Adsorption on an Equilateral Triangular Terrace Three Atomic Sites in Width:  Application to Chemisorption of CO on Pt(112)

Abstract

A model of monomer adsorption on equilateral triangular terraces three atomic sites in width is presented where step sites are considered first neighbors. Adsorbate-substrate interactions at the terrace step are treated differently than at bulk sites. Adsorbate-adsorbate first neighbor interactions are considered to be repulsive while second neighbors are allowed to be either repulsive or attractive. All low temperature phases have been identified under these conditions. The effect of increasing the temperature has also been investigated. Application of the model to chemisorption of CO on Pt(112) suggests experiments that would allow the various interaction energies to be obtained from a knowledge of the relatively low temperature phases and the conditions prevailing at the transitions between phases. Currently available experimental data is very extensive on the manner in which step sites are filled. However, there is insufficient data on the sequence of low temperature phases which appear when the pressure is gradually increased that would show the manner in which bulk sites are filled until full coverage of the terrace is reached.

Low Temperature Energy Phase Diagram for Adsorption on Fcc(112) Stepped Surfaces with Attractive First Neighbor Interactions

“…Phases P1, P2, P6, and P7 are phases already encountered in the repulsive first-neighbor case where they were listed as p 9 , p 10 , p 27 , and p 32 , respectively. Three of the remaining phases, P3, P4, and P5, consist of clusters of 4, 6, and 5 adsorbates, respectively, in contrast with the repulsive case where no clusters are observed …”

“…The eigenvalue of largest modulus, R ( x , y , z , u ), is therefore real and positive and provides the partition function of the system. The numerical results have been obtained for repulsive first-neighbors, or V < 0, and applied to the chemisorption of CO on Pt(112) as discussed in reference . In this article, we use the same transfer matrix, and the only difference is that first-neighbors are attractive, V > 0.…”

“…For completeness, we provide the basic equations found in ref still valid in this case. The partition function, Z 3 , for the adsorption on armchair, infinitely long, 3-atom wide terraces is Z 3 = [ R ( x , y , z , u ) ] 1 / 6 The statistical average of the coverage, θ 0 , the numbers per site of first- and second-neighbor adsorbates, θ and β, and the number per site of adsorbates on step sites, γ, are normalθ 0 = x 6 R ∂ R ∂ x normal, 1em normalθ = y 6 R ∂ R ∂ y normal, 1em normalβ = z 6 R ∂ R ∂ z normal, 1em normalγ = u 6 R ∂ italicR ∂ u The average energy per site ε, and the entropy per site divided by Boltzmann’s constant S follow as: normalε = normalμ normalθ 0 + V normalθ + W normalβ + normalγ U normal, 1em S = 1 6 .25em ln false( R false) − normalε k T A state of the system is determined by its occupational characteristics {θ 0 , θ, β, γ}. A phase is found when the occupational char...…”

“…A w -region between two w -boundary planes corresponds to 2-D phase diagrams having the same general structure. For repulsive first-neighbors, reference identified 31 w -regions and 37 phases. The information provided there focused on the application to CO/Pt(112). …”

Statistical Thermodynamics of Long Straight Rigid Rods on Triangular Lattices: Nematic Order and Adsorption Thermodynamic Functions

“…The problem is not only of theoretical interest, but also has practical importance. A complete summary about adsorption on triangular lattices can be found in refs − , and references therein.…”