Characteristics and catalytic behavior of Pd catalysts supported on nanostructure titanate in liquid-phase hydrogenation

Article abstract of DOI:10.1166/jnn.2013.7403

Titanate nanowire (TNW) and nanotube (TNT) structures were synthesized by the hydrothermal reaction using spherical shape anatase TiO₂ nanoparticles (TNP) as the starting material and employed as Pd catalyst supports for the liquid-phase selective hydrogenation of 1-heptyne to 1-heptene. Pd dispersion was significantly improved as the specific surface area of the supports increased in the order: Pd/TNT > Pd/TNW > Pd/TNP. While the hydrogenation rate increased with increasing number of active Pd⁰ surface, the selectivity to 1-heptene depended largely on the degree of interaction between Pd and the supports. The catalysts prepared by impregnation method led to a stronger metal-support interaction than those prepared by colloidal route. The selectivity of 1-heptene at complete conversion of 1-heptyne was obtained in the order: I-Pd/TNT > I-Pd/TNP > Pd/TNT ≈ Pd/TNW > Pd/TNP. Copyright

Full text of DOI:10.1166/jnn.2013.7403

Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 3062–3067, 2013
Characteristics and Catalytic Behavior of
Pd Catalysts Supported on Nanostructure
Titanate in Liquid-Phase Hydrogenation
1
2
Somjit Putdee , Okorn Mekasuwandumrong ,
1
1ꢀ∗
Apinan Soottitantawat , and Joongjai Panpranot
1Faculty of Engineering, Center of Excellence on Catalysis and Catalytic Reaction Engineering,
Department of Chemical Engineering, Chulalongkorn University, Bangkok 10330, Thailand
2
Faculty of Engineering and Industrial Technology, Department of Chemical Engineering,
Silpakorn University, Nakorn Pathom 73000, Thailand
Titanate nanowire (TNW) and nanotube (TNT) structures were synthesized by the hydrothermal
reaction using spherical shape anatase TiO2 nanoparticles (TNP) as the starting material and
employed as Pd catalyst supports for the liquid-phase selective hydrogenation of 1-heptyne to
1
-heptene. Pd dispersion was significantly improved as the specific surface area of the supports
increased in the order: Pd/TNT > Pd/TNW ꢀ Pd/TNP. While the hydrogenation rate increased with
0
increasing number of active Pd surface, the selectivity to 1-heptene depended largely on the degree
of interaction between Pd and the supports. The catalysts prepared by impregnation method led
to a stronger metal-support interaction than those prepared by colloidal route. The selectivity of
Delivered by Ingenta to: Chinese University of Hong Kong
1
-heptene at complete conversion of 1-heptyne was obtained in the order: I-Pd/TNT > I-Pd/TNP >
IP: 5.62.159.194 On: Mon, 16 May 2016 10:57:40
Pd/TNT ≈ Pd/TNW > Pd/TNP.
Copyright: American Scientific Publishers
Keywords: Titanate Nanotube, Nanowire, Selective Hydrogenation, Pd Catalysts.
1
. INTRODUCTION
double bond migration reaction, phenol hydrogenation,
and selective hydrogenation of o-chloronitrobenzene.
7
–11
Mesoporous materials especially those with metal-organic
frameworks have high potential applications in the fields
of gas storage, separations, and catalysis. Nanostructure
titanate has recently received considerable attention as both
photocatalyst and catalyst support due to its elongated mor-
The use of nanotubular titanate has often shown to result
in better metal dispersion and improved catalytic perfor-
mances. In particular cases, the metal-support interaction
was altered and charge transfer from the titanate nanotubes
1
–4
10ꢀ11
to metal particles were observed.
5
phology and high surface area. Typically, there are three
The selective alkyne hydrogenation is an important reac-
tion for the manufacturing of fine chemicals and biologi-
main methods for preparation of the titanate nanotubes:
template method, anodic oxidation, and hydrothermal
method. Among these methods, hydrothermal treatment
12
cally activity compounds. Pd-based catalysts have been
widely used in both laboratory and industry because they
exhibit high selectivity with respect to the semihydrogena-
tion of alkyne to alkene. A number of previous studies
have shown that a strong metal-support interaction (SMSI)
of TiO in base is probably the simplest and the low-
2
est cost method to generate nanotubular structure. Crys-
talline phase and morphology of the resulting materials are
strongly dependent on the synthesis conditions such as
effect that occurred on Pd/TiO catalysts is beneficial for
2
6
aging time, temperature, and base concentration. Titanate
improving the catalytic performance in the liquid-phase
1
3–14
nanotubes synthesized by this route have been employed
as catalyst supports for various metals (i.e., Au, Cu, Pt,
Ru, Pd) in many catalytic reactions such as low tem-
perature CO oxidation, ammonia decomposition, selective
selective hydrogenation reactions.
catalytic hydrogenation activity has also been reported for
A remarkable high
1
5–17
Pd nanoparticles encapsulated in titania matrix.
In the present work, the characteristics and cat-
alytic properties of Pd nanoparticles supported on TiO2
nanoparticles (Pd/TNP), titanate nanotubes (Pd/TNT),
and titanate nanowires (Pd/TNW) were investigated and
reduction of NO by NH , cinnamaldehyde hydrogenation,
3
∗
Author to whom correspondence should be addressed.
3
062
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 4
1533-4880/2013/13/3062/006
doi:10.1166/jnn.2013.7403

Putdee et al.
Characteristics and Catalytic Behavior of Pd Catalysts Supported on Nanostructure Titanate
compared in the liquid-phase selective hydrogenation of
-heptyne under mild reaction conditions. The cata-
lysts were characterized by X-ray diffraction (XRD),
N2 physisorption, transmission electron microscopy
using Micromeritics ChemiSorb 2750 and ASAP 2101C
V.3.00 software. XPS analysis was performed originally
using an AMICUS spectrometer equipped with a MgKꢁ
X-ray radiation. For a typical analysis, the source was
operated at the voltage of 15 kV and current of 12 mA. The
pressure in the analysis chamber was less than 10 Pa.
The AMICUS system is computer controlled using the
AMICUS “VISION 2” software.
1
(TEM), CO pulse chemisorption, and X-ray photoelectron
−6
spectroscopy (XPS).
2. EXPERIMENTAL DETAILS
2
.1. Preparation of Titanate Supports
2.4. Catalytic Reaction
Preparation of the nanostructured titanates was carried out
The selective hydrogenation of 1-heptyne (Aldrich) was
carried out in a magnetically stirred 50 cm stainless steel
3
by the hydrothermal method using anatase TiO (Sigma-
2
Aldrich) as the starting material. First, 1.5 g of TiO pow-
autoclave reactor (JASCO, Tokyo, Japan). Prior to the
reaction testing, the catalyst sample was reduced by hydro-
2
der was suspended with 40 ml of 10 M NaOH and then the
mixture was sonicated for 10 min. Thereafter the mixture
was transferred to a Teflon-lined stanless steel autoclave,
3
ꢁ
gen gas at a flow rate of 50 cm /min at 40 C for 2 h.
The reactant containing 0.2 ml of 1-heptyne and 9.8 ml of
toluene were mixed in the volumetric flask. Next, the mix-
ture and 20 mg of catalyst were introduced into the auto-
clave reactor. The reaction was carried out under hydrogen
and heated at various reaction temperatures ranging from
ꢁ
1
10 to 200 C for 24 h in an oven. After hydrothermal
reaction, the autoclave was cooled to room temperature by
natural cooled down. Then the sample was washed with
ꢁ
atmosphere at 1 atm and 30 C. After the reaction, the vent
0
.1 M HCl for several time and followed by deionized
valve was slowly opened to prevent the loss of product.
Then the product was analyzed by gas chromatography
with flame ionization detector (FID).
water until pH value approached deionized water. Finally,
the sample was dried at 110 C for 24 h.
ꢁ
2
.2. Preparation of Titania and Titanate
Supported Pd Catalysts
3
. RESULTS AND DISCUSSION
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3.1. Characterization of the Supports
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The Pd nanoparticles supported on TNP, TNT, and TNW
Copyright: American Scientific Publishers
were prepared using wet impregnation of colloidal Pd
nanoparticles which were synthesized by the reduction
by solvent method according to that described in our
previous study. An appropriate volume of Pd nano-
particles/methanol dispersion was mixed with the support
to obtain a final loading of ca. 1 wt% Pd. The mixture
was gently stirred at room temperature for 3 days and
Figure 1 shows the XRD patterns of the starting anatase
TiO nanoparticles and the samples obtained by hydrother-
mal treatment at different reaction temperatures. The XRD
pattern of the products obtained from hydrothermal syn-
thesis at 110 C remained anatase phase TiO
2
1
8
ꢁ
. When the
2
ꢁ
ꢁ
reaction temperature increased to 150 C and 200 C,
the XRD characteristic peaks of titanate were observed
ꢁ
ꢁ ꢁ
at 2ꢂ degrees 24.5 and 48.5 which corresponded to the
heated at 60 C in order to remove the methanol solution.
The collected solid was washed with ethanol and water
1
9
reflection (100) and (200) plane of H-titanate. The TEM
micrographs of various samples are shown in Figure 2. The
(50/50% V/V) for several times and dried overnight. For
comparison purposes, TNT supported Pd catalyst was also
prepared by the conventional impregnation using PdCl2
precursor. After impregnation, the catalyst was dried at
morphology of TiO commercial is spherical with diam-
2
eter larger than 100 nm. After hydrothermal treatment at
ꢁ
110 C, some tubes like structure and sheet were observed.
ꢁ
ꢁ
1
10 C overnight followed by calcination in air at 450 C
All the sheets transformed into tubular structure titanate
ꢁ
for 3 h and was denoted as I-Pd/TNT.
at 150 C. The tubular materials were several hundreds of
nanometers which outer and inner diameter of nanotubes
were approximately 10–12 nm and 4–6 nm, respectively.
Increasing temperature to 200 C led to a transformation
2
.3. Catalyst Characterization
ꢁ
XRD patterns were determined using a Siemens D5000
of nanotubes to nanowire with non-hollow morphology
with a length of several micrometers and width variations
between 60–100 nm.
using nickel filtered CuK radiation. The morphology and
ꢁ
particle size were obtained using a Hitachi s-3400N scan-
ning electron microscope and JEOL-JEM 2010 transmis-
sion electron microscope. Specific surface area of sample
was measured by nitrogen gas adsorption at liquid nitrogen
Specific surface areas of the samples were deter-
mined from the N physisorption data using Brunauer–
2
Emmett–Teller (BET) technique. The BET surface area
ꢁ
2
temperature (−196 C) using a Micromeritics ChemiSorb
of TiO2 commercial as starting material was 5 m /g.
ꢁ
2
750 system. The amount of active sites of catalysts were
After hydrothermal treatment at 110 C resulted in an
2
determined by carbon monoxide chemisorption technique
increased BET surface area to 158 m /g. Further increasing
J. Nanosci. Nanotechnol. 13, 3062–3067, 2013
3063

Characteristics and Catalytic Behavior of Pd Catalysts Supported on Nanostructure Titanate
Putdee et al.
that palladium was highly dispersed on the supports or
2
0
it was due to the low amount of Pd loaded. The XRD
patterns of titania and titanate supported Pd nanoparticles
catalysts prepared by solvent reduction method were not
significantly different from the as-synthesized support. On
the other hand, impregnation of Pd chloride on TNT sup-
port resulted in the disappearance of titanate peaks and
exhibited the characteristic peaks of rutile phase of TiO2
at 2ꢂ = 27ꢃ6 (major), 36.3, 41.5, 44.3, 55.5, 56.9, 63.3,
ꢁ
21
6
4.6, and 69.4 . The phase transformation of titanate to
rutile TiO was due to the presence of acid precursor of
2
palladium chloride and calcination at high temperature.
2
2
According to Murakami et al. TNT could transform into
−
rutile TiO in the presence of Cl during hydrothermal
2
1
7
treatment. Zhu et al. also reported the transformation of
TNT to rutile TiO in acidic environment at 60 C (2.65 M
ꢁ
2
of HNO ꢄ. Normally phase transformation of TNT to rutile
3
ꢁ
23–24
Fig. 1. XRD patterns of the starting TNP and the samples obtained by
TiO occurs after annealing at above 900 C.
2
hydrothermal treatment at different reaction temperatures.
The dispersion of Pd on the different titania and titanate
supports was observed by TEM and the results are dis-
played in Figure 3. It appears that the particle size and
shape of the catalysts prepared via reduction by sol-
vent method were not affected by loading of palladium
(no changes in the particle size and shape). The smaller
dark spots in catalyst represented the Pd particles. Cat-
alysts prepared by the colloidal method showed average
particle sizes of Pd about 4–5 nm. Larger Pd particle sizes
ꢁ
hydrothermal temperature to 150 C, the BET surface
2
area increased to 192 m /g. Increasing of BET surface
areas was due to the transformation of morphology from
nanosheet to nanotubes with hollow structure that pos-
sessed higher surface area. The sample synthesized at
ꢁ
2
2
00 C resulted in a low BET surface area (23.2 m /g) due
to non-hollow structure of the TNW.
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were observed on the catalysts prepared by impregnation
IP: 5.62.159.194 On: Mon, 16 May 2016 10:57:40
method. In addition, morphologies of the catalysts were
Copyright: American Scientific Publishers
.2. Characterization of Supported Pd Catalysts
3
markedly changed after impregnation and calcination. The
presence of acid precursor of palladium chloride and cal-
cination at high temperature resulted in the collapse of
nanotubes structure and as a consequence the large rods
and non-hollow structures were obtained.
The XRD patterns of Pd/TNP, Pd/TNT, Pd/TNW, and
I-Pd/TNT catalysts were collected at 2ꢂ degrees 10–80
ꢁ
0
(results not shown), however, the diffraction peak of Pd or
PdO were not observed for all the catalysts. It is suggested
Table I summarizes the physicochemical properties of
supported Pd catalysts. The BET surface area of solvent
Fig. 2. TEM micrographs of the starting TNP and the samples obtained
by hydrothermal treatment at different reaction temperatures.
Fig. 3. TEM images of the various supported Pd catalysts.
J. Nanosci. Nanotechnol. 13, 3062–3067, 2013
3
064

Putdee et al.
Characteristics and Catalytic Behavior of Pd Catalysts Supported on Nanostructure Titanate
Table I. Physicochemical properties of the TNP, TNT, and TNW supported Pd catalysts.
BET surface
area (m /g)
Binding energy
Pd 3d5/2 (eV)
CO chemisorption
(×10−18 molecule CO/gcat.)
2
a
b
0
c
Catalysts
Pd dispersion (%)
dP Pd (nm)
d
Pd/TNP
Pd/TNT
Pd/TNW
I-Pd/TNT
4.4 (4.8)
n.d.
335.1
n.d.
nꢃdꢃ
11ꢃ2
7ꢃ1
n.d.
35.7
26.1
14.4
n.d.
3.1
4.3
7.8
190.7 (192)
19.8 (23.2)
22.1 (192)
337.4
10ꢃ4
a
b
c
Notes: Based on XPS results; Based on total amount of palladium loading; Based on d = 1ꢃ12/D (nm), where D = fractional metal dispersion.
reduction-made catalysts slightly decreased after deposi-
tion process. This was suggested that Pd were deposited
in the pores of supports. The impregnation catalysts pos-
sessed low BET surface area due to the transformation of
of 1-heptyne conversion and 1-heptene selectivity as
a function of reaction time are shown in Figures 4
and 5, respectively. The hydrogenation rates were in the
order: Pd/TNT > Pd/TNW > Pd/TNP ≈ I-Pd/TNT ≈ I-Pd/
TNP. The conversion of 1-heptyne was completed in
30 min for Pd/TNT and Pd/TNW and 40 min for Pd/TNP,
I-Pd/TNT, and I-Pd/TNP under the conditions used.
Alkyne hydrogenation has often been regarded as structure
insensitive reaction, in which hydrogenation rate increases
with increasing number of exposed Pd surface. The results
follow the trend in the literature that high surface area
titanate nanotubes to rutile TiO during calcination step.
2
The rutile TiO exhibited a rod-like shape that can be
2
clearly observed from TEM images. The actual amount of
Pd loading was determined by ICP-OES. For the catalysts
prepared with Pd colloid nanoparticles, the Pd loading tar-
get 1 wt% was not achieved and the palladium contents
were in the range of 0.48–0.57 wt%, indicating some metal
loss during the synthesis, probably during the washing
step. However, Pd loading 1 wt% was achieved by the
impregnation method as expected.
The relative amounts of surface Pd atoms on the cata-
lyst samples were calculated from the CO chemisorption
experiments at room temperature. The calculation based
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on the assumption that one carbon monoxide molecule
IP: 5.62.159.194 On: Mon, 16 May 2016 10:57:40
adsorbs on one palladium site.25 The amounts of CO
Copyright: American Scientific Publishers
chemisorption on the catalysts, the percentages of Pd dis-
persion, and the average Pd metal particle sizes calculated
based on the CO chemisorption results are also given in
Table I. The active sites of catalysts varied from 7.1 to
1
8
1
1ꢃ21 × 10 site/g-catalyst in the order: Pd/TNT > I-Pd/
TNT > Pd/TNW. The amount of CO chemisorption could
not be determined on the Pd/TNP due probably to the
amount of Pd active site was lower than the detectability
limit of the instrument. The highest Pd active site and dis-
persion was found on the Pd/TNT. The high surface area
Fig. 4. The conversion of 1-heptyne as a function of reaction time
2
6–27
of titanate nanotubes promoted dispersion of Pd metal.
The Pd dispersion of catalysts prepared via reduction by
solvent was also higher than that obtained by impregnation
method.
XPS was performed to determine the oxidation state of
Pd on the catalysts and the binding energies of Pd 3d5/2
are reported in Table I. For the Pd/TNT, the binding ener-
gies of Pd 3d5/2 were observed at 335.1 eV, which corre-
0
6–8
sponded to metallic Pd . The distinctive peaks for Pd
d5/2 of impregnation-made catalyst appeared at 337.4 eV,
corresponding to the formation of PdO by calcination.
3
3
.3. Catalyst Evaluation
The catalytic performance of catalysts was investigated
in the liquid-phase selective hydrogenation of 1-heptyne
to 1-heptene under mild reaction conditions. The plots
Fig. 5. The selectivity of 1-heptene as a function of reaction time
J. Nanosci. Nanotechnol. 13, 3062–3067, 2013
3065

Characteristics and Catalytic Behavior of Pd Catalysts Supported on Nanostructure Titanate
Putdee et al.
and porosity of nanotubular catalysts were advantageous
for enhancing catalytic activity in various hydrogenation
reactions.10 However, it is interesting to note that the
I-Pd/TNT exhibited relatively high Pd dispersion (com-
pared to Pd/TNP) but it exhibited poor hydrogenation
activity in 1-heptyne hydrogenation. From the XPS results,
Pd was in the form of PdO in the I-Pd/TNT sample after
Pd catalyst supports and tested in the selective hydrogena-
tion of 1-heptyne to 1-heptene. Loading of Pd by the col-
loidal route resulted in well dispersion of metallic Pd and
0
the degree of dispersion increased in the order: Pd/TNT >
Pd/TNW ꢀ Pd/TNP. In contrast, conventional impregna-
tion of PdCl solution on TNT resulted in the formation
2
of PdO and changes of the TNT structure upon calcination
at high temperature in acidic environment. The improved
1-heptene selectivity over the I-Pd/TNT and I-Pd/TNP was
attributed to the stronger metal-support interaction exerted
calcination at high temperature. Although, all the catalysts
ꢁ
were reduced in H at 40 C for 2 h prior to the reaction
2
test, it is possible that the PdO was not completely reduced
0
or some of the metallic Pd was transformed back into
by impregnation of PdCl inside the pores of the supports.
2
PdO due to the easy oxidation of Pd upon contact with
There were no changes in the electronic properties of the
various supported Pd catalysts when Pd was deposited on
the supports by colloidal route.
2
8
air at room temperature. In addition, the metal-support
interaction is usually stronger when pore size of the sup-
2
9
port was very small. This could also be the case for the
I-Pd/TNT in this study.
Acknowledgments: Financial supports from the
Thailand Research Fund (TRF), the Office of Higher
Education Commission (CHE), and the National Research
Council of Thailand under the Joint Research Program
(NRCT-JSPS) are gratefully acknowledged.
The selectivity of 1-heptene after complete conversion
of 1-heptyne was in the order: I-Pd/TNT > I-Pd/TNP >
Pd/TNT ≈ Pd/TNW > Pd/TNP. The Pd/TNT and Pd/TNW
0
contained metallic Pd but differed in the degree of Pd
dispersion. However, they exhibited similar selectivity of
1
-heptene. In other words, the changes in Pd dispersion
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Received: 20 October 2012. Accepted: 28 November 2012.
Delivered by Ingenta to: Chinese University of Hong Kong
IP: 5.62.159.194 On: Mon, 16 May 2016 10:57:40
Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 13, 3062–3067, 2013
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