CALORIMETRIC HEATS OF ADSORPTION FOR CO ON NICKEL SINGLE-CRYSTAL SURFACES

An adsorption calorimeter for studies on well-defined single crystal surfaces under ultrahigh vacuum conditions is now available, based on supersonic molecular beam dosing onto ultrathin metal single crystals. Here we discuss the relationship between the calorimetric heat of adsorption as measured in this system and the related parameters: the differential heat of adsorption, the isosteric heat, and the Arrhenius desorption energy. Coverage-dependent calorimetric heats of adsorption and sticking probabilities for CO on Ni{111}, {110}, and {100} are presented, and comparisons made with literature values for isosteric heats and Arrhenius desorption energies. At intermediate coverages some significant discrepancies occur which are attributed to a temperature-dependent adlayer structure. By combining sticking probability with heat measurements at high coverage, at 300 K, where significant desorption occurs, the desorption preexponential has been accurately determined; differential entropies of adsorption are also obtained. Differences in initial heats of adsorption and in the coverage dependencies for the three crystal planes are discussed, particularly in relation to surface stoichiometry, and to CO-CO interactions.