Electrocatalytic oxidation of methanol

A very interesting approach to electrocatalysis has been developed in recent years, for the oxidation of methanol in the so-called direct-methanol fuel cell (DMFC). Employing a Pt-Ru alloy was found to increase the rate of oxidation of methanol very substantially, compared to the performance of each of the two metals in their pure form. It turns out that an OHads species is formed on Ru at a less positive potential, where the platinum surface is still bare and highly active for adsorbing methanol. The rate-determining step is assumed to be the interaction between adjacent adsorbed species 

Further interaction between Ru-OH and the partially oxidized organic molecule on the surface leads eventually to the total oxidation of methanol to CO2 and H2O. 

In this section we will discuss the role of surface modification to enhance electrocatalytic oxidation of methanol, one of the interesting components for fuel cell technology. Perhaps the most successful promoter of methanol electrooxidation is ruthenium. Pt/Ru catalysts appear to exhibit classical bifunctional behavior, whereas the Pt atoms dissociate methanol and the ruthenium atoms adsorb oxygen-containing species. Both platinmn and ruthenimn atoms are necessary for eomplete oxidation to occur at a significant rate. The bifunctional mechanism can account for a decrease in poisoning from methanol, as observed for Pt/Ru alloys. Indeed, CO oxidation has been attributed to a bifimctional mechanism that reduces the overpotential of this reaction by 0.1 V on the Pt/Ru surface. 

Furthermore, a modifier may alter the electronic nature of the electrode. By changing the electric field at the surface, a modifier may affect the reactant-substrate interactions. A change in reactant-substrate interactions may be manifested, for instance, in a change in molecular orientation of the reactant molecule adsorbed on the surface. Clear evidence does exist for the influence of surface electronic properties on catalytic reactions. It is apparent that a modifier which acts duough an electronic effect could influence both reaction kinetics and the tendency to poison. 

Pt/Ru alloys exhibit lower susceptibility to poisoning than does pure platinum, as can be shown by a slower decay in current-time curves following a potential step. The reaction of methanol on Pt/Ru alloys results in CO2 production at lower potential than does reaction on pine Pt, indicating enhanced complete oxidation by die ruthenium. When a high coverage of adsorbed CO develops on the Pt sites, the Ru sites facilitate its oxidation. 

The promotion of methanol electrooxidation by tin modifier has been studied, too. The presence of tin appears to enhance the reaction at low potential, increasing the production of CO2. The kinetic enhancement observed most likely result from an increase in the number of active catalyst sites for reaction due to decreased poisoning. 

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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... 

The electrocatalytic oxidation of methanol has been thoroughly investigated during the past three decades (see reviews in Refs. 21-27), particularly in regard to the possible development of DMFCs. The oxidation of methanol, the electrocatalytic reaction, consists of several steps, which also include adsorbed species. The determination of the mechanism of this reaction needs two kinds of information (1) the electrode kinetics of the formation of partially oxidized and completely oxidized products (main and side products) and (2) the nature and the distribution of intermediates adsorbed at the electrode surface. 

Attwood PA, McNicol BD, Short RT. 1980. Electrocatalytic oxidation of methanol in acid electrolyte—Preparation and characterization of noble-metal electrocatalysts supported on pretreated carbon-fiber papers. J Appl Electrochem 10 213-222. 

Clavilier J, Lamy C, Leger JM. 1981a. Electrocatalytic oxidation of methanol on single-crystal platinum-electrodes—Comparison with polycrystalline platinum. J Electroanal Chem 125 (1) 249-254. 

The electrocatalytic oxidation of methanol was discussed on page 364. The extensively studied oxidation of simple organic substances is markedly dependent on the type of crystal face of the electrode material, as indicated in Fig. 5.56 for the oxidation of formic acid at a platinum electrode. 

Wu, B., et al., Functionalization of carbon nanotubes by an ionic-liquid polymer Dispersion ofPt and PtRu nanoparticles on carbon nanotubes and their electrocatalytic oxidation of methanol. Angewandte Chemie International Edition, 2009. 48(26) p. 4751-4754. 

The methanol oxidation current increases because Pt-OH helps electrocatalytic oxidation of methanol, but decreases again once the siurface is completely covered with Pt-OH. This causes an oxidation ciurent peak at 850 mV. The oxidation current increases again after the potential is high enough (1200 mV) to oxidixe methanol on Ptr-OH surface. 

J. Luo, M. M. Maye, N. N. Kariuki, L. Wang, P. Njoki, Y. Lin, M. Schadt, H. R. Naslund, and C. J. Zhong, Electrocatalytic oxidation of methanol Carbon-supported gold-platinum nanoparticle catalysts prepared by two-phase protocol, Catal. Today 99, 291-297 (2005). 

Platinum carbonylate anion clusters like [Pt3(CO)6] can be obtained by alkaline reduction of [PtCh] in a CO atmosphere. From [Pt3(CO)s] other higher nuclear-ity anions can be obtained. In this context, several examples have been reported in which this type of anionic cluster is used in the preparation of catalysts by impregnation or exchange methods. Salts of [Pt3 (CO)6 ] (n = 3, 5) have been used to prepare, by impregnation, dispersed platinum on ZnO and MgO [49] and, by ion exchange methods, to prepare Pt3 /C electrodes for the electrocatalytic oxidation of methanol [50]. A salt of [Pti2(CO)24] has recently been used to prepare... 

Janik, M.J., Taylor, C.D. and Neurock, M. (2007) First principles analysis of the electrocatalytic oxidation of methanol and carbon monoxide. Top. Catal., 46, 306-319. 

Electrocatalytic oxidation of methanol on platinum based catalysts... 

Electrocatalytic oxidation of methanol was conducted over Pt and Pt-Ru-WOs deposited graphite electrodes in H2SO4 medium. Hydrogen adsorption experiment was conducted with H2SO4 in absence of methanol. XPS analysis was done to confirm existence of Pt, Ru and WO3 on electrodes. Constant potential oxidation was used to monitor the activity of electrodes with respect to time. 

Pt-Ru-WOs deposited graphite electrode shows better activity than pure Pt deposited graphite electrode. Presence of WO3 with Pt prevents the measurement of real surface area of platinum on the electrode. Increase of both H2SO4 and methanol concentration increases methanol oxidation. Potential around 0.5-0.6 V vs SCE appears to be the best for electrocatalytic oxidation of methanol. At all potentials Pt-Ru-WOs/C shows higher activity than Pt/C during long term polarisation. 

In general, a complex structure is designed toward a particular end, perhaps to facilitate an electrode process (such as the electrocatalytic oxidation of methanol), or to inhibit a reaction (such as metallic corrosion), or to produce selectivity toward a particular process (such as the enzyme-catalyzed oxidative determination of glucose in whole blood). The end is... 

In order to improve the electrocatalytic properties of methanol electrodes, and to reduce the poisoning phenomenon usually observed with bulk platinum, different platinum based alloys were considered such as Pt-Ru, and Pt-Sn, etc. [153]. Therefore such alloys were also dispersed into electron conducting polymers. Hable et al. [53] were apparently the first authors to disperse Pt-Sn catalyst particles in a polyaniline matrix, in order to activate the oxidation of methanol. They evaluated the Pt/Sn ratio by X-ray Photoelectron Spectroscopy and found that small amounts of Sn (e.g. Pt/Sn ratios of 10/1) were sufficient to enhance the electrocatalytic oxidation of methanol. Pt was found to be in the Pt(0) state whereas Sn was in an oxidized form. Similar observations concerning the enhanced electrocatalytic activity of Pt-Sn particles incorporated in PAni films were made by Laborde et al. [154]. Such Pt-Sn alloys are also very active for the electrocatalytic oxidation of ethanol [68,154]. 

Y Takasu, W. Sugimoto, and Y Murakami, Electrocatalytic oxidation of methanol and related chemical species on ultra fine Pt and Pt-Ru particles supported on carbon. Catalysis Surveys from Asia, 7(1) (2003) 21-29. 

F.-J. Liu, L.-M. Huang, T.-C. Wen, C.-F. Li, S.-L. Huang, and A. Gopalan, Effect of deposition sequence of platinum and ruthenium particles into nanofihrous network of polyaniline-poly (styrene sulfonic acid) on electrocatalytic oxidation of methanol, Synth. Met., 158, 603-609 (2008). 

P.O. Esteban, J.M. Leger, C. Lamy, and E. Genies, Electrocatalytic oxidation of methanol on platinum dispersed in polyaniline conducting polymers, J. Appl. Electrochem., 19, 462-464 (1989). 

M. Hepel, The electrocatalytic oxidation of methanol at finely dispersed platinum nanoparticles in polypyrrole films, J. Electrochem. Soc., 145, 124-134 (1998). 

S.M. Golabi and A. Nozad, Electrocatalytic oxidation of methanol on electrodes modified by platinum microparticles dispersed into poly(o-phenylenediamine), J. Electroanal. Chem., 521, 161-167 (2002). 

Figure 9.4 The initial steps in dual path mechanism for the electrocatalytic oxidation of methanol over Pt to COj, adapted from Neurock. ° )...

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