorption isotherms revealed indirect evidences on local temperature rise on the surface indicating photo-induced thermal chemistry upon charge recombination. Desorption was observed during CO2 adsorption over TiO2 P25 under illumination. Calculation of adsorbed amounts according to estimation for local temperature rise (to 160°C) on the surface yielded a reasonable adsorption isotherm under illumination. Similarly, H2O adsorption isotherm under illumination showed Langmuir type of adsorption isotherm indicating presence of only strongly bound molecules under illumination. In this study, weakly adsorbed molecules were claimed to be desorbed due to local temperature rise on the catalyst surface upon charge recombination resulting in similar chemistry with heating of the sample cell to 150°C. Additionally, the attempts on direct heat release measurement upon charge recombination via microcalorimeter were explained. Moreover, H2 and CO2 adsorption isotherms over Pd/TiO2 surface were investigatedusing a chemisorption manifold connected to a Setaram C80 microcalorimeter to understand the effect of presence of precious metal on TiO2 support. 0.5% Pd/TiO2 showed relatively well dispersed palladium on support. Enhanced CO2 adsorption observed on 0.5% Pd/TiO2 was related to increase in oxygen vacancy concentration due to spilled hydrogen during the reduction of surface. Evidences for presence of palladium bulk on 2.0% Pd/TiO2 were presented via H2-TPR results, especially. According to these evidences, it is understood that the effect of heat release upon charge recombination on chemistry is highly underrated in literature. The local temperature rise can affect the chemistry significantly;however, uncontrollable temperature conditions are not desired. Last but not least, metal hydrogen systems over metal oxide support can offer a strategy for enhanced reduction reactions by utilizing stored hydrogen on surface.