DISPLACEMENT OF CO CHEMISORBED ON METALS BY HYDROGEN

Chemisorbed CO can be displaced from both the Ni(100) and Pt(111) surfaces by hydrogen in a temperature range where thermally activated desorption is substantially slower. This result is unexpected since hydrogen is adsorbed more weakly than CO on both the Pt and Ni surfaces. This paper describes a series of experiments that clearly establish that displacement occurs on these two surfaces. Displacement of a strongly adsorbed species by a more weakly adsorbed species is a different process than surface exchange where one isotopic form of an adsorbed species is exchanged by another. The kinetic results presented here indicated that the displacement reaction is thermally activated and that the thermal activation barrier is substantially smaller than the thermal activation energy required for CO desorption in the absence of hydrogen. The first-order dependence on surface CO coverage suggests that a single adsorbed CO molecule is involved in the rate-limiting step. The positive order (about 0.5) observed in hydrogen pressure suggests that the atomic hydrogen coverage plays a role in the rate-limiting step. No isotope effect is observed with deuterium substitution. He, Ne, Ar, N2, and CH4 do not cause displacement under the conditions studied (up to 10-3 Torr). Adsorbed hydrogen is rapidly displaced by CO as expected since hydrogen is more weakly adsorbed than CO. The reaction energetics and probabilities observed are consistent with substantial weakening of adsorbed CO by high coverages of coadsorbed hydrogen. © 1990, American Chemical Society. All rights reserved.