THE ADSORPTION AND DECOMPOSITION OF CARBON MONOXIDE ON Ni(100) AND THE OXIDATION OF THE SURFACE CARBIDE BY OXYGEN

The interaction of carbon monoxide with nickel single crystals has been studied by many authors [l-24]. It has been observed that the adsorption of CO on nickel is completely reversible below temperatures of 18O”C, but that at more elevated temperatures, exposure of monocrystalline Ni surfaces to CO results in the buildup of carbon. Below 180°C buildup of carbon can also be brought about by the incidence of electrons of sufficiently high energy [22-251. A number of theoretical analyses have dealt with the interaction of CO with transition metal surfaces below 28O’C [27]. Disproportionation of CO (2 CO C,, f CO,,) is thermodynamically favoured, but there is evidence that dissociative adsorption is also possible, with formation of a Ni-C and a Ni-0 bond (2Ni + CO NipC + Ni-0) [26]. The surface carbon is carbidic and does not react to graphite upon heating in vacuum, but rather diffuses into the bulk [28]. According to Coad and Rivike [33] nickel carbide is not stable above 400°C and most of the evidence suggests that it starts to decompose