Study of Oxygen Adsorption on Ag/α-Al2O3 Catalysts under Ultrahigh/High Vacuum Conditions
Kinetics and Catalysis
, E-ISSN: 1608-3210
||SILVER SURFACE; PARTIAL OXIDATION; SUPPORTED SILVER; SINGLE-CRYSTAL; DESORPTION; CHEMISTRY; ETHYLENE; AG(111); AG; PARTICLES
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Adsorption of oxygen on Ag/alpha-Al2O3 catalysts with mean sizes of silver particles of 160, 560, and 1000 Angstrom is studied by thermal desorption at 300-500 K and pressures below 10(-2) Pa and in the range 500-15 000 Pa. It is found that for both initial catalysts and those activated under the conditions for ethylene epoxidation, several forms of oxygen thermal desorption are observed: molecular (T-max = 350-400 K), atomic (560-580, 700 K), and the form caused by the penetration of oxygen into subsurface layers (610-630 K). The kinetics and parameters of oxygen adsorption on small particles qualitatively and quantitatively differ from those on the species of a large size. A kinetic scheme that explains the temperature dependence of the initial sticking coefficient of oxygen is suggested for the dissociative adsorption of oxygen. The amount of molecular oxygen adsorbed can be considered as a measure of defects (roundness) on the surface of silver particles.