Surface diffusion of tetramethylsilane and neopentane on Ru(001)

Abstract

The surface diffusion coefficients for tetramethylsilane and neopentane on Ru(001) were measured using laser induced thermal desorption (LITD) techniques. The surface diffusion coefficient for tetramethylsilane at 125 K was constant at D ≈ 6.5 × 10 −8 cm 2 s for all coverages. In contrast, the surface diffusion of neopentane displayed a strong dependence on surface coverage. The neopentane surface mobility at 130 K decreased from D = 5.5 × 10 −7 cm 2 s at θ = 0.10 θ s to D = 8.0 × 10 −9 cm 2 s at θ = θ s. The surface diffusion coefficients for both tetramethylsilane and neopentane displayed Arrhenius behavior. For tetramethylsilane, a diffusion activation energy of E dif = 3.3 ± 0.1 kcal mol and preexponential of D 0 = 5.0 × 10 −2 ± 0.1 cm 2 s were measured at θ = 0.40 θ s. For neopentane at θ = 0.10 θ s, the Arrhenius parameters were E dif = 3.0 ± 0.3 kcal mol and D 0 = 3. cm 2 s At θ = 0.50 θ s, the neopentane surface diffusion activation energy and preexponential decreased to E dif = 2.5 ± 0.2 kcal mol and D 0 = 9.5 × −4 ± 0.3 cm 2 s . The kinetics for tetramethylsilane and neopentane desorption from Ru(001) were also measured using temperature programmed desorption (TPD) techniques. The parameters for tetramethylsilane desorption from Ru(001) were E des = 12.3 ± 0.5 kcal mol and v des = 3 × 10 15 ± 0.1 s −1. Likewise, the parameters for neopentane desorption f E des = 10.7 ± 0.2 kcal mol and v des = 4 × 10 13 ± 0.1 s −1. The surface corrugation ratio, Ω, was defined as t diffusion and desorption activation energies, Ω = E dif E des . The surface corrugation ratios for tetramethylsilane and neopent low coverage on Ru(001) were Ω = 0.27 and Ω = 0.28, respectively. The coverage dependent surface diffusion of neopentane was analyzed assuming either attractive adsorbate-adsorbate interactions, neopentane decomposition or isomerization, or multiple site-hopping. The various experimental results suggested that the multiple site-hopping mechanism was the most likely explanation. Monte Carlo simulations demonstrated that the multiple site-hopping model provided an excellent fit to the coverage dependence of the neopentane surface diffusion coefficient when the jump length was ten sites.