A kinetic investigation of the reduction of NO by CH4 on silica and alumina-supported Pt catalysts

The activity and selectivity of alumina and silica-supported Pt catalysts have been investigated for the simultaneous destruction of NO and CH4. The results show that these catalysts can be effective for the reduction of NO to N-2 using CH4 under a variety of experimental conditions. Pt/silica is more active than Pt/alumina, especially at lower temperatures. This difference may arise because of the influence of support effects on the oxidation state of Pt, or possibly because of particle size or morphological effects. A kinetic investigation has shown that the order of reaction for the NO reduction reaction is very high with respect to NO (ca. 2.5-3.3), reflecting a strong sensitivity to the partial pressure of NO. The order of reaction with respect to CH4 is smaller (ca. 0.5-1.4), being higher for the alumina-supported Pt catalyst. In both cases the orders vary with the reaction temperature. In the NO/O-2/CH4 reaction, the rate of NO conversion initially increases as O-2 is introduced, but the rate then goes through a maximum as the O-2 concentration is further raised. It is suggested that for the NO reduction reaction there is an optimum surface coverage with adsorbed hydrocarbonaceous and oxygen species. It is observed that these catalysts produce a substantial amount of N2O at low temperatures and NH3 at high temperatures. These results are interpreted on the basis of variations in the surface coverage with molecular N-oads, H-ads, and N-ads as a function of temperature and gas composition. It is concluded that under conditions close to the stoichiometric CH4:air ratio, as might be found with a natural gas engine, Pt may be an effective catalyst for the removal of NO and CH4 although it may be difficult to optimise the simultaneous conversion of both components under the same experimental conditions. (C) 1998 Elsevier Science B.V.