Metal Hexacyanometallate Nanoparticles: Spectroscopic Investigation on the Influence of Oxidation State of Metals on Catalytic Activity

Article abstract of DOI:10.1007/s10562-018-2411-7

Abstract: A series of metal hexacyanometallate nanoparticles KxMy[M′(CN)₆]z·qH₂O, with M=M′=Co (1) or Fe (2), M=Co and M′=Fe (3) and M=Fe and M′=Co (4) were prepared by either co-precipitation or reverse micro-emulsion. These nanopar

Full text of DOI:10.1007/s10562-018-2411-7

Catalysis Letters
https://doi.org/10.1007/s10562-018-2411-7
Metal Hexacyanometallate Nanoparticles: Spectroscopic Investigation
on the Influence of Oxidation State of Metals on Catalytic Activity
Lisa K. Parrott1 · Elizabeth Erasmus1
Received: 7 February 2018 / Accepted: 12 May 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
A series of metal hexacyanometallate nanoparticles KxMy[M′(CN)6]z·qH2O, with M=M′=Co (1) or Fe (2), M=Co and M′=Fe
(3) and M=Fe and M′=Co (4) were prepared by either co-precipitation or reverse micro-emulsion. These nanoparticles were
characterised by ATR FTIR, TEM and XPS, which revealed the presence of CoII/III and/or FeII/III within the compounds. The
average binding energies found for the Co 2p3/2 photoelectron lines for CoII and CoIII were ca. 781.7 and 782.9 eV respectively,
while the binding energies for FeII and FeIII of the Fe 2p3/2 photoelectron lines were located at 709.5 and 711.4 eV. TEM reveal
the particle size ranged between 180 and 520 nm. The eight different compounds’ catalytic activity were tested for the solvent-
free oxidation of benzyl alcohol using hydrogen peroxide as the oxidant. The progression of the reaction was monitored using
ATR FTIR, by following the appearance of the carbonyl stretching frequency of the oxidation products, which showed the for-
mation of not only benzyl aldehyde but also benzyl benzoate, which is the condensation product of benzyl alcohol and benzoic
acid (another oxidation product). The TOF of the catalytic oxidation reaction was correlated with the mean particle diameter,
% occurrence of CoII/III and/or FeII/III, total metal electronegativity and the degree of covalence of the Co- and Fe–ligand bond.
Graphical Abstract
Keywords Hexacyanoferrate · Hexacyanocobaltate · XPS · Heterogeneous catalysts · Oxidation states
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article (https://doi.org/10.1007/s10562-018-2411-7) contains
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1 3

L. K. Parrott, E. Erasmus
2.3 Preparation of 1a
1 Introduction
Potassium hexacyanocobaltate, K3[CoIII(CN)6] (503 mg,
1.5 mmol) was dissolved in water (15 ml), creating an
aqueous solution with a concentration of 0.1 M. To this
cobalt-cyano- containing solution was added 15 ml of a
0.1 M aqueous solution of cobalt(II) chloride hexahydrate,
CoCl2·6H2O (356 mg, 1.5 mmol). The resulting combined
solution turned light pink and was stirred at room tem-
perature for 15 min. Following this the solution was cen-
trifuged at 8500 rpm for 30 min at 15 °C. The supernatant
was discarded and the precipitate dried overnight in vacuo
at 60 °C. The resulting purple powder was crushed to yield
0.301 g (93.7%) of pure 1a.
Transition metal hexacyanometallates are coordination com-
plexes with a number of special physiochemical properties
such as having the ability to accommodate two different tran-
sition metal ions within the same compound [1], and these
two metals both have mixed valencies [2, 3], it can act as an
electron transfer mediator [4], some show supermagnetism,
[5], electrochromism [6], energy storage capability [7], and
it is insoluble in water [8]. Even upon oxidation or reduc-
tion of the metals, the particles do not dissolve due to the
zeolitic-like structure which allows for the movement of ions
in and out to maintain neutrality of the compound [9]. Due to
these and many other useful properties, transition metal hexa-
cyanometallates have found applications in many different
fields. One field that is not fully exploited is the use of metal
hexacyanometallates as heterogeneous catalysts. There are
reports available on the catalytic polymerization of ethylene
oxide [10], manufacturing of polyether polyols [11], copo-
lymerization of carbon dioxide and propylene oxide using
metal hexacyanometallates [12]. As for the heterogeneous
catalytic transformation of fine chemicals, the oxidation of
alcohols have been reported [13, 14]. However, little is known
about the effect of the different metals, the oxidation state of
the metals or even particle size on the catalytic oxidation
of alcohols. Here we report the preparation of metal hexcy-
anometallate nanoparticles by two different methods namely
co-precipitation and reverse micro-emulsion to obtain nano-
particle of different sizes. A spectroscopic characterisation
study was conducted to determine the percentage of the II and
III oxidation state of both iron and cobalt. The influence of
the variations of the spectroscopic data are related to the turn
over frequency (TOF) of the heterogeneous catalytic oxida-
tion benzyl alcohol using hydrogen peroxide as an oxidant.
ATR FTIR v(C≡N)=2166 (MII–CN–M′III), 2052 (MII–
CN–M′II) cm−1
.
XPS: Binding energy = 781.31 eV (CoII 2p3/2); 782.59 eV
(CoIII 2p3/2).
2.4 Characterisation Data for 2a
Yield 44.5% green powder
ATR FTIR v(C≡N) = 2174 (MIII–CN–M′III), 2166 (MII–
CN–M′III) cm−1
.
XPS: Binding energy = 709.48 eV (FeII 2p3/2); 711.03 eV
(FeIII 2p3/2).
2.5 Characterisation Data for 3a
Yield 73.1% black powder
ATR FTIR v(C≡N) = 2160 (MII–CN–M′III), 2108 (MII–
CN–M′II) cm−1
.
2 Experimental
XPS: Binding energy = 708.20 eV (FeII 2p3/2); 709.86 eV
(FeIII 2p3/2); 781.53 eV (CoII 2p3/2); 783.01 eV (CoIII 2p3/2).
2.1 Synthesis of Nanoparticles
All solvents and solid reagents namely: potassium hexacy-
anocobaltate, potassium hexacyanoferrate(III), cobalt(II)
chloride hexahydrate, anhydrous iron(III)chloride and
dioctyl sulfosuccinate sodium salt (AOT) (purchased from
Aldrich) were used as received. Distilled water was used
in all procedures.
2.6 Characterisation Data for 4a
Yield 23.4% pale yellow powder
ATR FTIR v(C≡N)=2175 (MII–CN–M′III) cm−1
.
XPS: Binding energy = 709.66 eV (FeII 2p3/2); 711.17 eV
(FeIII 2p3/2); 781.89 eV (CoII 2p3/2); 782.67 eV (CoIII 2p3/2).
2.2 Co‑precipitation Method
1a–4a were all prepared according to the same synthetic
procedure, 1a will be shown as an example.
1 3

Metal Hexacyanometallate Nanoparticles: Spectroscopic Investigation on the Influence of…
ATR FTIR v(C≡N) = 2175 (MIII–CN–M′III), 2082 (MII–
CN–M′II) cm−1
2.7 Reverse Micro‑emulsion Method
.
1b–4b were all prepared according to the same synthetic
procedure, 1b will be shown as an example. M(AOT)2 with
M=Co and Fe was prepared as described in Ref [15].
XPS: Binding energy = 710.73 eV (FeII 2p3/2); 713.50 eV
(FeIII 2p3/2); 781.92 eV (CoII 2p3/2); 782.84 eV (CoIII 2p3/2).
2.12 Characterisation
2.8 Preparation of 1b
Potassium hexacyanocobaltate, K3[CoIII(CN)6] (664 mg,
2 mmol) was dissolved in a mixture of 2,2,4-trimethylpen-
tane (10 ml) and water (10 ml). To this was added a solution
of Na-AOT (450 mg, 1 mmol) dissolved in 2,2,4-trimethyl-
pentane (10 ml), constituting Solution “A”. Solution “B” was
prepared by dissolving the previously prepared Co(AOT)2
surfactant (2.289 g, 2.6 mmol) in 2,2,4-trimethylpentane
(20 ml) and water (400 ml). Solution “A” was added drop-
wise to Solution “B” while stirring at room temperature for
15 min. The combined solution was sonicated for 15 min
after which it was centrifuged (at 8500 rpm for 30 min at
15 °C). The clear supernatant was discarded and the precipi-
tate washed with methanol to be centrifuged (at 8500 rpm
for 30 min at 15 °C) again. The methanol supernatant was
discarded and the pink precipitate was allowed to dry over-
night in vacuo at 60 °C, yielding 252 mg (58.9%) of pure 1b.
Infrared spectra of neat samples were recorded with a
Thermo Scientific IR spectrometer and NICOLET iS50 ATR
attachment, running OMNIC software (Version 9.2.86).
2.13 X‑ray Photoelectron Spectroscopy
XPS data was recorded on a PHI 5000 Versaprobe system,
with a monochromatic Al Kα X-ray source. Powered sam-
ples were mounted on the sample holder by means of carbon
tape. Spectra were obtained using the aluminium anode (Al
Kα = 1486.6 eV), operating at 50 µm, 12.5 W and 15 kV
energy (97 X-ray beam). A low energy neutraliser electron
gun was used to minimise charging of the samples. The
instrument work function was calibrated to give a binding
energy of 284.8 eV for the lowest binding energy peak of
the carbon 1s envelope, corresponding to adventitious car-
bon. Survey scans were recorded at constant pass energy
of 187.85 eV, while detailed region scans were recorded at
constant pass energy of 29.35 eV for C and O, and 93.90 eV
for the metals, with an energy step of 0.1 eV; the analyzer
resolution is ≤0.5 eV. Charge neutralisation was enhanced
by shooting the mounted sample with an Ar gun during
data recording. The resolution of the PHI 5000 Versaprobe
system is FWHM = 0.53 eV at a pass energy of 23.5 eV
and FWHM = 1.44 eV at a pass energy of 93.90 eV. The
background pressure was 2×10−8 mbar. Spectra have been
charge corrected to the main line of the carbon 1s spectrum,
which was set to 284.8 eV. XPS data was analysed utilising
Multipak version 8.2c computer software [16], and apply-
ing Gaussian/Lorentz fits (the Gaussian/Lorentz ratios were
always > 95%). The photoelectron lines were charge cor-
rected against the lowest binding energy of the fitted adven-
titious C 1s peak at 284.8 eV (the normal position of C–C
according to the XPS Handbook [16]).
ATR FTIR v(C≡N)=2075 (MII–CN–M′II) cm−1
.
XPS: Binding energy = 780.91 eV (CoII 2p3/2); 782.59 eV
(CoIII 2p3/2).
2.9 Characterisation Data for 2b
Yield 1.7% dark green powder
ATR FTIR v(C≡N)=2174 (MIII–CN–M′III) cm−1
.
XPS: Binding energy = 710.33 eV (FeII 2p3/2); 712.59 eV
(FeIII 2p3/2).
2.10 Charaterisation Data for 3b
Yield 15.3% brown-maroon powder
ATR FTIR v(C≡N) = 2157 (MII–CN–M′III), 2088 (MII–
2.14 Transmission Electron Microscopy
CN–M′II) cm−1
.
Transmission electron microscopy (TEM) was performed
with a Phillips (FEI) CM100 equipped with a Megaview III
digital camera and coupled to an Oxford X-Max (80 nm2),
energy-dispersive X-ray spectroscope (EDS). The digital
images were analysed utilizing Soft Imaging System (analy-
SIS) software. Multiple dispersions were prepared by mix-
ing the powder with methanol and deposited on a 300-mesh
nickel grid. Digital images were obtained after preparation.
XPS: Binding energy = 708.49 eV (FeII 2p3/2); 710.44 eV
(FeIII 2p3/2); 781.41 eV (CoII 2p3/2); 782.94 eV (CoIII 2p3/2).
2.11 Charaterisation Data for 4b
Yield 0.2% pale yellow powder
1 3

L. K. Parrott, E. Erasmus
Infra-red spectroscopy is useful to distinguish between
different functional groups within a sample. Stretching
frequencies of hydroxyl groups (vOH) can be identified
between 3650 and 3100 cm−1 [17], cyanide groups (vC≡N)
between 2300 and 2000 cm−1 [18, 19], cyanates (v-NCO) at
ca. 2250 cm−1 [20], metal-bound carbon monoxide (vC≡O)
between 1900 and 2200 cm− 1 [21, 22], while carbonyls
stretching frequencies (vCO) can be found between 1600 and
1800 cm−1 [23, 24]. Factors such as the electronic influence
of the chemical surroundings also affects the wavenumber of
the stretching frequency of the functional group [17].
The ATR FTIR spectra of the metal hexacyanometallates
and their starting materials are shown in the Figures S1–S5,
in the Supplementary Information. From the ATR FTIR
spectra of 1–4 revealed the presence of water within the
samples, adsorbed on the outside as well as being trapped
in the inside of the zeolite-structure of the metal hexacy-
anometallates. The OH stretching frequency (vOH) of water
adsorbed on the outside of the structure is located at ca.
3600–3100 cm− 1, while the δHOH bending vibration of
the coordinated water molecules which are trapped inside
the structure in the intercalated positions can be detected at
ca. 1600 cm− 1 [25]. With the exception if 1b and 4a, two
peaks were observed in the characteristic cyanide stretch-
ing frequency area. Since it is known that shifts in vibration
frequencies is an indicator of different oxidation of the metal
to which the CN is bound, the different peaks were assigned
to the different possible MII/III–CN–M′II/III chains in correla-
tion to published reports [18, 19], and the data summarised
in Table 1.
2.15 Heterogeneous Catalytic Oxidation of Benzyl
Alcohol
A mixture of benzyl alcohol (5.2 ml, 50 mmol) and 0.030 g
of the catalyst (either 1–4) were heated till reflux while stir-
ring. Hydrogen peroxide (2.3 ml, 96.7 mmol) was added
with the evolution of gas (bubbling occurred). Small sam-
ples were periodically remove for ATR FTIR analysis to
follow the progression of the reaction.
3 Results and discussion
Metal hexacyanometallate nanoparticles which has a general
formula of KxMy[M′(CN)6]z·qH2O, with x, y, z and q rep-
resenting stoichiometric numbers and M=M′=Co (1) or Fe
(2), M=Co and M′=Fe (3) and M=Fe and M′=Co (4) were
prepared by two different methods. The first method is a
simple co-precipitation reaction which involves the mixing
of the water soluble starting materials, a metal halide and the
potassium hexacyanometallate at room temperature in ambi-
ent air. This resulted in the formation of the water-insoluble
metal hexacyanometallates (1a–4a). The yields obtained
from co-precipitation ranged from 23% (4a) to 93% (1a).
The second method forms a surfactant stabilized nano-
particle utilising the anionic surfactant sodium bis(2-
ethylhexyl)sulfosusuccinate (Na-AOT). This allowed for
the confinement of the synthesis to occur in nanoscale
droplets, an additional measure to ensure the formation
of nanoparticles. The preparation involves the vigorous
mixing of two micro-emulsions, the surfactant metal salt
(M(AOT)2 with M=Co or Fe) [15] and the potassium
hexacyanometallate surfactant-containing solution. This
resulted in a milky but colourful fine suspension, which
indicates that the exchange of cations between different
droplets (of the two different micro-emulsions) are suf-
ficient to activate nucleation and growth. Yields obtained
for the reverse micro emulsion procedure were very low
(ranging between 0.2–58.9%).
To further characterise the metal hexacyanometallate
nanoparticles, X-ray Photoelectron Spectroscopy (XPS)
was conducted. The binding energy measured of each ele-
ment corresponds with the atomic potential which provides
insight into the oxidation status as well as the chemical and
electronic environment of the element [26–31].
In the survey scans of all the compounds C, N, O, Fe, Co
and K were detected. All the binding energies of the photo-
electron spectra were referenced against the lowest binding
Table 1 The ATR FTIR
Compound
No.
vC≡N
vC≡N
vC≡N
vOH
δHOH
measured stretching frequencies
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
)
detected for 1a–4b
MIII–CN–M′III
MII–CN–M′III
MII–CN–M′II
Absorbed
outside
Intercalated
Co–CN–Co
Fe–CN–Fe
Fe–CN–Co
Co–CN–Fe
1a
1b
2a
2b
3a
3b
4a
4b
2166
2166
2052
2075
3287
3328
3369
3389
3367
3379
3195
3338
1605
1586
1605
1607
1601
1608
1604
1586
2174
2174
2160
2157
2108
2088
2175
2175
2082
1 3

Metal Hexacyanometallate Nanoparticles: Spectroscopic Investigation on the Influence of…
energy of the C–C simulated adventitious C 1s photoelectron
line, which was set at 284.8 eV [16]. The carbon of nanopar-
ticles, M–C≡N–M′ was measured at ca. 288 eV, while the
N 1s photoelectron line from the –C≡N– group is located
at ca. 398 eV (see Figure S6).
binding energy of the FeII within ferrocene anchored onto
an aminoalkyl silane-capped silicon-wafer (709.7 eV) [32].
While the calculated average binding energy for the FeIII
(711.4 eV) was found to be slightly higher than reported
data for iron(III) tris(β-diketonato) complexes (710.9 eV)
[57], but in good agreement with sodium iron(III) oxide
(711.5 eV) [33], and iron(III) chloride (711.3 eV) [34].
The detail scans of the cobalt Co 2p photoelectron lines,
did not show the CoII and CoIII as separate peaks like the
iron. However, a small shoulder is clearly present on the low
energy side, while the photoelectron line is asymmetric in
shape towards the high energy side. The Co 2p photoelec-
tron lines were fitted with three simulated peaks (see Fig. 1
bottom left). The simulated fit of the Co 2p3/2 photoelectron
lines were assigned in order from the low energy side: CoII
at a binding energy of ca. 781.7 eV, CoIII at a binding energy
of ca. 782.9 eV (see Table 2), and the last simulated peak at
ca. 787.2 eV is a shake-up peak. The allocation correlates
well with the 781.7 eV reported for CoII in a bimetallic car-
boxylate complex spin-coated onto a Si-wafer [35], and the
782.4 eV reported for CoIII in CoF3 [34]. The average atomic
ratio between the amount of CoII and CoIII present is 2.1:1
(see Table 2).
Figure 1 shows the detail scan of Fe 2p and Co 2p area,
including the simulated peak fittings, of 3a as an example.
The XPS data of the metal hexacyanometallates, 1a–4b,
with respect to the N 1s, Fe 2p3/2 and Co 2p3/2 photoelectron
lines are presented in Table 2.
Both the Fe 2p1/2 and Fe 2p3/2 photoelectron lines of the
iron-containing compounds, 2–4, shows splitting of the
photoelectron line into two sharp well-defined peaks (see
Fig. 1 top), which is assigned to FeII and FeIII (at the higher
binding energy). The Fe 2p3/2 photoelectron lines assigned
to FeII have an average binding energy of ca. 709.5 eV (see
Table 2), with a spin orbit splitting between the Fe 2p3/2 and
Fe 2p1/2 photoelectron lines of ca. 12.9 eV. Whereas the
Fe 2p3/2 photoelectron lines assigned to FeIII is located at a
binding energy of ca. 711.4 eV (see Table 2), an average of
1.9 eV higher than the FeII. The average atomic ratio (AR)
between the amount of FeII and FeIII present is 2.1:1 (see
Table 2). The Fe 2p photoelectron lines are slightly asym-
metric towards the high energy side, where a small shake-up
peak could be fitted at ca. 2.8 eV higher than the main Fe
2p envelope.
Using the atomic ratios obtained between the potas-
sium, cobalt and/or iron, as well as the relative % of the
oxidation states from the XPS, it is possible to derive the
a quantified stoichiometric formula for 1–4 in the form of
KxMy[Fe(CN)6]z:
The calculated average binding energy for the FeII
(709.5 eV) correlates good with published values for the
Fig. 1 XPS detail scans of the
Fe 2p (top) and Co 2p areas
of 3a
1 3

L. K. Parrott, E. Erasmus
Table 2 Binding energies (eV)
of N 1s photoelectron line,
the Fe 2p3/2 photoelectron line
of FeII and FeIII, the Co 2p3/2
photoelectron line of CoII and
CoIII
BE N 1s (eV) BE FeII 2p3/2 (eV) BE FeIII 2p3/2 (eV) BE CoII 2p3/2 (eV) BE CoIII 2p3/2 (eV)
F2 397.84
F3 398.03
C3 398.87
708.61
710.08
786.79
782.59 (36.3%)
1a
398.06
781.31 (63.6%)
AR 2.8
1
1b 397.68
AR 3.2
780.91 (70.9%)
1
782.59 (29.1%)
2a
398.92
AR 4.6
709.48 (66.7%)
1
710.33 (73.8%)
1
708.20 (66.1%)
1
708.49 (72.1%)
1.2
709.66 (66.7%)
1
710.73 (70.1%)
1.1
711.03 (33.3%)
712.59 (26.2%)
709.86 (33.8%)
710.44 (27.9%)
711.17 (33.3%)
713.50 (29.9%)
2b 397.90
AR 3.5
3a
AR 3.9
3b 398.27
AR 5.3
4a
AR 5.3
398.02
781.53 (79.9%)
1
781.41 (33.4%)
1
781.89 (74.6%)
1.3
781.92 (40.3%)
1
783.01 (20.1%)
782.94 (66.6%)
782.67 (25.4%)
782.84 (59.7%)
398.39
4b 398.12
AR 5.4
The atomic ratios (AR) and % occurrence (given in brackets) are also presented. F2=K3(FeII(CN)6);
F3=K3(FeIII(CN)6) and C3=K3(CoIII(CN)6)
•
•
•
•
•
1a: K0.7Co0.4[Co(CN)6]1
1b: K0.9Co0.7[Co(CN)6]1
2a: K1.5Fe0.7[Fe(CN)6]1
2b: K1.1Fe0.75[Fe(CN)6]1
mean particle distribution (nm) of 1–4 range between ca.
180 nm (for 3b) and 520 nm (for 3a), and are summarised
in Table 3. The size depends on the different metals as
well as the preparation method. In general, it was found
3a: K1.3Co1[Fe(CN)6]1 or K1.3CoII0.8/CoIII0.2[FeII
FeIII0.34(CN)6]1
/
/
/
/
that the nanoparticles prepared by reverse micro-emulsion
are slightly smaller. The shape and size detected by TEM
image are similar to the variety of Prussian blue analogues
(including iron hexacyanocobaltate (Fe–Co) and manga-
nese-iron hexacyanocobaltate (Mn0.5Fe0.5–Co), which is
comparable with 1a–b) prepared by a “co-polymer-co-
morphology” concept using PVP [36]. This shows that it
is possible to obtain similar shape using different methods
of preparation.
0.66
3b: K0.97Co1.69[Fe(CN)6]1 or K0.97CoII
•
•
•
0.87
CoIII0.33[FeII0.33/FeIII0.67(CN)6]1
4a: K1.2Fe0.8[Co(CN)6]1 or K1.2FeII0.5/FeIII0.3[CoII
CoIII0.25(CN)6]1
0.75
0.40
4b: K1.2Fe1.1[Co(CN)6]1 or K1.2FeII0.78/FeIII0.32[CoII
CoIII0.6(CN)6]1
The Transmission electron microscopy (TEM) images
Considering the physical properties such mixed valencies,
high surface area, insolubility (making the metals self-sup-
porting) of metal hexacyanometallates, which are all favour-
able for utilisation as a heterogeneous catalyst, the catalytic
of 1a and b, the cobalt hexacyanocobaltate nanoparticle
prepared respectively by co-precipitation and reverse
micro-emulsion are shown in Fig. 2 as examples. The
Fig. 2 TEM image of the nano-
particles of (left) cobalt hexa-
cyanocobaltate (1a) prepared
by co-precipitation and (right)
cobalt hexacyanocobaltate (1b)
prepared by reverse micro-
emulsion (right)
1 3

Metal Hexacyanometallate Nanoparticles: Spectroscopic Investigation on the Influence of…
Table 3 Summary of the TOF
(determined from catalysis),
the pseudo first order rate
constant (kobs as determine
from following the kinetic of
the catalysis process), mean
diameter (from TEM) and
oxidation state ratio of the Fe
and Co (from XPS) in 1–4
Compound
No. TOF (h−1
)
kobs
(h−1
Combined Mean
% FeII % FeIII % CoII % CoIII
)
metal EN
diameter
(nm)
Co–CN–Co 1a
88
95
65
122
48
0.744 6.723
1.076 6.467
0.871 6.071
1.651 5.900
0.510 6.109
1.667 6.885
0.889 6.202
1.433 6.783
278
316
438
187
521
181
274
271
63.6
70.9
36.3
29.1
1b
2a
2b
Fe–CN–Fe
66.7
73.8
66.1
72.1
66.7
70.1
33.3
26.2
33.8
27.9
33.3
29.9
Fe–CN–Co 3a
79.9
33.4
74.6
40.3
20.1
66.6
25.4
59.7
3b
Co–CN–Fe 4a
4b
128
78
119
activities of 1–4 was tested for the solvent-free oxidation of
benzyl alcohol using the environmentally friendly oxidant
hydrogen peroxide [14].
vCO at 1715 cm−1 started to appear. This is on contrast to
what was observed by Ali et al. who found benzaldehyde as
the only oxidation product [14].
The progression of the catalytic reaction was monitored
using Attenuated Total Reflection Fourier Transformed
Infrared (ATR FTIR) analysis, see Fig. 3. From the ATR
FTIR spectra the formation of the oxidation products bearing
a carbonyl (C=O) groups could be detected following the
appearance of the characteristic FTIR stretching frequency
at ca. 1700 cm−1. This approach could also be used to moni-
tor the consumption of the starting material, the benzyl alco-
hol, by following the disappearance of the carbon-hydroxy
Determination of the oxidation products responsible for
the carbonyl stretching frequencies observed at 1698 and
1715 cm−1, were identified by comparison to commercially
available benzaldehyde and benzoic acid. From the results
obtained of the commercial samples, the vCO at 1698 cm−1
was assigned to benzaldehyde, however no carbonyl stretching
frequency for benzoic acid (which appeared at 1676 cm−1
)
could be detected in the FTIR spectra of the oxidation products
(see Fig. 3 Right). However, the vCO at 1715 cm−1 obtained
from the oxidation product was detected at the same the car-
bonyl stretching frequency of commercially available benzyl
benzoate, located at 1718 cm−1 (see Fig. 3 Left).
C–O–H stretching frequency at 1006 cm−1
.
Closer inspection of the carbonyl stretching frequency
(vCO) revealed that it consisted of two overlapping peaks at
1698 and 1715 cm−1, see Fig. 3 Left. The carbonyl stretch-
ing frequency at 1698 cm−1 started as the favourable product
but as the reaction progressed the oxidation product with a
Since benzyl benzoate is the condensation product formed
between benzyl alcohol and benzoic acid, it is implied that
benzoic acid must have formed and that the characteristic
Fig. 3 Left: ATR FTIR spectra sections showing the appearance and
disappearance of stretching frequencies at 1698 and 1006 cm−1 over
time of the solvent-free catalytic oxidation of benzyl alcohol using
the environmentally friendly oxidant H2O2 using 4b as the heteroge-
neous catalyst. Right: Comparative ATR FTIR spectra of the carbonyl
stretching frequency area showing from top to bottom benzoic acid,
benzaldehyde, the progress of the catalytic reaction and benzyl ben-
zoate
1 3

L. K. Parrott, E. Erasmus
carbonyl stretching frequency is probably hidden underneath
the water peak or that as soon as the benzoic acid is formed it
condenses with the benzyl alcohol to form benzyl benzoate.
The formation of this series of oxidation products (benzalde-
hyde, benzoic acid and benzyl benzoate) along with the for-
mation of toluene (not detectable/distinguishable from ATF
FTIR) has been reported in literature [37].
significantly. To determine the cause of this variation as well
as to determine other factors influencing the TOF, relation-
ships were established between the TOF and mean particle
diameter, % occurrence of the different oxidation states of Fe
and Co, combined metal electronegativity and the degree of
covalence of the metal–ligand bonds (see Figs. 4, 5).
In general, as the mean particle diameter of the nano-
particles decreased, the TOF of the catalytic solvent-free
oxidation of benzyl alcohol increased (Fig. 4 Left), which is
consistent with many data reported on literature on various
catalytic systems [38–41]. Even though the metal hexacy-
anometallate systems are not normal supported catalyst, but
self-supporting catalyst, the smaller particles exposes more
active metal sites to the surface giving rise to the higher
TOF.
From the data collected for the solvent-free catalytic oxida-
tion of benzyl alcohol using hydrogen peroxide as the oxidant
over 1–4, the turn over frequencies (TOF) were calculated after
180 min and summarised in Table 3.
The turn over frequencies (TOF) was calculated after
180 min using Eq. 1. The amount of molecules converted
was determined using the intensity ratio between the vCO (at
1698 cm−1) for benzaldehyde and vC–OH (at 1006 cm−1) for
benzyl alcohol. From a calibration curve the percentage alde-
hyde/alcohol (% convertion) present after 180 min could be
measured (see Figure S7). Using data of catalyzed by 4b as
an example, after 180 min (3 h) the intensity ratio between
vCO:vC–OH=1.8. From the calibration curve it is measured
that ca. 78% of the molecules converted. Since the catalytic
reaction was started with 50 mmol (3.011×1022 molecules)
benzyl alcohol and 78% of the molecules converted, the
amount of molecules converted is 2.3×1022. Considering that
ca. 1.1×10−4 mol (30 mg) of catalyst was added to the reac-
tion, the amount of active sites present is ca. 6.6×1019. Thus
the TOF (for 4b)=[(2.3×1022/6.6×1019)/3 h]≈119 h−1 or
The metal hexacyanometallates are a combination of dif-
ferent metals (for 3–4) with each metal also having different
oxidation states namely II and III, which would influence
the TOF differently. The influence of the % occurrence of
the II and III oxidation state of both Fe and Co on the TOF
is shown in Fig. 4 Middle and Right. Inversely proportional
relationships were obtained for the correlation between TOF
and % occurrence of FeIII and CoII, while directly propor-
tional relationships were found for the correlation between
TOF and % occurrence of FeII and CoIII. This shows that a
higher % occurrence of FeII and CoIII increases the TOF,
while a higher % occurrence of FeIII and CoII decreases the
TOF. The % occurrence of FeII and FeIII does not play as big
a role in TOF as Co oxidation state as can be seen from the
slope of the graphs.
1.99 min−1
.
(
)
amount of molecules converted
amount of active sites
(1)
TOF =
Seeing as it is well reported that oxidation reaction uti-
lising H2O2 as an oxidant according the Fenton mechanism
starts with a FeII source as the catalyst [24, 42, 43], the
higher TOF found by the higher % occurrence of FeII thus is
expected. A number of results published the use of homog-
enous CoII/III and FeII/III complexes for oxidation reactions.
It has been indicated that the use of a homogenous FeIII
time in h
The TOF of 1–4 ranged between 48 h− 1 (for 3a) and
122 h−1 (for 2b), which is comparable to 123 h−1 reported in
literature [14]. The TOF of the catalyst with the same com-
position e.g. Fe–CN–Co 3a (48 h−1) and 3b (128 h−1) varied
Fig. 4 Left: Relationship between the Mean particle diameter (as
determined from TEM) and the TOF (as determined from the cataly-
sis) of 1–4. Middle: Relationship between the % occurrence of FeIII
(●) and CoII (♦), as determined from XPS, and the TOF (as deter-
mined from the catalysis) of 1–4. Right: Relationship between the %
occurrence of FeII (●) and CoIII (♦), as determined from XPS, and the
TOF (as determined from the catalysis) of 1–4
1 3

Metal Hexacyanometallate Nanoparticles: Spectroscopic Investigation on the Influence of…
Fig. 5 Left: Relationship between the Total electronegativity and the
TOF (as determined from the catalysis) of 1–4. Middle: Relation-
ship between the degree of covalence of the Co (an indication from
XPS) and the TOF (as determined from the catalysis) of 1–4. Right:
Relationship between the degree of covalence of the Fe (an indication
from XPS) and the TOF (as determined from the catalysis) of 1–4
complex having the same ligand system than its CoII coun-
terpart is superior in its catalytic activity for the oxidation of
alcohols [44], and the use of a CoII Schiff base complex has
also been reported to need the addition of a bromide ion as
promotor [45, 46]. All confirming that the variation in the %
occurrence of Fe has a smaller effect on the oxidation than
the % occurrence of Co.
compound (see Table 3). With the exception of 2b, a directly
proportional relationship was displayed between the com-
bined metal electronegativity and the TOF. Thus, the more
electronegative compounds resulted in higher TOF’s. The
normal Fenton mechanism involves the oxidation of the FeII
and the reduction in FeIII by hydrogen peroxide, where the
net effect is the disproportionation of H2O2 to produce.
two different radicals (HO· and HOO·) and water. It can be
deduced that the more electronegative (more electron poor)
the compound in totality, the easier it is to disproportionate
hydrogen peroxide, causing the higher TOF.
There are numerous reports available on how the elec-
tronegativity of molecular fragments influences the physical
properties of the compounds. For instance, many linear rela-
tionships have been established to indicate the dependence
of physical properties such as reaction rates [47], oxidation
potentials [48–52], binding energies [53–57] on electron-
egativity, using the Gordy scale group electronegativity of
molecular fragments. Electronegativity also influence TOF
and since it is known that electronic environment of met-
als also influence the catalytic activity [61], an attempt was
made to demonstrate how the electronegativity of 1–4 influ-
ence the TOF. However, seeing as the Gordy group electron-
egativity scale (scale used for organic molecular fragments)
could not be used, the electronegativity of metal ions as
determined by Li et al. [58] was used. A combined metal
electronegativity for each compound (1–4) was determined
using the following equation:
It is known that the shake-up peak (satellite peak) of
metal photoelectron lines in XPS represents the charge trans-
fer process and the separation (in eV) between the main and
shake-up peak is a measure of the degree of covalence of the
metal–ligand bond [59]. The lower the degree of covalence
of a metal–ligand bond, the more “ionic” character will be
displayed by the bond. An inversely proportional relation-
ship was established between degree of covalence of the
metal–ligand bond (for both Fe and Co) and the TOF (see
Fig. 5 middle and right).
Though the degree of covalence of the metal–ligand
bond is not the same as the metal–ligand bond strength it
is a feature of molecular orbital energy [59], which cor-
[(
)
(
)]
[(
)
(
)]
% occurrence of FeII
×
EN of FeII
+
% occurrence of FeIII × EN of FeIII
)] [( ) (
% occurrence of CoIII × EN of CoIII
[(
)
(
)]
+
% occurrence of CoII × EN of CoII
+
The % occurrence of each oxidation state for each metal
responds to bond strength (the higher the molecular orbital
energy the higher the bond strength). Considering this, the
inversely proportional relationship found for the degree of
covalence of the metal–ligand bond and TOF is consist-
ent with reported data. During catalytic oxidation reac-
tions using metal oxides, the M–O bond strength has been
reported to be an important factor influencing the catalytic
reaction [60, 61]. An inversely proportional relationship
exist between M–O bond strength and catalytic activity,
was obtained from XPS (Table 2) and the electronegativ-
ity (EN) of each metal ion is: EN of FeII = 2.636, EN of
FeIII = 3.835, EN of CoII = 2.706 and EN of CoIII = 4.520
[58]. For compounds 1 and 2, having only Co and Fe respec-
tively, the calculated combined metal electronegativity was
doubled to compensate for the fact that only two types of
metal ions are present within one compound where as 3
and 4 has four types of metal ions are present within one
1 3

L. K. Parrott, E. Erasmus
100
80
60
40
20
0
2.0
1.6
1.2
0.8
0.4
0.0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2b
3b
4b
1b
1a
4a
2a
3a
y = 0.0148x - 0.2527
R² = 0.8967
y = 1.6147x
R² = 0.9006
0
1
2
3
4
5
0.0
0.5
1.0
Time (h)
1.5
2.0
40
60
80
100 120 140
TOF (h-1)
Time (h)
Fig. 6 Left: Time trace showing the conversion of benzyl alcohol
to its oxidation products at 25 °C using H2O2 as oxidant and 4b as
the catalyst. Middle: A kinetic plot of data for this process that leads
to the observed first order rate constant kobs. Right: Relationship
between the TOF and the kobs measure during the catalytic oxidation
of benzyl alcohol using 1–4 as the catalysts
Acknowledgements The author would like to acknowledge generous
which is consistent with our findings. When the M–O bond
strength is high the catalytic activity is low, up to a point
where the M–O bond strength is so strong that the catalyst
could be inactive. While a weak bond strength could lead
to total and even over oxidation.
financial support from Sasol and UFS during the course of this study.
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Affiliations
Lisa K. Parrott1 · Elizabeth Erasmus1
1
* Elizabeth Erasmus
Department of Chemistry, University of the Free State,
erasmuse@ufs.ac.za
Bloemfontein 9300, South Africa
1 3