Electrooxidation, with catalysts

The AIREs have a higher surface selectivity and sensitivity thus, the interface/surface reactions can be selectively monitored with less interference from the bulk solution. Generally, in situ FTIRS studies are concerned with the dissociative adsorption and oxidation of organic molecules, the formation, adsorption and oxidation of intermediates, the nature of adsorbed species and their interaction with catalysts, the determination of reaction selectivity, and also effects of catalyst composition, size, and morphology. The dual-path mechanism and active/poisoning intermediates in the electrooxidation of small organic molecules (SOMs) have been well characterized by in situ FTIRS methods. ... 

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. ... 

The mechanism of electrooxidation of methanol is now nearly well understood. From the considerable effort made during the past 20 years, it is now possible to propose electrocatalysts with acceptable activities for DMFCs, even though further improvement is still necessary. Despite considerable research efforts, R-Ru alloys are the only acceptable catalysts for the electrooxidation of methanol at low anode potentials. Two questions still remain unanswered ... 

In this chapter, two carbon-supported PtSn catalysts with core-shell nanostructure were designed and prepared to explore the effect of the nanostructure of PtSn nanoparticles on the performance of ethanol electro-oxidation. The physical (XRD, TEM, EDX, XPS) characterization was carried out to clarify the microstructure, the composition, and the chemical environment of nanoparticles. The electrochemical characterization, including cyclic voltammetry, chronoamperometry, of the two PtSn/C catalysts was conducted to characterize the electrochemical activities to ethanol oxidation. Finally, the performances of DEFCs with PtSn/C anode catalysts were tested. The microstmc-ture and composition of PtSn catalysts were correlated with their performance for ethanol electrooxidation. 

From the above experimental results, it can be seen that the both PtSn catalysts have a similar particle size leading to the same physical surface area. However, the ESAs of these catalysts are significantly different, as indicated by the CV curves. The large difference between ESA values for the two catalysts could only be explained by differences in detailed nanostructure as a consequence of differences in the preparation of the respective catalyst. On the basis of the preparation process and the CV measurement results, a model has been developed for the structures of these PtSn catalysts as shown in Fig. 15.10. The PtSn-1 catalyst is believed to have a Sn core/Pt shell nanostructure while PtSn-2 is believed to have a Pt core/Sn shell structure. Both electrochemical results and fuel cell performance indicate that PtSn-1 catalyst significantly enhances ethanol electrooxidation. Our previous research found that an important difference between PtRu and PtSn catalysts is that the addition of Ru reduces the lattice parameter of Pt, while Sn dilates the lattice parameter. The reduced Pt lattice parameter resulting from Ru addition seems to be unfavorable for ethanol adsorption and degrades the DEFC performance. In this new work on PtSn catalysts with more... 

Heteroatom Oxidation, Dehydrogenation Electrooxidative kinetic resolution of rac alcohols mediated with a catalytic amount of an optically active A-oxyl was performed in an undivided cell at constant current conditions. A high enantiomeric purity for the recovered alcohol was found, which could be increased by electrolysis at lower temperatures. The optically active A-oxyl was recovered and used repeatedly without change in efficiency and selectivity [368]. Cyclovoltammetry with the A-oxyl (GR, 7S, 10/f)-4-oxo-2,2,7-trimethyl-10-isopropyl-l-azaspiro[5.5]undecane-A-oxyl as catalyst showed for rac-1-phenylethanol a highly enhanced catalytic... 

The formation of these sites could be responsible for the progressive catalyst deactivation which is observed for sustained CO electrooxidation at gold and other metals.(I6b,c) Such sites could be associated with the formation of more tightly bound, and presumably less reactive, oxygen species that are required for CO oxidation. Interestingly, recent combined kinetic/in situ infrared studies of the deactivation of CO oxidation by O2 at Ru- and Rh-Si02 catalysts also exposed a connection between the appearance of higher frequency Vqq bands and the occurence of catalyst deactivation.(21)... 

The chemisorption of sulfur from mixtures of H,S and H2 has been widely studied we have discussed some of the results. Nevertheless, introduction of irreversible and reversible adsorbed sulfur, which is in line with adsorption stoichiometries varying from more than 1 to 0.4 sulfur atom by accessible platinum atom, shows that different adsorbed species are involved in sulfur chemisorption. In fact, electrooxidation of adsorbed sulfur on platinum catalysts occurs at two different electrochemical potentials (42) in the same way, two different species of adsorbed sulfur were identified on gold by electrochemical techniques and XPS measurements (43,44). By use of 35S (45) it was pointed out that, according to the experimental conditions, reducible PtS2 or nonreducible PtS mono-layers can be created. 

The hydrogenation of p-nitrotoluene in the presence of a supported Pd catalyst was carried out in a microreactor with stacked plates with complete selectivity [283,320]. The Pd catalyst was prepared in three different ways. Conversions were 58-98% for an impregnated aluminum oxide wash-coat catalyst, depending on the process conditions. The conversions for an electrodeposited catalyst and an impregnated catalyst on electrooxidized nanoporous substrate were 58 and 89%, respectively. The best latter result is similar to that of a conventional fixed-bed reactor (85%), while the maximum yield of 30% in a microreactor was superior because of the high selectivity. 

It can also be observed from this figure that Sn-containing catalyst is a more effective catalyst for the oxidation of CO than that containing Ru, as a lower onset potential of the oxidation wave is obtained with the former catalyst. It has also to be noted that PtSn catalysts are less active towards methanol electrooxidation than PtRu catalysts (see Section IV. 1). ° However, adsorbed CO species are proposed as reaction intermediates of methanol electro-oxidation, which seems to lead to a paradoxical behavior of PtSn based catalysts. In CO stripping experiments, a negative shift of the onset potential for the oxidation of adsorbed CO on PtSn also occurs. " On the basis of in situ infrared spectroscopy studies coupled with electrochemical measurements, Mo-... 

Figure 10 presents the in situ SPAIRS spectra of CO electrooxidation recorded as a function of potential at a Pt E-TEK modified by Ru with a surface coverage close to 0.20 (RuEiec20%/Pt-C catalyst). (A) spectra are calculated with a reference spectrum taken at 0.1 V vs. RHE, in order to detect the appearance of the band located close to 2345 cm related to CO2 (B) spectra are calculated with reference spectrum at 0.95 V, in order to determine the absorption band located close to 2071 cm assigned to adsorbed COl on Pt (COL/Pt) ° ""° (C) spectra are calculated with reference spectrum at 0.5 V vs. RHE in order to make visible the absorption band related to COl adsorbed on Ru (COl/Ru) located close to 2025 cm-. "°... 

Oxidation carried out in the presence of iodide and bromide ions have been extensively developed during the last two decades. They may often be used with advantage within a potential range in which other kinds of catalyst are not numerous (e.g., organic mediators) or need to be used ex-cell. Moreover, oxidized species obtained from I and Br are of a very broad applicability and are very active, even in water, toward substrates that are more difficult to oxidize than the halides. In most cases it has been demonstrated that such electrooxidized species often play a specific role at the anode (or close to it), whereas the same halogens in bulk solution are much less efficient. Only reactions offering that advantage are reviewed here. 

Cp—Cp coupling occurs, probably via the first-formed palladium phenolate (315) to give the bisquinone methide (316), and the latter spontaneously undergoes intramolecular Diels-Alder reaction to the natural lignan carpanone (317) in 46% yield, with stereocontrol at five chiral centers. High yields, up to 94%, have been recorded using oxygen as oxidant with a metal(II)-salen complex as catalyst, e.g. cobalt(II) salen. A low yield of carpanone was also obtained in electrooxidation. 8... 

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