Methanol oxidation electrode reaction study

Another important difference in the poison formation reaction is observed when studying this reaction on Pt(lll) electrodes covered with different adatoms. On Pt(lll) electrodes covered with bismuth, the formation of CO ceased at relatively high coverages only when isolated Pt sites were found on the surface [Herrero et al., 1993]. For formic acid, the formation takes place only at defects thus, small bismuth coverages are able to stop poison formation [Herrero et al., 1993 Macia et al., 1999]. Thus, an ideal Pt(lll) electrode would form CO from methanol but not from formic acid. This important difference indicates that the mechanism proposed in (6.17) is not vahd. It should be noted that the most difhcult step in the oxidation mechanism of methanol is probably the addition of the oxygen atom required to yield CO2. In the case of formic acid, this step is not necessary, since the molecule has already two oxygen atoms. For that reason, the adatoms that enhance formic acid oxidation, such as bismuth or palladium, do not show any catalytic effect for methanol oxidation. 

These new experimental approaches gave renewed motivation to the study of classical organic electrode reactions for direct electrochemical energy conversion. The present contribution intends to give a survey of the recent progress in the study of methanol oxidation attained by application of the above mentioned techniques. 

Understanding the oxidation mechanism is important. Impedance spectroscopy was recently used to study methanol electrooxidation, and kinetic parameters can be deduced from impedance spectra. Figure 6.58 shows an equivalent circuit that was developed for methanol oxidation on a Pt electrode, but which is common for all electrochemical reactions. In this circuit, a constant phase element was used rather than a double-layer capacitance, since a CPE is more realistic than a simple capacitor in representing the capacitive behaviour. 

The apparent transfer coefficient of the cathodic reaction, ac, is a measure of the sensitivity of the transition state to the drop in electrostatic potential between electrolyte and metal [109,112]. According to Ref. 113, it is ac = 0.75 for the O2 reduction on Pt in aqueous acid electrolytes. In Ref. Ill the value ac = 1.0 was reported instead. Since the cathodic reaction is a complex multistep process, it might follow several reaction pathways, and the competition between them is affected by the operation conditions (rj, p, T). Therefore, different values of ac have been reported in different regimes of operation. Although in the simple reactions the transfer coefficient is a microscopic characteristic of the elementary act [112], for complex multistage reactions in fuel cell electrodes it is rather an empirical parameter of the model. The dependence of effective a for methanol oxidation on the catalyst layer preparation was recently studied [114]. 

Shortly after our publications on the sonoelectrochemical oxidation of pheny-lacetate [186,187], a parallel study was reported by Japanese workers [191] who employed crossed Kolbe electrolyses of phenylacetates and succinates, variously deuterated, to produce deuterated derivatives of 4-phenylbutyric acids. In control experiments to produce deuterated bibenzyls from phenylacetate without succinate present, they obtained 11% of dimer with pyridine present. Under normal conditions, this rose to 47% yield of and 41% of dimer under ultrasound from a cleaning bath, although here the reaction time is stated to be reduced threefold. However, it is ambiguous whether this shortened reaction-time benefit also applies to the reaction with pyridine but without ultrasound. The authors state that ultrasound helps to keep the electrode surface clean and it would seem that in their conditions, which employ aqueous solution instead of methanol, the electrode is not completely switched off by the insulating film under normal conditions. The authors did not examine the system with both pyridine present and ultrasound, but the observed yield drop from 47 to 41% might suggest the same trend towards the two-electron pathway under ultrasound, although other products were not identified and quantified. 

Methanol oxidation on CNF- and CNT-supported Pt-Ru particles in liquid electrolytes has been studied using cyclic voltammetry, chronoamperometry [13,231-237], and electrochemical impedance spectroscopy [236]. Rotating disk electrode and linear potential sweep voltammetry in liquid electrolytes were used to study the oxygen reduction reaction on Pt supported on CNTs and CNFs (see,... 

The first studies of methanol oxidation s special features and of the kinetics and mechanism of anodic methanol oxidation at platinum electrodes began in the early 1960s, in the period known as the first boom of work in fuel cells. In the years after that, this reaction was the subject of countless studies by many groups in different countries. In summary one can say of all this work that, by now, the mechanism of this reaction has been established rather reliably (for reviews see Bagotsky et al., 1977 Iwasita and Vielstich, 1990 Kauranen et al., 1996), while conflicting views persist on certain detailed aspects. Work on these questions is continuing even now. 

In the following, the catalysts that have been investigated for the methanol oxidation reaction (MOR) will be presented. No attempt will be made to be exhaustive neither in terms of all the materials that have been studied nor in terms of the historical development of those materials. Most of the initial studies of the electrocatalysis of the MOR were carried out on massive electrodes and using electrochemical techniques. Later, the feasibility of the direct methanol fuel cell (DMFC) precluded the use of massive electrodes. Electrochemical reactions are surface reactions, so it is apparent that there is much to be gained by using large surface area electrodes, which led to the development of diffusion electrodes where the catalyst is in the form of nanoparticles. These electrodes have large specific surface areas which not only favor intrinsically the reaction but also allow for the use of minimal amounts of catalyst metals, usually rather expensive and, in some cases, scarce. 

The most frequent use of DBMS is for studies of possible fuels in fuel cells. Figure 5 shows the faradaic and ion currents for CO2 and methylformate during methanol oxidation at carbon-supported Pt nanoparticles. Note that the formation of methylformate starts at a slightly lower potential than that of CO2. The ratio of the CO2 formation rate to the faradaic current yields a current efficiency of 90 % in this case. Under flow and at smooth Pt electrodes, the current efficiency for CO2 remains at 30 % for all flow rates [4]. This proves the parallel reaction mechanism suggested by Bagotsky [30]. One path leads to formaldehyde and formic acid. Under flow, these molecules diffuse away fi om the electrode, while under stagnant conditions as in the pores of a porous electrode, they are further oxidized to CO2. The other path leads to CO2 via adsorbed CO and is independent of flow rate. 

In addition, Shen et al. [75] prepared a novel fliree-dimensional electrode using polypyrrole (Ppy) and polystyrene spheres (PS) covered by a platinum catalyst instead of the conventional gas diffusion electrode, in order to reduce the sealing effect in liquid fuel cells. This new type of porous structured electrode allows liquid alcohol to penetrate the catalyst layer quite easily. The approach results in an increased active surface area for electrochemical reactions. The electrochemical active areas of platinum in Pt/Ppy/PS electrodes and E-TEK Pt/C electrodes, calculated by cyclic voltammograms [76, 77], are 4.5 and 23.6 cm g respectively, indicating a larger EAS for the three-dimensional electrode. Preliminary studies show an improved performance for methanol oxidation on a three-dimensional electrode as compared with a conventionally prepared electrode with the same platinum loading. 

Transition metal carbides and nitrides are two major kinds of electrode materials. This is due to their good electrical conductivity, corrosion resistance, and electrocatalytic activity. In the past several decades, some studies have been done on these materials as electrocatalysts for hydrogen and methanol oxidation and oxygen reduction reactions in alkaline and acid solutions [59-63]. This section will briefly review transition metal carbides and nitrides and then introduce current state-of-the-art catalysts explored in acidic media. 

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