Effects of temperature and rich-phase composition on the performance of a commercial NOx-Storage-Reduction material

Abstract

A thin self-supported wafer of a commercial NOx-Storage-Reduction (NSR) material was investigated by operando transmission FT-IR to assess the effects of the operating temperature and the composition of the rich period (in particular with respect to H2, CH4, CO, CO2 and C3H6 reactants) on the deNOx activity. The DeNOx activity was high at 300°C under the simulated exhaust gas composition, but dropped sharply at 200°C and 100°C, primarily due to a loss of the NOx storage ability. The origin of the trapping loss depended on the temperature. Pt-bound CO(ads) were observed at 200°C, pointing to a significant poisoning of metallic Pt sites by strongly bound carbon monoxide, hindering NOx decomposition and reduction. Surprisingly, no Pt-CO was observed at 100°C, revealing the absence of accessible metallic Pt at this temperature, most likely due to the presence of a fully oxidized (or nitrated) Pt surface. In contrast, an important NSR activity could be obtained at 200°C and even at 100°C when H2 was used as the only reducing agent in a C-free feed. Propene had a similar detrimental effect as that of CO, while methane only induced a minor loss of trapping efficiency. The presence of a large concentration of CO2 also induced a trapping loss, albeit significantly less marked than those due to propene and CO. The combination of all the C-containing compounds (i.e. CO, CO2, C3H6, CH4) led to an almost complete inhibition of the NSR activity at temperatures equal to or lower than 200°C. The operando analysis also revealed that the nature of the NOx species trapped varied with the temperature; nitrates were the dominant species at 300°C, while nitrites were more prominent at 200°C and 100°C.