High catalytic activity for selective catalytic reduction of NO with dimethyl ether and methanol (DME- and methanol-SCR) has been found over a gamma-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-CH3) 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 NO2 reactions between NOx 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/Al2O3 a partly similar reaction mechanism may be operational for DME-SCR over gamma-Al2O3 despite the fact that Ag/Al2O3 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 gamma-Al2O3 than over Ag/Al2O3 (C) 2010 Elsevier Inc All rights reserved