Insights into the Selectivity Determinant and Rate-Determining Step of CO2 Hydrogenation to Methanol

CO2 hydrogenation to methanol has attracted much attention. The mechanism, the factors affecting selectivity, and the rate-determining step of the reaction have not been clearly concluded. Here, the reaction mechanism on the Cu/ZnO/Al2O3, the Pd/ZnO, and the ZnZrOx catalysts was studied by in situ infrared spectroscopy and HCOOH temperature-programmed surface reaction (HCOOH-TPSR) experiment. It is shown that the HCOO* mechanism is a feasible mechanism, and the more stable HCOO* on the catalysts is, the higher the selectivity of methanol accompanied with the less CO produced via the decomposition of HCOO*. H2–D2 isotope exchange reaction is inhibited in the presence of CO2, which indicates that H2 activation and H* migration are inhibited by CO2 adsorbed on the catalysts. As for CO2 hydrogenation to methanol, the reaction orders of H2 and CO2 are close to 0.5 and 0, respectively, indicating that activated H* on the catalysts is insufficient. Comparing CO2 hydrogenation to methanol reaction and H2–D2 isotope exchange reaction, their H2 reaction orders are both 0.5 and the two reaction rates show a linear relationship when the temperature changes. It is considered that the rate-determining step of CO2 hydrogenation to methanol is the migration of H* on the catalysts.