Formic acid, electrolyte

Benzidine.—That nitrobenzene, by electrolytical reduction in acid solution, can directly yield benzidine, was first proved by Hiiussermann,1 who used sulphuric acid. Lob 2 later proved the same to be true for hydrochloric-, acetic- and formic-acid electrolytes. However, several reactions predominate in this direct acid reduction, which prevent the carrying out of the reaction up to hydrazobenzene, or the formation of benzidine. Phenylhydroxylamine may particularly be mentioned in this connection. In alcoholic-acid solution it is partly rearranged to amidophenol or its ethers, and partly reduced to aniline. Azoxybenzene, in acid solution, is the starting-point in the benzidine formation however, in this case, the combining velocity of nitrosobenzene and phenylhydroxylamine is not very great, so that the latter is to a very considerable extent subject to the more rapidly acting influence of the acid. 

PurpurogaHin (5), a red-brown to black mordant dye, forms from electrolytic and other mild oxidations of pyrogaHol (1). The reaction is beHeved to proceed through 3-hydroxy-(9-benzoquinone (2) and 3-hydroxy-6-(3,4,5-trihydroxyphenyl)-(9-benzoquinone (3). The last, in the form of its tautomeric triketonic stmcture, represents the vinylogue of a P-diketone. Acid hydrolysis leads to the formation of (4), foHowed by cyclization and loss of formic acid... 

In both cases, the hydride ion approaches the double bond from the sterically more accessible side of the molecule. Reduction of imines by metals and acids, electrolytically or by formic acid gives saturated secondary amines (38,255). 

On the other hand, metals such as Ta, Nb, Ti, Zr, Al, etc. (the valve metals ) do not exhibit transpassive behaviour, and in appropriate electrolyte solutions film growth at high fields rather than corrosion and/or oxygen evolution is the predominant reaction thus aluminium can be anodised to 500 V or more in an ammonium borate buffer titanium can be anodised to about 400 V in formic acid and tantalum can be anodised to high voltages in most acids, including hydrochloric acid. 

For the determination of the molecular weight (Mw) by light scattering, the number of solvent systems is limited. The refractive index difference should be at least 0.1 and the solvent should not have an electrolytic effect. Useful solvents include formic acid containing KC1 salt and fluorinated alcohols. 

Cathodic reduction is the most promising approach to the removal of carbon dioxide from a closed atmosphere. Methods developed so far provide for electrode materials, electrolytes, and electrolysis conditions where CO2 can be reduced to hquid organic products of low molecular weight such as formic acid. More complex systems are required to regenerate foodstuffs from the rejects of human vital activities during... 

While very limited data ate presented here, the kinetics of adsorption/decomposition of formic acid molecules [Rice et al., 2002] have been measured by BB-SFG, as shown in Fig. 12.14. A Pt(lll) electrode and a 0.1 M H2SO4 electrolyte containing 0.1 M formic acid were used. The families of spectra at 0.200, 0.025, and 0.225... 

Figure 12.14 SFG spectra of the carbonyls formed during formic acid decomposition on a Pt(lll) electrode in 0.1 M H2SO4 electrolyte containing 0.1 M formic acid. The spectral position is typical of atop CO on the Pt(l 11) surface. Times at which the spectra have been recorded are from 2 to 496 s, yielding HCOOH decomposition kinetics at three electrode potentials, -0.200, -0.025, and 0.225 V vs. Ag/AgCl.

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 will present and discuss cyclic voltammetry and potential-step DBMS data on the electro-oxidation ( stripping ) of pre-adsorbed residues formed upon adsorption of formic acid, formaldehyde, and methanol, and compare these data with the oxidative stripping of a CO adlayer formed upon exposure of a Pt/ Vulcan catalyst to a CO-containing (either CO- or CO/Ar-saturated) electrolyte as reference. We will identify adsorbed species from the ratio of the mass spectrometric and faradaic stripping charge, determine the adsorbate coverage relative to a saturated CO adlayer, and discuss mass spectrometric and faradaic current transients after adsorption at 0.16 V and a subsequent potential step to 0.6 V. 

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...

Figure 13.6 Potential-step electro-oxidation of formaldehyde on a Pt/Vulcan thin-film electrode (7 p,gpt cm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCHO upon stepping the potential from 0.16 to 0.6 V (electrolyte flow rate 5 pL at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCHO oxidation to CO2 dotted line, difference between the net faradaic current and that for CO2 formation, (b) Solid line, m/z = 44 ion current transients gray line potential-step oxidation of pre-adsorbed CO derived upon HCHO adsorption at 0.16 V, in HCHO-free sulfuric acid solution, (c) Current efficiency transients for CO2 formation (dashed line) and formic acid formation (dotted line).

The adsorption and oxidation of the Ci molecules methanol, formaldehyde, and formic acid over a carbon-supported Pt/C fuel cell catalyst under continuous electrolyte flow have been investigated in a quantitative, comparative online DBMS study. 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

The electrochemical cell can again be of the regenerative or electrosynthetic type, as with the photogalvanic cells described above. In the regenerative photovoltaic cell, the electron donor (D) and acceptor (A) (see Fig. 5.62) are two redox forms of one reversible redox couple, e.g. Fe(CN)6-/4 , I2/I , Br2/Br , S2 /S2, etc. the cell reaction is cyclic (AG = 0, cf. Eq. (5.10.24) since =A and D = A ). On the other hand, in the electrosynthetic cell, the half-cell reactions are irreversible and the products (D+ and A ) accumulate in the electrolyte. The most carefully studied reaction of this type is photoelectrolysis of water (D+ = 02 and A = H2)- Other photoelectrosynthetic studies include the preparation of S2O8-, the reduction of C02 to formic acid, N2 to NH3, etc. 

Because of the close distance between electrode and window the concentration of methanol in the thin electrolyte layer diminishes at positive potentials and can only slowly be supplied by diffusion. In order to have measurable quantities of formic acid (or methyl formate) one has to work with methanol concentrations in the order of 1 M or more. 

A little later, Russell et al.19 tried to obtain methanol from carbon dioxide by electrolysis. Reduction of carbon dioxide to formate ion took place in a neutral electrolyte at a mercury electrode. On the other hand, formic acid was reduced to methanol either in a perchloric acid solution at a lead electrode or in a buffered formic acid solution at a tin electrode. The largest faradaic efficiency for methanol formation from formic acid was ca. 12%, with poor reproducibility, after passing 1900 C in the perchloric acid solution at Pb in a very narrow potential region (-0.9 to -1.0 V versus SCE). In the buffered formic acid solution (0.25 M HCOOH + 0.1 M... 

Recently, results of careful experiments were reported by Ito et a/.101 They claimed that formic acid, formaldehyde, and methanol, which had been previously reported as photoelectrochemical reduction products of carbon dioxide, were observed also by photolysis of cell materials, such as electrolytes, including 15-crown-5 ether, and epoxy resin, which has often been used as the molding material of semiconductor electrodes in aqueous solutions. Previously reported reduction products were obtained also under nitrogen with (Table 4) and without (Table 5) a p-GaP photocathode under illumination. These precise experiments under improved conditions, where no photolytic products were observed, gave the result that the main reduction product of carbon dioxide at a p-GaP photocathode in aqueous electrolytes was formic acid. Thus, many kinds of products reported in previous papers83,97,100 were suggested to be due to photolysis of cell materials. 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

Electrolyte chemistries continue to be discovered and need to be carefully controlled, e.g., there is a need to track nitric acid molarity as well as total acid molarity in the catholyte to avoid silver deposition, and the deposition of lead dioxide on cell electrodes and in electrode cavities, which has required the development of a formic acid wash. 

Kostelecka and Haller have determined procaine in mass-produced and extemporaneous pharmaceuticals by capillary isotachophoresis [152]. The method was carried out using pH 4.85 acetate buffer solution, and 0.01 M formic acid as leading and terminating electrolytes, respectively. 

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

Many investigators have actively studied the electrochemical reduction of C02 using various metal electrodes in organic solvents because these solvents dissolve much more C02 than water. With the exception of methanol, however, no hydrocarbons were obtained. The solubility of C02 in methanol is approximately 5 times that in water at ambient temperature, and 8-15 times that in water at temperatures below 0°C. Thus, studies of electrochemical reduction of C02 in methanol at —30°C have been conducted.148-150 In methanol-based electrolytes using Cs+ salts the main products were methane, ethane, ethylene, formic acid, and CO.151 This system is effective for the formation of C2 compounds, mainly ethylene. In the LiOH-methanol system, the efficiency of hydrogen formation, a competing reaction of C02 reduction, was depressed to below 2% at relatively negative potentials.152 The maximum current efficiency for hydrocarbon (methane and ethylene) formation was of 78%. 

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