Methanol oxidation mechanism determination

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]). 

Rate determining step (cont.) electrocatalysis and, 1276 methanol oxidation, 1270 in multistep reactions, 1180 overpotential and, 1175 places where it can occur, 1260 pseudo-equilibrium, 1260 quasi equilibrium and, 1176 reaction mechanism and, 1260 steady state and, 1176 surface chemical reactions and, 1261 Real impedance, 1128, 1135 Reciprocal relation, the, 1250 Recombination reaction, 1168 Receiver states, 1494 Reddy, 1163... 

From these results, a mechanism of methanol electrooxidation at PtRu can be proposed. The first step may consist in the dissociative adsorption of methanol at platinum and formation of an adsorbed CHO species according to the schema presented in Fig. 12. This mechanism of methanol adsorption and dehydrogenation is generally admitted." Then, for the co-reduced catalysts (alloy), the number of involved electrons from methanol stripping as determined by DBMS is higher than 2, then adsorbed CHO and CO species seem to be involved in the mechanism. Moreover, the number of electrons for the oxidation of bulk methanol is greater... 

The kinetics of methanol oxidation over metal oxide catalysts were elegantly derived by Holstein and Machiels [16], The kinetic analysis demonstrated that the dissociative adsorption of water must be included to obtain an accurate kinetic model. The reaction mechanism can be represented by three kinetic steps equilibrated dissociative adsorption of methanol to a surface methoxy and surface hydroxyl (represented by K,), equilibrated dissociative adsorption of water to two surface hydroxyls (represented by K ), and the irreversible hydrogen abstraction of the surface methoxy intermediate to the formaldehyde product and a surface hydroxyl (the rate determining step, represented by kj). For the case of a fully oxidized surface, the following kinetic expression was derived ... 

Figure 13. Estimated CO/Pt coverages in 0.3 methanol for the catalysts depicted in Fig. 10, and the indicated regions where each CO oxidation mechanism dominates, as determined by the binding site of the active OH in the CO oxidation. The brackets indicate the net amount of CO stripped via either the BF or Dsl mechanism due to mobile CO moving toward the Ru islands.

Similar IR experiments were performed to establish the reaction mechanism for methanol oxidation on unsupported and sihca-supported vanadia, which are more selective catalysts for formaldehyde synthesis than vanadia-titania. The formation of methoxy groups from methanol dissociative or condensative adsorption was determined while it was established that formaldehyde (directly adsorbed or produced by methoxy group oxidation) mainly adsorbs in the form of dioxymethylene species, stable only at relatively low temperatures. It was concluded that dioxymethylene can react with methanol at low conversion to give rise to dimethoxymethane while it preferentially desorbs as formaldehyde at higher conversions and temperatures. The weakness of the adsorption of formaldehyde was considered to be the key feature of catalysts allowing high selectivity in formaldehyde synthesis. 

Rousseau, S., Marie, O., Bazin, R, et al. (2010). Investigation of methanol oxidation over Au/Catalysts using operando IR spectroscopy Determination of the active sites, intermedi-ate/spectator species and reaction mechanism, J. Am. Chem. Soc., 132, pp. 10832-10841. 

Perhaps the first investigation into the chemical composition of the anodic oxide of HgCdTe was reported by Nemirovsky and Finkman in 1979 (50). Anodic oxide films were grown on n-type slush grown (Hg,Cd)Te (Ej = 0.1 eV) in 0.1 N KOH in methanol and various concentrations (< 0.1 N) of KOH in 90% ethylene glycol /10% HjO. The material surface was mechanically polished and etched in 20% Br in methanol prior to anodization. The refractive index and dielectric constant of the oxide were determined to be closer to TeOj than CdO or HgO, so the authors concluded that the... 

The slightly different physical properties of deuterium allow its concentration in ordinary hydrogen (or the concentration of a deuterium-containing compound in a hydrogen compound) to be determined. Exchange of deuterium and hydrogen occurs and can be used to elucidate the mechanism of reactions (i.e. the deuterium is a non-radioactive tracer). Methanol exchanges with deuterium oxide thus ... 

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. 

The crucial aspect is thus to determine the fate of the ( CHO), species. Possible mechanisms for its oxidative removal are schematically shown in Fig. 9. From this scheme, it appears that the desorption of the formyl species can follow different pathways through competitive reactions. This schematic illustrates the main problems and challenges in improving the kinetics of the electrooxidation of methanol. On a pure platinum surface, step (21) is spontaneously favored, since the formation of adsorbed CO is a fast process, even at low potentials. Thus, the coverage... 

This may be explained by the bifunctional theory of electrocatalysis developed by Watanabe and Motoo [14], according to which Pt activates the dissociative chemisorption of methanol to CO, whereas Ru activates and dissociates water molecules, leading to adsorbed hydroxyl species, OH. A surface oxidation reaction between adsorbed CO and adsorbed OH becomes the rate-determining step. The reaction mechanism can be written as follows [15] ... 

Data were obtained in acetonitrile solution containing 0.1 mol dm-3 Bu"NBF4 as supporting electrolyte. Solutions were 3 x 10"3 mol dm-3 in compound and potentials were determined with reference to SCE at 21 1°C at 50 mV s"1 scan rate. The CVs of [28], [29] and [31] consisted of a main current wave (reversible for [30] and [32] and EC mechanism for [28], [29] and [31]) corresponding to the Fc+/Fc couple and minor current waves (irreversible or quasi-reversible) from the oxidation of the amino groups. p, represents the anodic current peak potential of the Fc+/Fc couple. "Anodic shifts of the anodic peak potential of the Fc+/Fc couple produced by the presence of metal cations (1 or 2 equiv added as their perchlorate salts). For [28], [29] and [31], after addition of cations, the current waves from the respective amino groups disappeared and that of the Fc+/Fc couple became reversible. Obtained in methanol, instant oxidation by silver cations. 

---