On the different roles of isocyanate and cyanide species 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 (single bondNCO) and cyanide species (single bondCN), 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 single bondNCO and single bondCN species did not show a similar correlation. Consequently, in conflict with the literature, we propose two parallel, independent reaction pathways for single bondNCO and single bondCN species. In one of these reaction pathways, surface nitrates react with C-containing surface species either directly to single bondNCO or via Rsingle bondNO2 species. In the other pathway, C-containing surface species react with gas-phase NOx or short-lived N-containing surface species to Rsingle bondNO species and subsequently to single bondCN species.