Formic acid oxidation

Various 2,6-di8ubstituted p-benzoquinones have been prepared by oxidation of the corresponding 2,6-disubstituted phenols with potassium nitrosodisulfonate or lead dioxide in formic acid. Oxidative coupling of 2,6-disubstituted phenols to poly-2,6-disubstituted phenylene ethers followed by treatment of the polymers in acetic acid with lead dioxide is reported to give low yields of the corresponding 2,6-disubstituted p-benzoquinones. 

For the Pt(llO) electrode, there are some contradictory results regarding its catalytic performance compared with Pt(lOO) some studies indicate that the activity is higher for Pt(llO), whereas others suggest the opposite [Chang et al., 1990 Clavilier et al., 1981 Lamy et al., 1983]. The differences are probably associated with different surface states of the Pt(l 10) electrode. The acmal surface strucmre of the Pt(llO) electrode is strongly dependent on the electrode pretreatment. Since formic acid oxidation is a surface-sensitive reaction, different electrocatalytic behavior can be obtained for the same electrode after different treatments. 

Palladium electrodes are also very active for formic acid oxidation, with higher current densities than platinum electrodes [Capon and Parsons, 1973c]. Oxidation occurs almost exclusively through the active intermediate path, without poison formation. The reaction is also very sensitive to the surface stmcture, and the activity of the... 

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. 

Baldauf M, Kolb DM. 1996. Formic acid oxidation on ultrathin Pd films on Au(hkl) and Ft hkl) electrodes. J Phys Chem 100 11375-11381. 

Chen YX, Heinen M, Jusys Z, Behm RJ. 2006a. Bridge-bonded formate Active intermediate or spectator species in formic acid oxidation on a Pt film electrode Langmuir 22 10399-10408. 

Clavilier J, Parsons R, Durand R, Lamy C, Leger JM. 1981. Formic acid oxidation on single crystal platinum electrodes. Comparison with polycrystalline platinum. J Electroanal... 

Feliu JM, Herrero E. 2003. Formic acid oxidation. In Vielstich W, Lamm A, Gasteiger H, eds. Handbook of Fuel Cells. Volume 2. New York Wiley-VCH. p 625-638. 

Femandez-Vega A, Feliu JM, Aldaz A, Clavilier J. 1991. Heterogeneous electrocatalysis on well-deflned platinum surfaces modifled by controlled amounts of irreversibly adsorbed adatoms Part IV. Formic acid oxidation on the Pt(lll)-As system. J Electroanal Chem 305 229-240. 

Samjeske G, OsawaM. 2005. Current oscillations during formic acid oxidation on a Pt electrode Insight into the mechanism by time-resolved IR spectroscopy. Angew Chem 44 5694-5698. 

Figure 7.14 Current density for formic acid oxidation as a function of the fraction of Pt surface atoms blocked by adatoms on two different electrodes (a) Bi/Pt(lll) (b) Sb/Pt(100). (Reprinted with permission from Leiva et al. [1997].)...

The good coincidence between the model and the experimental data depicted in Fig. 7.14 supports the idea that this model captures the essential aspects of the effect of different adatoms on the electrocatalysis of formic acid oxidation. 

Llorca MJ, Herrero E, FeUu JM, Aldaz A. 1994. Formic acid oxidation on Pt(l 11) electrodes modified by irreversible adsorbed selenium. J Electroanal Chem 373 217-225. 

Macia MD, Herrero E, Fehu JM. 2003. Formic acid oxidation on Bi-Pt(lll) electrode in perchloric acid media. A kinetic smdy. J Electroanal Chem 554 25-34. 

Schmidt TJ, Behm RJ, Grgur BN, Markovic NM, Ross PN. 2000a. Formic acid oxidation on pure and Bi-modified Pt(lll) Temperature effects. Langmuir 16 8159-8166. 

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...

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