The role of NO2 in the selective catalytic reduction of nitrogen oxides over Fe-ZSM-5 catalysts: Active sites for the conversion of NO and of NO/NO2 mixtures

The selective catalytic reduction by NH3 of NO (“standard SCR”) and NO/NO2 mixtures (“fast SCR”) were compared over Fe-ZSM-5 catalysts of different Fe content ranging from 0.2 to 5 wt%. The Fe site structure was monitored by UV–vis spectroscopy. From the response of the reaction rates to the variation of the site structure, it was concluded that standard and fast SCR require different Fe sites; standard SCR proceeds both on isolated and oligomeric Fe oxo sites, whereas a small concentration of isolated Fe sites is sufficient to sustain the high reaction rates of fast SCR, with no discernible contribution of different sites. The response of both reactions to hydrothermal stress (with or without SO2 present) differs; strong deactivation in standard SCR accompanied by increasing NH3 oxidation activity being opposed to stable activity in fast SCR, suggesting that those isolated sites, which are responsible for fast SCR are very resistive toward clustering under the conditions of automobile exhaust catalysts. The potential of preparation procedures to improve the activity by increasing the Fe content is very different. The best performance was achieved using solid-state ion exchange with FeCl3, with NOx conversions >90% between 473 and 773 K obtained over the best catalyst. But if a concomitant increase of the aggregated site abundance cannot be avoided, then NH3 oxidation will effectively compete with fast SCR already at relatively low temperatures. The role of free NO2 in the reaction mechanism of standard SCR was assessed by comparing the SCR reaction rate with that of NO oxidation for all catalysts used. NO oxidation was significantly slower than NO reduction in all cases, which excludes free NO2 produced by NO oxidation as an intermediate of the standard SCR reaction.