Mechanistic insights in the activation of oxygen on oxide catalysts for the oxidative dehydrogenation of ethane from pulse experiments and contact potential difference measurements

The oxidative dehydrogenation of ethane (ODE) was studied over Na2O/CaO, Sm 2O3/CaO and Sm2O3 catalysts at 923 K using different oxygen partial pressur es. The type of interaction of oxygen with these catalysts was derived from measurements of contact potential difference (CPD) between a gold electrod e and the catalysts and pulse experiments in the Temporal Analysis of Produ ct (TAP) reactor. Modelling of TAP oxygen responses in the temperature rang e from 673 to 873 K showed that a reversible dissociative adsorption via a molecular precursor provides a good description of the measured transient r esponses for all catalysts. Supplementary CPD results confirmed the transfo rmation of adsorbed molecular oxygen species to atomic ones. Rate constants and activation energies of the elementary reaction steps of oxygen adsorpt ion, desorption, dissociation and association were estimated from the TAP o xygen responses. The dependence of ethylene selectivity both on doping and oxygen partial pressure was explained by the ratio of Theta(O)/Theta(O2). T his ratio is strongly affected by the constants of adsorption of oxygen and dissociation of molecular adsorbed oxygen. (C) 2000 Elsevier Science B.V. All rights reserved.