Precipitants Protons, exchangeable

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

The simple surface hydration and proton exchange enables the metal cation complexes also to adsorb due to ligand exchange. Equally with the solvent association and condensation processes this adsorption may lead to the formation of extended gel structures and surface precipitation. However, as the surface site distribution and surface potential influence these processes, the physicochemical conditions (pS, pH, pi, pe) where they occur do not match those for the solution species. ... 

Loss of manganese leads to the formation of a defect spinel structure with reduced or no 4 V capacity [104, 116-121]. Additionally, proton exchange [116-121], phase separation, film formation, and precipitation of MnO and Mnp2 may occur, increasing cell impedance and exacerbating the capacity fading. More... 

Figure 18.3. A schematic showing the mechanism of particle growth by dissolution/precipitation. The chemical potential of smaller particles is higher than that of larger particles [32]. (Reproduced by permission of ECS— The Electrochemical Society, from Virkar AV, Zhou Y. Mechanism of catalyst degradation in proton exchange membrane fuel cells.)...

Yong MacDonald (1998) show that upon apparent completion of metal sorption, measurements of the equilibrium pH of the system generally showed a reduction below initial pH. This reduction in pH was attributed to the resultant effect of the many reactions in the system. These reactions included the release of hydrogen ions by metal/proton exchange reactions on surface sites, hydrolyses of metals in the soil solution, and precipitation of metals. It was apparent that more detailed information was needed to distinguish between surface and solution reactions responsible for release of hydrogen ions. However, it was evident that if surface complexation models are to be used, the relationship between metal adsorption and proton release needs to be established. That is, net proton release or consumption is due to all the chemical reactions involving proton transfer. 

Hydroxide and carbonate typically form insoluble precipitates with polyvalent cations in natural waters. The activity of both of these species increases with pH. The presence of surface functional groups that are capable of exchanging a proton creates pH dependent-charge, whereby the ionic character of the surface increases with pH [158,284,285]. 

Once precipitation begins, a quasi-steady state will eventually be attained in which the soil pe and pH are poised by the redox and precipitation equilibria operating. In the transition to the steady state, protons will be provided by dissociation of acids in the soil solution—e.g. H2CO3 derived from C02-and by reactions with the soil exchange complex. The course of reduction and the eventual steady state will depend on these reactions and it is therefore necessary to allow for them in predicting what the steady state conditions will be. 

Complex formation between Nb(OH)s and methylamine has been established582 by an n.m.r. study. Two signals are observed in the n.m.r. spectrum of an aqueous solution of methylamine containing Nb(OH)5. One of these is due to the methyl protons of the amine and the other to the rapidly exchanging protons of the OH, NH2, and H20 groups. The i.r. spectra of solutions and precipitates formed by Nb(OH)5 and methylamine or monoethanolamine have been interpreted583 in terms of Nb—N co-ordination. Niobium oxohalide complex formation with tri-n-octylamine has been studied by reverse ebullioscopy.584... 

In the electrolysis of solutions of M(S04)2 dHyO, protons move toward the cathode and the metal to the anode, so these compounds can be formulated as H2[M0(S04)2] -3H20 (disulfatozirconic and -haftiic acids). The reaction of sodium acetate with Zr0Cl2-8H20 yields diacetatozirconic acid (equation 9). They do not precipitate with acid dyes, and they react with anion exchange resins rather than with their cation exchange counterparts. 

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