H₂/D₂ Kinetic Isotope Effects in Methanol Synthesis on an In₂O₃ Catalyst

Indium oxide (In2O3) has become a very promising catalyst in recent years for its high selectivity of CO2 hydrogenation to methanol, an ideal fuel for green energy. For rational design of this catalytic system, further kinetics and mechanistic insights into this reaction are demanded. Here, based on a recently optimized c-In2O3 catalyst, we investigate H2/D2 isotope effects on the catalytic methanol synthesis reactivity up to 16 bar and 270 °C by online MS measurements, in both steady and transient states. In addition to a normal kinetic isotope effect (RH > RD, where R is the isotope-corresponding reaction rate), the Arrhenius plot yields a higher activation energy (Ea) for D2/CO2 than for H2/CO2 (121.5 ± 5.8 vs 100.6 ± 4.9 kJ·mol–1) inputs. The activation energies remain almost constant at 8 and 16 bar, and the pressure dependence study confirms that the reaction rate is nearly linear to the total input pressure. Quantitative transient analysis of the exchanged D/H isotopic species reveals a much smaller Ea (61 ± 5.2 kJ·mol–1) for methanol formation from surface D/H species. The results also indicate that the active surface is highly reduced by hydrogen and there is a high efficiency of converting surface hydrogen to methanol. These new findings will hopefully provide useful information toward the rational design of active and stable catalysts based on In2O3 for CO2 hydrogenation to methanol