COVERAGE DEPENDENCE OF CO DISSOCIATION ON CLEAN AND HYDROGEN PRESATURATED FE(100) SURFACE

A detailed thermal desorption spectroscopy (TDS) study of very low and intermediate exposures of CO on clean and H-2 presaturated Fe(100) surfaces was conducted. At extremely low CO exposures, CO adsorbs only in the pi-bonded CO state at low temperature. Molecules in this state exhibit an extremely weak carbon-oxygen bond and exclusively undergo dissociation when the temperature is raised above 440 K. At higher coverages, both dissociation and desorption is observed around 440 K even though all the molecules are still bound in the pi-bonded CO state at lower temperatures. A model describing the partitioning between desorption and dissociation is proposed in which the dissociation fragments displace the strongly bound CO in the pi-bonded state. The stoichiometry of this reaction allows a maximum of one half of pi-bonded CO molecules to dissociate. Preadsorbed hydrogen strongly influences the ratio of dissociation to desorption as a function of coverage and suggests that hydrogen may induce island formation in the CO adlayer.