Interaction of CO with Ni 0 in zeolites and on alumina studied by isotopic exchange

Abstract

Isotopic exchange between 13C 16O and 12C 18O was carried out at 240°C over Ni 0 in and on zeolites and on alumina to characterize the dissociation of CO molecules. The scrambling rate was not affected by the Ni reduction level (higher than 10%) similarly to the sorption of hydrogen, which exhibited only weak dependence on the extent of Ni reduction. This sorption is assumed to reflect the exposed Ni 0 area, so that the rate of CO mixing can be related to the Ni 0 area. Such behaviour was found for Ni supported particles; Ni reduced in the cationic positions of NaY zeolite was almost inactive and exhibited a substantial activity increase only after reduction following reoxidation, most probably due to a change in the catalyst. As the amount of sorbed hydrogen was similar on both samples, the Ni 0 area cannot be the only factor governing the CO isotope scrambling. The isotopic exchange of 18O of carbon oxides complicated the CO isotope scrambling reaction. The ability to scramble the CO isotopes was compared, for completely reduced samples, with the methanation activity. This reaction is also assumed to proceed via dissociated CO. The rates of both reactions — isotope mixing and methanation — exhibited the same sequence over the individual catalysts, which supports the assumption of a common CO dissociation step; differences in the subsequent reaction pathways are discussed.