Photoelectrochemical Reduction of CO

Light energy may be used to reduce the necessary electrical potential in photoelectrochemical reactions. The overpotential is decreased by 700 mV for the photoelectrochemical reduction of CO on p-CdTe, compared to that on indium - the best metal electrode for CO2 reduction. For these semiconductors which involve a high concentration of surface states, the double layer at the semiconductor-electrolyte interface plays an important role in the kinetics of photoelectrochemical reactions. In this paper, we report spectroscopic and impedance aspects of the electrode-electrolyte interface as affected by reactants and radicals involved in CO reduction. 

The surface state capacitance calculated by a similar procedure is shown in Fig. 19 as a function of bias potential. Fig. 20 shows the surface states density as a function of bias potential. Surface states density is an order of magnitude less than that at the CdTe interface under similar conditions. This is consistent with the fact that the photocurrent for GaP is les compared to CdTe for the photoelectrochemical reduction of CO (cf. the model suggested of mediator surface states). The surface state density increases from its minimum at 0.2V NHE, a potential which lies close to the pzc value determined for the system (0.17V NHE). Correspondingly, the form of the surface state density as a function of bias potential resembles an adsorption isotherm. These results support the concept that surface dates are induced by adsorbed ions at the interface. 

Ammonium ions are adsorbed at the semiconductor electrolyte interface and the reduced ammonium ion radical acts as mediator for the photoelectrochemical reduction of CO. ... 

The semiconductors which are suitable for photoelectrochemical reduction of CO are shown in Figure 1. 

The photoelectrochemical reduction of CO at semiconductor o produces in appreciable amour depends on the nature of the semiconductor surface. 

The mediation of photoelectrochemical reduction of CO by ammonium ions has been reported by Bockris et al (15). The first step in... 

Considerable progress has been made on C02 fixation in photochemical reduction. The use of Re complexes as photosensitizers gave the best results the reduction product was CO or HCOOH. The catalysts developed in this field are applicable to both the electrochemical and photoelectrochemical reduction of C02. Basic concepts developed in the gas phase reduction of C02 with H2 can also be used. Furthermore, electrochemical carboxyla-tion of organic molecules such as olefins, aromatic hydrocarbons, and alkyl halides in the presence of C02 is also an attractive research subject. Photoinduced and thermal insertion of C02 using organometallic complexes has also been extensively examined in recent years. 

Photoelectrochemical reduction of COj in high pressure CO -methanol solution... 

PHOTOELECTROCHEMICAL REDUCTION OF CARBON PIQXIDEFUSING SEMICONDUCTOR ELECTRODES. Several different strategies for carbon dioxide reduction on semiconductors electrodes have been used to produce CO, formic acid, or even methanol (lA) These include ... 

Tetraalkylammonium perchlorate (TBAP)(Fluka) was recrystallized from ethanol. Dimethylformamide was used without further purification. Triply distilled and pyrolyzed water was used. 0.1 M tetraalkylammonium perchlorate in dimethylformamide - 5% water mixtures were used as electrolyte for the photoelectrochemical reduction of C0 to CO. 

Cooper JA, Compton RG (1998) Photoelectrochemical reduction of p-bromonitrobenzene mechanistic discrimination via channel microband array voltammetry. Electroanalysis 10 (17) 1182-1187. doi 10.1002/(sici)1521-4109(199811)10 17<1182 aid-elanll82>3.0.co 2-k... 

Cole EB, Bocarsly AB (2010) Photochemical, electrochemical, and photoelectrochemical reduction of carbon dioxide, in carbon dioxide as chemical feedstock. Wiley-VCH Verlag GmbH Co. KGaA, Weinheim, p 291-316... 

A number of advances have been made recently in the understanding of the catalysts able to selectively activate CO2 [26-29]. Another key issue is the need to optimize the multielectron transfer necessary for the reduction of CO2. This issue has been examined recently by Barton Cole et al. [30], Most electrochemical and photoelectrochemical systems for the reduction of CO2 produce only the 2 e reduction products of CO and formate, while few reported the formation of methane and methanol and even fewer reported >C1 products, as briefly discussed before. The main limitation in achieving such products has been attributed to the inability of most catalysts to affect multielectron transfers along with the required multiproton transfers. 

Polyhaloacetic acids and their partially hydrodehalogenated products represent a second important family of herbicide-/pesticide-derived substrates. In their review on the environmental applications of industrial electrochemistry, Juttner and co-authors (Juttner et al. 2000) documented the electroreductive dechlorination of dichloroacetic acid (a by-product of monochloroacetic acid), a way to recover the valuable compound and avoid wastes. The electrochemical reduction of polychloro- and polybromo-derivatives was performed by Korshin and Jensen (2001) on Cu and Au cathodes. Complete dehalogenation was obtained for all substrates, but for monochloroacetic acid. To overcome the intrinsic poor reactivity of the monochloro-derivative the photoelectrochemical properties of a p-doped SiC electrode were investigated (Schnabel et al. 2001) however, the dehalogenation stopped at monochloroacetic acid. 

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. ... 

The redox electrochemistry of a Fe /Fe couple is easily accomplished on a phthalocyanine-coated electrode with peak separations comparable to that of platinum [141,142,163,164]. Phthalocyanine-coated electrodes are found to be efficient electrocatalysts to catalys catechol, p-benzoquinone and oxalic acid oxidations [120,150]. The electrochemical activity of these electrodes may be due to the high voltage, surface area, high electronic conductivity and redox behaviour of phthalocyanine, vanadium phthalocyanine and other phthalocyanines have been prepared by vapour deposition and show photoelectrochemical responses when dipped in aqueous electrolytes [244-249]. Polymeric phthalocyanines of Co and Fe are coated on active carbon and are shown to give catalytic properties for dioxygen reduction and thiol oxidations. Dioxygen chemisorption and ammonia absorption of metallo... 

The phthalocyanine containing polymer films were electrochemically investigated for their electrochromic reductions and reoxidations [406,411]. Under irradiation the reduction of O2 to water was studied in photoelectrochemical cells [407,409,412]. Especially Zn(II)-phthalocyanine in poly(vinylidene fluoride) shows high cathodic photocurrents. Also the electrochemical carbox dioxide and proton reduction by Co(II)-phthalocyanines in a low concentration monomolecular in a polyvinylpyridine matrix were investigated as part of a photoenergy systems [413,414]. As an active catalyst for proton reduction also a bipyridyl platinum complex in a polymer Nafion membrane was found [415]. In order to construct such a photochemical energy conversion system, the research in this field was extended for the electrocatalytic water oxidation to O2 [416-419]. The Ru-complexes cj5-[Ru(bpy)2Cl2] and especially Ru-red ([(NHsjs Ru >-Ru(NH3)4-0-Ru(NH3)5] ) are active as electrocatalysts. 

The Co" complex [CoPcFig] have advantageous properties as electrocatalyst for the reduction of oxygen as compared to [CoPc] [39]. The Ru" complex [RuPcF, ] appeared to be an effective catalyst for the room tanperature oxidation of cycloalkanes [9]. Encapsulated in zeolite these species were used for catalytic oxidation of alkanes and alcohols [46]. In photoelectrochemical and (photo)conductivity studies it was shown that [ZnPcFig] behaves as n-type semiconductor in vacuo and photoconductor in the presence of oxygen [37, 47]. Due to its enhanced solubility in different solvents as compared to non-substituted [ZnPc] it has advantages for clinical application in photodynamic tumor therapy [11]. Perfluorinated In ", Ti and Zd phthalocyanines exhibit higher performance as optical limiters than non-fluorinated species [42]. 

Most electrochemical and photoelectrochemical systems produce only the two-electron reduction products of CO and formate, the products of two-electron reductions, evidencing once again the kinetic bottleneck associated wth multielectron and multiproton processes. As in the oxidation of water, efforts have been directed towards the development of transition-metal based electrocatalysts with multiple metal centres to facilitate charge accumulation in highly reduced intermediates and allow multiredox processes to occur. " " ... 

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