A. A. Deckert

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.

Effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001)

The effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001) were studied using laser‐induced thermal desorption techniques. The surface mobility of hydrogen was measured as a function of CO surface coverage at 260 and 280 K. At both temperatures, the surface diffusion of hydrogen displayed an abrupt reduction at a coadsorbed CO coverage of ΘCO =0.12 ML. LEED studies revealed that a CO coverage of ΘCO =0.12 ML corresponded to the onset of the formation of ordered √3×√3 CO islands at 260 and 280 K. Temperature programmed desorption results demonstrated that a lateral repulsive interaction exists between hydrogen and carbon monoxide on Ru(001). Assuming that the lateral repulsive interaction leads to a hydrogen exclusion area around each CO admolecule, a hydrogen exclusion radius of rCO =2.2–2.7 A was determined. The lateral repulsive interaction would also result in hydrogen exclusion from the interior of ordered √3×√3 CO islands and provide an explanation for the hydrogen surf...

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.

Coverage-dependent surface diffusion expected from a multiple-site hopping model

Monte Carlo simulations have been employed to examine the coverage-dependent surface diffusion expected from a multiple-site hopping model. The Monte Carlo simulations modeled the prepare-and-probe laser-induced thermal desorption (LITD) experiment used to measure surface diffusion on macroscopic single-crystal surfaces. For multiple-site hopping, the simulated refilling curves for the diffusion experiment revealed a strong decrease in the surface diffusion coefficient versus coverage. This coverage-dependent decrease was more pronounced for larger jump lengths. As coverage increased from θθs = 0.01 to 1.00, the surface diffusion coefficient decreased by factors of 6 and 23 for jump lengths of r = 4 and r = 8 sites, respectively. The magnitude of the surface diffusion coefficient was also proportional to the square of the jump length at low coverage. Assuming a multiple-site hopping mechanism, the expected coverage dependence of the surface diffusion coefficient was calculated for various jump lengths. These results can be utilized to obtain a jump length for a system that displays a coverage-dependent surface diffusion coefficient. A jump length of sixteen sites was estimated for the coverage-dependent surface diffusion coefficient of neopentane on Ru(001).

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...

Effect of sulfur on the decomposition kinetics of methanol on Ru(001)

Abstract The effects of sulfur on the decomposition kinetics of methanol on Ru(001) were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. TPD studies showed that molecular desorption became more probable as a function of sulfur coverage. In addition, no new surface pathways were observed to compete with the primary pathway involving decomposition to carbon monoxide and hydrogen. Isothermal LITD studies of methanol decomposition were performed at methanol coverages of θ sat for various temperatures from 188 to 223 K. These LITD measurements revealed that the methanol decomposition rate was very dependent on sulfur coverage. Both the decomposition activation barrier and preexponential were observed to increase versus sulfur coverage. The coverage-dependent decomposition rate versus sulfur coverage was fit to k = v 0 10 ( cθ s ) exp[−( E 0 + aθ CO + bθ S )/ RT ] where E 0 = 6.9 kcal/mol, v 0 = 4.2×10 6 s − , a = 35 kcal/(mol ML), b = 27 kcal/(mol ML) and c = 13 ML −1 . The increase in the decomposition activation barrier and preexponential were qualitatively consistent with the electron withdrawing properties and long-ranged electronic effects of surface sulfur.