Electrochemical reduction of COj

Figure 3.47 A summary of the processes taking place during the electrochemical reduction of COj, From Gressin et al. (1979).

In addition to control of the product distribution, the attainment of much higher reaction rates is essential for practical use. Recently, the electrochemical reduction of COj with high current density has been studied by many researchers, e.g., using high pressure aqueous systems [7] and gas diffusion electrodes [8], We... 

First, the electrochemical reduction of COj on the Cu electrode was studied in a COj-methanol medium at various pressures of COj. Under both atmospheric and high pressure, CO, CH, CjHj, and Hj were detected as products in the gas phase. In the liquid phase, methyl formate, HCOOCHy and dimethoxymethane, CH3OCH2OCH3, were detected. Cyclic voltammograms at various CO pressures are presented in Figure 3. The cathodic current was observed with an onset potential of -1.0 V under 1 atm of N. A shoulder was observed around -1.2 V during the scan in the negative direction. When was replaced with 1 atm of... 

Hydrocarbon formation is more interesting in the electrochemical reduction of COj, since multielectron transfer is required in this process. In the electrochemical reduction of concentrated COj in the COj-methanol medium, the major products are still the two-electron transfer products, CO and methyl formate, at the Cu electrode, when tetrabutylammonium salts are used (Table 3). However, when tetraethylammonium salt was used as the supporting electrolyte, efficient formation of methane and ethylene was observed with good reproducibility. We defined the hydrocarbon selectivity as the ratio of the... 

The electrochemical reduction of COj in aqueous solution on a functional dual-film electrode consisting of Prussian blue and polyaniline doped with a metal complex using a solar cell as the energy source led to the formation of lactic acid, formic acid, methanol, etc., and the maximum current efficiency for the COj reduction was more than 20 % at -0.8 V vs Ag I AgCI. 

Figure 11.27 Faradaic yields of the products In the electrochemical reduction of COj atCu-electrodes in KHCOj aqueous solutions of various concentrations a C2H4 o CH4 Hj,- <> EtOH V PrOH (after [98]).

K. Ito, S. Ikeda, T. lida and A. Nomura, Electrochemical reduction of COj dissolved under high pressure. 3. In non-aqueous electrolytes, Denki Kagaku, 50, 1982,463-469. 

G.K.S. Prakash, F.A. Viva and G.A. Olah, Electrochemical reduction of COj over Sn-Nafion (R) coated electrode for a fuel-cell-like device, /. Power Sources 223, 2013, 68-73. 

The individual steps of the multistep chemical reduction of COj with the aid of NADPHj require an energy supply. This supply is secured by participation of ATP molecules in these steps. The chloroplasts of plants contain few mitochondria. Hence, the ATP molecules are formed in plants not by oxidative phosphorylation of ADP but by a phosphorylation reaction coupled with the individual steps of the photosynthesis reaction, particularly with the steps in the transition from PSII to PSI. The mechanism of ATP synthesis evidently is similar to the electrochemical mechanism involved in their formation by oxidative phosphorylation owing to concentration gradients of the hydrogen ions between the two sides of internal chloroplast membranes, a certain membrane potential develops on account of which the ATP can be synthesized from ADP. Three molecules of ATP are involved in the reaction per molecule of COj. 

The electrochemical reduction of CO in fhe COj-methanol solution was carried out under high COj pressure. The high pressure apparatus was assembled from a SUS-316 sfainless steel tube. A glass inner tube was used to avoid contact of the electrolyte with the metal apparatus. Various metal electrodes were used in this study. The details have been described previously [9]. A Ft counter electrode and a silver quasi-reference electrode were used. Reagent grade methanol was used as the solvent. Tetrabutyl or tetraethyl ammonium salts were used as supporting electrolytes. 

Time-resolved Infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR spectroscopy (ns), has been outstandingly successful in identifying reactive intermediates [5] and excited states [6] of metal carbonyl complexes in solution at room temperature. We have used infrared spectroscopy to probe the mechanism of photo-17] and electrochemical [8] catalytic reduction of COj. We have used TRIR to study organometallic reactions in supercritical fluids on a nanosecond time-scale [9-10]. 

Electrochemical reduction of COg is one of the most important topics in connection with environment, energy and natural resources. A few papers have reported spectroscopic detection of intermediate species. COg or adsorbed CO is detected in the eiectroreduction of COj on a Pb or a Pt eiectrode.[1,2] However, these eiectrodes scarceiy produce hydrocarbons from COj or CO. 

Cu electrodes electrochemically produced CH4, CgH and alcohols from COg and CO in aqueous media at high current densities, and Ni electrodes at lower current densities.[3-7] Fe, not active in COj reduction, can reduce CO to hydrocarbons.[8] It is thus interesting to reveai intermediate species on these eiectrodes in the electrochemical reduction of CO2 and CO by in-situ spectroscopic method. 

It has been reported that the concentration of proton and adsorbed hydrogen can be controlled by adjusting the anodic and cathodic bias in the pulsed method [7]. The hydrogen adsorbed on the electrode surface seems to interrupt the reaction for the electrochemical reduaion of COj. The CO2 coverage on the electrode surface may be increased by the elimination of adsorbed hydrogen during anodic period. In the subsequent cathodic period, the electron transfer to CO2 was promoted, yielding CO2 radical anions. The selectivity of products for the electrochemical reduction of CO2 was determined in association with electrode material and CO2 radical anion [10,11]. CO is intermediate species in the reaction process of hydrocarbonization [8]. 

The interest in these highly reduced states stems from their potential use as catalysts for the electrochemical or photochemical reduction of COj to The mechanism involves binding of COj as an axial ligand to the reduced metalloporphyrin followed by two-electron transfer. Fe and Co porphyrins do not react with CO2 but when they are reduced to the M° oxidation state they react rapidly. 

The electrochemical formation of carbon film can be achieved by either the anodic oxidation of 3 ion or the cathodic reduction of COj ion [3-5]. The principle of each reaction is shown in Figure 7.1.5a,b. 

Other important alternate electrochemical methods under study for pCO rely on measuring current associated with the direct reduction of CO. The electrochemistry of COj in both aqueous and non-aqueous media has been documented for some time 27-29) interferences from more easily reduced species such as O2 as well as many commonly used inhalation anesthetics have made the direct amperometric approach difficult to implement. One recently described attempt to circumvent some of these interference problems employs a two cathode configuration in which one electrode is used to scrub the sample of O by exhaustive reduction prior to COj amperometry at the second electrode. The response time and sensitivity of the approach may prove to be adequate for blood ps applications, but the issue of interfering anesthetics must be addressed more thorou ly in order to make the technique a truly viable alternative to the presently used indirect potentiometric electrode. 

The reduction of CO2 at metallic cathodes has been studied with almost every element in the periodic table °. This reaction can be driven electrochemi-cally or photochemically " and semiconductors have been used as cathodic materials in electrochemical or photoelectrochemical cells . The aim of these studies has been to find cathodes that discriminate against the reduction of H2O to H2 and favor the reduction of CO2 and also to find a cathode selective for one product in the reduction of CO2. A fundamental requirement is that the latter process occurs at a lower overpotential on such electrodes. However the purposes mentioned before in metallic cathodes depends on a series of factors such a solvent, support electrolyte, temperature, pressure, applied overpotential, current density, etc. (we will see the same factors again in macrocyclic electro-catalysis). For instance when protons are not readily available from the solvent (e.g., A,A -dimethylformamide), the electrochemical reduction involves three competing pathways-oxalate association through self-coupling of COj anion radicals, production of CO via O-C coupling between and COj and CO2, and formate generation by interaction of C02 with residual or added water. ... 

Whipple, D.T. and Kenis, P.J.A. (2010) Prospects of COj utilization via direct heterogeneous electrochemical reduction. The Journal of Physical Chemistry Letters, 1,3451-3458. 

The natural assumption made by a large number of researchers in the field of electrochemical C02 reduction was that the intermediate was COj, as postulated by Haynes and Sawyer (1967). The observation of oxalate as a major product in addition to, or in competition with, the formation of CO, COj , HCOj and HCOO , increased the attention focused on the reactive intermediate and the mechanisms by which it reacted. However, controversy has arisen over whether the subsequent reaction of the COj" was via dimerisation (the EC mechanism) or via attack on another C02 molecule (the ECE mechanism). In addition, the existence of such species as COj" (ads) and HCOO (ads) have also been suggested but, as we shall see, these are not now thought to play a major role on simple metals. 

Some Ni(I) complexes bind CO2. The COj-binding properties have been investigated in Me2SO by electrochemical methods. The Ni(I) complex of cyclam binds CO2 irreversibly, whereas the Ni(I) complex of Me6[14]ane (Lig) shows no observable reaction. The Ni(II) and Ni(I) complexes of dieneN4(Lig) show a very low affinity for CO2 binding (151). The Ni(I) complexes of 9,11, and 12 have been observed to react with CO2 to form Ni(II) complexes, although the reduction products of CO2 are not easily identifiable (63). 

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