1. Introduction
The hydroxides of transition metals are known to catalyze the water
oxidation.[1] Nowadays, investigations are focused
mostly on mixed (Ni,Fe) hydroxide which is extremely effective in this
process. The iron moiety of the hydroxide is commonly considered as
responsible for overall activity of this
material.[2,3] One of the major open questions in
this field is the electron state of “active” iron cation and the
detailed mechanism of the O-O coupling. The up-to-date experimental and
DFT-computation results are the following. On base of operando Mössbauer
spectroscopic studies the FeIV=O ferryl site of the
(Ni,Fe) hydroxide was suggested to be responsible for the water
oxidation.[4] The ferryl site can appear via
proton-coupled electron-transfer (PCET) from the
FeIII−OH species.[4] The ferryl
model contradicts conclusion by Freibel, Bell, and Nørskov with
coauthors[5] that in
Ni1-xFexOOH the actual active site for
oxidation of water is FeIII. The latter conclusion was
put forward on base of operando X-ray absorption spectroscopy (XAS)
combined with high energy resolution fluorescence detection (HERFD) and
DFT modeling.[5] The ferric iron is claimed to
occupy under-coordinated octahedral positions appeared on high-index
surfaces (\(01\overset{\overline{}}{1}2\)) or
(\(01\overset{\overline{}}{1}4\)) of NiOOH.[5]Alternatively, Goddard with coauthors suggested that the
FeIV-O• species is a key intermediate which determines
the activity of the (Ni,Fe) hydroxide in the water
splitting.[6] However, in the latter work the
FeIV-O• species only assisted the O-O coupling which
actually proceeds on the NiIV-O• site. Anyway, the
idea that the oxyl intermediate is an indispensable participant of the
water splitting by hydroxides nicely agrees with suggestion by Siegbahn,
that the O-O bond association on natural photosynthetic center
necessarily involves an endergonic formation of oxygen
radical.[7] The only question arises how this
intermediate can appear in the hydroxide, taking into account obvious
metastability of such radical species.
In our previous work on the O-O coupling, with the use of tetramer
cluster model Fe4O4(OH)4of the FeIII hydroxide, the ferryl
FeIV=O species was formed from
FeIII-OH group via the first PCET.[8] So-formed
ferryl oxygen attacks on the cubane edge forming metastable
FeIII-O• oxyl group with negative spin density on
oxygen. It is the spin polarization of the oxyl that makes
FeIII-O• group responsible for further O-O coupling
with a low barrier of 12 kcal/mol.[8] This route
happens to be energetically favorable. A competing process blocking this
scenario is the hydroxylation of the ferryl center by water molecule to
form the HO-FeIV-OH moiety. However, the second PCET
restores a bare oxo site (having the HO-FeIV-O• or
HO-FeV=O configuration) capable of the O-O coupling
with neighboring cubane corner oxygen. In addition, the presence of
hydroxo group in the neighborhood of terminal oxo site creates a
possibility of the oxo-hydroxo association at the same Fe center to form
the OOH species. So-obtained O-O coupling on a single iron site is
though less probable due to a relatively high barrier of 18
kcal/mol.[8]In this work the O-O coupling is investigated for water nucleophilic
attack on the HO-FeIV-O• oxyl site.
As far as the above described formation of the -O• or =O terminal oxo
sites are concerned, a question arises whether unavoidable hydroxylation
process can deactivate these sites in water solution. One may guess that
electrophilic attack of water molecule on oxo sites resulting in the
FeIV=OH and FeV=OH formation
competes with the nucleophilic water attack on the same sites to form
hydroperoxo species Fe-OOH. To answer this question, the DFT comparative
modeling of the hydroxylation and oxidation was performed using simple
cubane cluster
Fe4O4(OH)4 treated in
our previous works.
Model
Active sites of water-oxidation catalysts based on iron hydroxides are
believed to have much in common with the structure of gamma-FeOOH
hydroxide. The latter consists of the double chain of edge-sharing
Fe(O,OH)6 octahedra. Major structural motif of this
Fe-hydroxide is a trimer consisting of iron-centered octahedra (Figure
1). The Fe-O-Fe angle is about 100 degrees, the oxo bridge sites are
always three-coordinated, while hydroxo groups couple two or one Fe
cation. Monocoordinated hydroxyl can appear only on the vertex of the
terminating octahedron and is most probably the subject of the first
PCET step initiating various scenarios for the O-O coupling process.
The cubane tetramer (FeOOH)4O (Figure 2) was chosen for
modeling of the reactive center. Previously this model was proved to be
quite useful to study O-O coupling since it allows to simulate oxidation
acts and the O-O coupling on the vertex of terminal octahedron in the
edge-sharing M(O,OH)6 (M=Co,Fe) octahedra
chain.[8–10]