On the promotional effect of Pd on the propene-assisted decomposition of NO on chlorinated Ce0.68Zr0.32O2

The selective catalytic reduction (SCR) of NOx assisted by propene is investigated on Pd/Ce0.68Zr0.32O2 catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO2 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 C3H6. These catalysts display very stable deNOx activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N2 selectivity obtained on Pd/CZ suggests that the lean deNOx mechanism occurring on these catalysts is different from that occurring on Pd0/SiO2. A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO2 and NO decomposition to N2, whereas PdOx activates C3H6 via ad-NO2 species, intermediately producing R-NOx compounds that further decompose to NO and CxHyOz. The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C3H6-assisted NO decomposition mechanism stresses the key role of NO2, R-NOx and CxHyOz as intermediates of the SCR of NOx by hydrocarbons.