Methanol oxidation mechanism

To the best of our knowledge, no consensus has been reached in the hterature on methanol electro-oxidation by MDH enzymes. Hence, a detailed theoretical investigation is carried on the already proposed methanol oxidation mechanisms (A-E and H-T) by using more extended MDH active site models considering protein environment. 

According to step 1 of the H-T methanol oxidation mechanism (Fig. 2b), there should be a direct hydride transfer (H17) from methanol to C5 of PQQ in concert with proton abstraction (HI6) by 014 of ASP, thus resulting in the formation of by-... 

Energy Barriers (kcal/mol) Corresponding to Steps One to Four of the H-T Methanol Oxidation Mechanism Calculated at the BLPY/DNP Theory Level for Models A and B. 

Hampson, N.A., Willars, M.J., McNicol, B.D. (1979) The methanol air fuel cell a selective review of methanol oxidation mechanisms at platinum electrodes in acid electrolytes. Journal of Power Sources, 4, 191-201. 

Perhaps the most important paradigm in research on the mechanism of the electrocatalytic oxidation of small organic molecules is the dual pathway mechanism introduced in Capon and Parsons [1973a, b], and reviewed in Parsons and VanderNoot [1988]. In terms of methanol oxidation, the dual pathway may be summarized in a simplified way by Fig. 6.1. The idea is that the complete oxidation of methanol to carbon dioxide may follow two different pathways ... 

A so-called direct pathway involving a more weakly adsorbed perhaps even partially dissolved intermediate. Likely candidates for such intermediates are formaldehyde and formic acid. The oxidation mechanism of formic acid is discussed in Section 6.3. The idea is that the formation of a strongly adsorbed intermediate is circumvented in the direct pathway, though in practice this has appeared difficult to achieve (the dashed line in Fig. 6.1). Section 6.4 will discuss this in more detail in relation to the overall reaction mechanism for methanol oxidation. 

In the oxidation of methanol to CO2, six electrons ate involved. This high number of electrons implies that the mechanism is inevitably very complex, with several intermediate species participating in the mechanism. In spite of its complexity, it has been proposed that the oxidation mechanism follows the same general scheme as the oxidation of formic acid, i.e., a dual path mechanism with active and poisoning intermediates (see the reaction Scheme 6.16) [Parsons and VanderNoot, 1988]. For that reason, we will compare the behavior with that of formic acid to highlight the similarities and differences. 

The first part of the mechanism is a sequential reaction yielding formic acid, and from that point the typical dual path mechanism for formic acid occurs. In fact, it has been proposed that the mechanisms of formic acid and methanol oxidation consist of the same dominating elemental steps [Okamoto et al., 2005]. However, experiments have revealed that the mechanism is much more comphcated than that. 

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. 

The FTIR studies revealed that the formation of CO2 is only detected when the CO starts to be oxidized (Fig. 6.18). Therefore, it was proposed that the mechanism has only one path, with CO as the C02-forming intermediate [Chang et al., 1992 Vielstich and Xia, 1995]. This has two important and practical consequences. First, methanol oxidation will be catalyzed by the same adatoms that catalyze CO oxidation, mainly ruthenium. Second, since the steric requirements for CO formation from methanol are quite high, the catalytic activity of small (<4nm) nanoparticles diminishes [Park et al., 2002]. 

Lai SCS, LehedevaNP, Housmans THM, Koper MTM. 2007. Mechanisms of carbon monoxide and methanol oxidation at single-crystal electrodes. Top Catalysis 46 320-333. 

Vielstich W. 2003. CO, formic acid, and methanol oxidation in acid electrol3ftes—mechanisms and electrocatalysis. In Bard AJ, Stratmann M, Calvo EJ, eds. Encyclopedia of Electrochemistry. Volume 2. New York Wiley, p 466-511. 

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

In the following, after a brief description of the experimental setup and procedures (Section 13.2), we will first focus on the adsorption and on the coverage and composition of the adlayer resulting from adsorption of the respective Cj molecules at a potential in the Hup range as determined by adsorbate stripping experiments (Section 13.3.1). Section 13.3.2 deals with bulk oxidation of the respective reactants and the contribution of the different reaction products to the total reaction current under continuous electrolyte flow, first in potentiodynamic experiments and then in potentiostatic reaction transients, after stepping the potential from 0.16 to 0.6 V, which was chosen as a typical reaction potential. The results are discussed in terms of a mechanism in which, for methanol and formaldehyde oxidation, the commonly used dual-pathway mechanism is extended by the possibility that reaction intermediates can desorb as incomplete oxidation products and also re-adsorb for further oxidation (for the formic acid oxidation mechanism, see [Samjeske and Osawa, 2005 Chen et al., 2006a, b Miki et al., 2004]). 

For both methanol oxidation and formic acid oxidation, a dual-pathway mechanism has been proposed (for methanol oxidation, see Lamy et al. [1983] Jarvi and Stuve [1998] Cuesta [2006] Housmans et al. [2006] Iwasita [2003] for formic acid oxidation, see Parsons and VanderNoot [1988] Sun et al. [1988] Willsau and Heitbaum [1986] Miki et al. [2002] Samjeske and Osawa [2005] Chen et al. [2006a, b, c] Samjeske et al. [2005, 2006] Miki et al. [2004], Chang et al. [1989]), in which one reaction pathway proceeds via formation and subsequent oxidation of COad (P, indirect pathway ), while the other leads, via one or more reaction intermediates RI, directly to CO2 ( direct pathway ) (Fig. 13.8a). 

In the original proposal of the dual-pathway mechanism (for formic acid oxidation, see [Capon and Parsons, 1973a, b, c] for methanol oxidation, see [Parsons and VanderNoot, 1988 Jarvi and Stuve, 1998 Leung and Weaver, 1990 Lopes et al., 1991 Herrero et al., 1994, 1995]), both pathways lead to CO2 as the final product, as illustrated in the reaction scheme depicted in Fig. 13.8a [Jarvi and Smve, 1998]. In this mechanism, desorption of incomplete oxidation products was not included. The existence of a direct reaction pathway for methanol oxidation, following the dual-pathway mechanism, was justified by the observation of a methanol oxidation current at potentials where COad oxidation is not yet active [Sriramulu et al., 1998, 1999 Herrero et al., 1994, 1995]. The validity of this interpretation was questioned, however, by Vielstich and Xia (1995), who claimed that CO2 formation is observed only with the onset of COad oxidation and that the faradaic current measured at lower potentials is due to the formation of the incomplete oxidation products formaldehyde and formic acid. The latter findings were later confirmed by Wang et al. [2001], Korzeniewski and Childers [1998], and Jusys et al. [2001, 2003]. In more... 

Similar ideas can be applied to formaldehyde oxidation. For bulk formaldehyde oxidation, we found predominant formic acid formation under current reaction conditions rather than CO2 formation. Hence, it cannot be ruled out, and may even be realistic, that formaldehyde is first oxidized to formic acid, which can subsequently be oxidized to CO2. The steady-state product distribution at 0.6 V is much more favorable for such a mechanism as in the case of methanol oxidation. On the other hand, because of the high efficiency of COad formation from formaldehyde, this process is likely to proceed directly from formaldehyde adsorption rather than via formation and re-adsorption of formic acid. Alternatively, the second oxygen can be introduced via formaldehyde hydration to methylene glycol, which could be further oxidized to formic acid and finally to CO2 (see the next paragraph). 

Herrero E, Chrzanowski W, Wieckowski A. 1995. Dual path mechanism in methanol oxidation on a platinum electrode. J Phys Chem 99 10423-10424. 

A simplified scheme of the dual pathway electrochemical methanol oxidation on Pt resulting from recent advances in the understanding of the reaction mechanism [Cao et al., 2005 Housmans et al, 2006] is shown in Fig. 15.10. The term dual pathway encompasses two reaction routes one ( indirect ) occurring via the intermediate formation of COads. and the other ( direct ) proceeding through partial oxidation products such as formaldehyde. 

Although the complete mechanism for each of the previously described reactions is not known, substantial details have been worked out. First, it is clear that Ti is incorporated into the framework of the silicalite structure. Too much Ti (more than about 2.5%) in the preparation steps forms nonframework TiOz crystallites, which decompose H202. Second, the rate enhancement due to methanol suggests a tight association at the Ti active site as shown in Fig. 6.8.37,38 This is supported by the fact that methanol oxidizes much more slowly than other alcohols.47 This tight coordination of methanol is proposed to increase the electrophilicity of the Ti-coordinated H202 and facilitate oxygen transfer to the alkene.31... 

Reactions alcohols, 29 36-49 adsorption, 29 36-37 clean surfaces, 29 37-38 ethanol oxidation, 29 44—48 methanol oxidation, 29 38-44 oxidation on copper and silver, 29 38-48 oxidation reaction, silver, 29 48-49 base-catalyzed, of hydrocarbons, 12 117 free radical mechanism in, of hydrogen peroxide, 4 343... 

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. 

High area platinum showed different voltammetric features from smooth platinum for methanol oxidation and provided slightly higher sustained current density. These results provided evidences that the morphology of platinum affects the mechanisms and the kinetics of methanol oxidation. 

As stated before, the enhancement effects of ruthenium and tin on the catalytic activity for the methanol oxidation are different in their mechanisms, i. e. ruthenium facilitates the oxidation of hoCOad while tin has the same effect on eoCOad One may naturally ask what if both of them are present on platinum. To answer this question, the oxidation of COad and methanol on Pt-Ru-Sn electrode were investigated. 

Combined with methanol crossover, slow anode kinetics lead to a power density of a DMFC that is three to four times lower than that of a hydrogen fuel cell. Much work has been focused on the anodic oxidation of methanol. The mechanism of the... 

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