Indium oxide modified with alkali metals: A selective catalyst for the reverse water-gas shift reaction at high pressure

Alkali (K, Rb, Cs)-modified In2O3 displays outstanding performance in the reverse water-gas shift (RWGS) reaction under high pressure. For instance, alkali-containing catalysts achieved nearly stoichiometric selectivity at 350 degrees C and 50 bar, while pristine In2O3 exhibited 58.5 % CO selectivity. Furthermore, the presence of Rb and Cs improved CO2 conversion by approximately 1.8-fold compared to In2O3, reaching equilibrium conversion. Conversely, Li and Na significantly reduced catalytic activity. CO formation followed the trend: Li/In2O3 < Na/In2O3 < In2O3 < K/In2O3 < Rb/In2O3 < Cs/In2O3. The Alkali/In2O3 catalysts also demonstrated greater flexibility under varying reaction conditions. Compared to In2O3, Cs/In2O3 operated at a higher maximum temperature (600 vs. 500 degrees C) and significantly reduced byproduct formation under high space velocity and high H-2/CO2 ratios. Notably, Cs/In2O3 achieved a remarkable CO productivity of 0.26 mol<middle dot>L-1<middle dot>h(-1) at a GHSV of 100 L<middle dot>h(-1)<middle dot>g(-1) at 400 degrees C and 50 bar, outperforming previously reported selective catalysts for high-pressure RWGS. Characterization of fresh and spent samples suggests that the alkali metals are dispersed on the In2O3 surface as carbonates and bicarbonates, particularly on Cs/In2O3, which enhances CO2 uptake and catalytic behavior. DFT and diffuse reflectance infrared Fourier transform spectroscopy reveal that oxygen vacancies play a critical role in the catalytic activity of In2O3 for CO2 conversion, with alkali metal promotion, particularly Cs, further boosting performance. DFT calculations indicate that alkali metals lower the formation energy of oxygen vacancies and enhance CO2 and CO adsorption. Combining computational and experimental data shows that Cs/In2O3 promotes CO formation via the carboxyl pathway while suppressing methanol formation through the formate pathway.