Mechanistic studies of NOx reduction reactions under oxidative atmosphere on alumina supported 1wt%Pt and 1wt%Pt-0.5wt%Zn catalysts (Part I)

Our study is focussed on the (NO)-N-15 reduction with alkane (propane C3H8) and alkene (propene C3H6) in the presence of a large excess of O-18(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 degrees C, (CO2)-O-18, (COO)-O-18-O-16 as well as (NO)-N-15-O-18 are formed which shows that O-18(2) is dissociated on these catalysts, that nitrogen-oxygen bond is broken and oxygen exchange reactions take place. The participation of O-16-adsorbed from the support cannot be excluded. About the possible mechanisms involved we noticed that on platinum catalyst, propene and (NO)-N-15 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 (NO)-N-15 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. (C) 1999 Elsevier Science B.V. All rights reserved.