Mechanistic studies of NO x reduction reactions under oxidative atmosphere on alumina supported 1 wt% Pt and 1 wt% Pt–0.5 wt% Zn catalysts (Part I)

Abstract

Our study is focussed on the 15 NO reduction with alkane (propane C 3H 8) and alkene (propene C 3H 6) in the presence of a large excess of 18 O 2 . Alumina supported platinum catalysts are used as well as platinum–zinc bimetallic catalysts. The catalytic experiments were performed in a recirculation set up coupled to a magnetic mass spectrometer. Since 150°C, C 18O 2 , C 18O 16O as well as 15 N 18 O are formed which shows that 18 O 2 is dissociated on these catalysts, that nitrogen–oxygen bond is broken and oxygen exchange reactions take place. The participation of 16 O-adsorbed from the support cannot be excluded. About the possible mechanisms involved we noticed that on platinum catalyst, propene and 15 NO disappearance reactions have the same activation energy ( E a) values at low reaction temperatures and propene and 18-oxygen have the same activation energy values at high reaction temperatures. On the other hand, on Pt–Zn catalyst two groups of similar activation energy values are obtained for propene and 18-oxygen consumption reactions at low and high reaction temperatures, respectively. These observations lead us to propose that, on platinum catalyst at a low temperature, an additive compound with propene and 15 NO may be involved, and on Pt–Zn catalyst a partially oxidised hydrocarbon may participate in the catalytic reaction as well as on platinum catalyst at a high reaction temperature. Only one type of sites seems to be involved on platinum and platinum–zinc catalysts, but at high temperature, on the latter catalyst, at least two types of sites are involved with different activity. This deviation may explain why Pt–Zn catalysts have a higher activity than Pt catalysts.