CO isotopic mixing measurements on nickel: Evidence for irreversibility of CO dissociation

The isotopic mixing reaction, 12C18O + 13C16O → 12C16O + 13C18O, and the methanation reaction (3H2 + CO → CH2 + H4O) have been studied at 2 Torr CO pressure over a Ni(100) single crystal between 300 and 700 K. At 600 K the rate of the exchange reaction is a factor of 50 slower than CO hydrogenation indicating irreversibility of the CO dissociation reaction step. Below 850 K, the turnover number for the isotopic mixing reaction is less than 4 × 10−3 reactive events site−1 sec−1. This corresponds to a reactive sticking coefficient of < 2 × 10−8. The steady state reaction becomes significant at ~850 K at which temperature a graphite layer begins to decompose, opening up free Ni sites. Various models to explain these observations are discussed.