Platinum on graphite

Oxidation of Highly Dispersed Platinum on Graphite Catalysts in Gaseous and Aqueous Media... 

Catalyst. A highly dispersed platinum on graphite catalyst was prepared following a method described by Richard and Gallezot 20). 

Figure 5 Effect of degree of bum-off on the surface area and degree of dispersion of a 1% platinum on graphitized carbon black...

Single pulse, shock tube decomposition of acetic acid in argon involves the same pair of homogeneous, molecular first-order reactions as thermolysis (19). Platinum on graphite catalyzes the decomposition at 500-800 K at low pressures (20). Ketene, methane, carbon oxides, and a variety of minor products are obtained. Photochemical decomposition yields methane and carbon dioxide and a number of free radicals, which have complicated pathways (21). Electron impact and gamma rays appear to generate these same products (22). Electron cyclotron resonance plasma made from acetic acid deposits a diamond [7782-40-3] film on suitable surfaces (23). The film, having a polycrystalline structure, is a useful electrical insulator (24) and widespread industrial exploitation of diamond films appears to be on the horizon (25). 

The addition of various Kolbe radicals generated from acetic acid, monochloro-acetic acid, trichloroacetic acid, oxalic acid, methyl adipate and methyl glutarate to acceptors such as ethylene, propylene, fluoroolefins and dimethyl maleate is reported in ref. [213]. Also the influence of reaction conditions (current density, olefin-type, olefin concentration) on the product yield and product ratios is individually discussed therein. The mechanism of the addition to ethylene is deduced from the results of adsorption and rotating ring disc studies. The findings demonstrate that the Kolbe radicals react in the surface layer with adsorbed ethylene [229]. In the oxidation of acetate in the presence of 1-octene at platinum and graphite anodes, products that originate from intermediate radicals and cations are observed [230]. 

A major obstacle is related to the anode material. The active component in the anode is a highly dispersed metal supported on graphite that is pressed against the membrane. Platinum is chosen as the active metal because of its efficiency in dissociating hydrogen, but, unfortunately, platinum is also very sensitive towards trace amounts of impurities (e.g. CO) in the hydrogen gas. 

Blurton KF, Kunz HR, Rutt DR. 1978. Surface area loss of platinum supported on graphite. Electrochim Acta 23 183-190. 

An interesting catalyst is the platinum supported on graphite. Fig. 12 shows a bright field image. The... 

Shaikhutdinov, S.K., Moller, F.A., Mestl, G. and Behm, R.J., Electrochemical deposition of platinum hydrosol on graphite observed by scanning tunneling microscopy, J. Catal., 163,492, 1996. 

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

The potential window can be limited by the decomposition potential of a solute, not just a solvent. In particular, reactions of anodic oxidation of halides (Cl-, Br, and I-) on diamond are highly irreversible and have much higher overvoltage (for Cl, by 1 V) than on platinum or graphite electrodes [97, 123, 124], In all probability this is due to poor adsorption of intermediates, that is, Cl, Br, and I atoms, on the diamond electrode surface. We recall that the outer-sphere reactions discussed in Section 6.1 generally do not involve adsorption of intermediates and thus are not... 

With regard to electrode material, it can be seen (Tables 4 and 5) that cathodic limits on mercury are displaced by a few tenths of a volt to more negative potentials than on platinum. On the anodic side, the number of practically useful electrode materials is limited to noble metals and different types of carbon one case (anodic limit of pyridine nos. 35 and 36) shows that the anodic limit is lower on graphite than on platinum, and this seems to be a general trend for the comparison of carbon based anode materials, except possibly for vitreous carbon (Table 5) and bright (smooth, polished or shiny) platinum. 

An investigation of the anodic oxidation of mesitylene in nitrate-ion based electrolytes but with aprotic solvents revealed little more to illuminate the mechanistic picture (Nyberg, 1971d). Again, a very pronounced shift of the voltammetric curve was observed upon addition of the substrate when platinum was the anode material, whereas on graphite a small shift toward less positive potentials was noted. Product distributions are shown below eqn (67). The forma-... 

The foregoing discussion serves to show that disordered carbon structures are oxidized more readily than well-ordered graphite planes and that dislocations and active sites provide nucleation points for attack of the carbon crystallite. Another factor that must be considered is that dispersed electrocatalysts, such as platinum, on the carbon surface are not benign. The electrocatalysts interact with the carbon causing local oxidation or corrosion, i.e., the platinum catalyzes the corrosion of the carbon itself. In the presence of oxygen, which is the condition under which the electrocatalyst will operate, reduction intermediates from the oxygen (e.g., HOj) can have an accelerated corrosion effect. 

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