Reaction pathways and the role of the carbonates during CO_2 hydrogenation over hexagonal In_2O_3 catalysts

摘要

Complete reaction pathways for CO2 hydrogenation to CH3OH and CO on the h-In2O3(012) and h-In2O3(104) flat and step surfaces are investigated by density functional theory (DFT) calculations. By analyzing the complete reaction pathways, bi-HCOO* hydrogenation to H2COO* is found to be the rate-determining step (RDS) for CH3OH formation on the h-In2O3(012) flat surface and h-In2O3(104) flat and step surfaces, OH* protonation to H2O* is determined as the RDS for CH3OH formation on the h-In2O3(012) step surface and that for CO formation on the h-In2O3(012) flat and step surfaces, and bent CO2 configuration (bt-CO2*) protonation to COOH* is predicted to be the RDS for CO formation on the h-In2O3(104) flat and step surfaces. On the h-In2O3(012) step surface and the h-In2O3(104) flat and step surfaces, the carbonate(carb-CO2*) as one of CO2 adsorption configurations can be seen as a spectator species at the O-v sites during the reaction, whereas on the h-In2O3(012) flat surface, protonation of the carb-CO2* contributes more to CO formation than protonation of the bt-CO2*. Meanwhile, the O-v sites on the h-In2O3(012) and h-In2O3(104) step surfaces show lower catalytic activities than corresponding flat surfaces, and thus they are predicted to play little role in determining the catalytic activity of the hexagonal In2O3 catalysts.