Porous nickel

The most important example of this category is Raney nickel, which is extensively used in hydrogenation reactions in fine chemistry. The catalyst has been named after Murray Raney who invented this catalyst in 1924. It is prepared by the reaction of a powdered nickel-aluminium alloy with aqueous sodium hydroxide to selectively remove a large fraction of the aluminium component (.see Figure 3.12). The product consists of porous nickel with a high... 

Beeck at Shell Laboratories in Emeryville, USA, had in 1940 studied chemisorption and catalysis at polycrystalline and gas-induced (110) oriented porous nickel films with ethene hydrogenation found to be 10 times more active than at polycrystalline surfaces. It was one of the first experiments to establish the existence of structural specificity of metal surfaces in catalysis. Eley suggested that good agreement with experiment could be obtained for heats of chemisorption on metals by assuming that the bonds are covalent and that Pauling s equation is applicable to the process 2M + H2 -> 2M-H. 

Molten Carbonate Fuel Cell The electrolyte in the MCFC is a mixture of lithium/potassium or lithium/sodium carbonates, retained in a ceramic matrix of lithium aluminate. The carbonate salts melt at about 773 K (932°F), allowing the cell to be operated in the 873 to 973 K (1112 to 1292°F) range. Platinum is no longer needed as an electrocatalyst because the reactions are fast at these temperatures. The anode in MCFCs is porous nickel metal with a few percent of chromium or aluminum to improve the mechanical properties. The cathode material is hthium-doped nickel oxide. 

Raney nickel A porous nickel catalyst produced from a nickel-aluminum alloy. 

The impregnation of porous nickel discs with CoPc was difficult because of the limited solubility of the chelate in the usual solvents. CoPc cathodes with carbon as substrate were therefore prepared for use in H2/O2 fuel cells. A mixture of 72 mg CoPc and 48 mg acetylene black, with PTFE as binder, was pressed into a nickel mesh of area 5 cm2. Electrodes of this type were tested in an H2/O2 fuel cell with 35% KOH electrolyte in an asbestos matrix at 80° C. Figure 5 compares the current/voltage characteristics of CoPc cathodes (14 mg/cm2) with those of other catalysts, including platinum (9 mg/cm2), silver (40 mg/cm2), and pure acetylene black (20 mg/cm2). An hydrogen electrode (9 mg Pt/cm2) was used as the anode in all tests. To facilitate comparison of the activity of different cathodes, the pure ohmic internal resistance of the cells (of the order of 0.02 ohm) was eliminated. 

The use of porous nickel and nickel foam anodes in these experiments was shown to be advantageous in terms of providing very high specific surface areas (and therefore low current densities), excellent heat dissipation, and thorough mixing. 

Hydrogen-Tritium Exchange Studies. The alloy to be studied was placed between porous nickel frits in a tube 4 in. long with %-in. i.d. This sample tube was inserted as part of a closed loop of y4-in. stainless steel tubing. 

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). 

Other common anode materials for thermal batteries are lithium alloys, such as Li/Al and Li/B, lithium metal in a porous nickel or iron matrix, magnesium and calcium. Alternative cathode constituents include CaCr04 and the oxides of copper, iron or vanadium. Other electrolytes used are binary KBr-LiBr mixtures, ternary LiF-LiCl-LiBr mixtures and, more generally, all lithium halide systems, which are used particularly to prevent electrolyte composition changes and freezing out at high rates when lithium-based anodes are employed. 

High pressure can increase efficiency anil this concept has been under development for many years. A commercial electrolyzer (Lurgii is available which operates at a pressure of 30 atmospheres and 90 C. requiring 3(HI amperes of electric current at 217 volts In the mid-1960s, bipolar cells of porous nickel electrodes were developed which operate at current densities of BIX) and 1600 amperes per square foot (11.09 square meler)... 

One group of NADH oxidants, which does not fit the proposed reaction scheme in Fig. 2.4 are the metal complexes. Examples of this type include nickel hexacyanoferrate deposited on porous nickel electrodes [29], gold electrodes modified with cobalt hexacyanoferrate films [30] and adsorbed l,10-phenanthroline-5,6-dione complexes of ruthenium and osmium [31]. It is unclear how these systems work and no mechanism has been proposed to date. It may be worth noting that dihydronicotinamide groups have been shown to reduce aldehydes in a non-enzymatic reaction when the reaction is catalysed by zinc, a metal ion [15]. In a reaction between 1,10-phenanthroline-2-carboxaldehyde and N-propyl-l,4-dihydronicotinamide, no reaction was seen in the absence of zinc but when added to the system, the aldehyde was reduced and the nicotinamide was oxidised. This implies that either coordination to, or close proximity of, the metal ion activates... 

Gaseous Diffusion. In the gaseous diffusion process, the UFfi flows through a porous nickel membrane called the barrier. The heavier U-238F6 flows more slowly than the U-235F6, and the theoretical separation factor for an equilibrium stage is ... 

The anode in the diagram is a porous nickel structure with the pores containing some molten carbonate. The cathode is porous nickel oxide also containing molten carbonate. 

Raney nickel is an alternative to dispersing nickel on a support to obtain high surface area particles. It is made by treating of a Ni-Al alloy with a concentrated alkaline solution. Aluminium is selectively dissolved, forming soluble aluminates, and leaving porous nickel metal that retains, at least in part, the structure of the starting alloy with channels easily accessible to the reactants. 

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. 

A model for the nano-structural evolution of Raney-type nickel catalysts (widely used in hydrogenation reactions) from the constituent intermetallic phases present in nickel-aluminium precursor alloys is presented here. Nano-porous nickel catalysts are prepared via a caustic leaching process where the NiAl alloy powder (typically 50-50 at.%) is immersed in concentrated NaOH solution in order to leach away the aluminium present to leave a highly-porous nickel catalyst (often referred to as spongy nickel). 

Firstly, a kinetic Monte Carlo (kMC) [8] for the nano-structural evolution of so-called spongy nickel from the constituent intermetallic phases present in nickel-aluminium precursor alloys is described. Experimental data concerning nano-porous nickel catalyst powder used in this paper are derived from leached NiAl alloy powder produced via a spray-atomization route rather than the conventional cast-and-crushed route. 

N. Guillou, Q. Gao, M. Nogues, R. Morris, M. Hervieu, G. Ferey, and A. Cheetham, Zeolitic and Magnetic Properties of a 24-membered Ring Porous Nickel(II) Phosphate, VSB-1. C. R. 

The electrodes are flat. The anode is composed of porous sintered nickel along with additives, which inhibit the loss of surface area during operation. The anode is in direct contact with the electrolyte matrix. The cathode is a porous nickel oxide, which is initially fabricated in the form of a porous sintered nickel and is subsequently oxidized during the cell operation. 

Bychin, V.P. Zvezdkin, V.A. Samatov, O.M. Porous nickel anodes for molten-carbonate fuel cells. Russ. J. Electrochem. 1993, 29 (11), 1346-1349. 

Antolini, E. Ferretti, M. Gemme, S. Preparation of porous nickel electrodes for molten carbonate fuel cells by non-aqueous tape casting. J. Mater. Sci. 1996, 31 (8), 2187-2192. 

Porous alloy electrodes of Raney nickel are often used in electrolyzers. Raney nickel is produced by first making an alloy composed of 50% aluminum and 50% nickel. This composite is then treated with potassium hydroxide, which eats away the aluminum and leaves a porous nickel sponge material, known as Raney nickel after Murray Raney, the inventor of the process. Electrolyzer electrodes of this material have a large surface area due to their porosity and will produce more gas with smaller electrodes, compared to electrodes made of materials with less surface area, such as sheet or screen. Although Raney nickel is preferred, it is more expensive than either sheet or screen. The porous texture that creates a larger surface area also acts as a filter for small particles. The sediment which forms reduces the active surface area over time, inhibiting gas formation and thus efficiency. 

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