The NO2, NO (O-2) adsorption and temperature programmed desorption (TPD) were studied systematically to probe into the selective catalytic reduction of NO by methane (CH4-SCR) over CoH-ZSM-5 (SiO2/Al2O3 = 25, Co/Al = 0.132-0.312). Adsorption conditions significantly affect the adsorption of NO, NO2 and NO + O-2. Adsorbed NO species are unstable and desorbed below the reactive temperature 523 K. Increasing adsorption temperature results in the decrease of the adsorbed NO species amount. The amount of -NOy species formed from NO2 adsorption increases with the increase of NO2 concentration in the adsorption process, while decreases significantly with the increase of adsorption temperature. Though NO species are adsorbed weakly on CoH-ZSM-5, competitive adsorption between NO and -NOy species decreases the amount of adsorbed -NOy, species. Similar desorption profiles of NO2 were obtained over CoH-ZSM-5 while they were contacted with NO2 or NO + O-2 followed by TPD. If NO2 was essential to form adsorbed -NOy species, the amount of adsorbed -NOy species for NO + O-2 adsorption should be the least among the adsorptions of NO2, NO + O-2 and NO + NO2 because of the lowest NO2 concentration and highest NO concentration. In fact, the amount of adsorbed -NOy,, species is between the other two adsorption processes. These indicate that the formation of adsorbed -NOy species may not originate from NO2. (c) 2006 Elsevier Ltd. All rights reserved.