Fabrication of highly dispersed/active ultrafine Pd nanoparticle supported catalysts: A facile solvent-free: In situ dispersion/reduction method

Article abstract of DOI:10.1039/c7gc00262a

The development of sustainable routes for the synthesis of noble metal supported catalysts is of high importance because of their wide applications on a large scale in the catalysis field. Herein we report a facile solvent-free solid-state dispersion rout

Full text of DOI:10.1039/c7gc00262a

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: X. Guo, Y. Shen, X.
Bo, Z. Tian, Y. Wang, X. Guo, M. Xie, F. Gao, M. Lin and W. Ding, Green Chem., 2017, DOI:
10.1039/C7GC00262A.
This is an Accepted Manuscript, which has been through the
Volume 18 Number
7 7 April 2016 Pages 1821–2242
Royal Society of Chemistry peer review process and has been
accepted for publication.
Green
Chemistry
Accepted Manuscripts are published online shortly after
Cutting-edge research for a greener sustainable future
www.rsc.org/greenchem
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1463-9262
CRITICAL REVIEW
G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
rsc.li/green-chem

Please do not adjust margins
Green Chemistry
Page 1 of 8
View Article Online
DOI: 10.1039/C7GC00262A
Journal Name
ARTICLE
Fabrication of Highly Dispersed/Active Ultrafine Pd Nanoparticles
Supported Catalyst: a Facile Solvent-free In-situ
Dispersion/Reduction Method
Received 00th January 20xx,
Accepted 00th January 20xx
Yu Shena, Xiangkun Boa, Zhengfang Tianb, Yongzheng Wanga, Xiangke Guoa, Mingjiang Xiea, Fei
Gaoa, Ming Linc, Xuefeng Guo*ab, and Weiping Dinga
DOI: 10.1039/x0xx00000x
www.rsc.org/
Development of sustainable routes for synthesis of noble metal supported catalysts is of high importance because of their
wide applications at large scale in catalysis field. Herein we report a facile solvent-free solid-state dispersion route to
fabricate highly dispersed ultrafine palladium nanoparticles supported catalyst. In the first step, noble metal precursor
Pd(acac)2 was dispersed spontaneously by treating the physical mixture of Pd(acac)2 and catalyst support hydroxyapatite
(HAP) at 120 oC in a flow of N2. SubsequentH2 reduction results in the formation of two kinds of Pd particles. In-situ
reduction at 120 oC is essential for preparing highly dispersed Pd nanoparticles (~1.2 nm, a1Pd/HAP-SSD) and cooling-down
reduction leads to the formation of larger Pd nanoparticles (~4 nm, b1Pd/HAP-SSD). The as-prepared a1Pd/HAP-SSD
exhibits higher activity for phenol hydrogenation than b1Pd/HAP-SSD and that obtained by traditional wet impregnation
method, due to the highly dispersed ultrafine Pd0 nanoparticles obtained by in-situ dispersion/reduction. Compared with
conventional wet chemistry-based methods, the synthesis route in this work simplifies the synthesis process, avoids
producing large polluted wastes, and enhances the dispersion of noble metals. This may open a new way to prepare
highly dispersed/active noble metal supported catalysts and is also potentially of high importance for green production of
noble metal supported catalysts at large scale in the future.
where plenty of solvents such as water are necessary. The use
of the water solvent leads to a large amount of polluted
wastes, and the forced condensation during the following
Introduction
Metal supported catalysts have received considerable
attention for decades based on their potential applications in
energy conversion, environmental remediation, and catalysis.1
Numerous studies find that properties of these kinds of
supported catalysts are strongly dependent on the metal
particle size, degree of dispersion, and interaction between
metal and support, etc.2-6 Ultrafine metal nanoparticles (<2
nm) with high dispersion usually exhibit exceptional catalytic
performance for their unique properties distinct from those in
large-size and bulk counterparts, probably due to their
increased surface areas and number of edge and corner
atoms. To prepare supported catalysts with ultrafine metal
particle size, various routes have been explored including, wet
deposition precipitation technique,7 photo-deposition,8,9
colloidal method,10 ion exchange,11 atomic layer deposition,12
and so on. Usually, the synthesis of metal nanoparticle
supported catalysts is performed under solution conditions,
drying procedure usually results in undesirable dispersion of
metal precursors.13 The synthesis of ultrafine, uniform, high-
dispersed and catalytically active noble metal nanoparticles in
a controllable manner still remains an unmet challenge,
despite recent developments in the dispersion and
stabilization of noble metal nanoparticles.
Solid state dispersion (SSD) is a solvent-free technique based
on spontaneous monolayer-dispersion principle, which
suggests that salts can disperse spontaneously onto the
surfaces of supports to form a thermodynamically stable
monolayer or sub-monolayar structure.14 Compared with the
above-mentioned solution-based methods, no solvents are
involved in this procedure therefore circumventing drying
steps normally involved in preparing such catalysts. Thus, it is
not only a time-saving and energy-saving process without the
removal of solvent, but also provides a better dispersion of
metal precursors due to the avoidance of forced condensation
during drying procedure. Up to now, many transition metal
oxides or transition metals, such as CuO, NiO and Co, have
been successfully dispersed onto various supports by SSD
method,15-17 and this technique was also extended to a few
organic compounds.18,19 To the best of our knowledge, the
preparation and catalytic performance of noble metal
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 2 of 8
View Article Online
DOI: 10.1039/C7GC00262A
ARTICLE
Journal Name
supported catalysts by using this solid state dispersion method Pd loading of catalyst was analyzed by an Optima 5300 DV ICP-
are far less investigated. OES.
Herein, we explore the solvent-free solid state dispersion Temperature-programmed desorption of hydrogen (H2-TPR):
route to synthesize highly dispersed ultrafine Pd nanoparticles The catalysts was performed on Tianjin XQ TP-5080
on hydroxyapatite (HAP). TEM, H2-TPR, in-situ XPS with H2 chemisorption instrument. First, the sample was calcined at
o
treatment and IR spectra of chemisorbed CO are used to 100 C for 30min with a 10 oC min-1 heating rate in N2 in order
investigated the dispersion/particle size and the chemical to remove any adsorbed gas. Then, the sample was cooled to
nature of Pd/HAP. Both dispersion and reduction conditions 30 oC and the temperature was raised from room temperature
strongly affect the size of Pd nanoparticles, and in-situ to 500 oC with a 10 oC/min heating rate. The profile was
dispersion/reduction leads to the highly dispersed ultrafine continuously registered with a TCD.
~1.2 nm Pd nanoparticles (a1Pd/HAP-SSD). Together with Temperature-programmed desorption of hydrogen (H2-TPD) of
Pd/HAP obtained from traditional wet impregnation method the catalysts was performed on Tianjin XQ TP-5080
(WI), all Pd/HAP samples were evaluated in phenol chemisorption instrument. First, the sample was calcined at
hydrogenation reaction with a view to correlate with Pd 100 ◦C for 30min with a 10 ◦C min-1 heating rate in Ar in order
nanostructures. Remarkably enhanced activities for phenol to remove any adsorbed gas. Then, the sample was cooled to
hydrogenation were observed for Pd/HAPs obtained from SSD 30 ◦C and exposed to H2 flow (20 mL min-1) for 1 h. Hydrogen
a
b
method (73.5%, 1Pd/HAP-SSD; 20.45%, 1Pd/HAP-SSD; 3.8%, flow was stopped and the sample was flushed with Ar for 1 h
1Pd/HAP-WI), especially that with smaller Pd particle size to remove any physically adsorbed hydrogen. Finally, the
obtained by in-situ dispersion/reduction.
temperature was raised from room temperature to 600 ◦C
with a 10 ◦C/min heating rate and the hydrogen desorption
profile was continuously registered with a TCD.
Experimental
X-ray photoelectron spectra (XPS): The X-ray photoelectron
spectroscopy (XPS) measurements were performed in
a
Synthesis of Pd/HAP catalyst
commercial XPS system (PHI 5000 VersaProbe) equipped with
a hemispherical electron analyzer and monochromatic Al Kα X-
ray exciting source. The sample was first evacuated at 298 K
for 1h and the spectra was measured. Then the sample was
HAP is utilized as metal support and synthesized according to
the previous report by our group, which presents nanorod
morphology with several micrometers in length and ca. 8-50
nm in diameter (Fig. S1
†
, PDF#09-0432).20 The hydroxyapatite
o
o
reduced at 85 C in a 5 vol. % H2/N2 (30 mL/min) for 1h, after
(HAP) was first dried in an oven at 120 C for 12 h to remove
the adsorbed water or gas on the surface. Then, it was
physically mixed with the palladium salt Pd(acac)2 (97%) by
grinding to have a nominal content of 1 wt. % of Pd (denoted
as 1Pd(acac)2/HAP). Then, it was thermally treated at 120 oC
for 4 h in a flow of N2. Before cooling to the room
temperature, N2 was switched to H2, and catalysts denoted as
a1Pd-HAP-SSD was obtained by being in-situ reduced for 30
the sample was cooled to room temperature the spectra was
measured. All binding energies (BEs) were referenced to the
C1s peak at 284.6 eV.
In situ FT-IR: Diffuse-reflectance infrared Fourier transform
spectroscopy (DRIFTS) measurements were performed on a
Bruker Tensor 27 spectrophotometer. The catalysts were first
activated at 100 °C in ultrahigh-purity N2 (99.9%, 4 mL/min) for
30 min to remove the adsorbed water or gas on the surface.
After the DRIFTS cell was cooled to 25 °C, 10 vol% CO in He
with a flow rate of 10 mL/min was introduced into the cell for
30 min to saturate the Pd surface. After the CO saturation,
another 20 min N2 purge at a flow rate of 4 mL/min was
performed to remove gas-phase CO from the DRIFTS cell.
Finally, the spectrum was recorded.
o
min at 120 C. For comparison, the catalyst was also reduced
after cooling down to room temperature in N2 flow and
catalysts denoted as b1Pd-HAP-SSD was obtained by being
heated at 5 oC min-1 up to 120 oC in a H2 flow for 30 min.
1Pd-HAP-WI was prepared by conventional wet impregnation
route as follows: 0.4g HAP were placed in 80 ml ethanol
solution containing 0.0115g Pd(acac)2 in order to have 1 wt. %
of Pd in the final solid. The suspension was stirred at room
Catalytic Tests
o
Catalytic tests were conducted in a 25 mL round flask with a
magnetic bar. Heterogeneous Pd catalyst powder (5 mol %
relative to phenol), 0.05 mmol phenol, and 3 ml water were
introduced into the round flask, and then it was vacuumed and
purged with H2 three times before it was finally pressurized
with 1.0 atm of H2 gas. Subsequently, the reaction mixture was
heated and stirred to 85 oC (10 oC/min) for a certain time. After
cooling to room temperature, excess H2 was carefully released,
and the internal standard (toluene) was added. The resultant
product mixtures were analyzed by an Agilent gas
chromatograph (GC-9860).
temperature for 12 h and then heated at 60 C in a vacuum
evaporator to remove excess ethanol. Finally, the sample was
reduced following a similar procedure for the sample b1Pd-
HAP-SSD.
Characterization
X-ray diffraction patterns (XRD) were recorded on a Philips
X’Pert X-ray diffractometer with a Cu Ka radiation (40 kV, 40
mA). The TEM images were taken on a JEOL JEM-1010 electron
microscope operated at 100 kV. High-angle annular dark-field
scanning TEM (HAADF-STEM) studies were carried out on an
FEI Titan 80-300 electron microscope (200 kV). N2 sorption
isotherms were measured using a Micromeritics ASAP2020
analyzer at 77 K and the Brunauer-Emmett-Teller (BET)
method was utilized to calculate the specific surface areas. The
Results and discussion
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 3 of 8
View Article Online
DOI: 10.1039/C7GC00262A
Journal Name
ARTICLE
Obtaining a monolayer dispersed Pd(acac)2 on HAP is critical
for the preparation of ultrafine Pd nanoparticles. So measuring
the dispersion capacity of supported Pd(acac)2 is of obvious
significance. Fig. 2a depicts the XRD patterns of Pd(acac)2/HAP
with different Pd contents (x, denoted as xPd(acac)2/HAP). No
Pd(acac)2 crystalline phase in the XRD patterns was observed
on 1Pd(acac)2/HAP, indicating that Pd(acac)2 species were
highly dispersed on the HAP support without the formation of
any highly crystalline Pd(acac)2 species or the Pd(acac)2 content
was below the detection of the XRD instrument. At the Pd
loading of 5 wt. %, the weak diffraction peak of crystalline
Pd(acac)2 was detected. The intensity of crystalline Pd(acac)2
diffraction peak increases proportionally with Pd(acac)2
loading. Since the disappearance of XRD patterns might not
be a strong criterion for monolayer spreading, its
uncertainty can be further alleviated by employing an
extrapolation procedure.24,25 The dispersion capacity of
Pd(acac)2 on HAP was determined by the amount of residual
crystalline Pd(acac)2. As a function of Pd(acac)2 loadings, the
intensity ratios of IPd(acac)2/IHAP are plotted in Fig. 2b, in which
the highest intensity peaks of Pd(acac)2 (12.3o) and HAP (31o)
were chosen for the calculation. The intercept of X-axis
corresponds to the utmost monolayer dispersion capacity of
Pd(acac)2 on the surface of HAP and was determined to be
0.0552 g Pd(acac)2 / g HAP, which corresponds to 1.9 wt. % Pd.
If the Pd(acac)2 content in the mixture exceeds this critical
dispersion capacity, there will be an aggregation of Pd(acac)2
and formation of crystalline phase after heat treatment,
probably leading to form larger Pd particles after reduction.
Besides, we found that the dispersion threshold of
Pd(acac)2/HAP-SSD was much higher than that of
Fig. 1 Schematic diagram for the preparation of Pd/HAP with
a) solid state dispersion and b) wet impregnation methods.
The schematic diagram for the preparation of Pd/HAP using
SSD method is shown in Fig. 1a. Pd(acac)2 with characteristic of
low melting point (116 oC measured with melting point
apparatus) and high decomposition temperature (206 oC) is
selected as metal precursor. In a typical synthesis process (for
details see experimental section), Pd(acac)2 with loading
quantity below its monolayer dispersion capacity was first
thoroughly ground with HAP to form a physical mixture and
then put into a quartz tube under N2 flow. With the increase of
the temperature, Pd(acac)2 turned into a liquid phase before
decomposition and was dispersed onto the surface of HAP to
form a thermodynamically stable monolayer, similar to the
dispersion behavior of transition metal salts on the surface of
silica.15,16,21 Due to the decrease of Gibbs free energy of the
whole system, this procedure can be assumed to be a
spontaneous process.13,22 Besides, driven by the energy of
elastic stresses induced by grinding, virtual melting of
Pd(acac)2 on the surface can be occurred at
a lower
temperature than its standard melting point.14,23 Finally,
1Pd/HAP-SSD are obtained after being in-situ reduced in H2
with temperature keeping (a1Pd/HAP-SSD) or being reduced
after cooling down (b1Pd/HAP-SSD). As
a comparison,
Pd(acac)2/HAP-WI (less than 1 wt. % Pd, Fig. S2†). Such
1Pd/HAP-WI is also prepared via conventional wet
impregnation method and subsequently H2 reduction (Fig. 1b).
behavior can be rationalized by taking into consideration that
competitive adsorption of solvent ethanol molecules will
disturb the guest-host interaction and hinder the anchoring of
the Pd(acac)2 species on the surfaces.14 More importantly, the
forced condensation of Pd(acac)2 during drying process
generally results in Pd(acac)2 aggregation.
Fig. 3 Wide and enlarged XRD patterns of Pd(acac)2/HAP
Fig. 2 a) XRD patterns of Pd(acac)2/HAP with different Pd loadings
treated at 120 oC for 4 h; b) dispersion capacity of Pd(acac)2 on HAP
measured by XRD.
treated in different temperatures (a) and for different periods
(b).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 4 of 8
View Article Online
DOI: 10.1039/C7GC00262A
ARTICLE
Suitable temperature is also desirable for monolayer
Journal Name
Based on the above results, the optimal experimental
dispersion. Fig. 3a shows the X-ray diffraction (XRD) patterns conditions are determined. With the Pd(acac)2 loading under
of 1Pd(acac)2/HAP with a nominal content of 1 wt. % Pd its dispersion capacity, physically mixed Pd(acac)2 and HAP are
thermally treated at different temperatures. With Pd(acac)2 first thermally treated at 120 oC for 4 h in a flow of N2.
loading below its monolayer dispersion capacity, Pd(acac)2 was Subsequently, Pd/HAP was obtained after H2 treatment and
first physically mixed with HAP and a small diffraction peak of here two reduction methods are compared. All the samples
Pd(acac)2 appeared at the low angle of 12.3° compared with exhibited high similarity between XRD patterns of HAP and
the pure HAP. Then thermal treatment at different Pd/HAP (Fig. S4†), indicating that Pd was highly dispersed on
temperatures for 24 h, the diffraction peak of crystalline phase the HAP support without the formation of any large Pd
o
Pd(acac)2 still appeared until the temperature reached 100 C, particles. The little variation of nitrogen adsorption-desorption
and it disappeared at 120 oC, indicative of the well-dispersion isotherm curves and the surface areas (Fig. S5
† and Table S1†)
of Pd(acac)2. If thermal treatment temperature was kept at between HAP and Pd/HAP also suggests the retaining of HAP
120 oC for different times (Fig. 3b), 4 h was enough to allow for structures, which is in agreement with XRD patterns.
the occurrence of Pd(acac)2 monolayer dispersion. Based on
TEM and HAADF-STEM are used to investigate the
these results, we speculated that the critical dispersion morphology and distribution of the supported Pd
temperature of Pd(acac)2 is higher than 100 oC, which is in nanoparticles on HAP. In-situ reduction with temperature
agreement with the actual melting temperature of Pd(acac)2 keeping at 120 oC leads to the formation of ultrafine
o
at 116 C observed in melting point apparatus. Obviously, at nanoparticles (black dots) with an average size approximately
the lower temperature, the solid dispersion would not proceed at 1.2 nm and the Pd loading at 1.08 wt.% determined by
fast enough to overcome kinetic resistance and require a emission spectrometer (ICP-AES) (Fig. 4a), in agreement with
longer heating time. At 120 oC, slightly above the actual the amount of raw material added. HAADF-STEM image
melting temperature of Pd(acac)2, Pd(acac)2 powder will melt further confirms that Pd nanoparticles (white dots, Fig. 4b) are
and spread on the surface of HAP quickly, forming a uniform uniformly anchored on the surface of HAP and no
layer of coating on it. High temperature treatment (150 oC) will agglomerations are observed. However, reduction after
decompose the Pd(acac)2 and change the color of mixture cooling down leads to the formation of much larger Pd
from light yellow to gray. In this synthesis, 120 oC is the nanoparticles (~4.0 nm, Fig. 4c). We speculate that due to the
appropriate treatment temperature. Besides, the smaller the weak interaction between Pd(acac)2 and HAP, cooling down
particles, the more thorough the mixing, then the more readily might cause the reaggregating of already monolayer dispersed
this process proceeds and the less time it takes to reach its precursor to form larger stable Pd(acac)2 particles. Therefore,
limit.26
In contrast, 1Pd(acac)2/HAP-WI clearly gave rise to the peak Pd nanoparticles. As a comparison, Pd nanoparticles up to 10
of the crystalline Pd(acac)2 species (Fig. S2 ), suggesting the nm are observed for sample 1Pd/HAP-WI (Fig. 4d), which may
failure of wet impregnation method to highly dispersed be attributed to the forced condensation of Pd(acac)2 during
in-situ dispersion/reduction is essential for preparing ultrafine
†
Pd(acac)2. This may be caused by the forced condensation of drying (Fig. S2†and Fig. S3†). Thus, the SSD method can be
metal precursors during drying procedure.27 This phenomenon considered as an alternative to traditional wet impregnation
is also confirmed by H2 temperature programmed reduction and drying procedures, which provides a better dispersion for
(H2-TPR) analysis in Fig. S3†, which shows that 1Pd(acac)2/HAP- metal supported catalysts.
WI with large Pd(acac)2 particle size is much easier to be
reduced than 1Pd(acac)2/HAP-SSD in highly dispersed state.
The particle size variation and the chemical nature of
Pd/HAP were further investigated by X-ray photoelectron
spectroscopy (XPS), as shown in Fig. 5. Samples are first
treated in vacuum at 298 K to remove the volatile impurities
weakly adsorbed on the surface and the Pd 3d spectra all
present two sets of doublet peaks corresponding to Pd 3d5/2
a
and Pd 3d3/2 (Fig. 5A). In the case of 1Pd/HAP-SSD (Fig. 5A-a),
peaks of a1Pd/HAP-SSD centered at Pd 3d5/2 (336.1 eV) and Pd
3d3/2 (341.5 eV) are attributed to Pd0 species, while the peaks
at Pd 3d5/2 (336.8 eV) and Pd 3d5/2 (341.9 eV) are related to
Pd2+ species.28,29,30 It is well known that binding energies of
small Pd clusters generally increased with the reduced cluster
size.31 In detail, for Pd0 3d5/2 peak, the characteristic value of
bulk Pd at >4 nm are approximately 335.0-335.2 eV, and that
of the supported Pd clusters at approximately 1 nm is
increased to 335.9-336.9 eV.32
Fig. 4 TEM images of a) a1Pd/HAP-SSD, c) b1Pd/HAP-SSD and d)
a
1Pd/HAP-WI; b) HAADF-STEM image of 1Pd/HAP-SSD. Insets
in a) and c) are the corresponding particle size distributions.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 5 of 8
View Article Online
DOI: 10.1039/C7GC00262A
Journal Name
ARTICLE
Fig. 5 XPS spectra of a) a1Pd/HAP-SSD, b) b1Pd/HAP-SSD and c) 1Pd/HAP-WI experienced in-situ H2 treatment.
Thus, compared with Pd 3d signal of b1Pd/HAP-SSD and
1Pd/HAP-WI, the shift towards a higher binding energy could
be an indication of the decreased Pd cluster size for a1Pd/HAP-
SSD. Besides, the Pd nanoparticles of a1Pd/HAP-SSD at
approximately 1.2 nm revealed 336.1 eV for Pd0 3d5/2 peak,
which is consistent with the reported ones. Further, according
a
to XPS data, Pd0 percentage of 1Pd/HAP-SSD is calculated to
be 47.1% which is lower than that of b1Pd/HAP-SSD (58.1%).
The main reason is that smaller Pd particles with more surface
metal atoms are inclined to be oxidized in the air.33 However,
after treated at 85 oC (the temperature of following phenol
hydrogenation reaction) in the H2, the Pd0 percentage (72.5%)
a
a
of 1Pd/HAP-SSD reaches the highest of all the samples and Fig. 6 IR spectra of chemisorbed CO on samples: a) 1Pd/HAP-
Pd0 is formed as the major phase of Pd (Fig. 5B-a). This SSD, b) b1Pd/HAP-SSD and c) 1Pd/HAP-WI
indicates that smaller Pd particles on a1Pd/HAP-SSD are also
likely to be reduced in H2 and with more Pd0 species, which Thus, in conjunction with the preceding TEM and XPS analysis,
benefit the hydrogenation reactions.34
the dominant CO liner adsorption for a1Pd/HAP-SSD further
As a sensitive probe to identify the nature and the exposed confirms the decrease of Pd nanoparticle size.
sites of the metal particles, IR spectra of CO chemisorption has
The catalytic performances of Pd/HAP were investigated in
been extensively used.35 As shown in Fig. 6a, a1Pd/HAP-SSD the selective hydrogenation of phenol, which is
a
exhibits two peaks at 2090 and 1920 cm-1, which can be fundamentally important reaction to produce cyclohexanone
assigned to the linear and bridged adsorption of CO on Pd, as intermediates for chemical industry.41-43 All catalysts are
respectively.36 It should be noted that the intensity of the tested and the results are shown in Table 1. Catalyst
linear adsorption band was higher than that of the bridged a1Pd/HAP-SSD showed an excellent phenol conversion of
one, indicating that the linear adsorption of CO is dominant on 73.5% within only 1 h, while under the same reaction
this catalyst. For b1Pd/HAP-SSD and 1Pd/HAP-WI, the low band condition, b1Pd/HAP-SSD
and 1Pd/HAP-WI showed
centred at 2057 cm-1 could also be ascribed to liner CO conversions of 20.4% and 3.8%, respectively. According to the
adsorption.37 However, its intensity is much lower than their analysis using TEM, XPS and IR spectra of chemisorbed CO, the
a
bridged adsorption peaks at 1920 and 1868 cm-1. Previous Pd particle sizes in 1Pd/HAP-SSD are much smaller than that
studies have found that the liner adsorption is attributed to CO of b1Pd/HAP-SSD and 1Pd/HAP-WI, which is in favor of
adsorbed on Pd atoms at edges and corners with single atom catalytic hydrogenation reactions. The higher performance of
or small clusters, whereas bridged adsorption corresponds to catalyst a1Pd/HAP-SSD can also be attributed to the higher Pd0
CO adsorbed on Pd planes of larger particles.38-40
percentage, which has
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 6 of 8
View Article Online
DOI: 10.1039/C7GC00262A
ARTICLE
Journal Name
interaction between Pd(acac)2 and HAP, cooling down might
Table 1 Hydrogenation of phenol with different Pd/HAP cause the reaggreagting of monolayer dispersed liquid
catalysts.
Catalyst
Pd(acac)2, leading to the formation of larger Pd nanoparticles
(~4 nm, b1Pd/HAP-SSD). By contrast, traditional wet
impregnation method results in the forced condensation of
Pd(acac)2 during drying procedure. Larger and uneven
aggregated Pd particles were obtained (~10 nm). The as-
prepared a1Pd/HAP-SSD catalyst exhibited a higher catalytic
activity than those of b1Pd/HAP-SSD and their counterparts
prepared by wet impregnation method for phenol
hydrogenation, which can be attributed to the decrease of Pd
cluster size with more edge and corner atoms, the higher
active Pd0 percentage and the enhanced hydrogen adsorption
capacity. With this method, no solvents are involved,
circumventing drying steps normally involved in preparing such
noble metal supported catalysts, which further avoids the
forced condensation of metal precursor. This methodology
may provide an alternative to classical solution-based
preparation methods and open a new green way to prepare
highly dispersed ultrafine metal supported catalysts with
higher activity.
Time Conversion
Selectivity /
%
/h
/%
73.5
20.4
3.8
C=O
>99
>99
>99
C-OH
n.d
a1Pd/HAP-SSD
b1Pd/HAP-SSD
1Pd/HAP-WI
1
1
1
n.d.
n.d.
Reaction condition: phenol (0.05 mmol), Pd (5 mol % relative to phenol),
water solvent (2 mL), 85 oC, 0.1Mpa of H2, stirring at 1000 rpm, unless
otherwise noted.
Acknowledgements
Fig. 7 Phenol conversion and cyclohexanone selectivity of
a1Pd/HAP-SSD varied with time.
This work was financially supported by the National Science
Foundation of China (21173119, 21273109, 21303083), the
Natural Science Foundation of Jiangsu Province (BK20130563),
the Specialized Research Fund for the Doctoral Program of
Higher Education (20130091120045) and the Fundamental
Research Funds for the Central Universities.
been supposed as the active sites for hydrogenation
reactions.34,44,45 Besides, higher hydrogen adsorption
capacity/ability (higher hydrogen desorption temperature) of
a1Pd/HAP-SSD exhibited by H2-TPD (Fig. S6
†) may also account
for the enhancement of activity.46 It is particularly remarkable
that the selectivity values remain very close to 100% until
conversion values exceed 90% (Fig. 7). This specific selectivity
is supposed to be related with the abundant basic sites on HAP
support.11 When the reaction time was prolonged to 5 h the
final 96.5% selectivity together with 97.9% conversion on
a1Pd/HAP-SSD was achieved, which satisfied the task of
attainment of high selectivity (>95%) at elevated conversion
(>80%) simultaneously,41,42,47 indicating that the highly
dispersed ultrafine Pd nanoparticles on hydroxyapatite
prepared via the present green solvent-free route hold great
potential for the further application in hydrogenation of
phenol to cyclohexanone.
Notes and references
1
2
3
4
P. Munnik, P. E. Jongh and K. P. Jong, Chem. Rev., 2015, 115,
6687-6718.
M. Chen and D. W. Goodman, Acc. Chem. Res. , 2006, 39,
739-746.
Y. Xie, K. Ding, Z. Liu, R. Tao, Z. Sun, H. Zhang and G. An, J.
Am. Chem. Soc., 2009, 131, 6648-6649.
M. Turner, V. B. Golovko, O. P. Vaughan, P. Abdulkin, A.
Berenguer-Murcia, M. S. Tikhov, B. F. Johnson and R. M.
Lambert, Nature, 2008, 454, 981-983.
5
6
7
8
9
J. L. Lu, B. S. Fu, M. C. Kung, G. M. Xiao, J. W. Elam, H. H.
Kung and P. C. Stair, Science, 2012, 335, 1205-1208.
V. Polshettiwar and R. S. Varma, Green Chem., 2010, 12, 743-
754.
F. Moreau and G. C. Bond, Applied Catalysis A: General,
2006, 302, 110-117.
S. Shironita, K. Mori, T. Shimizu, T. Ohmichi, N. Mimura and
H. Yamashita, Appl. Surf. Sci., 2008, 254, 7604-7607.
P. Liu, Y. Zhao, R. Qin, S. Mo, G. Chen, L. Gu, D. M. Chevrier,
P. Zhang, Q. Guo, D. Zang, B. Wu, G. Fu and N. Zheng,
Science, 2016, 352, 797-801.
Conclusions
In conclusion, a facile solvent-free solid state dispersion route
was reported to prepare highly dispersed Pd clusters onto
hydroxyapatite (HAP) support. With Pd(acac)2 loading below its
monolayer dispersion capacity, physically mixed Pd(acac)2 and
HAP were first thermally treated at a temperature slightly
higher than its melting temperature for a short time and
Pd(acac)2 were monolayer dispersed spontaneously. After in-
situ H2 reduction of monolayer dispersed Pd(acac)2, highly
dispersed Pd nanoparticles with ultrafine sizes were obtained
(~1.2 nm, a1Pd/HAP-SSD). However, due to the weak
10 H. Lang, R. A. May, B. L. Iversen and B. D. Chandler, J. Am.
Chem. Soc., 2003, 125, 14832-14836.
11 G. Xu, J. Guo, Y. Zhang, Y. Fu, J. Chen, L. Ma and Q. Guo,
ChemCatChem, 2015, 7, 2485-2492.
12 H. Yan, H. Cheng, H. Yi, Y. Lin, T. Yao, C. Wang, J. Li, S. Wei
and J. Lu, J. Am. Chem. Soc., 2015, 137, 10484-10487.
13 Y. M. Wang, Z. Y. Wu, H. J. Wang and J. H. Zhu, Adv. Funct.
Mater., 2006, 16, 2374-2386.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 7 of 8
View Article Online
DOI: 10.1039/C7GC00262A
Journal Name
ARTICLE
14 Y. Xie, Y. Zhu, B. Zhao and Y. Tang, Stud. Surf. Sci. Catal., 45 X. Xu, M. H. Tang, M. M. Li, H. R. Li, Y. Wang, ACS Catal.,
1998, 118, 441-449. 2014, 4, 3132-3135.
15 P. Gaudin, S. Dorge, H. Nouali, M. Vierling, E. Fiani, M. 46 T. L. Chen, D. Q. Li, H. Jiang, C. R. Xiong, Chem. Eng. J., 2015
,
Molière, J. F. Brilhac and J. Patarin, Applied Catalysis B:
Environmental, 2016, 181, 379-388.
16 T. M. Eggenhuisen, J. P. Breejen, D. Verdoes, P. E. de Jongh
and K. P. de Jong, J. Am. Chem. Soc., 2010, 132, 18318-
18325.
259, 161-169.
47 Y. Wang, J. Yao, H. Li, D. Su and M. Antonietti, J. Am. Chem.
Soc., 2011, 133, 2362-2365.
17 M. Wolters, L. J. W. van Grotel, T. M. Eggenhuisen, J. R. A.
Sietsma, K. P. Jong and P. E. Jongh, Catal. Today, 2011, 163,
27-32.
18 W. Wang, J. Zhang, H. Huang, Z. Wu and Z. Zhang, Appl. Surf.
Sci., 2007, 253, 5393-5399.
19 A. K. Medina-Mendoza, M. A. Cortés-Jácome, J. A. Toledo-
Antonio, C. Angeles-Chávez, E. López-Salinas, I. Cuauhtémoc-
López, M. C. Barrera, J. Escobar, J. Navarrete and I.
Hernández, Applied Catalysis B: Environmental, 2011, 106,
14-25.
20 X. Guo, L. Yu, L. Chen, H. Zhang, L. Peng, X. Guo and W. Ding,
J. Mater. Chem. B, 2014, 2, 1760-1763.
21 W. B. Yue and W. Z. Zhou, Chem. Mater., 2007, 19, 2359-
2363.
22 P. E. Jongh and T. M. Eggenhuisen, Adv. Mater., 2013, 25,
6672-6690.
23 V. I. Levitas, B. F. Henson, L. B. Smilowitz and B. W. Asay,
Phys. Rev. Lett., 2004, 92, 1773-1787.
24 D. Jiang, B. Zhao, Y. Xie, G. Pan, G. Ran and E. Min, Applied
Catalysis A: General, 2001, 219, 69-78.
25 Y. S. Tan, L. Q. Dou, D. S. Lu and D. Wu, J. Catal., 1991, 129,
447-456.
26 X. Y. Wang, B. Y. Zhao, D. E. Jiang and Y. C. Xie, Applied
Catalysis A: General, 1999, 188, 201-209.
27 W. Dai, L. M. Zhou, S. Li, X. Hu and H. Huang, Energy Fuel,
2013, 27, 816-821.
28 H. Y. Jin, T. Y. Xiong, Y. Li, X. Xu, M. M. Li and Y. Wang, Chem.
Commun., 2014, 50, 12637-12640.
29 Z. Z. Wei, Y. T. Gong, T. Y. Xiong, P. F. Zhang, H. R. Li and Y.
Wang, RSC Adv., 2013, 3, 10973-10982.
30 H. K. Sadhanala, R. Nandan and K. K. Nanda, Green
Chem.,2016, 18, 2115-2121.
31 M. Che and C. O. Bennett, Adv. Catal., 1989, 36, 55-172.
32 Z. Karpin’ski, Adv. Catal., 1990, 37, 45-100.
33 X. C. Kang, H. Z. Liu, M. Q. Hou, X. F. Sun, H. L. Han, T. Jiang,
Z. F. Zhang and X. B. Han, Angew. Chem. Int. Ed., 2016, 55,
1080-1084.
34 X. Xu, H. Li, Y. Wang, ChemCatChem, 2014, 6, 3328-3332.
35 M. Baumer, H. J. Freund, Prog. Surf. Sci., 1999, 61, 127-198.
36 B. T. Qiao, J. Lin, L. Li, A. Q. Wang, J. Y. Liu, and T. Zhang,
ChemCatChem, 2014, 6, 547-554.
37 Y. Zhou, Z. H. Xiang, D. P. Cao, and C. J. Liu, Ind. Eng. Chem.
Res., 2014, 53, 1359-1367.
38 T. Schalow, B. Brandt, D. E. Starr, M. Laurin, S. K.
Shaikhutdinov, S. Schauermann, J. Libuda*z and H. J. Freund,
Phys.Chem.Chem.Phys., 2007, 9, 1347-1361.
39 T. Lear, R. Marshall, J. A. Lopez-Sanchez, S. D. Jackson, T. M.
Klapotke, M. Baumer, G. Rupprechter, H. J. Freund, D.
Lennon, J. Chem. Phys., 2005, 123, 174706-174713.
40 G. X. Pei, X. Y. Liu, A. Q. Wang, A. F. Lee, M. A. Isaacs, L. Li, X.
L. Pan, X. F. Yang, X. D. Wang, Z. J. Tai, K. Wilson and T. Zhang,
ACS catal., 2015, 5, 3717-3725.
41 H. Liu, T. Jiang, X. B. Han, S. Liang and Y. Zhou, Science, 2009
326, 1250-1252.
,
42 J. Zhong, J. Chen and L. Chen, Catal. Sci. Technol., 2014, 4,
3555-3569.
43 Y. Z. Xiang, L. Ma, C. S. Lu, Q. F. Zhang and X. N. Li, Green
Chem., 2014,16, 2664-2669.
44 Z. L. Li, J. L. Li, J. H. Liu, Z. L. Zhao, C. G. Xia and F. W. Li,
ChemCatChem, 2014, 6, 1333-1339.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Green Chemistry
Page 8 of 8
View Article Online
DOI: 10.1039/C7GC00262A