Platinum oxygen species

Above 1,0 V, a platinum-oxygen species is formed which, upon cathodic potential sweep, reduces to Pt in one step, without forming an [OH] intermediate (124). 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

Most of the studies have involved the alloying of a second metal to platinum. The second metal was generally chosen because of its ability to increase the concentration of oxygenated species on the electrode surface, but also for its corrosion resistance. Even if some discrepancies exist in the literature, R-Ru is now widely accepted as the most interesting one, and hence our analysis will focus on this alloy in the next subsection. Other alloys such as R-lr, R-Os, or R-Re have also been reported to be good candidates, and R-Mo under specific conditions of preparation was claimed to have the desired properties. The Pt-Sn alloy is still a subject... 

Aity quantitative inteqjretation is still difficult to make, since the mass variations result from several coupled processes replacement of adsorbed hydrogen, water molecules and supporting anions by strongly adsorbed species from methanol chemisorption, reorganization of the double layer, formation of oxygenated species at the platinum surface, etc. 

From these results it appears that the addition of tin to platinum greatly favors the formation of AA eomparatively to AAL. This can be explained by the bifunctionnal mechanism where ethanol is adsorbed dissociatively at platinnm sites, either via an 0-adsorption or a C-adsorption process followed by the oxidation of these adsorbed residues by oxygenated species formed on Sn at lower potentials giving AA. 

A recent study of Bozo et al concerning platinum deposited onto ceria-zirconia solid solution has been published [45]. Pt/CeOo 7ZrOo33 was a most attractive catalysts which showed an activity much higher than for platinum deposited onto alumina T50 for methane combustion was lowered from 470 to 300 C according to their experimental conditions. This activity was attributed to enhanced oxygen species mobility onto ceria containing solids. Unfortunately a continuous... 

Several surface oxygen species have been postulated based on kinetic parameters, stable intermediates, and in situ observations during oxygen reduction or evolution. As in the case of hydrogen, platinum is the most well-studied electrocatalyst. Optical measurements (122-126) show that a freshly developed oxygen layer on platinum behaves reversibly up to 0.95 V. However, rapid aging yields an irreversibly bound layer. Chemisorption of OH is assumed to occur in this potential region (123,124,127,128) formed by z. + + H- + e (27)... 

PtO2 (752, 757), similar to some electrochemical oxygen layers. Figure 12 shows a possible structure of platinum oxides on various planes (752). The [1(X)] plane has a PtOj composition (752,757), while the bulk corresponds to a PtO oxide. Present information does not unambiguously point toward either surface oxide formation or chemisorption. Because of the apparent similarity of some surface oxygen species on catalysts and electrocatalysts, coordinated efforts in both fields using standardized techniques and procedures could resolve uncertainties. 

Hydrocarbon oxidations on platinum metal catalysts readily proceed in a liquid phase in the presence of co-reductant, most simply, dihydrogen. Under the action of co-reductant the active oxygen species are generated from dioxygen. The properties of active oxygen are close to the properties of hydroperoxides, therefore, the two type systems operate in a similar way. With O2/H2 mixture the oxidative transformation of aromatics [1-4], cyclohexene [5-6] and saturated hydrocarbons [7-8] have been studied. It should be noted that some of the systems are effective enough to generate interest of industrial chemistry. 

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