Anode catalysts electrochemical oxidation

Alternately, with a different anode catalyst, electrochemical oxidative coupling of methane to produce C2 hydrocarbons may be performed in a SOFC cCMR, which... 

The purple permanganate ion [14333-13-2], MnOu can be obtained from lower valent manganese compounds by a wide variety of reactions, eg, from manganese metal by anodic oxidation from Mn(II) solution by oxidants such as o2one, periodate, bismuthate, and persulfate (using Ag" as catalyst), lead peroxide in acid, or chlorine in base or from MnO by disproportionation, or chemical or electrochemical oxidation. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

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. 

Again, points on the curve were the measured acrolein production rates, and the line is the predicted production rate based on the current and the stoichiometry according to eq 9. At higher conversions, we observed significant amounts of CO2 and water, sufficient to explain the difference between the acrolein production and the current. It should be noted that others have also observed the electrochemical production of acrolein in a membrane reactor with molybdena in the anode. The selective oxidation of propylene to acrolein with the Cu—molybdena— YSZ anode can only be explained if molybdena is undergoing a redox reaction, presumably being oxidized by the electrolyte and reduced by the fuel. By inference, ceria is also likely acting as a catalyst, but for total oxidation. 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

Mediated electrochemical oxidation (MEO) is an ex situ treatment technology that uses electricity, acid, and a metal catalyst to destroy organic wastes at low temperatures and pressures. The proprietary CerOx Corporation MEO configuration uses cerium metal as a catalyst to oxidize organic waste into carbon dioxide and water. The process occurs in an acidic solution, typically nitric acid. The first step involves the generation of an oxidant at the anode, followed by the reduction of water or another chemical species at the cathode. This technology serves as a nonthermal alternative to incineration. 

The conditions encountered in studying the stability of catalysts under electrochemical load are very complicated. Stability depends strongly on the potential and on the nature of the working substance. For example, pure CoTAA, when used as an oxygen catalyst at potentials of about 800 mV, is active only for a period of some hours. If, however, it is used in the anode for the oxidation of formic acid at 350 mV, it will give more than 6 months (4000 hours ) continuous service under the same conditions. 

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... 

Anodic addition to an electron-rich heteroaromatic compound is used to transform furan to 2,5-dimethoxy-2,5-dihydrofuran, a valuable synthetic intermediate. Again, an indirect electrochemical process occurs. The bromide ion as redox catalyst is electrochemically oxidized to give bromine, which then acts as chemical oxidant for furan [7] ... 

Anodic substitution can also occur easily in the a-position to heteroatoms like nitrogen or oxygen. Thus, the indirect electrochemical oxidation of ethylene glycol dimethyl ether in methanol using tris(2,4-dibromophenyl)amine as redox catalyst leads to the formation of 2-methoxyacetaldehyde dimethylacetal [25] ... 

Several, different, electrochemical oxidations of 26 to 27 have been reported. Using a variety of electrodes (copper, Monel metal, nickel, or silver), 26 was oxidized in aqueous potassium hydroxide solution containing potassium chromate or potassium permanganate, to afford 27 in 70-85% yield.118,119 This electrochemical oxidation has been conducted in aqueous, alkaline solution in the presence of a surfactant, but with added metal catalyst, to give 27 in 85-95% yield.120 Alternatively, the oxidation has been performed by using an anode on which nickel oxide was deposited. This anode, in a solution of 26 at pH >9, with or without nickel salts, afforded 27 in >90% yield.121 A number of additional publications described122-140 other modifications of the... 

A library of 35 different catalysts fixed on electrochemically oxidized aluminum either in oxalic acid (Lib 1) or sulfuric add (lib 2) was tested at 450 °C and 1.1 bar. The methane-to-oxygen ratio was set to 1 in order to establish the potential of the catalyst to form intermediates. Figure 3.20 shows experimental results for a residence time of 550 ms and a screening time of 60 s. The conversion rate followed directly the platinum content in the catalysts. The higher the platinum content, the higher is the degree of conversion. Catalyst carrier formed by anodization of... 

The electrochemical reaction proceeds most effectively in the presence of a catalyst, and the nature of the catalyst can have a significant effect upon the electrode overpotentials. As a matter of convenience, all of the early work in the electrolyzer development used platinum as both the anode (SO2 oxidation electrode) and cathode (H2 generation electrode) catalyst. It was recognized, however, that although platinum might be a technically satisfactory catalyst for the cathode, it was only marginally suitable as the anodic catalyst. 

Chen, K.Y., Sun, Z., and Tseung, A.C.C., Preparation and characterization of high-performance Pt-Ru/WOj-C anode catalysts for the oxidation of impure hydrogen, Electrochem. Solid-State Lett., 3, 10, 2000. 

Adcock, P.A. et al.. Transition metal oxides as reconfigured fuel cell anode catalysts for improved CO tolerance polarization data, J. Electrochem. Soc., 152, A459, 2005. 

Kuchynka et al. [125] studied the electrochemical oxidative dimerization of methane to C2 hydrocarbon species using perovskite anode electrocatalysts. Three designs of solid oxide fuel cells were used, including tubular and flat plate solid electrolytes. The maximum current density for the dimerization reaction at these electrocatalysts was related to the oxygen binding energies on the catalyst surface. The anodic reaction was ... 

The anode catalyst should possess high electrochemical oxidation activity and minimal conventional oxidation activity the cathode catalysts should exhibit selectivity for the reduction of O2 to. Single chamber fuel cell studies using Ni-SDC as an anode and Smo.sSr 0.5C0O3 as a cathode showed that the anode catalyst was active for partial oxidation of C2H6 to produce CO and H2 for further electrochemical oxidation on the anode catalyst surface. 

Tail-to-tail coupling of radicals obtained in the anodic oxidation of triphenyl-amines results in the formation of tetraphenylbenzidines. Oxidation of triarylamines to the di-cation results in the formation of the carbazoles, as observed for Ai -alkyl-p,p -disubstituted diphenylamines [1-3, 78]. The cation radicals of triarylamines with substituents in the para position of the aryl groups, which can protect them against nucleophilic attack, are very stable and can be used as organic redox catalysts for indirect electrochemical oxidation reactions. Depending on the substitution pattern on the phenyl group the oxidation potentials of the triarylamines can be tuned over a wide range [Eoy. = 0.7-2.0 V) and many of these have been used as redox catalysts in numerous indirect electrochemical reactions [1-3, 79-83]. 

Hamakawa S, Shiozaki R, Hayakawa T, Suzuki K, Murata K, Takehira K, Koizumi M, Nakamura J, and Uchijima T. Partial oxidation of methane to synthesis gas using Ni/Cao.8Sro.2Ti03 anode catalyst. J. Electrochem. Soc. 2000 147 839-844. 

Because the direct electrochemical oxidation of NAD(P)H has to take place at an anode potential of +900 mV vs. NHE or more, only rather oxidation-stable substrates can be transformed without loss of selectivity, thus limiting the applicability of this method. The electron transfer between NADH and the anode may be accelerated by the use of a mediator. At the same time, electrode fouling, which is often observed in the anodic oxidation of NADH, can be prevented. Synthetic applications have been described for the oxidation of 2-hexene-1 -ol and 2-butanol to 2-hexenal and 2-butanone catalyzed by yeast alcohol dehydrogenase (YADH) and the alcohol dehydrogenase from Thermoanaerobium brockii (TBADH), respectively, with indirect electrochemical regeneration of NAD" and NADP", respectively, using the tris(3,4,7,8-tetramethyl-l,10-phenan-throline) iron(II/III) complex as redox catalyst at an anode potential of 850 mV vs. NHE [106]. Under batch electrolysis conditions using a carbon felt anode, the turnover number per hour was 40. The current efficiency reached between 90 and 95%. 

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