In Situ-DRIFTS Study of Selective Catalytic Reduction of NOx by NH3 over Cu-Exchanged SAPO-34

The intrinsic mechanism of the selective catalytic reduction (SCR) reaction over a Cu-exchanged SAPO-34 catalyst at low temperature was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), coupled with mass spectrometry to measure inlet and outlet gas concentrations. The evolution of the surface intermediates, as well as the reactivity of NH3 with surface NOx species and NOx with surface NH3 species, was evaluated. In terms of NOx adsorption, surface nitrates and nitrites are the main NOx adsorption species at low temperature. When NO was exposed to the sample with NH3 preadsorbed, surface NH3 was not reactive because of the low surface coverage of nitrates and nitrites. However, the reactivity is significantly enhanced by the inclusion of O2 in the feed, which promotes an increase in the concentration of surface nitrates and nitrites. DRIFTS results also reveal that the low temperature SCR reaction involves the formation of an NH4NO3 intermediate and its subsequent reduction by NO. The NH4NO3 was formed on Lewis acid sites on the Cu-SAPO-34 sample. The Brønsted acid sites act as an NH3 reservoir that supplies additional NH3 via migration to the Lewis acid sites for the SCR reaction. The migration of NH3 between different acid sites was confirmed in an NH3-temperature-programmed desorption (TPD) study. The presence of NO in the feed reduces surface NH4NO3 to produce N2 at temperatures as low as 100 °C. Since NH4NO3 is typically considered an inhibitor, the onset temperature of the reaction between NO and NH4NO3 is much lower than that reported for other SCR zeolite catalysts; therefore, it is likely the key factor that results in the low temperature SCR activity of Cu-SAPO-34.