Polypyrrole supported catalysts

Another problem encountered in this work was that bofii the uncatalysed polypyrrole/PSS support and catalysed samples lost conductivity during storage. The conductivity of an uncatalysed polypyrrole/PSS sample decreased firom 3 S cm to 0.1 S cm over a period of 9 months under vacuum, while the conductivity of a Pt catalysed sample dropped firom 0.3 S cm to 10 S cm. Th e observations indicate that polypyrrole supported catalysts are unlikely to be suiteble for oxygen reduction in fuel ceils. 

All report studies of methanol oxidation at polypyrrole supported catalysts have involved the use of electrochemically prepared films of the polymer on a solid electrode. Strike et al. (20), who electrochemically deposited Pt onto such electrodes from a H2PtCl5 solution, reported current densities for methanol oxidation that were enhanced by factors of 10-100 over those at bulk Pt and platinized gold electrodes. Furthermore, a less rapid decay of the current with time... 

Polypropylenes, and metallocene catalysts amorphous PP, 4, 1052 flexible and elastomeric isotactic PP, 4, 1064 isotactic PP, 4, 1056 syndiotactic PP, 4, 1070 Polypyrroles, with ferrocene groups, 12, 305 Polypyrrole supports, for diphosphine borane complex,... 

Qi, Z. and Pickup, P.G., Novel supported catalysts platinum and platinum oxide nanoparticles dispersed on polypyrrole/polystyrenesulfonate particles, Chem. Com-mun., 15, 1998. 

Interesting supports are the polymeric materials, notwithstanding their thermal instability at high temperatures. In the electrocatalysis field, the use of polypyrrole, polythiophene and polyaniline as heteropolyanion supports was reported [2]. The catalytically active species were introduced, in this case, via electrochemical polymerization. Hasik et al. [3] studied the behavior of polyaniline supported tungstophosphoric acid in the isopropanol decomposition reaction. The authors established that a HPA molecular dispersion can be attained via a protonation reaction. The different behavior of the supported catalysts with respect to bulk acid, namely, predominantly redox activity versus acid-base activity, was attributed to that effect. 

Rajeshwar and coworkers (4-6) took the important step of using colloidal Pt particles, which are necessary to provide efficient use of the expensive Pt catalyst They electrochemically deposited polypyrrole onto glassy carbon electrodes from a colloidal Pt suspension containing pyrrole. These authors draw the interesting conclusions that the polypyrrole support itself shows some activity for oxygen... 

Although electrochemically prepared polypyrrole films are useful for fiindam tal studies, they are not practical for use in fuel cells. Apart firom the difficulty of their large scale production, their permeability is insufficient for generation of the large current densities (> 500 mA cm required of commercial cells. To circumvent these problems, we have used chemically prepared polypyrrole/poly(styr e-4-sulphate) powders, which we have raidered c tatalytic by the chemical deposition of Pt particles by various methods (7-9). These polypyrrole supported Pt catalysts can easily be mass-produced, and can be formed into catalyst layers for fuel cell gas diffiision electrodes using the technology currently used commercially for carbon supported catal> ts. 

The best oxygen reduction performance observed to date for a polyaniline supported catalyst is comparable to the best obtained with polypyrrole (Fig. 4) (8). As for polypyrrole, polyaniline supported catalysts lose conductivity during storage and are therefore unlikely to be suitable for use in fuel cells. 

The relatively high operating potentials of methmiol anodes, and slowness and mechanistic complexity of the medianol oxidation reaction provide considerable incentive to develop polymer supported catalysts, and this has resulted in much research activity. Polypyrrole has been most widely used as a support, presumably because its conductivity extends to lower potentials than for most other conducting polymers. Polyaniline has also attracted significant attention, and some polythiophenes are attractive for their enhanced stability. 

An electronic conductive polymer was found by Strike al. as a support for platinum with a co-catalysts. The platinum particles were deposited on electrodeposited polypyrrole. 

The real problem area with CdS concerns its ability to photo-oxidize water to O2. Earlier reportsthat forms of CdS loaded with Pt and RUO2 were effective photocatalysts for the cyclic cleavage of water have not been reproduced and there are serious doubts about their validity. However, some Support for this work has been obtained with CdS electrodes coated with RUO2 and covered with polypyrrole, but the experimental evidence showing O2 formation is inadequate for such an important conclusion. Other workers have failed to observe O2 formation with catalyst-loaded CdS and, in fact, irradiation of CdS in aerated solution results in rapid consumption of O2. In... 

H.B. Zhao, L. Li, J. Yang, and Y.M. Zhang, Nanostructured polypyrrole/carbon composite as Pt catalyst support for fuel cell applications, J. Power Sources, 184, 375-380 (2008). 

The heat-treated M/N/C composite materials, such as CoPPy/C, can also be considered as multifunctional catalysts, featuring Co nanoparticles coated with Co oxides and Co " species associated with N-C moieties that originate from the polypyrrole structures [82]. An illustration of the CoPPy/C catalyst surface and the ORR processes is shown in Fig. 15.30. The Co-N type site (shown as a C0-N4 complex) supports the initial adsorption of the O2 molecule and conversion of O2 to the intermediate reaction prcxluct, H02, by a 2e reduction reacticm. The H02 species can further react at a decorating Cof)y/Co nanopaiticle phase. Chu et al. [126] found that the mixture of the heat-treated Co- and Fe-tetraphenylporphyrins (CoTPP/FeTPP) had better catalytic performance for ORRs in acid media than that of the respective heat-treated single components. All of the above cited research results point to the fact that carefully designed bifuncticmal or multifunctional catalysts can be much more active for ORR than their single components. 

The more porous, fibrous structure of polyaniline (26) makes it a more attractive support than polypyrrole, and the high dispersion of the catalyst that can be achieved has produced current densities for methanol oxidation as high as 65 mA cm (26). The polyaniline support also appears to prevent the absorption of CO on Pt, resulting in less poisoning (28). 

Relatively high iron contents are found for self-supported iron-polypyrrole catalysts which were prepared by spray pyrolysis. The authors proposed high site densities however, comparatively low current densities with respect to other Fe-N-C catalysts make it most probable that only a small fraction of the overall irrai is bonded in active sites. Nevertheless, the shape of these catalysts is interesting because porous carbon spheres with diameters of 100 up to 1,000 nm are formed [194],... 

Conductive polymers, such as polyacetylene, polythiophene, polypyrrole, polyisothianaphthene, polyethylene dioxythiophene, polyaniUne, and so on, have interesting properties that make them suitable for use in PEMFCs (Heeger, 2001 Shirakawa, 2001). Their electroconductivity and noncarbon functionalities allow some of them to perform effectively as alternative carbon catalysts or with carbon supports to enhance their catalytic effects. Huang et al. utilized polypyrrole as a conductive polymer support for a platinum catalyst active for the ORR (Huang et al., 2009). Their results show significant resistance to carbon corrosion and improved conductivity over traditional Pt/C catalysts. They report that the platinum on polypyrrole catalyst (Pt/Ppy) has well-dispersed platinum particles of about 3.6 nm in diameter. CV scans up to 1.8 V revealed that there was httle carbon support corrosion on the Pt/Ppy and a twofold increase in activity than Pt black at 0.9 V. 

Huang, S. et al. 2009. Development of conducting polypyrrole as corrosion-resistant catalyst support for polymer electrolyte membrane fuel cell (PEMFC) application. Applied Catalysis B Environmental 93 75-81. 

The half-cell shown in Fig. 12.1 can also be used for evaluating non-Pt-based catalysts. For example, Lee et al. [2] investigated the ORR kinetics and mechanisms on carbon-supported cobalt polypyrrole (Co-PPy/C) catalysts by using both RDE and RRDE techniques. Figure 12.5 presents the ORR current-voltage (I-V) curves in 0.5 M H2SO4 with several Co-PPy/C catalysts, which shows that the catalyst heat treated at 800 °C can yield the highest catalytic... 

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