Mechanistic aspects of the selective catalytic reduction of NO x by dimethyl ether and methanol over γ-Al 2O 3

Abstract

High catalytic activity for selective catalytic reduction of NO x with dimethyl ether and methanol (DME- and methanol-SCR) has been found over a γ-alumina catalyst. The surface species involved in the DME-SCR reaction were studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in combination with mass spectrometry and compared to the corresponding species observed during methanol-SCR. Dimethyl ether adsorbs mainly as methoxy groups ( O CH 3) on the catalyst surface, while methanol adsorbs as methoxy groups and molecularly. With increasing temperature, DME desorbs in two steps, whereas methanol desorbs first as methanol and at higher temperatures as DME. At higher temperatures, the two reducing agents display similar DRIFTS spectra showing first formaldehyde-like species and then formates on the surface. In the presence of NO or NO 2, reactions between NO x species and carbon-containing species occur. Formohydroxamic acid (CHO N(H)OH) forms isocyanates (NCO), and both are observed at temperatures relevant for DME-SCR. Since these species are likely intermediates for hydrocarbon-SCR over Ag/Al 2O 3, a partly similar reaction mechanism may be operational for DME-SCR over γ-Al 2O 3 despite the fact that Ag/Al 2O 3 does not catalyze DME-SCR efficiently. The difference is thus due to the reaction steps leading to the formation of formohydroxamic acid that with DME follow a more efficient route over γ-Al 2O 3 than over Ag/Al 2O 3.