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 NO x with hydrocarbons (HC-SCR-NO x ) on Cu-zeolites is an alternative for the depletion of NO x 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 (C 3 H 8 -SCR-NO x ) 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, Cu 2+ NO 3 – is formed in the two Cu-zeolites and that there exists an interaction of Cu 2+ with C 3 H 8 in Cu-TNU-9 but not in Cu-Y. The formation of Cu 2+ - - -C 3 H 8 species in Cu-TNU-9 is confirmed by HYSCORE EPR experiments and theoretical calculations. Cu 2+ 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 C 3 H 8 -SCR-NO x reaction requires the activation of the alkane at accessible isolated Cu 2+ and that this generates ammonia which reacts with Cu 2+ NO 3 − and NO acting as the reductant. The low activity of Cu-Y zeolite for the C 3 H 8 -SCR-NO x reaction proves that C 3 H 8 in the gas phase does not react with Cu 2+ NO 3 –, the activation of C 3 H 8 being mandatory on accessible Cu 2+ sites. A general reaction scheme for HC-SCR-NO x involving Cu 2+ /Cu + redox cycles and the formation of ammonia that acts as a reductant of NO is proposed.