J. L. Brand

Surface diffusion of hydrogen on Ru(001) studied using laser‐induced thermal desorption

The surface diffusion coefficient for hydrogen on Ru(001) at low coverage was measured using laser‐induced thermal desorption techniques. In the temperature range between 260 and 330 K, the diffusion coefficients displayed Arrhenius behavior with an activation barrier Ediff=4.0±0.5 kcal and a preexponential factor D0=6.3×10−4 cm2/s. Agreement between the experimental and theoretical parameters suggests that hydrogen diffuses on the surface by moving from a threefold site to a neighboring threefold site via a twofold site. Surface contaminants such as carbon and oxygen were observed to produce dramatic effects on the hydrogen surface diffusion rate.

Surface diffusion of n-alkanes on Ru(001)

The surface diffusion of n‐alkanes on Ru(001) was measured using laser‐induced thermal desorption (LITD) techniques. The surface diffusion coefficients for propane, n‐butane, n‐pentane, and n‐hexane all displayed Arrhenius behavior. The surface diffusion activation energies increased linearly with carbon chain length from Edif =3.0±0.1 kcal/mol for propane to Edif =4.8±0.2 kcal/mol for n‐hexane. In contrast, the surface diffusion preexponentials remained nearly constant at D0 ≂0.15 cm2 /s. Measurements performed at different coverages also revealed that the surface diffusion coefficients were coverage‐independent for all the n‐alkanes on Ru(001). The surface corrugation ratio Ω was defined as the ratio of the diffusion activation energy to the desorption activation energy, Ω=Edif /Edes . The surface corrugation ratio was observed to be remarkably constant at Ω≂0.3 for all the n‐alkanes. This constant corrugation ratio indicated a linear scaling between the diffusion activation energy and the desorption ac...

Surface diffusion of carbon monoxide on Ru(001) studied using laser-induced thermal desorption

The surface diffusion of CO on Ru(001) was measured using laser-induced thermal desorption techniques. The surface diffusion coefficients displayed a strong dependence on the CO coverage. For CO coverages below θ = 0.33 ML, the surface diffusion coefficient at 290 K was approximately constant at D 1× 10−8 pcm2s. As the CO coverage increased from θ = 0.33 to θ = 0.58 ML, the surface diffusion coefficient at 290 K increased dramatically from 1 × 10t8 to 1 × 10t6cm2s. The surface diffusion coefficients at 250 K also exhibited a similar increase versus CO coverage. At various temperatures and CO coverages, the surface diffusion coefficients displayed Arrhenius behavior. For θ = 0.27 ML, the Arrhenius parameters were Edif = 11kcalmol and D0 = 0.38 pcm2s in the temperature range between 300 and 370 K. For θ 0.45 ML, Edif and D0 both decreased as a function of increasing CO coverage at temperatures between 210 and 290 K. At θ = 0.45 ML, the Arrhenius parameters were Edif = 8.0 kcalmol and D0 = 0.28 cm2s. At θ = 0.58 ML, the Arrhenius parameters were Edif = 6.2 kcalmol and D0 = 0.06 cm2s. The coverage dependence of CO surface diffusion on Ru(001) was consistent with strong repulsive nearest-neighbor CO interactions. A repulsive CO-CO pairwise interaction energy of Ω = −1.4 kcalmol was obtained using a model for D(θ) derived from the quasichemical approximation.

Surface diffusion of hydrogen on sulfur-covered Ru(001) surfaces studied using laser-induced thermal desorption

Abstract The effects of surface sulfur coverage on the surface diffusion of hydrogen on Ru(001) were studied using laser-induced thermal desorption techniques. Measurements at T = 270 and 300 K revealed that the surface mobility of hydrogen decreased rapidly as a function of increasing surface sulfur coverage. At T = 300 K the hydrogen surface diffusion coefficient dropped approximately a factor of 30 from 8.5 × 10 −7 cm 2 /s to less than 3 × 10 −8 cm 2 /s as the surface sulfur coverage increased from θ s = 0 to θ s = 0.25 monolayer. The reduction of hydrogen surface mobility versus surface sulfur coverage was compared to predictions from a site-blocking model. In order to reproduce the rapid decrease in hydrogen surface mobility as a function of sulfur coverage, the site-blocking model demonstrated that each sulfur adatom must block ten hydrogen adsorption sites. The effect of sulfur on hydrogen surface mobility was attributed to both steric and long-range electronic effects. Reduced surface mobility versus surface sulfur coverage may help to explain the effect of sulfur as a poison of catalytic reactions.

Coverage dependence of the surface diffusion coefficient for hydrogen on Ru(001)

The coverage dependence of the surface diffusion coefficient for hydrogen on Ru(001) was studied using laser-induced thermal desorption(LITD) techniques. The LITD measurements were performed for a wide range of initial hydrogen surface coverages in the temperature range 230–270 K. At a given temperature, the surface diffusion coefficient was found to be constant as a function of coverage. The absence of coverage dependence in the surface diffusion coefficient indicates that adsorbate-adsorbate interactions between adsorbed hydrogen atoms on Ru(001) are negligible. This observation suggests that the coverage-dependent features observed in work function, high resolution electron energy loss spectroscopy and possibly temperature programmed desorption studies of hydrogen on Ru(001) are associated with two inequivalent hydrogen sites rather than repulsive lateral hydrogen interactions. Monte Carlo simulations reveal that the surface diffusion coefficient should be coverage independent on a surface with two inequivalent sites in the absence of adsorbate-adsorbate interactions.

Surface diffusion of hydrogen on carbon‐covered Ru(001) surfaces studied using laser‐induced thermal desorption

The effects of surface carbon on the surface diffusion of hydrogen on Ru(001) were studied using laser‐induced thermal desorption techniques. The surface mobility of hydrogen decreased by approximately a factor of 60 as a function of increasing surface carbon coverage from θC=0 to θC=0.42 monolayer at T=300 K. The observed reduction of hydrogen surface mobility vs surface carbon coverage was consistent with the trapping of hydrogen atoms by carbide species on the Ru(001) surface. A simple trapping model suggests that the potential energy wells of the carbon trap sites are ΔE>2.4 kcal/mol deeper than regular hydrogen adsorption sites. This estimate is also consistent with the results of Monte Carlo simulations.

Isotope effect in the surface diffusion of hydrogen and deuterium on Ru(001)

The surface diffusion of hydrogen and deuterium on Ru(001) at low coverage was studied using laser-induced thermal desorption techniques. In the temperature range from 260 to 300 K, the surface diffusion coefficients could be expressed in Arrhenius form as D H =6.9×10 −4 cm 2 /s exp(−3.6±0.5 kcal mol −1 / RT ) and D D =4.6×10 −4 cm 2 /s exp(−4.1±0.5 kcal mol −1 / RT ) for hydrogen and deuterium, respectively. The observed isotope effect was somewhat larger than predicted by simple transition state theory but was within the limits of experimental error. Quantum mechanical tunneling can be playing, at best, only a minor role in the surface migration of hydrogen on Ru(001) at these temperatures.

The decomposition of methanol on Ru(001) studied using laser induced thermal desorption

The decomposition reaction of methanol on Ru(001) was studied using laser induced thermal desorption (LITD). The LITD studies, combined with temperature programmed desorption and Auger electron spectroscopy measurements, allowed absolute product yields for the three competing surface pathways to be determined over the entire range of chemisorbed methanol coverages at a heating rate of β=2.6 K/s. At the lowest methanol coverages of θ≤0.07θs, where θs is the surface coverage of a saturated chemisorbed layer, all the methanol reacted between 220–280 K. This methanol decomposition reaction yielded desorption‐limited H2 and CO as reaction products. At higher coverages, molecular desorption and the second methanol decomposition reaction involving C–O bond breakage became increasingly important. At θ=θs, 50% of the initial methanol coverage desorbed, 24% produced H2 and CO and 26% left C on the surface. Isothermal LITD kinetic measurements were carried out at low methanol coverages of θ≤0.07θs at various tempera...

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.

CO desorption kinetics from clean and sulfur‐covered Ru(001) surfaces

The desorption of CO from clean and sulfur‐covered Ru(001) surfaces was studied using laser‐induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. CO was observed to desorb from clean Ru(001) with coverage‐dependent kinetics. The isothermal desorption of CO was monitored with LITD measurements. The rates for CO desorption were determined using a simple Pade approximant method to evaluate coverage‐dependent kinetic parameters. On the clean Ru(001) surface, the desorption activation energy and preexponential dropped sharply from Ed=34 kcal/mol and νd=5×1015s−1 for ΘCO 0.33 ML. The clean Ru(001) surface results agreed very well with earlier studies of CO desorption. The presence of surface sulfur shifted the TPD peaks for CO on Ru(001) to lower temperatures. Likewise, isothermal LITD measurements revealed that the CO desorption parameters at ΘCO=0.06 ML decreased from Ed=36 kcal/mol and νd=1×1016s−1 to Ed=22 kcal/mol and ν...