Transient kinetics and mechanism of oxygen adsorption over oxide catalysts from the TAP-reactor system

For elucidating the mechanism of oxygen adsorption and its effect on selectivity in the oxidative coupling of methane (OCM) transient experiments were carried out in the TAP-2 reactor by pulsing oxygen over Na2O/CaO catalysts at temperatures between 623 and 873 K. The response signals were fitted to three different models for oxygen adsorption. Model discrimination showed that only a reversible and dissociative adsorption via the molecular precursor provides a good description of the transient oxygen responses over all catalysts studied. Rate constants and activation energies of the elementary reaction steps of oxygen adsorption, desorption, dissociation and association were estimated. Doping CaO with sodium oxide influenced the ratio of k(ads)/k(dis) determining the coverage of the catalyst surface with molecular and atomic oxygen. The steady-state surface coverages with molecular and atomic adsorbed oxygen species were simulated for different partial oxygen pressures (0.5-15 kPa) using the kinetic parameters from transient experiments. These results may, however, be affected by extrapolating the pressure from 10(-4) Pa to 15 kPa. It was derived that an increase of C-2 selectivity in OCM on Na2O/CaO can be ascribed to a decrease in the coverages of adsorbed molecular oxygen, which appears to be a plausible interpretation confirming previous findings of the dependence of C-2 selectivity on oxygen partial pressure.