Phase transformation and oxygen vacancies in Pd/ZrO2 for complete methane oxidation under lean conditions

A series of ZrO2 materials were prepared by calcining ZrO2·xH2O at temperatures from 600 to 900 °C. The oxide materials were used to prepare Pd/ZrO2 catalysts coated on the monolith. Under lean conditions, the monolithic catalysts were tested for complete oxidation of CH4 in the presence and absence of water. In process of the preparation of supports, the ratio of monoclinic ZrO2 (m-ZrO2) to tetragonal ZrO2 (t-ZrO2) increased with calcination temperature. Strong interaction between palladium and ZrO2 further induced the transformation of t-ZrO2 to m-ZrO2 when supporting Pd on ZrO2. Phase transformation in samples including ZrO2 supports and Pd/ZrO2 catalysts was accompanied by an increase in the concentration of surface oxygen vacancies, improving the mobility of oxygen species over the samples. It was found that the Pd catalyst using ZrO2 calcined at 900 °C as the support had the highest oxygen vacancy concentration among the four catalysts as synthesized. Under dry reaction conditions, this catalyst showed considerably higher activity in methane oxidation promoted by oxygen vacancies. However, under wet reaction conditions (10 vol% H2O), water poisoned the surface-active sites for all the catalysts. The promoting effect of increasing oxygen vacancies on the activity of CH4 oxidation was thus weakened significantly by the water poisoning, resulting in slight differences in activity among the catalysts.