Hydrogen electrooxidation

Such information can be obtained from cyclic voltammetric measme-ments. It is possible to determine the quantity of electricity involved in the adsorption of hydrogen, or for the electrooxidation of previously adsorbed CO, and then to estimate the real surface area and the roughness factor (y) of a R-C electrode. From the real surface area and the R loading, it is possible to estimate the specific surface area, S (in m g ), as follows ... 

In addition to these different types of alloys, some studies were also devoted to alternatives to platinum as electrocatalysts. Unfortunately, it is clear that even if some catalytic activities were observed, they are far from those obtained with platinum. Nickel tungsten carbides were investigated, but the electrocatalytic activity recorded for methanol oxidation was very low. Tungsten carbide was also considered as a possible alternative owing to its ability to catalyze the electrooxidation of hydrogen. However, it had no activity for the oxidation of methanol and recently some groups showed that a codeposit of Pt and WO3 led to an enhancement of the activity of platinum. ... 

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. 

Figure 4. Typical polarization curve of the electrooxidation of hydrogen and electroreduction of oxygen the exchange current density, io, determined by extrapolation of E vs, log i to the reversible potential...

Electrooxidation of carbon monoxide to carbon dioxide at platinum has been extensively studied mainly not least because of the technological importance of its role in methanol oxidation in fuel cells [5] and in poisoning hydrogen fuel cells [6]. Enhancing anodic oxidation of CO is critical, and platinum surfaces modified with ruthenium or tin, which favor oxygen atom adsorption and transfer to bound CO, can achieve this [7, 8]. 

Recently, the electrochemical behavior saturated alcohols, that is, propargyl alcohol (HCSCCH2OH, PA) [145], benzyl alcohol (C6H5CH2OH, BA) [146] andallylalcohol [147], has been studied at Pd electrodes in an acid medium by cyclic voltammetry, chronoamperometry, and on-line mass spectrometry. For BA, it was observed that the fragmentation of the molecules occurs at potentials in the hydrogen ad-sorption/absorption region of palladium, whereas for PA the adsorbates maintain the C3-chain. On the other hand, the yields of the electroreduction and electrooxidation products for both PA and BA differ from those obtained at Pt [146,148,149]. 

It has been known that the electrolysis in an MeCN- NaClO system generates an acid The hydrogen has to originate from the solvent. A mechanism for hydrogen abstraction from acetonitrile by the electrooxidatively generated radical 104- to produce perchloric acid has been proposed, but no evidence for the succinonitrile formation appeared (Eq. (5)). The detection of the 104- radical by the aid of HSR was tried But it was found to be difficult to differentiate between the perchlorate radical and the radical from chlorine dioxide The electrolysis in a CH Clj—... 

Electrochemical oxidation of alkyl aryl ethers results m oxidative dealkylation and coupling of the intermediate radicals Electrooxidation in the presence of hydrogen fluoride salt leads to fluonnated dienones [66] (equation 58)... 

The amperometric response, described above, is based on the electrooxidation at the platinum-modified electrode of the enzymatically generated hydrogen peroxide at + 0.60 V vs. Ag/AgCl. 

Thin-layer Studies. The thin-layer electrochemical system was developed to address the lack of sensitivity of a preliminary bulk amperometric activity assay (77). The first set of thin-layer studies was taken to characterize the thin-layer cells in soluble enzyme solutions and to determine if there were any interferences to the detection of hydrogen peroxide. Preliminary thin-layer studies (23) indicated that the oxidation of hydrogen peroxide could be detected at approximately 1080 mV with only minimal interference from the oxidation of glucose by gold. The addition of chloride ion to the solution further suppressed the glucose electrooxidation interference. 

Do JS, Yeh WC. In situ paired electrooxidative degradation of formaldehyde with electrogenerated hydrogen peroxide and hypochlorite ion. J Appl Electrochem 1998 28 703-710. 

It was pointed out that adsorption of propanethiol or hexanethiol occurs according to the following reaction (58) RSH —> RSads + H+ + e desorption can occur by an electrooxidation, RSads + 3H20—> RSO,H + 5H + + 5e. A comparative study of the amount of thiol adsorbed and the effect of this adsorption on hydrogen chemisorption showed that one molecule of thiol is adsorbed on one accessible platinum atom (58). 

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 has studied process of hydrogen electrooxidation with initial surface diamond (1) and after heat-treated (2) from concentration of culfuric acid (Fig. 2). 

Figure 3. The exchange current density of hydrogen electrooxidation from acetylene black AD-100 (1), WC(2), VC(3), the initial nanodispersed diamond (4), the initial nanodispersed diamond after heat - treated treatment (5).

However, for some electrocatalytic reactions, such as the electrooxidation of alcohols, aldehydes or acides, and also the electro reduction of oxygen, lead adatoms can exhibit a promoting effect (3-7). Moreover, lead can change the selectivity in the case of electrocatalytic reductions of nitrocompounds (8), whereas it inhibits the adsorption of hydrogen on platinum (9,10),... 

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