Comparative study of the chemisorptive and catalytic properties of supported Pt catalysts related to the selective catalytic reduction of NO by propylene

Abstract

The chemisorptive properties of Pt/Al2O3, Pt/CeO2 and Pt/ZrO2 catalysts and of the bare supports toward nitrogen monoxide and propylene have been studied with the use of TPD and FTIR techniques and were related to their performance for the selective catalytic reduction (SCR) of NO by C3H6 in the presence of excess oxygen. It has been found that interaction of the investigated materials with NO at 25°C results in the formation of three types of adsorbed species, namely nitrosyl, nitrite and nitrate. The relative population and thermal stability of adsorbed NOx species depend on the nature of the metal oxide used as support and on the deposition or not of platinum on its surface. Pt/ZrO2 promotes NO dissociation and nitrite/nitrate decomposition at lower temperatures, compared to Pt/Al2O3 and Pt/CeO2 catalysts. Platinum crystallites supported on Al2O3 are partially oxidized following interaction with NO, while ZrO2 and CeO2 are able to maintain dispersed platinum in its reduced state. Propylene is activated on the surface of the three catalysts by forming oxygenated species with the participation of oxygen originating from the support. These species are progressively oxidized with increasing temperature yielding CO and CO2 in the gas phase, with intermediate formation of adsorbed CO species on platinum. The process, which is promoted by platinum, is more pronounced over Pt/CeO2, followed by Pt/ZrO2 and Pt/Al2O3 catalysts. The catalytic performance of the three catalysts for the SCR of NO by propylene has been investigated with respect to the effects of the nature of the support and of platinum loading (0.1–5.0wt.%) on activity and selectivity. It has been found that the Pt/ZrO2 catalyst is much more active than Pt/CeO2 for all metal loadings investigated, with Pt/Al2O3 exhibiting an intermediate performance. It is concluded that the nature of the support influences the chemisorptive properties of the catalysts investigated and concomitantly their activity for the SCR of NO by propylene.