Structural Influences on Adsorption Energy. II. CO on Ni(100)

Abstract

The adsorption of CO on Ni(100) has been studied using LEED, Auger, and work function measurements and was found to be completely reversible. The isosteric heat of adsorption was approximately 1.30 eV/molecule independent of coverage up to 0.61 monolayers. Three surface structures were observed: (1) a disordered phase at high temperature and/or low coverage, (2) a c(2× 2) phase at coverages up to and including 0.5 monolayers at sufficiently low temperature, and (3) a compressible hexagonal phase between 0.61 and 0.69 monolayers. Evidence is presented which supports an attractive CO–CO interaction at coverages of less than 0.5 monolayers. A number of two-dimensional phase transitions were observed and suggest an interesting T—θ phase diagram with a eutecticlike triple point. The importance of scrupulous cleaning procedures in allowing all possible adsorbed phases to be observed has again been demonstrated.