Sulfur Coverage Research Articles

Acetylene and Ethylene Adsorption during Floating Fe Catalyst Formation at the Onset of Carbon Nanotube Growth and the Effect of Sulfur Poisoning: a DFT Study.

Here, we investigated the adsorption of acetylene and ethylene on iron clusters and nanoparticles, which is a crucial aspect in the nascent phase of carbon nanotube growth by floating catalyst chemical vapor deposition (FCCVD). The effect of sulfur on adsorption was also studied due to its indispensable role in the process and its commonly known impact on metal catalyst poisoning. We performed systematic density functional theory (DFT) computations, considering numerous adsorption configurations and iron particles of various sizes (Fen, n = 3-10, 13, 55). We found that acetylene binds significantly more strongly than ethylene and prefers different adsorption sites. The presence of sulfur decreased the adsorption strength only in the immediate proximity of the adsorbate, suggesting that the effect of sulfur is mainly of steric origin while electronic effects play only a minor role. Higher sulfur coverage of the catalyst surface significantly weakened the binding of acetylene or ethylene. To further investigate this interaction, Bader's atoms in molecules (AIM) analysis and charge density difference (CDD) were used, which showed electron transfer from iron clusters or nanoparticles to the adsorbate molecules. The charge transfer exhibited a decreasing trend as sulfur coverage increased. These results can also contribute to the understanding of other iron-based catalytic processes involving hydrocarbons and sulfur, such as the Fischer-Tropsch synthesis.

Sulfur and phosphorus co-doped FeCoNiCrMn high-entropy alloys as efficient sulfion oxidation reaction catalysts enabling self-powered asymmetric seawater electrolysis

Seawater electrolysis (SWE) represents a promising approach to green hydrogen (H2) production but currently faces substantial challenges such as the interference of chlorine chemistry and high energy consumption. In this work, we demonstrate that by replacing the energy-demanding oxygen evolution reaction (OER) with the sulfion oxidation reaction (SOR) and by implementing the concept of bipolar membrane (BPM) electrolysis in an acid-base dual electrolyte system, not only can the notorious chlorine evolution reaction (CER) be completely circumvented, but the energy consumption of SWE be significantly reduced. To do so, we develop a sulfur and phosphorus co-doped FeCoNiCrMn high entropy alloy (HEA-SP) catalyst, which shows good electrocatalytic performance for the SOR in alkaline-saline water. This can be attributed to the abundant lattice defects and strains in HEA-SP, leading to a high density of active sites and an optimized electronic structure favorable for the SOR. Moreover, density functional theory calculations and in situ Raman spectroscopy characterization reveal the crucial role of imperfect sulfur coverage on the HEA in facilitating the formation of Sx clusters during the SOR. Using the HEA-SP as anode catalysts, the SOR-assisted SWE only needs electrical energy of 0.253 kWh to produce one cubic meter of H2 at 100 mA cm−2, in the presence of a BPM. Impressively, chlorine-free H2 production from seawater and upgrading of sulfions to valuable sulfur can occur simultaneously and spontaneously at 10 mA cm−2, highlighting the great potential of the HEA-SP catalysts and the asymmetric cell design to enable energy- and cost-effective seawater electrolysis.

Theoretical insight into the rearrangement of sulfur atoms on the Ni- and Cu-doped MoS2 S-edge induced by hydrogen adsorption under HDS reaction conditions.

Density functional theory (DFT) calculations and an atomistic thermodynamic approach were used to study the geometric rearrangement of sulfur atoms on the Ni- and Cu-doped MoS2 S-edge upon hydrogen adsorption. Under HDS conditions, thermodynamically stable hydrogenated structures were identified as SH groups on the undoped S-edge with 100% sulfur coverage, on the Ni-doped S-edge with 50% sulfur coverage and on the Cu-doped S-edge with 25% sulfur coverage. It was found that the rearrangement of the S atoms is essential to reach the most stable state at the edge for the undoped and Ni-doped S-edge. Hydrogen adsorption on the Ni-doped S-edge leads to the greatest amount of S rearrangement (ΔERearrang = 0.93 eV/H2). Our results suggest that under the reaction conditions, the H2 dissociative adsorption process is strongly coupled to the rearrangement of the sulfur atoms. By examining the differential hydrogen adsorption energy on the most stable edge structures, we found a plausible explanation for the trend in the hydrogenation activity of the doped edges. Our results suggest that Ni enhances the hydrogenation activity of the S-edge by decreasing the S-H bond strength, while Cu poisons it by increasing the S-H bond strength.

Self-Constructing 100% Water-Resistant Metal Sulfides through In Situ Acid Etching for Effective Elemental Mercury (Hg0) Capture.

Metal sulfides (MSs) can efficiently entrap thiophilic components, such as elemental mercury (Hg0), and realize environmental remediation. However, there is still a critical problem challenging the extensive application of MSs in related areas, i.e., how to self-regulate their water (H2O) resistance without complexing the sorbent preparation procedure. This work for the first time developed an in situ acid-etching method that self-engineered the water affinity of MSs through changing the interfacial interaction between MSs and Hg0/H2O. The introduction of abundant, undercoordinated sulfur onto the sorbent surface was the primary reason accounting for the significantly improved H2O resistance. The high surface coverage of undercoordinated sulfur induced the formation of polysulfur chains (Sx2-) that stabilized Hg0 via a bridging bond and repelled H2O, attributed to the favorable electron configurations. These properties made the surface of MSs highly hydrophobic and increased the adsorption selectivity toward Hg0 over H2O. The MSs exhibited 100% H2O resistance even in the presence of 20% H2O, which is much higher than the H2O concentration under most practical scenarios. From these perspectives, this work for the first time overcame the detrimental effects of H2O on MSs through a self-regulating way that is scalable and negligibly complexes the sorbent preparation pathway. The highly water-resistant and cost-effective MSs as prepared can serve as efficient Hg0 removal from industrial flue gas in the future.

Elemental sulfur corrosion of nickel-based alloy 825 in CO2–H2S-containing environment at high temperature and high pressure

The elemental sulfur corrosion resistance and corrosion mechanisms of nickel-based alloy 825 at high temperature and high pressure are still missing. This work investigates the elemental sulfur corrosion of nickel-based alloy 825 in CO2–H2S-containing environment at high temperature and high pressure using weight loss measurement, surface characterization and in-situ electrochemical techniques including electrochemical noise, electrochemical impedance spectroscopy as well as polarization curves. The results show that in the absence of elemental sulfur nickel-based alloy 825 hardly corrodes in the environment containing 0.5 MPa CO2 and 1.6 MPa H2S at 100 °C, since the passive film on the substrate could offer an enough protective performance. However, with the increasing elemental sulfur content, the corrosion of nickel-based alloy 825 becomes more and more obvious. Under the condition of full coverage of elemental sulfur, the uniform corrosion rate of the specimen can be as high as 0.327 mm/y. Moreover, under such a condition, serious localized corrosion could also be observed on the specimen surface. Based on results of electrochemical measurement and surface analysis, it is considered that underneath the elemental sulfur deposit the formation of a strong acidic environment generated from the hydrolysis of elemental sulfur largely destroys the passive film, leading to serious corrosion. Therefore, nickel-based alloy 825 used in high sulfur-containing environment should pay close attention to safety risk from elemental sulfur corrosion. The results of this work could provide a reference for the material selection in high sulfur natural gas field.

Influence of coverage on adsorbate diffusion measurements at electrode surfaces by in situ linear optical diffraction.

In situ linear optical diffraction is a new method for studies of surface mass transport in electrochemical environments that is based on the equilibration of coverage gratings in an adlayer on the electrode surface. We, here, discuss the temporal evolution of the diffraction intensity on the basis of experimental data for sulfur adsorbates on Pt(111) electrodes in 0.1M H2SO4 and simulations of the time-dependent diffusion profiles. At low and medium sulfur coverage, the decay of the signal exhibits two time scales, which can be explained by the influence of coverage-dependent diffusion rates on the evolution of gratings with large coverage modulation. At high coverage, a further ultra-slow decay process or even a complete termination of the decay is observed, which we attribute to the presence of high-density, ordered, adlayer phases with low sulfur mobility. These results provide insight into the approaches required for extracting quantitative surface transport rates from linear optical diffraction measurements.

Deactivation of a steam reformer catalyst in chemical looping hydrogen systems: experiments and modeling

The utilization of real producer gases such as raw biogas or gasified wood for chemical looping hydrogen production implies the introduction of harmful contaminants into the process. Hydrogen sulfide represents one of the most challenging trace gases in the reformer steam iron cycle. The aim of the present work was an in-depth investigation of steam reforming with pure methane and synthetic biogas contaminated with selective concentrations of 1, 5 and 10 ppm of hydrogen sulfide. To validate the experimental data, the fixed-bed reactor system was modeled as one-dimensional pseudo-homogeneous plug flow reactor by an adapted Maxted model. In a preliminary thermodynamic study, the dry equilibrium composition was determined within a deviation of 4% for steam methane reforming (SMR) and 2% for synthetic biogas reforming compared to the experimental results. The impact of hydrogen sulfide on the reactivity of the catalyst was characterized by the residual methane conversion. The deactivation rate and extent is directly proportional to the concentration of H2S, as higher hydrogen sulfide concentrations lead to a faster deactivation and lower residual methane conversion. A comparison of the methane conversion as a function of sulfur coverage between experimental and simulated data showed good agreement. The predicted results are within <10% deviation for SMR and synthetic biogas reforming, except for sulfur coverages between 0.6 and 0.8. The temperature in the catalyst bed was monitored throughout the deactivation process to gather additional information about the reaction behavior. It was possible to visualize the shift of the reforming reaction front towards the bottom of the reactor caused by catalyst deactivation. The impact of sulfur chemisorption on the morphology of the steam reformer catalyst was analyzed by scanning electron microscope (SEM/EDS) and Brunnauer–Emmet–Teller techniques. SEM patterns clearly indicated the presence of sulfur as a sort of dust on the surface of the catalyst, which was confirmed by EDS analysis with a sulfur concentration of 0.04 wt%.

Optical reflectance studies on the oxidation of chemisorbed sulfur at the Pt(111) electrode

The oxidation of chemisorbed sulfur on Pt(111) was studied in 0.1 M H2SO4 using combined electrochemical and in situ optical reflectance. The adsorbed sulfur adlayer, prepared by immersion of the Pt electrode in Na2S solution, was successively removed by potential cycling between 0.05 and 1.22 V vs. RHE. Correlation of the electrochemical data with the parallel measured changes in optical reflectance provides insight into the intermediate steps of the sulfur oxidation process. In addition to features related to sulfur oxidation and Pt oxide formation and reduction, two transient cathodic current peaks at rather negative potentials (0.19 and 0.41 V) are observed the first cycles, which can be assigned to intermediates of the sulfur oxidation process. The sulfur coverage oxidized in each cycle is initially large but then decreases rapidly, becoming negligibly small after about 10 cycles. Furthermore, a nonlinear change of the reflectance with decreasing sulfur coverage is observed, which suggests changes in the nature of the S-Pt bond with coverage.

Determination of subsidy and emission control coverage under competition and cooperation of China-Europe Railway Express and liner shipping

Silk Road Economic Belt (SREB) and Maritime Silk Road (MSR) provide important avenues for cargo transport between China and Europe. This paper addresses the issues of the subsidy for China-Europe Railway Express (CERE) on the SREB and the sulfur emission control for liner shipping on the MSR from a perspective of building a community of shared future for mankind. Two welfare maximization models are proposed for determining the optimal subsidy and the optimal sulfur emission control coverage within which the sulfur in ships’ fuel oil is required to be less than 0.1%. One considers the competition between two transport modes (CERE and liner shipping), in which their own net profit is maximized. The other concerns the cooperation between two modes, in which their total net profit is maximized. The properties of the two models are analytically explored and compared. The results show that intermodal competition outperforms intermodal cooperation in terms of social welfare, consumer surplus, and environmental pollution, though at the cost of carrier profit and total subsidy from the regulator. The regulator may efficiently control the adverse impacts of an increase in the fuel oil price or in the marginal cost of air pollution on the social welfare of the system through adjusting the CERE subsidy and the sulfur emission control coverage.

Understandings about functionalized porous carbon via scanning transmission x-ray microscopy (STXM) for high sulfur utilization in lithium-sulfur batteries

Functionalized porous carbons are necessary for the reversible operation of lithium-sulfur batteries (LSBs), however, their appropriate utilization and physicochemical states in the sulfur cathode are still unclear. Although numerous studies regarding distribution of sulfur in the pores of the carbon framework in the cathode have been reported, sulfur distribution and oxygen coverage on the carbon surface after sulfur infiltration have not been clearly investigated. In this study, we scrutinized the role of the pores in the oxygen-functionalized mesoporous carbon nanoparticle (O-MCN). Furthermore, we elaborated sulfur distribution, oxygen coverage, and pore utilization of O-MCN under the three different sulfur infiltration process. The soft x-ray scanning transmission x-ray microscopy (STXM) was used for the first time in the LSB system. The STXM enabled probing of local distribution of sulfur and the chemical nature of sulfur and oxygen in O-MCNs. Our findings provide a comprehensive understanding about how the sulfur infiltration process impacts the sulfur utilization during the redox reaction of sulfur in the oxygen-functionalized mesoporous carbon.

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.

First-principles study of the oxygen evolution reaction on Ni3Fe-layered double hydroxides surfaces with varying sulfur coverage

Ni3Fe-layered double hydroxides (Ni3Fe-LDHs) are promising catalysts for the oxygen evolution reaction (OER). Recently, Ni3Fe (oxy) hydroxides incorporating S have demonstrated enhanced OER activity, but the effects of varying the S coverage have not been studied. The present work used first-principles calculations to explore the OER performance of Ni3Fe-LDH specimens with 1/4, 1/2 and 3/4 S monolayer coverage on their (110) surfaces. The performances of the three Ni3Fe-LDH with different S coverages were compared to that with a clean surface at both Ni and Fe sites. The overpotential values calculated for Ni sites decreased in the order of clean > 1/2 monolayer > 3/4 monolayer > 1/4 monolayer, while the values for Fe sites decreased in the order of clean > 3/4 monolayer > 1/2 monolayer > 1/4 monolayer. In addition, the overpotentials at Fe sites were determined to be lower than those at Ni sites for the same coverage. In the case of a 1/4 S monolayer coverage, the overpotential at Fe sites was as low as 153 mV. Comparison of the Bader charge values and partial density of states data between the O covered and S covered surfaces indicated that a surface with a 1/4 S monolayer readily adsorbed the reaction intermediate OH.

Uniform coverage of high-loading sulfur on cross-linked carbon nanofibers for high reaction kinetics in Li–S batteries with low electrolyte/sulfur ratio

Aggregation of high loading sulfur on host materials at low carbon/sulfur ratio results in the limited S↔Li2S reaction kinetics, which is a bottleneck for the development of lithium sulfur batteries with high energy densities.

Graphene-substrate decoupling by S segregation. A LEEM/LEED study

The growth of graphene on Ni(110) is studied with low energy electron microscopy, low energy electron diffraction (LEED) and associated methods at several sulfur coverages obtained by surface segregation and compared with growth on the clean surface. On the clean surface graphene grows nearly epitaxial with small azimuthal alignment fluctuations. The segregated S reduces the interaction between graphene and Ni, which results in the formation of several azimuthal orientations. The reduced film-substrate interaction is evident in the details of the LEED pattern, in the work function change and the energy dependence of the specular reflected intensity. This study clarifies the differences between earlier studies of graphene growth on Ni(110) and suggests using impurity surface segregation as means for decoupling of graphene from the substrate.

Sulfur dioxide interaction with thin iron oxide films on low-index surfaces of iron

The adsorption of sulfur dioxide (SO2) at room temperature on iron oxide surfaces has been studied using core-level photoelectron spectroscopy. A variety of iron oxides, from adsorbate structure to thin film, with different stoichiometries and terminations were grown on common low-index single crystal iron surfaces to model a range of structures in the initial stages of atmospheric oxidation. This permits well-controlled comparisons of differences and similarities in SO2 interaction.Both non-dissociative and dissociative adsorption of SO2 were observed, to different relative and absolute coverages, on all surfaces. The only identified non-dissociated species is SO4. For all surfaces, at least some amounts of atomic sulfur are observed while only for the submonolayer adsorbate structure, also tentative dimerization of sulfur into S2 or formation of S-Osurface is observed. For the two oxides terminated by a complete well-ordered oxygen layer, the absolute sulfur coverage is low. The complete oxygen layer terminated oxide film exhibiting a moiré-pattern had a saturation coverage of one SO4 species per surface super cell, which indicates a tentative preferred adsorption site. The surface showing the highest SO4 formation is the Fe3O4(100) thin film surface with a corrugated row structure. The SO4 formation is suggested between surface oxygen atoms within the same row.

Experimental studies of catalyst deactivation due to carbon and sulphur during CO2 reforming of CH4 over Ni washcoated monolith in the presence of H2S

AbstractThis study presents the CO2 reforming of CH4 over Ni coated monolith catalyst at 800°C and 101.325 kPa. The high CH4 to CO2 ratio employed in this study is similar to the CH4:CO2 ratio of &gt;1 found in biogas. Cordierite monolith samples (0.258 channels per m2) washcoated with alumina are used for the experimental purpose. The study considers the combined deactivation effect due to sulphur poisoning and fouling due to carbon deposition. Four different cases with respect to the introduction and removal of H2S are considered. The rate of deactivation due to simultaneous carbon deposition and sulphur poisoning is much faster than the individual poisoning processes. The catalyst shows almost stable operation for 6 h without the presence of in the feed stream. From the conversion studies, it appears that the pre‐treatment of catalyst samples with H2S leads to negligible sulphur coverage. The sulphur poisoning effect is also found to be reversible.

How Stable Are 2H-MoS2 Edges under Hydrogen Evolution Reaction Conditions?

Transition metal dichalcogenides (TMDs), especially MoS 2 , have emerged as a promising class of electrocatalysts for the production of H 2 via the hydrogen evolution reaction (HER) in acidic conditions. The edges of MoS 2 are known for their HER activity, but their precise atomistic nature and stability under HER conditions is not yet known. In contrast to other typical uses of MoS 2 as a catalyst, under HER there is no external source of sulfur. Therefore, the sulfidation of the edges can only decrease under operating conditions and the thermodynamics of the process are somewhat illdefined. Our results suggest that the 50%S S-edge may be active for HER via the Volmer-Tafel mechanism and is, despite a high H coverage, stable with respect to H 2 S release. At the 50%S Mo-edge, the adsorbed hydrogen opens the way for H 2 S release, leading to the 0%S Mo-edge, which was previously investigated and found to be HER active. HER being a water-based process, we also considered the e↵ect of the presence of H 2 O and the in-situ formation of OH. For the 50%S Mo-edge, H 2 O is only very weakly adsorbed and OH formation is unfavorable. Nevertheless, OH assists the loss of sulfur coverage, leading to OH-based HER active sites. In contrast, OH is strongly adsorbed on the 50%S S-edge. By explicitly considering the electrochemical potential using grand-canonical density functional theory, we unveil that the Volmer-Heyrovsky mechanism on sulfur sites is still accessible in the presence of surface OH at the 50%S S-edge. However, the 50%S S-edge is found to be mildly unstable with respect to H 2 S in the presence of water/OH. Hence, we suggest that the 50%S S-edge evolves over time towards a 0%S S-edge, covered by surface OH that will block permanently the active sites.

Experimental Study on Sulfur Deactivation and Regeneration of Ni-Based Catalyst in Dry Reforming of Biogas

The dry reforming of methane (DRM) using biogas and a Ni-based catalyst for syngas production was studied experimentally in this study under the presence of H2S. Using the nonpoisoned DRM performance as a comparison basis, it was found that the catalyst deactivation by the sulfur chemisorption onto the catalyst surface depends on both reaction temperature and time. With low reaction temperatures, a complete sulfur coverage was resulted and could not be regenerated. With higher reaction temperatures, the H2S coverage decreased, and the poisoned catalysts could be regenerated. The experimental results also indicated that a catalyst deactivation could not be avoided by using the bi-reforming of methane by adding O2 or H2O simultaneously in the reactant due to the stronger chemisorption capability of sulfur. The catalyst could only be regenerated after it was poisoned. The experimental results indicated that the high-temperature oxidation process was the most effective process for regenerating the poisoned catalyst.

Catalytic activity of synthesized 2D MoS2/graphene nanohybrids for the hydrodesulfurization of SRLGO: experimental and DFT study.

Hydrodesulfurization (HDS) of straight run light gas oil (SRLGO) using novel highly active two-dimensional (2D) MoS2/graphene (G) nanohybrid catalysts is a precursor technology for the production of clean heavy fuel. The aim of this research is the synthesis of 2D MoS2/G nanohybrid catalysts by use of exfoliation method from commercial bulky MoS2 and graphite using hydrothermal ball milling system, which is a low-cost, high-yield, and scalable method. These nanohybrid catalysts were characterized by XRD, Raman spectroscopy, XPS, SEM, TEM, STEM, ICP, BET surface, TPR, and TPD techniques. Also, catalytic activities of 2D MoS2/G nanohybrid catalysts were evaluated under different operating conditions such as temperature, pressure, LHSV, and H2/Feed (SRLGO) ratio in the HDS reaction. The conversion of the HDS of SRLGO with 14000 ppm sulfur showed a considerably higher activity of 2D MoS2/G nanohybrid catalyst (99.95% HDS efficiency) compared with the Co-Mo/γAl2O3 as a commercial catalyst (90% HDS efficiency) in the operation condition (340 °C, 40 bars, LHSV: 1 h-1and H2/oil: 600 NL L-1) which is economically valuable. Using density functional theory calculations, the detailed mechanism of the HDS process over MoS2/G catalyst was explored. It was found that sulfur coverage on the Mo edge of MoS2 plays an important role in the hydrogenation of sulfur components.

Sulfur functions as the activity centers for high-capacity lithium ion batteries in S- and O-bifunctionalized MXenes: A density functional theory (DFT) study

MXenes materials, a new family of two-dimensional carbides or nitrides, are promising electrode materials with high capacity and high charging rate. Its electrochemical performance depends strongly on the surface functions. A series of bifunctionalized Ti3C2 MXenes with different coverages of sulfur and oxygen functions were designed in this study, and the electrochemical properties and lithium (Li) storage performance of these bifunctionalized MXenes in different solvents were investigated based on the density functional theory. Ti3C2O4/3S2/3 possessed the best performance with relatively low barrier and high capacity. Single sulfur function surrounded by oxygen functions was activated as an adsorption center under the combined influence by the Li adsorption layer and the MXenes surface. Simulations in this study revealed the mechanism of sulfur substitution to Li storage, as well as provided a solid theoretical support to the modification of MXene materials.