ML Of Sulfur Research Articles

Ab Initio Thermodynamics Examination of Sulfur Species Present on Rh, Ni, and Binary Rh–Ni Surfaces under Steam Reforming Reaction Conditions

The stable form of adsorbed sulfur species and their coverage were investigated on Rh, Ni, and Rh-Ni binary metal surfaces using density functional theory calculations and the ab initio thermodynamics framework. S adsorption, SO(x) (x = 1-4) adsorption, and metal sulfide formation were examined on Rh(111) and Ni(111) pure metals. Both Rh and Ni metals showed a preference for S surface adsorption rather than SO(x) adsorption under steam reforming conditions. The transition temperature from a clean surface (<(1)/(9) ML) to S adsorption was identified on Rh(111), Ni(111), Rh(1)Ni(2)(111), and Rh(2)Ni(1)(111) metals at various P(H(2))/P(H(2)S) ratios. Bimetallic Rh-Ni metals transition to a clean surface at lower temperatures than does the pure Rh metal. Whereas Rh is covered with (1)/(3) ML of sulfur under the reforming conditions of 4-100 ppm S and 800 °C, Rh(1)Ni(2) is covered with (1)/(9) ML of sulfur at the lower end of this range (4-33 ppm S). The possibility of sulfate formation on Rh catalysts was examined by considering higher oxygen pressures, a Rh(221) stepped surface, and the interface between a Rh(4) cluster and CeO(2)(111) surface. SO(x) surface species are stable only at high oxygen pressure or low temperatures outside those relevant to the steam reforming of hydrocarbons.

Effect of surface contamination on the UHV tribological behavior of the Cu(111)/Cu(111) interface

Friction and adhesion measurements have been made using two Cu(1 11) samples modified by the presence of atomic adsorbates. Copper surfaces with 10–15 A thick contaminant films resulting from exposure to the atmosphere exhibited a static friction coefficient of μs = 0.75 ± 0.18. A sharp increase in the friction coefficient was observed when the contaminant layer was reduced to submonolayer thickness by sputtering and for the cleanest Cu(111) surfaces studied the static friction coefficient was μs = 4.4 ± 1.3. To further investigate the tribological effect of submonolayer coverages of surface contamination adhesion experiments were performed using surfaces modified with sulfur adsorbed at coverages in the range θs = 0.0–0.39 ML (saturation). The adhesion coefficient of the clean surfaces (μad = 0.69 ± 0.20) was markedly reduced by the presence of 0.05 ML of sulfur and decreased monotonically with increasing sulfur coverage to a limiting value of μad = 0.26 ± 0.07 at θs = 0.39.

The determination of the decomposition mechanism and kinetics of methoxy on clean Ni(100) and the effects of coadsorbed hydrogen, CO, and sulfur by time-resolved Fourier transform infrared spectroscopy

Time-resolved Fourier transform infrared spectroscopy (TR-FTIR) was used to simultaneously examine the mechanism and kinetics of methoxy decomposition on Ni(100) under varying conditions. The use of TR-FTIR to obtain kinetic parameters was first validated by comparison to temperature-programmed desorption using annealed CO adlayers. Assuming a first-order reaction with a constant pre-exponential factor of 5.2 × 10 12 s −1, we find the activation energy for methoxy decomposition to CO and hydrogen to increase as the reaction proceeds according to 16.9 + 10 θ CO ± 0.3 kcal/mol, where θ CO is the coverage of CO product in monolayers, for initial methoxy coverages between 0.18 and 0.056 ML. The addition of 0.10 ± 0.02 ML of CO increases the energy for CH bond activation to 19.2 ± 0.4 kcal/mol. Coadsorbed sulfur was found to inhibit both methoxy formation and decomposition. The addition of 0.14 ± 0.02 ML of sulfur raises the activation energy for methoxy decomposition by 1 ± 0.4 kcal/mol, an effect similar in magnitude to coadsorbed CO. The preadsorption of 0.4 ML of hydrogen reduces the amount of methoxy formed from a methanol multilayer but does not raise the activation energy for methoxy decomposition. These results are compared to methoxy decomposition studies on Ni(111) and Ni(110).

Interaction of S with W(001)

Abstract Sulfur chemisorbed on W(001) has been studied by soft X-ray and ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy and work function measurements. Two distinct S 2p photoemission states have been identified as a function of sulfur coverage and sample annealing temperature. Below 0.5 ML, virtually all of the sulfur exists in a single state. Above 0.5 ML, a second state emerges at a lower binding energy. At 1 ML of sulfur, the two states have comparable intensity if the sample is annealed to 600 K. However, if the sample is annealed to 900 K nearly all of the sulfur resides in the low binding energy state. The high and low binding energy states have been associated with sulfur adsorbed in hollow and bridge sites, respectively. Possible reasons for the change in binding energy for sulfur adsorbed in the two different sites include an initial state (charge transfer) effect and a change in reference potential due to a change in height above the surface in the two types of adsorption site. The UPS, work function change and S(LMM) Auger lineshape results all indicate a change in S-W bonding at sulfur coverages above 0.5 ML.

Interaction of S with W(001)

Sulfur chemisorbed on W(001) has been studied by soft X-ray and ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy and work function measurements. Two distinct S 2p photoemission states have been identified as a function of sulfur coverage and sample annealing temperature. Below 0.5 ML, virtually all of the sulfur exists in a single state. Above 0.5 ML, a second state emerges at a lower binding energy. At 1 ML of sulfur, the two states have comparable intensity if the sample is annealed to 600 K. However, if the sample is annealed to 900 K nearly all of the sulfur resides in the low binding energy state. The high and low binding energy states have been associated with sulfur adsorbed in hollow and bridge sites, respectively. Possible reasons for the change in binding energy for sulfur adsorbed in the two different sites include an initial state (charge transfer) effect and a change in reference potential due to a change in height above the surface in the two types of adsorption site. The UPS, work function change and S(LMM) Auger lineshape results all indicate a change in S-W bonding at sulfur coverages above 0.5 ML.

ChemInform Abstract: Cyclopropane Hydrogenolysis on Clean and Chemically Modified Mo(100) Surfaces

Catalytic hydrogenolysis of cyclopropane (CP) to propane, ethane, and methane has been investigated over initially clean Mo(100) single-crystal surfaces and over surfaces chemically modified by adsorbed carbon, oxygen, and sulfur. Catalyst preparation was performed in ultrahigh vacuum with surface characterization by LEED and Auger. Catalytic hydrogenolysis under 1 atm pressure was monitored by gas chromatography. Initially clean Mo(100) surfaces and surfaces modified by 1 ML of atomic carbon or oxygen are active for single hydrogenolysis to propane and double hydrogenolysis to ethane and methane. The Mo(100) surface with 0.8 ML of sulfur is completely inactive. The reaction is first order in hydrogen and zero order in CP. The activation energy, determined from Arrhenius plots over the temperature range 300–523 K, is 37 ± 4 kJ/mol and is the same for all active surfaces. The product distribution (ca. 17% CH4, 17% C2H6, and 66% C3H8) is also the same for all active surfaces and is independent of temperature. The initially clean Mo(100) surface is ca. five times more active than either the carbon- or oxygen-modified surfaces which have approximately the same activity. Since the selectivity of CP hydrogenolysis has previously been shown to be sensitive to Mo chemical state, the data indicate that the same catalytic site is operative on all active surfaces. Comparison to literature data for supported Mo catalysts indicates that the observed selectivity is characteristic of low-valent Mo rather than of the oxide or carbide. Prior knowledge that adsorbed carbon and oxygen block fourfold hollow sites on the (100) crystal plane coupled with the higher activity observed for initially clean Mo(100) suggests (1) that the active sites for hydrogenolysis are open fourfold hollows and (2) that the open hollow sites are present as defects on the carbon- or oxygen-modified surfaces. The catalysts are inactive for hydrogenolysis of propane, indicating that CH4 and C2H6 are primary products. Likewise, propene is not an intermediate since propene conversion in H2 is exclusively to propane and metathesis products. The absence of the C4 products of metathesis from cyclopropane suggests that hydrogenolysis does not proceed via a metallacyclobutane surface intermediate. Therefore, the most likely mechanism involves a 1,3-diadsorbed surface intermediate.

Cyclopropane hydrogenolysis on clean and chemically modified Mo(100) surfaces

Catalytic hydrogenolysis of cyclopropane (CP) to propane, ethane, and methane has been investigated over initially clean Mo(100) single-crystal surfaces and over surfaces chemically modified by adsorbed carbon, oxygen, and sulfur. Catalyst preparation was performed in ultrahigh vacuum with surface characterization by LEED and Auger. Catalytic hydrogenolysis under 1 atm pressure was monitored by gas chromatography. Initially clean Mo(100) surfaces and surfaces modified by 1 ML of atomic carbon or oxygen are active for single hydrogenolysis to propane and double hydrogenolysis to ethane and methane. The Mo(100) surface with 0.8 ML of sulfur is completely inactive. The reaction is first order in hydrogen and zero order in CP. The activation energy, determined from Arrhenius plots over the temperature range 300–523 K, is 37 ± 4 kJ/mol and is the same for all active surfaces. The product distribution (ca. 17% CH 4, 17% C 2H 6, and 66% C 3H 8) is also the same for all active surfaces and is independent of temperature. The initially clean Mo(100) surface is ca. five times more active than either the carbon- or oxygen-modified surfaces which have approximately the same activity. Since the selectivity of CP hydrogenolysis has previously been shown to be sensitive to Mo chemical state, the data indicate that the same catalytic site is operative on all active surfaces. Comparison to literature data for supported Mo catalysts indicates that the observed selectivity is characteristic of low-valent Mo rather than of the oxide or carbide. Prior knowledge that adsorbed carbon and oxygen block fourfold hollow sites on the (100) crystal plane coupled with the higher activity observed for initially clean Mo(100) suggests (1) that the active sites for hydrogenolysis are open fourfold hollows and (2) that the open hollow sites are present as defects on the carbon- or oxygen-modified surfaces. The catalysts are inactive for hydrogenolysis of propane, indicating that CH 4 and C 2H 6 are primary products. Likewise, propene is not an intermediate since propene conversion in H 2 is exclusively to propane and metathesis products. The absence of the C 4 products of metathesis from cyclopropane suggests that hydrogenolysis does not proceed via a metallacyclobutane surface intermediate. Therefore, the most likely mechanism involves a 1,3-diadsorbed surface intermediate.

A hreels, tpd study of nitric oxide adsorption, desorption and reaction on clean and sulfur covered palladium (100)

The effect of sulfur poisoning on the adsorption, desorption and reactions of NO on the Pd(100) surface was studied using TPD, EELS, and LEED at sulfur precoverages ranging from zero to saturation (0.5 monolayer) and for NO coverages between zero and saturation (0.65 ML). NO adsorption is primarily associative. Simple steric blocking accounts for the decrease in NO saturation coverage with adsorbed sulfur. Correspondingly, a 25% decrease in the saturation coverage of NO was observed at a sulfur coverage of 0.1 ML, as expected for simple atop site blockage by sulfur. Thereafter the NO coverage declined more slowly, with detectable NO adsorption at θ S(sat). Three different vibrational modes were detected by EELS at 1510, 1720, and 1750 cm −1, which were assigned respectively to a bridge site, an atop site and a weakly bound NO. The 1720 cm −1 loss frequency is coverage dependent. NO adsorption on the clean surface gives rise to atop site occupation in a c(2 × 2) LEED structure at 300 K, and a p(4 × 2) structure at 410 K, corresponding to coverages of 0.5 and 0.25 ML, respectively. The p(4 × 2) structure was generally observed to form a single domain. In the presence of 0.03 ML of sulfur the p(4 × 2) LEED pattern is not observed at any temperature, but the c(2 × 2) LEED pattern persists up to at least 0.2 ML sulfur. Correspondingly, the atop to bridge site transfer was not observed, and the high temperature desorption state was eliminated by adsorbed sulfur, implying that the most strongly bound state is bridge bonded. Only small amounts of N 2 and N 2O are observed to desorb during heating of adsorbed NO, and oxygen is not observed as a product. At sulfur coverages above 0.10 ML these side reactions were suppressed.