The Electro-Oxidation of Methanol

The oxidation of methanol has been thoroughly studied for many years, so that the reaction mechanism is now well established. The overall oxidation reaction involves six electrons, and one water molecule, as follows  

It was first shown by electrochemically modulated infrared reflectance spectroscopy (EMIRS) that the main poisoning species formed during the chemisorption and oxidation of methanol on a platinum electrode is carbon monoxide CO, either linearly bonded, or bridge bonded to the surface. The coverage degree of the electrode surface by linearly bonded CO can reach 90% on a pure platinum electrode, so that most of the active sites are blocked 

Besides (CO)ads, some other adsorbed species, such as (-CHO)ads or (-COOH)ads, were identified, on a platinum electrode, by infrared reflectance spectroscopy, i.e., EMIRS (Fig. 11) and Fourier transform infrared reflectance spectroscopy.  

Intermediate products, like formaldehyde HCHO and formic acid HCOOH, and final product (CO2), were formed on polycrys-talline smooth platinum electrodes and were analyzed quantitatively by liquid or gas chromatography.  

All these results may be smmnarized in Fig. 12, where all the adsorbed species (identified by in situ IR Reflectance Spectroscopy) and intermediate products (analyzed by HPLC) are shown. 

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The activation energy for the reaction, a, was determined for the above Pt-porous nanoparticles from the first cycle of CV measurement in the temperature range between 30 and 60 °C, Figure 13c. The activation energy was obtained from the slope, —EJR, of the Arrhenius relationship and equal to SOklmoP. This value was similar to some of those obtained for the electro-oxidation of methanol on electrodes of Pt particles dispersed in Nation [50, 51]. 

Takasu Y, Iwazaki T, Sugimoto W, Murakami Y. 2000. Size effects of platinum particles on the electro-oxidation of methanol in an aqueous solution of HCIO4. Electrochem Commun 2 671-674. 

O 3.2—The Electro-Oxidation of Methanol at Platinum in Acid Solution... 

The first example of the application of the EMIRS technique was reported by Beden et al. (1981) and concerned the electro-oxidation of methanol. 

The authors thus concluded that HCOOH is an intermediate in the electro-oxidation of methanol and a product of the chemical reaction of methanol with water. C02 is the major product of the electrochemical reaction. 

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electro-oxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

Figure 3.35 shows the potential dependence of the integrated band intensity of the linear CO observed in the experiment described above and the corresponding variation in the methanol oxidation current. The latter was monitored as a function of potential after the chemisorption of methanol under identical conditions to those employed in the IRRAS experiments. As can be seen from the figure the oxidation of the C=Oads layer starts at c. 0.5 V and the platinum surface is free from the CO by c. 0.65 V. The methanol oxidation current shows a corresponding variation with potential, increasingly sharply as soon as the CO is removed strong evidence in support of the hypothesis that the adsorbed CO layer established at 0.4 V acts as a catalytic poison for the electro-oxidation of methanol. 

In the case of ethanol, Pd-based electrocatalysts seem to be slightly superior to Pt-based catalysts for electro-oxidation in alkaline medium [87], whereas methanol oxidation is less activated. Shen and Xu studied the activity of Pd/C promoted with nanocrystalline oxide electrocatalysts (Ce02, C03O4, Mn304 and nickel oxides) in the electro-oxidation of methanol, ethanol, glycerol and EG in alkaline media [88]. They found that such electrocatalysts were superior to Pt-based electrocatalysts in terms of activity and poison tolerance, particularly a Pd-NiO/C electrocatalyst, which led to a negative shift of the onset potential ofthe oxidation of ethanol by ca 300 mV compared... 

The catalytic properties of Pt and Ru particles on T102-CNT catalysts in the methanol electro-oxidation were investigated by He et Pt and Ru nanoparticles, approximately of 3 nm in diameter, were uniformly electro-deposited on the as synthesized Ti02-supported C nanotubes. An enhanced and stable catalytic activity was obtained in the electro-oxidation of methanol due to the uniformly dispersed Pt and Ru nanoparticles on the... 

Genera/. The central goal of fundamental electrochemical kinetics is to find out what electrons, ions, and molecules do during an electrode reaction, hr this research, one is not only concerned with the initial state (Le., the metal and the reactants in the solution next to the electrode surface before the reaction begins) and the final product of the reaction, one also has to know the intermediate species formed along the way. Thus, all practical electrode reactions (say, the electro-oxidation of methanol to C02) consist of several consecutive and/or parallel steps, each involving an intermediate radical, e.g., the adsorbed C-OH radical. I Iowcver, one finds that intermediates can be classed into two types. 

C. Herrero, W. Chrzanowski, and A. Wieckowski, J. Phys. Chem. 99 10423 (1995). Dual-path mechanism in the electro-oxidation of methanol. 

Young HC, Yong GS, Won CC, Seong IW, Hak SH (2003) Evaluation of the Nafion effect on the activity of Pt-Ru electrocatalysts for the electro-oxidation of methanol. J Power Sources 118 334 11... 

Melnick RE, Palmore GT (2001) Impedance spectroscopy of the electro-oxidation of methanol on pohshed polycrystalline platinum. J Phys Chem B 105(5) 1012—25... 

Iwasita et al. [107] also studied the electro-oxidation of methanol on Ru-evaporated Pt(lll) modified electrodes with different Ru coverage. The surface compositions were characterized by cyclic voltammetry and Auger spectroscopy. The topography of the UHV-prepared deposits was observed by STM. Figure 11 shows STM data for a Pt(l 11) electrode without (a) and with (b-f) Ru layers formed by vapor deposition. 

Until now, for methanol oxidation the best bimetallic catalyst was found to be Pt-Ru. Several papers deal with the electro-oxidation of methanol at Pt-Ru bimetallic system dispersed in polyaniline [33,46]. From results with bulk alloys, the optimum Pt/Ru ratio of around 6 1 to 4 1 was found [49] and confirmed [50]. The electroactivity of Pt-Ru-modified polyaniline is much better than that displayed by pure Pt particles dispersed into the PAni film. The optimum composition of the Pt-Ru bimetallic system was confirmed from these results [33]. The decrease of the poisoning phenomenon is the consequence of a low coverage in adsorbed CO resulting from the chemisorption of methanol. This was checked by considering the oxidation of CO at the same Pt-Ru/PAni-modified electrode [34], which occurs at low overvoltages (150 mV) in the presence of Ru. 

The potentials at which CO2 is formed, during the electro-oxidation of methanol or that of CO (from gaseous CO), are summarized in Table 1. These threshold potentials were obtained from infrared data. It clearly appears that the distribution of COads at the surface of the electrocatalyst depends greatly on the source of adsorbed CO, confirming undoubtedly the presence of other adsorbed species in the case of methanol. 

K. Lasch, L. Jorissen, J. Garche, The effect of metal oxides as co-catalysts for the electro-oxidation of methanol on platinum-ruthenium. J. Power Sources 1999, 84, 225-230. 

Identification of participants in electrode reactions with high chemical specificity. A knowledge of chemical participants is indispensible to achieving an understanding of electrode processes that will permit manipulation and improvement of important processes, such as the electro-oxidation of methanol or the adsorption of olefins on platinum. Among established techniques for chemical identification, vibrational spectroscopies offer the best opportunities for improvement. The current high level of effort with these techniques should be... 

Similar observations concerning the electro-oxidation of methanol were made by Ulmann et al, using platinum micro-particles dispersed into polypyrrole films (from 100 to 700 nm thickness) deposited by cyclic voltammetry on a gold electrode [152]. The platinum loading was varied from 10 pg/cm to 300 pg/cm, leading to an increase in the current density (recorded after 12 hours of methanol oxidation at a constant potential) for platinum loadings up to 150 pg cm after which the current density reaches a plateau. 

The electrocatalytic oxidation of alcohols is possible by using modified conductive polymer electrodes. One of the most interesting examples of such a reaction is the electro-oxidation of methanol with highly dispersed platinum-based particles inserted in a polymeric matrix. 

A detailed mechanism for the electro-oxidation of methanol on platinum electrodes in acidic media is shown in Scheme 2. The important features of the reaction mechanism include the formation of reactive intermediates, such as (CHO)a 5, which form on the electrode surface and are further oxidized to either (CO)ads, which leads to the poisoning species, or the adsorbed formyl species is oxidized to (COOH)ads or directly to COj. The mechanistic pathways described... 

Because of the great potential of methanol as a fuel for low-temperature fuel cells, the electro-oxidation of methanol on Pt or Pt-based alloy electrodes has been studied extensively in the past decades [112-115]. It is generally accepted that methanol is oxidized to CO2 by the so-called dual-path mechanism [112] via adsorbed CO (poison) and non-CO reactive intermediates. The formation of CO by dehydrogenation of methanol has been well confirmed, but no consensus has been reached so far on the nature of the reactive intermediates in the non-CO pathway. Various adsorbates such as CHxOH [116], -COH [116], formyl (-HCO), [117] carboxy (-COOH) [117], a dimer of formic acid [35], and COO [38] have been claimed to be the reactive intermediates from IRAS and other physicochemical measurements. However, the spectra of the reaction intermediates are not well reproduced by other groups. 

PANI-NTs synthesized by a template method on commercial carbon cloth have been used as the catalyst support for Pt particles for the electro-oxidation of methanol [501]. The Pt-incorporated PANl-NT electrode exhibited excellent catalytic activity and stabUity compared to 20 wt% Pt supported on VulcanXC 72R carbon and Pt supported on a conventional PANI electrode. The electrode fabrication used in this investigation is particularly attractive to adopt in solid polymer electrolyte-based fuel cells, which arc usually operated under methanol or hydrogen. The higher thermal stabUity of y-Mn02 nanoparticles-coated PANI-NFs on carbon electrodes and their activity in formic acid oxidation pomits the realization of Pt-free anodes for formic acid fuel cells [260]. The exceUent electrocatalytic activity of Pd/ PANI-NFs film has recently been confirmed in the electro-oxidation reactions of formic acid in acidic media, and ethanol/methanol in alkaline medium, making it a potential candidate for direct fuel cells in both acidic and alkaline media [502]. 

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