Method for selective removal of supported platinum particles from external zeolite surfaces: characterisation of and application to a catalyst for the selective reduction of nitrogen oxide by hydrocarbons

A method for selectively removing Pt particles supported on H-ZSM-5 (Pt/H-ZSM-5) from the external surfaces of zeolite crystals was investigated, using a reagent system composed of halogen-tetraethylammonium halide-organic solvent. The influence of the nature of the halogen and the treatment time on the quantity of Pt removed was studied. After treatment of the Pt/H-ZSM-5, the characterisation by hydrogen chemisorption, XPS and TEM indicated that the method is suitable for preparing a zeolite material that contains exclusively Pt particles confined in the grain boundaries of zeolite crystals (enveloped Pt). As an example of its application, the catalytic activity for the reduction of nitrogen oxides (NOx) with n-dodecane in the presence of oxygen was studied. The results indicate a beneficial effect of removing Pt from the outermost surface of Pt/H-ZSM-5, with an increase in the selectivity of NOx reduction to N-2 from 25 to 45% at the maximum of NOx conversion. This effect was mainly attributed to the hindered access of the bulky hydrocarbon molecules to the enveloped Pt, thus favouring the reaction of the hydrocarbon with NO2 formed by the oxidation of NO on the Pt sites. (C) 1999 Elsevier Science B.V. All rights reserved.