Methanol Oxidation results

The work presented shows that an increase of the electrocatalytic activity can be obtained, if a suitable method for the catalyst synthesis is employed. In this sense, the Alcohol Reduction Method showed a positive effect, probably due to the good particle dispersion at the carbon surface and the suitable particle size distribution that this method produces. For the methanol oxidation results, an increase in the cell potential by PtRu/C electrocatalyst on Vulcan XC72 system was observed compared to the PtRu/C E-TEK formulation. This can be explained due to the better conductivity of this Carbon Suport, enhancing the speed of the electron transference in the Methanol Oxidation Reaction (MOR).These results can also be attributed to the good particle distribution at... 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

This results from the slow kinetics of methanol oxidation and oxygen reduction. An additional loss is due to the cell resistance (arising mainly... 

Using the colloidal Pt(i t ) + RU c/C catalysts described above, the optimal atomic ratio depends upon methanol concentration, cell temperature, and applied potential, as shown by the Tafel plots recorded with methanol concentrations of 1.0 and 0.1 M at T = 298K (Fig. 11.4) and 318K (Fig. 11.5). Some authors have stated that for potentials between 0.35 and 0.6 V vs. RHE, the slow reaction rate between adsorbed CO and adsorbed OH species must be responsible for the rate of the overall process [Iwasita et al., 2000]. From these results, it can be underlined that, at a given constant potential lower than 0.45-0.5 V vs. RHE, an increase in temperature requires an increase in Ru content to enhance the rate of methanol oxidation, and that, at a given constant potential greater than 0.5 V vs. RHE, an increase in temperature requites a decrease in Ru content to enhance the rate of methanol oxidation. 

In this section, we present results of potentiodynamic DBMS measurements on the continuous (bulk) oxidation of formic acid, formaldehyde and methanol on a Pt/ Vulcan catalyst, and compare these results with the adsorbate stripping data in Section 13.3.1. We quantitatively evaluate the partial oxidation currents, product yields, and current efficiencies for the respective products (CO2 and the incomplete oxidation products). In the presentation, the order of the reactants follows the increasing complexity of the oxidation reaction, with formic acid oxidation discussed first (one reaction product, CO2), followed by formaldehyde oxidation (two reaction products) and methanol oxidation (three reaction products). 

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

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. 

The SNIFTIRS results presented here confirm the presence of formic acid and methyl formate as by-products of methanol oxidation. Other by-products such as formaldehyde could not be detected under our experimental conditions. In fact, formaldehyde hydrolyses (99.99%) in aqueous solutions to a gemdiol H2C(OH)2, and the typical aldehyde bands are, therefore, not expected. 

The fact that electrodes prepared with Sn02 [95] also show catalytic properties upon methanol oxidation does not invalidate the result that Sn(II) species are actually responsible for the observed effects. It could be possible for SnOz to be reduced to SnO (or SnOH+) at the potentials where the catalytic effect is observed. 

Treating ethylene oxide with sodium methoxide (in the presence of a small amount of methanol) can result in the formation of a polyether. 

The work of Kunimatsu and Kita (1987) is very powerful evidence in favour of linearly adsorbed CO being the catalytic poison for methanol oxidation at a smooth platinum electrode in acid solution and has resulted in this hypothesis being generally accepted. However, there is some conflict between the IR results and those obtained by Vielstich and colleagues using chronocoulometry, ECTDMS and DEMS. 

Methanol still proceeds through an initial C H bond scission, but reacts with water before the OH bond breaks. Alternatively, formaldehyde formation likely occurs along the same pathway as CO formation. This is true if HCO is an intermediate in the decomposition pathway. Furthermore, the lack of a kinetic isotope effect for CH3OD indicates that formaldehyde is not the product of an initial O-H scission.94 Because formaldehyde and formic acid are not the thermodynamically favored products of methanol oxidation, they must be the result of kinetic limitations preventing the full oxidation to C02, analogous to the production of H202 for the reduction of oxygen (see next section). 

The methanol oxidation current rapidly increases after the platinum metal surface is revealed when the reduction of Pt-OH occvirred at 900 - 800 mV. The current decreases again by the combined effects of the accumulation of poisoning adsorbates, consumption of Pt-OH and the potential decrease. This results in an oxidation peak at 700 mV on the cathodic sweep. 

Chemically and electrochemically platinized platinum electrode (180 as roiighness factors for both electrodes) were used as high area platinum electrodes. While they showed slightly different COad oxidation characteristics, their methanol oxidation characteristics were virtually the same. Therefore, the results will be shown for only chemically platinized platinum electrodes. 

From this result, the lai er methanol oxidation current of the Nafion SPE electrode seems to be attributed to this resistance against the contamination rather than the intrinsic activity for the methanol oxidation. 

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