Methanol oxidation reaction

An Eley-Rideal/Mars-van Krevelen type of mechanism was found for the partial oxidation of methane to formaldehyde. The differences in the rate equations were due to differences in the amounts of oxygen present in methane rich and lean conditions. Methanol was not an intermediate in the reaction. Methanol oxidation experiments indicated that methanol was oxidised sequentially to formaldehyde, carbon monoxide and hence to carbon... 

Direct methanol fuel cell is an electrochemical device that converts the chemical energy of a reaction directly into electrical energy. The principle and schematic diagram of a DMFC is shown in Figure 9.1. In a typical DMFC, methanol and water molecules are simultaneously electro-oxidized at anode to produce CO2, electrons, and protons through the reaction (methanol oxidation reaction, MOR) ... 

Ion implantation has also been used for the creation of novel catalyticaHy active materials. Ruthenium oxide is used as an electrode for chlorine production because of its superior corrosion resistance. Platinum was implanted in mthenium oxide and the performance of the catalyst tested with respect to the oxidation of formic acid and methanol (fuel ceU reactions) (131). The implantation of platinum produced of which a catalyticaHy active electrode, the performance of which is superior to both pure and smooth platinum. It also has good long-term stabiHty. The most interesting finding, however, is the complete inactivity of the electrode for the methanol oxidation. 

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

Methanol oxidation on Pt has been investigated at temperatures 350° to 650°C, CH3OH partial pressures, pM, between 5-10"2 and 1 kPa and oxygen partial pressures, po2, between 1 and 20 kPa.50 Formaldehyde and C02 were the only products detected in measurable concentrations. The open-circuit selectivity to H2CO is of the order of 0.5 and is practically unaffected by gas residence time over the above conditions for methanol conversions below 30%. Consequently the reactions of H2CO and C02 formation can be considered kinetically as two parallel reactions. 

Methanol oxidation on Ag polycrystalline films interfaced with YSZ at 500°C has been in investigated by Hong et al.52 The kinetic data in open and closed circuit conditions showed significant enhancement in the rate of C02 production under cathodic polarization of the silver catalyst-electrode. Similarly to CH3OH oxidation on Pt,50 the reaction exhibits electrophilic behavior for negative potentials. However, no enhancement of HCHO production rate was observed (Figure 8.48). The rate enhancement ratio of C02 production was up to 2.1, while the faradaic efficiencies for the reaction products defined from... 

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. 

Redox reactions with metal porphyrins (MPs) as photocatalysts. A spectacular example here is the reaction that couples upon illumination with the sunlight, methanol oxidation to formaldehyde with the formation of hydrogen peroxide in be nzene-methanol mixture (90 10)... 

The purpose of this work was to increase the A3 selectivity at low conversion through a catalyst modification. Previous studies of phenol alkylation with methanol (the analogue reaction) over oxides and zeolites showed that the reaction is sensitive to acidic and basic properties of the catalysts [3-5]. It is the aim of this study to understand the dependence of catalyst structure and acidity on activity and selectivity in gas phase methylation of catechol. Different cations such as Li, K, Mg, Ca, B, incorporated into y-Al203 can markedly modify the polarisation of the lattice and consequently influence the acidic and basic properties of the surface [5-8] which control the mechanism of this reaction. 

The most recent innovation in this field is scanning tunnelling microscopy, which has the capability of atomic resolution. In the work reported here two surface reactions are examined using this technique. These reactions are of relevance to automobile catalysis (CO oxidation on Rh) and methanol oxidation/synthesis on Cu. It is proposed that active sites are imaged in these reactions and that these active sites can indeed be extremely dilute on the surface. 

Similar size effects have been observed in some other electrochemical systems, but by far not in all of them. At platinized platinum, the rate of hydrogen ionization and evolution is approximately an order of magnitude lower than at smooth platinum. Yet in the literature, examples can be found where such a size effect is absent or where it is in the opposite direction. In cathodic oxygen reduction at platinum and at silver, there is little difference in the reaction rates between smooth and disperse electrodes. In methanol oxidation at nickel electrodes in alkaline solution, the reaction rate increases markedly with increasing degree of dispersion of the nickel powders. Such size effects have been reported in many papers and were the subject of reviews (Kinoshita, 1982 Mukerjee, 1990). 

Adatoms produce a strong change in catalytic properties of the metal on which they are adsorbed. These catalytic effects are highly specific. They depend both on the nature of the metal and on the nature of the adatoms they also depend on the nature of the electrochemical reaction. For instance, tin adatoms on platinum strongly (by more than two orders of magnitude) enhance the rate of anodic methanol oxidation. 

Figure 12. Cyclic voltammograms of direct methanol oxidation catalyzed by the porous Pt nanoparticle membrane and as-made Pt nanoparticles. The reaction solution was made of an aqueous mixture containing O.IMHCIO4 and 0.125 M methanol. (Reprinted with permission from Ref [31], 2005, Wiley-VCH.)...

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. 

The qualitative voltammetric behavior of methanol oxidation on Pt is very similar to that of formic acid. The voltammetry for the oxidation of methanol on Pt single crystals shows a clear hysteresis between the positive- and negative-going scans due to the accumulation of the poisoning intermediate at low potentials and its oxidation above 0.7 V (vs. RHE) [Lamy et al., 1982]. Additionally, the reaction is also very sensitive to the surface stmcture. The order in the activity of the different low index planes of Pt follows the same order than that observed for formic acid. Thus, the Pt(l 11) electrode has the lowest catalytic activity and the smallest hysteresis, indicating that both paths of the reaction are slow, whereas the Pt( 100) electrode displays a much higher catalytic activity and a fast poisoning reaction. As before, the activity of the Pt(l 10) electrode depends on the pretreatment of the surface (Fig. 6.17). 

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. 

Nichols RJ, Bewick A. 1988. SNIFTIRS with a flow cell the identification of the reaction intermediates in methanol oxidation atplatinum anodes. Electrochim Acta 33 1691-1694. 

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. 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

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. 

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