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-O-2-He also suggests the presence of two R-NOx species, C3H5-O-NO (propenyl nitrite) and C3H5-NO2 (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 R-NOx are not active intermediates in C3H6-SCR on Pd/WZ catalysts but are more likely spectators. The activity for C3H6-SCR is attributed to Pd-0 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 R-NOx, results reported previously on a Pd/Ce0.68Zr0.32O2 catalyst [C. Thomas, O. Gorce, C. Fontaine, J.-M. Krafft, F. Villain, G. Djega-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 Pd-0 sites, are widened. (C) 2008 Elsevier Inc. All rights reserved.