Catalytic CO2 reduction

Molecular tailoring of organometallic polymers for efficient catalytic CO2 reduction mode of formation of the active species... 

Zhao Z-H, Fan J-M, Wang Z-Z (2007) Photo-catalytic CO2 reduction using sol-gel derived titania-supported zinc-phthalocyanine. J Clean Prod 15 1894—1897... 

The synthesis of catalytic photocathodes for H2 evolution provides evidence that deliberate surface modification can significantly improve the overall efficiency. However, the synthesis of rugged, very active catalytic surfaces remains a challenge. The results so far establish that it is possible, by rational means, to synthesize a desired photosensitive interface and to prove the gross structure. Continued improvements in photoelectrochemical H2 evolution efficiently can be expected, while new surface catalysts are needed for N2 and CO2 reduction processes. 

Since the work of Inoue a wide variety of metal complexes have been shown to be highly active in CO2 reduction. These are usually hydrides or halides with phosphines as neutral ligands, and Rh and Ru proved to be the most active metals.9,100-104 Variation of the ligand has a considerable effect on catalytic activity. 

Re diimine complexes act as photocatalysts and/or electrocatalysts for CO2 reduction to CO. Examples include the tricarbonyl complexes yac-[Re(Q -diimine)(CO)3L]" [n = 0, L = halide n = 1, L = NCMe, P(OR)3 a-diimine = 1,4-disubstituted 1,4-diazabuta-l,3-dienes or bpy and related chelating N-heterocycles], for example, fac-[Re(dmb)(CO)3(NCMe)]+, 5 [Re(dmb)(CO)3]2" and fac-[Re(bpy)(CO)3 P(OPfl)3 ]+. Electron-transfer from an amine electron donor (e g. triethanolamine or triethylamine) to the excited state complex is usually considered as the initiation of the photocatalysis, and metallocarboxylates and metallo-carboxyUc acids have been proposed as intermediates in the formation of CO. The electrocatalytic process is triggered by a 1-electron or a 2-electron cathodically induced chloride (X) or L ligand dissociation to form the catalytic species. ... 

Photochemical CO2 reduction to CO (and formate in some cases) has been reported in a catalytic system using Ru(bpy)3 + as the sensitizer, nickel or cobalt macrocycles as the electron relay catalyst, and ascorbate as a sacrificial reductive quencher [9, 15, 16]. These systems also produce H2 via water reduction. Although Ni(cyclam) + is an efficient and selective catalyst for electrochemical CO2 reduc-... 

Complexes of the general formula /ac-Re(a-diimine)(CO)3X and Re(a-diimine)(CO)2XX (where ot-diimine = bpy, phen, substituted bpy or phen, etc. and X, X = halide, solvent, alkyl, benzyl, monodentate phosphine, CO, etc.), have attracted interest since the mid-1970s [51-53]. Many of these complexes show emission from their lowest long-lived MLCT state at room temperatme in solution. Their catalytic properties for CO2 reduction have also been investigated. Electrolysis of a solution containing/uc-Re(bpy)(CO)3 Cl and 0.1 M BU4NPF6 in freshly distilled C02 saturated MeCN at —1.5 V (vs. SCE) produces both CO and C03 with cmrent efficiencies of 98 and 110 %, respectively [54]. Further, yhc-Re(bpy)(CO)3X (X = Cl, Br ) has been used successfully as a photocatalyst for CO2 reduction to CO with TEOA in DMF [55-58]. When X = Cl , a quantiun yield of 0.14 has been measured in the presence of excess Cl". A formato-rhenium complex,/ac-Re(bpy)(C0)3(02CH), has been isolated in the absence of excess Cl". 

Metal complexes have also been used as catalysts for CO2 reduction, but only few examples are known where the major product is oxalate [148 and refs, therein]. The reductions are inner-sphere processes, and oxalate formation takes place at significantly higher potentials than by direct reduction or reduction via organic catalysts (which typically have values of Ff in the range —1.95 V to —2.25 V). Catalytic dimerization of CO2 in MeCN has been reported to take place at —1.5 V using the complex [(RhCp )3(/ 3-S)2]" , at —0.7 V using the complex [(CoCp )3(/A3-S)9] " , and at —1.6 V using [(IrCp )3(/ 3 — 8)2]-+ [148]. 

This paper describes the use of polydentate ligands to optimize the performance of palladium catalysts for CO2 reduction and to probe mechanistic aspects of catalytic reactions. Polydentate ligands can be used to precisely control coordination environments, electronic properties, and specific steric interactions that can lead to new insights into the relationship between catalyst structure and activity. 

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. 

The electrocatalytic activity of various nickel macrocycles in aqueous solution was studied. Cyclic voltammograms indicate that 7 / S -NiHTIM2+, NiMTC2+ and NiDMC + are better catalysts than Ni(cyclam)2+ in terms of more positive potentials and/or their larger catalytic currents [26], Bulk electrolyses with 0.5 mM Ni complexes confirm that these complexes are excellent catalysts for the selective and efficient CO2 reduction to CO. The macrocycles with equatorial substituents showed increased catalytic activity over those with axial substituents. These structural factors may be important in determining their electrode adsorption and CO2 binding properties. 

An induction period of 30 min was observed at the initial stage of the CO formation in the photocatalysis for CO2 reduction on CdS-DMF, suggesting that the surface structures change photochemically during this period, giving catalytically active sites. 

This result indicates that HjO acted as a proton donor in an AN solution for hydrocarbon production. A naked p-Si electrode gave mainly H2 together with small amounts of CO, HCOOH and CH4, indicating that the Si surface has low catalytic activity for CO2 reduction. The particulate-Cu/p-Si... 

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. 

There has been a growing interest in the utilization of CO2 as a potential Cl source for chemicals and fuels to cope with the predictable oil shortage in the near future. Insertion reactions of CO2 into M-H, M-0, M-N, and M-C bonds are well documented, where these reactions are explained in terms of the electrophilicity of CO2 il, 2). Catalytic syntheses of lactones (3-9) and pyrones (10-16) are also established by incorporation of CO2 into dienes and alkynes activated on low-valent metal complexes. Carbon dioxide shows only an electrophilicity under usual reaction conditions, but it exhibits a nucleophilicity upon coordination to low-valent metals because of the intramolecular charge transfer from metals to CO2. Metal-C02 formation may be the key species in electro- and photochemical CO2 reductions. Since the first characterization of [Ni(PCy3)2(T) (C,0)-C02)] (17), a variety of metal... 

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. 

In photochemical reduction of CO2 by metal complexes, [Ru(bpy)3] is widely used as a photosensitizer. The luminescent state of [Ru(bpy)3] is reductively quenched by various sacrificial electron donors to produce [Ru(bpy)3] . Metal complexes used as catalyst in the photochemical reduction of CO2 using [Ru(bpy)3] are prerequisites which are reduced at potentials more positive than that of the [Ru(bpy)3] " redox couple (-1.33 V vs SCE) (72). Irradiation with visible light of an aqueous solution containing [Co (Me4(14)-4,ll-dieneNJ], [Ru(bpy)3], and ascorbic acid at pH 4.0 produces CO and H2 with a mole ratio of 0.27 1 (73). Similarly, photochemical reduction of CO2 is catalyzed by the [Ru(bpy)3] /[Ni(cyclam)] system at pH 5.0 and also gives H2 and CO. However, the quantum efficiency of the latter is quite low (0.06% at X = 400 nm), and the catalytic activity for the CO2 reduction decreases to 25% after 4 h irradiation (64, 74, 75). This contrasts with the high activity for the electrochemical reduction of CO2 by [Ni(cyclam)] adsorbed on Hg. 

Since that time there have been many reports of intramolecular hydridic-protonic bonds [78-86]. Recently an intermediate with an intramolecular RuH—HN interaction has been implicated in the catalytic asymmetric reduction of ketones [87] and another, in the reduction of CO2 to formic acid [83]. In the last process, the RuH—HN bond is proposed to form via the heterolytic splitting of dihydrogen (see Scheme 4, Section 1.9). 

Table 1 Products, catalytic activities and quantum yields of photocatalytic CO2 reduction using single-component systems...

Table 2 Products, catalytic activities, and quantum yields of photochemical CO2 reduction using...

The supramolecular complex (34a) bridged by a -CH2CH(OH)CH2- chain with two dmb units as a peripheral ligands on the Ru center exhibited a high photocatalytic ability for CO2 reduction to CO ( co = 0.12, TNco = 170). This supramolecule was a much better photocatalyst than the 1 1 mixed system of mononuclear model complexes [Ru(dmb)3] (35) and /ac-Re(dmb)(CO)3Cl (36) (first example of a supramolecular photocatalyst exhibiting a high catalytic activity for this reaction [54]. 

A second question is, where does CO bind when added externally It is known that acetyl-CoA synthesis is much faster in the presence of CO2 plus reduc-tant than with externally added CO [ 145]. This argues for different initial CO binding modes in the two cases and it is conceivable that the mechanism differs depending on whether CO is added externally or it arrives through the tunnel after CO2 reduction at the C-cluster. More studies will be required before the ACS catalytic mechanism is fully elucidated. 

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