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 °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 °C) and significantly reduced byproduct formation under high space velocity and high H₂/CO₂ ratios. Notably, Cs/In₂O₃ achieved a remarkable CO productivity of 0.26 mol·L⁻¹·h⁻¹ at a GHSV of 100 L·h⁻¹·g⁻¹ at 400 °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.