In situ X-ray absorption study during methane combustion over Pd/ZrO2catalysts

Abstract

In situ X-ray absorption fine structure (EXAFS) spectroscopy combined with on-line gas analysis was used to get insight into the oxidation state of the constituents and of the structure of a Pd/ZrO₂ catalyst during activation and catalytic combustion of methane. The sample was prepared from an amorphous Pd₃₃Zr₆₇-alloy by oxidation at 400 °C. In situ XANES/EXAFS together with thermal analysis showed that the Pd and Zr constituents were oxidized at the same time. Catalytic combustion activity was higher if the catalyst was previously reduced by hydrogen than if the fully oxidized sample was used. According to EXAFS complete reduction of PdO/ZrO₂ to Pd/ZrO₂ was achieved by hydrogen exposure at room temperature. In situ EXAFS studies during temperature programmed reaction in methane/oxygen and under reaction conditions uncovered that palladium is mainly in an oxidized state. However, during methane combustion in 1% CH₄/4% O₂/He at 500–550 °C the active catalyst contains some metallic Pd, as evidenced by in situ EXAFS, which seems to be beneficial for the activity. The catalyst can be fully reduced by methane and re-oxidized by oxygen in separate steps at this temperature.