Environmental Engineering Reference
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(a)
H
O
H
Ca
O
(b)
Mn
O
H
2
O
2
O
O
O
Mn
Mn
O
O
2
Ca
Mn
O
O
O
Mn
O
sCheme 25.3
Proposed mechanisms of oxygen evolution by the WOC in (a) Photosystem II and (b) calcium manganese oxides.
Using density functional calculations, Nørskov's group for the biological catalysts and active catalysts for hydrogen reduction
showed that in terms of “being able to stabilize intermediates involving atomic hydrogen they have very similar properties” [62].
In other words, they considered the following mechanism for hydrogen evolution:
+
++→
He H
*
*
(25.1)
2
HH
*
→+
2
*
(25.2a)
2
+
++→+
HeHH
*
*
(25.2b)
2
By calculating the free energy of atomic hydrogen bonding to the catalyst, the group compared different metal surfaces as
catalysts. They found that for a fast hydrogen evolution, reaction steps cannot be associated with large changes in the free
energy. Thus, the compound that forms strong bonds to atomic hydrogen is not a good catalyst because the hydrogen release
step will be slow (eqs. 25.2a and 25.2b) [62]. The compound that does not bind to atomic hydrogen is also not a good catalyst
because the proton/electron transfer step will be slow. This approach is a quantifier to the Sabatier principle [63]: the interaction
between the catalysts and the adsorbate should be neither too strong nor too weak (Fig. 25.4). regarding biological hydroge-
nase with FeMo cofactor, Nørskov's group found that hydrogen atoms can only bind exothermically to the three equatorial
sulfur ligands (μ
2
S ligands) on the FeMo cofactor (Fig. 25.6). In this condition, it results in a binding energy close to that of Pt.
Interestingly, MoS
2
showed a similar diagram (Fig. 25.6).
The calculations show that the metal ions and the first atoms coordinated to them (usually S and O) are important as
reactions. Thus, in addition to enzymes, related metal sulfides or oxides with similar arrangements could be efficient catalysts
for the related reactions (Table 25.1).
However, the 3 billion years of evolutionary experiments by
Nature
have provided better catalysts with regard to appropriate
residues around inorganic cores. In other words, it is important to note that, in addition to cofactors, enzymes have hundreds of
amino acids; however, only a small fraction of the residues come into contact with the inorganic cores, and an even smaller fraction,
three to four residues on an average, are directly involved in metal ions. The roles of the residues that come into direct contact with
the inorganic cores could be the regulation of charges and electrochemistry of the inorganic cores, helping in coordinating water
molecules at appropriate metal sites, and proton transfer in the stability of these inorganic cores. Other residues are not in direct
contact with the inorganic cores but play very important roles in enzymes, and their deletion from enzymes causes a dramatic
decline in the rate of reactions. Many of these amino acid residues are important in the substrates for proton transfer. To design a
biomimetic
catalyst
, a deep understanding of the roles of these amino acid residues in related enzymes is necessary, but these are
not discussed here. The amino acid side groups could improve catalytic reactions efficiently. Thus, the addition of these groups to
metal oxide or sulfide models could increase the efficiency of catalytic activity (Table 25.1). recently, we considered the ability of
manganese oxide monosheets to self-assemble and synthetize layered structures of manganese oxide including guanidinium and
imidazolium groups. The compounds can be considered new structural models for the WOC of Photosystem II [64].
On the other hand, Nakmora et al. used in situ spectroelectrochemical techniques to show that the stabilization of surface-
associated intermediate Mn(III) species, on the surface of manganese oxide, relative to charge disproportionation is an effective
strategy to lower the overpotential for water oxidation by MnO
2
[65]. The formation of N-Mn bonds via the coordination of
