Operando resonance Raman spectroscopic characterisation of the oxidation state of palladium in Pd/γ-Al2O3 catalysts during the combustion of methane

Resonance Raman spectroscopy is used in order to investigate the behaviour of Pd/gamma-alumina (2 wt.%) in the catalytic combustion of methane. Simultaneous spectroscopic and catalytic measurements allow an operando spectroscopic investigation of the catalyst under operational conditions. Temperature cycles were performed by alternately heating and cooling the system under the reaction conditions after various in situ gaseous pretreatments of the catalyst. Our results show that neither the Raman features nor the conversion of methane are influenced by initial thermal activation treatment under reducing or oxidizing atmospheres. Pure metallic palladium was found to be inactive for the catalytic combustion of methane. Under the reaction conditions, the Pd/gamma-Al2O3 catalyst is always in an oxide PdO form with its surface in an intermediate state between surface lacunary PdO and crystalline PdO species.