High Sulfur Coverage Research Articles

Effect of H2S and HCl contaminants on nickel and ceria pattern anode solid oxide fuel cells

In this study, with the motivation of elucidating the effect of H2S and HCl on solid oxide fuel cell anodes, nickel and ceria pattern anodes are prepared on yttrium-stabilized zirconia electrolyte, and the effect of H2S and HCl on their performance is tested using electrochemical impedance spectroscopy. However, it has been found that while H2S adversely impacts both nickel and ceria, the poisoning caused is reversible for nickel and only partially reversible for ceria. Poisoning kinetics are similar and fast for both materials, while recovery kinetics are slower for ceria than nickel. High sulfur coverage is the rate-limiting factor inferred from the elementary kinetic modeling. Unlike H2S, the presence of HCl appeared to be favorable for electrochemical oxidation as the polarization resistance of both pattern electrode cells decreased upon feeding HCl contaminated hydrogen gas. Similar behavior has not been reported previously, and the conclusion regarding underlying mechanisms requires further investigation.

Degradation behavior of Ni-YSZ anode-supported solid oxide fuel cell (SOFC) as a function of H2S concentration

This study investigated the effect of H2S concentration (5, 10 and 50 ppm) on the degradation and performance of Ni-YSZ anode supported solid oxide fuel cells. When supplied with hydrogen fuel containing H2S, the cell voltage dropped rapidly, and with increasing H2S concentration, voltage drop % increased (due to higher sulfur coverage on the Ni surface) and saturated more rapidly. A high concentration (50 ppm) of H2S led to an additional, slow rate voltage loss. In all cases, cell performance did not completely recover even after being supplied with H2S-free hydrogen fuel, because of the incomplete desorption of sulfur from the Ni surface. After the performance tests, nickel sulfides were detected on the Ni surface by Raman spectra, which were produced by the reaction of the remaining adsorbed sulfur with Ni during the cooling process. This indicates that the formation of nickel sulfides was not responsible for the secondary voltage drop. SEM/EDS analyses combined with FIB revealed that the reason for the additional 2nd drop was Ni oxidation; at a high sulfur coverage ratio (50 ppm), the outer layer of the Ni particle was oxidized by oxygen ions transported from the electrolyte. This indicates that H2S concentration as well as current density is a critical factor for Ni oxidation, and gives rise to the second voltage drop (irreversible cell degradation). The present work showed that the degradation behavior and phenomenon can differ significantly depending on the concentration of H2S, i.e., permanent changes may or may not occur on the anode (such as Ni oxidation) depending upon H2S concentration.

Reactivity of CO on Sulfur-Passivated Graphene-Supported Platinum Nanocluster Arrays

Graphene-supported nanocluster arrays are interesting model systems to bridge the materials gap to real catalysts. This model system was used to investigate the poisoning of platinum particles by sulfur, which is crucial for many industrial processes. The adsorption and desorption of the probe molecule CO on sulfur-poisoned platinum nanoparticles on graphene were studied using high-resolution X-ray photoelectron spectroscopy. We observed a strong blocking of edge adsorption sites and a weaker blocking of on-top sites on facets; bridge sites are only blocked at higher sulfur coverages. During heating, CO diffuses to edge sites, thereby displacing the sulfur to the facets. Our results show interesting similarities to stepped platinum single crystals.

Sulfur poisoning of Ni/Gadolinium-doped ceria anodes: A long-term study outlining stable solid oxide fuel cell operation

This work presents an analysis of the long-term behavior of nickel/gadolinium-doped ceria (CGO) anode-based solid oxide fuel cells (SOFC) under sulfur poisoning conditions. A parameter study of sulfur-induced irreversible long-term degradation of commercial, high-performance single cells was carried out at 900 °C for different H2/N2/H2S fuel gas atmospheres, current densities and Ni/CGO anodes. The poisoning periods of the cells varied from 200 to 1500 h. The possibility of stable long-term Ni/CGO anode operation under sulfur exposure is established and the critical operating regime is outlined. Depending on the operating conditions, two degradation phenomena can be observed. Small degradation of the ohmic resistance was witnessed for sulfur exposure times of approximately 1000 h. Moreover, degradation of the anode charge transfer resistance was observed to be triggered by the combination of a small anodic potential step and high sulfur coverage on Ni. The microstructural evolution of altered Ni/CGO anodes was examined post-mortem by means of SEM and FIB/SEM, and is correlated to the anode performance degradation under critical operating conditions, establishing Ni depletion, porosity increase and a tripe phase boundary density decrease in the anode functional layer. It is shown that short-term sulfur poisoning behavior can be used to assess long-term stability.

Atomic Sulfur Anchored on Silicene, Phosphorene, and Borophene for Excellent Cycle Performance of Li-S Batteries.

Dissolution of intermediate lithium polysulfides (LiPS) is an inevitable obstacle for the solid sulfur-based cathode in Li-S batteries. For the first time, herein, atomic sulfur is incorporated into silicene, phosphorene, and borophene to intrinsically eliminate the dissolution of LiPS. The small molecular sulfur species are anchored on the silicene surface with stronger Si-S interaction than the P-S and B-S ones. Meanwhile, a high atomic sulfur coverage (63.1 wt %) is achieved in silicene and concomitantly stabilizes the silicene layer. For the S3-covered silicene, a high theoretical capacity of 857 mA h g-1 is achieved with slight dissolution of LiPS originated from the loss of interior S atoms that are not directly bound with silicene surface. By realizing the elemental S2 coverage on silicene with large surface area, the Li+ ions can react fast with the S2 species, leading to a high theoretical capacity of 891 mA h g-1 without dissolution and migration of the intermediate LiPS. Most interestingly, the discharge products of atomic layer of lithium sulfides on silicene surface exhibit completely different behaviors from the traditional discharge products of solid Li2S, which can function as effective adsorption and activation sites for the conversion of LiPS from long chain to short chain by accelerated redox reaction. The present study gains some key insights into how the atomic sulfur contributes to the intrinsic shuttle inhibition and offers a feasible way to design the atomic sulfur-based cathode materials of Li-S batteries with better electrochemical performance.

Density Functional Theory Study of Sulfur Poisoning on Nickel Anode in Solid Oxide Fuel Cells: Effects of Surface and Subsurface Sulfur Atoms

Impurities such as sulfur compounds cause performance degradation in solid oxide fuel cells (SOFCs). We have studied the sulfur poisoning mechanism on an SOFC nickel anode using density functional theory method, focusing on the effects of surface and subsurface sulfur atoms. The binding energy of surface sulfur atoms decreases with an increase in sulfur coverage on the nickel surface. Subsurface sulfur atoms become stable relative to surface sulfur atoms at high sulfur coverage. A subsurface sulfur phase also appears between sulfur adsorption phase and nickel sulfide phase in the calculated phase diagram for Ni–S systems. Influences of sulfur atoms on intermediate adsorbates during surface reactions are also investigated. Sulfur hinders the reaction by destabilizing the reaction intermediates on the nickel surface.

Thiophene hydrodesulfurization catalysis on supported Ru clusters: Mechanism and site requirements for hydrogenation and desulfurization pathways

Kinetic and isotopic methods were used to probe elementary steps and site requirements for thiophene hydrogenation and desulfurization on Ru metal clusters. Turnover rates for these reactions were unaffected by whether samples were treated in H 2 or H 2S to form metal and sulfide clusters, respectively, before reaction. These data, taken together with the rate and extent of sulfur removal from used samples during contact with H 2, indicate that active structures consist of Ru metal clusters saturated with chemisorbed sulfur at temperatures, pressures, and H 2S levels relevant to hydrodesulfurization catalysis. Turnover rates and isotopic data over a wide range of H 2, H 2S, and thiophene pressures are consistent with elementary steps that include quasi-equilibrated H 2 and H 2S heterolytic dissociation and thiophene binding with η 1(S) or η 4 coordination onto sulfur vacancies. We conclude that hydrogenation proceeds via addition of protons (H δ+, as –S–H δ+ from H 2 or H 2S dissociation) to η 4 thiophene species, while desulfurization involves C–S activation in η 1(S) species aided by H δ− species formed via H 2 dissociation. Reactant concentrations influence hydrogenation and desulfurization turnover rates to the same extent, suggesting that the involvement of similar active structures, consisting of vacancies on sulfur-covered Ru clusters. Smaller turnover rates and stronger H 2S inhibition on smaller Ru clusters for hydrogenation and desulfurization routes reflect the stronger sulfur binding and the smaller vacancy concentrations on small clusters, which contain exposed atoms with lower average coordination. A preference for η 1(S) over η 4 thiophene species at the higher sulfur coverages that prevail on smaller Ru clusters causes desulfurization and hydrogenation rate ratios to increase with decreasing cluster size. We conclude that hydrogenation and desulfurization routes require similar active sites and that weaker M–S bonds lead to higher concentrations of kinetically-relevant sulfur vacancies. These elementary steps and site requirements are likely to also prevail on metals and sulfides with M–S bond strengths similar or higher than Ru–S, for which vacancy sites are also present as minority species.

Hydrogenation of Toluene over Supported Pt and Pd Catalysts: Influence of Structural Factors on the Sulfur Tolerance

Pt, Pd, and PtPd metallic precursors are deposited on dealuminated FAU zeolites and on one amorphous silica–alumina. The catalytic activity is measured in the hydrogenation of toluene at 30 MPa total pressure, either in the absence or in the presence of sulfur (200 ppm in toluene). In the absence of sulfur, alloying Pt with Pd decreases the hydrogenation activity. In general, catalysts made from ammoniated precursors are the most active, with one remarkable exception. In the presence of sulfur, thioresistance is a function of reaction temperature, i.e., sulfur coverage of the metallic phase. Pd increases substantially the sulfur tolerance. The effect of support acidity on S tolerance is observed only at high sulfur coverage.

Surface structures of sulfur and carbon overlayers on Mo(100): A detailed analysis by automated tensor LEED

Detailed surface structures have been determined for three ordered sulfur overlayers and one ordered carbon overlayer on Mo(100), by automated tensor low-energy electron diffraction (LEED). The three ordered sulfur structures occur at 0.5, 0.67 and 0.75 monolayers (ML) and have c(2 × 2), c(3√2 × √2)R45° and c(4 × 2) periodicities with one, two and three atoms per unit cell, respectively, while the carbon structure has c(2 × 2) periodicity at 0.5 ML. In addition, a previously reported p(2 × 1) structure at high sulfur coverage is shown to actually be a poorly ordered version of the c(4 × 2) structure. In all four ordered structures the substrate is unreconstructed (unlike the clean surface) but relaxed down to its second layer, while the adatoms reside at or near fourfold hollow sites. In the c(2 × 2) structures, the smaller carbon penetrates deeper into the hollow sites than does sulfur: C approaches the second-layer Mo atoms closer than the first-layer Mo atoms, while S does the opposite. Except for the c(4 × 2) structure, the second-layer Mo atoms are pushed deeper into the substrate by adatoms directly above them. In the c(4 × 2) structure, large lateral displacements due to crowded S packing complicate this picture. In the two higher-coverage S structures, adatoms in adjacent hollow sites repel each other, forcing each other away from the hollow centers by 0.13 to 0.2 Å. Comparison with results of parallel STM experiments is made. A brief discussion is also given of a very high-coverage sulfur structure interpreted as MoS 2-like.

Hydrogen adsorption on sulfur and nitrogen covered Ru(001) surfaces

Hydrogen adsorption on nitrogen- and sulfur-modified Ru(001) surfaces was investigated using X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), low-energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS). On a S-covered Ru(001), two thermal desorption peaks of hydrogen at 135 and 265 K were clearly observed at sulfur coverages > 0.1, at which p(2 × 2)-S islands start to form. The intensities of these two peaks increase with θ S up to 0.21, and then decrease rapidly at higher sulfur coverages. At θ S = 0.33, corresponding to a complete ( 3 × 3 )R30° LEED pattern, hydrogen adsorption was fully suppressed. Compared with hydrogen desorption from H 2S decomposition on Ru(001), it was concluded that the hydrogen desorption peak at 135 K is correlated with the formation of p(2 × 2)-S unit cells. Our experiments further demonstrate that preadsorbed N-atoms also inhibit the adsorption of hydrogen with each N-adatom blocking four hydrogen adsorption sites.

Structure of the sulfur-adsorbed Cu 3Au(110) surface

Abstract The sulfur-adsorbed Cu 3 Au(110) surface (precise bulk composition is 71.7% Cu and 28.3% Au) has been studied with RHEED and Auger spectroscopy. The sulfur is segregated from inside of the bulk crystal. It has been found that the composition and the structure of the surface depend on the surface sulfur concentration. The ratio of Au and Cu concentration on the surface drops drastically in a uniform way with the increase of the sulfur coverage. At different sulfur coverages (2 × 2), (2 × 3), (2 × 4) and (3 × 2) structures and some incommensurate (3 × 2) ∗ structures have been observed. It is found that under low sulfur coverage the original (2 × 1) lattice at the first layer of the clean surface remains mostly unchanged, although some Au atoms are lost from the surface due to the sulfur adsorption. Under very high sulfur coverage the surface appears to be similar to the sulfur-adsorbed Cu(110) surface, in which the sulfur atoms are adsorbed at the hollow site of the substrate lattice. The linear relation of surface composition changes implies that the hollow site model is valid for all the surfaces with different sulfur concentration. The different reconstructions are formed by different periodical distributions of occupied and unoccupied sites, which depend on the sulfur coverage.

Structure of the sulfur-adsorbed Cu 3Au(110) surface

The sulfur-adsorbed Cu 3Au(110) surface (precise bulk composition is 71.7% Cu and 28.3% Au) has been studied with RHEED and Auger spectroscopy. The sulfur is segregated from inside of the bulk crystal. It has been found that the composition and the structure of the surface depend on the surface sulfur concentration. The ratio of Au and Cu concentration on the surface drops drastically in a uniform way with the increase of the sulfur coverage. At different sulfur coverages (2 × 2), (2 × 3), (2 × 4) and (3 × 2) structures and some incommensurate (3 × 2) ∗ structures have been observed. It is found that under low sulfur coverage the original (2 × 1) lattice at the first layer of the clean surface remains mostly unchanged, although some Au atoms are lost from the surface due to the sulfur adsorption. Under very high sulfur coverage the surface appears to be similar to the sulfur-adsorbed Cu(110) surface, in which the sulfur atoms are adsorbed at the hollow site of the substrate lattice. The linear relation of surface composition changes implies that the hollow site model is valid for all the surfaces with different sulfur concentration. The different reconstructions are formed by different periodical distributions of occupied and unoccupied sites, which depend on the sulfur coverage.

Effect of surface orientation and sulfur coverage on the oxidation of carbon on Ni surfaces

The reaction between adsorbed oxygen and segregated carbon on a cylindrical nickel single-crystal has been examined with Auger electron spectroscopy (AES) and temperature programmed desorption (TPD), for a range of surface orientation, oxygen exposure, and sulfur coverage. It was found that for small oxygen exposures, surface carbon and surface oxygen react during TPD to form a CO desorption peak, labeled β 1. The β 1 CO peak temperature and peak shape vary with orientation. At higher oxygen coverages, the CO desorption peak split into low-temperature and high-temperature peaks. The behavior of the β 1 CO desorption peak for large oxygen exposures is consistent with a model of the carbon-oxygen recombination reaction in which the morphologies of the initial carbon and oxygen phases change during oxygen exposure as a result of repulsive lateral interactions. High oxygen exposures result in the formation of large regions of contact between the two phases; this is believed to produce the low-temperature β 1 CO desorption peak. Small segregated-sulfur coverages, and low oxygen exposures, caused the β 1 CO peak to shift to lower temperatures for all orientations. Sulfur is believed to cause more frequent contact between carbon and oxygen for small oxygen exposures because it compresses the adsorbed oxygen and segregated carbon into the sulfur-free areas of the surface. Large coverages of segregated sulfur inhibited carbon segregation on some, and oxygen adsorption on most, orientations. The absence of reactant species explains the disappearance of the β 1 CO peak during TPD from orientations which had a high sulfur coverage.

Sulfur coverage effects on the reduction of dilute nitric oxide at platinum black gas diffusion electrodes

To study the effect of sulfur coverages on porous platinum black gas diffusion during the reduction of nitric oxide in a quantitative manner, electrodes were prepared with fractional sulfur coverages between 0.0 and1.0 (based on total platinum surface sites as measured by electrochemical hydrogen adsorption). The deposited sulfur was obtained through reduction of adsorbed SO 2 in a controlled system. The reduction of dilute streams of nitric oxide (3% in nitrogen) was performed in electrogenerative ( iegalvanic) cells at positive potentials. At fractional sulfur coverages above 0.2, cell current began to diminish, though NO conversion could be controlled at a high level (80–95% in a single pass). The product selectively during NO reduction was markedly effected at higher sulfur coverages (θ s > 0.2). Nitrous oxide formation was diminished, and ammonia production was greatly inhibited at these higher sulfur coverages. Hydroxylamine became the dominant product on sulfur-covered electrodes at lower voltages. Results from voltammetric experiments for NO reduction on a smooth bulk platinum electrode at comparable sulfur coverages did not reflect the results obtained with the platinum black-Teflon gas diffusion electrodes, illustrating some structure sensitivity. Some of the sulfur adsorbed on Pt-black gas diffusion electrodes is postulated to exist as sulfur doublets adsorbed between every other platinum atom.

The adsorption and coadsorption of sulfur and carbon monoxide on rhenium single crystal surfaces

The structure of sulfur overlayers on Re(0001) and Re(10 10) , and their effect upon CO chemisorption have been studied using Auger electron spectroscopy, thermal desorption spectroscopy, and low energy electron diffraction. In addition, calculations based upon a kinematic analysis of electron scattering have yielded possible two-dimensional (2D) real space structures based upon the observed LEED patterns for sulfur adsorbed on Re(0001). Four sulfur overlayer structures were seen on Re(0001): starting with a p(2×2) at 0.25 monolayers ( θ s = 0.25), a “ (3 3 ×3 3 ) R30° ” with missing spots at θ s = 0.35, a 3 1 1 3 at θ s = 0.4, and a (2 3 ×2 3 ) R30° at θ s = 0.5, with saturation occurring at approximately θ s ≅ 0.6. Two sulfur structures are observed on Re(10 10) , a p(2×2) at θ s = 0.5, and a p( l ×2) at θ s = 1.0. Desorption energies for sulfur on both surfaces were determined to range from 104 kcal/mol at low sulfur coverage to 85 kcal/mol near saturation. Comparisons with calculations based upon a kinematic analysis of the high sulfur coverage diffraction patterns on Re(0001) suggest that close proximity between adsorbed sulfur atoms may be responsible for lowering the adsorption energy. On Re(10 10) , an adsorption site for sulfur is proposed which involves first and second layer atoms. CO chemisorption was inhibited linearly by increasing the concentration of pre-adsorbed sulfur. A slight decrease in the CO desorption peak temperature at low sulfur coverage on Re(0001) was also observed. Comparison with other systems such as Ru(0001) enables us to conclude that the decrease in peak temperature is due to repulsive interactions between CO and S. On the other hand, pre-adsorbed sulfur on Re(10 10) blocks CO desorption peaks at low temperature, suggesting that pre-adsorbed sulfur blocks CO-CO repulsion.