1.2 Water Reduction
Effective process to achieve a sustainable large-scale hydrogen economy is still in
challenge [21–24]. The reduction of protons to produce hydrogen by photochemistry
and electrochemistry is a green and renewable method, while the reduction of
protons to hydrogen has a large activation barrier and thus leads to a high
overpotential and a slow reaction rate. Therefore, an efficient catalyst is necessary
to lower the barrier and improve the performance. (Eq. 4) [25–28].
In nature, a class of metal enzymes, hydrogenase, can catalyze hydrogen to
convert protons and electrons, and can also perform its reversible reaction to give
hydrogen [29]. Hydrogenase exists widely in bacteria, archaea, and some eukarya in
nature [30–34], and can be divided into [FeFe]-, [NiFe]-, and [Fe]-hydrogenases
according to the metal ions in their active centers (Fig. 3) [30, 31, 35, 36]. In the
active center of [FeFe]-hydrogenase, the diiron center coordinated with carbonyl and
cyanide ligands is bridged by a dithiolate ligand, and one of the two Fe ions is
connected to the [4Fe4S] cluster via a cysteinyl residue. One iron center has an
octahedral coordination environment, while the other iron has a five-coordinated
tetragonal pyramid geometry. It is precisely because of the unsaturated iron central
coordination that it can bind protons in the reduced state or hydrogen molecules in
the oxidized state, thus realizing the reversibility between protons and hydrogen. In
this process, the proton relay (-NH-) in the molecule plays an important role. The
TOF of hydrogen production catalyzed by [FeFe]-hydrogenase can reach as high as
9,000 s
À1 [37, 38]. However, the [NiFe]-hydrogenase with heteronuclear bimetals
maintains a certain difference with [FeFe]-hydrogenase. The active sites are mostly
coordinated by cysteine residues, carbonyl and cyanide ligands. The full-coordinated
octahedral structure is maintained around the iron atom, while the penta-coordinated
triangular bipyramidal geometry is adopted for the nickel atom. It was found that the
active center of [NiFe]-hydrogenase was on the nickel atom, while the iron atom
existed as a redox auxiliary metal ion, which may lead to its catalytic activity
different from that of [FeFe]-hydrogenase. Nickel-iron hydrogenase has excellent
catalytic properties in hydrogen-activated, but its TOF for reducing proton to
hydrogen evolution is about 700 s
À1 [38]. Inspired by hydrogenase, researchers
have developed a large number of hydrogenase models to explore their ability of
hydrogen production from proton reduction [29, 39, 40].
Platinum-based compounds are known as excellent proton reduction catalysts
[41, 42], and are still used in the industrial field of hydrogen production from water
electrolysis. Even though its catalytic performance has tended to be perfect, platinum
is a rare precious metal with limited abundance, which limits its application in largescale industrial hydrogen production. Therefore, it is of great significance to develop
a cheap catalyst for water reduction and hydrogen evolution based on the first
transition metal. In addition to the exploration of proton reduction by mimicking
382
H.-T. Zhang and M.-T. Zhang
Effective process to achieve a sustainable large-scale hydrogen economy is still in
challenge [21–24]. The reduction of protons to produce hydrogen by photochemistry
and electrochemistry is a green and renewable method, while the reduction of
protons to hydrogen has a large activation barrier and thus leads to a high
overpotential and a slow reaction rate. Therefore, an efficient catalyst is necessary
to lower the barrier and improve the performance. (Eq. 4) [25–28].
In nature, a class of metal enzymes, hydrogenase, can catalyze hydrogen to
convert protons and electrons, and can also perform its reversible reaction to give
hydrogen [29]. Hydrogenase exists widely in bacteria, archaea, and some eukarya in
nature [30–34], and can be divided into [FeFe]-, [NiFe]-, and [Fe]-hydrogenases
according to the metal ions in their active centers (Fig. 3) [30, 31, 35, 36]. In the
active center of [FeFe]-hydrogenase, the diiron center coordinated with carbonyl and
cyanide ligands is bridged by a dithiolate ligand, and one of the two Fe ions is
connected to the [4Fe4S] cluster via a cysteinyl residue. One iron center has an
octahedral coordination environment, while the other iron has a five-coordinated
tetragonal pyramid geometry. It is precisely because of the unsaturated iron central
coordination that it can bind protons in the reduced state or hydrogen molecules in
the oxidized state, thus realizing the reversibility between protons and hydrogen. In
this process, the proton relay (-NH-) in the molecule plays an important role. The
TOF of hydrogen production catalyzed by [FeFe]-hydrogenase can reach as high as
9,000 s
À1 [37, 38]. However, the [NiFe]-hydrogenase with heteronuclear bimetals
maintains a certain difference with [FeFe]-hydrogenase. The active sites are mostly
coordinated by cysteine residues, carbonyl and cyanide ligands. The full-coordinated
octahedral structure is maintained around the iron atom, while the penta-coordinated
triangular bipyramidal geometry is adopted for the nickel atom. It was found that the
active center of [NiFe]-hydrogenase was on the nickel atom, while the iron atom
existed as a redox auxiliary metal ion, which may lead to its catalytic activity
different from that of [FeFe]-hydrogenase. Nickel-iron hydrogenase has excellent
catalytic properties in hydrogen-activated, but its TOF for reducing proton to
hydrogen evolution is about 700 s
À1 [38]. Inspired by hydrogenase, researchers
have developed a large number of hydrogenase models to explore their ability of
hydrogen production from proton reduction [29, 39, 40].
Platinum-based compounds are known as excellent proton reduction catalysts
[41, 42], and are still used in the industrial field of hydrogen production from water
electrolysis. Even though its catalytic performance has tended to be perfect, platinum
is a rare precious metal with limited abundance, which limits its application in largescale industrial hydrogen production. Therefore, it is of great significance to develop
a cheap catalyst for water reduction and hydrogen evolution based on the first
transition metal. In addition to the exploration of proton reduction by mimicking
382
H.-T. Zhang and M.-T. Zhang