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

Abstract

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 ZnZrO_x 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.