Mechanism and structure sensitivity of methanol synthesis from CO2 over SiO2-supported Cu nanoparticles

Abstract The size of copper nanoparticles affects the intrinsic rate of methanol and CO formation in the hydrogenation of CO 2 at 230–270 °C and total pressure 8 bar. Cu/SiO 2 catalysts with different mean Cu particle sizes were prepared by water-in-oil microemulsions and suitably pretreated to obtain samples with mean particle size between 4 and 36 nm. The intrinsic rates of methanol and CO formation are 3-fold higher for large particles (>10 nm) than for smaller ones (ca. 4 nm). The reaction path was studied by means of kinetic experiments, H/D isotopic substitution, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). Results demonstrate that the hydrogenation of formic acid is the rate-determining step (rds) for methanol synthesis, whereas for CO formation the reaction proceeds with the assistance of H atoms and the dissociation of the carboxyl intermediate is the rds. This reaction scheme implies that the ratio of CO to methanol formation rates is inversely proportional to H 2 partial pressure, which is also consistent with results for similar catalysts reported in earlier studies. The similar kinetic parameters and H/D kinetic isotope effect observed for catalysts with significantly different Cu mean particle sizes suggests that active surface sites have similar topology regardless of Cu particle size. Step-edge sites whose relative abundance increases with particle size in a way similar to activity are suggested as the active sites for methanol and CO formation reactions.