Oxidation methanol electrooxidation

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 ... 

Figure 13.3 Potentiodynamic electrooxidation of (a) formic acid, (b) formaldehyde, and (c) methanol on a Pt/Vulcan thin-film electrode (7 xgpt cm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH (a), HCHO (b), or CH3OH (c). The potential scan rate was 10 mV s and the electrolyte flow rate was 5 p-L s at room temperature). The top panels show the faradaic current (solid lines), the partial currents for Ci oxidation to CO2 (dashed lines) and for formic acid formation (dash-dotted line), calculated from the respective ion currents, and the difference between the measured faradaic current and the partial current for CO2 oxidation (formic acid oxidation (a), formaldehyde oxidation (b)), or the difference between faradaic current and the sum of the partial currents for CO2 formation and formic acid oxidation (methanol oxidation, (c)) (dotted line). The solid lines in the lower panels in...

The catalytic properties of a Pt/Sn combination were observed on different kinds of electrode materials alloys [90], electro co-deposits of Pt and Sn [89, 90], underpotential deposited tin [42] or a mixture of tin oxide and platinum deposited on glass [95], All different materials present a marked influence on methanol electrooxidation. 

Although most of the research in methanol fuel cells is performed in acidic PEM systems, substantial work is carried out in alkaline systems as well. Interestingly, methanol oxidation in alkaline media is, like hydrogen, more facile. In parallel to the PEM in acidic media, the anion alkaline exchange membrane (AAEM) is adopted for methanol electrooxidation in alkaline solutions. Like PEM, AAEM is also studied as a barrier for methanol crossover. 

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-... 

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 electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

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... 

Fig. 1.20 Steady-state galvanostatic polarization curves for the oxidation of 1.0 M methanol on Pt/Sn electrodes in 0.5 M sulfuric acid at room temperature. Effect of perfiuoroalkanesulfonic acid additives upon ease of methanol electrooxidation.

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. 

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]. 

Takasu et al. [27] prepared a homogenized Pt-Ru/C electrocatalyst with a high-specific activity for methanol oxidation from carbon black and ethanolic solutions of Pt(NH3)2(N02)2 and RuN0(N03). The specific activity for methanol electrooxidation increased with an increase in the Pt/Ru particle size. The concept of larger particle size aiding in the activity of methanol oxidation was experimentally verified [28-33]. 

Besides, ILs unit could be attached to the sidewall of CNTs by radical grafting, in which acid-oxidation pretreatment of CNTs could be avoided. Chen et al. reported that thermal-initiation free radical polymerization of the IL monomer 3-ethyl-l-vinylimidazolium tetrafluoroborate ([VEIM]BF4] on the CNT surface (Fig. 4.18a] [62]. Then under similar method, the Pt and PtRu nanoparticles with narrow size distribution (average diameter (1.3 0.4] nm for PtRu, (1.9 0.5] nm for Pt] are dispersed uniformly on the CNTs and show better performance in methanol electrooxidation than that without ILs units (Fig. 4.18b]. 

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. 

Fig. 1.8 DFT investigations of methanol oxidation on PtML supported on different substrates. The DFT-calculated variation of the lowest potential to proceed methanol electrooxidation on the PtML supported on Cu(lll), Ru(OOl), Rh(l 11), Re(OOl), Pd(l 11), Os(001), Ag(ll 1), and...

CV and CA experiments were performed to examine the electrochemical activity of Pt, Pd, WC, PtAVC, and PdAVC activity toward methanol electrooxidation. Experiments were performed with Pt, 0.8 ML PtAVC, and WC samples in a 0.2 M CH3OH and 0.05 M H2SO4 electrochemical environment [20]. The R foil showed methanol oxidation peaks at 0.9 V on the forward scan at 0.7 V on the... 

A lot of papers have been addressed to the methanol oxidation on Pt-Ru catalyst in acidic media, and excellent reviews have been done by Spendelow et al. [24] and Petrii [25]. Conversely, few works have been addressed to the MOR on Pt-Ru catalysts in alkali media. Firstly, Petrii et al. [26] compared the polarization curves of methanol electrooxidation in an alkaline solution under steady-state conditions... 

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