Coverage dependent surface diffusion of noble gases and methane on Pt(111)

Abstract

The surface diffusion of krypton, xenon, and methane on the Pt(111) surface was studied using laser induced thermal desorption (LITD) techniques. The surface diffusion coefficient of krypton on Pt(111) at 45 K decreased dramatically with coverage from D = 1.4 × 10 −7 cm 2 s at θ = 0.05θ s to approximately D ≈ 1 × 10 −9 cm 2 s at θ = θ s Similarly, the surface diffusion coefficient of xenon on Pt(111) at 80 K decreased from D = 8 × 10 −7 cm 2 s at θ = 0.05θ s to D> = 2 × 10 −8 cm 2 s at θ = θ s . The coverage-dependent surface diffusion coefficients for the noble gases on Pt(111) were consistent with attractive interactions between the adsorbed atoms and a multiple site diffusion mechanism. In contrast, the surface diffusion coefficient of methane on Pt(111) at 45 K was independent of surface coverage at D = 1 × 10 −6 cm 2 s . This coverage independence indicated that latera interactions between the adsorbed methane molecules did not influence the surface diffusion process. Thermal desorption kinetics for krypton, xenon, and methane on Pt(111) were determined using temperature programmed desorption (TPD) experiments. Using the variation of heating rates method, the desorption energies were E des = 3.7 ± 0.1 kcal mol for Kr/Pt(111), E des = 6. kcal mol for Xe/Pt(111), and E des = 3.6 ± 0.15 kcal/ mol for CH 4/Pt(111). The TPD peaks for krypton and xenon on Pt(111) shifted to higher temperature versus initial coverage, consistent with attractive interactions between the adsorbed atoms. In contrast, the coverage-independent TPD peak temperature for methane on Pt(111) provided further evidence for the absence of interactions between the adsorbed methane molecules.