Effect of precious metals and NOx storage materials on hydrogen reduction of stored NOx on millisecond time scale Original Research Article

The effect of precious metals (PMs) and NOx storage materials (NSMs) on the reduction of stored NOx over PM/NSM/Al2O3 (PM = Pt, Rh, Pd; NSM = Ba, K, Na) catalysts was studied via transient reaction analysis using pulsed gases with submillisecond resolution. The time profiles of the product gases were analyzed with a high time resolution of 0.25 ms for the first 50 ms after supplying a hydrogen gas pulse.These results were explained with a model that correlates with the spillovers of released NOx species from the NSM and the activated hydrogen on the PM. The catalyst Pt/Na/Al2O3 had the highest reduction rate of stored NOx among the nine catalysts examined. This performance is due to both the lowest decomposition temperature of Na and the high reactivity of hydrogen on Pt. On the other hand, the Rh-containing catalysts exhibited a lower NH3 production capability than the Pt- and Pd-containing catalysts, and the Rh/K/Al2O3 catalyst had the least NH3 production capability and the lowest reduction rate of stored NOx. The lower NH3 production capability of Rh is caused both by the low hydrogen reactivity on Rh and the lowest reduction rate of stored NOx for Rh/K/Al2O3, which has the lowest surface area among the Rh-containing catalysts.