Formaldehyd oxide

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

When the final methylation of either product is effected with formaldehyde, oxidation of the secondary alcohol group occurs simultaneously in each case, and of the two resulting ketones that from product (XIX) proved to be dZ-hygrine, which must therefore have formula (XVII) given above. Another synthesis of dZ-hygrine has been effected recently by Sorm. ... 

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]). 

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). 

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...

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. 

Formaldehyde Oxidation The general characteristics of the faradaic current dependence on the potential (solid line in the top panel of Fig. 13.3b) are... 

The partial faradaic current for formaldehyde oxidation to CO2, calculated from the m/z = 44 ion current, is plotted as a dashed hne in Fig. 13.3b (upper panel). Complete oxidation of formaldehyde to CO2 contributes only one-third (positive-going scan) or one-quarter (negative-going scan) of the corresponding faradaic current peaks (sohd hne in the upper panel of Fig. 13.3b). The difference between the measured net current and the calculated faradaic current, which is plotted as a dotted line in Fig. 13.3b (upper panel), reflects the partial current for incomplete formaldehyde oxidation to... 

The current efficiencies for CO2 formation and formic acid formation during poten-tiodynamic formaldehyde oxidation, calculated from the data in Fig. 13.3b as the ratio of the partial currents to the total faradaic current (in %), are plotted in Fig. 13.4a. 

Formaldehyde Oxidation The faradaic current for formaldehyde oxidation (solid line in Fig. 13.6a) is largely suppressed during the first minute after stepping to 0.6 V, and then increases with time, resulting in an S-shaped transient. It reaches its maximum current about 4-5 minutes after the potential step, followed by a slow and nearly linear decay of the faradaic current with time (solid line in Fig. 13.6a). In comparison with formic acid oxidation (solid line in Fig. 13.5a), three major differences can be noted ... 

On the other hand, during potentiodynamic formaldehyde oxidation (solid line in the upper panel of Fig. 13.3b), there is only a small faradaic current at 0.6 V in the positive-going scan, in contrast to the much higher steady-state value (about 0.55 mA) attained in the potentiostatic experiment. 

Conversion of the m/z = 44 ion current into a partial faradaic reaction current for formaldehyde oxidation to CO2 (four-electron reaction) shows that, under these experimental conditions, formaldehyde oxidation to CO2 is only a minority reaction pathway (dashed line in Fig. 13.6a). Assuming CO2 and formic acid to be the only stable reaction products, most of the oxidation current results from the incomplete oxidation to formic acid (dotted hne in Fig. 13.6a). The partial reaction current for CO2 formation on Pt/Vulcan at 0.6 V is only about 30% of that during formic acid... 

MS measurements, the actual height of the initial spike may be signihcantly greater than indicated in Fig. 13.7b.) The steady-state MOR current of 0.12 mA at the end of the measurement is signihcantly lower than that for formic acid oxidation and much lower than that for formaldehyde oxidation under similar conditions. 

The current efficiencies for the different reaction products CO2, formaldehyde, and formic acid obtained upon potential-step methanol oxidation are plotted in Fig. 13.7d. The CO2 current efficiency (solid line) is characterized by an initial spike of up to about 70% directly after the potential step, followed by a rapid decay to about 54%, where it remains for the rest of the measurement. The initial spike appearing in the calculated current efficiency for CO2 formation can be at least partly explained by a similar artifact as discussed for formaldehyde oxidation before, caused by the fact that oxidation of the pre-formed COacurrent efficiency. The current efficiency for formic acid oxidation steps to a value of about 10% at the initial period of the measurement, and then decreases gradually to about 5% at the end of the measurement. Finally, the current efficiency for formaldehyde formation, which was not measured directly, but calculated from the difference between total faradaic current and partial reaction currents for CO2 and formic acid formation, shows an apparently slower increase during the initial phase and then remains about constant (final value about 40%). The imitial increase is at least partly caused by the same artifact as discussed above for CO2 formation, only in the opposite sense. 

A similar reaction scheme is also likely for formaldehyde oxidation [Loucka and Weber, 1968]. 

Whereas in the indirect pathway, COad is clearly identified as a reaction intermediate, the specific nature of the intermediate(s) in the direct pathway is under debate. For methanol oxidation, species such as COH [Xia et al., 1997 Iwasita et al., 1987, 1992 Iwasita and Nart, 1997], CHO [Zhu et al., 2001 Willsau and Heitbaum, 1986 Wilhehn et al., 1987], COOH [Zhu et al., 2001], and adsorbed formate species [Chen et al., 2003] have been proposed. Adsorbed formate species were identified during formaldehyde oxidation [Samjeske et al., 2007], methanol oxidation [Nakamura et al., 2007 Chen et al., 2003, unpublished], and fornfic acid oxidation [Miki et al., 2002, 2004 Samjeske and Osawa, 2005 Chen et al., 2006a, b, c Samjeske et al., 2005, 2006]. 

Does methanol (or formaldehyde) oxidation along the direct pathway lead directly to CO2 or are incomplete oxidation products formed first, which are then oxidized further to finally result in CO2 formation (dashed arrows in Fig. 13.8b) ... 

The much lower amount of formic acid formation during methanol oxidation compared with formaldehyde oxidation agrees with expectations if we assume that formic acid is predominantly formed by further oxidation of (free) molecular formaldehyde produced in a first step of methanol oxidation. Under the present reaction conditions, only a very small fraction, about 1 part per thousand, of the total reactant passing through the cell reacts to give formaldehyde, formic acid, or CO2. The rest... 

Similar ideas can be applied to formaldehyde oxidation. For bulk formaldehyde oxidation, we found predominant formic acid formation under current reaction conditions rather than CO2 formation. Hence, it cannot be ruled out, and may even be realistic, that formaldehyde is first oxidized to formic acid, which can subsequently be oxidized to CO2. The steady-state product distribution at 0.6 V is much more favorable for such a mechanism as in the case of methanol oxidation. On the other hand, because of the high efficiency of COad formation from formaldehyde, this process is likely to proceed directly from formaldehyde adsorption rather than via formation and re-adsorption of formic acid. Alternatively, the second oxygen can be introduced via formaldehyde hydration to methylene glycol, which could be further oxidized to formic acid and finally to CO2 (see the next paragraph). 

The hnding of very substantial amounts of incomplete oxidation products for methanol and formaldehyde oxidation can have considerable consequences for technical applications, such as in DMFCs. In that case, the release of formaldehyde at the fuel cell exhaust has to be avoided not only from efficiency and energetic reasons, but in particular because of the toxicity of formaldehyde. While in standard DMFC applications the catalyst loading is sufficiently high that this is not a problem, i.e., only CO2 is detected [Arico et al., 1998], the trend to reducing the catalyst loading or applications in micro fuel cells may lead to situations where the formation of incomplete oxidation products could indeed become problematic (see also Wasmus et al. [1995]). For such purposes, one could dehne a maximum space velocity above which formation of incomplete oxidation products may become critical. 

Batista EA, Iwasita T. 2006. Adsorbed intermediates of formaldehyde oxidation and their role in the reaction mechanism. Langmuir 22 7912-7916. 

JusysZ. 1994. H/D substitution effect on formaldehyde oxidation rate at a copper anode in alkaline medium studied by differential electrochemical mass spectrometry. J Electroanal Chem 375 257-262. 

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