Synthesis and decomposition of formate on a Cu/SiO2 catalyst: Comparison to Cu(111)

The kinetics of formate synthesis from CO2 and H-2 and from formate decompo sition on Cu/SiO2 were studied by temperature-programmed decomposition expe riments 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 H-2 at 323-353 K over the Cu/SiO2 cat alyst was in good agreement with those on Cu(111) and Cu(110), At 353 K, fo r example, the initial rate was measured to be 5.3 x 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 de composition rate of formate was first-order in formate coverage, and the ra te constant was 1.13 x 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 x 1 0(11) s(-1) respectively, in good agreement with those obtained for Cu(111) , 107.9-112.8 kJ mol(-1) and (1.87-4.02) x 10(11) s(-1), rather than those for Cu(110) (145.2 kJ mol(-1) and 1.22 x 10(16) s(-1)) and Cu(100) (130-155 .0 kJ mol(-1)). The promotional effect of H-2 upon the decomposition of for mate was observed on the Cu/SiO2 catalyst as observed for Cu(111) but not f or Cu(110). The decomposition rate of formate on Cd/SiO2 was promoted by a factor of 6 at an H-2 pressure of 457 Torr. These kinetic results clearly i ndicate 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 equil ibrium formate coverage on Cu/SiO2 was greater than that measured for Cu(11 1) under the same reaction conditions because the hydrogen-promoting decomp osition 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 caus ed by the spillover of hydrogen atoms from Cu onto the SiO2 surface. Thus, the effect of SiO2 was observed in the hydrogen-promoting decomposition kin etics on Cu. (C) 2000 Academic Press.