Precursor and overlayer structural effects in the interaction of CO with Ni(110)

Abstract

Structural and electronic modification of Ni, Pd, and Fe strained metal overlayers on W(110) and W(100) The interaction of Ag and Al overlayers with InP (110): Surface and diode studies of the effect of Sb interlayers J.

Mechanism, configurational degeneracy, and mass action in transition-state rate functions for adsorption-desorption reactions

“…Figure 1 shows isothermal measurements of the desorption rate vs coverage for temperatures between 399 and 423 K arranged according to eq 42. 22 The collinearity is excellent, demonstrating that the adsorbed layer behaves as a disordered lattice gas of independent molecules at these temperatures. The common slope of the lines gives « 0.68, which is in reasonable agreement with the value = 0.625 expected for the ideal c(8X2) saturation coverage.…”

“…We have recently completed a study of CO adsorption and desorption on Ni( 110) that includes measurements of the rates of both processes as functions of surface coverage at constant temperature. 22 The question addressed here is whether the state of order in the adsorbed layer at low coverages, ó < 0.6 ML (ML * monolayer), can be identified from the coverage dependenceof the desorption rate for temperatures above 400 K. In this coverage regime at temperatures below 150 K a c(8x2) LEED pattern is observed, indicative of an ordered structure with a saturation coverage of = 5/8 ML. 23 To preserve the generality of this treatment, let us assume that the adsorbed layer is known to include a single ordered structure with cells covering a lattice sites and including molecules per cell, in equilibrium with a disordered lattice gas of the same species that occupies single sites.…”

“…This corresponds very well to what one would expect if molecules leaving the precursor state rotate and translate freely in two dimensions, which would yield an estimate of about 4 X 104 for this quantity. 22 Clearly, the scenario just outlined gives a good account of the desorption data for temperatures near 400 K. However, the singleparticle lattice gas description of the configurational degeneracy fails to account for the fact that c(8X2) order appears in LEED measurements at temperatures below 150 K for coverages as low as 0.2 ML. 23 The contrasting behaviors can be reconciled if an order-disorder transition occurs for this system somewhere in the temperature range between 150 and 400 K. Campbell, Vajo, and Becker have recently reported measurements of surface coverage as a function of time at various temperatures when initially clean Ni(l 10) is exposed to a constant pressure (3 X 10™8 Torr) of CO. 25 For T -386 K, where adsorption and desorption compete, the coverage vs time curve shows a sharp transition when a coverage near 0.4 ML is reached, and the authors report that typical kinetic models cannot describe this curve and curves for higher temperatures with a single set of rate parameters.…”

“…The resulting evaporation rate and condensation probability are given by '»»-"(t)(£)"p(-T3?!) (22) and 5*<(1^ ")" (" ) (23) Note that the absence of configurational degeneracy in eq 20 leads directly to the anticipated absence of mass actions for occupied or vacant sites in the activities and rate functions, resulting in zero-order kinetics. Several principles were exposed in the preceding analyses that should be enumerated for emphasis at this point:…”

“…The large magnitude of this ratio reflects not only the fact that each molecule entering the precursor state gains frustrated translational and rotational degrees of freedom at the expense of vibrational degrees of freedom, but that the three molecules left behind "relax" from the crowded environment of a (2X1) cell with four molecules to the less constrained environment of a c(4X2) cell with three molecules, which increases their respective vibrational partition functions. The reader is directed to ref 22 for a complete discussion of this important effect. Hydrogen adsorption on the dimer surface is known to be dissociative, and in the so-called "monohydride" phase, paired Si atoms lose their «--bond to accommodate one H atom each until a saturation coverage of*= 1.0 ML is reached.…”