Proton electrocatalytic reduction

ELECTROCATALYTIC REDUCTION OF PROTONS AND HYDRIDE TRANSFER REACTIONS... 

Other aza macrocyclic complexes, such as Schiff bases or tetraazaannulene derivatives, have been also used with some success for the electrocatalytic reduction of C02 in organic solvent in the presence of proton sources.166-168... 

Electrochemical and Electrocatalytic Reduction. The one-electron reduction of C02 yields the radical anion C02-, which reacts with an H source to give the formate ion. The reaction, however, is not selective because various other reactions may take place. An alternative and more promising approach is the two-electron reduction of C02 in the presence of a proton source to afford formic acid. The latter process requires a considerably lower potential (—0.61 V) than does the one-electron reduction (—1.9 V) consequently, the electrolysis in the presence of catalysts may be performed at lower voltages. The control of selectivity, however, is still a problem, since other two-electron reductions, most importantly reduction to form CO and H2, may also occur.101127 The reduction of C02 to CO, in fact, is the subject of numerous studies. Electrochemical and electrocatalytic reductions of C02 in aqueous solutions have been studied and reviewed.11,128-130... 

A cathodic electrocatalyst, which uses the protons and electrons produced in the anodic section for the electrocatalytic reduction of C02. 

The features of the electrode used in this gas-phase electrocatalytic reduction of C02 are close to those used in PEM fuel cells [37, 40, 41] (e.g. a carbon cloth/Pt or Fe on carbon black/Nafion assembled electrode, GDE). The electrocatalysts are Pt or Fe nanoparticles supported on nanocarbon (doped carbon nanotubes), which is then deposited on a conductive carbon cloth to allow the electrical contact and the diffusion of gas phase C02 to the electrocatalyst. The metal nanoparticles are at the contact of Nation, through which protons diffuse. On the metal nanoparticles, the gas-phase C02 reacts with the electrons and protons to be reduced to longer-chain hydrocarbons and alcohols, the relative distributions of which depend on the reaction temperature and type of metal nanoparticles. Isopropanol forms selectively from the electrocatalytic reduction of C02 using a gas diffusion electrode based on an Fe/N carbon nanotube (Fe/N-CNT) [14, 39, 40]. Not only the nature of carbon is relevant, but also the presence of nanocavities, which could favor the consecutive conversion of intermediates with formation of C-C bonds. 

Electrocatalytic Reduction (Hydrogenation) Reactions in a Solid Polymer Eeectroeyte (Proton Exchange Membrane) Reactor... 

The importance of reactions of cobalt(I) complexes with proton donors in the context of understanding the photo- and electrocatalytic reduction of water to H2 and of carbon dioxide to HCO2 has been highlighted. [146] In a series of reports the thermodynamics and kinetics of proton binding to the cobalt(I) complex of the macrocycle, 5,7,7,12,14,14,-hexamethyl-... 

On the other hand, one electron reduction of other substrates, for instance [MH(C0)2(L-L)2] (M = Cr, Mo, W L-L = dppm, dppe) [24] and [CpCoH(PR3)2]+ [25], leads to expulsion of Hj. This type of reaction is of fundamental importance in the electrocatalytic reduction of protons to dihydrogen [7] and could play a fundamental role in other electrocatalytic reduction processes. 

A newly-designed photoelectrocatalytic (PEC) reactor for CO2 reduction, which combines photocatalysis by Ti02 and electrocatalysis by carbon nanotubes (CNT), has recently been proposed (Fig. 7) [152]. A proton-conductive Nafion membrane connects the Ti02 and CNT. Irradiation of the combined system of nano-structured Ti02 deposited on a metal Ti electrode with Pt modified CNT deposited on carbon sheet caused water splitting to H2 and O2. A half-cell for the cathodic electrode, i.e., Pt or Fe modified CNT electrode, produces various organic molecules such as 2-propanol due to electrocatalytic reduction of CO2 on the electrode. The proposed PEC reactor is incomplete in its present state. However, these systems are expected to couple water splitting and CO2 reduction, and thus it may establish a new artificial photosynthetic system. 

Interest in the electrocatalytic reduction of CO2 with a view to Ihe construction of artificial photosynthetic systems has been increasing very rapidly in die last 10 years (i, 2). A large variety of transition metal complexes has been studied as electrocatalysts (i), however only a few appear to be effective in pure aqueous electrolyte. The accompanying reduction of protons limits the efficiency of die electroca ysis. One solution to this problem is to use complexes in a polymer-confined system deposited on metallic cathode surfaces. Some attempts have been reported recently in the literature (4), but successes obtained with this type of device have been very scarce. 

Metal porphyrins act as catalysts for chain transmission in radical polymerisation [55] and for electrochemical oxidation of sulphur dioxide [64], as well as for oxidation of a wide range of organic substrates [65]. Cobalt and nickel complexes with LI5 are effective in the electrocatalytic reduction of carbon dioxide [66]. Porphyrin complexes of cobalt and tetraazamacrocyclic complexes of nickel catalyse reduction of the proton [67-71]. 

Scheme 7.2 Possible mechanism for the electrocatalytic reduction of protons by 4.

G. Bidan, E. M. Genies, and M. Lapkowski, Modification of polyaniline films with heteropolyanions Electrocatalytic reductions of oxygen and protons, J. Chem. Soc., Chem. Commun. 533 (1988). 

McNamara WR, Han Z, Alperin PJ, Brennessel WW, Holland PL, Eisenberg R. A cobalt-dithiolene complex for the photocatalytic and electrocatalytic reduction of protons. J Am Chem Soc 2011 133 15368-15371. 

The reduction of protons is one of the most fundamental chemical redox reactions. Transition metal-catalyzed proton reduction was reviewed in 1992.6 The search for molecular electrocatalysts for this reaction is important for dihydrogen production, and also for the electrosynthesis of metal hydride complexes that are active intermediates in a number of electrocatalytic systems. 

Several other polypyridyl metal complexes have been proposed as electrocatalysts for C02 reduction.100-108 For some of them HCOO- appears as the dominant product of reduction. It has been shown for instance that the complexes [Rhin(bpy)2Cl2]+ or [Rh n(bpy)2(CF3S03)2]+ catalyze the formation of HCOO- in MeCN (at —1.55 V vs. SCE) with a current efficiency of up to 80%.100,103 The electrocatalytic process occurs via the initially electrogenerated species [RhI(bpy)2]+, formed by two-electron reduction of the metal center, which is then reduced twice more (Rlr/Rn Rh°/Rh q. The source of protons is apparently the supporting electrolyte cation, Bu4N+ via the Hoffmann degradation (Equation (34)). 

A number of electrocatalytic reactions have been reported in which the EGB is derived by initial reduction of an aldehyde or a ketone that at the same time functions as the electrophile in a coupling reaction [136-139]. It is Kkely that the actual EGB is a dimer dianion of the carbonyl compound or a dianion of the carbonyl compound formed by disproportionation. The general principle is outlined in Scheme 38. The reactions become catalytic when the product anion, P , is protonated by the weak acid, NuH, whereby the nucleophile, Nu , is regenerated. 

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

It was reported that cobalt-tetraphenylporphyrin complex (CoTPP) coated on an electrode catalyzes electrocatalytic proton reduction,215 but the activity was not very high. We have found that metal porphyrins and metal phtahlocyanines when incorporated into a polymer membrane coated on an electrode show high activity in electrocatalytic proton reduction to produce H2.22,235 Some data are summarized in Table 19.2. It was shown that this catalyst is more active than a conventional platinum base electrode. 

The ORR cannot be studied by feedback mode in neutral or acidic buffers. The tip-generated ORR product, that is, hydroxide ions, immediately react with aqueous protons to form water and do not have time to diffuse toward the substrate. A similar situation is encountered when the HOR is probed in neutral or alkaline media. Fernandez et al. [125] employed the TG/SC mode to study the ORR on the substrate in acidic solutions. The tip was placed close to the substrate and biased at a potential at which water was oxidized to oxygen. The substrate potential was fixed at a value corresponding to oxygen reduction to water. Under these conditions, the oxygen reduction rate at the substrate surface reflected its electrocatalytic activity. 

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