Cu-ZSM-5 zeolite highly active in reduction of NO with decane under water vapor presence: Comparison of decane, propane and propene by in situ FTIR

Abstract

Selective catalytic reduction of NOx (SCR-NOx) with decane, and for comparison with propane and propene over Cu-ZSM-5 zeolite (Cu/Al 0.49, Si/Al 13.2) was investigated under presence and absence of water vapor. Decane behaves in SCR-NOx like propene, i.e. the Cu-zeolite activity increased under increasing concentration of water vapor, as demonstrated by a shift of the NOx–N2 conversion to lower temperatures, in contrast to propane, where the NOx–N2 conversion is highly suppressed. In situ FTIR spectra of sorbed intermediates revealed similar spectral features for C10H22– and C3H6–SCR-NOx, where –CHx, R–NO2, –NO3−, Cu+–CO, –CN, –NCO and –NH species were found. On contrary, with propane –CHx, R–NO2, NO3−, Cu+–CO represented prevailing species. A comparison of the in situ FTIR spectra (T–O–T and intermediate vibrations) recorded at pulses of propene and propane, moreover, under presence and absence of water vapor in the reaction mixture, revealed that the Cu2+–Cu+ redox cycle operates with the C3H6–SCR-NOx reactions in both presence/absence of water vapor, while with C3H8–SCR-NOx, the redox cycle is suppressed by water vapor. It is concluded that decane cracks to low-chain olefins and paraffins, the former ones, more reactive, preferably take part in SCR-NOx. It is concluded that formation of olefinic compounds at C10H22–SCR-NOx is decisive for the high activity in the presence of water vapor, while water molecules block propane activation. The increase in NOx–N2 conversion due to water vapor in C10H22–SCR-NOx should be connected with the increased reactivity of intermediates. These are suggested to pass from R–NOx→–CN→–NCO→NH3; the latter reacts with another activated NOx molecule to molecular nitrogen. The positive effect of water vapor on the NOx–N2 conversion is attributed to increased hydrolysis of –NCO intermediates.