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Abstract: In this study, the parameters governing the activity of Pd/ceria-zirconia catalysts in the selective catalytic reduction (SCR) of NO x assisted by methane are investigated using a combination of temperature-programmed spectroscopic and thermogravimetric techniques and transient SCR conditions. By DRIFTS of adsorbed CO, it is established that Pd species on Ce0.2Zr0.8O2 are mainly present in cationic form but exhibit high reducibility. As found by temperature-programmed surface reaction (TPSR) in CH4 +NO2 atmosphere, the CH4-SCR reaction is initiated at 280°C on Pd/Ce0.2Zr0.8O2 and yields almost 100% N2 above 500°C. DRIFTS-MS and TGA experiments performed under transient SCR conditions show that DeNO x activity is due to a surface reaction between some methane oxidation products on reduced Pd sites with ad-N x O y species presumably located on the support. The detrimental effect of O2 on DeNO x is explained by the promotion of the total combustion of methane assisted by the ceria-zirconia component at the expense of the SCR reaction above 320°C. [Copyright &y& Elsevier]