Sulfur adsorption thermodynamics, metal

THERMODYNAMICS OF SULFUR ADSORPTION ON METAL SURFACES IN WATER... 

Thermodynamics of Sulfur Adsorption on Metal Surfaces in Water... 

A. Thermodynamics of Sulfur Adsorption 1. Affinity of Sulfur for Metals... 

Only a limited amount of thermodynamic data are available for 2-D sulfides of metals other than nickel. Data reported for Ag (57), Co (252), Cu (255), Fe (252, 154), Mo (255, 255), Ru (256), and Pt (84, 85) indicate that the heats of sulfur adsorption are generally 20-40% larger than the heats of formation of the most stable bulk sulfides. Indeed, Benard et al. (255) have shown a linear correlation between the heats of adsorption of sulfur and the... 

As demonstrated in the case of H2S on Ni, thermodynamic data such as AG° and A//ad are of great value in understanding the extent and reversibility of sulfur poisioning. Clearly, more thermodynamic data for adsorption of sulfur are needed for important metals of catalytic interest such as Co, Fe, Mo, Pd, Pt, Re, Rh, Ru, etc. Since sulfur adsorption on Co, Pd, Rh, and Ru... 

Oudar et al. (112) have studied the influence of sulfur on the hydrogenation of 1,3-butadiene and H2-D2 equilibration over Pt(110). The rates decayed linearly with sulfur coverage, so that each sulfur atom poisons one dissociation site for hydrogen without influencing the activation energies or mechanism. The authors established the first isotherm for sulfur adsorption under actual catalytic reaction conditions. The adsorbed hydrocarbons influenced the equilibrium coverage of sulfur on the Pt surface. The thermodynamics of adsorbed sulfur on several metal single crystal surfaces have been presented by Bernard et al. (114). 

Most data on the thermodynamics of sulfur adsorption were derived from adsorption isotherms achieved in H2S-H2 gas mixtures [28,29]. At high temperature, sulfur is reversibly adsorbed on a metal surfece in a range of H2S partial pressures below that required for the formation of the most stable sulfide. At a fixed temperature the... 

In this equation, B stands for adsorption coefficients and C for concentrations. The thermodynamic control imposes the use of very high pressures, low space velocities, and very active catalysts. For the specific case of aromatic saturation and in the presence of H2S or any other sulfur compound, NiW is the recommended catalyst [66], However, in those cases where a precious metal catalyst may be used then, it becomes the preferred choice [67],... 

Adsorption of sulfur on monocrystalline, polycrystalline, and supported metallic catalysts has been extensively studied. For all adsorption reactions, thermodynamics states that A//llds = Ea - Ed (where A//ads is the enthalpy of adsorption, Ea is the activation energy of adsorption, and Ed... 

Previously we mentioned studies of adsorption of sulfur introduced as H2S and the thermodynamics of the sulfur-metal bond. But a great variety of sulfur compounds can be present in the feeds of industrial hydrogenations. The variation of the poisoning effect as a function of the sulfur compound type has been recognized for a long time, but is not yet fully understood. It is thus of the greatest interest to study the adsorption of different types of sulfur compounds. 

The Pt-Ir catalyst exhibits thioresistance similar to monometallic Pt catalysts. Although thermodynamic calculations (ref. 21) and adsorption studies (ref. 22) have shown that Ir would be more deactivated by sulfur than Pt, it has to be emphasized that all conmercial Pt-Ir reforming catalysts should contain only a small amount of Ir (0.04S in catalyst II-E) as this metal presents a high activity for hydrogenolysis reactions. The present results are in line with previous studies (ref. 3) showing that commercial Pt-Ir catalysts are slightly more thioresistant than monometallic Pt catalysts. 

In the first sections of this chapter, the basic effects of adsorbed sulfur on anodic dissolution, on passivation, and on the breakdown of passive films are presented. In a subsequent section the effects of alloyed elements (essentially Cr and Mo) are presented and the way in which they can counteract the detrimental influence of sulfur is emphasized. In the next section, implications of the mechanisms of sulfur-induced corrosion for different areas of practical importance are given, with connections to the related chapters of this book. In the last section of this chapter, the thermodynamic predictions of the conditions of adsorption of sulfur on metal surfaces in water are given. 

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