Methanol electrooxidation process

Ralph etal. (2003) give, at Figure 2, a complex methanol electrooxidation process at a platinum particle, which highlights the extent of development problems. The same topic was discussed at Palm Springs by University of Washington authors. 

In the above reactions, the oxidation process takes place in the anode electrode where the methanol is oxidized to carbon dioxide, protons, and electrons. In the reduction process, the protons combine with oxygen to form water and the electrons are transferred to produce the power. Figure 9-1 is a reaction scheme describing the probable methanol electrooxidation process (steps i-viii) within a DMFC anode [1]. Only Pt-based electrocatalysts show the necessary reactivity and stability in the acidic environment of the DMFC to be of practical use [2], This is the complete explanation of the anodic reactions at the anode electrode. The electrodes perform well due to the presence of a ruthenium catalyst added to the platinum anode (electrode). Addition of ruthenium catalyst enhances the reactivity of methanol in fuel cell at lower temperatures [3]. The ruthenium catalyst oxidizes carbon monoxide to carbon dioxide, which in return helps methanol reactivity with platinum at lower temperatures [4]. Because of this conversion, carbon dioxide is present in greater quantity around the anode electrode [5]. 

An electrooxidation process was developed by Asahi Chemical Industry ia Japan, and was also piloted by BASF ia Germany. It produces high purity sebacic acid from readily available adipic acid. The process consists of 3 steps. Adipic acid is partially esterified to the monomethyl adipate. Electrolysis of the potassium salt of monomethyl adipate ia a mixture of methanol and water gives dimethyl sebacate. The last step is the hydrolysis of dimethyl sebacate to sebacic acid. Overall yields are reported to be about 85% (65). 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

The steady-state surface coverage by the carbon monoxide residue can be studied by anodic stripping voltammetry. By this technique, it is possible to separate the adsorption residue contribution from the bulk electrooxidation process. The micro-flux cell is adapted with a big flask containing the supporting electrolyte, which is used to wash the cell until there is no trace of methanol in solution. The current vs. potential profile mn from the adsorption potential upward is the tripping profile for the oxidation of the adsorbed residue. An example is presented in Figure 2.4. 

Further reported examples include electrocatalytic processes and their intermediates [313, 314]. Formate could be identified as an active intermediate of methanol electrooxidation at a polycrystalline platinum electrode [315]. Water molecules coadsorbed during methanol adsorption on platinum were identified as those species that react subsequently with COad that was formed as a result of methanol chemisorption [316]. The high sensitivity of SEIRAS allows mapping of two-dimensional spectra (for selected examples, see [285]). Finally, two-dimensional correlation analysis of electrochemical reactions becomes possible [317]. 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

Compared with methanol electrooxidation, ethanol electrooxidation seems to be a more complicated process because it involves 12-electron transfer per ethanol molecule and cleavage of C—C bond. In order to speed up DEFCs development, it is necessary and important to develop a novel electrocatalyst with a high activity to ethanol electrooxidation. 

The crucial aspect is thus to determine the fate of the ( CHO), species. Possible mechanisms for its oxidative removal are schematically shown in Fig. 9. From this scheme, it appears that the desorption of the formyl species can follow different pathways through competitive reactions. This schematic illustrates the main problems and challenges in improving the kinetics of the electrooxidation of methanol. On a pure platinum surface, step (21) is spontaneously favored, since the formation of adsorbed CO is a fast process, even at low potentials. Thus, the coverage... 

The electrooxidations of allyl- and propenyl-phenols may follow even more different pathways leading to a rich diversity of products. At a carbon anode in methanol-lithium perchlorate, ferulic acid formed the tricyclic diacid (306 c/. Section 2.9.5), in a concentration-dependent process probably involving [4 + 2] dimerization of the primary oxidation product, the dienone acetal (307). 

The DMFC operates, as its name suggests, by direct, complete electrooxidation of methanol to CO2 at the cell anode. The methanol anode is coupled in the DMFC with an air cathode, completing a cell schematically shown in Fig. 50. In the majority of recent development efforts, the DMFC has been based on a protonconducting polymeric membrane. The uniqueness of this type of fuel cell is the direct anodic oxidation of a carbonaceous fuel. Such a direct electrochemical conversion process of liquid fuel and air to electric power at low temperatures can provide a basis for a very simple fuel-cell system. 

Electrochemical oxidation of complex 1 in die presence of methanol leads to considerable enhancement of the oxidative currents (Figure 2), consistent with an electrocatalytic oxidation process. The onset of this catalytic current coincides with the irreversible Pt(II/IV) oxidative wave at 1.70 V. The bulk electrolysis of 1 and dry methanol were performed at 1.70 V (onset of catalytic current) in 0.7 M TBAT/DCE. Gas chromatographic analysis of the solution indicated that dimethoxymethane (DMM, formaldehyde dimethyl acetal) and methyl formate (MF) are formed (Scheme 1). This result is consistent with the electrooxidation of dry methanol on PtRu anodes, which yields DMM after acid-catalyzed condensation of the formaldehyde product with excess methanol (36). Bulk electrolysis of methanol in the presence of heterobimetallic complex 1 resulted in higher current efficiencies than those obtained from the mononuclear model compound CpRu(PPh3)2Cl (2) (Table II), No oxidation products were found when the electrolysis was performed at 1.70 V in the absence of a Ru complex or in the presence of the Pt model compound (ri -dppm)PtCl2. These results suggest that Pt enhances the catalytic activity of the Ru metal center. 

Cyclic voltammograms of complexes 8 and 9 both exhibit a single oxidation wave assigned to the Ru(II/III) oxidation process (Table 1). The electrooxidation of neat methanol with complexes 8 and 9 was performed at 1.2S V and at 1.40 V. Higher current efficiencies for the electrooxidation of methanol are obtained with the Ru/TPPMS complexes 8 and 9 than are obtained with complexes 2 and 7 (Table IV). Complex 7 has been shown to form significantly more DMM than MF at lower potentials this effect is also observed during the electrolysis with complex 9. When the potential was decreased from 1.40 V to 1.25 V the amount of DMM in the product mixture increased from 90.5% to 100%. To our knowledge, no other examples of selective electrooxidation of methanol to DMM have been reported. 

Due to a large number of electrons involved in Eq. (4.4), the equilibrium potential of 0.02 V cannot be readily achieved, the reaction is slow, and the total oxidation process consists of a pattern of parallel reactions. On pure Pt, a complete electrooxidation of methanol to CO2 takes... 

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