H2S/H2 ratio

The interaction of H2S, organic sulfides, and other sulfur compounds may involve a number of consecutive steps including reversible molecular adsorption of the sulfur compound, its dissociation, reorientation or reconstruction of the metal surface, formation of a 2-D surface sulfide, and at still higher H2S/H2 ratios, formation of a three-dimensional (3-D) (bulk) metal sulfide. Kinetic information about these processes may generally be helpful in elucidating the adsorption mechanism. Unfortunately, such quantitative kinetic information is not adequately available, with one exception, formation of bulk sulfides (9, 96). 

At equilibrium, the sulphur coverage of the nickel surface, 0, can be calculated by a Temkin-like adsorption isotherm dependent on the temperature and the H2S/H2-ratio 1,9) with the parameters found by Alstrup et al (P) equation 5. 

Weaver and Winnick [111] studied the performance of a nickel/nickel sulfide cathode for the electrochemical removal of hydrogen sulfide gas from a gas stream. At 650 °C, the porous nickel cathode was converted in situ to Ni3+ S2 by the H2S in the feed gas stream. The exact composition of the nickel sulfide was found to be a function of the H2S/H2 ratio in the gas stream. A current density of 150 mA/cm was attained at an iR free cathodic overpotential of 300 mV. A maximum H2S removal of 40% was reported. The low removal percentage was due to mass transport limitations of the reactant gas to the electrode. 

This surface layer has a structure like a two-dimensional sulfide. The adsorption takes place well below the H2S/H2 ratio that is needed to form bulk nickel sulfides. 

With an H2S/H2 ratio in the gas of 1 ppb, the equilibrium surface coverage of nickel at 500°C is around 70%. This means that all sulfur in the feed is quantitatively adsorbed on the nickel catalyst of a prereformer. The result is not only the deactivation of the prereformer catalyst even at very low sulfur levels, but also the protection of downstream catalysts from poisoning. Sulfur uptake on the catalyst will initially take place as shell poisoning and because of pore diffusion restrictions, it may take years before sulfur reaches the center of the particle. ... 

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 ... 

Sulfur exchange (and uptake) experiments are performed usually applying H2/H2S gas mixtures of different H2 H2S ratio. Comparison of H2/H2 S (with 90% content of the stable sulfur isotope ( S)) experiments with those of Ar/H2 S, indicatedthat the presence of H2 had no essential influence on the extent of S exchange, whereas the introduction of H2 S resulted in a sharp evolution of H2, indicating that H2S adsorption displaced H2 quickly. Consequently, different amounts of H2 at different H2 H2S ratio should not affect the S-exchange and uptake. The differences in results can be assigned to the differences in H2S pressure. 

The noble-metal (Pt, Pd. Ru and Rh) sulfides in combination with Mo, supported on AC exhibited higher activity for HYD of carbonyl and carboxylic groups than Mo/AC catalyst alone. They also accelerated hydrogenolysis of the etheric bonds such as CH3 O and Car-0. However, bimetallic catalysts without Co had no activity for decarboxylation. The H2S/H2 ratio had a different effect for every noble metal. In this study, GUA, 4-MA, ED, 4-methyl phenol and 2-octanone were used as model compounds. Thus study was conducted in an autoclave at 553 K and 7 MPa of H2. 

Effect of sulfur on carburisation mass gain of Alloy 800 in CH4/H2/H2S atmospheres at 900, 1000 and 1100 °C plotted versus ratio H2S/H2, and decrease of mass gain by carburisation with increasing H2S/H2 ratio up to an optimum value, above which sulfidation occurs (35). 

Under conditions where desulfurization occurs at finite H2S/H2 ratio, the surface of M0S2 will be covered with S and not contain any vacancies, whereas on the promoted catalyst the presence of vacancies to adsorb a hydrocarbon will be present. 

In a similar way, theory has been used not only to establish the nature of active sites on the surface but, in addition, to map out a full phase diagram for M0S2 as a function of the H2/H2S ratio. The results provided the ability to predict the state of the surface at specific conditions and to establish the region of phase space. This can ultimately be used in order to aid in the design of optimal op>erating conditions. 

This takes place at H2S/H2 ratios far below those required for formation of bulk sulphides [376]. 

Figure 5-24. Effect of H2S on the carburization of alloy 800 in CH4-H2-H2S (carbon activity = 1) at 900, 1000, and 1100°C. Total mass gain after 100 h as a result of carburization and sulfidation is plotted against H2S/H2 ratio increasing the ratio reduces the extent of carburization but also results in sulfidation (Rahmel et al., 1998).

It can be calculated that a H2S/H2 ratio of 4.7 x 10 is required to establish the Fe/FeS equilibrium at TOOK, using data from Barin et but a H2S/H2 ratio of only 10 to affect the surface properties as extrapolated from Grabke s data. Similarly, a H2O/H2 ratio of 0.15 is required to establish the equilibrium between magnetite and iron at 700 K, but only ppm levels of water are required to affect the catalytic activity of an ammonia synthesis catalyst. 

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