Reduction-sulfidation

Cyclohcxcnyllithium by Reductive Sulfide Cleavage with Lithium Naphthalenide32 ... 

Figure 10. Comparison of calculated weight change curve with the experimental data for simultaneous reduction-sulfidation.

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). 

Figure 22. Comparison of the sulfur content of a commercial C0M0 catalyst (Procatalyse HR 306) activated by simultaneous reduction-sulfidation (RS) or by reduction followed by sulfidation (R + RS ). (a) Simultaneous reduction-sulfidation by a H2S/H2 mixture (15% H2S by volume). Except for the experiment at 573 K, the samples were first reacted for 4h at 673 K, then progressively heated to the reaction temperature indicated in the figure and maintained at that temperature for 4h. (b) Reduction at the indicated temperature for 4 h, followed by sulfidation with the H2S/ H2 mixture at 673 K for 4h (RSs, subscript denotes standard temperature) [138].

The final step in the preparation of a catalyst is the transformation of the precursor to the active phase (e.g. metal, sulfide) which is sometimes called activation. The activation may be a reduction (e.g. by H2), reduction-sulfidation (e.g. by H2 + H2S), dehydroxyla-tion (e.g. by removal of H20 from zeolite) or oxidation (e.g. by O2). The details of the activation process must be stated (e.g. partial pressure and purity of gas, method of heating and its rate, gas reacting flow rale, sample size). 

On the other hand, morphological changes can occur on the minute scale [8], or transformations during activation of a catalyst (temperature-programmed reaction/ reduction/sulfidation), ignition of a reaction, or oscillations can even occur on the subsecond timescale [11, 14, 15],... 

Photoelectrochemical Cells with Polycrystalline Cadmium Sulfide as Photoanodes Light induced HER driven with reductants (sulfide, EDTA) in a photoelectro chemical cell containing three compartments. Also see Refs. 496 and 497 by same group. 495... 

Garrels et al. (1973) believe that the BIF must have been formed in restricted basins in semi-enclosed water bodies, periodically communicating with the ocean via channels or over bars. Deposition of silica occurred mainly during evaporation, but deposition of iron was complex and is explained both by oxidation (hematite facies) and by evaporation (silicate and carbonate facies) and sulfate reduction (sulfide facies). It is suggested that the spatial distribution of the sedimentary facies of the BIF will correspond to the well-known scheme of James (1954), but to explain the similarity of banding in the face of different causes of precipitation of the iron raises difficulties. 

How does the composition of the metal surface change during pyrolysis Surface reactions that have been identified on Incoloy 800 surfaces include oxidation, reduction, sulfidation, desulfidation, and coke formation (14). Do increased concentrations of nickel and chromium ever occur in the surface in view of the fact that iron is incorporated into the coke Tsai and Albright (14) found increased iron concentrations on inner surfaces of tubes used for pyrolyses. 

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). 

Walter and co-workers (Walter and Burton, 1990 Walter et al., 1993 Ku et al., 1999) have made extensive efforts to demonstrate the importance of dissolution of calcium carbonate in shallow-water carbonate sediments. Up to — 50% carbonate dissolution can be driven by the sulfate reduction-sulfide oxidation process. In calcium carbonate-rich sediments there is often a lack of reactive iron to produce iron sulfide minerals. The sulfide that is produced by sulfate reduction can only be buried in dissolved form in pore waters, oxidized, or can diffuse out of the sediments. In most carbonate-rich sediments the oxidative process strongly dominates the fate of sulfide. Figure 6 (Walter et al., 1993) shows the strong relationship that generally occurs in the carbonate muds of Florida Bay between total carbon dioxide, excess dissolved calcium (calcium at a concentration above that predicted from salinity), and the amount of sulfate that has been reduced. It is noteworthy that the burrowed banks show much more extensive increase in calcium than the other mud banks. This is in good agreement with the observations of Aller and Rude (1988) that in Long Island Sound siliciclas-tic sediments an increased bioturbation leads to increased sulfide oxidation and carbonate dissolution. 

FT-IR spectroscopy. Self-supporting wafers (4-8 mg/cm ) were analyzed by FT-IR (Fourier Transform Infe-Red) spectroscopy (mod. 2000, PERKIN-ELMER). All treatments were performed in situ in a pyrex cell with KBr windows and spectra were registered at 20°C. The following experiments were performed (i) acidity determination by pyridine adsorption-desorption method [11]. After evacuation (300°C, Ih, 10 mbar) the catalyst was contacted by 10 mbar pyridine vapor at 200°C for Ih desorptions were performed for Ih at 200 and 300°C in dynamic vacuum (ii) hydroxyl evolution after different in situ treatments (reduction, sulfidation, H2 adsorption). The wafer was heated at 300°C in a dynamic vacuum, contacted by a known amount of H2S/H2 mixture for 1 hr at the same temperature, reduced for 2 h at 300°C in H2 flow and cleaned in N2 flow at the same temperature. After this pretreatment, H2 chemisorption was performed at different temperatiues (20-100°C), followed by pyridine adsorption-desorption at 200°C. 

Catalytic hydrogenation Bechamp reduction Sulfide reduction... 

Gas-solid surface reactions take place in the last two steps oxidation during the air-calcination step resulting to oxidic catalysts and reduction/sulfidation in the activation step resulting in metallic/sulfided catalysts. Sintering... 

Typically, sulfiding is done through the injection of H2S or an organic sulfide into the unit at the end of reduction. Sulfiding is continued to specified sulfur level is reached or until sulfur is no longer adsorbed by the catalyst and is detected at the outlet of the catalyst bed. 

When hemiacetal formation is followed by reduction, sulfides can be obtained from aldehydes in a one-pot manner under mild reaction conditions (eq 15). Vinyl sulfides can be directly prepared from silyl enol ethers upon treatment with PhSTMS in the presence of equimolar amounts of BF3 -OEt2 (eqs 16 and 17). Ketones under the same reaction conditions give only trace amount of vinyl sulfides. 

Although all high-temperature corrosion is considered oxidation, there are other terms that are also encountered, such as oxidation-reduction, sulfidation, fuel ash corrosion, carburization, and nitridation, to name a few. 

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