The influence of adsorbate interactions on kinetics and equilibrium for CO on Ru(001). II. Desorption kinetics and equilibrium

Abstract

Articles you may be interested inFermi resonance distortion of the Ru-CO stretching mode of CO adsorbed on Ru(001) J. Chem. Phys. 108, 5035 (1998); 10.1063/1.475910 CO desorption kinetics from clean and sulfurcovered Ru(001) surfaces J. Chem. Phys. 92, 4483 (1990); 10.1063/1.457759Layer resolved spectroscopy of potassium adsorbed on a Ru(001) surface: Photoemission and thermal desorption study A variety of methods [temperature programmed desorption via pressure rise and via work function changes (.:1~); isothermal desorption via .:1~: quasiequilibrium measurements via isobars monitored by .:1~. in combination with sticking coefficients] has been used to obtain detailed data on the coverage dependence of the adsorption equilibrium and desorption kinetics for CO on the basal Ru(OOI) face. While the deviation from reversibility varies strongly over these methods, no significant influence of the degree of irreversibility on the results has been found. Desorption energies and isosteric heats are constant at 160 kJ/mol for 0 < e < 0.2, then rise slowly up to 175 kJlmol at e = 0.33, where they fall abruptly to 120 kJ/mol and more gradually at higher coverage. The "first order" frequency factor (Arrhenius preexponential normalized by the coverage) is 10'6 s-' at e = 0, rises precipitously, especially in the range 0.2 < e < 0.33, to 1019 s-' at e :::::0.33, where it drops abruptly to :::::10" s-'. The main conclusions drawn are: (1) The dependence on coverage of the desorption energies and preexponentials can be understood in terms of the equilibrium statistical mechanics of the chemisorption layer, governed by lateral adsorbate-adsorbate interactions. In particular, the high preexponentials and their strong increase close to e = 0.33 are due to strong localization in the adlayer. (2) No strong influence of precursor kinetics exists in desorption.(3) Possible dynamic effects have constant influence throughout the range of measurements and can be described by the behavior of the sticking coefficient. These conclusions are discussed in connection with other recent findings.

Formation of Ru subcarbonyls by reaction of CO with stepped Ru single crystal surfaces

“…Such an inte~retation would be in accord with the results of Klein [I] and Kraemer and Menzel [2] who also used Ru field emitter tips and observed the main part of the CO, layer to desorb thermally between 350 and 500 K. However, it comes as a surprise that thermal desorption occurs in the millisecond time scale. In fact, a longer time constant would have been expected in view of the results obtained by Pfniir et al [20] for desorption from flat Ru(OO1). The PFDMS technique always samples the most stable (longest lifetime) adsorption states of the equilibrated layer and it is likely that these states are located at the step sites of the field emitter surface.…”

Evidence of stable high-temperature Dx-CO intermediates on the Ru(0001) surface

“…The CO TPD traces develop in a characteristic fashion with increasing coverage, consistent with previous reports of CO desorption from Ru(0001). 17,23,24 The doublet structure at ∼475 K is fully saturated at θ CO ∼ 1/3 ML. When θ CO exceeds this value a second lower temperature desorption peak begins to develop.…”

“…The surface cleanliness was checked by reference to the TPD spectra of CO (Ref. 17) and NO. 18 CO-covered surfaces were prepared by background dosing of CO, which was performed at sample temperatures <200 K. Absolute CO coverage (θ CO ) values were determined by comparison with the CO saturation coverage on the basis of the total integrated areas of the TPD traces.…”

From Adlayer Islands to Surface Alloy: Structural and Chemical Changes on Bimetallic PtRu/Ru(0001) Surfaces

“…The CO saturation coverage q CO (sat) decreases for the bimetallic surface to 0.55 ML compared to the value of 0.68 ML reported for pure Ru(0001). [32] It is important to note in this context that the structure of the bimetallic surface may change slightly during this first TPD flash, at least above 293 K. This effect will influence the results of the experiment, since the TPD spectrum reflects the chemical properties of the surface at the respective desorption temperature.…”

“…CO desorption from Ru(0001) shows two characteristic peaks, the a 1 peak with its maximum at 461 K and the a 2 peak (maximum at 406 K), which are well known from the literature. [31,32] After 0.45 ML Pt deposition at 293 K, CO desorption in the temperature range of the a peaks of the non-modified Ru(0001) surface (350-500 K) is reduced to~50 % of that on the Pt-free surface. Two distinct desorption peaks are still discerned, although the minimum between them is much less pronounced.…”