Uniform Distribution Of Sulfur Research Articles

Sulfur-impregnated porous carbon for removal of mercuric chloride: optimization using RSM

This study relates to optimization of process conditions for sulfur impregnation process on to a virgin steam-activated commercial carbon, with the objective of maximizing the mercuric chloride adsorption. Process optimization was performed using response surface methodology adopting Box–Behnken design. The chosen key process parameters being impregnation temperature (IT), carbon to sulfur ratio (CSR), and impregnation time (ITE), with lower and upper bounds for process conditions being 250–600 °C IT, 0.5–4 CSR, and 15–45 min ITE. The optimum conditions were identified to be an IT of 544 °C, CSR of 0.53 and ITE of 43 min with the HgCl2 adsorption capacity of 85 mg/g. The optimized process conditions were further ensured with repeat runs with reproducible results within acceptable variation. Langmuir and Freundlich equilibrium adsorption isotherm models fitted well with the experimental data. The maximum monolayer adsorption capacity was found to be 294 mg/g which was quite high as compared with the maximum reported in literature. Further, the SEM–EDX analysis evidence the higher amount of sulfur and uniform distribution of sulfur over the surface of activated carbon.

Effect of sulfur loading on the desulfation chemistry of a commercial lean NOx trap catalyst

We investigate the effects of initial sulfur loadings on the desulfation chemistry and subsequent final activity of a commercial LNT catalyst. Identical total amounts of SO2 are applied to the samples, albeit with the frequency of desulfation varied. The results indicate that performance is better with less frequent desulfations. The greater the amount of sulfur deposited before desulfation, the more amount of SO2 evolution before H2S is observed during desulfation, which can be explained by two sequential reactions; initial conversion of sulfate to SO2, followed by the reduction of SO2 to H2S. After completing all sulfation/desulfation steps, the sample with only a single desulfation results in a fairly uniform sulfur distribution along the z-axis inside of the monolith. We expect that the results obtained in this study will provide useful information for optimizing regeneration strategies in vehicles that utilize the LNT technology.

High temperature desulfurization over nano-scale high surface area ceria for application in SOFC

In the present work, suitable absorbent material for high temperature desulfurization was investigated in order to apply internally in solid oxide fuel cells (SOFC). It was found that nano-scale high surface area CeO2 has useful desulfurization activity and enables efficient removal of H2S from feed gas between 500 to 850°C. In this range of temperature, compared to the conventional low surface area CeO2, 80–85% of H2S was removed by nano-scale high surface area CeO2, whereas only 30–32% of H2S was removed by conventional low surface area CeO2. According to the XRD studies, the product formed after desulfurization over nano-scale high surface area CeO2 was Ce2O2S. EDS mapping also suggested the uniform distribution of sulfur on the surface of CeO2. Regeneration experiments were then conducted by temperature programmed oxidation (TPO) experiment. Ce2O2S can be recovered to CeO2 after exposure in the oxidation condition at temperature above 600°C. It should be noted that SO2 is the product from this regeneration process. According to the SEM/EDS and XRD measurements, all Ce2O2S forming is converted to CeO2 after oxidative regeneration. As the final step, a deactivation model considering the concentration and temperature dependencies on the desulfurization activity of CeO2 was applied and the experimental results were fitted in this model for later application in the SOFC model.

Effect of Nafion® ionomer aggregation on the structure of the cathode catalyst layer of a DMFC

The effect of Nafion® ionomer aggregation in solution on the structure of the cathode catalyst layer of a direct methanol fuel cell (DMFC) was investigated by dynamic light scattering (DLS), laser particle sizer, electron probe microanalysis (EPMA) and scanning electron microscopy (SEM). The results showed that large aggregation particles were suppressed by the addition of NaOH to aqueous solutions of Nafion®, which led to a smaller agglomerate particle size distribution in the catalyst ink. The cathode catalyst layer made from the catalyst ink with NaOH addition showed a more uniform distribution of sulfur and Pt elements, a higher electrochemical active surface area (48% increase) and achieved a better performance in the DMFC than one made from the catalyst ink without NaOH addition.

Sulfur Impregnation on Activated Carbon Fibers through H2S Oxidation for Vapor Phase Mercury Removal

Sulfur was impregnated onto activated carbon fibers (ACFs) through H2S oxidation catalyzed by the sorbent surface in a fixed-bed reactor. By changing the temperature and duration of the sulfur impregnation process, ACFs with different sulfur contents were developed. Characterization of ACFs before and after sulfur impregnation was conducted by surface area analysis, energy dispersive X-ray analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy, and temperature programmed desorption. Vapor phase mercury adsorption experiments were carried out in a fixed-bed reactor. Sulfur was impregnated mainly as elemental sulfur and the amount of sulfur deposited on the ACF increased with an increase in impregnation temperature. Higher temperature leads to more uniform sulfur distribution inside the sorbent pores. The impregnation process can be explained by a combination of pore filling and monolayer adsorption, with the former mechanism predominating at low temperatures. In the absence of sulfur, the mercury adsorption capacity can be correlated with surface area and pore volume.

The influence of sorbent properties and reaction temperature on sorbent attrition, sulfur uptake, and particle sulfation pattern during fluidized-bed desulfurization

The influence of operating parameters such as sorbent properties and reaction temperature on sorbent attrition, sulfur uptake, and particle sulfation pattern during fluidized-bed desulfurization is assessed. Sulfur distribution throughout the particles is evaluated by means of a novel quantitative automated statistical procedure. With the aid of this technique, energy dispersive X-ray sulfur mappings of cross sections of sorbent particles are converted into sulfur distribution density functions, which can be directly related to the prevailing particles sulfation pattern. This procedure is applied to samples of three different sorbents (two limestones and one dolomite) sieved in three particle size ranges and batch-wise sulfated in a fluidized bed at three different temperatures. This analysis is complemented by parallel measurement of calcium conversion degree and elutriated calcium mass during the sulfation tests, as well as by visual inspection of scanning electron microscope micrographs of cross sections of spent sorbent particles discharged at the end of the tests. Experimental results show that the two limestones achieve a larger final sulfation degree than the dolomite. For one of the limestones it was found that the maximum sulfation degree was higher the smaller the particle size and the lower the bed temperature. Results of the statistical analysis on spent sorbent particles reveal that, for most samples, a core-shell sulfation pattern is established. Departure from the core-shell pattern is shown by the finest sorbent particles and by sorbent reacted at the lowest bed temperature investigated, in which a uniform sulfur distribution is achieved consistently with sulfation degree results. The influence of particle size and reaction temperature on sulfur uptake is interpreted in light of the significance of kinetic and intraparticle diffusional resistances assessed by the evaluation of particle Thiele moduli.

Effects of precipitates in Cu upon impact fracture: an ultra-high-vacuum study with local probe Scanning Auger/Electron Microscopy

In situ fracture under ultra-high vacuum (UHV) conditions of copper-alloys containing copper sulfide precipitates exhibits areas in the form of pits. The wide variety of morphologies depends significantly on the size of the existing precipitate. For large precipitates, the fractured surface reveals loops surrounded by step-terrace like structures with sulfur remaining preferentially on straight ledges as nanometer scale Auger images reveal in comparison with scanning electron images. For smaller precipitates (<3–4 μm), only well defined facets are formed having an almost uniform distribution of sulfur. Qualitatively the loop and facet formation could be explained due to accommodation of the Cu sulfide to the Cu matrix, leading for small precipitates to facet formation, and for large ones to step-terrace structures. Asymmetry in loop structures could be attributed to asymmetry of precipitate shape, while in some areas plasticity effects are also present. Finally, a comparison with distinctly different behavior in Cu–Bi alloys is made.

Decomposition of the sulfates of copper, iron (II), iron (III), nickel, and zinc: XPS, SEM, DRIFTS, XRD, and TGA study

The bulk and surface characteristics during decomposition of the transition metal sulfates of copper, iron (II), iron (III), nickel, and zinc are investigated utilizing various spectroscopic techniques. An oxidized form of sulfur was detected on the surface during decomposition of all metal sulfate samples, except zinc sulfate. Surface characteristics were not necessarily representative of the bulk characteristics. Oxy-sulfate was observed with copper sulfate only. Lower decomposition temperatures were observed in vacuum as compared to those at atmospheric pressure. Uniform sulfur distribution was observed across sample cross sections. Analysis consisted of Scanning electron microscopy/X-ray microanalysis, X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis, and X-ray diffraction.

Effect of Sulfur Impregnation Method on Activated Carbon Uptake of Gas-Phase Mercury

The dynamics of granular activated carbon (GAC) adsorbers for the uptake of gas-phase mercury was evaluated as a function of temperature, influent mercury concentration, and empty bed contact time. Sulfur-impregnated carbons exhibited enhanced mercury removal efficiency over virgin carbon due to the formation of mercuric sulfide on the carbon surface. The effect of the sulfur impregnation method on mercury removal efficiency was examined through experi ments conducted on commercially available sulfur-impregnated carbon (HGR) and carbon impregnated with sulfur in our laboratory (BPL-S). Although HGR and BPL-S possess similar sulfur contents, BPL-S is impregnated at a higher temperature, which promotes a more uniform distribution of sulfur in the GAC pore structure. At low influent mercury concentrations and low temperatures, HGR and BPL-S performed similarly in the removal of mercury gas. However, as the temperature was increased above the melting point of sulfur, the performance of HGR deteriorated significantly, while the performance of BPL-S slightly improved. At high influent mercury concentrations, HGR performed better than BPL-S, regardless of temperature. For both HGR and BPL-S, the observed dynamic mercury adsorptive capacities were far below the capacities predicted by the stoichiometry of mercuric sulfide formation. In HGR carbon the sulfur is very accessible, but ag glomeration that occurs at high temperatures causes the sulfur to be relatively unreactive. In BPL-S carbon, on the other hand, the sulfur remains in a highly reactive form, but its location deep in the internal pores makes it relatively inaccessible and prone to blockage by HgS formation.

Uptake of Elemental Mercury Vapors by Activated Carbons.

The adsorptive capacities of virgin and sulfur-impregnated activated carbons (GAC) for gas-phase mercury were evaluated as a function of temperature and influent mercury concentration. The virgin activated carbon showed little adsorptive capacity, especially at temperatures above 90 °C. The isothermal representation of the adsorptive capacity for virgin GAC exhibited a semi-logarithmic relationship at 50 °C, 90 °C, and 140 °C. The pronounced effect of temperature on the adsorptive capacity evidences a physical adsorption mechanism between the mercury and virgin GAC. Sulfur-impregnated activated carbons exhibited enhanced mercury removal efficiency over the non-impregnated varieties, due to formation of mercuric sulfide on the carbon surface. This chemisorption process is enhanced by increased temperatures between 25 °C and 90 °C, yielding increased removal efficiency of elemental mercury. However, at 140 °C a decrease in adsorptive capacity occurs, indicating reduced formation of mercuric sulfide. The method used for impregnating GAC with sulfur had a pronounced effect on mercury removal capacity. The chemical bonding of sulfur at 600 °C provides a more uniform distribution of sulfur throughout the GAC pore structure than is achieved by conventional condensation techniques, yielding improved performance.

Characterization of lacustrine iron sulfide particles with proton‐induced X‐ray emission

Black particles, collected by filtration (1.2‐µm pore size) from the anoxic waters of a soft‐water lake, were examined by a scanning proton microprobe which permitted quantitative elemental analysis by proton‐induced X‐ray emission (PIXE) and Rutherford backscattering (RBS). There was a uniform distribution of sulfur across the filter, but Fe, and to a lesser extent, Mn, was localized in ∼5‐µm‐diameter clusters. Elemental analysis with 1‐µm‐diameter beams revealed that the Fe clusters were mainly comprised of iron oxides. Iron sulfide material not in the Fe clusters had stoichiometric proportions of Fe1.0S0.60P0.60Ca0.24K0.14. Although a purely biogenic origin for P, Ca, and K cannot be ruled out, the composition is consistent with the particles originating as authigenic iron oxides which react with sulfide as they sink through the water column. The iron sulfide particles are richer in Cu (4,000 ppm) and Zn (6,000 ppm) than the iron oxides, suggesting that these elements are also concentrated as their insoluble sulfides. The coexistence of iron oxides and sulfides indicates that either the supply of sulfide is limiting or that some iron oxide particles are unreactive.