Platinum oxidation mechanisms

The reduction of iminium salts can be achieved by a variety of methods. Some of the methods have been studied primarily on quaternary salts of aromatic bases, but the results can be extrapolated to simple iminium salts in most cases. The reagents available for reduction of iminium salts are sodium amalgam (52), sodium hydrosulfite (5i), potassium borohydride (54,55), sodium borohydride (56,57), lithium aluminum hydride (5 ), formic acid (59-63), H, and platinum oxide (47). The scope and mechanism of reduction of nitrogen heterocycles with complex metal hydrides has been recently reviewed (5,64), and will be presented here only briefly. 

Given the results obtained on platinum electrodes discussed in some detail in the previous section, it is clearly of fundamental interest to study the mechanism of CO oxidation on other transition metal electrodes, and to compare the results with platinum. Rhodium has been the electrode material that has been studied in greatest detail after platinum, and results obtained with rhodium have provided some very significant insights into some of the general issues about the CO oxidation mechanism. 

Figure 9.6 Visual representation of the platinum oxide growth mechanism, (a) Interaction of H2O molecules with the Pt electrode occurring in the 0.27 V < < 0.85 V range, (b) Discharge of 5 ML of H2O molecules and formation of 5 ML of chemisorbed oxygen (Ochem)- (c) Discharge of the second ML of H2O molecules the process is accompanied by the development of repulsive interactions between (Pt-Pt) -Ofi m surface species that stimulate an interfacial place exchange of Ochem and Pt surface atoms, (d) Quasi-3D surface PtO lattice, comprising Pt and moieties, that forms through the place-exchange process. (Reproduced with permission...

The reason for this is that platinum is able to adsorb organic compounds dissociatively forming adsorbed hydrogen and organic residues and the oxidation mechanism of the latter involves extremely slow steps. 

Although CO is the simplest Ci component, its electrochemical oxidation mechanism is yet to be fully understood even on platinum electrodes. One of the most important issues on this topic is the sovurce of oxygen since CO needs one oa en atom to be folly oxidized to CO2. 

These effects are most striking on silver since it is, itself, a very unreactive surface. There is every reason to expect, however, that oxygen will behave similarly on other metals. More complex reaction behavior will, of course, be observed as the intrinsic reactivity of the metal increases. Oxygen adsorbed on platinum should show similar properties. In fact the formation of surface OH groups from HjO and 0(a) was recently reported 145). The ability of platinum itself to break C-H and C-C bonds complicates oxidation mechanisms, but future work should provide a greater understanding of the relative role of surface oxygen in oxidation catalysis. 

The oxidation mechanism of nitric oxide at single and polycrystalline platinum and other metals was surveyed in [82]. 

Tasaka, A. and Tojo T. (1985) Anodic oxidation mechanism of hypochlorite ion on platinum electrode in alkaline solution. J. Electrochem. Soc. 132,1855-1859. 

Activation (of noble metal electrodes) — Noble metal electrodes never work well without appropriate pretreatment. Polycrystalline electrodes are polished with diamond or alumina particles of size from 10 pm to a fraction of 1 pm to obtain the mirror-like surface. The suspensions of polishing microparticles are available in aqueous and oil media. The medium employed determines the final hydrophobicity of the electrode. The mechanical treatment is often followed by electrochemical cleaning. There is no common electrochemical procedure and hundreds of papers on the electrochemical activation of -> gold and platinum (- electrode materials) aimed at a particular problem have been published in the literature. Most often, -> cyclic and - square-wave voltammetry and a sequence of potential - pulses are used. For platinum electrodes, it is important that during this prepolarization step the electrode is covered consecutively by a layer of platinum oxide and a layer of adsorbed hydrogen. In the work with single-crystal (- monocrystal) electrodes the preliminary polishing of the surface can not be done. 

The explanation for the mechanism of exchange with thiophene is particularly important since a number of workers have found poor reproducibility with prereduced platinum, i.e., some prereduced catalysts actually catalyze exchange, whereas no deuteration is observed in duplicate samples.105 The reason for this apparent anomaly is presumably that reduction of platinum oxide with hydrogen is very slow and inefficient. Unless complete reduction is achieved, some PtO or Pt02 remains on which self-activation can readily occur. 

The size effect is probably the most proposed interpretation for the CO oxidation mechanism. For example, this effect on the adsorption of CO was investigated by Rice et al. using in situ infrared reflectance spectroscopy from five different samples of platinum particles with mean diameters of 2.0, 2.5, 3.2, and 3.9 nm, respectively. These authors found that the infrared COl stret-... 

Several studies have demonstrated the ability to observe a complete catalytic cycle in the gas-phase. Wallace and Whetten, and Woste and coworkers combined gas-phase experiments and theoretical calculations to elucidate the fuU catalytic cycle of CO oxidation including intermediate reaction steps [27-29]. Schwarz et al. have also demonstrated a full gas-phase catalytic cycle for the oxidation of CO in the presence of cationic platinum oxide clusters [30]. Furthermore, Armentrout and co-workers have studied the energetics of the individual steps in the overall catalytic cycles and produced a wealth of information on the thermochemistry, structure, and bond energies of transition metal clusters [31]. Clearly, the ability to probe the active sites and intermediates of complex catalytic reactions through gas-phase ion-molecule studies has yielded significant insight into the mechanisms of condensed-phase catalytic processes. 

For other types of systems such as highly branched reaction networks for homogeneous gas-phase combustion and combined homogeneous and catalytic partial oxidation, mechanism reduction involves pruning branches and pathways of the reaction network that do not contribute significantly to the overall reaction. This pruning is done by using sensitivity analysis. See, e.g., Bui et al., "Hierarchical Reduced Models for Catalytic Combustion H Air Mixtures near Platinum Surfaces, Combustion Sci. Technol. 129(l-6) 243-275 (1997). 

From EXAFS characterisation, the presence of platinum oxide was established which has several implications on the possible mechanism. As in the case of Cu-ZSM5 catalyst type, a promoting effect of the oxygen concentration was observed [5] as well as a volcano shape curve [6] for the NO conversion. On the contrary, an inhibiting effect of the partial pressure of oxygen was found in the case of the reaction of N2O decomposition which would Indicate that N2O is not a rection intermediate. 

As for the mechanism of the Pt(02) gas electrode s work in the nitrate melt, the authors of Ref. [225] found it to become metal-oxide owing to the formation of a platinum oxide film on the electrode surface. 

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