Adsorption on Cu ion-exchanged titanium pillared clay (Cu-Ti-PILC) was investigated by in situ infrared spectroscopy to provide insight into the reaction intermediates present in the selective catalytic reduction (SCR) of NO by propene in the presence of oxygen. The NO/O-2, adsorption produced different nitrate species due to the presence of terminal and bridged Cu2+-OH groups. These nitrates evolved into N-2 and N2O in small amounts once the NO catalytic cycle was finished. It can be concluded that the Cu2+-OH groups reacted with the nitro group, thus forming nitrates. C3H6 adsorption was higher and stronger than NO adsorption on the active sites of the catalyst. C3H6 reacted in the active site producing hydrocarbon intermediates (an organic nitro compound and acetate), which were responsible for the NO reduction. (c) 2004 Elsevier B.V. All rights reserved.