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 NH 3 with surface NO x species and NO x with surface NH 3 species, was evaluated. In terms of NO x adsorption, surface nitrates and nitrites are the main NO x adsorption species at low temperature. When NO was exposed to the sample with NH 3 preadsorbed, surface NH 3 was not reactive because of the low surface coverage of nitrates and nitrites. However, the reactivity is significantly enhanced by the inclusion of O 2 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 NH 4 NO 3 intermediate and its subsequent reduction by NO. The NH 4 NO 3 was formed on Lewis acid sites on the Cu-SAPO-34 sample. The Brønsted acid sites act as an NH 3 reservoir that supplies additional NH 3 via migration to the Lewis acid sites for the SCR reaction. The migration of NH 3 between different acid sites was confirmed in an NH 3 -temperature-programmed desorption (TPD) study. The presence of NO in the feed reduces surface NH 4 NO 3 to produce N 2 at temperatures as low as 100 °C. Since NH 4 NO 3 is typically considered an inhibitor, the onset temperature of the reaction between NO and NH 4 NO 3 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.