The influence of NOx on soot oxidation rate: molten salt versus platinum

A systematic study was carried out to assess the influence of simulated diesel exhaust on the activity of molten salt, Cs2SO4·V2O5, supported on ceramic foam and Pt/γ-alumina catalysts in the oxidation of diesel soot. Gas compositions containing O2, NOx, CO, C3H6, and SO2 were used. The activity of molten salt catalyst, an active catalyst for the oxidation of soot with O2, is slightly affected by the gas component due to NO2 already present in NOx. In contrast, the presence of NOx, significantly increases the soot oxidation rate with platinum catalyst. These changes were due to the catalytic oxidation of NO to NO2 with platinum, followed by soot oxidation with NO2. Three configurations are compared, viz. a fixed bed containing a physical mixture of Pt catalyst and soot, Pt catalyst upstream of a fixed bed containing soot, and Pt catalyst upstream of soot loaded on ceramic foam supported molten salt. The reaction cycle of oxidation of NO, followed by soot oxidation with the NO2 produced, was observed only in a physical mixture of platinum catalyst and soot.