Intermediate species formed on ZSM-5 zeolites during the selective catalytic reduction of NO by acetylene (C2H2-SCR) were investigated by transmittance FTIR and temperature-programmed desorption, to elucidate the different catalytic performance of the proton and sodium forms of ZSM-5. Bidentate nitrates formed exclusively on HZSM-5 were very active toward the reductant, whereas the nitrates associated with the Na+ ions on NaZSM-5 were inert. Although the saturation adsorption amount of acetylene on NaZSM-5 was about five times higher than that on HZSM-5 at 80 degrees C, most of the acetylene on NaZSM-5 was desorbed below 200 degrees C, whereas a considerable amount of acetylene was detected above 250 degrees C on HZSM-5 in TPD. The strongly adsorbed species formed on acetylene adsorption was vinyl alcohol (CH2=CH-OH) bound to the Bronsted acid sites that could be converted to acetate species at elevated temperatures on HZSM-5. Based on our results, a mechanism of the C2H2-SCR on HZSM-5 was proposed in which the bidentate nitrates and acetate species formed exclusively on HZSM-5 react with each other, leading to NOx elimination, which explains why the behavior of HZSM-5 is quite different from that of NaZSM-5 in the C2H2-SCR. (c) 2006 Published by Elsevier Inc.