On the different roles of NCO and CN in propene-SCR over silver/alumina

Silver/alumina is one of the most promising catalyst materials for HC-SCR. The reaction mechanism remains incompletely understood, however. Isocyanate species (–NCO) and cyanide species (–CN), important intermediates in the reaction path in HC-SCR, were investigated by gas-phase FTIR and DRIFT spectroscopy in step-response experiments. In accordance with the literature, the corresponding gas-phase species, HNCO and HCN, were always detected at the same time at the different steps of the experiment; however, the accumulation and consumption of surface –NCO and –CN species did not show a similar correlation. Consequently, in conflict with the literature, we propose two parallel, independent reaction pathways for –NCO and –CN species. In one of these reaction pathways, surface nitrates react with C-containing surface species either directly to –NCO or via R–NO2 species. In the other pathway, C-containing surface species react with gas-phase NOx or short-lived N-containing surface species to R–NO species and subsequently to –CN species