Transient isotopic kinetic study of the NO/H2/O2 (lean de-NOx) reaction on Pt/SiO2 and Pt/La-Ce-Mn-O catalysts

Steady-state isotopic transient kinetic analysis (SSITKA) coupled with temperature-programmed surface reaction (TPSR) methods and using in situ mass spectroscopy and DRIFTS have been applied for the first time to study essential mechanistic aspects of the NO/H-2/O-2 reaction at 140degreesC under strongly oxidizing conditions over 0.1 Wt % Pt/SiO2 and 0.1 wt % Pt/La-Ce-Mn-O catalysts. The nitrogen-pathway of the reaction from NO to form N-2 and N2O gas products was probed by following the (NO)-N-14/H-2/O-2 --> (NO)-N-15/H-2/O-2 isotopic switch at 1 bar total pressure. It was found that the chemical structure of active intermediate NOx species strongly depends on support chemical composition. In the case of the Pt/SiO2 catalyst, the reaction route for N-2 and N2O formation passes through the interaction of one reversibly and one irreversibly NOx species chemisorbed on the Pt surface. On the other hand, in the case of a Pt/La-Ce-Mn-O catalyst, the reaction route passes through the interaction of two different in structure irreversibly chemisorbed NOx species on the support. For the latter catalyst, the mechanism of the reaction must involve a hydrogen-spillover process from the Pt metal to the support surface. A surface coverage 0 = 1.8 (based on Pt metal surface) of active NOx intermediate species was found for the Pt/La-Ce-Mn-O catalyst. A large fraction of it (81.5%) participates in the reaction path for N-2 formation, whereas in the case Of Pt/SiO2, this fraction was found to be 68.4% (active NOx, 0 = 0.65). These important results provide an explanation for the lower N-2 reaction selectivity values observed on Pt/SiO2 compared to Pt/La-Ce-Mn-O catalyst. Inactive adsorbed NOx species (spectators) were found to accumulate on both Pt and support surfaces. It was found via the NO/H-2/O-16(2) --> NO/H-2/O-18(2) isotopic switch that the reaction path from NO to form N2O passes through the oxidation step of NO to NO2 with the participation of gaseous O-2, where the extent of it depends on support chemical composition.