Promoting effect of H₂O over macroporous Ce-Zr catalysts in soot oxidation

Gasoline particulate filters (GPFs) contain small amounts of O₂ and excess H₂O; however, the effect of H₂O on soot oxidation has not been thoroughly explored to date. Thus, it is necessary to understand the effect of H₂O on soot oxidation to improve the catalytic performance in GPFs. This study investigates the role of H₂O in soot oxidation on macroporous Ce-Zr mixed oxide catalysts (M-CeZr). The results revealed an improvement in the catalytic activity of soot oxidation in the presence of H₂O over that afforded under oxygen-only conditions. Since the mechanism of soot oxidation under dry conditions involves the conversion of gaseous oxygen to active oxygen (O_x⁻) species on the oxygen vacancies of the catalyst, O_x⁻ and oxygen vacancy are critical factors that affect the catalytic performance in the absence of H₂O. Notably, when H₂O was introduced into the reaction, it was predominantly used as an oxidant rather than gaseous oxygen. Further, the dissociation of H₂O into active oxygen over the catalyst surface was not related to the number of oxygen vacancies. Therefore, even when the catalyst comprised few oxygen vacancies, its activity improved under wet conditions. In addition, although the catalysts were damaged by high temperatures, the catalytic performance was maintained in the presence of H₂O, unless the morphology of the catalysts collapsed.