Methanol electrooxidation reactions

Kazemi R, Kiani A (2012) Deposition of palladium submonolayer on nanoporous gold film and investigation of its performance for the methanol electrooxidation reaction. Int J Hydrogen Energy 37 4098 106... 

Page T, Johnson R, Hormes J, Noding S, Rambabu B. A study of methanol electrooxidation reactions in carbon membrane electrodes and structural properties of Pt alloy electro-catalysts by EXAFS. J Electroanal Chem 2000 485 34-41. 

From the results obtained with in situ reflectance spectroscopy and on-line analytical methods, investigators at Universite de Poitiers proposed a complete mechanism for the electrooxidation of methanol at a platinum electrode. The first step of the electrooxidation reaction is the dissociative adsorption of methanol, leading to several species according to the following equations ... 

The rate-determining step (rds) of the reaction on platinum is the oxidation of adsorbed CO with adsorbed hydroxyl species [step (26)]. The current density of the methanol electrooxidation can be obtained from the following equatiorf ... 

Sriramulu S, Jarvi TD, Stuve EM. 1999. Reaction mechaiusm and dynamics of methanol electrooxidation on platinum (111). J Electroanal Chem 467 132-142. 

The electrooxidation of methanol has attracted tremendous attention over the last decades due to its potential use as the anode reaction in direct methanol fuel cells (DMFCs). A large body of literature exists and has been periodically reviewed [130,131,156], [173-199]. Unlike for formic acid, a generally accepted consensus on the specific mechanistic pathways of methanol electrooxidation is still elusive. 

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. 

It can also be observed from this figure that Sn-containing catalyst is a more effective catalyst for the oxidation of CO than that containing Ru, as a lower onset potential of the oxidation wave is obtained with the former catalyst. It has also to be noted that PtSn catalysts are less active towards methanol electrooxidation than PtRu catalysts (see Section IV. 1). ° However, adsorbed CO species are proposed as reaction intermediates of methanol electro-oxidation, which seems to lead to a paradoxical behavior of PtSn based catalysts. In CO stripping experiments, a negative shift of the onset potential for the oxidation of adsorbed CO on PtSn also occurs. " On the basis of in situ infrared spectroscopy studies coupled with electrochemical measurements, Mo-... 

SCHEME 2.1 General scheme of methanol electrooxidation considering series and parallel pathways to form carbon dioxide as the product. Solid and dashed arrow lines indicate the demonstrated and possible reaction pathways, respectively. Path 1 denotes the formyl intermediate mechanism and Path 2 the methoxyl intermediate mechanism. 

Many efforts have been undertaken to enhance the electrocatalytic performance of catalytic reactions that have technological importance. The case of organic fuel electrooxidation is a major point for study, especially the possibility of achieving long-term, less-polluting fuel cells. In the case of methanol electrooxidation, the reaction occurs by a self-poisoning mechanism, so it is clear that the catalysts performances must be improved to impede the formation of carbon... 

Carbon-supported bimetallic or even ternary catalysts are of increasing interest in electrocatalytic reactions such as methanol oxidation. In this sense, the preparation routes are of pristine importance in determining the catalytic performance. As an example, it has been shown that Pt/Rn/Ni = 5 4 1 nanoparticles have a higher catalytic activity for methanol electrooxidation than does Pt/Ru = 1 1... 

Metal-doped carbon gels have been used as catalysts in several reactions, including the removal of pollutants in either gas or aqueous phase, ORR and methanol electrooxidation for fuel cell applications, C=C double-bond hydrogenations, skeletal isomerization and hydrodechlorination reactions, and other organic syntheses. The activity and/or selectivity of metal-doped carbon gels was better than that of other, more widely used supported catalysts in many of the... 

Further reported examples include electrocatalytic processes and their intermediates [313, 314]. Formate could be identified as an active intermediate of methanol electrooxidation at a polycrystalline platinum electrode [315]. Water molecules coadsorbed during methanol adsorption on platinum were identified as those species that react subsequently with COad that was formed as a result of methanol chemisorption [316]. The high sensitivity of SEIRAS allows mapping of two-dimensional spectra (for selected examples, see [285]). Finally, two-dimensional correlation analysis of electrochemical reactions becomes possible [317]. 

In this section we will discuss the role of surface modification to enhance electrocatalytic oxidation of methanol, one of the interesting components for fuel cell technology. Perhaps the most successful promoter of methanol electrooxidation is ruthenium. Pt/Ru catalysts appear to exhibit classical bifunctional behavior, whereas the Pt atoms dissociate methanol and the ruthenium atoms adsorb oxygen-containing species. Both platinmn and ruthenimn atoms are necessary for eomplete oxidation to occur at a significant rate. The bifunctional mechanism can account for a decrease in poisoning from methanol, as observed for Pt/Ru alloys. Indeed, CO oxidation has been attributed to a bifimctional mechanism that reduces the overpotential of this reaction by 0.1 V on the Pt/Ru surface. 

The promotion of methanol electrooxidation by tin modifier has been studied, too. The presence of tin appears to enhance the reaction at low potential, increasing the production of CO2. The kinetic enhancement observed most likely result from an increase in the number of active catalyst sites for reaction due to decreased poisoning. 

In the above reactions, the oxidation process takes place in the anode electrode where the methanol is oxidized to carbon dioxide, protons, and electrons. In the reduction process, the protons combine with oxygen to form water and the electrons are transferred to produce the power. Figure 9-1 is a reaction scheme describing the probable methanol electrooxidation process (steps i-viii) within a DMFC anode [1]. Only Pt-based electrocatalysts show the necessary reactivity and stability in the acidic environment of the DMFC to be of practical use [2], This is the complete explanation of the anodic reactions at the anode electrode. The electrodes perform well due to the presence of a ruthenium catalyst added to the platinum anode (electrode). Addition of ruthenium catalyst enhances the reactivity of methanol in fuel cell at lower temperatures [3]. The ruthenium catalyst oxidizes carbon monoxide to carbon dioxide, which in return helps methanol reactivity with platinum at lower temperatures [4]. Because of this conversion, carbon dioxide is present in greater quantity around the anode electrode [5]. 

A significant size effect in activity has also been observed for CO [34] and methanol electrooxidation [44,45]. For these reactions, a significant drop in activity was observed when the particle diameter was below 2-3 nm. 

Various surface intermediates are formed during methanol electrooxidation. Methanol is mainly decomposed to CO which is then further oxidized to CO2. Other CO-like species are also formed such as COHa s, HCOads. HCOOads [13]. Main by-products are formaldehyde and formic acid. Some of these intermediates are not easily oxidizable and remain strongly adsorbed to the catalyst surface. Consequently, they prevent methanol molecules adsorbing and undergoing further reaction. Thus the electrooxidation of the reaction intermediates reveals to be the rate limiting step. 

Methanol electrooxidation and ethanol electrooxidation are complex reactions occurring in a pattern of parallel reaction pathways (Fig. 1.1) [5-14]. Although detailed reaction mechanisms remain obscure, a number of reaction intermediates and products have been identified by spectroscopic methods such as in situ Fourier transform infrared spectroscopy (FTIR), on-line differential electrochemical mass spectrometry (OEMS), and other techniques [6, 12-14]. 

Much of the effort on the electrooxidation of ethanol has been devoted mainly to identifying the adsorbed intermediates on the electrode and elucidating the reaction mechanism by means of various techniques, as differential electrochemical mass spectrometry, in situ Fourier transform infrared spectroscopy, and electrochemical thermal desorption mass spectroscopy. The established major products include CO2, acetaldehyde, and acetic acid, and it has been reported that methane and ethane have also been detected. Surface-adsorbed CO is still identified as the leading intermediate in ethanol electrooxidation, as it is in the methanol electrooxidation. Other surface intermediates include various Ci and C2 compounds such as ethoxy and acetyl [102]. There is general agreement that ethanol electrooxidation proceeds via a complex multi-step mechanism, which involves a number of adsorbed intermediates and also leads to different byproducts for incomplete ethanol oxidation, as shown in Figure 1.22. 

Figure 4.63 compares the effect of the novel extended reaction zone supports on the electrocatalytic activity of PtRu for methanol electrooxidation. The catalyst characteristics arc presented in Table 4.3. The catalyst surface morphology on all three supports could be characterized as predominantly mesoporous coating composed of nanoparticle agglomerates (Figure 4.64). 

The bi-functional mechanism, although simple, can explain very well the promoted MOR activity of Pt-Ru alloy catalysts. This mechanism is also well adapted by other binary alloys such as Pt-Sn [48]. It has been identified that CO does not bind to the Sn sites, with the result that OH can more easily adsorb on the Sn sites without competition from CO. The synergetic effect on Pt and Sn sites gives rise to Pt-Sn, a very active CO electrooxidation catalyst. However, the strong adsorption of OH species on Sn sites, particularly at high potentials, makes the Pt-Sn catalyst inferior to the Pt-Ru catalyst for the methanol oxidation reaction. 

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