Transition metal ionomers

Reactions of Potential Catalytic Interest Transition Metal Ionomers... 

As indicated in the previous section, a range of reactions of transition metal ionomers of potential catalytic interest have been studied (14-23). While space does not permit presenting the results in detail here, it is appropriate to illustrate several of the types of results that have been obtained. The first involves reactions that can be compared readily to reactions of the same metals or ions on other supports. The second type demonstrates the formation of metal particles by reduction of metal ionomers. And, the third type concerns the catalytic potential of these types of systems. 

Copolymers of methacrylic add and ethylene termed as ethylene ionomers have been used as the base polymer for binding alkali, alkaline earth and transition metal ions. Organic amines such as n-hexylamine, hexamethylene tetraamine, 2,2,6,6-tetramethyM-hydroxy piperazine, ethylene diamine and polymeric diamines such as silicone diamine, polyether diamine and polymeric diamines such as silicone diamine, polyether diamine and polyamide oligomers considerably enhance the complex formation characteristics of Zn(II) ethylene ionomers thereby enhancing the physico-chemical properties [13]. 

The stability and durability of Pt alloys, especially those involving a >d transition metal, are the major hurdles preventing them from commercial fuel cell applications. "" The transition metals in these alloys are not thermodynamically stable and may leach out in the acidic PEM fuel cell environment. Transition metal atoms at the surface of the alloy particles leach out faster than those under the surface of Pt atom layers." The metal cations of the leaching products can replace the protons of ionomers in the membrane and lead to reduced ionic conductivity, which in turn increases the resistance loss and activation overpotential loss. Gasteiger et al. showed that preleached Pt alloys displayed improved chemical stability and reduced ORR overpotential loss (in the mass transport region), but their long-term stability has not been demonstrated. " These alloys experienced rapid activity loss after a few hundred hours of fuel cell tests, which was attributed to changes in their surface composition and structure." ... 

It should be acknowledged that Risen utilized the concept of the ionic domains in ionomers (Nafion sulfonates, sulfonated linear polystyrene) as microreactors within which transition metal partides can be grown and utilized as catalysts (23-25). Transition metal (e.g. Rh, Ru, Pt, Ag) cations were sorbed by these ionomers from aqueous solutions and preferentially aggregated within the pre-existing clusters of fixed anions. Then, the ionomers were dehydrated, heated and reduced to the metallic state with Hg. Risen discussed the idea of utilizing ionomeric heterophasic morphology to tailor the size and size distributions of the incorporated metal particles. The affected particle sizes in Nafion were observed, by electron microscopy, to be in the range of 25-40 A, which indeed is of the established order of cluster sizes in the pre-modified ionomer. 

Ionomers are certainly not the only useful support for transition metals and ions. Indeed, inorganic oxides, such as silica, zeolites and aluminas, are the most widely used at present (13). Among the organic polymeric supports now used, the most closely related to the ionomers are the well known ion-exchange resins. While they are polyelectrolytic, as are the PFSA and PSSA ionomers, they are not thought to possess the potentially useful morphological properties of ionomers. 

In order to test the feasibility of the notion that transition metals in ionomers would exhibit interesting reactivity, several important types of metal ions in FSA ionomers were reacted with CO and NO under mild conditions. The ions Ag and Cu were chosen because their carbonyls are known to be quite difficult to forgi without chemicaUy forcing conditions on high surface area supports. The ions Fe, Ni 2 and Co+ were chosen because, although their elements have well known carbonyls (e.g. Fe(CO)r, Ni(CO)4, and Co2(CO)g), the carbonyls typically are prepaired by reaction witn the reduced forms of the metal. In fact, Ni(CO)4 forms spontaneously by reacting metallic Ni with CO at low pressure. On the other hand, most of these ions form nitrosyls by reaction with NO under mild conditions. 

The proton exchange membrane can be a source of fluoride ions as well [143]. Hydroxyl radicals, formed via crossover gases or reactions of hydrogen peroxide with Fenton-active contaminants (e.g., Fe +), could attack the backbone of Nafion, causing the release of fluoride anions these anions in turn promote corrosion of the fuel cell plates and catalyst, and release transition metals into the fuel cell [143]. Transition metal ions, such as Fe, then catalyze the formation of radicals within the Nafion membrane, resulting in a further release of fluoride anions. On the other hand, transition metal ions also can cause decreased membrane and ionomer conductivity in catalyst layers, as discussed in section 2.4 of this chapter. 

In the case of transition metal complexes with large g anisotropy in disordered matrices, mw frequencies <9.4 GHz are sometimes preferable, because local heterogeneities (strain) of the matrix lead to a distribution of the principal values of the g- and A-tensors g- and A-strain) and thus to field-dependent line broadening. Such a situation is illustrated in Fig. 11 for Cu(II) in Nation perfluorinated ionomers swollen by acetonitrile the line width of the parallel components was measured at four mw frequencies in the range 1.2-9.4 GHz, and the narrowest line widths were detected for the two low-field lines of the parallel quartet at C band (4.7 GHz) and L band (1.2 GHz). In this way, clear superhyperfine splittings from nuclei were resolved, in addition of course to the hyperfine splittings from Cu(ll). 

Motyakin, M.V., Comet, N., Gebel, G., Schlick, S. (2000) Morphology of sulfonated polyimide ionomers from ESR spectra of paramagnetic transition metal cations and nitroxide spin probes. Bulletin of the Polish Academy of Sciences. Chemistry, 48, 273-292. 

The interactions of various polar agents with the ionic groups and the ensuing property changes are unique to ionomer systems. This plasticization process is also important in membrane applications. A different application of ionic cluster plasticization involves the interaction of metal stearates to induce softening transitions. The plasticization process is required to achieve the processability of TPEs based on this technology. 

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