Fourier transform-infrared investigation of adsorption of methane and carbon monoxide on HZSM-5 and Mo/HZSM-5 zeolites at low temperature

The low-temperature (173 K) methane adsorption and coadsorption of methane and CO on HZSM-5 and Mo/HZSM-5 were investigated by FT-IR spectroscopy, Five distinct bands of adsorbed methane at 3008, 3002, 2900, 2890, and 1303 cm(-1) were observed for HZSM-5 zeolite. The assignments of these bands are the nu(1) (2900, 2890 cm(-1)), nu(3) (3008, 3002 cm(-1)), and nu(4) (1303 cm(-1)) modes of methane, respectively. The infrared inactive nu(1) mode (2917 cm(-1)) of free methane became active and shifted to lower frequencies when methane was adsorbed on the zeolite sample. The weakness of the C-H bond of methane caused by distortion of the methane molecule via its interaction with the zeolite surface is possibly the first step in methane activation, Very possibly the bands at 3002 and 2890 cm(-1) are due to the methane that interacts with the hydroxyl groups, whereas the band at 2900 cm(-1) is from the methane that interacts with the oxygen anion species formed by dehydroxylation HZSM-5 zeolite. Three strong bands of adsorbed CO at 2173, 2162, and 2138 cm(-1) were detected for HZSM-5. The 2173 cm(-1) band was assigned to the CO that interacts with surface Lewis acidic sites, whereas the band at 2162 cm(-1) is attributed to the CO that interacts with hydroxyl groups and the band at 2138 cm(-1) originates from the weakly adsorbed CO. The introduction of molybdenum into HZSM-5 considerably reduces the amount of adsorbed methane and CO because molybdenum may eliminate the surface hydroxyl groups and block the Lewis acidic sites available for methane and CO adsorption. (C) 1996 Academic Press, Inc.