THERMODESORPTIONS AND THERMOREACTIONS WITH HYDROGEN OF CARBON-MONOXIDE AFTER ITS CHEMISORPTION ON A NICKEL-CATALYST .1. MECHANISM OF THE METHANATION

After chemisorption of CO at room temperature on silica-supported Ni, the spectrum of TPSR (Temperature Programmed Surface Reaction) with H-2 is very similar to that obtained after chemisorption of CO2. In particular the CH4 and H2O peaks are obtained simultaneously. The same surface heterogeneity as already evidenced by the TPD (Temperature Programmed Desorption) spectra of CO2 is also evidenced by the TPD spectra of CO. However the latter show that supplementary CO adspecies, less tightly bonded, are formed during CO adsorption as compared to the CO2 case. Only a part of the CO is reversibly chemisorbed, most of it being subjected during heating to disproportionation or dissociation with re-association and desorption at T > 400 degrees C. It is the irreversibly chemisorbed CO which is involved in the methanation reaction. The latter proceeds quickly after H-assisted breaking of the C-O bond.