In situ infrared study of NO reduction over Pd/Al_2O_3 and Ag-Pd/Al_2O_3 catalysts under H_2-rich and lean-burn conditions

摘要

Catalytic reduction of NO under H_2-rich and lean-burn conditions over Pd/Al_2O_3 and Ag-Pd/Al_2O_3 has been studied by temperature programmed reaction (TPR) coupled with in situ infrared (IR) and mass spectrometry (MS) techniques. Under H_2-rich conditions, Pd/Al_2O_3 produces, Pd-NCO, Pd~+-NO, Pd~0-NO, and adsorbed nitrate at temperatures below 398 K. N_2 and adsorbed NH_3 are produced at temperatures above 400 K. Addition of Ag to Pd/Al_2O_3 suppresses the Pd~+-NO formation and lowers the formation temperature of NH_3 species to 373 K. Under lean-burn conditions, Pd/Al_2O_3 produces Pd-NCO, Pd~+-NO, Pd~0-NO, and adsorbed nitrate at temperatures below 373 K, while the addition of Ag suppresses the Pd-NCO and Pd~+-NO formation, and enhances the Pd~0-NO formation. Infrared results reveal that oxygen under lean-burn conditions does not oxidize Pd~0 sites to Pd~+ on the Ag-Pd/Al_2O_3 catalyst. Addition of Ag enhances NO_2 formation and shifts the N_2 formation to lower temperatures, suggesting that NO_2 is effectively reduced to N_2.