Nickel sulfide, coatings

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. 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

For a long time it has been industrial practice to activate nickel cathodes by depositing coatings of nickel sulfide on their surface. Two different methods are used cathodic deposition of NiS, and chemical sulfidization of nickel electrodes. 

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... 

Microscopic and spectroscopic investigations (SEM and XPS) reveal the relatively fast change of the chemical composition of nickel sulfide coatings upon the onset of cathodic hydrogen evolution (74). Indeed, at 90°C all nickel sulfide phases are reduced to porous nickel within several days to a week s time. They lose some catalytic activity with time with an increase in overvoltage between 0.15 and 0.3 V after continuous operation for 1 year. It is clear that the catalyst after I week is already no longer nickel sulfide but some type of Raney nickel. Thus far the initial catalytic activity of the NiS, coating is of little relevance. The respective results and data are due to be published by the present authors (73). 

Cathodically released sulfide anions are anodically oxidized to sulfate anions, which enhance surface corrosion and stress corrosion cracking of mild and stainless steels and enhance surface corrosion even of nickel. Therefore, nickel sulfide coatings can only be used in nonpressurized electrolyzers working at relatively moderate temperatures (80°C). 

The Ni3S2 catalyst on china clay used for the catalytic removal of organic sulfur compounds from coal gas has an average life of three months (8). Nickel sulfide catalysts lose activity, as a rule, because they become coated with reaction products which are insufficiently volatile or soluble under the prevailing conditions. However, a nickel sulfide... 

The first step in regenerating the coated, inactive nickel sulfide catalysts is to remove the reactants and products of the catalytic reaction aloi with the extraneous material coating the catalyst. In most cases, extraction of the catalytic mass with solvents, or purging with steam is adequate for this clean-up. The more resistant film of coke or tar is then removed by oxidation with air under carefully controlled conditions to avoid over-heating the catalyst. During this oxidation, the nickel sulfide is usually oxidized to NiO. This NiO is then reduced and sulfided in the same manner as in the initial preparation of the catalyst. 

Maintaining high levels of polyunsaturates is desirable in producing coating fats. A sulfided nickel catalyst is used to reduce IV to 70 and produce about 65 percent trans isomers with production of saturates (Cl8 0) minimized to 2-4 percent increase131. 

Catalyst selectivity is somewhat meaningless unless the term is defined. There also are selective catalysts that do not meet the technical or practical definition of hydrogen selectivity. Such catalysts are sulfur-poisoned catalyst. Sulfided nickel catalyst produces high trans-isomers, has lower activity than conventional nickel, exhibits longer reaction times, and is used for specialty applications (e.g., coating fats and hard butters). 

The supported Ni catalysts (5 wt.%) for H2S oxidation were prepared by incipient wetness impregnation of the two supports, i.e. SiC grains and graphite felt coated with carbon nanofibers, with an aqueous solution of Ni(N03)2.6H20 (Merck). After drying overnight at 120°C, the catalysts were calcined at 350°C for 2h in order to decompose the nitrate salt and to form the nickel oxide. The corresponding sulfidic catalysts were obtained by sulfidation of NiO by reaction with a H2S/He flow at 300°C. 

The bulk metal is oxidized by air or steam only at high temperatures, but Raney nickel (see Section 21.2) is pyrophoric. Nickel reacts with F2 to give a coherent coating of NiF2 which prevents further attack hence the use of nickel and its alloy Monel metal in apparatus for handling F2 or xenon fluorides. With CI2, Bt2 and I2, Ni(II) halides are formed. At elevated temperatures, Ni reacts with P, S and B and a range of different phosphide (see Section 14.6), sulfide and boride (see Section 12.10) phases are known. 

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