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The Selective Catalytic Reduction (SCR) of NO x assisted by propene is investigated on Pd/Ce 0.68 Zr 0.32 O catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO 2 reference catalyst. Physico-chemical characterisation of the studied catalysts along with their catalytic properties indicate that Pd is not fully reduced to metallic Pd for the Pd/CZ catalysts. This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C 3 H 6. These catalysts display very stable deNO x activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N 2 selectivity obtained on Pd/CZ suggests that the lean deNO x mechanism occurring on these catalysts is different from that occurring on Pd 0 /SiO 2. A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO 2 and NO decomposition to N 2 , whereas PdO x activates C 3 H 6 via ad-NO 2 species, intermediately producing R-NO x compounds that further decompose to NO and C x H y O z. The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C 3 H 6-assisted NO decomposition mechanism stresses the key role of NO 2 , R-NO x and C x H y O z as intermediates of the SCR of NO x by hydrocarbons.

The identification of a Rh-oxidised species of a Rh(0.29)/Ce0.68Zr0.32O2 catalyst that exhibits peculiar CO-O2 kinetics [I. Manuel, J. Chaubet, C. Thomas, H. Colas, N. Matthess and G. Djéga-Mariadassou, J. Catal. 2004, 224, 269] is addressed. For this purpose, various catalysts are studied by XANES, CO- and N2-FTIR, and benzene hydrogenation. The results obtained, particularly from N2-FTIR, which is, to our knowledge, reported for the first time on this kind of catalyst, suggest that Rh is mainly stabilised as electron-deficient clusters (Rhndelta+) on a Rh(0.29)/Ce0.68Zr0.32O2 catalyst after reduction at 500 degrees C under H2. The existence of these species, which may be caused by either electron perturbation induced on the metal by the reduced support or electron withdrawal from the metal clusters by an inductive effect of the neighbouring Cl anions, is also revealed through CO-FTIR experiments. In the presence of CO, however, evidence of RhI(CO)2 species is also provided.

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