Hydrogen-nickel sulfide equilibrium

Nickel sulfide coatings are therodynamically unstable at the hydrogen equilibrium potential. NiS2 decomposes simply by immersion in aqueous KOH. In aqueous alkaline solution at hydrogen-evolving cathodes, all NiS, phases are reduced according to... 

Since nickel sulfide catalysts are often used in systems containing hydrogen, it is necessary to consider the equilibrium between hydrogen and nickel sulfide. Badger, Grifiith, and Newling (4) examined the equilibrium... 

The equilibrium ratios of hydrogen-to-hydrogen sulfide for the reaction, derived (34) from available thermodynamic data (35), are plotted in Figure 10 as a function of temperature. When Ph2/Ph2s over the catalyst is less than the equilibrium value, the nickel can be sulfided and hence poisoned. Conversely, when this ratio is greater than the equi-... 

Table 8.8 presents these numbers in a slightly different way, in which the equilibrium constant is multiplied by the pressure term to whatever power is appropriate and the result is the ratio of naphthenes to aromatics, calculated for four temperatures—400°F, 600°F, 800°F, and 1000°F—at 2500 psi hydrogen partial pressure. High values of Keq pH2 correspond to low aromatic levels and are favored by low temperatures, hence the use of highly active noble metals and nickel catalysts for this purpose. Base metal sulfide catalysts have also been successfully used for this purpose, but are less likely to achieve very low product polyaromatic levels. 

Complete removal of thiophene at temperatures below 500° may be brought about by using an intimate mixture of Ni and Ni3S2 as proposed by Szayna (19). The free nickel reacts with the hydrogen sulfide as it is liberated from the thiophene, driving the equilibrium in the above equation to the right. 

---