Palladium nanoparticles supported on CNT functionalized by rare-earth oxides for solvent-free aerobic oxidation of benzyl alcohol

Article abstract of DOI:10.1016/j.cattod.2015.07.021

A series of rare-earth oxide functionalized multi-walled carbon nanotubes (CNTs) were prepared through wet impregnation process. Palladium nanoparticles were deposited over surfaces of functionalized CNT through metal ion adsorption-reduction method and w

Full text of DOI:10.1016/j.cattod.2015.07.021

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Catalysis Today
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Palladium nanoparticles supported on CNT functionalized by
rare-earth oxides for solvent-free aerobic oxidation of benzyl alcohol
Yibo Yan, Xinli Jia, Yanhui Yang∗
School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of rare-earth oxide functionalized multi-walled carbon nanotubes (CNTs) were prepared through
wet impregnation process. Palladium nanoparticles were deposited over surfaces of functionalized CNT
through metal ion adsorption–reduction method and were examined using X-ray diffraction, trans-
mission electron microscopy, in situ adsorbed pyridine Fourier Transform Infrared spectroscopy, and
electrochemical measurements. Compared to the non-functionalized Pd/CNT catalyst, the catalytic activ-
ities of different rare-earth oxide functionalized CNT supported Pd catalysts were improved for aerobic
solvent-free oxidation of benzyl alcohol due to the fine-tuned properties of catalytic active sites, e.g., Pd
particle size, size distribution, valence status, metal–support interactions, electron density, surface acid-
ity and basicity. Sm2O3 functionalization displayed the highest improvement in catalytic activity whereas
the content of Sm2O3 had impact on Pd particle size, electrochemical surface area, and metal–support
interactions which further influenced the benzyl alcohol conversion and selectivity toward benzyl alde-
hyde. The catalyst functionalized by an appropriate amount of Sm2O3 afforded a remarkably high turnover
frequency of 318,760 h−1 for aerobic oxidation of benzyl alcohol based upon palladium electrochemical
Received 18 September 2014
Received in revised form 22 July 2015
Accepted 24 July 2015
Available online xxx
Keywords:
Rare-earth oxides
CNT surface functionalization
Supported Pd catalyst
Electrochemical measurement
Aerobic oxidation of benzyl alcohol
active surface areas.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Catalyst support, such as metal oxides [17] and carbonaceous
materials [18,19], play a pivotal role in the catalytic reactions.
Rare-earth elements have demonstrated a broad range of
applications in catalysis. Rare-earth oxides-based heterogeneous
catalysts have been applied in various organic chemistry reactions
such as the redox, aldolization, alcohol dehydration, ketone forma-
tion, aromatic compounds alkylation, ring-open, coupling reactions
and so forth due to the redox and acid–base properties [1,2].
Numerous cationic rare-earth metal complexes have been synthe-
sized to perform as the catalysts for olefin polymerization [3,4].
Furthermore, rare-earth oxides also attracted various curiosities as
a structural and electronic promoter to improve the catalyst perfor-
mance over conventional supports such as silica, alumina, titania
[5–10]. Well known for their lattice oxygen storage capability, rare-
earth oxides have been applied into automotive catalysts [11,12],
improved catalyst for soot oxidation [13], and industrial catalysts
for removal of organics from waste water [14,15]. Owing to the
improved basic strength and their contribution to the reducibility
increase, rare-earth oxides have highly promoted the performance
after their modifying Pt/SiO2 catalysts for CO oxidation [16].
Carbon nanotubes (CNTs) are among the headlines of nanotechnol-
ogy, particularly catalysis [20], due to their high purity eradicating
self-poisoning, thermal stability, electrical conductivity, impres-
sive mechanical properties, high accessibility of the active phase
and absence of micro-porosity thus reducing the mass diffusion
and intra-particle transfer effects in reaction medium [20–23]. As
for catalysis, the metal oxide functionalities contribute to the con-
trol of the properties such as the electron conductivity, polarity,
hydrophobicity and surface chemistry including amphoteric prop-
erty in order to develop the CNT-based catalysts with enhanced
catalytic performance for certain reactions. Zhou et al. reported
that the manganese dioxide functionalized CNTs supported plat-
inum catalysts possessed higher electrochemical active surface
area and better methanol electro-oxidation activity than Pt/CNTs
[24].
CNT as catalyst support has been extensively explored [25,26].
Nevertheless, reports related to rare-earth oxides as surface func-
tionalities of CNT-supported catalysts are limited. Hence, this
study is aimed to prepare rare-earth oxides LnOx (Ln = La, Ce,
Sm, Gd, Er, Yb) functionalized CNTs supported palladium catalysts
considering the possible surface chemistry and electron property
modification that the rare-earth oxides would bring to the catalyst.
∗
Corresponding author.
E-mail address: yhyang@ntu.edu.sg (Y. Yang).
http://dx.doi.org/10.1016/j.cattod.2015.07.021
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Yan, et al., Palladium nanoparticles supported on CNT functionalized by rare-earth oxides for
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bromide (KBr) pellets (4000–450 cm−1, resolution of 1 cm−1). Sam-
ples were pressed with KBr into pellets and placed into an in situ IR
cell. Prior to measurements, each sample was dehydrated at 250 ◦C
for 1 h afterward introducing pyridine vapor flow at 50 ◦C till sat-
uration. In situ IR cell was purged by helium for 30 min at a series
of step-like temperatures, 50, 100, 150, and 200 ◦C. Spectra were
recorded at room temperature after strip of background.
Cyclic voltammetry (CV) and CO stripping voltammetry were
performed in a 1.0 M KOH solution at 50 mV s−1 using the Versa
STAT 3 Potentiostat/Galvanostat (Princeton Applied Research). Cat-
alyst ink was prepared by ultrasonically suspending 2 mg of catalyst
into 3 mL of 0.025 wt.% Nafion in ethanol solution. The working
electrode was prepared through dropping 30 ␮L of ink onto a glassy
carbon electrode. Hg/HgO (1.0 M KOH) electrodes and Pt foil were
respectively employed as reference and counter electrodes. Poten-
tials of CV test were controlled in a range between −0.8 and 0.3 V.
CO stripping was carried out in this way: after purging nitrogen
into electrolyte for 20 min, CO was purged for 15 min to form CO
layer on the catalyst surfaces while the potential was maintained
at −0.8 V. Surplus CO was expelled by nitrogen bubbles. Potentials
of CO stripping were also ranged between −0.8 and 0.3 V.
The obtained catalysts are evaluated through the aerobic oxida-
tion of benzyl alcohol with molecular oxygen, which is a vitally
important intermediate reaction to transform functional groups in
organic synthesis and meanwhile a model aromatic alcohol oxi-
dation reaction employed by laboratory study [27–29]. Since the
first study of the Pd-catalyzed oxidation of benzyl alcohol, mas-
sive investigations have explored the role of Pd catalysts along
with their applications in benzyl alcohol oxidation [30]. Surface-
functionalized carbon or CNTs supported Pd catalysts [30–32] have
become one of the most alluring topics after the early usage of Pd/C
[33] or Pd/CNT [34] catalysts for alcohol oxidation. This investiga-
tion concerning rare earth oxides functionalized CNTs supported
Pd for benzyl alcohol oxidation and the discussion on the corre-
lation of the catalyst structure, surface chemistry, electrochemical
activity and catalytic performance will be rendered in detail.
2. Experimental
2.1. Synthesis
Commercial multi-walled CNTs powder (>95%, Cnano) was
purified with pretreatment in concentrated nitric acid and oxygen-
containing groups were generated on CNT surfaces: slurry of CNTs
powder (2 g) suspended in 100 mL of concentrated nitric acid (69%,
Sigma–Aldrich) was refluxed at 120 ◦C for 4 h. After cooling to
room temperature, vacuum filtration was performed followed by
rinsing with deionized water for several times and subsequently
drying at 80 ◦C overnight. Then the purified CNT was surface-
functionalized using rare-earth nitrides Ln(NO3)3·6H2O (Ln = La,
Ce, Sm, Gd, Er, Yb) prior to adsorbing palladium precursors. 0.25 g
of pretreated CNTs powder and a certain amount of rare-earth
nitride Ln(NO3)3·6H2O were suspended in 20 mL of deionized
water, where the content of rare-earth element was controlled at
5 wt.%. After stirring for 4 h and aging for 12 h, the mixture was
evaporated at 80 ◦C in a water bath and dried at 80 ◦C overnight, fol-
lowed by calcination in tube furnace at 400 ◦C for 4 h under nitrogen
flow to afford LnOx/CNT composites. Adsorption–reduction method
reported by Chi et al. [35] was employed to deposit palladium
nanoparticles onto LnOx/CNT functionalized support. 379.7 ␮L of
0.05 M PdCl2 aqueous solution was added to 0.2 g of LnOx/CNT
suspended in 20 mL of deionized water, followed by refluxing
at 80 ◦C for 5 h. The suspension was filtered, washed and dried
overnight. After reduction in hydrogen flow at 400 ◦C for 2 h, the
catalysts, denoted as Pd/LnOx-CNT (Ln = La, Ce, Sm, Gd, Er, Yb), were
obtained.
2.3. Aerobic oxidation of benzyl alcohol
The catalytic reaction was carried out using molecular oxygen
under solvent-free conditions. Benzyl alcohol (5.174 mL, 50 mmol)
was loaded in a glass flask pre-charged with 0.01 g of catalyst. The
mixture was immersed in a 160 ◦C of oil bath while gaseous oxygen
was purged (20 mL min−1) to initiate the reaction. The reaction was
allowed to go on for 1 h under vigorous stirring (1200 rpm). After
the reaction, the solid catalyst was filtered off while the liquid phase
was analyzed by Agilent gas chromatograph 6890 equipped with
HP-5 capillary column. Dodecane was used as the internal standard
to measure the percentage of remaining reactant and products so as
to calculate benzyl alcohol conversion and benzaldehyde selectivity
values [36]. The conversion, selectivity, turnover frequency (TOF)
as well as the quasi-turnover frequency (qTOF) is defined as follows
[37,38].
moles of reactant converted
moles of reactant in feed
Conversion (%) =
Selectivity (%) =
× 100%
moles of product formed
moles of reactant converted
× 100%
moles of reactant converted
TOF (h−1) =
qTOF (h−1) =
dispersion =
moles of Pd × reaction time (h)
moles of reactant converted
moles of Pd × dispersion × reaction time (h)
2.2. Characterizations
electroactive surface area
(1/atomic weight of Pd) × (NA × 4␲R2
)
Powder X-ray diffraction (XRD) was conducted on a Bruker
AXS D8 X-ray diffractometer, using filtered Cu-K␣ radiation
(ꢀ = 0.15406 nm), under ambient conditions; operation condition
was 40 kV and 40 mA. Data collections were between 10 and 80◦
(2ꢁ) with the step length of 0.02◦ (2ꢁ) per second. Inductively
coupled plasma (Dual-view Optima 5300 DV ICP-OES system)
was attempted to measure the metallic element contents of each
catalyst using concentrated nitric acid (>69%) to dissolve the sam-
ples prior to the measurements. Transmission electron microscope
(TEM) was conducted on a JEOL 2010, operating at 200 kV. Sam-
ples suspended in ethanol were dropped onto holey carbon-coated
Cu grids. X-ray photoelectron spectroscopy (XPS) characterization
was conducted on VG Escalab 250 spectrometer with aluminum K␣
1846.6 eV anode. The 1s peak of carbon at 284.6 eV was employed
as the standard reference for calibration of binding energy.
Pd
3. Results and discussion
3.1. Effect of rare-earth oxides functionalities
Fig. 1 displays the XRD patterns of Pd/LnOx-CNT catalysts with
different rare-earth oxides. The diffraction peaks at 26.01◦ and
42.80◦ are respectively ascribed to the (0 0 2) and (1 0 0) facets of
graphitic CNT [32]. The diffraction peaks at 40.00◦, 46.07◦, 68.30◦
shown in each catalyst are in correspondence with the (1 1 1),
(2 0 0), and (2 2 0) facets respectively of palladium face-centered
cubic (FCC) structure. According to Bragg’s law [39], the lattice d-
spacing of (1 1 1) plane calculated is approximately 0.225 nm which
is further verified by HRTEM shown in Fig. 2. Thus, rare-earth oxides
surface functionalities did not result in lattice contraction within Pd
In situ pyridine adsorbed Fourier Infrared spectra were recorded
on a PerkinElmer Spectrum One FTIR spectrometer with potassium
Please cite this article in press as: Y. Yan, et al., Palladium nanoparticles supported on CNT functionalized by rare-earth oxides for
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1586 (or 1570, 1582, 1585, 1587) cm−1, 1500 (or 1496, 1487) cm−1
crystal lattice. The approximate full width at half max (FWHM) of
(1 1 1) diffraction peak at 40.00◦ is in agreement with the Pd par-
ticles average size of 8 nm for 1Pd/CNT catalyst according to the
Scherrer equation [40,41]. Similarly, the FWHM of 1.14◦ at 40.00◦
is indexed to the average Pd size around 7 nm for 1Pd/5Er-CNT and
1Pd/5Yb-CNT catalysts. The FWHM of 1.24◦ at 40.00◦ corresponds
to the Pd particle average size around 6 nm for 1Pd/5Gd-CNT, and
1Pd/5La-CNT catalysts. The FWHM of 1.43◦ at 40.00◦ indicates the
Pd grain average size of about 4 nm in Pd/5Sm-CNT and 1Pd/5Ce-
CNT catalysts. The rare-earth oxides functionalities can improve
the hydrophilicity of CNT surfaces, and then can increase the affin-
ity to the aqueous palladium chloride. Therefore, the deposition of
Pd precursor and the formation of small Pd particles are favored.
Moreover, the positively charged rare-earth cation possesses great
affinity to the [PdCl2(OH)2]2− precursor, also contributing to the
formation of small and highly dispersed Pd nanoparticles. This
result is further corroborated by the TEM observations and Pd par-
ticle size distribution histograms shown in Fig. 2. No distinct XRD
diffraction peak for rare-earth oxides can be detected, implying that
the rare-earth oxides have been homogeneously dispersed over
the CNT surfaces. Since the amount of rare-earth oxides are not
remarkably high, grains of rare-earth oxides as well as their con-
tact with Pd particles are hardly perceived on HRTEM images. The
rare-earth cationic precursors are chemically anchored to oxygen-
containing groups on CNT surfaces. Thus the rare-earth oxides are
finely deposited and the grain size is well-controlled on the level
of nano-dimensions.
and 1453 (or 1447, 1452, 1454, 1457) cm−1 have been indexed to
the 8a and b, 19a and b pyridine vibrational modes attributed to
electron transfer at Lewis acidic surface sites respectively. Whereas
the bands at 1642 (or 1635, 1640, 1644) cm−1 and 1544 (or 1537,
1545, 1546, 1547) cm−1 are assigned to ␯8a and ␯8b vibrational
modes of pyridine due to proton transfer (i.e., hydrogen bond) at
Brønsted acidic sites [45]. The density and strength of Lewis acid
site are both higher than those of Brønsted acidic site for 1Pd/CNT
and 1Pd/5Ln-CNT catalysts, implying that electron deficient sites
are dominant. At 50 ◦C, the more intense bands at 1586 cm−1
,
1500 cm−1 and 1543 cm−1 on 1Pd/CNT indicates that it possesses
higher density of Lewis acid sites than those functionalized by
rare-earth oxides nanoparticles under relatively mild conditions.
This indicates that at a relatively low temperature (e.g., 50 ◦C), the
amount of the surface oxygen-containing species (e.g., carbonyl and
carboxyl) on CNT decreases after the surface-functionalization with
rare-earth oxides. However, an abrupt decline of Lewis acid site
density occurs on 1Pd/CNT as the temperature increases, which, on
the contrary has not been observed in spectra of 1Pd/5Ln-CNT cat-
alysts, implying that the oxygen-containing species on CNT surface
are unstable at high temperatures, in agreement with the results
by Guo et al. [47]. The rare-earth oxides functionalized CNT sup-
ported Pd catalysts possess higher thermal stability of surface Lewis
acid sites than 1Pd/CNT catalyst. Moreover, the shifts or splits of
each band demonstrate the different types of Lewis/Brønsted acidic
sites that the rare-earth oxides introduce onto the CNT surface after
functionalization. Among all the catalysts, the most complexity of
multiple IR bands indicates the largest number of acid site types
presented on Pd/5Sm-CNT with eminent thermal stability. These
acidic sites can play a vital role in alcohol dehydrogenation reaction
[48–52], which is commonly present in the mechanism of benzyl
alcohol oxidation reaction with molecular oxygen [53].
In situ Fourier Infrared spectroscopy with pyridine adsorbed on
catalyst surfaces has been widely applied to investigate the sur-
face Lewis and Brønsted acidic sites [42–45]. As shown in Fig. 3,
the pyridine molecule is retained on the surface in three modes:
(1) interaction between the N electron pair and the hydrogen
atom of -OH group, (2) proton transfer from surface to pyridine
(Brønsted acidity) and (3) pyridine molecule coordination to elec-
tron deficient sites (Lewis acidity) [46]. Ring vibrations in the range
from 1400 to 1700 cm−1 distinguish between pyridine adsorbed on
The surface basicity is associated with their electro-chemical
properties, especially the electron donor property [46]. Cyclic
voltammetry measurements are carried out on 1Pd/CNT and
1Pd/5Ln-CNT catalysts and the results are shown in Fig. 4. The larger
Lewis and Brønsted acidic sites. FTIR bands centered at 1609 cm−1
,
Fig. 1. XRD diffraction patterns of Pd/5Ln-CNT.
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area enclosed by CV background implies a higher electron density
[47]. The rare-earth oxide functionalities remarkably improve the
electron densities of the catalysts. As shown in Fig. 4, the back-
ground of CV signals are observed with an evident decrease in the
area enclosed in the sequence of: 1Pd/5La-CNT > 1Pd/5Ce-CNT >
1Pd/5Sm-CNT ≈ 1Pd/5Gd-CNT > 1Pd/5Er-CNT > 1Pd/5Yb-CNT>1Pd/
CNT. Rare-earth oxides are well known for their surface electron
donor properties [54–57], which favors the enlargement of areas
enclosed by CV curves, since the more redox active sites or electron
donors on CNT, the larger capacitance and CV background are [58].
The sequence mentioned above is in tune with lanthanide con-
traction: basicity of rare-earth oxides, which is related to electron
donor properties, declines with the contraction of rare-earth cation
radius [59]. The absence of palladium oxide (Pd-O) reduction peak
at 0.1 V for rare-earth oxides modified CNT supported Pd catalysts,
except 1Pd/5Gd-CNT, indicates that Pd nanoparticles are more
easily reduced to zero-valent state (Pd0) over rare-earth oxides
surface functionalized CNT supports. Surface basicity contributes
to the formation of small Pd nanoparticle size and narrow size
Fig. 2. TEM images and Pd particle size distribution histograms: (a) Pd/5La-CNT; (b)
Pd/5Ce-CNT; (c) Pd/5Sm-CNT; (d) Pd/5Gd-CNT; (e) Pd/5Er-CNT; (f) Pd/5Yb-CNT; (g)
Pd/CNT.
Fig. 3. FTIR spectra of pyridine adsorption on 1Pd/CNT, 1Pd/5La-CNT, 1Pd/5Ce-CNT,
1Pd/5Sm-CNT, 1Pd/5Gd-CNT, 1Pd/5Er-CNT and 1Pd/5Yb-CNT catalysts at different
temperatures.
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distribution along with an enhanced metal–support interaction,
which has been reported by Chen et al. and they suggested that the
basicity can significantly impact the catalytic performance in the
aerobic oxidation of benzyl alcohol [37]. Furthermore, the electron
density is another crucial factor for catalytic performance.
catalysts. Accordingly, the rare-earth oxides functionalized CNT
supported Pd catalysts possess the higher onset catalytic activity
than non-functionalized Pd/CNT catalyst, which may due to the
improved electron density, electron transition capability and the
enhanced metal–support interactions. Among all these catalysts,
the 1Pd/5Sm-CNT affords the highest onset catalytic activity for
CO oxidation. The EAS areas of Pd particles are calculated with the
assumption that the oxidation of CO monolayer adsorbed on sur-
face of 1 cm2 of Pd nanoparticles requires 420 ␮C of electric charge
[62,63]. The results show that the electrochemical active surface
areas are in correspondence with the XRD and TEM measurements
(shown in Table 1). Modification with rare-earth oxides, especially
Sm2O3 and CeO2, facilitates EAS increase in catalysts due to the
smaller particle sizes and the higher dispersion of Pd nanoparticles.
We applied the solvent-free oxidation of benzyl alcohol with
molecular oxygen as a model reaction to examine these Pd/CNT and
Pd/5Ln-CNT catalysts, in order to elucidate the effect of surface rare-
earth oxides functionalities on catalytic performance. The results
are listed in Table 1, where qTOF was determined as the number
of converted benzyl alcohol molecules per hour over the number
of active sites where the number of active sites (i.e., Pd atoms
exposed on the particle surface) was determined by EAS value
with the Pd content obtained from ICP measurements, subtracting
the non-catalytic effect (i.e., ∼5% of benzyl alcohol conversion at
160 ◦C). According to ICP test results in Table 1, the increased Pd
contents upon functionalization with rare-earth oxides suggest the
enhanced surface hydrophilicity which can reduce the adsorption
barrier of palladium aqueous precursor onto the support surfaces.
Dehydrogenation of benzyl alcohol already occurs to form benz-
aldehyde and co-product molecular hydrogen in the absence of
O2. Oxygen is a great hydrogen acceptor to consume H2 [53]. The
main product benzaldehyde is a non-enolizable aldehyde, which
limits the number of byproducts. Primary byproducts include
toluene from hydrogenolysis of C O bond of benzyl alcohol where
CO stripping voltammetry was performed through electro-
oxidation reaction of carbon monoxide. In CO stripping, CO
monolayer adsorbed on catalysts will be electro-oxidized to CO2 at
initially a negative voltage and along with the increased sweeping
voltage of the working electrode immersed in 1 M of KOH aqueous
electrolyte solution. Based on cyclic voltammetry, CO stripping is
a practical method to measure the electrochemical active surface
(EAS) area of Pd nanoparticles at low Pd loadings [60] because of
the low electrical signals ascribed to the release of hydrogen atoms
bonded to Pd surfaces as well as the uncertainty caused by the back-
ground subtraction [47]. The Langmuir–Hinshelwood mechanism
is normally accepted as the model for CO electro-oxidation, where
adsorbed OH formed on the Pd surface by dissociative adsorption
of H2O reacts with the adsorbed CO [61].
k
1
H2Oads−→ OHads + H+ + e−
(1)
(2)
(3)
k
2
OHads + H+ + e−−→ H2Oads
k
3
COads + OHads−→ CO2 + H+ + e−
The KOH aqueous electrolyte neutralizes the H+ and pushes
the reaction moving forward. Thus CO electro-oxidation poten-
tial relies upon the kinetics, thermodynamics and electrochemical
state of Pd nanoparticles. Fig. 5 presents the CO stripping voltam-
mograms of 1Pd/CNT and 1Pd/5Ln-CNT catalysts. The negative
shifts of onset potentials for CO electro-oxidation indicate the
higher catalytic activity for the onset of CO electro-oxidation
reaction, suggesting that it is easier to initiate the CO oxi-
dization using rare-earth oxides functionalized 1Pd/5Ln-CNT
Fig. 4. (a) Cyclic voltammetry curves (CVs) of 1Pd/CNT and 1Pd/5Ln-CNT catalysts in 1 M KOH aqueous solution with a scan rare of 50 mV s−1; (b) magnification of the selected
area in (a) presenting Pd-O reduction peaks.
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In addition, the surface basicity of catalysts favors the dis-
proportionation of benzaldehyde (Cannizzaro reaction), producing
benzoic acid and benzyl alcohol [64].
base
2PhCHO + H2O−→ PhCH2O + PhCOOH
(8)
As shown in Table 1, the catalytic activity is remarkably
improved upon the functionalization with rare-earth oxides.
Among all the catalysts, 1Pd/5Sm-CNT presents the best catalytic
performance with a conversion of 29.6% and an outstanding qTOF
of 253,842 h−1, which is 1.5 times higher than that of 1Pd/CNT,
owing to the smallest size of Pd nanoparticles [65], improved
metal–support interaction, appropriate surface basicity and the
abundant thermal stable surface acid sites over 1Pd/5Sm-CNT sur-
faces. The surface basic site has been reported to cleave the O
H
bond to produce alkoxide intermediate while the surface acid site
participates in producing and transferring H2 in dehydrogenation
of benzyl alcohol [52], a significant step in its oxidation reaction,
since surface acid sites possess better affinity to H2 [66–68]. Nev-
ertheless, the high activities are accompanied by a large amount
of toluene as byproduct, so as to decrease the benzaldehyde selec-
tivity. The formation of toluene further verifies that the Pd is in
reduced metallic state (Pd0) [53], which facilitates the hydrogenol-
ysis of benzyl alcohol when O2 is not sufficiently supplied or the
reaction rate of O2 consuming H2 is not rapid enough. The rel-
atively lower selectivity toward toluene over 1Pd/5Sm-CNT than
other rare-earth oxides functionalized catalysts is caused by the
abundant amount of types of acid sites absorbing the hydrogen
from Pd surface on 1Pd/5Sm-CNT, reducing the amount of H2 on Pd
surface. Considering the highest conversion and appropriate selec-
tivity toward benzaldehyde, the Sm2O3 was selected as the optimal
surface functionality in the following study.
3.2. Effect of Sm2O3 amount
CNT functionalized with 2.5 wt.%, 5 wt.%, 7.5 wt.%, 10 wt.% con-
tent of Sm in the form of Sm2O3 and loaded with 1 wt.% of Pd were
prepared for comparison. XRD patterns of samarium oxide func-
tionalized CNT supported Pd catalysts are presented in Fig. 6. The
peak width at half height of the peak at 40.00◦, related to the charac-
teristic (1 1 1) plane of Pd, increases with the increasing the Sm2O3
content, implying the decrease of Pd nanoparticle size according to
Scherrer equation, which has been further substantiated by TEM
observations shown in Fig. 7. The Pd nanoparticle size decreases in
the series of 4.3 nm, 3.7 nm, 3.3 nm and 3 nm, as the Sm2O3 con-
tent increases in the order of 2.5 wt.%, 5 wt.%, 7.5 wt.% and 10 wt.%.
The more amount of Sm2O3 functionality facilitates the higher
hydrophilicity of CNT surface which favors smaller Pd nanoparti-
cles. Meanwhile, peaks centered at 27.76◦ and 31.94◦, assigned to
(1 1 1) and (2 0 0) planes of Sm2O3, emerge and intensify as the Sm
Fig. 5. CO striping voltammograms of 1Pd/5La-CNT, 1Pd/5Ce-CNT, 1Pd/5Sm-CNT,
1Pd/5Gd-CNT, 1Pd/5Er-CNT, 1Pd/5Yb-CNT and 1Pd/CNT catalysts.
hydrogen molecule is formed from dehydrogenation reaction on
Pd in reduced state (Pd0) and benzoic acid formed by hydration of
benzaldehyde followed by the dehydrogenation reaction [53].
PhCH2OH → PhCHO + H2
(4)
(5)
(6)
(7)
PhCH2OH + H2 → PhCH3 + H2O
PhCHOad + H2Oad → PhCH(OH)2,ad
PhCH(OH)2,ad → PhCOOHad + 2Had
Table 1
Catalytic results of benzyl alcohol oxidation over 1Pd/CNT and 1Pd/5Ln-CNT.a
qTOFd (h−1
)
Catalyst
Pd content (wt.%)b
Ln content (wt.%)b
EASc (m2 g−1
)
Conversion (%)
Selectivity (%)
Benzaldehyde
Toluene
Benzoic Acid
1Pd/CNT
0.78
0.81
1.25
0.79
1.08
0.93
0.91
12.8
18.9
18.4
17.2
14.7
13.9
13.0
17.2
26.7
25.8
29.6
24.5
23.4
23.0
91.3
88.6
85.2
90.4
81.4
84.2
85.6
7.1
1.6
0
0.4
2.4
0
169,163
203,777
201,597
253,842
235,437
234,942
245,746
1Pd/5La-CNT
1Pd/5Ce-CNT
1Pd/5Sm-CNT
1Pd/5Gd-CNT
1Pd/5Er-CNT
1Pd/5Yb-CNT
2.81
1.64
2.06
2.59
2.78
2.86
11.4
14.4
7.2
18.6
15.8
13.9
0
0.5
a
Reaction conditions: catalyst, 10 mg (the amount of Pd is 1 wt.% of catalyst, 0.1 mg); benzyl alcohol, 50 mmol; O2, 20 mL min−1; temperature, 160 ◦C; time, 1 h.
Real metal content was tested by ICP.
EAS areas were deduced from data of CO stripping.
qTOF is defined as the number of converted benzyl alcohol molecules in 1 h over one active site; the number of active sites (Pd atoms exposed on the surface of particle)
b
c
d
is determined by EAS value [63] considering Pd content obtained from ICP, and subtracted the non-catalytic effect (conversion ∼5%).
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Fig. 6. XRD patterns for 1Pd/Sm2O3-CNT with Sm content of 2.5 wt.%, 5 wt.%, 7.5 wt.% and 10 wt.%.
Fig. 7. TEM images and Pd particle size distribution histograms of various amount of Sm2O3 functionalized CNT supported Pd catalysts: (a) 1Pd/2.5Sm-CNT; (b) 1Pd/5Sm-CNT;
(c) 1Pd/7.5Sm-CNT; (d) 1Pd/10Sm-CNT.
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Fig. 8. Cyclic voltammograms (CVs) of 1Pd/2.5Sm-CNT, 1Pd/5Sm-CNT, 1Pd/7.5Sm-CNT and 1Pd/10Sm-CNT catalysts in 1 M KOH electrolyte solution with a scan rate of
50 mV s−1
.
content increases from 2.5 wt.% to 10 wt.% in the form of Sm2O3,
which enables the formation of larger amount of crystal structure
of Sm2O3.
CV measurements are performed and cyclic voltammograms
are shown in Fig. 8. The areas enclosed by CV curves are
observed with an evident decrease in the sequence of: 1Pd/2.5Sm-
CNT > 1Pd/5Sm-CNT > 1Pd/7.5Sm-CNT > 1Pd/10Sm-CNT. Small con-
tent of rare-earth functionalized CNT supported Pd catalyst has
larger capacitance and CV background due to the remarkable elec-
tron donating properties of Sm2O3. Nevertheless, the capacitance
and CV background will decline with the further increase of rare-
earth oxides content due to the abrupt decrease of the electron
conductor (i.e., CNT) content. The redox signal peak of Pd O bond
at ca. −0.09 V is only discernible for 1Pd/2.5Sm-CNT, indicating that
the higher content of Sm2O3 favors fully reduced Pd (Pd0) genera-
tion and stabilizes the zero-valent status of Pd (Pd0) nanoparticles.
CO stripping voltammetry was carried out to measure the EAS
areas of 1Pd/Sm2O3-CNT, as shown in Fig. 9. The EAS area ini-
tially increases when Sm content rises from 2.5 wt.% to 5 wt.%
and subsequently declines as Sm content rises up to 10 wt.%. The
Sm2O3 functionalities facilitate the surface hydrophilicity of CNT
and hence favor the formation of smaller Pd nanoparticles which
possess higher EAS area without the presence of agglomeration at
low Sm2O3 loadings. Nonetheless, when Sm2O3 content gets high
enough to form Sm2O3 crystalline grains, the metal–support inter-
action is enhanced with smaller Pd nanoparticles which may reduce
the surface exposure of Pd nanoparticles. The mass transfer resis-
tance resulted from the uneven surface after functionalization by
large amount of Sm2O3 also hampers the contact between Pd and
CO molecules and thus results in a smaller EAS area. In addition,
higher Sm2O3 loadings lead to more direct contact between small
Pd nanoparticles and Sm2O3 which may reduce the exposure of Pd
effective surface to the liquid phase. Among all these samarium
oxide functionalized CNT-supported Pd catalysts, 1Pd/5Sm-CNT
retains the largest EAS area. Meanwhile, the 1Pd/5Sm-CNT cata-
lyst possesses the lowest onset potential for CO electro-oxidation
reaction, indicating its highest onset electrochemical activity for
CO electro-oxidation as the model reaction.
Benzyl alcohol oxidation over 1Pd/Sm2O3-CNT with different
content of Sm2O3 was investigated and illustrated in Table 2. Pd
Fig. 9. CO stripping voltammograms of 1Pd/2.5Sm-CNT, 1Pd/5Sm-CNT, 1Pd/7.5Sm-
CNT and 1Pd/10Sm-CNT catalysts.
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Table 2
Catalytic results of benzyl alcohol oxidation over 1Pd/2.5Sm-CNT, 1Pd/5Sm-CNT, 1Pd/7.5Sm-CNT and 1Pd/10Sm-CNT catalysts.a
qTOFd (h−1
)
Catalyst
Pd content (wt.%)b
Ln content (wt.%)b
EASc (m2 g−1
)
Conversion (%)
Selectivity (%)
Benzaldehyde
Toluene
Benzoic Acid
1Pd/CNT
0.78
0.72
0.79
0.83
0.95
12.8
15.9
17.2
11.7
10.3
17.2
28.1
29.6
26.1
23.5
91.3
83.3
90.4
93.6
95.7
7.1
15.8
7.2
4.4
1.5
1.6
0.9
2.4
2.0
2.8
169,163
257,853
253,842
318,552
318,760
1Pd/2.5Sm-CNT
1Pd/5Sm-CNT
1Pd/7.5Sm-CNT
1Pd/10Sm-CNT
0.66
2.06
4.20
6.52
a
Reaction conditions: catalyst, 10 mg (the amount of Pd is 1 wt.% of catalyst, 0.1 mg); benzyl alcohol, 50 mmol; O2, 20 mL min−1; temperature, 160 ◦C; time, 1 h.
Real metal content was tested by ICP.
EAS areas were deduced from data of CO stripping.
qTOF is defined as the number of converted benzyl alcohol molecules in 1 h over one active site; the number of active sites (Pd atoms exposed on the surface of particle)
b
c
d
is determined by EAS value [63] considering Pd content obtained from ICP, and subtracted the non-catalytic effect (conversion ∼5%).
loading of each catalyst was determined through ICP measure-
ment. The catalytic activity is improved when Sm2O3 is doped on
CNT surface due to the decreased Pd particle size and synergistic
effect from metal–support and metal–functionalities interactions.
1Pd/5Sm-CNT catalyst possesses the highest benzyl alcohol conver-
sion value of 29.6%. Further increasing the Sm2O3 loading elevates
the qTOF value and selectivity toward benzaldehyde at the sacrifice
of EAS area along with the lower conversion value of the substrate
benzyl alcohol. The explanation for decrease in EAS area has been
illustrated that large amount of Sm2O3 functionality enhances the
metal–support interaction and the contact area between Pd, Sm2O3
and CNT. Both of them decline the exposure of Pd effective area,
leading to lower conversion of benzyl alcohol. The trend of qTOF
value implies the activity of supported Pd catalysts is primarily
dependent on the electrochemical active surface, electron density
and surface chemistry surrounding the active phase. Small amount
of Sm2O3 facilitates the formation of toluene and hence the selec-
tivity toward benzaldehyde is low (83.3%). As the Sm2O3 content
increases, the selectivity toward benzaldehyde is improved, indi-
cating that the functionalization with an appropriate amount of
Sm2O3 reduces the formation of byproducts. On account of the
highest yield toward the main product benzaldehyde, 1Pd/5Sm-
CNT is considered as the optimal catalyst in this study.
3.3. Studies of 1Pd/5Sm-CNT catalyst
The XPS spectra of Pd 3d and O 1s have been investigated as
shown in Fig. 10. The Pd 3d peaks were convoluted to Pd0 and Pd2+
two states. The ratios of Pd0/(Pd0+Pd2+) in Pd/CNT and Pd/5Sm-
CNT are 42.3% and 69.7% respectively, indicating the electron donor
property of Sm2O3 in consistent with the CV results which subse-
quently enhances the Pd electron density. The shift of 3d3/2 and
3d5/2 binding energy peaks of Pd2+ toward reduced forms after
surface-functionalization with Sm2O3 illustrates the lower oxida-
tion extent of Pd or the formation of Pd-Sm2O3 complex [69]. The
fractions of C O/C O of Pd/CNT and Pd/5Sm-CNT are quite close.
Fig. 10. XPS spectra: (a) Pd 3d spectra; (b) O 1s spectra of Pd/CNT and Pd/5Sm-CNT.
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Fig. 11. Time course of 1Pd/5Sm-CNT catalyst for aerobic oxidation of benzyl alco-
hol (benzyl alcohol/Pd = 500 mol/g; O2 flow rate, 20 mL min−1; temperature, 160 ◦C;
stirring rate, 1200 rpm).
Fig. 12. Recyclability of 1Pd/5Sm-CNT catalyst for solvent-free aerobic oxidation
of benzyl alcohol (benzyl alcohol/Pd = 500 mol/g; oxygen flow rate, 20 mL min−1
reaction temperature, 160 ◦C; time, 1 h; stirring speed, 1200 rpm).
;
batch of reaction, indicating that no leaching of Pd occurs during
reactions and the immobilization of metal phase on support has
been stabilized after the surface-functionalization of Sm2O3. There-
fore, the samarium oxide surface-functionalized CNT supported Pd
catalysts possess not merely the improved catalytic performance in
benzyl alcohol oxidation, but also the superiority in the resistance
against deactivation.
Nonetheless the apparent increase in lattice oxygen of Pd/5Sm-CNT
compared to Pd/CNT is explained by the presence of lattice oxygen
in Sm2O3 surface functionality.
Investigations on 1Pd/5Sm-CNT, which is selected as the best
catalyst in solvent-free oxidation of benzyl alcohol, were carried
out in detail. Time course of 1Pd/5Sm-CNT in target reaction was
detected periodically as shown in Fig. 11. The conversion of ben-
zyl alcohol monotonically increases with the passage of reaction
time followed by a monotone decrease of the qTOF value. During
the reaction time course, more and more substrate is converted,
while due to the decrease of the concentration of substrate, the
reaction rate becomes slower which is reflected as the decline in
qTOF value. The selectivity toward benzaldehyde is 82.5% at the first
0.5 h. This low selectivity is caused by the high selectivity toward
toluene from hydrogenolysis reaction of the C O bond in benzyl
alcohol with the residual H2 absorbed on Pd from the dehydrogena-
tion reaction. This is in consistence with the mechanism reported by
Keresszegi et al. that the dehydrogenation reaction of benzyl alco-
hol initially exists whether at the absence of molecular oxygen or
not [53]. The coproduct H2 from dehydrogenation reaction is sub-
sequently consumed (1) in the hydrogenolysis of benzyl alcohol to
from toluene; and (2) in the oxidation reaction by O2, which are
two parallel competitive reactions. Evidently, O2 is a much supe-
rior hydrogen acceptor than benzyl alcohol, consuming almost all
the H2 and suppressing the extent of hydrogenolysis reaction [30].
With the reaction time going on, more benzaldehyde is formed
and partially further oxidized to benzoic acid, which explains the
decrease of selectivity toward benzaldehyde after 1 h.
4. Conclusions
In this work, Pd nanoparticle catalysts supported on rare-earth
oxide functionalized CNT surfaces Pd/LnOx-CNT (Ln = La, Ce, Sm, Gd,
Er, Yb) have been prepared using wet impregnation method fol-
lowed with a metal ion adsorption–reduction process. Rare-earth
oxides functionalities presented great influence on the catalytic
performances. The XRD, TEM, HRTEM, ICP, pyridine adsorption
FTIR, and electrochemical measurements such as CV and CO
stripping analysis suggested the impact of rare-earth oxides func-
tionalities on Pd nanoparticle size, Pd valence, electron densities,
EAS areas, chemistry environment of active phase and synergis-
tic effect from metal–support interactions, along with the impact
on catalytic performance for solvent-free selective aerobic oxida-
tion of benzyl alcohol. Sm2O3 surface functionalized CNT with an
appropriate amount of Sm2O3 loading exhibited the best improve-
ment. Among all the catalysts, 1Pd/5Sm-CNT possessed the highest
conversion value of 29.6% whereas 1Pd/10Sm-CNT had the highest
qTOF value of 318,760 h−1 and selectivity of 95.7% toward benzal-
dehyde.
The recyclability is regarded as a vital factor in the evalua-
tion of heterogeneous catalyst in a multiphase reaction. The active
phase with poor stability and reusability may leach out during
the reaction, and the metallic components producing susceptible
by-products in a reaction might get poisoned, bringing about the
loss of catalytic activity for subsequent runs of target reaction. The
recyclability of 1Pd/5Sm-CNT in the solvent-free aerobic oxidation
of benzyl alcohol using gaseous oxygen has been investigated, as
displayed in Fig. 12. The catalyst was manually recovered after
each reaction batch by washing with acetone, drying at 60 ◦C and
recharged in the next reaction batch. The operation was repeated
four times and 1Pd/5Sm-CNT was used in five consecutive runs.
The conversion, selectivity and qTOF are accompanied by a mod-
erate decrease, which is ascribed to the loss of active surface area
of metallic phase. The Pd content after the five consecutive cycles
remains almost the same as the as-prepared catalyst. The Pd con-
tent is not detectable by ICP analysis in the filtrate phase after any
Acknowledgements
We acknowledge the financial support from the National
Research Foundation (NRF), Prime Minister’s Office, Singapore
under its Campus for Research Excellence and Technological Enter-
prise (CREATE) program and AcRF Tier 1 grant (RG129/14), Ministry
of Education, Singapore.
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solvent-free aerobic oxidation of benzyl alcohol, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.021