Evidence for the facile formation of nitrogen-containing compounds from NO x and propene species on tungstated zirconia-based catalysts: Are these compounds active or spectator species in the selective catalytic reduction of NO x by C 3H 6?

Abstract

The propene selective catalytic reduction (C 3H 6-SCR) of NO x is investigated on WO x –ZrO 2 and Pd/WO x –ZrO 2. Temperature-programmed surface reaction (TPSR) experiments provide evidence of the decomposition of organic nitrogen-containing species formed through the interaction of C 3H 6 and ad-NO x , for which a C/N ratio of 3 is estimated. The TPSR of preadsorbed NO x in C 3H 6–O 2–He also suggests the presence of two R NO x species, C 3H 5 O NO (propenyl nitrite) and C 3H 5 NO 2 (nitropropene), decomposing in distinct temperature regions. The addition of Pd to WO x -ZrO 2 promotes the decomposition of nitropropene, whereas that of propenyl nitrite is scarcely affected. It is also suggested that R NO x are not active intermediates in C 3H 6-SCR on Pd/WZ catalysts but are more likely spectators. The activity for C 3H 6-SCR is attributed to Pd 0 species, formed by in situ reduction of oxidized Pd species by C 3H 6, via the well-established decomposition mechanism. In light of the new findings about the formation–decomposition of R NO x , results reported previously on a Pd/Ce 0.68Zr 0.32O 2 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 PdO x reduction and C 3H 6-SCR reaction mechanism is different from that on Pd 0 sites, are widened.