Evidence of a Cu2+–Alkane Interaction in Cu-Zeolite Catalysts Crucial for the Selective Catalytic Reduction of NOx with Hydrocarbons

The selective catalytic reduction of NOx with hydrocarbons (HC-SCR-NOx) on Cu-zeolites is an alternative for the depletion of NOx present in the exhaust gas of diesel engines. The role played by copper and the general reaction pathway are investigated here using propane as reductant (C3H8-SCR-NOx) and Cu-TNU-9 and Cu-Y zeolites as catalysts, giving 80% and 10% NO conversion, respectively, at 623 K. In situ EPR shows that, even at room temperature, Cu2+NO3– is formed in the two Cu-zeolites and that there exists an interaction of Cu2+ with C3H8 in Cu-TNU-9 but not in Cu-Y. The formation of Cu2+- - -C3H8 species in Cu-TNU-9 is confirmed by HYSCORE EPR experiments and theoretical calculations. Cu2+ is reduced to Cu+ after heating with the reaction mixture to an extent that is related with the catalyst activity. In situ NMR spectra have allowed identifying nitrile and isocyanate as reaction intermediates, which are partially hydrolyzed to carboxylic acid and ammonia. The results allow concluding that the C3H8-SCR-NOx reaction requires the activation of the alkane at accessible isolated Cu2+ and that this generates ammonia which reacts with Cu2+NO3− and NO acting as the reductant. The low activity of Cu-Y zeolite for the C3H8-SCR-NOx reaction proves that C3H8 in the gas phase does not react with Cu2+NO3–, the activation of C3H8 being mandatory on accessible Cu2+ sites. A general reaction scheme for HC-SCR-NOx involving Cu2+/Cu+ redox cycles and the formation of ammonia that acts as a reductant of NO is proposed.