Methanol, oxidation

The direct methanol fuel cell is a special form of low-temperature fuel cells based on PEM technology. In the DMFC, methanol is directly fed into the fuel cell without the intermediate step of reforming the alcohol into hydrogen. Methanol is an attractive fuel option because it can be produced from natural gas or renewable biomass resources. It has the advantage of a high specific energy density, since it is liquid at operation conditions. The DMFC can be operated with liquid or gaseous methanoFwater mixtures. 

Very few electrode materials have been shown to be capable of adsorbing methanol in acidic media, and of these only Pt-based materials display a high enough sta-bihty and activity to be attractive as catalysts. The overall reaction mechanism for methanol oxidation is (Eq. 9-34)  

The scheme shows that CO is formed during the oxidation of methanol. This CO species can block the surface of the catalyst and hinder any further reaction. For this reason a munber of co-metals are usually added to the Pt catalyst to facilitate CO re- 

Much research has been carried out on catalysts for methanol oxidation (see also Section 9.3.4) to find a catalyst which can avoid the poisoning effect of the CO species. Several promoters have been found to increase the activity of the Pt catalyst. One of the most important and most investigated promoter is Ru. A bimetallic alloy consisting of Pt and Ru supported on carbon has thus far been one of the major research interests in DMFC technology. The action of Ru can be explained as follows. The adsorption of H2O molecules at Ru surfaces takes place with lower overvoltages (Eq. 9-35). 

The subsequent oxidation of COad occurs by the adsorbed OH species (Eq. 9-36). 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were 

Although ORR catalysts for DMFCs are mostly identical to those for the PEM fuel cell, one additional and serious drawback in the DMFC case is the methanol crossover from the anode to the cathode compartment of the membrane electrode assembly, giving rise to simultaneous methanol oxidation at the cathode. The 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic 

Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

In principle, this operation consists in passing a mixture of air and methanol vapor over a catalyst bed at about atmospheric pressure, and absorbing the product in water. Two main methods are available, which essentially dtfler in the type of catalyst employed, and which lead either to dehydrogenation combined with partial oxidadon, or to oxHatioo. Many variants have been developed around these two basic situations. 

This means that the following oxidation reaction partially occurs  

The equilibrium values of transformation (1.4) are 50 per cent at 4C0 C, 90 per cent at. 500 C, and 99 per cent at 700°C. It is therefore necessary to operate at about 600 C to obtain high once-through conversions. 

This highly exothermic reaction is complete. It can be carried out at the optimal temperature to guarantee high selectivity and high conversion. 

Since the air/methanol mixture is flammable in a methanol concentration range between 6 to 25 and 9 to 37 per cent volume according to temperature and pressure, operations can be carried out in two ways  

Significant changes in the activity and the selectivity in this system can be made by varying the nature of the support. The use of amphoteric oxide supports such as Zr02 and Ti02 catalyzed the selective formation of methyl formate, whereas the use of non- 

The redox properties for these systems were also found to be quite sensitive the nature of the interaction between the HPVMo Keggin structure and the oxide support. 

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Magnesium Air, beryllium fluoride, ethylene oxide, halogens, halocarbons, HI, metal cyanides, metal oxides, metal oxosalts, methanol, oxidants, peroxides, sulfur, tellurium... 

Ion implantation has also been used for the creation of novel catalyticaHy active materials. Ruthenium oxide is used as an electrode for chlorine production because of its superior corrosion resistance. Platinum was implanted in mthenium oxide and the performance of the catalyst tested with respect to the oxidation of formic acid and methanol (fuel ceU reactions) (131). The implantation of platinum produced of which a catalyticaHy active electrode, the performance of which is superior to both pure and smooth platinum. It also has good long-term stabiHty. The most interesting finding, however, is the complete inactivity of the electrode for the methanol oxidation. 

Liver alcohol dehydrogenase (ADH) is relatively nonspecific and will oxidize ethanol or other alcohols, including methanol. Methanol oxidation yields formaldehyde, which is quite toxic, causing, among other things, blindness. Mistaking it for the cheap... 

C.G. Vayenas, and S. Neophytides, Non-Faradaic Electrochemical Modification of Catalytic Activity 3. The Case of Methanol Oxidation on Pt, J. Catal. 127, 645-664 (1991). 

Methanol oxidation on Pt has been investigated at temperatures 350° to 650°C, CH3OH partial pressures, pM, between 5-10"2 and 1 kPa and oxygen partial pressures, po2, between 1 and 20 kPa.50 Formaldehyde and C02 were the only products detected in measurable concentrations. The open-circuit selectivity to H2CO is of the order of 0.5 and is practically unaffected by gas residence time over the above conditions for methanol conversions below 30%. Consequently the reactions of H2CO and C02 formation can be considered kinetically as two parallel reactions. 

Qualitatively similar behaviour for methanol oxidation on Pt/YSZ was reported by Cavalca, Larsen, Vayenas and Haller51 who used the single chamber design51 instead of the fuel-cell type design of the earlier study of Neophytides and Vayenas.50 Cavalca et al51 took advantage of the electrophobic... 

Methanol oxidation on Ag polycrystalline films interfaced with YSZ at 500°C has been in investigated by Hong et al.52 The kinetic data in open and closed circuit conditions showed significant enhancement in the rate of C02 production under cathodic polarization of the silver catalyst-electrode. Similarly to CH3OH oxidation on Pt,50 the reaction exhibits electrophilic behavior for negative potentials. However, no enhancement of HCHO production rate was observed (Figure 8.48). The rate enhancement ratio of C02 production was up to 2.1, while the faradaic efficiencies for the reaction products defined from... 

W.Y. Lee The group of Professor Lee in Korea has made interesting NEMCA studies of methanol oxidation on Ag/YSZ (Chapter 8). 

Methane oxidation and partial oxidation, electrochemical promotion of, 308 dimerization, 470 reforming, 410 Methanol dehydrogenation electrochemical promotion of, 403 selectivity modification, 404 Methanol oxidation electrochemical promotion of 398 selectivity modification, 400 Microscopy... 

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. 

Fig. 2 shows the dynamic response of stack voltage to the step changes of various applied current densities. Like the former case of applied current pulses, the response exhibits the overshooting and relaxation which is caused by the methanol oxidation kinetics on the catalyst surface. The steady state stack voltage was found to be the same for both pulse and step loads with the same current density. 

Wasmus S, Kiiver A (1999) Methanol oxidation and direct methanol fuel cells a selective review. J Electroanal Chem 461 14-31... 

Bolivar H, Izquierdo S, Tremont R, Cabrera CR (2003) Methanol oxidation at Pt/MoOx/ MoSc2 thin film electrodes prepared with exfohated MoSe2. J Appl Electrochem 33 1191-1198... 

This results from the slow kinetics of methanol oxidation and oxygen reduction. An additional loss is due to the cell resistance (arising mainly... 

III. ELECTRODE KINETICS AND ELECTROCATALY SIS OF METHANOL OXIDATION—ELECTROCHEMICAL AND SPECTROSCOPIC INVESTIGATIONS... 

Figure 11. Tafel plots for methanol oxidation on (a) an E-Tek Pt-C electrode and (b) an E-Tek PtojRuoj-C electrode (1 M CH3OH in 0.5 M HQO4, 50 C, metal loading 0.1 mg cm" ).

The use of adatoms of foreign metals obtained by imderpotential deposition on the platinum surface is another convenient method for investigating the effect of a promoter on the electrocatalytic properties of platinum. However, the effect of adatoms in this case has been shown to be not as effective for electrooxidation of methanol as for the oxidation of other organic molecules such as formic acid adatoms of tin, however, showed a positive effect on the rate of methanol oxidation. ... 

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. ... 

What is the stability of such a bimetallic electrode, as well as that of other bimetallic and multimetallic electrodes, which exhibit high activity for methanol oxidation ... 

The exchange current density for methanol oxidation depends on the methanol concentration, i.e. ... 

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