Evidence for the facile formation of nitrogen-containing compounds from NOx and propene species on tungstated zirconia-based catalysts: Are these compounds active or spectator species in the selective catalytic reduction of NOx by C3H6?

The propene selective catalytic reduction (C3H6-SCR) of NOx is investigated on WOx–ZrO2 and Pd/WOx–ZrO2. Temperature-programmed surface reaction (TPSR) experiments provide evidence of the decomposition of organic nitrogen-containing species formed through the interaction of C3H6 and ad-NOx, for which a C/N ratio of 3 is estimated. The TPSR of preadsorbed NOx in C3H6–O2–He also suggests the presence of two RNOx species, C3H5ONO (propenyl nitrite) and C3H5NO2 (nitropropene), decomposing in distinct temperature regions. The addition of Pd to WOx-ZrO2 promotes the decomposition of nitropropene, whereas that of propenyl nitrite is scarcely affected. It is also suggested that RNOx are not active intermediates in C3H6-SCR on Pd/WZ catalysts but are more likely spectators. The activity for C3H6-SCR is attributed to Pd0 species, formed by in situ reduction of oxidized Pd species by C3H6, via the well-established decomposition mechanism. In light of the new findings about the formation–decomposition of RNOx, results reported previously on a Pd/Ce0.68Zr0.32O2 catalyst [C. Thomas, O. Gorce, C. Fontaine, J.-M. Krafft, F. Villain, G. Djéga-Mariadassou, Appl. Catal. B Environ. 63 (2006) 201], for which metal–support interactions prevent PdOx reduction and C3H6-SCR reaction mechanism is different from that on Pd0 sites, are widened.