CALORIMETRIC ADSORPTION HEATS ON LOW-INDEX NICKEL SURFACES

A new method has been devised for detecting the heat evolved when a gas is adsorbed onto a crystal surface. The method forms the basis of an adsorption calorimeter which provides, for the first time, accurate heats of adsorption for adsorbates on metal single crystal surfaces as a detailed function of surface coverage. Coverage-dependent calorimetric heats of adsorption and sticking probabilities for CO on the (111) {100} and {110} planes of nickel have been obtained, enabling comparisons to be made with literature values for isosteric heats. Data for CO adsorption onto nickel surfaces with varying amounts of predosed K have been obtained, demonstrating an unexpectedly large effect of promotion on the heat of adsorption. A model is presented to explain this data. Finally, heats of adsorption for oxygen chemisorption have been obtained for the clean and K predosed nickel surfaces. The data provide a direct measure, of the lateral interaction energies between adatoms. Unexpectedly large values for the pairwise repulsive interaction between 0 adatoms on Ni surfaces have been obtained.