Further evidence of multiple NOx sorption sites on NOx storage/reduction catalysts

NOx storage/reduction (NSR) catalysts are a potential solution for meeting the upcoming, diesel engine exhaust emissions regulations. A single-site adsorption kinetic mechanism for NOx storage on NSRs is commonly accepted in the literature; however, there is growing evidence that more than one type of active site or reaction pathway is involved. Bench reactor data described in this work using a model Pt/Ba/Al2O3 NSR catalyst provide additional evidence in support of a more complex kinetic model of NOx storage. This issue has direct practical significance for optimization of the NSR catalyst operation, and for controlled design of more efficient NSR catalyst formulations. First, the impacts of CO2 and H2O on NOx adsorption were investigated, since these species are believed to be competing for the same adsorption sites as NOx. These two components were found to strongly influence the NOx adsorption process, although in very different manners and to different extents depending on the operating conditions. The resulting phenomenological picture is complex and cannot be described using a single type of adsorption site. Additional, NOx speciation experiments showed that the commonly accepted NO2 disproportionation mechanism clearly dominates at the later stages of the adsorption process, such that a satisfactory N-balance can be obtained using this mechanism alone. However, at the early stages of adsorption the stoichiometric relationships for this mechanism are not observed. Experimental evidence strongly suggests that this is due to presence of two distinct types of storage sites, most likely based on the proximity of Ba and Pt components.