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Synergistic Coupling of Photo and Thermal Conditions for Enhancing CO2 Reduction Rates in the Reverse Water Gas Shift Reaction

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journal contribution
posted on 31.12.2019, 12:33 by Joel Y. Y. Loh, Yufeng Ye, Nazir P. Kherani
The photocatalytic activity of nanostructured In2O3–x(OH)y for the reverse water gas shift (RWGS) reaction CO2 + H2 → CO + H2O can be greatly enhanced by substitution of Bi­(III) for In­(III) in the lattice of BizIn2–zO3–x(OH)y. This behavior was hypothesized as the effect of the population and location of Bi­(III) on the Lewis acidity and Lewis basicity of proximal hydroxide and coordinately unsaturated metal surface sites in BizIn2–zO3–x(OH)y acting synergistically as a frustrated Lewis acid–base pair reaction. Nonetheless, such photocatalytic activity is usually optimized in a specific batch reactor setup sequence, with H2 as an initial gas input under photo and thermal conditions before introducing CO2. Hence, the chemical interplay between environment parameters such as photoillumination, thermal input, and gas reactant components with the effects of Bi substitution is unclear. Reported herein, glovebox-protected X-ray photoelectron spectroscopy (XPS) interrogates this photochemical RWGS reaction transiting from vacuum state to similar conditions in a photocatalytic reactor, under dark and ambient temperatures, 150°C in dark and 150 °C under photoillumination. Binding energy shifts were used to correlate the material system’s Lewis basicity response to these acidic probe gases. In-situ gas electronic sensitivity and in-situ UV–vis-derived band-gap trends confirm the trends shown in the XPS results, hence showing its equivalency with in situ methods. The enhanced photocatalytic reduction rate of CO2 with H2 with a low doped 0.05% a.t Bi system is thus associated with an increased gas sensitivity in H2 + CO2, a greater expansion in the OH shoulder than that of the undoped system under heat and light conditions, as well as a greater thermal stability of dissociated H adatoms. The photoinduced expansion of the OH shoulder and the increased positive binding energy shifts show the important role of photoillumination over that of thermal conditions. The poor catalytic performance of the high doped system can be attributed to a competing H2 reduction of In3+. The results provide new insight into how pairing photo and thermal conditions with the methodical tuning of the Lewis acidity and Lewis basicity of surface frustrated Lewis acid–base pair sites by varying z amount in BizIn2–zO3–x(OH)y enables optimization of the rate of the photochemical RWGS.