Key role of NO + CH reaction for the elimination of NO in automobile exhaust by three-way catalyst.

Pd-only three-way catalysts with improved catalytic activity for NO elimination were prepared. In order to explore the catalytic reaction rules of NO reduction under a three-way catalytic system, a series of single reactions related to NO reduction were evaluated. It was found that the reaction temperatures of NO + H or NO + CO or NO + CH reactions were below 250 °C, while that of NO + CH was up to 350 °C. Thus, the reaction NO + CH served as the key reaction in determining the purification efficiency of NO at the high-temperature stage. By in situ FTIR, we proposed that three possible steps were involved in NO + CH reaction. The first step was the oxidation of CH and NO to acetone and nitrate species by active oxygen species, respectively (CH + O* → CHO, NO + O* → NO). XPS results revealed that the amount of active oxygen species in Pd/CeO-ZrO-AlO (Pd/CZA, 73.7%) was much higher than that in Pd/CeZrO+AlO (Pd/CZ+A, 64.1%). This was in line with the higher reaction efficiency of the first step over Pd/CZA. Then the NO + CH reaction was accelerated by the first step, which consequently contributed to the higher NO elimination efficiency of Pd/CZA.