Proposed molecular mechanism for the colloidal nanocatalysis of the hexacyanoferrate III-thiosulfate electron transfer reaction: On the involvement of a Prussian blue analogue complex intermediate

Article abstract of DOI:10.1016/j.jcat.2010.07.002

Attenuated total reflectance (ATR-FTIR) spectroscopic methods were used to study the mechanism of the reaction between hexacyanoferrate III (HCFIII) and thiosulfate catalysed, by colloidal platinum nanostar (PtNSs). A dynamic flow cell was fabricated and used for in situ FTIR and optical measurements for the reaction near zero-time after mixing the reacting materials. It is proposed here that the mechanism of this reaction takes place via a Prussian blue analogue complex intermediate, whereby the HCFIII reacts with the PtNS surface atoms prior to reacting with the thiosulfate reactant. Further, it is proposed here that the complex occurs on the PtNSs surface and that platinum atoms move through multiple redox cycles as they are oxidized and reduced by the reactants. When the reaction is complete, the Pt atoms largely return to their ground state through reduction by excess thiosulfate in solution. In this way, the majority of the Pt atoms are thought to remain associated with the same nanoparticle throughout the reaction, thus providing evidence for a heterogeneous particle surface mediated nanocatalysis.

Full text of DOI:10.1016/j.jcat.2010.07.002

Journal of Catalysis 274 (2010) 215–220
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Proposed molecular mechanism for the colloidal nanocatalysis
of the hexacyanoferrate III–thiosulfate electron transfer reaction:
On the involvement of a Prussian blue analogue complex intermediate
M.A. Mahmoud
Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Attenuated total reflectance (ATR-FTIR) spectroscopic methods were used to study the mechanism of the
reaction between hexacyanoferrate III (HCFIII) and thiosulfate catalysed, by colloidal platinum nanostar
(PtNSs). A dynamic flow cell was fabricated and used for in situ FTIR and optical measurements for the
reaction near zero-time after mixing the reacting materials. It is proposed here that the mechanism of
this reaction takes place via a Prussian blue analogue complex intermediate, whereby the HCFIII reacts
with the PtNS surface atoms prior to reacting with the thiosulfate reactant. Further, it is proposed here
that the complex occurs on the PtNSs surface and that platinum atoms move through multiple redox
cycles as they are oxidized and reduced by the reactants. When the reaction is complete, the Pt atoms
largely return to their ground state through reduction by excess thiosulfate in solution. In this way,
the majority of the Pt atoms are thought to remain associated with the same nanoparticle throughout
the reaction, thus providing evidence for a heterogeneous particle surface mediated nanocatalysis.
Ó 2010 Elsevier Inc. All rights reserved.
Received 13 May 2010
Revised 30 June 2010
Accepted 1 July 2010
Available online 12 August 2010
Keywords:
Platinum nanostars
Mechanism
Nanocatalysis
Colloidal
1. Introduction
nanoparticles undergo shape changes [9]. Some research was
carried out studying the mechanism of the gas nanocatalysis, e.g.
One of the unique properties of the nanoparticles is their high
surface area compared with the number of surface atoms [1–3].
When the size of the material decreases to the nanoscale, the sur-
face area of the material largely increases as well as the number of
chemically unsaturated thermodynamically active surface atoms.
In addition to the high surface to volume ratio, the shape and size
of the nanoparticles proved to have a role in the activity of the cat-
alyst [4,5]. An important question that arises in this field is
whether catalysis with nanoparticles is heterogeneous [6,7] or
homogenous [4,8,9]. The common and acceptable definition of col-
loidal nanocatalysis is heterogeneous when the reaction occurs on
the surface of nanoparticles and formation of a complex on the sur-
face atoms themselves. If a complex forms on the nanoparticles
surface, however and dissolves away from the surface the reaction
is homogenous.
the oxidation of CO by O2 catalysed by palladium nanoparticles
supported on the surface of Fe3O4, the mechanism of this reaction
was found to be size dependent [10], while there is no sufficient
work carried out on the mechanism of colloidal nanocatalysis. To
make the colloidal nanocatalysis useful, one should understand
the mechanism of the reaction and the role of the nanocatalyst.
Here, a simple example for colloidal nanocatalysis is discussed, in
which the electron transfer reaction between hexacyanoferrate III
(HCFIII) and thiosulfate in the presence of platinum nanostars
(PtNSs) as nanocatalyst is studied.
Former studies were carried out on the HCFIII–thiosulfate reac-
tion using PtNSs with different shapes and sizes [4,8,9]. Reshaping
of the nanocatalyst was observed for all shapes but the mecha-
nism of the electron transfer process is still unclear. In this study,
the proposed molecular mechanism for the HCFIII–thiosulfate
electron transfer reaction, catalysed by platinum nanoparticles,
is discussed.
The attenuated total reflection FTIR (ATR-FTIR) technique is
used here to measure the IR of the compounds in aqueous solution,
because it depends on the multi-reflection of the IR source through
the ATR cell. However, it monitors the adsorbed materials on the
surface of PtNSs. The in situ optical and FTIR spectroscopies in stea-
dy state and in time-resolved (mixing of the reacting materials
during the measurement) technique were applied.
It has been reported that the activation energy of the hexacya-
noferrate III and thiosulfate electron transfer reaction catalysed
with platinum nanoparticles was found to be shape dependent.
Nanocatalysts possess more catalytic activity when they have
more atoms on edges or corners (i.e. more valence unsaturated
atoms) [4]. Furthermore, it was found that the catalytically active
E-mail address: mmahmoud7@mail.gatech.edu
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.07.002

216
M.A. Mahmoud / Journal of Catalysis 274 (2010) 215–220
case with an in-out (0.2 ꢁ 0.5 ꢁ 2 cm) quartz cell. To prepare PtNPs
2. Experimental
from Pt II, a 5 mL solution of K2PtCl4 (0.001 M) and thiosulfate
(0.01 M) was mixed and left for 4 h.
Platinum nanostars (PtNSs) were prepared by a seed-growth
method [5] where tetrahedral platinum nanoparticles (THPN) were
used as a seed, which grew to PtNS. THPN were prepared by dis-
solving 0.0667 g of PVP (MW 360,000) in 33 mL of deionized water,
four drops of 1 M HCl were added and the solution was heated un-
der reflux to boil, and then 3 mL of 0.01 M K2PtCl4 was added. After
2 min of boiling, 14 mL of ethanol was quickly injected and the
solution was left to cool to room temperature. Forty microlitres
of the tetrahedral platinum nanoparticle was allowed to grow to
PtNS in a hot solution of: 0.133 g of PVP (MW 360,000) dissolved
in 100 mL of deionized water, 9.6 mL of 0.01 M K2PtCl6, and 8 mL
of 1 M HCl. The resulting mixture was heated under reflux to boil
and then 36 mL of ethanol was injected, followed by 3 h reflux. A
JEOL 100C transmission electron microscope (TEM) was used to
image PtNS before and after the reaction, while JEOL 4000EX used
for the high-resolution TEM measurements.
The setup used in this experiment depends on a continuous
flow of two reacting materials through two separate tubes and fi-
nally allows them to mix together and react inside the ATR with
a germanium crystal [2]. By this method, it is possible to generate
and detect short lived compounds formed during or at the early
stages of the reaction. FTIR (Nicolet 860 Magna-IR) operating in
the range of 4000–600 cmꢀ1 with 4 cmꢀ1 resolution is used for
the ATR-FTIR measurement in which the ATR was fixed at 45° from
the IR source.
3. Results
3.1. Imaging of Platinum nanostar
The TEM image of the platinum nanostar before and after the
catalysis reaction is useful to study the mechanism of the reaction.
Fig. 1A shows the TEM image of PtNS prepared by the above men-
tioned method; the particles seem to have multiple arms. The sta-
tistical analysis performed for 300 particles taken from three
different preparations shows that the particle diameter between
two facing arm tips was found to be 11.2 2.7 nm. A good size dis-
tribution was observed using the Gaussian fitting of size distribu-
tion histogram shown in Fig. 1B. The high-resolution TEM
imaging proved the PtNS is a single crystal (Fig. 1C). After the PtNS
involved in the reaction, the TEM in Fig. 1D shows that the size of
the particles seems larger than the original catalyst (before the
reaction); the reason for increasing particle size of the PtNS after
the reaction is due to the precipitation of some products on its sur-
face. Further, the electron beam diffraction pattern in Fig. 1E
proves that this precipitate is crystalline and similar to the re-
ported results by Zhang et al. [11]. It shows that the Prussian blue
analogue complex crystallizes on the surface of the nanoparticle.
Forty-five microlitres of 0.5 M sodium thiosulfate was mixed
with different volumes of the PtNSs prepared by the above method,
and the volume was raised to 50 mL by deionized water.
3.2. Effect of thiosulfate and hexacyanoferrate III on PtNSs
The HCFIII was flowed through one tube and freshly mixed
PtNSs (0.2 mM) and potassium thiosulfate (0.5 M) was flowed
through the second tube using a Cole-Parmer pump (model
77202–60). For the UV–Vis. measurement (Ocean optics,
HR4000Cg-UV-NIR) was used; the ATR cell was substituted in this
In order to understand the mechanism of the HCFIII–thiosulfate
reaction catalysed by PtNSs, one should study the effect of each
reactant on the PtNSs. Optical and IR measurements were used
to monitor the reaction of each reactant separately with the PtNSs
surface.
[2]
[1]
[2]
C
150
A
[1]
particle size = 11.2 2.7 nm
B
[2]
120
[1]
[1]
[1]
[2]
90
60
30
[2]
200
-
0
111
20 nm
7
8
9
10 11 12 13 14 15
size (nm)
50 nm
4 nm
D
E
Fig. 1. TEM image of PtNS (A), size distribution histogram with Gaussian fit (B), high-resolution TEM, the inset is the diffraction pattern of the PtNS (C), and TEM of the PtNS
after the reaction (D). Electron beam diffraction pattern of the crystalline products, which is deposited on the surface of PtNS (E).

M.A. Mahmoud / Journal of Catalysis 274 (2010) 215–220
217
PtNPs, however the PtNPs acts as a catalyst and reactant at the same
time [11].
HCFIII is reported to react with PtNSs to form the red Prussian
blue analogue according to the following equations [2]:
The thiosulfate anion possesses C3v symmetry [12], and it shows
two IR vibration bands centred at 996 and 1117 cmꢀ1 assigned to
the symmetric and asymmetric SO vibrations, respectively [1].
Thiosulfate forms a complex with platinum II through the sulphur
atom [13], the coordinated thiosulfate molecules have SO stretch-
ing vibration appearing at a higher frequency compared with the
corresponding free thiosulfate due to changing the symmetry from
C3v to different symmetry. Fig. 2 shows the ATR-FTIR spectrum of
thiosulfate after mixing with different amounts of PtNSs. Two IR
bands were observed at 996 and 1117 cmꢀ1 corresponding to sym-
metric and asymmetric SO stretching vibration of thiosulfate ions
[8,14]. While for all the samples containing thiosulfate mixed with
different amounts of PtNSs (5, 10, and 15 mL), three bands were
detected. Two corresponding to the symmetric and asymmetric
vibration of SO in addition to a new band appear at 1046 cmꢀ1 as-
signed as the symmetric SO vibration for adsorbed thiosulfate on
the surface of PtNSs. As the amount of PtNSs increases, the inten-
sity of the band corresponds to the adsorbed thiosulfate increases
at the expense of the bands corresponding to the free thiosulfate.
2K3½FeðCNÞ ꢂ þ Ptðon the surface of PtNPsÞ þ 2Kþ
6
! Pt2þ þ 2K4½FeðCNÞꢂ ꢂ
ð1Þ
ð2Þ
6
K3½FeðCNÞ ꢂ þ Pt2þ ! 2KPtII½FeIIIðCNÞ6ꢂ þ 2Kþ
6
Two molecules of HCFIII react with the platinum atoms on the sur-
face of PtNSs leading to the formation of Pt II ions, then one mole-
cule of the platinum ion quickly combined with another HCFIII
molecule forming a Prussian blue analogue molecule with an opti-
cal absorption spectrum peak centred at 560 nm [2]. Zhang et al.
have synthesized platinum nanoparticles (PtNPs) modified with
Prussian blue by heterogeneous catalytic reaction of HCFIII and
Concentration of PtNPs
0.26
0 mL
5 mL
10 mL
0.24
15 mL
0.22
3.3. FTIR measurement for the hexacyanoferrate III–thiosulfate
reaction
0.20
0.18
0.16
0.14
0.12
0.10
0.08
The reaction of PtNSs and the HCFIII leads to the formation of a
red Prussian blue analogue absorbing at 560 nm, as mentioned in
the former section [2]. To prove whether this complex is formed
during the reaction, HCFIII (0.5 M) was mixed with a solution con-
taining PtNSs and 0.5 M thiosulfate. No optical absorption peak
was observed at 560 nm. This could be due to one of two reasons:
either the complex forms quickly and takes part in the reaction or
it does not form during the reaction. Therefore, its concentration
becomes too minute to be sensed. To confirm the formation of
the complex during the reaction, the in situ dynamic flow optical
measurement was carried out. The thiosulfate–PtNSs solutions
mixture was mixed with HCFIII inside a flow quartz cell; this tech-
nique made it possible to generate the short lived complexes
formed at nearly zero-time of the reaction. An optical peak was ob-
served at 556 nm which was assigned to KPtII[FeIII(CN)6] Prussian
blue analogue complex as shown in Fig. 3A. The rates of flow of
1250 1200 1150 1100 1050 1000
Wavenumber (cm-1)
950
Fig. 2. ATR-FTIR spectral region showing the free symmetric (996 cmꢀ1
) and
asymmetric (1117 cmꢀ1) S–O stretching of thiosulfate. A shifted vibrational band
appears at 1046 cmꢀ1 whose intensity is proportional to the concentration of
platinum nanoparticles in solution; this band is attributed to the bound symmetric
SO stretching of the thiosulfate.
0.5
0.07
12.50 mL/sec
9.09 mL/sec
A
B
8.33 mL/sec
6.25 mL/sec
0.06
0.4
0.3
0.2
0.1
10 to 1
5 to 1
1 to 1
1 to 5
1 to 10
5 mL/sec
4.17 mL/sec
3.13 mL/sec
1.85 mL/sec
1 mL/sec
0.05
0.04
0.03
0.02
556
2200
2100
2000
1900
cm-1
1200
1100
1000
500
550
600
650
700
Wavelength (nm)
Fig. 3. (A) Visible absorption during the initial mixing of 0.5 M HCFIII and 0.2 M PtNSs at various flow rates (1–12.5 mL/s). The Prussian blue analogue species has an
absorption band at 556 nm which forms and quickly disappears. (B) The FTIR spectrum of the hexacyanoferrate III and thiosulfate reaction with ratios of 0.5 M:0.05 M,
0.5 M:0.1 M, 0.5 M:0.5 M, 0.1 M:0.5 M, and 0.05 M:0.5 M at zero-time (mixing the reacting materials inside the ATR cell during the detection).

218
M.A. Mahmoud / Journal of Catalysis 274 (2010) 215–220
the reacting materials were changed to obtain the optimum flow
that produces the maximum amount of this complex. The opti-
mum flow was found to be 4.2 mL/s.
1 to 1
1 to 5
1 to 10
After 10 min
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
The reaction of HCFIII–thiosulfate produced short lived species,
because the cyanide ligand in HCFIII is capable of binding with two
metal ions forming a bridged complex, so the electron transfer
could take place through the formation of bridged complex similar
to that previously reported [2]. Fig. 3B shows the ATR-FTIR spectra
of the HCFIII–thiosulfate reaction at concentration ratios of 10:1,
5:1, 1:1, 1:5, and 1:10 at approximately zero-time (mixing the
reacting materials during the IR detection inside the ATR cell).
The flow rate was adjusted to 4.2 mL/s (the rate that produced
the highest amount of the intermediate see Fig. 3A). The products
of this reaction are well known to be tetrathionate and HCFII [15],
and the bands at 2114 and 2038 cmꢀ1 are assigned to be CN
stretching of HCFIII and HCFII, respectively [16]. The band at
2052 cmꢀ1 corresponds to the CN stretching of the Prussian blue
analogue KPtII[FeIII(CN)6] complex, while that at 2074 cmꢀ1 corre-
sponds to PtIV[FeII(CN)6] complex formation [2,16] and the band
which corresponds to the bridged cyanide appears at 2140 cmꢀ1
[16]. The bands at 996 and 1117 cmꢀ1 are assigned to the symmet-
ric and asymmetric SO stretching of free thiosulfate, respectively.
The adsorbed thiosulfate appears at 1046 cmꢀ1. The intensity of
band at 2052 and 2076 cmꢀ1 corresponds to the formation of
KPtII[FeIII(CN)6] Prussian blue analogue [2] and was found to be
more intense in the case of 10 to 1 (hexacyanoferrate III to thiosul-
fate) ratio compared to the other concentration ratios.
4.8
3.8
2.5
2200 2150 2100 2050 2000
1200
1100
1000
cm-1
Fig. 5. ATR-FTIR spectrum of hexacyanoferrate III mixed with thiosulfate after
10 min, the hexacyanoferrate III to thiosulfate concentration ratio is 0.5 M:0.5 M,
0.1 M:0.5 M, and 0.05 M:0.5 M.
solution was diluted with deionized water to 10 mL. Fig. 4A shows
the decrease in the absorption peak spectra of the Prussian blue
analogue complex with time, after mixing with thiosulfate solu-
tion. The rate of the reaction of the Prussian blue analogue complex
and the thiosulfate solution depends on the concentration of the
thiosulfate ions; however, the rate constant of this reaction in-
creases in the following order 0.035, 0.098, and 0.147 minꢀ1 for
0.01 M thiosulfate with volume of 0.5, 1, and 2 mL, respectively.
In order to examine the mechanism of the HCFIII–thiosulfate
catalysed by PtNSs, the reaction was carried out at different con-
centration ratios of the reacting materials. The concentration of
thiosulfate remains constant and the concentration of HCFIII var-
ied. After ten minutes of mixing, the IR spectrum was measured
for each mixture (1:1, 1:5, and 1:10 HCFIII to thiosulfate ratio), a
new band appears corresponding to the product formation, but
the peaks corresponding to the Prussian blue complexes become
minute or even disappear in some cases. Fig. 5 shows the ATR-FTIR
spectrum of the HCFIII–thiosulfate reaction. For the three concen-
tration ratios, the bands at 996 and 1117 cmꢀ1, assigned to sym-
metric and asymmetric SO stretching vibration of free thiosulfate
appear with different intensities. As the amount of HCFIII in-
creases, the free thiosulfate bands decreases. While the band corre-
sponds to the bound thiosulfate to PtNSs appeared at 1046 cmꢀ1
and was found to have similar intensities. This is because the
amount of adsorbed thiosulfate is constant and depends only on
the surface area of the catalyst. Since the PtNSs are small compared
with the reacting materials, the ATR-FTIR technique can detect
both the free and adsorbed reactants and products. In addition to
the thiosulfate, two new bands appear at 1229 and 1017 cmꢀ1 as-
signed to be asymmetric and symmetric SO stretching vibration of
tetrathionate product, respectively [8]. Since the concentration of
thiosulfate used in the experiments is the same, the tetrathionate
IR band intensity increases in opposite order compared with the
thiosulfate.
3.4. Reaction of thiosulfate and the Prussian blue analogue
KPtII[FeIII(CN)6] complex
HCF III reacts with PtNSs and a red Prussian blue complex pro-
duces, with a maximum absorption peak around 560 nm. In order
to prove that this complex involved in the HCFIII–thiosulfate reac-
tion catalysed with PtNSs, the reactivity of this complex towards
the thiosulfate should be examined. This complex was prepared
according to the previously reported method [2] (0.2 mM PtNSs
mix with 0.5 M HCFIII for 4 h), then 2 mL of the produced red Prus-
sian blue analogue complex allow to react with 2 mL (0.1 M) thio-
sulfate and the kinetics of this reaction studied optically after the
0.80
2 mL 0.1 M Thio 2+.6 2 mL 0.5 M Ferri Pt complex
Different amounts of Thio +
0.75
2.4
+ 6 mL Water
2mL 0.5 M Ferri Pt complex
2.2
k
= 0.14702 0.01006 min-1
A
k21 = 0.09823 0.00969 min-1
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0 min
2 min
4 min
6 min
8 min
10 min
12 min
k
3 = 0.03503 0.0009 min-1
B
2
4
6
8
10 12 14 16 18 20
time (min)
500
550
600
650
700
750
800
The stretching vibration of cyano group in the HCFIII appears at
2114 cmꢀ1, while in HCFII it appears at 2038 cmꢀ1; however, the
band intensity ratio between HCFII and HCFIII was increased in
the following order 1:10, 1:5, and 1:1 for the HCFIII to thiosulfate
concentration ratio, respectively. The reason for this order is that
the rate of the reaction increases as the concentration of HCFIII in-
creases at constant thiosulfate concentration.
Wavelength (nm)
Fig. 4. (A) Absorption spectra showing the disappearance of the Prussian blue
analogue complex peak at (556 nm) after mixing with thiosulfate solution. (B) The
depletion rate of the Prussian blue analogue complex in the presence of various
concentrations of thiosulfate, as the concentration of thiosulfate increases the rate
of loss of the Prussian blue analogue complex increases.

M.A. Mahmoud / Journal of Catalysis 274 (2010) 215–220
219
3.5. Reduction of Platinum II with thiosulfate
sulfate reduction. The thiosulfate acts here as a reducing agent as
well as capping material; this result is supported by what was ob-
served from the ATR-IR measurement which is the thiosulfate ad-
sorbed on the surface of the PtNSs.
The above measurements involve the formation of Prussian
blue analogue based on Pt (II) ions. If it is the case, this means that
the PtNSs are not a catalyst but one of the reacting materials and it
keeps on dissolving with time leading to size reduction, but what
was reported is only reshaping of the platinum nanocatalyst
[4,9]. To determine the possibility of the reduction in Pt II ions
by the thiosulfate, the potentials of these ions should be studied.
Thiosulfate is well known as reducing agent with reduction po-
tential of 0.304 V while the platinum II is 0.758 V. Therefore, it is
electrochemically possible to reduce the Pt II ions to Pt metal by
thiosulfate. Fig. 6 shows the TEM image of PtNSs prepared by thio-
4. Discussion
In order to use nanoparticles more effectively to catalyse a
chemical reaction, we should understand their role in the mecha-
nism. In other words, determining if it is heterogeneous (in which
the reaction takes place on the surface of the nanoparticle catalyst)
or homogeneous (in which the reaction takes place wholly in solu-
tion). The reason for studying the mechanism is that in some cases
the small size of the nanoparticles means that they are close in size
of reactants and this question of the reaction mechanism is not
easily answered. The mechanism of the HCFIII–thiosulfate reaction
catalysed with PtNSs has been investigated using the results ob-
tained here and has been shown to be a heterogeneously catalysed
reaction.
The ATR-FTIR measurements for the thiosulfate–PtNSs mixture
showed a new vibrational band appeared at 1046 cmꢀ1 which cor-
responds to the adsorbed thiosulfate on the surface of PtNSs, and
its intensity increases as the amount of PtNSs increases. Moreover,
the in situ UV–Vis. optical detection of HCFIII–thiosulfate reaction
catalysed by PtNSs observes an optical spectrum peak at 556 nm
corresponding to the Prussian blue analogue. The UV–Vis. optical
measurement shows that the Prussian blue analogue complex, pre-
pared by mixing HCFIII with PtNSs for 40 min, reacts with thiosul-
fate and the rate of the reaction depends on the concentration of
thiosulfate. The reactivity of Prussian blue analogue towards the
thiosulfate helps to prove its role during the reaction.
The ATR-IR measurements in summary proves that the HCFIII–
thiosulfate catalysis reaction takes place through the adsorption of
thiosulfate on the surface of PtNSs catalyst and the Prussian blue
analogue forms during the reaction.
The thiosulfate has ability to form a complex with platinum II so
the Prussian blue analogue will bind to the surface of the PtNSs.
Fig. 6. TEM image of Pt nanoparticles synthesized via K2PtCl4 reduction by
thiosulfate.
-O3S
-O3S
Pt[Fe(CN)6]-
PtNSs
+ 3 K3[Fe(CN)6]
+ 2K4[Fe(CN)6]
-
SSO3
PtNSs
-
SSO3
2-
+ 2 K3[Fe(CN)6]
- S4O6
-
PtNSs
SSO3
Pt
S
-O3S
Fig. 7. Schematic diagrams for the mechanism of the reaction of hexacyanoferrate III and thiosulfate catalysed by PtNS.

220
M.A. Mahmoud / Journal of Catalysis 274 (2010) 215–220
It has been reported that the shape of the nanocatalyst changes
the Prussian blue analogue leading to the formation of tetrathio-
nate and Pt atoms which return back to the surface leading to
reshaping of the PtNS. The complete reaction is described by Eqs.
(1)–(3).
after the catalysis reaction, while the size does not change much
[4]. The TEM image in Fig. 1D and E showed a deposition of the
Prussian blue analogue complex on the surface of the nanocatalyst
after the reaction. On combining these results, one concludes that
the reshaping during the reaction is due to the movement of the
corner and edge atoms to other sites on the surface of the nanocat-
alyst. Because if the Prussian blue complex dissolves away in the
solution, the size will decrease and small nanoparticles will form.
Since no tiny particles were observed after the reaction, therefore
the complex forms on the surface the PtNSs and the Pt ion present
in the complex reduce to atomic Pt on the surface of PtNSs by the
thiosulfate adsorbed on its surface.
KPt½FeðCNÞ ꢂ þ 2S2O2ꢀ þ 2Kþ ! þK3½FeðCNÞ ꢂ þ S4O2ꢀ þ Pt
ð3Þ
6
3
6
6
Acknowledgment
I would like to thank Prof. Mostafa El-Sayed for helpful discus-
sion, Prof. Z.L. Wang for the TEM microscope imaging, and DOE
support under Grant No. (ER 46606).
The reaction takes place by the attack on the PtNSs by the HCFIII
and formation of Prussian blue analogue complex, the complex
formed on the surface of the PtNSs. The thiosulfate will reduce
platinum II in the KPtII[FeIII(CN)6] complex to Pt atom and return
it back to the surface leading to the reshaping of the particles. Ost-
wald ripening (size reduction) will take place if some KPtII[-
FeIII(CN)] is leached from the PtNSs surface [4]. This is why the
amount of adsorbed thiosulfate does not change on changing the
concentration ratio of the reacting materials, but it depends on
the concentration of thiosulfate. Pt involved in the reaction will
regenerate after oxidation because thiosulfate will reduce Pt II in
the Prussian blue complex to platinum atoms.
The above measurements and discussion summarized in Fig. 7;
two molecules of HCFIII attacks the PtNS surface and oxidize Pt
into Pt II and the HCFIII reduced to HCFII. Then Pt II forms a Prus-
sian blue analogue complex with another HCFIII molecule. The
complex binds with two thiosulfate ions on the surface of PtNSs.
Therefore, a red Prussian blue analogue forms on the catalyst sur-
face. The electron transfer takes place between the thiosulfate and
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