Oxidation of formic acid

The net reaction for the anodic oxidation of formic acid has been established [1, 46, 47] in acid electrolytes as  

This was confirmed by recent measurements [40] by the author on platinized platinum. There is some controversy in the literature as to the reaction in alkaline solutions. While a uniform two-electron process throughout the pH range was reported in some papers [1, 7, 46—48], other work [10,11,49] indicates that the formate ion is not easily oxidized in alkaline electrolytes. The subsequent discussion is largely based on the extensive studies of formic acid oxidation in acid electrolytes in which the formic acid molecule is the reacting species [13]. 

Chemisorbed carbonaceous species are formed during the anodic oxidation [9, 39, 40, 48, 50—57] of HCOOH on platinum or during the interaction [39,52,58] at open circuit. The net composition COOH d, suggested in references 9 and 56 for smooth platinum in acid electrolytes is not in agreement with results obtained [40, 58] by the combination of charging curves and gas chromatography on platinized platinum. 

Curve 1 determined by anodic pulses, curve 2 determined by hydrogen deposition, curve 3 computed from the initial part of I —t curves 

As discussed in section 10 of chapter IX, the latter results suggest the simultaneous adsorption of more than one type of species, possibly of HCO and COOH. Slightly more than two electrons are required on the average for the anodic formation of one molecule CO2 from one adsorbed particle C,HpOq. 

Srivastava and Ghosh report that the kinetics are first-order with respect to peroxodisulphate and zero-order with respect to formic acid, but Kappana reports first-order kinetics with respect to each reactant. The effect of trace amounts of metal ions and of oxygen on the rate is uncertain, and discussion of the mechanism is of doubtful significance at present. However, the reported observations definitely indicate a chain mechanism. Thus Srivastava and Ghosh found an induction period in the oxidation, and report that halide ions inhibit the reaction (inhibition by halide ions is a feature of reactions involving hydroxyl radicals). In a study of the silver ion-catalysed oxidation, Gupta and Nigam found that the reaction is approximately first-order with respect to both peroxodisulphate and the catalyst, and zero-order with respect to the substrate. 

The y-ray-initiated oxidation of formic acid by peroxodisulphate has been studied by Hart , who reports the dependence of the yield of carbon dioxide on various factors, but does not give any kinetic data. 

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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. 

The permanganate oxidation of formic acid has attracted much attention. The reaction is pH-independent above pH 5 and involves formate ion. At lower pH s the rate is much lower until permanganic acids begins to be formed at very low pH ... 

Oxidation of formic acid by mercuric chloride is the subject of several early kinetic studies. Dhar showed the reaction to be first-order in oxidant and substrate and to be subject to strong retardation by added chloride ions in agreement with earlier work. The reaction is also subject to retardation by added acid and presumably involves formate ion as the principal reactant. 

The oxidations of formic acid by Co(III) and V(V) are straightforward, being first-order with respect to both oxidant and substrate and acid-inverse and slightly acid-catalysed respectively. The primary kinetic isotope effects are l.Sj (25°C)forCo(IU)and4.1 (61.5 C°)for V(V). The low value for Co(lII) is analogous to those for Co(IIl) oxidations of secondary alcohols, formaldehyde and m-nitrobenzaldehyde vide supra). A djo/ h20 for the Co(III) oxidation is about 1.0, which is curiously high for an acid-inverse reaction . The mechanisms clearly parallel those for oxidation of alcohols (p. 376) where Rj and R2 become doubly bonded oxygen. 

The oxidation of formic acid by Ce(IV) sulphate which is reported as being very slow, is accelerated by X-irradiation OH- is the active oxidant. 

The free-radical scheme, however, fails to account for the following (i) It cannot be easily generalised to cover the identical kinetics of the Mn(lII) sulphate oxidation if -CH(C02H) has an oxidation potential comparable with Mn(Ill)/ Mn(II) pyrophosphate then it cannot appreciably reoxidise Mn(ll) sulphate, (a) If -CH(C02H) reoxidises Mn(II) sulphate then it should be capable of re-oxidising both V(1V) sulphate (of the V(V)/V(IV) pair, potential 1.0 V) and Mn(II) sulphate in the V(V) oxidation of malonic acid that it does neither can be seen from the rate laws of these oxidations which show no Mn(II)-retardation vide infra). Hi) The not dissimilar kinetics of the Mn(III) sulphate oxidation of formic acid vide supra) and mercurous ion °. 

Cerium(III) also proved to be an effective inhibitor of the oxidation of formic acid. As the oxidation of cerium(rri) to cerium(IV) is a 1-equivalent process, the inhibition furnishes additional evidence for the chromium(IV) species as intermediate. 

Earher results on the oxidation of formic acid on Pt electrodes have been extensively reviewed [Parsons and VanderNoot 1988 Jarvi and Stuve, 1998 Sun, 1998 Vielstich, 2003 Eeliu and Herrero, 2003]. Here, we will summarize previous results, but wUl focus on the most recent results. 

In the oxidation of methanol to CO2, six electrons ate involved. This high number of electrons implies that the mechanism is inevitably very complex, with several intermediate species participating in the mechanism. In spite of its complexity, it has been proposed that the oxidation mechanism follows the same general scheme as the oxidation of formic acid, i.e., a dual path mechanism with active and poisoning intermediates (see the reaction Scheme 6.16) [Parsons and VanderNoot, 1988]. For that reason, we will compare the behavior with that of formic acid to highlight the similarities and differences. 

Beltramo G, Shubina TE, Koper MTM. 2005. Oxidation of formic acid and carbon monoxide on gold electrodes studied by surface-enhanced Raman spectroscopy and DFT. ChemPhysChem 6 2597-2606. 

Capon A, Parsons R. 1973a. Oxidation of formic acid at noble metal electrodes. I. Review of previous work. J Electroanal Chem 44 1-7. 

Hoshi N, Kida K, Nakamura M, Nakada M, Osada K. 2006. Structural effects of electrochemical oxidation of formic acid on single crystal electrodes of palladium. J Phys Chem B 110 12480-12484. 

Okamoto H, Kon W, Mukouyama Y. 2005. Five current peaks in voltammograms for oxidations of formic acid, formaldehyde, and methanol on platinum. J Phys Chem B 109 15659-15666. 

Samjeske G, Miki A, Ye S, Osawa M. 2006. Mechanistic study of electrocatal3dic oxidation of formic acid at platinum in acidic solution by time-resolved surface-enhanced infrared absorption spectroscopy. J Phys Chem B 110 16559-16566. 

Sun SG, Clavilier J, Bewick A. 1988. The mechanism of electrocatalytic oxidation of formic acid on Pt(lOO) and Pt(lll) in sulphuric acid solution An EMIRS study. J Electroanal Chem 240 147-159. 

Wolter O, Willsau J, Heitbaum J. 1985. Reaction pathways of the anodic oxidation of formic acid on platinum evidenced by oxygen-18 labeling—A DEMS study. J Electrochem Soc 132 1635-1638. 

Herteto E, Llorca MJ, Feliu JM, Aldaz A. 1995b. Oxidation of formic-acid on Pt(lOO) electrodes modified by irreversibly adsorbed tellurium. J Electroanal Chem 383 145-154. 

For the purpose of demonstrating the effects of surface coverage by Pd, 0pd, on the rate of electro-oxidation of formic acid and the ORR, Fig. 8.17 reveals that the i versus 0Pd relationship again has a volcano-like form, with the maximum catalytic activity being exhibited for 1 ML of Pd. The examples that we have given indicate that volcano relationships are the rule rather than the exception, emphasizing the importance of a systematic evaluation of the catalyst factors that control catalytic activity. A thorough... 

Figure 8.17 Activities of Pt(l 1 l)-wML Pd electrodes from rotating disk electrode measurements, with corresponding ball models (a) electro-oxidation of formic acid in 0.1 M HCIO4 ...

Bulk Oxidation of Formic Acid, Formaldehyde, and Methanol Potentiodynamic Measurements... 

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 13.5 Potential-step electro-oxidation of formic acid on a Pt/Vulcan thin-film electrode (7 p,gptcm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH upon stepping the potential from 0.16 to 0.6 V (electrol)Te flow rate 5 p,L s at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCOOH oxidation to CO2. (b) Solid line, m/z = 44 ion current transients gray line, potential-step oxidation of pre-adsorbed CO derived upon HCOOH adsorption at 0.16 V, in HCOOH-ftee H2SO4 solution.

Xia XH. 1999. New insights into the influence of upd Sn on the oxidation of formic acid on platinum in acidic solution. Electrochim Acta 45 1057-1066. 

Adzic RR, Tripkovic AV, Markovic NM. 1983. Structural effects in electrocatalysis oxidation of formic acid and oxygen reduction on single-crystal electrodes and the effects of foreign metal adatoms. J Electroanal Chem 150 79-88. 

Zhang X-G, Arikawa T, Murakami Y, Yahikozawa K, Takasu Y. 1995. Electrocatal3ftic oxidation of formic acid on ultrafine particles supported on glassy carbon. Electrochim... 

The electrocatalytic oxidation of methanol was discussed on page 364. The extensively studied oxidation of simple organic substances is markedly dependent on the type of crystal face of the electrode material, as indicated in Fig. 5.56 for the oxidation of formic acid at a platinum electrode. 

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