Formic acid formation

H0(CH20)A increased capacity elevated temperature storage, formic acid formation... 

Ameisen-. formic, -aldehyd, n. formaldehyde. -amylMther, -amylester, m. amyl formate, -ather, m. formic ether (ethyl formate), geist, m. (Pharm.) spirit of ants (a mixture of formic acid, alcohol and water), -naphta, n. = Ameisenather. -persaure, /. peroxy-formic acid, performic acid, ameisensauer, a. of or combined with formic acid, formate of. — ameisensaures Salz, formate. 

Ameisensaure, /. formic acid, -nitril, n. formo-nitrile (HCN). -salz, n. salt of formic acid, formate, -sulfonsiure, /. eulfoformic acid. Ameisen-spiritus, m. = Ajneisengeist. -wein-ather, m. ethyl formate. 

C18-0095. The pH of a formic acid/formate buffer solution is 4.04. Calculate the acid/conjugate base ratio for this solution. Draw a molecular picture that shows a small region of the buffer solution. (You may omit spectator ions and water molecules.) Use the following symbols ... 

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. 

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. 

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 mjz = 60 ion current (Fig. 13.7c), which results from methyl formate and hence is indicative of formic acid formation, equally increases in a pronounced step after raising the potential, and then decays slowly with time. (The higher noise in these data is due to the very low concentration of methyl formate species.)... 

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. 

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). 

Finally, we have discussed the effect of incomplete Cj oxidation product formation for fuel cell applications and the implications of these processes for reaction modeling. While for standard DMFC applications, formaldehyde and formic acid formation will be negligible, they may become important for low temperature applications and for microstructured cells with high space velocities. For reaction modeling, we have particularly stressed the need for an improved kinetic data base, including kinetic data under defined reaction and transport conditions and kinetic measurements on the oxidation of Ci mixtures with defined amounts of formaldehyde and formic acid, for a better understanding of cross effects between the different reactants at an operating fuel cell anode. 

Photocatalytic oxidation of ethanol on Pt/ri02 and Nafion coated Ti02 catalysts were studied using in situ infrared IR techniques. Infrared studies show that the reaction produced acetaldehyde, acetic acid, acetate, formic acid, formate, and CO2/H2O. Modification of the Ti02 catalyst by Pt and Nafion slowed down the oxidation reaction through site blocking. Incorporation of Pt was found to favor formation of formate (HCOO ), indicating Pt decreases the rate of oxidation of formate more than that of its formation. 

Acetaldehyde Acetic acid Acetate Formic acid Formate... 

Smallwood, C. J., McAllister, M. A., 1997, Characterization of Low-Barrier Hydrogen Bonds. 7. Relationship Between Strength and Geometry of Short-Strong Hydrogen Bonds. The Formic Acid-Formate Anion Model System. An Ab Initio and DFT Investigation , J. Am. Chem. Soc., 119, 11277. 

A calculation of the temperature dependence of the free energy for the reactions in Eqs. (15)-(18), and hence the electrochemical potential, showed that with an increase in temperature, formic acid formation became more unfavorable.4 In the case of formaldehyde, methanol, and methane formation, the calculation indicated a positive shift in the reduction potential, but of very small magnitude ca. 30 mV for a temperature change from 300 to 500 K, and ca. 20 mV from 500 to 1200 K.4... 

The most controversial issue is the number and exact stoichiometries of the iron(III)-sulfito complexes formed under different experimental conditions. Earlier, van Eldik and co-workers reported the formation of a series of [Fe(SO ) ]3-2" (n = to 3) complexes and the [Fe(S03)(0H)] complex (89,91,92). The stability constants of these species were determined by evaluating time resolved rapid-scan spectra obtained from the sub-second to several minutes time domain. The cis-trans isomerization of the complexes was also considered, under feasible circumstances. In contrast, Betterton interpreted his results assuming the formation and linkage isomerization of a single complex, [Fe(SC>3)]+ (93). In agreement with the latter results, Conklin and Hoffmann also found evidence only for the formation of a mono-complex (94). However, their results were criticized on the basis that the experiments were made in 1.0 M formic acid/formate buffer where iron(III) existed mainly as formato complex(es). Although these reactions could interfere with the formation of the sulfito complex, they were not considered in the evaluation of the results (95). Finally, van Eldik and co-workers re-examined the complex-formation reactions and presented additional data in support of... 

Multiple products are possible from C02 hydrogenation, but all of the products are entropically disfavored compared to C02 and H2 (Scheme 17.1). As a result, the reactions must be driven by enthalpy, which explains why formic acid is usually prepared in the presence of a base or another reagent with which formic acid has an exothermic reaction. Of the many reduction products that are theoretically possible, including formic acid, formates, formamides, oxalic acid, methanol, CO, and methane, only formic acid and its derivatives are readily prepared by homogeneous catalysis. 

Ion pair formation appears to become relevant only for stronger complexing agents. The partitioning of the protonated form of DMA increases significantly in the presence of formic acid/formate buffer at pH 3, and is most likely due to complex formation between the anilinium ion and formate [120]. 

Photoreduction. Only very strong reducing agents are able to perform the one-electron reduction of C02 to C02-, which is difficult to produce by photochemical means. Two-electron reduction, that is, the formation of formic acid (formate), in turn, is energetically highly favorable. 

The same workers have more recently used a similar catalyst system to produce formic acid (formate ion) from hydrogen and carbon dioxide (160). The catalyst system is again Pd(diphos)2 and Et3N in benzene solution under 25 atm each of C02 and H2. The reaction is run at room temperature or higher. A small amount of water dramatically accelerates the rate of the reaction, and a mechanism is proposed to account for this effect. Control experiments are stated to rule out the initial reduction of C02 to CO by H2, followed by reaction with water to yield the formic acid. 

The acid is unstable at the conversion temperatures (10), decomposing rapidly to carbon dioxide and hydrogen. Thus with the switch from protio to deuterio, the formic acid formation experiences a... 

Similar reaction pathways have also been found for the oxidation of dimethyl sulfide to dimethyl sulfoxide and dimethyl sulfoxide to dimethyl sulfone by [Ru(bpy)2(py)(0)]2+ with respective rate constants of 17.1 and 0.13 M l s"1 in MeCN at 298 K (48). The complex [Ru(bpy)2 (py)(0)]2+ has also been used electrocatalytically for the oxidation of alcohols, aldehydes, alkenes, and aromatics (23, 49). The kinetics of oxidation of formic acid/formate ion by [Ru(bpy)2(py)(0)]2 +, with a large kinetic isotope effect [ HC02-/ADCo2- = 19 (25°C, /r = 1.0 M)], has been reported (50). A two-electron hydride transfer has been suggested for the oxidation of HC02 by [Ru(bpy)2(py)(0)]2+. A similar mechanism has also been suggested for the oxidation of alcohols (51) and aromatics (52) by [Ru(bpy)2(py)(0)]2+ and other related Ru(IV) oxo complexes (28,... 

Acetic acid Acetaldehyde Acetone L-Alanine l-Alanine ion L-Alaninate ion L-Arginine dl-Aspartic acid L-Aspartic acid L-Aspartic acid ion L-Aspartate ion Benzene Butyric acid Butyrate ion Carbon dioxide Citric acid Citrate ion Creatine L-Cysteine L-Cystine Ethanol Ethyl acetate Formic acid Formate ion Fumaric acid Fumarate ion a-D-Glucose p-D-Glucose Glycerol L-Glutamic acid L-Glutamate ion... 

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