Mechanistic Studies on the Oxidative Dehydrogenation of Methanol over Polycrystalline Silver Using the Temporal-Analysis-of-Products Approach

The temporal-analysis-of-products approach was used to study the mechanism of the oxidative dehydrogenation of methanol over an unsupported polycrystalline silver catalyst. Pulse experiments revealed a distinct influence of the state of the silver surface on the adsorption of oxygen and on reactivity toward methanol. The surface-embedded oxygen species Oγ, which is stable at high temperatures, was found to provide a highly selective reaction pathway for the oxidation of methanol to formaldehyde above 550 K. A comparison of the pulse responses of formaldehyde, water, and hydrogen to methanol pulses showed that water is a primary gas-phase product and that only small amounts of hydrogen are formed. Furthermore, it was found that the Oγ species is not active in the oxidation of hydrogen or carbon monoxide. In contrast, the much more active adsorbed oxygen species Oα was observed to be involved in the nonselective deep oxidation of methanol at lower temperatures, yielding carbon dioxide at high coverages. However, at low coverages Oα also causes the selective formation of formaldehyde.