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

For elucidating the mechanism of oxygen adsorption and its effect on select ivity 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 precur sor 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 associat ion 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 a tomic adsorbed oxygen species were simulated for different partial oxygen p ressures (0.5-15 kPa) using the kinetic parameters from transient experimen ts. These results may, however, be affected by extrapolating the pressure f rom 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 adsor bed molecular oxygen, which appears to be a plausible interpretation confir ming previous findings of the dependence of C-2 selectivity on oxygen parti al pressure.