Chemisorption of hydrogen sulphide

Group VIII metals are susceptible to sulphur poisoning [39] and sulphur must be removed from the feed stream. Natural gas may be cleaned over active carbon, but normally all hydrocarbon feedstocks are treated by hydro-desulphurisation over CoMo (or Ni/Mo) catalyst followed by sulphur absorption on a zinc-oxide absorption mass (refer to Section 1.5.1). 

Poisoning effects are often correlated with the poison concentration in the feed stream, which, of course, is the important parameter in practical operation. However, in a more detailed analysis this approach can hardly be justified for other than isothermal tests in gradientless reactors. The adsorption equilibrium depends on temperature and composition of the gas phase which varies through the reactor as well as within the single catalyst pellet. Therefore it appears more rational to correlate the deactivation with the amount of poison present on the catalyst rather than 

Under reforming conditions, all sulphur compounds will be converted to hydrogen sulphide, which is chemisorbed on the metal surface (Me)  

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

Hydrogen sulphide chemisorbs dissociatively on nickel. Stable saturation uptakes of sulphur are observed at ratios of H2S/H2 from 10 10 up to close to 1000 10 above which bulk sulphide is formed. 

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Another method for determination of the nickel area is based on chemisorption of hydrogen sulphide [389], Hydrogen sulphide is the stable sulphur compound at conditions for tubular reforming. It is the most severe poison for the reaction (refer to Section 5.4). The adsorption of hydrogen sulphide on nickel is rapid even below room temperature [376] [381]. At temperatures of industrial interest hydrogen sulphide is chemisorbed dissociatively on nickel. 

A similar behaviour has been observed for other systems including the chemisorption of hydrogen sulphide on other metals (Ag, Fe, Cu, Co, Ru, Pt) [523],... 

The activation energy in the SPARG reaction in the presence of sulphur in Figure 5.45 is estimated to 227 kJ/mol and the value of the sulphur-free test is 110 kJ/mol. This can be explained by the heat of chemisorption of hydrogen sulphide by comparing the rate expressions for the sulphur-poisoned and the sulphur-free catalysts. 

Figure 4.5 Nickel surface areas determined by chemisorption of hydrogen and hydrogen sulphide, respectively (using procedures described with test) [415]. Reproduced with the...

Equilibrium constants for Reaction (5.26) are shown in Appendix 2. The equilibrium constant at 700 C is 3.3 10, which means that even a small amount of hydrogen will inhibit the conversion of hydrogen sulphide. Therefore, sulphur removal following this reaction pattern requires a total oxidation of the catalyst. If some part of the nickel surface is still exposed to the gas, hydrogen formed by Equation (5.14) will cause hydrogen sulphide to be retained at the surface by the chemisorption reaction [377] [389]. 

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