HC-SCR system combining Ag/Al2O3 and Pd/Al2O3 catalysts with resistance to hydrothermal aging for simultaneous removal of NO, HC, and CO

Abstract

A combined Ag/Al2O3 and Pd/Al2O3 catalyst was investigated for the selective catalytic reduction (SCR) of NOx by H2-assisted C3H6 in the presence of excessive O2. The coexistent H2 significantly lowered the active temperature window of the de-NOx reaction because of the enhanced partial oxidation of C3H6 and NO oxidation to NO2. A low Ag loading of 1 wt% exhibited better de-NOx performance than that attained using 2 wt% Ag. Although Ag/Al2O3 exhibited a high resistance to hydrothermal aging, a high reaction temperature (>500 °C) was required to oxidize CO and C3H6 completely. In contrast to Ag/Al2O3, Pd/Al2O3 suppressed C3H6 and CO slip at relatively low temperatures, and a complete oxidation of C3H6 and CO was achieved at 195–275 °C and 240–275 °C, respectively, under varying SCR conditions and Pd contents (0.5–1.5 wt%); however, 30% de-NOx performance in a narrow temperature window of 210–240 °C was attained. The best configuration was found to be a sequential arrangement of 1Ag/Al2O3 followed by 1Pd/Al2O3, which enabled the reduction of NOx, C3H6, CO, and H2 over wide reaction temperatures, unlike the stand-alone configuration. In this configuration, the reacted NOx was reduced to N2 with negligible formation of by-products, even after hydrothermal aging.