Proton reduction catalysts

Figure 13-3. Artificial photosynthesis [9] that combines a water oxidation catalyst (C2), a photoexcitation center (sensitizer) and a proton reduction catalyst (C ) through mediators (M2 and Ml).

The project by Sun et al. has been extended to photochemical production of H2 by a Ru(II) photosensitizer covalently linked to a proton reduction catalyst of dinuclear Fe carbonyl complex [128,129] (Fig. 25, left). The researchers synthesized (but did not isolate) a Ru(II) terpy complex connected to an [Fe2( x - SCH2N(Ph)CH2S)(CO)6] complex by an acetylene spacer [129] (Fig. 25, right). However, the photoproduction of H2 has not yet been reported. 

Cobalt macrocychc, or pseudo-macrocyclic, complexes have proven among the most widespread cobalt-based proton reduction catalysts. For example, Fisher and Eisenberg demonstrated in 1980 that some cobalt tetraazamacrocyclic complexes are active in both CO2 and reduction [75]. Similarly, porphyrins have been extensively investigated. Nocera and coworkers showed that cobalt(II) hangman porphyrins can catalyze proton reduction with less overpotential and weaker acids than their standard porphyrin cousins (Fig. 13d) [85, 86]. Both features are thought to be a result of the enhanced proton donation by the carboxylic acid of the hangman substituent. Bren and coworkers showed that the biologically derived cobalt-substituted microperoxidase-11 is stable with a turnover number of 25,000, but the catalytic rate is relatively low at 6.7 s [87]. 

First coordination spheres including nitrogen and sulfur donors have also been used to constmct highly active nickel proton reduction catalysts. Employing a polypyridyl ligand framework. Sun and coworkers have created the complex [Ni (L)2(H20)2](BF4)2 for L=2-(2-pyridyl)-l,8-naphthyridine, which, under optimal, basic, photocatalytic conditions, has a remarkable turnover number of 3230... 

The cationic complex [CpFe(CO)2(THF)]BF4 (23) can also catalyze the proton reduction from trichloroacetic acid by formation of Fe-hydride species and may be considered as a bioinspired model of hydrogenases Fe-H Complexes in Catalysis ) [44]. This catalyst shows a low overvoltage (350 mV) for H2 evolution, but it is inactivated by dimerization to [CpFe(CO)2l2-... 

Sometime probably two billion years before humans became interested in efficient catalysts for four-electron, four-proton reduction of O2 to H2O, the so-called oxygen reduction reaction (ORR),... 

It is well recognized that protonated phosphine complexes such as [M(dppe)(H)2]+ (dppe = 2-bis(diphenylphosphine)ethane), M = Co, Ir),39 [Fe(dppe)(L)H]+,40 or [Pt(PEt3)3H]+41 catalyze proton reduction at very negative potentials, 2 V vs. SCE. In contrast, the protonated [(,/s-CsI Is)CoIII P(OMe)3 2I I]1 complex is a catalyst for hydrogen production at —1.15 V vs. SCE at a Hg-pool cathode in pH 5 aqueous buffer.42 Dihydrogen is evolved from the reduced [(r/5-Cd fdCo1"-(P(OMe)3)2H]° form of the complex, which decays to H2 or reacts in a proton-hydride reaction. 

Proton reduction is an important catalysis in water photolysis. Pt and Pt02 have been the best known catalysts for process. However, these colloidal or powder catalysts are not well suited for the construction of a conversion system based on molecules, and, moreover, incorpuration of these strongly colored materials into photochemical conversion systems should be avoided because of their possible filter effect. From this point of view it is desirable to use a molecular catalyst if a highly active one is available. 

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. 

In view of its high overpotential for the production of hydrogen from water, the mercury electrode allows electrochemical evaluation of possible catalysts for proton reduction since redox potentials and efficiencies for hydrogen production can be measured. In general, a species that is catalytically active for hydrogen production will give an amplified wave in both polarography and cyclic voltammetry. 

A cobalt complex 8 containing a redox active tetradentate bis-iminopyridine framework has been reported to support a water reduction catalyst, with activities of observed rate constant, of 10 M s derived from voltammetry measurements (Scheme 3) [16]. Ligand-centered reduction of the coordinated imine function has been proposed as the first electrocatalytic step followed by protonation. Notably, this compound was shown to operate even under basic conditions at pH 8 (buffer) to give 10 liter of H (mol catalyst" h" ) albeit with a modest Faradaic efficiency of only 60%. 

Proton reduction is also an important catalysis in water photolysis. Pt and PtOa have been the most well-known and active catalysts for proton reduction to... 

---