Anode catalysts electrocatalytic oxidation

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

Platinum, ruthenium and PtRu alloy nanoparticles, prepared by vacuum pyrolysis using Pt(acac)2 and Ru(acac)3 as precursors, were applied as anode catalysts for direct methanol oxidation . The nanoparticles, uniformly dispersed on multiwaUed carbon nanotubes, were all less than 3.0 nm in size and had a very narrow size distribution. The nanocomposite catalysts showed strong electrocatalytic activity for methanol oxidation, which can... 

Materials effective as anode catalysts for epoxidation of 1-hexene by the method in Figure 2 were screened. Among various metal oxides, metal salts and metal blacks tested, the most active and selective anode catalyst for the formation of 1,2-epoxy hexane was Pt black (Table 1). The oxidation efficiency for the formation of epoxide defined by equation 9 was about 26% and its selectivity was 66%. Pt black samples obtained from different producers or prepared in this work showed quite low electrocatalytic activity. However, the calcination of these inactive Pt blacks in air at 673 K substantially enhanced the catalytic activities of these samples. XPS studies on various Pt black samples suggested that a Pt02 phase was associated with the active oxygen for the epoxidation. 

In this chapter, the authors review some developments in the application of Pd-based nanostructures for the electrocatalytic oxidation of alcohols in alkaline media. Special focus is given to the role of such nanostructures (mono-, hi-, or ternary metallic catalysts) and/or their supporting platforms in lowering the anodic overpotential (onset potential), enhancing the catal3dic current density, and improving the stability or lifetime of the catalysts. 

One possibility is the electrocatalytic preferential oxidation (cPrOx) (Zhang Datta, 2005), which is simply the electrochemical version of PrOx. By exploiting the strong selective CO adsorption on the anode Pt catalyst (Eqn (3.22)), coupled with the fact that H2O can be activated by the anode catalyst at certain electrode potentials to produce surface hydroxyl radicals, especially on Ru cocatalyst, the cPiOx process involves the following chemistry ... 

Figure 4.44. Anodic polarization curves of CH3OH on Wo,8Moo,2C catalyst as a function of temperature. Catalyst load 0.2 g cm , 4 M CH3OH and 1 M H2SO4 [213]. (Reproduced by permission of ECS—The Electrochemical Society, from Kawamura G, Okamoto H, Ishikawa A, Kudo T. Tungsten molybdenum carbide for electrocatalytic oxidation of methanol.)...

By working at higher temperatures of 170 °C, Arico et al. reported a CO2 efficiency of ca. 95% in a DEFC with PtRu anode electrode [116]. The relative product distribution for the electrocatalytic oxidation of ethanol in a DEFC with Pt and Pt-Ru anode catalysts has been also studied by Wang et al. [117]. They used on-line mass spectroscopy in the temperature range of 150-190 °C. Acetaldehyde was reported as the main reaction product and CO2 was detected as a minor product in the studied catalysts. Remarkably, the formation of CO2 increased with the H20/ethanol mole ratio used as feed. Fujiwara and coworkers [118] measured the carbon dioxide and acetaldehyde selectivity in ethanol electrooxidation by DBMS on electrodeposited Pt and PtRu and reported that Ru addition improves slightly CO2 selectivity, however, carbon dioxide remains as a minority product. 

Qi, J., Xin, L., Chadderdon, D.J., Qiu, Y., Jiang, Y., Benipal, N., Liang, C.H., and Li, W.Z. (2014) Electrocatalytic selective oxidation of glycerol to tartronate on Au/ C anode catalysts in anion exchange membrane fuel cells with electricity cogeneration. Applied Catalysis B Environmental, 154, 360-368. 

In Table 1 the ancxlic reactions that have been studied so far in small cogenerative solid oxide fuel cells are listed. One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus, the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) to the selectivity of Pt and Pt-Rh alloy catalysts for the corresponding commercial catalytic oxidation. The same applies for Ag, which turns out to be equally selective as an electrocatalyst for the anodic partial oxidation of methanol to formaldehyde, ... 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

The electrocatalytic activity of the nanostructured Au and AuPt catalysts for MOR reaction is also investigated. The CV curve of Au/C catalysts for methanol oxidation (0.5 M) in alkaline electrolyte (0.5 M KOH) showed an increase in the anodic current at 0.30 V which indicating the oxidation of methanol by the Au catalyst. In terms of peak potentials, the catalytic activity is comparable with those observed for Au nanoparticles directly assembled on GC electrode after electrochemical activation.We note however that measurement of the carbon-supported gold nanoparticle catalyst did not reveal any significant electrocatalytic activity for MOR in acidic electrolyte. The... 

Raney Ni with additives is also used [77, 276]. In particular, valve metals are added to stabilize the catalyst structure [102,410, 411], thus decreasing the recrystallization and sintering which always takes place as the solution temperature is raised [412] (which points to the high energy state of such an electrode structure). In this respect, potential cycling has also been observed to be detrimental since it can induce recrystallization [407]. This is probably the reason why surface oxidation may be deleterious with Raney structures [390] while it normally results in improved electrocatalytic properties with bulk Ni electrodes [386]. However, after prolonged cathodic load resulting in deactivation, Raney Ni electrodes can be reactivated (temporarily) by means of anodic sweeps [405]. 

In terms of heterocatalytic characteristics, it is worthy to note that the catalytic characteristics could be changed with the structure variation of the catalyst. For rare earth, there are many f-electron orbits of rare earth atoms and, when element of rare earth was doped into the DSA-coating materials, additional energy bands can be induced in the structure of coating metal oxides. This might help to develop convenient channels for electrons transition and help to enhance the electrocatalytic characteristics of the anodes also. 

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