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The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH3 ) on vanadium-based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time-resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH3 coordinated to Lewis sites on vanadia on tungsta-titania (V2 O5 -WO3 -TiO2 ), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono-oxo vanadyl group that reduces only in the presence of both nitric oxide and NH3 . We were also able to verify the formation of the nitrosamide (NH2 NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.

Practical catalysts often operate under dynamic conditions of temperature variations and sudden changes of feed composition that call for understanding of operation and catalyst structure under analogous experimental conditions. For instance, the copper exchanged small pore SSZ-13 catalyst used currently in the selective catalytic reduction of harmful nitrogen oxides from the exhaust gas of diesel fuelled vehicles operates under recurrent ammonia dosage. Here we report the design of unsteady state experiments that mimic such dynamic environment to obtain key mechanistic information on this reaction. Through the combination of time-resolved X-ray absorption spectroscopy and transient experimentation we were able to capture an ammonia inhibition effect on the rate-limiting Cu re-oxidation at low temperature. The practical relevance of this observation was demonstrated by the optimization of the ammonia dosage on a catalyst washcoat on cordierite honeycomb, resulting in lower ammonia consumption and the increase of NO conversion at low temperature.

The low-temperature behavior of the selective catalytic reduction (SCR) process with feed gases containing both NO and NO2 was investigated. The two main reactions are 4NH3 + 2NO + 2NO2 → 4N2 + 6H2O and 2NH3 + 2NO2 → NH4NO3 + N2 + H2O. The “fast SCR reaction” exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO and dominates at temperatures above 200 °C. At lower temperatures, the “ammonium nitrate route” becomes increasingly important. Under extreme conditions, e.g., a powder catalyst at T ≈ 140 °C, the ammonium nitrate route may be responsible for the whole NO x conversion observed. This reaction leads to the formation of ammonium nitrate within the pores of the catalyst and a temporary deactivation. For a typical monolithic sample, the lower threshold temperature at which no degradation of catalyst activity with time is observed is around 180 °C. The ammonium nitrate route is interesting from a standpoint of general DeNO x mechanisms:  This reaction combines the features typical to the SCR catalyst with the features of the NO x storage−reduction catalyst, i.e., NO x adsorption to a basic site.

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