Generation of hydrogen (H2) from water by solar-energy conversion is considered a promising way to deal with the energy crisis and climate change. One of the key challenges at this stage is to create catalysts for H2 production with high efficiency and low cost. [FeFe]-hydrogenase, an enzyme in algae, is the fastest proton reduction catalyst in nature known to date. The H2 production rate of the active site of [FeFe]-hydrogenase, a Fe2S2 subunit coordinated by CO and CN ligands, achieves turnovers as high as 6000–9000 molecular H2 per active site per second. Such an efficient catalyst with its noble-metal-free structure has aroused much interest in the last decade. With reference to the crystal structure of natural [FeFe]-hydrogenase, a large number of [FeFe]-hydrogenase mimics have been synthesized on the basis of the Fe2S2 cluster. From a photochemical point of view, molecular dyads and triads, multi-component systems, and assembled hybrid systems, have been developed. Although two systems with [FeFe]-hydrogenase mimics as catalysts performed H2 photo-production with a turnover number (TON) of over 200, most of analogous systems finished their photochemical H2 production with low turnover numbers (TON<5) in organic solutions or a mixture of organic solvents and water. In 2011, we designed a robust water-soluble [FeFe]-hydrogenase mimic by linking three hydrophilic ether chains to the Fe2S2 active site, and achieved for the first time photocatalytic H2 production in water. With this water-soluble [FeFe]-hydrogenase mimic as a catalyst, CdTe quantum dots (QDs) as a photosensitizer (PS), and ascorbic acid (H2A) as a proton source and sacrificial electron donor, the system exhibited a high efficiency for photocatalytic H2 production (TON=505). Since then, incorporation of the Fe2S2 active site to a water-soluble group, protein, and peptide, even the hydrophobic cavity of cyclodextrins, has been developed to realize photocatalytic H2 production in water because water is believed to be an ideal solvent for proton mobility and has non-toxic as well as economic advantages. Indeed, the TON of these water-soluble systems is enhanced in a range of 75 to 84. Very recently, a systematic comparison suggested that the efficiency and stability of photocatalytic [FeFe]-hydrogenase systems in water is much better than in organic solvents. Nevertheless, the efficiency for H2 evolution is far less than the natural [FeFe]-hydrogenase (turnover frequency (TOF) 6000–9000 molecule H2per active site per second).
Recently, a new set of water-soluble polymer catalysts PAA-g-Fe2S2 has been designed and successfully synthesized by Scientists from the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (TIPCCAS). A system, containing PAA-g-Fe2S2 as the catalyst, CdSe QDs as the photosensitizer, and ascorbic acid as proton source and sacrificial electron donor, shows high efficiency for the photocatalytic H2production in water. The TON of 27135, initial TOF of 3.6 s−1, are the highest known to date for [FeFe]-hydrogenase mimics, competitive with those from current state-of-the-art catalytic systems for H2 production. The PAA chain of PAA-g-Fe2S2 plays three roles in the system: 1) it is a framework to bring the Fe2S2 active site into aqueous solution; 2) it is a good stabilizer protecting the MPA-CdSe QDs from aggregation and enhancing the emission quantum yield of the CdSe QDs; 3) it narrows the distance between the photosensitizer and the Fe2S2 core for more efficient electron transfer (kET=6.15×1012 m−1 s−1) from the excited MPA-CdSe QDs to the Fe2S2 active site. The three functions jointly contribute to a highly efficient photocatalytic H2 production. Static-state and time-resolved spectroscopic studies demonstrate that the electron transfer from the excited MPA-CdSe QDs to the Fe2S2 catalyst center is the key step to trigger the catalytic cycle. The unique performance of the polymer-based [FeFe]-hydrogenase system indicates that this approach is a promising strategy to improve the photocatalytic efficiency of H2production in water. Extension of the present systems is ongoing in our laboratory.
Angewandte Chemie International Edition
|
