Kinetic Study of Heterogeneous Oxygen-Exchange Reactions and Bulk Self-Diffusion of Oxygen

Abstract

Apparent activation energies are determined for oxygen-isotope exchange reactions between O2, CO, or NO and preoxidized or prereduced CaO surfaces. Oxygen exchange between N16O or C16O and isotope labeled Ca18O surfaces produced an apparent activation energy of 2 kcal/mol. Similar values are obtained for single- isotope-exchange between 18O18O and prereduced Ca16O surfaces. Apparent activation energies of 15−18 kcal/mol are found for single and double exchange between 18O18O and preoxidized Ca16O surfaces, as well as for double exchange between 18O18O and prereduced Ca16O surfaces. The low apparent activation energies are believed to result from adsorbed intermediates, whereas the high values may involve the formation of a singlet O2 transient. It is shown that eventually the self-diffusion of oxygen ions in the bulk becomes the rate-determining step in the isotope-exchange reaction. Apparent activation energies are determined, and the values are found to depend on surface treatment, (i.e., 44 kcal/mol under reducing and 78 kcal/mol under oxidizing conditions). The involvement of oxygen vacancies under reducing conditions is discussed.

Oxygen Activation in Oxidative Coupling of Methane on Calcium Oxide

“…Now that impurities were removed, the onset of the oxygen-exchange activity decreases to a temperature of 50 °C, suggesting that even very little amounts of surface hydroxide or carbonate are able to quench the oxygen-exchange activity of calcium oxide completely. The apparent activation energy of 80 kJ/mol determined in this experiment is lower than the calculated dissociation barrier for oxygen on pristine flat CaO surfaces with 110 kJ/mol but comparable to what was found for the double oxygen exchange on a preoxidized CaO surface with 74 kJ/mol before …”

“…A gas-phase contribution due to the simultaneous presence of both 32 O 2 and 36 O 2 , as well as an activity of the inert material, was excluded by a blank experiment with silicon carbide (Figure ). Bulk contributions in this experiment cannot be analyzed, but they can be neglected due to the omnipresence of 16 O in the gas phase immediately regenerating exchanged oxygen atoms on the surface and the bulk diffusion being very slow compared to the surface reaction and therefore only weakly affecting the tailing of the outlet pulses …”

“…Bulk contributions in this experiment cannot be analyzed, but they can be neglected due to the omnipresence of 16 O in the gas phase immediately regenerating exchanged oxygen atoms on the surface and the bulk diffusion being very slow compared to the surface reaction and therefore only weakly affecting the tailing of the outlet pulses. 23 An advantage of this method lies in its applicability. Since steady-state isotopic transient kinetic analysis (SSITKA) experiments require a dead-zone-free reactor with optimized flow conditions to analyze the transients, the pulsed exchange can be applied in any standard catalysis test reactor.…”

“…The isotope exchange with the calcium oxide lattice has been found to be sensitive toward the pretreatment, such as reduction or oxidation, finding a prereduced surface more easy to exchange surface oxygen compared to the oxidized surface. 23 In the present experiments, CaO was activated in the reactor at 900 °C in synthetic air prior to starting oxygen exchange. In Figure 1 (black symbols) the 18 O distribution after oxygen scrambling on the activated calcium oxide at different temperatures in flowing oxygen is presented.…”

Liquid phase oxidation of cyclohexane using bimetallic Au–Pd/MgO catalysts

“…This diminishes the possibility of oxidation occurring via an  2 oxygen intermediate (i.e. with both oxygen atoms of molecular oxygen adsorbed over the catalyst surface), as it would involve a switch in the oxidation state of the metal [56]. Furthermore this process would be slower than for a diffusion controlled route where O 2 reacts directly with the C 6 H 11 · radical [18].…”