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We present a critical view of the analysis of experimental island densities acquired as a function of temperature in terms of barriers and prefactors for tracer diffusion at surfaces. We investigate the achievable precision for methods ranging from simple application of scaling laws, via integration of mean-field rate equations within various approximations for the capture rates, to kinetic Monte Carlo simulations that account for the various island shapes realized on square and hexagonal lattices. The discussion of theoretical models will be accompanied by variable temperature STM data for the nucleation of Ag on a Pt͑111͒ surface. We introduce experimental methods to test for dimer diffusion and dissociation, as well as for transient mobility of monomers. Density scaling is analyzed in the presence of post-deposition mobility and easy adatom attachment to islands and other monomers. From extended kinetic Monte Carlo simulations we establish density scaling for the various island shapes on square and hexagonal lattices for coverages up to percolation, which is particularly relevant for methods working in reciprocal space. ͓S0163-1829͑99͒01231-X͔

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The CO methanation reaction over nickel was studied at low CO concentrations and at hydrogen pressures slightly above ambient pressure. The kinetics of this reaction is well described by a first-order expression with CO dissociation at the nickel surface as the rate-determining step. At very low CO concentrations, adsorption of CO molecules and H atoms compete for the sites at the surface, whereas the coverage of CO is close to unity at higher CO pressures. The ratio of the equilibrium constants for CO and H atom adsorption, K(CO)/K(H), was obtained from the rate of CO methanation at various CO concentrations. K(H) was determined independently from temperature programmed adsorption/desorption of hydrogen to be K(H) = 7.7 x 10(-4) (bar(-0.5)) exp[43 (kJ/mol)/RT] and hence the equilibrium constants for adsorption of CO molecules may be calculated to be K(CO) = 3 x 10(-7) (bar(-1)) exp[122 (kJ/mol)/RT]. Furthermore, the rate of dissociation of CO at the catalyst surface was determined to be 5 x 10(9) (s(-1)) exp[-96.7 (kJ/mol)/RT] assuming that 5% of the surface nickel atoms are active for CO dissociation. The results are compared to equilibrium and rate constants reported in the literature.

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