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 CO2 at 230–270 °C and total pressure 8 bar. Cu/SiO2 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 H2 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.