Formaldehyde, electrochemical reduction

The conventional electrochemical reduction of carbon dioxide tends to give formic acid as the major product, which can be obtained with a 90% current efficiency using, for example, indium, tin, or mercury cathodes. Being able to convert CO2 initially to formates or formaldehyde is in itself significant. In our direct oxidation liquid feed fuel cell, varied oxygenates such as formaldehyde, formic acid and methyl formate, dimethoxymethane, trimethoxymethane, trioxane, and dimethyl carbonate are all useful fuels. At the same time, they can also be readily reduced further to methyl alcohol by varied chemical or enzymatic processes. 

Halmann reported in 1978 the first example of the reduction of carbon dioxide at a p-GaP electrode in an aqueous solution (0.05 M phosphate buffer, pH 6.8).95 At -1.0 V versus SCE, the initial photocurrent under C02 was 6 mA/ cm2, decreasing to 1 mA/cm2 after 24 h, while the dark current was 0.1 mA/cm2. In contrast to the electrochemical reduction of C02 on metal electrodes, formic acid, which is a main product at metal electrodes, was further reduced to formaldehyde and methanol at an illuminated p-GaP. Analysis of the solution after photoassisted electrolysis for 18 and 90 h showed that the products were 1.2 x 10-2 and 5 x 10 2 M formic acid, 3.2 x 10 4 and 2.8 x 10-4 M formaldehyde, and 1.1 x 10-4 and 8.1xlO 4M methanol, respectively. The maximum optical conversion efficiency calculated from Eq. (23) for production of formaldehyde and methanol (assuming 100% current efficiency) was 5.6 and 3.6%, respectively, where the bias voltage against a carbon anode was -0.8 to -0.9 V and 365-nm monochromatic light was used. In a later publication,4 these values were given as ca. 1% or less, where actual current efficiencies were taken into account [Eq. (24)]. 

In has been observed in the case of electroless Cu-Pd deposition from electroless solutions containing formaldehyde as reductant, that codeposition of small amounts of Pd with Cu results in appreciable reduction in the amount of H2 gas evolved [50], For example, codeposition of 0.3 at% Pd yields a reduction of 30% in the amount of evolved H2. It is difficult, however, to formulate stable electroless solutions involving Pd2+ since it has strong tendency in the electroless solution to become reduced due to it relatively high electrochemical series potential (ca. 0.95 V vs. RHE3). 

Electrochemical reduction of the nitrosyl group is possible, which ultimately yields ammonia thus, 4-electron reduction of an aqueous solution of [Fe(CN)5(NO)]2- gives [Fe(CN)5(NH20H)]3- (260), and reaction of [Cr(N0)(H20)5] or [Ru(NO)(NH3)s]3+ with chromous ion gives Cr2+(aq) -I- NH3 or [Ru(NH3)]e], respectively (261). Electrochemical reduction of [Ru(NO)(trpy)(bpy)] + (262) has been shown also to yield ammonia. Nitrido complexes can be also isolated in the reduction of [RuClslNO)] ". With SnCl2-HCl or aqueous formaldehyde, for example, [Ru2NCl8(H20)2] is formed (263). 

The electrochemical reduction of formaldehyde to the corresponding pinacol, dihy-droxyethane, has been closely examined as a possible technical scale process. Yields are very dependent on the choice of reaction conditions. Best results are obtained with a graphite cathode and sodium formate as electrolyte at 57°C [20]. The reduction of acetone to pinacol has also been examined from a technical point of view. Moderate yields of pinacol are obtained at a lead cathode in acid solution together with isopropanol and propane. The propane arises by hydrolysis of lead alkyl intermediates and under some conditions tetraisopropyllead is formed [21]. A pilot plant scale production of acetone... 

The solubility of carbon dioxide in aqueous and non-aqueous solutions depends on its partial pressure (via Henry s law), on temperature (according to its enthalpy of solution) and on acid-base reactions within the solution. In aqueous solutions, the equilibria forming HCO3 and CO3 depend on pH and ionic strength the presence of metal ions which form insoluble carbonates may also be a factor. Some speculation is made about reactions in nonaqueous solutions, and how thermodynamic data may be applied to reduction of CO2 to formic acid, formaldehyde, or methanol by heterogenous catalysis, photoreduction, or electrochemical reduction. 

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 same authors proposed a complex system for the electrochemically driven enzymatic reduction of carbon dioxide to form methanol. In this case, methyl viologen or the cofactor PQQ were used as mediators for the electroenzymatic reduction of carbon dioxide to formic acid catalyzed by formate dehydrogenase followed by the electrochemically driven enzymatic reduction of formate to methanol catalyzed by a PQQ-dependent alcohol dehydrogenase. With methyl viologen as mediator, the reaction goes through the intermediate formation of formaldehyde while with PQQ, methanol is formed directly [77],... 

Ponce de Leon C, Pletcher D. Removal of formaldehyde from aqueous solutions via oxygen reduction using a reticulated vitreous carbon cathode. J Appl Electrochem 1995 25 307-314. 

In the following subsections experiments are described which indicate that CO2 reduction to methylene-H4MPT is driven by a primary electrochemical Na potential generated by formaldehyde reduction to CH4. These experiments include (1) studies of the mode of energy transduction of the reverse reaction, the exergonic formaldehyde oxidation to CO2 and 2H2 (2) experiments on the effects of ionophores and inhibitors on CH4 formation from CO2/H2 and CH4 formation from formaldehyde/H2, and the determination of stoichiometries of primary Na" translocation. 

CO2 reduction to methylene-H MPT - coupled to Na uptake. If Reactions (l)-(4) of Fig. 3 and Table 2 are reversible (see Reactions 6—9 of Table 3, below), it is likely that CO2 reduction to the formaldehyde level is driven by an electrochemical Na potential. Evidence for this notion was obtained from the following experiments with cell suspensions of Methanosarcina barkeri [167]. 

The electroactive species A is generated by a reaction, generally an equilibrium displaced toward Z, that precedes the electron transfer step. Z is the reactant introduced in the cell or the predominant form of the reactant in the reaction medium. These reaction schemes were introduced to rationalize the various electrochemical phenomena observed during the reduction of certain aldehydes in aqueous solutions. Indeed, in water, formaldehyde, r-electron-poor heterocyclic aldehydes, and a few aldehydes with strongly electron-attracting groups exist as their nonreducible hydrated form in rapid equilibrium with the reducible carbonyl ... 

Different electron-conducting polymers (polyaniline, polypyrrole, polythiophene) are considered as convenient substrates for the electrodeposition of highly dispersed metal electrocatalysts. The preparation and the characterization of electronconducting polymers modified by noble metal nanoparticles are first discussed. Then, their catalytic activities are presented for many important electrochemical reactions related to fuel cells oxygen reduction, hydrogen oxidation, oxidation of Cl molecules (formic acid, formaldehyde, methanol, carbon monoxide), and electrooxidation of alcohols and polyols. 

In multielectron transfer processes, the reduction of CO2 can yield formic acid, carbon monoxide, formaldehyde, methanol, or methane that is, the primary electrochemical process supplies Ci compounds. These reactions can proceed at reasonable reduction potentials between —0.24 and —0.61 V (NHE) (Equations (6.12-6.16) the reduction potentials, E°, refer to pH 7 in aqueous solutions versus NHE), while the formation of the C02 radical anion is estimated to take place at —2.1 V.104 Reduction of CO (in the presence of H + ) supplies CH2" radicals that may yield methane directly or leads to higher hydrocarbons (e.g., ethene or ethane) by recombination.24,105 Efficient formation of ethene (together... 

Presuming the electrochemical mechanism electroless copper plating [19], namely, the catalytic reduction of Cu(II) ions by formaldehyde, the partial reactions occurring at equal rates under open-circuit conditions could be written (using the deuterium tracer to specify the origin of hydrogen) in a simplified form as follows ... 

FIGURE 19.9 Extraction of the partial reaction rates for formaldehyde oxidation (o) ((A) measured by DEMS, Figure 19.6A) and copper Cu(II) ion reduction (dotted line) ((B) measured by EQCM, Figure 19.8A) as the difference with the net current (solid line) in the positive-going scan in electroless copper-plating solution (for details see captions of Figures 19.6A and 19.8A). (From Pauliukaite, R. et al., J. Appl. Electrochem., 36, 1261, 2006.)... 

The highest rate of electroless copper deposition under open-circuit conditions is achieved at the most positive Em values both Em and process rate values are determined by electrochemical characteristics of coupled partial reactions. Notably, Cu(II) reduction partial reaction is more sensitive to the nature of the complexing agent compared to anodic formaldehyde oxidation. The decrease in the rate of electroless copper deposition in solutions with Em value becoming more negative corresponds to a negative shift of the Cu(II)/Cu potential, due to the increase in the pK value of the Cu(II) complexes, as well as due to kinetic and structural factors [37]. 

---