In summary, the novel catalyst is synthesized and characterized and it consists of clusters of
Pt atoms dispersed on a Ti0.9Mo0.1O2 nanosupport. This catalyst showed extremely high activity
for CO oxidation reaction. The chemical reactivity of our positively charged single Pt atoms.
The more vacant orbitals of the cluster of Pt atoms, because of the electron transfer from Pt
atoms to the Ti0.9Mo0.1O2 nanosupport surface, are responsible both for the strong binding and
stabilization of cluster of Pt atoms and for providing positively charged Pt atoms, which
ultimately account for the excellent catalytic activity of the cluster of Pt atoms/Ti0.9Mo0.1O2
nanosupport catalyst. Although we used CO oxidation to demonstrate the high activity of the
cluster of Pt atoms/Ti0.9Mo0.1O2 nanosupport catalyst, the observed behaviour of cluster of Pt
atoms is, nonetheless, not limited to CO oxidation. Moreover, the stabilization of cluster of Pt
atoms on practical oxide supports via the charge-transfer mechanism is not limited to the cluster
of Pt atoms/Ti0.9Mo0.1O2 nanosupport system, but can be extended further and made applicable to
other precious-metal systems. This catalyst not only proves the concept of cluster-atom
heterogeneous catalysis, but also has a great potential to reduce the high cost of commercial
noble-metal catalysts in industry.
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Journal of Science and Technology 54 (4B) (2016) 145-151
CO-OXIDATION REACTION ACTIVITY OF Pt CLUSTER
CATALYSTS ON Ti0.9MO0.1O2 SUPPORT
Trung Thanh Nguyen1, *, Bing Joe Hwang2
1An Giang University, 18 Ung Van Khiem, Long Xuyen city, An Giang province, Vietnam
2National Taiwan University of Science and Technology, 34 Kee Lung road, Taipei, Taiwan
*Email: ntthanh@agu.edu.vn
Received: 15 August 2016; Accepted for publication: 10 November 2016
ABSTRACT
Platinum-based heterogeneous catalysts are critical to many important commercial
chemical processes. However, their efficiency is extremely low on a per metal atom basis,
because only the surface active-site atoms are used. Catalysts with clusters of atom dispersions
are thus highly desirable to increase atom efficiency, but making them is challenging. Here we
report the synthesis of a catalyst that consists of isolated clusters of Pt atoms anchored to the
surfaces of Molybdenum-doped titanium oxide nanocrystallites. This cluster-atom catalyst has
extremely high atom efficiency and shows excellent stability and activity for CO oxidation. The
result showed that the highest activity and stability of catalyst with 0.2 wt.% loading of Pt are
observed. These could be due to the partially vacant 5d orbitals of the positively charged, high-
valent Pt atoms.
Keywords: cluster of Pt atoms, CO oxidation reaction, Ti0.9Mo0.1O2 nanosupport.
1. INTRODUCTION
Supported noble catalysts are the most widely used in industry due to they show high
activity and/or selectivity in the large number of different and important chemical reactions [1-
4]. Generally, the factors including size, shape, composition, oxidation state, geometry,
chemical/physical environments can play an important role in determining nanocrystal reactivity
[2]. Many experimental approaches for these nanocatalysts have focused on the fine dispersion
of noble metals on a support with a high surface area for the efficient use of catalytically active
component [1]. Therefore, in among of these factors, the size of noble metal catalysts is the
important consideration for enhancing the catalyst performances [2]. However, the catalytic
durability is a huge challenge for the smaller nanometre sized particles or clusters under the
effects of high temperature and potential etc. performances. Recently, theoretical and
experimental results demonstrated that the sub-nanometre clusters showed better catalytic
activity and/or selectivity than nanometre nanoparticles [3, 5, 6]. Low coordination, unsaturated
atoms often function as active sites [7], so downsizing the particles or clusters to single atoms is
highly desirable for catalytic reactions [1]. One highlighting example is Zhang et al. showed the
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194
single-atomic Pt catalyst on FeOx support has extremely high atom efficiency and shows that the
activity for both CO-oxidation and preferential oxidation of CO in H2.
Herein, the atomic Pt clusters including the isolated single atom and cluster-atoms were
anchored to the surfaces of Ti0.9Mo0.1O2 nanosupport by a simple hydrothermal method. The Pt
cluster catalyst shows very high activity and durability for CO oxidation. The higher activity of
the Pt cluster catalyst could be attributed to the partially vacant 5d orbital of positively charged,
high-valent Pt atoms. Therefore, the stability of catalyst with low Pt loading can be attributed to
the less effects of low CO poisioning.
2. MATERIALS AND METHODS
2.1. Experimental section
2.1.1. Hydrothermal synthesis of atomic Pt clusters on Ti0.9Mo0.1O2 nanosupport
Both sample A (with 0.2 wt.% loading of Pt) and sample B (with 2 wt. % loading of Pt)
were prepared by hydrothermal method of an aqueous solution of Chloroplatinic acid
(H2PtCl6.6H2O, 0.4 mg for sample A and 4 mg for sample B), Titanium Chloride (TiCl4, 28
mM), Molybdenum Chloride (MoCl5, 3.1 mM) and Ammonia solution (NH4OH). In the typical
preparation, the pH value of the good mixture including H2PtCl6, TiCl4 and MoCl5 precursors in
water solvent was adjusted into pH∼8 by an ammonia solution at room temperature under the
ambient condition. After 15 min for stirring, this mixture was transferred into a Teflon cylinder
reactor. Next, a sealed autoclave, which contained the reactor, was heated to 200 oC in an oven
and kept at this temperature for 2 hrs. The precipitate of Pt/Ti0.9Mo0.1O2 samples were collected
after the performances such as washing with deionized (DI) water and centrifugations several
times. The powders were obtained after drying in a vacuum oven at 80 oC overnight and calcined
at 400 oC. Prior to being characterized and tested, the samples were reduced in 10 vol. % H2/He
at 200 oC for 0.5 hrs. The actually Pt loadings in the two samples were determined by
inductively coupled plasma spectroscopy were 1.7 wt. % for sample A and 2.2 wt. % for sample
B. Noted that the procedure for Pt/Ti0.9Mo0.1O2 sample synthesis is developed from the
procedure for Ti0.9Mo0.1O2 synthesis. Please see the synthesis procedure and characterizations of
Ti0.9Mo0.1O2 nanomaterial in our previous work [8, 9].
2.1.2. Characterizations and CO-oxidation activity testing
High resolution-Transition Electron Microscopy (HR-TEM). These measurements were
obtained by using a Philips/FEI Tecnai 20G2 S-Twin TEM apparatus. Prior to these
measurements, the sample solution was prepared by suspending the catalysts in absolute ethanol
solution with ultrasonication. A drop of the specimen was applied onto the carbon-supported
copper grid, which was placed in a vacuum oven overnight at 25 oC.
X-ray absorption spectroscopy (XAS) measurements. Pt LIII-edge X-ray absorption
spectra (EXAFS) were performed on BL17C1 in National Synchrotron Radiation Research
Center (NSRRC), Taiwan, operated at ∼ 2.2 GeV with injection currents of 200 mA. The Pt foil
was used as reference sample and both of Pt foil measured in the transmission mode; both
sample A and B were measured in the fluorescence mode with the in-situ system. Before the
measurements, the sample A and B were reduced at 200 oC for 30 minutes under a mixing 10
vol. % H2/He environment (flow rate of 40 mL.min-1). Pt LIII-edge measurements data collection
CO-oxidation reaction activity of Pt cluster catalysts on Ti0.9MO0.1O2 support
195
modes, and calculation of errors were all done as per the guidelines set by the International
XAFS Society Standards and Criteria Committee [8, 10, 11, 12].
CO-oxidation reaction. The catalytic performance of samples A and B for CO-oxidation
reaction was evaluated in a fixed-bed reactor with an in-situ system. Approximated 80 mg of the
sample was loaded into a U-shaped quartz reactor, and then was reduced in-situ with 10 vol. %
H2/He at 200 oC for 0.5 hrs. For observing the effect of reaction temperature to CO-conversion
percentage, after cooling to 30 oC, the feed gas containing 1 vol. % CO, 1 vol. % O2 and balance
He was allowed to pass through the reactor at a flow of 25 mL.min-1 (corresponding to a space
velocity of 18,750 mL.h-1.gcat-1). The effluent gas composition (including before and after CO
reaction performances) was analyzed by a gas chromatograph (HP-6000) in the in-situ system.
The CO-conversion percentages were calculated by a comparison of CO-peak areas before and
after the gas flow pass through the catalyst bed.
3. RESULTS AND DISCUSSION
3.1. Synthesis of Pt atom clusters on Ti0.9Mo0.1O2 nanosupports
Generally, in the reported work, the Pt single atom or sub-nanometre cluster on different
supports were synthesized by co-precipitation method and used as hetero-structural catalysts
with extremely high activity for CO-oxidation reaction [1]. As a similarity, the Pt atom clusters
including single atom and bunch of atoms on Ti0.9Mo0.1O2 nanosupports were synthesized by a
simple hydrothermal method. The detail of this synthesis is described in the experimental
section. In brief, the Pt ions are adsorbed onto the Ti0.9Mo0.1O2 surface by replacing the proton
(hydrogen ions) of -OH groups on this surface (as the Scheme 1), and the Pt single atom/cluster
is caught on the oxide support by linking with oxygen. In this research, two levels of Pt4+ ion
concentration are 0.2 weight percent (wt. %) (denoted as sample A, with a Pt/(Ti + Mo) atomic
ratio of 1/1591, and a real Pt loading of 0.17 wt. %); 2 wt% (denoted as sample B, with a Pt/(Ti
+ Mo) atomic ratio of 1/159, and a real Pt loading of 2.2 wt. %). Both of samples were
synthesized, characterized, and tested.
Scheme 1. An adsorption mechanism of Pt4+ ion onto the Ti0.9Mo0.1O2 support surface to form the Pt
single atom on the Ti0.9Mo0.1O2 nanosupport.
3.2. Characterizations of Pt/ Ti0.9Mo0.1O2 nanocrystals.
3.2.1. Electron microscopy
The normal TEM machine was used to characterize the samples (see the experimental
conditions). Figure 1 shows TEM and Mapping-TEM images of the as-prepared 0.2 wt. %
atomic Pt clusters/Ti0.9Mo0.1O2 nanocrystals. The Pt clusters are observed and circled.
Trung Thanh Nguyen, Bing Joe Hwang
196
3.2.2. X-ray absorption fine structure studies
The results of electronic diffraction X-ray (EDX)-TEM and X-ray diffraction (XRD) results
did not show any Pt-presence in both of these samples due to the insensitivity of XRD
measurements to small clusters. However, it was known that the X-ray absorption spectroscopy
(XAS) measurements have already used to study the electronic properties of the subnanometre
and even single atom of Pt clusters on FeOx support [1]. Hence, the XAS measurements were
also used to examine the electronic properties of these atomic Pt clusters on Ti0.9Mo0.1O2
nanosupports. The XAS results were measured on the freshly reduced Pt catalyst and the K3-
weight Fourier transform spectra from EXAFS depicted after the normalizing with Pt mass in
the Figure 2. The data show a decreasing trend in the white-line intensities: 0.2 wt. %
Pt/Ti0.9Mo0.1O2 > 1 wt. % Pt/Ti0.9Mo0.1O2 > 2 wt. % Pt/Ti0.9Mo0.1O2, which indicates that the Pt
clusters in the catalyst A carry postive charges. (a.u. = arbitrary units).
Figure 1. TEM images of 0.2 wt. % atomic Pt clusters/Ti0.9Mo0.1O2 nanosupports.
Figure 2. The normalized XANES spectra at the Pt LIII edge of sample A and B and Pt foil.
CO-oxidation reaction activity of Pt cluster catalysts on Ti0.9MO0.1O2 support
197
3.2.3. Catalytic performance for CO-oxidation reactions
Figure 3. (A) CO-gas conversion, (B) Specific rate at 90 oC of Pt/Ti0.9Mo0.1O2 catalysts and (C) Catalytic
CO-gas oxidation stability of 0.2 wt. % Pt/ Ti0.9Mo0.1O2 catalyst at 180 oC.
The CO-oxidation performances of Pt catalysts on Ti0.9Mo0.1O2 nanosupport are showed
in Figure 3. It is noted that the homemade catalysts were pressed and sieved to enhance the
stability under the effect of high gas flow rate in the U-reactor. As shown in the Figure 3A, the
activated Pt/ Ti0.9Mo0.1O2 catalysts show very high activity for CO-oxidation reaction. The
temperature of totally CO-conversion of the sample B (2 wt. % Pt/Ti0.9Mo0.1O2 catalyst) is
lower than that of the sample A (0.2 wt. % Pt/Ti0.9Mo0.1O2 catalyst). For example, the totally
CO-conversion temperature for sample B, and sample A is 120 oC and 180 oC, respectively.
Additionally, the differences are observed for the conversion curves of two catalysts toward CO-
oxidation. For 2 wt. % Pt/Ti0.9Mo0.1O2 catalyst, the increasing step of CO-conversion curve is
so large. For 0.2 wt. % Pt/Ti0.9Mo0.1O2 catalyst, however, this observation is opposite. Herein,
at the reaction temperature less than 100 oC, these catalysts show the ability for CO-conversion
(see Fig. 3A). It is an important issue to applicate this catalyst in the CO contaminant cases with
low concentration. Interestingly, at 90 oC, the specific rate of catalyst A is observed to be higher
(~2.7-folds) than that of catalyst B (see Fig. 3B and Table 1 more detail). This says that the rate
of CO conversion of catalyst A is faster than that of catalyst B.
Figure 4. Mechanism of CO-oxidation reaction on Ti0.9Mo0.1O2 nanosupport with (A) single atomic Pt
catalyst model and (B) cluster Pt catalyst model. (1) CO adsorption on the Pt sites; (2) O2 adsorption on
oxygen vacancy sites; (3) Oxygen atom generation; (4) the reaction of oxygen atom with CO ads species
on the Pt surfaces; and (5) CO2 desorption from the Pt surfaces.
The Figure 4 could be used to explain these issues. For high CO oxidation activity at less
100 oC are observed for both of catalyst A and B, this can be due to oxygen vacancies are
contained on the surface of Ti0.9Mo0.1O2 nanoparticles and positive charge on Pt particle
surface. It is well known that the oxidation reaction is strongly supported from oxygen vacancies
Trung Thanh Nguyen, Bing Joe Hwang
198
[8, 9]. For the large of increasing step of CO-conversion curve, it could be effect of thermal
conduction from Pt sites. For the higher activity of 0.2 wt. % Pt/Ti0.9Mo0.1O2 catalyst, it can
be due to higher positive charged on Pt particle surface and high-valent Pt atoms [1].
3.2.4. Stability of 0.2 wt.% Pt/ Ti0.9Mo0.1O2 catalyst
Table 1. Effect of particle size of Pt catalyst on Ti0.9Mo0.1O2 nanosupport to CO-oxidation activity.
Sample Real Pt loading
(wt %)
CO-gas oxidation reaction
Specific rate (molCO h-1 gPt-1)
At 90 oC
CO conversion Initial-After 3 hrs
At 180 oC (%)
Sample A 0.17 4.563 98.4 - 98.2
Sample B 2.20 1.685 -
The stability test of 0.2 wt. % Pt/Ti0.9Mo0.1O2 catalyst is performed at 180 oC and the result
is showed in the Fig. 3C. The stability of catalyst is determined by comparing the CO-
conversion coefficients at the initial time and after 3 hrs for CO oxidation reaction. They have
not show any effect on the catalytic activity and catalyst A is good stability in the CO oxidation
reaction at 180 oC. It is implied that sintering of Pt clusters on nanosupport does not occur during
the catalytic reactions.
4. CONCLUSIONS
In summary, the novel catalyst is synthesized and characterized and it consists of clusters of
Pt atoms dispersed on a Ti0.9Mo0.1O2 nanosupport. This catalyst showed extremely high activity
for CO oxidation reaction. The chemical reactivity of our positively charged single Pt atoms.
The more vacant orbitals of the cluster of Pt atoms, because of the electron transfer from Pt
atoms to the Ti0.9Mo0.1O2 nanosupport surface, are responsible both for the strong binding and
stabilization of cluster of Pt atoms and for providing positively charged Pt atoms, which
ultimately account for the excellent catalytic activity of the cluster of Pt atoms/Ti0.9Mo0.1O2
nanosupport catalyst. Although we used CO oxidation to demonstrate the high activity of the
cluster of Pt atoms/Ti0.9Mo0.1O2 nanosupport catalyst, the observed behaviour of cluster of Pt
atoms is, nonetheless, not limited to CO oxidation. Moreover, the stabilization of cluster of Pt
atoms on practical oxide supports via the charge-transfer mechanism is not limited to the cluster
of Pt atoms/Ti0.9Mo0.1O2 nanosupport system, but can be extended further and made applicable to
other precious-metal systems. This catalyst not only proves the concept of cluster-atom
heterogeneous catalysis, but also has a great potential to reduce the high cost of commercial
noble-metal catalysts in industry.
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