Sub-reactions of the Silver Catalysed Conversion of Methanol to Formaldehyde

Abstract

Using an annular structured reactor the oxidation of CO and H2 over silver was studied at temperatures employed during the industrial partial oxidation of methanol to formaldehyde. Silver tubes were systematically exposed to different atmospheres and imaged at regular intervals using scanning electron microscopy to gain insight into both reaction activity and catalyst restructuring. The reactant feed has a profound effect on restructuring, and a “dynamic steady-state morphology” is identified for each gas mixture. The exposed catalysts show clear signs of stepped surface faceting, grain growth and formation of small pinholes (< 1 μm). H2 oxidation conditions promote the formation of numerous angular surface cavities (> 3 μm). Addition of steam to the feed, however, inhibits the formation of stepped facets. Silver is active towards both CO and H2 oxidation, and co-feeding both reactants has a synergistic effect. Addition of steam leaves the H2 oxidation activity unaffected but completely inhibits the formation of CO2 via CO oxidation. The results clearly show that the effects of H2O formation are very different from those of H2O addition and that CO is not a precursor towards CO2 in the silver-catalysed methanol to formaldehyde system, in which steam is usually co-fed. The systematic approach taken here allows the effects of temperature and gas composition to be decoupled, and the results can be used in future studies to ascribe restructuring features to various reactants.