Oxygen adsorption on silver catalysts during the course of partial oxidation of ethylene

To explore oxygen adsorption kinetics on silver catalyst surfaces during the course of ethylene oxidation, relations between the amount of oxygen consumed by reaction with ethylene (q  ) and oxygen partial pressure (pO2pO2) at a given ethylene pressure (pC2H4pC2H4), i.e., kinetic data, were measured using a pulse reaction technique on K2SO4-promoted Ag/α-Al2O3 catalyst and a flow reaction method on Cs and Re-copromoted Ag/(α-Al2O3-crystal) catalyst at 453–529 K. The kinetic data were analyzed by the use of three kinetic models derived on the basis of a redox model assuming that, under steady states, no oxygen desorption occurred and the rate of oxygen adsorption on silver surface was equal to the rate of surface reactions consisting of adsorbed oxygens and gaseous ethylene. One was the model that two-step consecutively dissociative oxygen adsorptions forming oxygen adatom and then admolecule were rate-determining (Model 1), and others were molecular oxygen adsorption rate-determining model (Model 2) and surface reaction rate-determining model when one-step dissociative oxygen adsorption forming oxygen adatom only lay in a complete equilibrium state (Model 3). The Model 1 showed the most excellent suitability to the kinetic data and reproduced accurately the distinctive features for experimental q–pO2q–pO2 curves. The coverage of oxygen admolecule estimated from the Model 1 was 0.48–0.96 and 0.12–0.34 on the K2SO4-promoted Ag/α-Al2O3 catalyst at pC2H4 ≦0.03pC2H4 ≦0.03 atom and on the Cs and Re-copromoted Ag/(α-Al2O3-crystal) catalyst at pC2H4≧0.30pC2H4≧0.30 atom, respectively. They were about thousand times bigger than those for oxygen adatom. These results indicated that the first dissociative adsorption on vacant active center was much slower than the second dissociative oxygen adsorption on adatom; the adatom seemed likely to act as active center for the second oxygen adsorption. Selectivity to ethylene oxide elevated almost linearly with increase in the coverage of oxygen admolecule. Activation energies and heats of adsorption obtained for dissociative oxygen adsorption were quite reasonable values compared to literature values and remarkedly reduced with increasing pC2H4pC2H4.