Synthesis and Decomposition of Formate on a Cu/SiO2 Catalyst: Comparison to Cu(111)

The kinetics of formate synthesis from CO2 and H2 and from formate decomposition on Cu/SiO2 were studied by temperature-programmed decomposition experiments and compared with those reported for Cu(111), Cu(110), and Cu(100) surfaces under similar reaction conditions. The initial rate of the formate synthesis from 380 Torr CO2/380 Torr H2 at 323–353 K over the Cu/SiO2 catalyst was in good agreement with those on Cu(111) and Cu(110). At 353 K, for example, the initial rate was measured to be 5.3×10−4 molecules site−1 s−1. The apparent activation energy was determined to be 58.8 kJ mol−1, which was also comparable with those obtained for Cu(111) (54.6–56.6 kJ mol−1), Cu(110) (59.8 kJ mol−1), and Cu(100) (55.6 kJ mol−1). The decomposition rate of formate was first-order in formate coverage, and the rate constant was 1.13×10−4 s−1 at 384 K. The activation energy and the pre-exponential factor were determined to be 115.7 kJ mol−1 and 5.38×1011 s−1, respectively, in good agreement with those obtained for Cu(111), 107.9–112.8 kJ mol−1 and (1.87–4.02)×1011 s−1, rather than those for Cu(110) (145.2 kJ mol−1 and 1.22×1016 s−1) and Cu(100) (130–155.0 kJ mol−1). The promotional effect of H2 upon the decomposition of formate was observed on the Cu/SiO2 catalyst as observed for Cu(111) but not for Cu(110). The decomposition rate of formate on Cu/SiO2 was promoted by a factor of 6 at an H2 pressure of 457 Torr. These kinetic results clearly indicate that the surface of Cu particles supported on SiO2 comprises Cu(111) planes. Equilibrium formate coverage on Cu/SiO2 during the hydrogenation of CO2 was well reproduced by the calculated coverage based on the kinetics of the formate synthesis and the formate decomposition. However, the equilibrium formate coverage on Cu/SiO2 was greater than that measured for Cu(111) under the same reaction conditions because the hydrogen-promoting decomposition rate on Cu/SiO2 is less than that on Cu(111). This is explained by a decrease in hydrogen coverage on Cu surfaces of the Cu/SiO2 catalyst caused by the spillover of hydrogen atoms from Cu onto the SiO2 surface. Thus, the effect of SiO2 was observed in the hydrogen-promoting decomposition kinetics on Cu.