Carbon monoxide electrochemical reduction

The reduction of carbon dioxide is another of the basic electrochemical reactions that has been studied at modified electrodes. The reduction at Co or Ni phthalocyanine in acidic solution yields formic acid or carbon monoxide A very high selectiv-... 

Ito et al.40 examined the electrochemical reduction of C02 in dimethylsulfoxide (DMSO) with tetraalkylammonium salts at Pb, In, Zn, and Sn under high C02 pressures. At a Pb electrode, the main product was oxalic acid with additional products such as tartaric, malonic, glycolic, propionic, and n-butyric acids, while at In, Zn, and Sn electrodes, the yields of these products were very low (Table 3), and carbon monoxide was verified to be the main product even at a Pt electrode, CO was mainly produced in nonaqueous solvents such as acetonitrile and DMF.41 Also, the products in propylene carbonate42 were oxalic acid at Pb, CO at Sn and In, and substantial amounts of oxalic acid, glyoxylic acid, and CO at Zn, indicating again that the reduction products of C02 depend on the electrode materials used. 

Mechanistic Aspects of the Electrochemical Reduction of Carbon Monoxide and Methanol to Methane at Ruthenium and Copper Electrodes... 

The electrochemical reduction of carbon monoxide also offers a route for the production of fuels from inorganic sources. For example, carbon monoxide is formed from coal in gasification... 

The electrochemical reductions of carbon monoxide and methanol to methane (Equations 1 and 2) have potentials, under standard conditions, of +0.019 and +0.390 V vs SCE respectively (or a... 

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]. 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

In aprotic solvents, direct electrochemical reduction of C02 (—2.23 V vs. SCE) yields carbon monoxide and carbonate ion.29 The (porT)Fe dianion also reduces C02 to CO, but at a less negative potential (-1.70 V).30 Hence, the estimated iron-carbon bond energy (—AGbF) for the (porT)Fera—C(0)0 dianion is at least 50 kJ mol-1 [—AGBF > 96.5(—1.70 + 2.23)]. 

As discussed above, reduction of metal-metal bonded complexes is common, particularly for carbon monoxide-substituted complexes. The electrochemical potential of these reactions has been widely studied (see Electrochemistry Applications in Inorganic Chemistry). In addition, Meyer has shown that the metal-metal bond strength can be estimated via electrochemical techniques. The results applied to Mn2(CO)io are in agreement with values obtained by other methods and could provide a means of generating a wide range of metal-metal bond strengths. The legs of the electrochemical cycle are shown in equation (99). 

Fig. 5. Proposed mechanism of ATP synthesis coupled to methyl-coenzyme M (CH3-S-C0M) reduction to CH4 The reduction of the heterodisulfide (CoM-S-S-HTP) as a site for primary translocation. ATP is synthesized via membrane-bound -translocating ATP synthase. CoM-S-S-HTP, heterodisulfide of coenzyme M (H-S-CoM) and 7-mercaptoheptanoylthreonine phosphate (H-S-HTP) numbers in circles, membrane-associated enzymes (1) CH3-S-C0M reductase (2) dehydrogenase (3) heterodisulfide reductase 2[H] can be either H2, reduced coenzymeF420 F420H2) or carbon monoxide the hatched box indicates an electron transport chain catalyzing primary translocation the stoichiometry of translocation (2H /2e , determined in everted vesicles) was taken from ref. [117] z is the unknown If /ATP stoichiometry A/iH, transmembrane electrochemical...

We here report the first example of an electrochemical polymerization process which leads to formation of a modified electrode having the generic formula [Ru (bpy)(CO)2Cl]n, and which displays outstanding electrochemical activity towards reduction of carbon dioxide to either carbon monoxide or formate. A crucial stereochemical effect of the leaving groups on the feasibility of polymerization is demonstrated. Formation of the polymer occurs stepwise, through the formation of a dimeric or a tetrameric intermediate. 

These modified electrode having the generic formula [Ru (bpyRR)(CO)2Cl] , display outstanding electrochemical activity towards the reduction of carbon dioxide to either (i) carbon monoxide, 100 % faradic yield in water at -1.2 V vs SCE, bpy = 2,2 -bipyridine, R = H (ii) or formate, 95 % faradic yield in aqueous electrolyte at -1.2 V vs Ag/Ag, R = isopropyl esters groups. 

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. 

The region of the cyclic voltammogram, corresponding to anodic removal of Hathermal desorption spectra of platinum catalysts. However, unlikely the thermal desorption spectra, the cyclic-voltammetric profiles for H chemisorbed on Pt are usually free of kinetic effects. In addition, the electrochemical techniques offer the possibility of cleaning eventual impurities from the platinum surface through a combined anodic oxidation-cathodic reduction pretreatment. Comparative gas-phase and electrochemical measurements, performed for dispersed platinum catalysts, have previously demonstrated similar hydrogen and carbon monoxide chemisorption stoichiometries at both the liquid and gas-phase interfaces (14). 

Studies are in progress to identify and quantify the products formed by the electrochemical reduction of CO2 on precious metal electrodes as well as on other electrodes such as Mo when nearly neutral electrolytes are used that minimize proton donor or acceptor reactions. A review of CO2 reduction on metal electrodes shows that CHi+ is produced on Ru and Cu (8, 9), CH3OH is a major product on Ru and Mo (6-8), carbon monoxide is formed on Ru, Pd, Pt, Co, Fe, Au, and Ag (7-9), HCOO is the main product on Cd, In, Sn, and Pb (, ), and a product more complex than formic acid is reported for Pt... 

Ammonium ions have been shown to act as a catalyst for the photo-electrochemical reduction of carbon dioxide to carbon monoxide (36). 

The first concept is the closed-loop-controlled three-way catalyst. In this, one type of catalyst, which is placed in the exhaust gas stream, is able to promote all the main reactions that lead to the simultaneous removal of carbon monoxide, hydrocarbons and nitrogen oxides. To balance the extent of the oxidation and the reduction reactions, the composition of the engine-out exhaust gas is maintained at or around stoichiometry. This is achieved by a closed-loop engine operation control, in which the oxygen content of the engine-out exhaust gas is measured up-stream of the catalyst with an electrochemical oxygen sensor, also called lambda sensor. 

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... 

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