When γ-Al2O3is applied as a support for noble metals in the catalytic oxidation of dichloromethane, the reaction primarily takes place on the alumina. In our flow study, dichloromethane was completely converted at 600 K on γ-Al2O3, CO being a main product, especially at higher temperatures. Methyl chloride was observed in substantial amounts as an intermediate product. The formation of CH3Cl, a reduction product formed under oxidative conditions, has been described before, but no satisfactory explanation has been given. To this end, reaction between alumina and CH2Cl2has been studied by FTIR spectroscopy as well. The spectrum of the adsorbed species at 523 K was clearly different from that at 298 K. Apparently, dichloromethane reacted with surface hydroxyl groups, which disappeared. Experiments with CD2Cl2enabled the identification of the surface species. The bands found could be attributed to surface methoxy groups and to adsorbed formate. With these species the products found in the flow experiments could be explained: the formates are converted into CO and the methoxy groups can react with HCl to form CH3Cl. The first step, reaction of the adsorbed dichloromethane with a surface hydroxyl group, yields a chloromethoxy species. This leads to a chemisorbed formaldehyde analogue, which appears to disproportionate due to some hydride shift, to form the observed methoxy and formate groups. The Cl from the CH2Cl2is released as HCl and/or reacts with the alumina to form aluminum chlorides. The observed rate enhancing effect of added water in the gas stream can be explained by reaction with chlorinated alumina and restoration of surface hydroxyl groups, the active sites in the reaction with dichloromethane.
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