Mechanism of methanol oxidation

Hamnett A. 1999. Mechanism of methanol oxidation. In Wieckowski A, ed. Interfacial Electrochemistry Theory, Experiments and Applications. New York Marcel Dekker. pp. 843-883. 

It is essential to understand the mechanisms of methanol oxidation including the adsorbate formation and removal. To this purpose, the electrochemical oxidation of adsorbed carbon monoxide (COad) and methanol was studied using electrochemical and two spectroscopic... 

As discussed in Chapter 3, the controversial argument about the strongly adsorbed poisonous spedes of methanol partial oxidation is almost settled, resvilting in the condusion that COad is at least one of the msgor adsorbates. Therefore, both for understanding the mechanisms of methanol oxidation and for searching for new catalysts, it is very important to understand the behavior of COad under various conditions. 

Bronkema and Bell (2007) analyzed the Raman bands of surface methoxy species and of supported vanadia to elucidate the mechanism of methanol oxidation to formaldehyde. In their detailed investigation, insight from Raman spectroscopy was combined with information from EXAFS and XANES spectroscopies. The authors discussed the reaction pathways in the presence and absence of 02, and identified the roles of various lattice oxygen sites. Formaldehyde was found to decompose to H2 and CO in the absence of 02 (Bronkema and Bell, 2007). Similar observations were reported by Korhonen et al. (2007) for methanol conversion on supported chromia catalysts. 

Methanol oxidation is a self-poisoning process, in which the intermediate adsorbate CO, formed from the dissociation of methanol, poisons the catalytic sites of Pt. The mechanism of methanol oxidation is as follows ... 

Methanol is the typical fuel with one carbon (Cl) atom for fuel cells. Methanol was one of the first small molecules chosen to study the oxidation on platinum group metals in the very early beginning of electrocatalysis. In that time, the oxidation of other Cl molecules such as formic acid and formaldehyde (interest in CO oxidation came latter with the oxidation of reformatted gases) were investigated as a model oxidation because their elementary steps were supposedly present in the mechanism of methanol oxidation. From the point of view of CO2 emission, methanol has, among the other small molecules, the highest energy production per unit of produced... 

Presently, the oxidation of methanol on pure platinum has more academic interest than practical application once DMFC universally employs platinum based materials having two or more metals as an anodic catalyst In absence of methanoUc inteimediate readsorption, the maximum reactiOTi rate for CO oxidation is 100-fold smaller than maximum reaction rate for CO adsorption from methanol dehydrogenation steps [11]. Indeed, the mechanism of methanol oxidation on platinum is expected to be equal to that on its alloys despite different kinetics which would result in a selection of pathway. In terms of complex activation theory, alloyed Pt is intend to lower the Ea barrier for CO adsorption, thus driving methanol oxidation to completion. As previously established [3], there are several factors that affect the calculated activation energy for the MOR at a given potential, such as coverage of methanoUc intermediates and anion adsorption from the electrolyte as well as pH and oxide formation processes. 

In Sects. 2.1 and 2.2 of this chapter the mechanism of methanol oxidation on platinum was discussed. This section is dedicated to clarify the impact of the main Pt-alloys, as already mentioned in Sects. 2.2.3 and 2.2.4, on the kinetics of the most relevant oxidative steps belonging to the MOR mechanism. Baring this in mind, the CO oxidation, the last step for complete methanol oxidation, is analyzed as well as the oxidation of other MOR intermediates such as formic acid and formaldehyde. 

The interest in formic acid oxidation (FAO) rose up in the 1970s with the aim of shedding light on the mechanism of methanol oxidation beyond the commercial interest in direct formic acid oxidation in fuel cells [90]. The FAO in acid solution was extensively investigated on surfaces of platinum [91-100] The FAO on other pure metallic surfaces seems to have been restricted to the palladium surface [98, 101-104]. In the 1980s, the remarkable contribution was done by the studies on the influence of the ad-atom in the activity of the platinum electrode [91—94]. In the 1990s, superficial spectroscopic techniques were employed to describe the electrochemical mechanism on palladium surface [98, 101—103] as well as platinum surface [97, 98, 105]. In the last 10 years, there was a triplication of publications about the FAO, specially driven by the use of nanoparticles. 

The reaction mechanisms of methanol oxidation at platinum electrodes in acid medium, according to the overall reaction,... 

One of the key problems in DMFC technology is sluggish kinetics of methanol oxidation. In spite of several decades of studies (Bagotzky and Vasilyev, 1967 Gasteiger et al., 1993 Kauranen et al., 1996), the reaction mechanism of methanol oxidation is still not fully understood. 

Since the oxidation of methanol to CO includes six electrons, the reaction process must involve several steps with several products or intermediates. The results from mass spectral measurements, high performance liquid chromatography (HPLC), and gas chromatography (GC) absorbance have showed that H O, HCOOH, HCOOCH, and CO were all produced during the oxidation of methanol on Pt in acid solutions. These species were formed initially but eventually became CO [92], In addition, some other adsorbed species such as (CHO)ads or (COOH)ads were identified by infrared reflectance spectroscopy or Fourier transform infrared reflectance spectroscopy. The detailed reaction mechanism of methanol oxidation on a Pt electrode is shown in Figure 1.20 [94]. 

Figure 1.20. Detailed reaction mechanism of methanol oxidation on a Pt electrode [94]. (Reprinted from Journal of Power Sources, 105(2), Lamy C, Lima A, LeRhun V, Delime F, Coutanceau C, Leger J-M, Recent advances in the development of direct alcohol fuel cells (DAFC), 283-96, 32002, with permission from Elsevier.)...

Hamnett A. (1999). Mechanism of Methanol Oxidation, in A. Wieckowski (ed.). Inteifacial Electrochemistry Theory, Experiments atul Applications, Marcel Dekker, New York, pp. 843-883. 

Busca, G., (1989). On the mechanism of methanol oxidation over vanadia-based catalysts A FTIR study of the adsorption of methanol, formaldehyde and formic acid on vanadia and vanadia-silica, J. Mol. Catal., 50, pp. 241-249. 

FIGURE 4.8 Promotion mechanism of methanol oxidation on PtRu catalyst. 

The mechanism of methanol oxidation on Pt-based catalysts has been studied for several decades [1-14]. Complex parallel and series reaction pathways in which several adsorbed species and soluble intermediates were involved in methanol oxidation were proposed by Bagotzky et al. [2]. The in situ application of infrared spectroscopy during methanol oxidation showed that adsorbed CO is formed on the Pt surface [15]. However, other adsorbed intermediates are still not identified. Formaldehyde, formic acid, methyl formate, and dimethoxy methane have been identified as soluble intermediates [8, 10, 16-18]. The quantitative analysis of methanol oxidation products changing with various parameters can help us better understand the mechanism of methanol oxidation and identify reactirai pathways. This can be achieved by online quantitative differential electrochemical mass spectrometry (OEMS), which will be discussed in Sect. 3. 

The facts that CO2 comes from the oxidation of adsorbed CO, and that large amounts of soluble intermediates are also generated during methanol oxidation on platinum, suggest that the mechanism of methanol oxidation on platinum can be simply formulated as follows ... 

Reaction kinetics and mechanism of methanol oxidation Pt(lOO) NaHCOj, Na2C03, and NaOH solution Electrode purchased from Metal Crystal and Oxides Ltd., Cambridge, UK Oxidation proceeds with some poisoning species formation. Main reaction product is formate and CO. Potential range for OH j is 0.4 V< <0.7 V Tripkovid et al. (19%)... 

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