The impact of annealing on the kinetic parameters for CO desorption from Ni(100) was investigated using isotopic mixing, temperature-programmed desorption (TPD), and work function Δφ measurements. Isotopic labeling TPD demonstrated that all of the CO molecules in a layer are kinetically equivalent above 200 K regardless of the order in which they were adsorbed; the CO molecules that initially desorb from the surface are consequently representative of all of the CO molecules in the layer at that coverage. Using the threshold TPD technique to sample the initial CO distribution, values of the activation energy Ed and preexponential factor Vd derived for the layers prepared with or without annealing converged to 135 ±5 kJ/mol and 1015±1 s-1, respectively, in the limit of zero coverage. Increasing the CO coverage in a layer adsorbed at 110 K without annealing prior to TPD produced a sharp decrease in Ed and vd from the zero coverage limits. At θ0 = 0.37 monolayer, values of 26 ± 5 kJ/mol and 103.4±1 s-1 were obtained. These values are not representative of the true binding parameters for CO on Ni(100) because the Polanyi-Wigner equation was applied to a reaction in which two steps, desorption and site conversion, occur simultaneously and with comparable rates. This analysis yielded kinetic parameters for the nonequilibrated adlayers that compensated strongly. The desorption rate parameters derived from CO layers adsorbed at 296 K without annealing prior to TPD show a much less pronounced change with coverage. At θ0 = 0.39 monolayer, values of 76 ± 6 kJ/mol and 109.3±1 s-1 were obtained. Annealing CO layers adsorbed at 110 K prior to TPD produced more normal kinetic behavior. At θ0 = 0.38 monolayer, an Ed of 121 ± 4 kJ/mol and a vd of 1015.3±1.5 s-1 were calculated. Layers saturated with CO at 110 K and annealed above approximately 300 K were quasi-equilibrated as shown by the close agreement between the Ed results and the steady-state measurements of the isosteric heat qst made using the Kelvin technique. The qst values fell gradually from 139 ± 7 kJ/mol near 0 eV to 106 ± 13 kJ/mol at 1.1 eV (0.5 monolayer). The kinetic parameters calculated from the TPD spectra of the annealed layers consequently reflect the true binding properties for CO on the Ni(100) surface.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry