Effect of NO2 on the selective catalytic reduction of NO with NH3 over Cu/SAPO-34 molecular sieve catalyst

Abstract

This study investigated the effects of NO2 on the selective catalytic reduction (SCR) of NO by NH3 over Cu/SAPO-34 catalyst at temperatures ranging from 100 to 500 ° C. The Cu/SAPO- 34 sample was hydrothermally treated at 750 °C for 4 h to obtain a de-greened sample and X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the structure of the catalyst. SCR activity test, kinetic analysis, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ-DRIFTS) were all applied to evaluate the changes in catalytic activity in the presence of various NO/NO2 ratios. The SCR results for different NO/NO2 molar ratios demonstrated that NO2 inhibited the NOx removal efficiency over the Cu/SAPO- 34 catalyst at low temperatures (100-280 °C), but enhanced the efficiency at high temperatures (above 280 °C). The amount of N2O was observed to increase with decreasing NO/NO2 ratios, owing to the decomposition of NH4NO3. The kinetic results showed that the fast SCR reaction exhibited a higher apparent activation energy (Ea =64.02 kJ{bullet operator}mol-1) than that of the standard SCR reaction (Ea =48.00 kJ{bullet operator}mol-1) over Cu/SAPO-34 catalyst. The results of in situ-DRIFTS showed that NO2 did not efficiently generate nitrate species on Cu2+ sites compared with NO, and that some nitrate species combined with NH4+ on Brønsted acid sites to generate NH4NO3. The inhibitory effect of NO2 at low temperatures is evidently caused by deposited NH4 NO3 covering the active sites of Cu/SAPO-34 catalyst, while these NH4NO3 species can be reduced by NO or thermally decomposed as the temperature increases. © Editorial office of Acta Physico-Chimica Sinica.