Metal sulfiding, thermodynamics

Sulfur compounds, whether organic or inorganic in nature, cause sulfidation in susceptible materials. The sulfide film, which forms on the surface of much con-stmction materials at low temperatures, becomes friable and melts at higher temperatures. The presence of molten sulfides (especially nickel sulfide) on a metal surface promotes the rapid conversion to metal sulfides at temperatures where these sulfides are thermodynamically stable. High-alloy materials such as 25% Cr, 20% Ni alloys are widely used, but these represent a compromise between sulfidation resistance and mechanical properties. Aluminum and similar diffusion coatings can be of use. 

In order to get a catalytic cycle it is necessary that the metal sulfide intermediate can react with hydrogen to form the reduced metal complex (or compound) and H2S. For highly electropositive metals (non-noble metals) this is not possible for thermodynamic reasons. The co-ordination chemistry and the oxidative addition reactions that were reported mainly involved metals such as ruthenium, iridium, platinum, and rhodium. 

A catalytic example of C-S bond breakage in benzothiophene has been reported by Bianchini [47], A catalytic desulfurisation was not yet achieved at the time as this is thermodynamically not feasible at such mild temperatures because of the relative stability of metal sulfides formed. Bianchini used a water-soluble catalyst in a two-phase system of heptane-methanol/water mixtures in which the product 2-ethylthiophenol is extracted into the basic aqueous layer containing NaOH. Figure 2.43 gives the reaction scheme and the catalyst. The 16-electron species Na(sulfos)RhH is suggested to be the catalyst. Note that a hydrodesulfurisation has not yet been achieved in this reaction because a thiol is the product. Under more forcing conditions the formation of H2S has been observed for various systems. 

The interconversions of the numerous aggregates in the Fe/S/SR systems have been successfully described from measurements of electronic spectra and H NMR.141>212 It is evident that some clusters such as [Fe4S4(SR)4]2 have thermodynamic advantage similarly, in the Cd/S/SPh system, [S4Cdl0(SPh)16 4 predominates, and M2+ + H2S + PEt3 reactions yield stable aggregates [SJMm(PEt3)tt]2+ instead of metal sulfides.52-250... 

Routes to prepare SH- complexes have, in principle, to overcome two major problems the formation of thermodynamically preferred metal sulfides and, via partial ligand elimination, the formation of polymeric species. This requires both a rigid stereochemistry brought about by bulky ligands and sufficient electron density on the metal centers. 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

The thermodynamic properties of bulk metal sulfides are fairly well documented 13-20). Multiple phases similar to those of Ni are observed for Fe, Co, and other metal-sulfur systems of catalytic interest. Figure 2 shows... 

Three primary reactions were observed between aqueous chlorine and the base-metal sulfides. Elemental sulfur was produced during the reaction with chalcocite, bornite, and covellite. A rapid oxidation of the pyrrhotite, pyrite, and arsenopyrite to the sulfate form was observed. The formation of sulfur monochloride was indicated with sphalerite, galena (under most conditions), and chalcopyrite. The ratio of sulfur to sulfate was close to what could be expected if the sulfur monochloride hydrolyzed to form sulfur. Thermodynamic considerations indicated sulfate formation as the primary product. 

In the same context, the metal sulfide substrate is a component of an Me-S-HjO system which may be far from equilibrium and which may contain a number of metastable molecular and ionic species. While the availability of the relevant thermodynamic data, especially for metastable systems, is far from complete, Eh-pH diagrams have been constructed in connection with... 

The complex nature of the HDS and HDN problems requires a broad, transdisciplinary approach in order to try to answer the most varied questions related to these important classes of reactions. The key issues include the practical aspects related to process and product engineering, a precise knowledge of the nature and the composition of petroleum and of refinery fractions, and the thermodynamics and detailed kinetics of the different processes involved. Also, a number of more fundamental solid-state and surface chemistry considerations regarding the preparation, the characterization, and the resulting properties of HDS and HDN catalysts, as well as the complicated reaction mechanisms involved for the various important families of substrates, need to be understood in depth. Even though some very impressive achievements have been disclosed over the last 30-40 years, it seems that some of the major new discoveries desired today may have been held back by the lack of a better understanding of some key issues. Of particular importance are the nature and the structure of HDS-HDN active sites on metal sulfide catalysts, and the intimate details of the elementary reactions implicated in the commonly accepted catalytic schemes. 

Consider the thermodynamics of metal sulfiding on the basis of the [H2S/H2] ratios calculated for synthetic auto exhaust gas (Table I) for an equilibrium reaction system involving the water-gas shift reaction, reactions between H2, CO, and oxygen, the conversion of S02 to H2S, and metal sulfiding. This reaction system model is as follows ... 

The author expresses his gratitude to his colleague Howard D. Simpson for his contributions to the thermodynamic model and calculations which were used to predict catalyst metal sulfiding in a synthetic exhaust gas atmosphere. 

RAN/BRE] De Ranter, C., Breckpot, R., The electrochemical and thermodynamic behaviour of transition metal sulfides. Bull. Soc. Chim. Belg., 78, (1969), 503-522. Cited on page 337. 

Fig. 10.16 Predicted dissociation pressures of various metal sulfides based on the thermodynamic data of Barin [148] and Taylor [146]...

Although other thermodynamically stable metal sulfides, such as those of Zn, Pb, Ni, and Hg, should form in wetland soils and sediment, they seldom form simply because these metals are not present in sufficient concentrations in most sediments to allow the formation of precipitated metal sulfides. However, at sites that are contaminated with heavy metals, metal sulfide formation is an important process in reducing heavy metal availability. 

---