Methanol partial oxidation and dehydration over silver catalyst

Abstract

This thesis describes an experimental and modelling study of the kinetics of silver-catalysed formaldehyde production via methanol partial oxidation and dehydrogenation. In methanol oxidation, the reaction is controlled by the gas phase diffusion of reactants towards the catalyst surface. The methanol-to-oxygen ratio at the catalyst wall is 100-200 times higher than that in the feed, which means that the reaction operates with large excess of methanol and therefore is solely controlled by the availability of O2 at the surface. A constant stoichiometry = 2.25 is found. In methanol dehydrogenation, the strongly-bound surface oxygen (Oγ) catalyses the formation of CH2O and H2 without any by-product. The decline in methanol conversion over time is attributed to the gradual consumption of Oγ due to the thermal decomposition. During the surface pre-oxidation, the surface oxygen penetrates into the silver bulk and diffuses back to the surface after O2(g) is withdrawn. The H2 production during methanol oxidation is solely via the dehydrogenation channel occurring on the whole catalyst surface independently but concurrently with the oxidation, which only happens on the surface that is in contact with O2(g). Without pre-treatment, the “oxidation surface” is the only active surface for both oxidation and dehydrogenation. A transient penetration layer of active oxygen species is formed and catalyses the methanol dehydrogenation. The near constant stoichiometry is a fundamental constant describing the ratio of the dehydrogenation channel over the oxidation channel on the same surface area. Surface characterization (ex-situ XPS and SEM analysis) were performed over silver foil showing the dynamic picture of oxygen evolution and further supporting the proposed reaction mechanisms in this work.