Reduction and sulfidation

The performance of a supported metal or metal sulfide catalyst depends on the details of its preparation and pretreatraent. For petroleum refining applications, these catalysts are activated by reduction and/or sulfidation of an oxide precursor. The amount of the catalytic component converted to the active ase cind the dispersion of the active component are important factors in determining the catalytic performance of these materials. This investigation examines the process of reduction and sulfidation on unsupported 00 04 and silica-supported CO3O4 catalysts with different C03O4 dispersions. The C03O4 particle sizes were determined with electron microscopy. X-ray diffraction (XRD), emd... 

Metal contaminants can in some cases be immobilized in situ by oxidation or reduction, or precipitated by reaction with sulfide. Sulfate reducing bacteria are sometimes stimulated to produce sulfide, or a sulfur-bearing compound such as calcium polysulfide can be injected into the subsurface as a reductant and sulfide source. In certain cases where the contamination poses little immediate threat, it can safely be left to attenuate naturally (e.g., Brady et al., 1998), a procedure known as monitored natural attenuation. 

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Measured rates of microbial oxidation of sulfide in lakes range from 0 to over 100,000 mmol/m2 per year (Table IV). These rates, which are comparable to measured rates of sulfate reduction (Table I), suggest that microbial oxidation of sulfide is capable of supplying sulfate at rates needed to sustain sulfate reduction. The majority of measurements are for photosynthetic bacteria in the water column. Symbiotic sulfate reduction and sulfide oxidation are known to occur and lead to dynamic cycling of S within anaerobic water... 

Pretreatment In order to compare results of catalyst treatments, e.g., reduction and sulfidation, precise conditions should be given, such as temperature, time, reagents, and partial pressures. Also, whether the catalyst has been treated under static, recirculation, or flow conditions is important. Finally, the desorption conditions after treatment should be specified as some adsorbed species are only slowly removed and could contribute to misleading results if sufficient time is not allowed for their removal. This is particularly true of H2S, H20, and other polar compounds as well as organic heterocyclics. 

This article covers the activation of supported catalysts. It examines the formation of the final catalysts, as achieved by three categories of treatment calcination, reduction, and sulfidation (more precisely reduction sulfidation). Detailed studies of these three categories of processes did not lead to an equally detailed analysis. Outside very general (phenomenological) observations, very little is known concerning the calcination of supported precursors. By contrast, some particular systems have been studied in some detail in reduction (e.g. NiO/support for activation to Ni/support) or sulfidation (Coj(Moj,Or/y-Al203 to sulfides). 

Since hydrotreating catalysts arc usually used in the presence of H2 and H2S, it is important to understand the influence of phosphorus on the reduction and sulfidation of the supported metal-oxo-specics. It is also important to know whether the phosphates arc sensitive to such treatments. In this section, activation of the catalysts is discussed on the basis of XPS, TPR, and temperature-programmed sulfidation results. Note that the bulk of the alumina support is not chemically modified by the reduction-sulfidation treatments. However, some hydrogen-reactive species and surface SH groups have already been detected on it (31, 70). 

Dalsgaard T. and Bak F. (1992) Effect of acetylene on nitrous oxide reduction and sulfide oxidation in batch and gradient cultures of Thiobacillus denitrificans. Appl. Environ. Microbiol. 58, 1601-1608. 

The ideal environment for bacterial sulfate reduction and sulfide mineral formation is anoxic, non-toxic, and near neutral pH. It is also characterized by adequate organic matter and mesophilic temperatures. In the absence of such an ideal environment, it must be remembered that microoi anisms are diversified and adaptable. For example, bacteria may function in a microenvironment that differs distinctly from the surrounding macroenvironment. Trudinger et al. (1972) concluded that few geochemical factors by themselves can prevent sulfate reduction and sulfide mineralization. 

Bacterial sulfate reduction and sulfide oxidation in groundwaters of sulfur deposits (After Ivanov, 1964)... 

In addition to sulfate reduction and sulfide oxidation, isotopic fractionation data have been obtained for other conversions in the sulfur cycle. With the exception of sulfate and sulfite reduction by Saccharomyces cerevisiae under varied concentrations of added panthothenate (McCready et al.,... 

REDUCTION AND SULFIDATION PROPERTIES OF IRON SPECIES IN FE-TREATED Y-ZEOLITES FOR HYDROCRACKING CATALYSTS... 

The objective of the present study is to examine the reduction and sulfidation properties of the Fe-treated Y-zeolite by using temperature-programmed reduction (TPR) and sulfiding (TPS), in order to quantitatively determine the active species and support the production control of the commercial Fe-treated Y-zeolite catalysts. An interpretation of the reduction and sulfidation mechanisms of several types of Fe-treated Y-zeolites is presented. 

Organic matter is oxidized in the suboxic zone through iron or manganese reduction. This may be a direct oxidation by metal reducing bacteria or an indirect oxidation via sulfate reduction and sulfide oxidation. Does it matter for the end products which pathway dominates ... 

The concentrations of Cu, Zn, Fig, Cd, and other metals in sediment pore water are controlled by the solubility of metal sulfides in the reduced zone where sulfate reduction and sulfide formation is dominant. Change in redox condition upon burial results in a system where the growth of diagenetic copper, zinc, and arsenic sulfides control the distribution and partitioning of metals and arsenic in the sediment. In polluted sediments, sulfate reduction plays a key role in the formation and retention of sedimentary S as metal sulfides. The majority of the iron and manganese in coastal lake sediments is associated with sulfidic forms, especially in saline area high in available sulfate that is reduced to sulfide. 

How does sulfate reduction and sulfide production influence heavy metal availability ... 

Mittleman and Danko [45] determined that sulfur cycling, that is, sulfate reduction and sulfide oxidation, by microorganisms... 

The elemental sulfur becomes colloidally stable in Sg molecules but can also add to sulfite and form thiosulfate S + S (0)3 S - S (0)3 (S203 ), which also loses the sulfur via disproportionation into H2S and SO4 . In hydrometeors and interfacial waters, however, sulfur reacts with oxygen (reactions 5.265 and 5.262) via SO2 to form hydrogen sulfite HSO3. Thiosulfate is an important intermediate in biological sulfur chemistry from both sulfate reduction and sulfide oxidation. Many hypothetical so-called lower sulfuric acids (Table 5.19) might appear as intermediates or in the form of radicals (such as SOH, HSO, HSO2, HSS and HS as seen from the structure formulas) in the oxidation chain from sulfide to sulfate ... 

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