The Role of Surface Oxides in NOx Storage Reduction Catalysts

The need for greater fuel economy in automotive vehicles has driven a surge in the development of diesel and lean-burn gasoline engines. Coupled with the increase in fuel efficiency, an increase in the air to fuel ratio also results in heightened production of NOx. [1] Several different strategies have been proposed for the handling of the higher NOx levels in lean-burn automotive emissions. One such strategy is the NOx storage reduction (NSR) system whereby excess NO is further oxidized to NO2 over the exhaust catalyst, which is typically platinum, and then NO2 is stored on a carrier material, such as BaO. [2] The catalyst system operates in this fashion for some minutes until the storage material has been fully converted. The system then cycles to a rich mode, during which a reductant is injected into the exhaust stream and reacts with the stored NOx, possibly at the interface between the carrier material and the metal particle, to produce the traditional exhaust products of N2, H2O, and CO2, all in a matter of seconds. Therefore, a requirement of a successful NSR catalyst is that the catalyst must be active for both NO oxidation and NOx reduction. In recent years, a debate has emerged as to the role of metal oxides in oxidation catalysis. [3–6] Although there have been some indications that oxide surfaces may be more active in simple oxidation reactions such as CO oxidation, [7] platinum catalysts have been preferred as the metal component in the NOx storage reduction system for their combination of high activity and ability to resist deactivation through oxidation. Ribeiro et al. have studied Pt NO-oxidation catalysts for NSR applications in considerable detail. They determined that large Pt particles are more active and that single crystal Pt(111) and Pt(100) outperform disperse Pt particles. [8, 9] One possible explanation for the higher activity of the single crystal surfaces is that oxygen is bound less strongly and that the catalysts do not deactivate due to oxide formation. Reports of platinum deactivation as a NO oxidation catalyst, however, concern catalysts that have been exposed to oxygen (and NO) for long periods. [9] As NSR systems operate under a cyclic operation