Carbon dioxide reduction metal electrodes

Mechanisms of carbon dioxide reduction in both aqueous and nonaqueous solutions have been studied mainly at metal electrodes. 

Semiconductor electrodes seem to be attractive and promising materials for carbon dioxide reduction to highly reduced products such as methanol and methane, in contrast to many metal electrodes at which formic acid or CO is the major reduction product. This potential utility of semiconductor materials is due to their band structure (especially the conduction band level, where multielectron transfer may be achieved)76 and chemical properties (e.g., C02 is well known to adsorb onto metal oxides and/ or noble metal-doped metal oxides to become more active states77-81). Recently, several reports dealing with C02 reduction at n-type semiconductors in the dark have appeared, as described below. 

Ru electrodes were prepared as previously described by plating Ru metal onto spectroscopic carbon rods, except for the electrode used for Auger analysis (before and after carbon dioxide reduction) which was plated on Ti (2.). Cu electrodes were prepared from Cu foil as previously described (Kim, J. J. Summers, D. P. Frese, K. W., Jr. J. Electroanal- Chem. in press.). Each entry in the tables and figures was obtained on different days with the electrode kept in ordinary laboratory air overnight between runs. 

Electrocatalysis at metal electrodes in aqueous (1.2) and non-aqueous ( ) solvents, phthalocyanine ( ) and ruthenium ( ) coated carbon, n-type semiconductors (6.7.8),and photocathodes (9,10) have been explored in an effort to develop effective catalysts for the synthesis of reduced products from carbon dioxide. The electrocatalytic and photocatalytic approaches have high faradaic efficiency of carbon dioxide reduction (1,6). but very low current densities. Hence the rate of product formation is low. Increasing current densities to provide meaningful amounts of product, substantially reduces carbon dioxide reduction in favor of hydrogen evolution. This reduction in current efficiency is a difficult problem to surmount in light of the probable electrostatic repulsion of carbon dioxide, or the aqueous bicarbonate ion, from a negatively charged cathode (11,12). 

Kapusta, S. and N. Hackerman (1984). Carbon dioxide reduction at a metal phthalo-cyanine catalyzed carbon electrode. J. Electrochem. Soc. 131(7), 1511-1514. 

Electrocatalysts for Carbon Dioxide Reduction Electrosynthesis in Supercritical Fluids Reactive Metal Electrode... 

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)]. 

Reduction of carbon dioxide takes place at various metal electrodes. The main products are formic acid in aqueous solutions and oxalate, CO, and formic acid in nonaqueous solutions. An indium electrode is the most potential saving for C02 reduction. Due to the difference in optimum conditions between those for C02 reduction to formic acid and those for formic acid reduction to further reduced products, direct reduction of C02 in aqueous solutions without a catalyst to highly reduced products seems to be difficult at metal electrodes. However, catalytic effects of metal electrodes themselves have recently become more clear for example, on Cu, methane was detected, while on Ag and Au, CO was produced effectively in aqueous solutions. Furthermore, at a Mo electrode, methanol was obtained. The power efficiency is, however, still low at any electrode. 

In contrast to a variety of oxidizable compounds, only a few examples for the detection of strong oxidants with transition metal hexacyanoferrates were shown. Among them, hydrogen peroxide is discussed in the following section. Except for H202, the reduction of carbon dioxide [91] and persulfate [92] by Prussian blue-modified electrode was shown. The detection of the latter is important in cosmetics. It should be noted that the reduction of Prussian blue to Prussian white occurs at the lowest redox potential as can be found in transition metal hexacyanoferrates. 

Carbon dioxide has been reduced to methanol with the Everitt s salt (ES)-mediated electrode in the presence of l,2-dihydroxybenzene-3,5-disul-phonato(iron) ferrate(III) complex (Scheme 103) [408, 409]. The reduction proceeds as follows a weak coordination bond is first formed between the central metal of ES and ethanol, then the subsequent insertion of CO2 onto... 

Electrochemical reduction of carbon monoxide in dry nonaqueous media at moderate to low pressures leads to the formation of the 1,3-cyclobutanedione dianion (squarate) at current efficiencies, up to about 45% depending on the cathode material [1,2]. In aqueous solution, electroreduction can lead to the formation of methane and other hydrocarbon products. The role of the metal/adatom in determining the extent of CO and hence hydrocarbon formation during the reduction of carbon dioxide is related to the ability of the electrode material to favor CO formation (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and stabilize HCCO [3, 4]. 

Transition-metal -phthalocyanines as catalysts in acid medium. To prevent carbonate formation by the carbon dioxide in the air or that produced by oxidation of carbonaceous fuels, an acid electrolyte is necessary hence it is important to find electrocatalysts for an acid medium. Independently of Jasinski, we were soon able to show 3>4> that under certain conditions the reduction of oxygen in dilute sulfuric acid proceeded better with phthalocyanines on suitable substrates than with platinum metal. The purified phthalocyanines were dissolved in concentrated sulfuric acid and precipitated on to the carbon substrate by addition of water. This coated powder was made into porous electrodes bound with polyethylene and having a geometrical surface of 5 cm2 (cf. Section 2.2.2.1.). The results obtained with compact electrodes of this type are shown in Fig. 6. 

Fig. 15.17. Current in milliamperes cm-2 for CO production from C02 using C02+ tetraphenyl porphyrins modified with various pyridyl derivatives. (Reprinted with permission from T. Atoguchi, A. Aramata, and M. Engo, C02 Reduction by Macrocyclic Transition Metal Complex-Modified Electrodes," in Proc. International Symposium on Chemical Fixation of Carbon Dioxide, p. 338, Fig. 5,1991.)...

Mahmood et al. [117] studied the electrochemical reduction of carbon dioxide using gas diffusion electrodes. The reduction was performed on metal (lead, indium, and tin)-impregnated Teflon -bonded carbon gas diffusion electrodes in a sulfuric acid electrolyte over a 1 - 5 pH range. A schematic of the cell is shown in Fig. 24. The following reactions occurred ... 

A reaction in which two gas diffusion electrodes were employed is the simultaneous reduction of carbon dioxide and nitrate with various metal catalysts at the gas diffusion and a hydrogen diffusion electrode as anode [72]. 

A p-type silicon (p-Si) electrode modified with copper particles (particulate-Cu/p-Si) was applied to photoelectrochemical (PEC) reduction of carbon dioxide (CO2) in acetonitrile electrolyte solutions with and without 3.0 M HjO. The particulate-Cu/p-Si electrode generated high photovoltages of 0.50 to 0.75 V, and produced methane, ethylene, etc., under addition of 3.0 M HjO, similar to a Cu metal electrode, indicating that the particulate-Cu/p-Si electrode acted as an efficient electrode for the PEC reduction of CO2 in non-aqueous solutions. 

CO2 reduction at metallic electrodes is generally poorly selective [151]. Monoelec -tronic reduction of carbon dioxide may occur at a platinum cathode in non-aqueous solvents, but at very negative potentials. Catalytic activation of CO2 has been described (e.g. at a cathode modified by a rhenium complex in a hydroorganic solvent) the observed conversions did correspond to the formation of CO and formic acid. In organic synthesis, CO2 was mainly used as an electrophile (toward electrogenerated anions from jt -acceptors or electrogenerated nucleophiles when adequate transition metals ions were present in situ) for the purpose of carboxylation. 

Carbon Dioxide. As the debate of the effect of greenhouse gases rages on, the simple fact remains that carbon dioxide production is one of the known side reactions of most metal-production operations. Carbon is an effective metal reductant. Coke is used to produce pig iron from iron oxide ores and lead from sulfide ores in blast furnaces, carbon electrodes are used to produce aluminum from bauxite leaching products, and coal is used in the reduction of zinc oxide in retorting furnaces. All told, the resulting product of metal reduction is the oxidation of carbon to carbon dioxide. It is important to keep in mind that the production of carbon dioxide has been reduced dramatically since the start of the Industrial Revolution of the late nineteenth-century. This is best exemplified by the history of steel making in the world. 

Determine kinetics and mechanisms of electrode reactions shown to be potentially useful (i.e., carbon monoxide and dioxide reduction, alkali metal deposition, solution redox reactions, and oxygen reduction) to permit the design of highly efficient electrolytic cells (55)... 

Value added products, such as the reduction of oxalic acid to glyoxylic acid, are nowadays developed at an industrial scale (BASF—Badische Anilin und Soda-Fabriken). This process uses non-precious metals such as lead. This electrode material reduces carbon dioxide to formic acid in an aqueous medium and to oxalic acid in an organic solvent. This approach can be used to develop environmentally friendly value added products from abundant anthropogenic carbon dioxide. 

A number of transition-metal complexes, both in solution and on electrode surfaces, have been shown to be effective in the electrocatalytic reduction of carbon dioxide. All of those complexes significantly decrease the overpotential for reduction of CO2 by up to IV (as compared to a 1-el reduction to the C02 radical), and yield various multielectron reduction products. Known electrocatalysts yield primarily carbon monoxide and formate anion as the major products of the CO2 reduction. Sullivan et al. did detailed mechanistic work on a sales of bipyridine complexes of transition metals, and made several suggestions concerning the design of new electrocatalysts that would be capable of reducing CO2 past the CO and formate step. Their major recommendation is to use as electrocatalysts "electron reservoir" complexes, i.e. compounds capable of storing multiple electrons. 

A major focus of more recent studies on adsorption at metal electrodes has been the investigation of the mechanism of electro-oxidation of organic fuels (methanol, formic acid, formaldehyde, etc. [55, 56]) and the electro-reduction of carbon dioxide. The former type of reaction is important in the context of the development of fuel cells a major problem has been the poisoning of the anode by carbon fragments and mechanistic insights are urgently needed. In the latter case, the development of C02 sensors has a high priority. 

Electrosorption and Reduction of CO2. - Nowadays the electrochemical reduction of carbon dioxide to useful organic materials and fuels is an important topic with both theoretical ° and practical interest. The CO2 reduction at metal electrodes in aqueous media yields CO, HCOO, CH4, C2H4, and alcohols. The metal electrodes that show activity in CO2 reduction can be divided according to the product selectivity into the following groups ... 

The biological reduction of nitrate is not heme-related a well-known family of Mo-oxo pterin enzymes facilitate O-atom transfers between nitrite and nitrate, sulfite and sulfate, and other substratesStill, the electrochemical reduction of nitrate on bare metal electrodes is well known, and can also be facilitated by addition of N4 chelates. For instance, Hobbs et al. have examined the electrochemical reduction of nitrate in basic solutions using iron and nickel electrodes coated with phthalocyanin yielding nitrite, hydroxylamine, and ammonia. The iron electrodes efficiency increased when coated with phthalocyanin while the nickel s activity decreased. A related study has been done by Shibata et al. to determine the synthetic applications of simultaneous reduction of nitrate and carbon dioxide using phthalocyanin complexes of most of the first-row transition metals and other metals with mixed success . 

Azuma, M., K. Hashimoto, M. Miramoto, M. Watanabe, and T. Sakata (1990). Electrochemical reduction of carbon dioxide in various metal electrodes in low-temperature aqueous KHCO3 media. J. Electrochem. Soc. 137(6), 1772-1778. 

Shibata, M., K. Yoshida, and N. Furuya (1998). Electrochemical synthesis of urea at gas-diffusion electrodes III. Simultaneous reduction of carbon dioxide and nitrite ions with various metal catalysts. J. Electrochem. Soc. 145(2), 595-600. 

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