Mechanism CO2 reduction

The information obtained can be used to give interesting information upon the CO2 reduction mechanism. Because the radical anion increases in concentration in the negative direction, it cannot be in equilibrium with the electrode. The increase in anion concentration at cathodic potentials may, however, be explained if CO2 is formed as an intermediate radical. Thus from equations 5-7... 

In the case of CO2 contamination, we have strong evidence that its reduction on noble metal electrodes in nonaqueous systems in the presence of Li ions (and the absence of water) forms Li2C03 and CO [17]. Figures 20 and 21 show typical FTIR spectra obtained from noble metal electrodes polarized to low potentials in C02-saturated nonaqueous Li salt solutions and provide clear evidence for Li2C03 formation as the major surface species that is precipitated [15,39]. The CO2 reduction mechanism for the reaction appears in the literature [43] and is described in the following equations ... 

The development of photocatalysts for the reduction of CO2 hy sunlight is an important research field. Recently, a mononuclear iridium(iii) complex, [Ir(tpy)(ppy)Cl], has been reported as a novel photocatalyst able to selectively reduce CO2 to CO under visible light (A = 480nm), without additional photosensitizers. A big advantage of this Ir-based system over Re complexes is the high photocatalytic activity, that is maintained even in the presence of water. The photocatalytic CO2 reduction mechanism has been obtained by spectroscopic (e.g. H-NMR) and ESI-MS investigations. During the photocatalytic reaction, [Ir(tpy)-(ppy)Cl] is transformed into an activated form, [Ir(tpy)(ppy)H]. 

In situ infrared observations show that the primary species present during the reduction of NO by CH4 over Co-ZSM-5 are adsorbed NO 2 and CN. When O2 is present in the feed NO2 is formed by the homogeneous and catalyzed oxidation of NO. In the absence of O2, NO2 is presumed to be formed via the reaction 3 NO = NO2 + N2O. The CN species observed are produced via the reaction of methane with adsorbed NO2, and transient response studies suggest that CN species are precursors to N2 and CO2. A mechanism for the SCR of NO is proposed (see Figure 10). This mechanism explains the means by which NO2 is formed from adsorbed NO and the subsequent reaction sequence by which adsorbed NO2 reacts with CH4 and O2 to form CN species. N2 and CO or CO2 are believed to form via the reaction of CN with NO or NO2. CH3NO is presumed to be formed as a product of the reaction of CH4 with adsorbed NO2. The proposed mechanism explains the role of O2 in facilitating the reduction of NO by CH4 and the role of NO in facilitating the oxidation of CH4 by O2. 

The mechanism of electrocatalytic CO2 reduction has been studied in some detail [57-60] but is not fully understood. A proposed mechanism from an early investigation ]55] is shown in Fig. 7. The initial step in the reaction is the reversible reduction of c-Re(bpy)(CO)3Cl (E° = — 1.1 V in CH3CN), which is assigned to the bipyridyl-centered electron transfer (Eq. 5). This is followed by a second step (Eq. 6) at ca —1.5 V, which is irreversible at room temperature and may be accompanied by loss of Cl ... 

The mechanisms and kinetics of CO2 reduction by Ni macrocycles have been investigated." ... 

Figure 5. Schematic reaction mechanism for photochemical CO2 reduction using TP and ML.

The subsequent reactions of the radical species depend on the nature of a-diimine, X, solvent and electron donor as also found in the electrochemical studies. The mechanism for photochemical CO2 reduction must account for reactions of the electron donor (e.g., TEOA or TEA) and its reaction products. TEOA or TEA can coordinate to metal complexes and change their redox properties [27]. The following reactions can all become important under different conditions ... 

In principle, two mechanisms of coupling can be envisaged (i) activation of CO2 occurs at the level of the substrate at the expense of ATP hydrolysis ( substrate activation ), or (ii) the redox potentials (E° ) of the electron required for CO2 reduction are pushed towards more negative values at the expense of electrochemical potentials of either or Na by the mechanism of reversed electron transport ( redox activation ). Since ATP-consuming synthetases are not involved in CO2 reduction to methylene-Fl4MPT (Reactions 1-4 of Table 2) the latter mechanism is more likely. 

CO2 reduction to formyl-MFR in cell extracts. Cell extracts of Methanobacterium thermoautotrophicum catalyze the formation of formyl-MFR from H2, CO2 and MFR. The rates of formation of formyl-MFR were stimulated by CH3-S-CoM [170,171] or CoM-S-S-HTP [172], suggesting that CO2 activation is coupled with the terminal steps of methane formation ( RPG effect [170], for literature see [173]). The mechanism of that coupling is far from clear. The heterodisulfide may act as an allosteric effector in the activation of low-potential electron carriers in electron flow from H2 to formyl-MFR [173]. It should be pointed out that the RPG effect was not observed in cell extracts of Methanosarcina barkeri[ 14],... 

In Methanogenium thermophilum, which can use both H2 and ethanol as electron donor for CO2 reduction, the energetics and the mechanism of ethanol oxidation was... 

Electron transfer between 53b and CO2 is too slow to be important because of the large difference (> 0.4 V) between the reductions potentials of 53b and CO2. The mechanism in Scheme 14 seems to be general for o , S-diactivated alkenes in their reaction with CO2 [161]. The kinetics of the reaction between the radical anions of fumarates, 52a-c, and maleates, 53a-c, with CO2 in DMF (BU4NI) has been studied by RRDE [162]. Reaction between the radical anion and CO2 prior to dimerization was confirmed for 53a by preparative and electroanalytical experiments at different substrate concentrations [165]. The radical anions 53 reacted with CO2 20-50 times faster than did the analogous species 52 , for which the pseudo-first-order rate constants were determined to be in the range 0.35-1.5 s [162]. 

Electrochemical studies of [Pd(triphosphine)(CH3CN)](BF 2 complexes were carried out under both catalytic and noncatalytic conditions to probe mechanistic aspects of CO2 reduction. The mechanism that we have proposed for this catalytic reaction is outlined in Scheme 1. In this scheme, L represents a triphosphine ligand and solv represents a solvent molecule such as DMF or acetonitrile. The data supporting the various steps shown will be summarized next. 

Problems related to the increase of greenhouse gases in the atmosphere and the depletion of fossil fuels have made the conversion of CO2 into useful chemicals and fuels an important area of research. However, CO2 reduction poses many scientific challenges. Despite intense interest in photochemical and electrochemical CO2 reduction, the kinetics and mechanism of the reduction remain unclear in many systems. 

Electrochemical reduction of CO2 in nonaqueous solutions is significant from the following viewpoints Firstly, hydrogen evolution reaction can be suppressed. Secondly, the concentration of water as a reagent can be accurately regulated and the reaction mechanism may be more easily studied. Thirdly, the solubility of CO2 in organic solvents is much liigher than in water. Various metal electrodes have been tested for CO2 reduction in some nonaqueous solvents, such as propylene carbonate (PC), acetonitrile (AN), DMF, and dimethyl sulfoxide (DMSO), as tabulated in Table 5. Methanol is also used for CO2 reduction, and mentioned in the next Section. 

Hori et al. pointed out that the deactivation takes place due to the presence of heavy metal impurities originally contained in chemical reagents used as the electrolytes. Heavy metal ions in the electrolyte solution are cathodically reduced and deposited on the electrode surface during the CO2 reduction, deteriorating the electrocatalytic properties of metal electrodes. They apphed a classically established technique of preelectrolysis to purification of electrolyte solutions since their early works. Frese also referred to the impurity heavy metals, and mentioned the presence of Fe and Zn on the Cu electrode after electrolysis on the basis of the surface analysis by XPS. The importance of the purity of the electrolyte solution was mentioned in Section I1.2(zz) as well. The mechanism of the deactivation was recently established, and sununarized below. ... 

Eyring and his coworkers analyzed polarization data of CO2 reduction to HCOO at a Hg electrode in aqueous electrolytes containing HCO3. and discussed the reaction mechanism with CO2 anion radical as the initial intermediate The resultant CO2 subsequently accepts a H and another electron, reduced to HCOO, ... 

The electrocatalysis of the electrochemical reduction of CO2, a reaction of great potential importance for the future, is treated in an authoritative chapter by Y. Hori, who for many years has been a leading expert in this area. The Chapter reviews critically the plethora of experimental investigations of CO2 reduction on different metals and provides deep and useful insight about the fundamental mechanisms leading to dramatically different product selectivity on different metals. 

However, little is known as to why selective generation of CO takes place in the electrochemical reduction by these Ni macrocycle species adsorbed on the surface of an Hg electrode in H2O. Sakaki et al. carried out SCF ah initio calculations for [NiF(NH3)4] as the model of [Ni(cyclam)] adsorbed on Hg (Fig. 5), and a NiX7ji-C02 adduct with an OCO angle of 135.3° is suggested as the active species during CO2 reduction (66, 67). The increase in electron density of an 0-atom of the terminal CO2 from -0.33e (free CO2) to -0.58e upon coordination to Ni supports the proposed mechanism for the catalytic cycle involving an hydroxycarbonyl intermediate. 

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