Selective catalytic reduction of NO under lean conditions by methane and propane over indium/cerium-promoted zeolites

The potential of zeolites MFI and MOR promoted by In and Ce as catalysts for the SCR of NOx by methane and propane was studied, particularly considering their stability in the presence of water vapour. A preparation route for a zeolite-based Ce/In composite catalyst was developed and the influence of several reaction conditions was studied to optimise the catalyst performance. TPR and FTIRS were used to check the preparation steps and to investigate the catalyst structure. These data were used together with results from characterisation by X-ray-based techniques to describe the generation and catalytic function of different indium species ((InO)(+), In2O3 Cluster) and their co-operation with ceria in the composite catalyst. The primary role of the CeOx promoter is to catalyse the oxidation of NO to NO2. Probably, NO2 reacts with methane on Lewis-acidic and redoxactive (InO)(+) species, but the further reaction of the intermediate formed needs the presence of residual Bronsted-acid sites. SCR of NOx by methane or propane is still partially inhibited by adsorption competition between water and the hydrocarbons on the (InO)(+) species. Higher SCR activity could be achieved by increased hydrocarbon partial pressure in the feed, shifting presumably the competitive adsorption to enhanced adsorption of methane and propane, respectively. Furthermore, combining of methane and propane as reductants opens an opportunity to broaden the 'temperature window' for the SCR of NOx. Different influence of hydrothermal ageing was obtained with methane and propane, respectively, as reducing agent. (C) 2002 Elsevier Science B.V. All rights reserved.