Electrolytic reactions catalysts

When first discovered in the eighties as a pronounced apparent violation of Faraday s law it looked like a phenomenon of limited importance, praised however already by several leading electrochemists and surface scientists including Bockris21 and Pritchard.22 The subsequent involvement of the groups of Comninellis, Haller, Lambert, Sobyanin, Anastasijevic, Smotkin and others and the continuous discovery of new electrochemically promoted reactions broadened substantially the horizons of electrochemical promotion as it became obvious that the phenomenon was not limited to any particular electrolyte, conductive catalyst or type of reaction. 

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

The general electrochemical procedure for the carbon dioxide incorporation was based on the use of one-compartment cells fitted with consumable anodes of magnesium or zinc [12]. Electrocarboxylations were carried out in DMF at constant current density, using tetrabutylammonium tetrafluoroborate (10 2 m) as supporting electrolyte. The catalyst was introduced in a 10% molar ratio with respect to the substrate and carbon dioxide was bubbled through the solution at atmospheric pressure. Electrolyses were generally run at room temperature and reactions were stopped when starting material was consumed or when the faradaic yield attained 30%. 

Time-resolved and spatially resolved photoelectron spectroscopic data along with reactor measurements demonstrate that EP of thin-hlm metal catalysts deposited on solid electrolyte supports is the result of spillover phenomena at the three-phase boundary between the electrolyte, the catalyst, and the gas phase. Ions from the electrolyte are discharged at the catalyst-electrolyte interface and migrate to cover the catalyst surface, whose properties are thereby strongly altered. This is illustrated by reference to a variety of metal-catalyzed reactions. Reaction mechanisms and the mode of promoter action are deduced, and it is shown how this understanding may be exploited to develop improved nano-particulate supported metal catalysts. 

Electrochemical kinetics and the magnitude of A Table 1 provides a list of the catalytic oxidation reactions studied so far from the view point of electrochemical promotion and of the measured A, p and P values. As shown in this table measured lAI values range from 1 to 3x10. It has been shown both theoretically [9] and experimentally [9,14] that the order of magnitude of the absolute value lAI of the Faradaic efficiency A can be estimated for any reaction, catalyst and solid electrolyte from the following approximate expression ... 

As discussed below, the porosity and surface area of the catalyst film is controllable to a large extent by the sintering temperature during catalyst preparation. This, however, affects not only the catalytically active surface area but also the length / of the tpb between the solid electrolyte, the catalyst film, and the gas phase (Figure 8). Elec-trocatalytic reactions, such as the transformation of from the zirconia lattice to oxygen adsorbed on the film at or near the tpb, which we denote by 0(a), (Equation [11]), have been found to take place primarily at these There is some experimental... 

The NEMCA effect does not appear to be limited to any specific type of catalytic reaction, metal catalyst or electrolyte, particularly in view of the recent demonstration of NEMCA using aqueous electrolytes. The catalyst, however, must be electronically conductive and the only report of NEMCA on an oxide catalyst is for the case of Ir02 which is a metallic oxide. It remains to be seen if NEMCA can be induced on semiconductor catalysts. 

However, for technical use of AFC, the long-term behavior of AFC components is important, especially that of the electrodes. Nickel can be used for the hydrogen oxidation reaction (catalyst in the anode) and on the cathode silver can be used as catalyst (see next section), no expensive noble metal (platinum) is necessary, because the oxygen reduction reaction kinetics are more rapid in alkaline electrolytes than in acids and the alkaline electrochanical environment in AFC is less corrosive compared to acid fuel cell conditions. Both catalysts and electrolyte represents a big cost advantage. The advantages of AFC are not restricted only to the cheaper components, as shown by Giilzow [1996]. 

GDEs typically consist, at least, of two layers a gas diffusion layer (GDL) and an active layer (AL). The GDL should provide mechanical support and electrical contact (current collector), optimal distribution of reactant gases and a pore structure suitable for the remova of liquid or vapor phase water (water management). The AL contains the catalyst, where the electrochemical reaction takes place, but only in those sites where reactant, electrolyte and catalyst meet, that is, in the three-phase zone or boundary (TPB). 

Many studies have been reported during the last almost 20 years regarding the effect of the electrochemical promotion of catalysis and its origin and application to several types of reactions of environmental and industrial interest. The effectiveness of NEMCA for catalytic oxidations, reductions, hydrogenerations, decompositions, and isomerizations using numerous types of soUd electrolytes and catalysts underlines the importance of this phenomenon in both catalysis and electrochemistry. Application of these... 

Reactions Catalyst (preparation method) Solid electrolyte T/°C Preagent/kPa P02/kPa Polarization time/min ymax Ref... 

The synthesis of formaldehyde from selective oxidation of methanol over a thin layer of electrolytic silver catalyst is a well-known industrial process that occurs in the temperature range of 850-923 K at atmospheric pressure. Since the total reaction is highly exothermic and fast, requiring very short contact time (0.01 s or less), the use of a silicon MSR was demonstrated to improve conversion up to 75% and 90% selectivity at safe conditions within the flammability limits [29]. 

Platinum catalyst particles are supported on larger and finely divided carbon particles. A carbon-based power XC-72 is commonly used. The reaction regions are characterized by the active surface area where electrode, electrolyte, and catalyst are present. 

As it turns out eiqierimentally (Figure 13.14) and can be explained theoretically, one can estimate or predict the order of magnitude of the absolute value of the enhancement factor A for any given reaction, catalyst, and catalyst/solid electrolyte interface from ... 

The maximum in U — Ap curves at constant i was interpreted [20] by a shortening of the three-phase boundary (gas/electrolyte/electro-catalyst) at small and large gas pressures on the basis of an idealized structure consisting of parallel cylindrical pores of different radii and equal length. Such a structure is called homoporous . The electrochemical reactions were assumed to occur at this boundary. The homo-porous structure is nearly drowned at low gas pressures while gas fills too many pores at high pressures. The insufficiency of the above interpretation of the U — Ap curves and i — Ap curves was pointed out [7]. A linear relation... 

Alkali AletalIodides. Potassium iodide [7681-11-0] KI, mol wt 166.02, mp 686°C, 76.45% I, forms colorless cubic crystals, which are soluble in water, ethanol, methanol, and acetone. KI is used in animal feeds, catalysts, photographic chemicals, for sanitation, and for radiation treatment of radiation poisoning resulting from nuclear accidents. Potassium iodide is prepared by reaction of potassium hydroxide and iodine, from HI and KHCO, or by electrolytic processes (107,108). The product is purified by crystallization from water (see also Feeds and feed additives Photography). 

Tripotassium hexakiscyanoferrate [13746-66-2] K2[Fe(CN)g], forms anhydrous red crystals. The crystalline material is dimorphic both orthorhombic and monoclinic forms are known. The compound is obtained by chemical or electrolytic oxidation of hexacyanoferrate(4—). K2[Fe(CN)g] is soluble in water and acetone, but insoluble in alcohol. It is used in the manufacture of pigments, photographic papers, leather (qv), and textiles and is used as a catalyst in oxidation and polymerisation reactions. 

Lithium Iodide. Lithium iodide [10377-51 -2/, Lil, is the most difficult lithium halide to prepare and has few appHcations. Aqueous solutions of the salt can be prepared by carehil neutralization of hydroiodic acid with lithium carbonate or lithium hydroxide. Concentration of the aqueous solution leads successively to the trihydrate [7790-22-9] dihydrate [17023-25-5] and monohydrate [17023-24 ] which melt congmendy at 75, 79, and 130°C, respectively. The anhydrous salt can be obtained by carehil removal of water under vacuum, but because of the strong tendency to oxidize and eliminate iodine which occurs on heating the salt ia air, it is often prepared from reactions of lithium metal or lithium hydride with iodine ia organic solvents. The salt is extremely soluble ia water (62.6 wt % at 25°C) (59) and the solutions have extremely low vapor pressures (60). Lithium iodide is used as an electrolyte ia selected lithium battery appHcations, where it is formed in situ from reaction of lithium metal with iodine. It can also be a component of low melting molten salts and as a catalyst ia aldol condensations. 

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