Hydrodeoxygenation of phenol as a bio-oil model compound over intimate contact noble metal-Ni2P/SiO2 catalysts

Article abstract of DOI:10.1039/c5ra11203f

This study investigates phenol hydrodeoxygenation on supported Ni₂P, prepared via the sol-gel and TPR methods, and noble metal (Pd, Pt and Ru)-Ni₂P catalysts, prepared from Ni₂P by the partial in situ reduction of a noble metal precursor. 10% Ni₂P/SiO₂ had a relatively uniform distribution of Ni₂P nanoparticles and a highly active activity for phenol hydrodeoxygenation. Phenol conversion increased with increasing reaction temperature and the main products on noble metal-Ni₂P/SiO₂ also changed from cyclohexanol at 453 K to cyclohexane at 493 K. In comparison with supported Ni₂P or noble metal catalysts and their physical mixture, Pd-Ni₂P/SiO₂ presented the highest conversion activity and cyclohexane selectivity. Physicochemical characterization showed that the number of active sites on the catalysts increased and electron transfer occurred from Ni₂P to the noble metal due to the intimate contact between Ni₂P and the noble metal. A synergistic effect of the deoxygenation and carbonyl hydrogenation from Ni₂P and the hydrodeoxygenation from Pd resulted in an improvement of the catalytic activity and differences in the selectivity of the catalysts.

Full text of DOI:10.1039/c5ra11203f

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PAPER
Hydrodeoxygenation of phenol as a bio-oil model
compound over intimate contact noble metal–
Ni P/SiO catalysts†
Cite this: RSC Adv., 2015, 5, 80388
2
2
Yunhua Li,* Xin Yang, Linhui Zhu, Hua Zhang and Binghui Chen
This study investigates phenol hydrodeoxygenation on supported Ni
methods, and noble metal (Pd, Pt and Ru)–Ni P catalysts, prepared from Ni
reduction of a noble metal precursor. 10% Ni P/SiO had a relatively uniform distribution of Ni
nanoparticles and a highly active activity for phenol hydrodeoxygenation. Phenol conversion increased
with increasing reaction temperature and the main products on noble metal–Ni P/SiO also changed
from cyclohexanol at 453 K to cyclohexane at 493 K. In comparison with supported Ni P or noble metal
catalysts and their physical mixture, Pd–Ni P/SiO presented the highest conversion activity and
cyclohexane selectivity. Physicochemical characterization showed that the number of active sites on the
catalysts increased and electron transfer occurred from Ni P to the noble metal due to the intimate
contact between Ni P and the noble metal. A synergistic effect of the deoxygenation and carbonyl
hydrogenation from Ni P and the hydrodeoxygenation from Pd resulted in an improvement of the
2
P, prepared via the sol–gel and TPR
2
2
P by the partial in situ
2
2
2
P
2
2
2
2
2
Received 12th June 2015
Accepted 11th September 2015
2
2
DOI: 10.1039/c5ra11203f
2
www.rsc.org/advances
catalytic activity and differences in the selectivity of the catalysts.
6
metal hypophosphites and converting metal nanoparticles into
1
. Introduction
7
metal phosphides.
At the same time, a series of composite catalysts for the
and clear biofuels and chemicals due to energy consumption hydrodeoxygenation of guaiacol or phenol, the common model
Many efforts have been focused on the production of renewable
8,9
and environmental deterioration. Lignin, typically composed of chemicals in biofuel, have also been studied. For example,
10
methoxy-substituted phenyl propanoid units, is the second Yan et al. catalyzed the hydrodeoxygenation of phenols using
most abundant component in biomass and can be regarded as a Ru nanoparticle catalysts combined with Brønsted acidic ionic
1
1
potential resource for the production of biofuels and special liquids. Hong et al. prepared Pd/WOx/g–Al
2
O
3
for guaiacol
1
chemicals. Bio-oil made from lignin, however, cannot be used hydrodeoxygenation. With respect to bimetallic catalysts con-
without prior rening due to its high viscosity, low heating value taining a transition metal phosphide, Pd–Ni
P/C has presented
2
and corrosiveness. Thus, hydrodeoxygenation (HDO) has been a 1.5 times higher power density than commercial Pd for direct
12
developed to convert this biofuel with these undesirable prop- formic acid fuel cells. Ni
erties into feasible vehicle fuel.
durability of the Pt anode catalyst in direct methanol fuel cells.
P could also enhance the activity and
2
13
Up to now, hydrodeoxygenation catalysts mainly include the In addition, some researchers also noticed that the pro-
traditional MoS , noble metal or transition metal phosphide grammed reduction temperature for phosphate decreased and
2
2–4
based catalysts. Among them, nickel phosphide has been the hydrodesulfurization activity increased with the addition of
14
2
prepared using the traditional temperature programmed Pd or Pt into a Ni P supported catalyst. However, these cata-
reduction (TPR) method. However, there are still problems such lysts have usually been obtained by impregnating a passivated
as high preparation temperatures and the large and uncon- phosphide or phosphate with a noble metal precursor solution.
trollable particle sizes of nickel phosphide with this method. It is difficult to ensure the directional loading of the noble metal
For this reason, several other processes have been developed, and the formation of an intimate contact between the phos-
5
such as the solution-phase method, the decomposition of phide and noble metal due to the nonselective deposition of the
noble metal on the support or phosphate. It is noteworthy that a
transition metal phosphide requires further passivation to
Department of Chemical and Biochemical Engineering, College of Chemistry and avoid the drastic oxidation of nickel phosphide when it is
Chemical Engineering, National Engineering Laboratory for Green Chemical
exposed to air. This means that phosphide has a certain
Productions of Alcohols-Ethers-Esters, Xiamen University, Xiamen 361005, PR
reducibility.
China. E-mail: yunhuali@xmu.edu.cn
Herein, supported nickel phosphide was rstly prepared
from the sol–gel and the traditional TPR method. Thus, Ni P
2
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra11203f
1
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was uniformly dispersed over SiO
2
. The as-prepared supported RuCl
3
, was injected using a syringe into the freshly prepared
nickel phosphide was then impregnated with a noble metal Ni P/SiO in the U-tube quartz reactor under ultrasonic condi-
2
2
precursor solution to obtain the noble metal–Ni P catalyst. This tions. The concentration of the noble metal precursor was 0.01
2
ꢀ
1
preparation process not only favors the distribution of the noble g mL and the corresponding liquid volume used was based on
metal and minimizes its usage due to the controllable distri- the nominal loading of the noble metal, 1%. Aer 40 min, the
bution of reductant Ni
intimate contact structure and intensies the interaction passivated under 1.5% O
between the noble metal and Ni P. Catalytic HDO evaluation For comparison, the supported pure noble metal catalysts
2
P, but also facilitates the formation of an sample was washed, dried under H
2
at 393 K for 2 h and
2
2
/N for 3 h.
2
and catalyst characterizations were conducted on the supported were prepared using the impregnation method. The prepara-
Ni P, noble metal and noble metal–Ni P catalysts. The results tion of the support SiO was similar to that of Ni/SiO except for
2
2
2
2
showed that noble metal–Ni P/SiO had higher catalytic the addition of the nickel precursor. Aer impregnating with
2
2
performances for phenol hydrodeoxygenation, due to more the noble metal precursor solution for 12 h, the samples were
active sites and electron transfer from Ni P to Pd, than the calcined under H at 723 K for 3 h.
supported Ni P or noble metal catalysts. Based on these
2
2
2
outcomes, the relationship between the structure and catalytic
performance were also discussed. The results may provide
2.3. Physicochemical characterization
valuable information for developing Ni
catalysts to upgrade bio-crude-oil to feasible fuel.
2
P or noble metal based The X-ray powder diffraction (XRD) measurements were carried
out on a PAN-alytical X-ray diffractometer with Cu Ka (l ¼
.15406 nm) radiation. A continuous scan mode was used to
0
ꢁ
ꢁ
ꢁ
collect the 2q data from 10 to 90 with a step width of 0.0167 .
Transmission electron microscopy (TEM) images were collected
using a JEOL JEM-1400 instrument. The samples were prepared
by dispersing the powders in ethanol and then dropping a small
volume onto a carbon-coated copper grid.
2. Experimental
2.1. General
Chemical reagents (GC grade) used for analysis, such as
benzene, phenol, cyclohexanol, cyclohexane, cyclohexene,
cyclohexanone, isooctane, dodecane and the noble metal
precursors, were from Aladdin Chemistry Co., Ltd. All other
chemicals were of analytical grade from Sinopharm Chemical
Reagent Co., Ltd, and were used as received without any further
purication.
The specic surface area was determined via the N
2
adsorption–desorption isotherm method at 77 K using a
Micromeritics ASAP 2020 system and calculated using the BET
(Brunauer–Emmett–Teller) method. The pore structure param-
eters were calculated using the BJH (Barrett–Joiner–Halenda)
method. Prior to the analyses, the samples were outgassed at
5
23 K for 3 h to eliminate volatile adsorbates on the surface.
CO chemisorption was performed (Micromeritics ASAP 2020)
2
Ni
2 2
.2. Preparation of the Ni P/SiO catalysts and noble metal–
2
P/SiO
2
to provide an estimate of the number of active sites on the
was prepared via a sol–gel process according to ref. 7. samples. Usually, 0.1 g of a passivated sample was loaded into a
In the typical preparation, a homogeneous solution of 2.62 g quartz reactor and re-reduced in a H
Ni
2
P/SiO
2
ꢀ
1
2
ow (40 mL min ) at 723
Ni(NO $6H O, 4.2 g urea and 225 mL H O was kept in a K for 2 h. Aer evacuating for 1 h at this temperature, the
3
)
2
2
2
ꢀ
1
conical ask at 303 K, and its pH value was adjusted to 2.5 using samples were cooled down to RT in a He ow (60 mL min ).
a diluted HNO3 solution. A solution of 22.4 mL tetraethyl Subsequently, the rst isotherm was taken. Aer the rst
orthosilicate (TEOS) and 20 mL ethanol was dropwise added to isotherm was measured, the sample was evacuated for 2 h. The
the above solution under stirring. The as-prepared sample was second isotherm was performed. The CO chemisorption
then heated to 363 K, ltered at pH 8.0, dried at 393 K for 12 h obtained was calculated by subtracting the amount of adsorbed
and calcined in U-tube quartz reactor at 823 K for 5 h to obtain CO in the second isotherm.
Ni/SiO
2
. Ni
2
P/SiO
2
was further prepared via the traditional TPR
To characterize the structure and composition of the cata-
method using a NH
4
H
2
PO solution to impregnate the as- lysts, high angle annular dark eld scanning transmission
4
prepared Ni/SiO sample. The nominal Ni/P ratio of the cata- electron microscopy (HAADF-STEM) and energy-dispersive X-
2
lyst was 1.0. Aer impregnation for 12 h, the sample was ray spectroscopy (EDS) were performed using a FEI TECNAI
calcined at 823 K in an air atmosphere for 4 h. Aer being F30. X-ray photoelectron spectroscopy (XPS) measurements
cooled to room temperature, the sample was reduced at 923 K in were performed using a Quantum 2000 Scanning ESCA Micro-
a H
2
ow for 3 h and passivated under 1.5% O
2
/N
2
for 6 h at probe system with Al Ka radiation under ultrahigh vacuum. The
room temperature.
binding energies were internally referenced to the C1s peak at
The noble metal–Ni
reduction of the noble metal precursors. The detailed procedure
2
P/SiO
2
was prepared by an in situ 284.6 eV.
Transmission infrared spectra of the catalysts were collected
is as follows. Aer the preparation of the fresh Ni P/SiO in the in situ in a reactor cell placed in a Bruker Vertex 70 Fourier
2
2
U-tube quartz reactor was completed, the H ow was switched transform infrared (FTIR) spectrometer. The IR cell was equip-
2
to a N atmosphere. This reactor lled with N was placed in an ped with CaF windows, connections for inlet and outlet ows,
2
2
2
ultrasonic cleaner (KQ-250DE). A certain concentration of a and thermocouples to monitor and control the temperature.
noble metal precursor solution, such as PdCl , H PtCl , or Before introducing CO, the samples were reduced in H at 723 K
2 2 6 2
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for 3 h, then cooled to room temperature in a He ow and diffraction peak at 5% Ni
2
P/SiO
2
compared with the other
exposed to CO for 3 h to achieve saturation. The samples were samples due to the low loading and high distribution of Ni P.
2
then purged in an Ar ow for 30 min to remove the gaseous and However, excess loading leads to larger particle sizes of Ni2P
weakly adsorbed CO species. The spectra were obtained in the and correspondingly decreases the hydrodeoxygenation activity.
absorbance mode and were presented aer the subtraction of a The appropriate catalyst is 10% Ni
2 2
P/SiO for phenol hydro-
background spectrum obtained on the freshly reduced samples. deoxygenation. The TEM micrographs in Fig. 2 present the
distribution of the Ni
With the increase in the loading from 5 to 15%, Ni
size increases from about 2 nm to 6 nm. 10% Ni
uniform particle size of 3.2 nm. This is similar to the XRD result
using the Scherrer formula.
2
P particle size in the Ni
2
P/SiO
2
catalysts.
2
P particle
2
.4. Phenol hydrodeoxygenation evaluation
2
2
P/SiO has a
For the hydrodeoxygenation reaction, phenol (0.27 g), dodecane
as a solvent, (20 mL) and the catalyst (0.1 g) were rstly loaded
into an autoclave (100 mL). The reactor was ushed with N to
remove air and then stably pressurized with H to 2.0 MPa at
room temperature. Aer the autoclave was heated to 453 or 493
K, the reaction lasted for a certain amount of time. At last, the
liquid product was analyzed with a gas chromatograph (SHI-
MADZU GC2010) equipped with an auto-sampler and a ame
ionization detector, using N
as the internal standard for the quantication of products.
2
Cyclohexane, cyclohexanol and cyclohexanone are the main
2
2 2
products over Ni P/SiO at 453 K (Fig. 1). Cyclohexanol and
cyclohexanone are intermediates for the phenol hydro-
16
deoxygenation to cyclohexane. The higher reaction tempera-
ture favors the formation of more cyclohexane. Thus, the
cyclohexane selectivity prominently rises and those of cyclo-
hexanol and cyclohexanone dramatically drop, accompanied by
an increase in phenol conversion when the reaction tempera-
ture increases from 453 K to 493 K.
2
as carrier gas. Isooctane was used
3
. Results and discussion
3
.1. Phenol hydrodeoxygenation and physicochemical
characterization over Ni P/SiO
Fig. 1 shows the phenol conversion and product selectivity at
53 and 493 K. With increasing Ni
activity increases and then decreases at 453 and 493 K. A similar Ni
change tendency is also found over Ni P/SiO for benzofuran noble metal, the catalytic activities of Ni P/SiO2 increase
2
2
3.2. Physicochemical characterization and phenol
hydrodeoxygenation of noble metal–Ni P/SiO
2
2
4
2
P loading, the catalytic Fig. 3 demonstrates the catalytic results over the noble metal–
P/SiO catalysts at 453 and 493 K. With the addition of a
2
2
2
2
2
15
hydrodeoxygenation. Fig. S1† demonstrates the XRD patterns dramatically. At the same time, the selectivity toward cyclo-
of Ni P/SiO with different loadings. There is no prominent Ni hexanol increases while for cyclohexanone it decreases at 453 K.
2
2
2
P
2 2
Fig. 1 Phenol conversion and product selectivities of the different Ni P/SiO catalysts: (a and b) 453 K; (c and d) 493 K.
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2 2
Fig. 2 TEM images and particle size distribution for the different Ni P/SiO catalysts: (a) 5%, (b) 10% and (c) 15%.
2 2
In other words, the presence of a noble metal leads to the effect between the noble metal and Ni P on 1% Pd–10% Ni P/
generation of a great deal of cyclohexanol with the increase in SiO2.
the phenol conversion over the noble metal–Ni P/SiO catalyst.
Thus, a series of physicochemical characterizations were
In comparison with the supported noble metal catalysts conducted to further unravel the root of the improvement of the
Fig. S2†), the catalytic activity over noble metal–Ni P/SiO catalytic performances of the noble metal–Ni P/SiO . Table 1
2
2
(
2
2
2
2
changes slightly. However, cyclohexane and cyclohexanol lists the BET surface area, pore volume, pore size and CO
selectivities increase and the cyclohexanone selectivity chemisorption for the different catalysts. As shown in Table 1,
decreases obviously. This is mainly because the existence of compared with the Si
Ni P intensies the deoxygenation and carbonyl hydrogenation volume of the 10% Ni
over noble metal–Ni P/SiO . When the reaction temperature phosphorus precursor. This indicates that parts of the catalyst
2
O support, the BET surface area and pore
2
2
P/SiO decrease with the addition of the
2
2
2
further increases up to 493 K, the effects of the addition of a pores were blocked by the phosphorus source during the
noble metal on the catalytic activities are not prominent, since impregnation process. On the other hand, with the addition of a
1
0% Ni
rise of the catalytic activities is limited. However, the main metal–Ni
products also change from cyclohexanol at 453 K to cyclohexane the consumption of part of the Ni
at 493 K on noble metal–Ni P/SiO . The effect of reaction time noble metal precursors in contact with dispersed Ni
on the product selectivities (Fig. S3†) also conrms that cyclo- convert into noble metal nanoparticles because of the Ni
2
P/SiO
2
has a high phenol conversion, 92.6%, and the noble metal, the pore size and surface area increase for noble
P/SiO . The increments of the pore sizes result from
P. At the same time, only
P can
2
2
2
2
2
2
2
P
hexanol can gradually convert into cyclohexane when prolong- reducing the noble metal precursors during the preparation
ing the reaction time over 1% Pd–10% Ni P/SiO . Thus, in process. As a result, the increasing surface area is a compromise
2
2
comparison with the pure noble metal or Ni P catalysts, the between the consumption of Ni P and the high dispersion of
2
2
synergistic effect of the hydrodeoxygenation from Pd and the the generated noble metal nanoparticles. Moreover, with the
deoxygenation and carbonyl hydrogenation from Ni P gives rise addition of the noble metal, the number of active sites also
to the increment in the hydrodeoxygenation activity and the increases, except for 1% Ru–10% Ni P/SiO . If it is assumed that
differences in the selectivity of the noble metal–Ni P catalysts as CO is linearly adsorbed on the nickel atoms of Ni P, as was
well as higher reaction temperatures and longer times. In theoretically and experimentally shown by Nelson et al. and
2
2
2
2
2
18
19
addition, the catalytic activities over the supported catalysts Layman and Bussell, respectively, and the same type of
were compared with that of a mixture of 1% Pd/SiO and 10% adsorption exists for the noble metal atoms, except for partially
2
17
Ni P/SiO2 (Fig. 4 and Table S1†). The result showed that 1% bridged CO over Pd and Ru, then increased CO chemisorption
2
2 2
Pd–10% Ni P/SiO has the higher phenol conversion and uptake implies that more active sites for hydrodeoxygenation
cyclohexane selectivity than the pure and physically mixed are generated with the addition of a noble metal. As shown in
samples. This implies again the occurrence of a synergistic Table 1, the trend for CO chemisorption uptake is 1% Pd–10%
2 2 2 2 2 2
Ni P/SiO > 1% Pt–10% Ni P/SiO > 10% Ni P/SiO z 1%
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2 2
Fig. 3 Phenol conversion and product selectivities of the different noble metal–Ni P/SiO catalysts: (a and b) 453 K; (c and d) 493 K.
Ru–10% Ni
result of the catalysts. Thus, the high hydrodeoxygenation palladium, nickel and phosphorus in Fig. 6. The loading site of
capacity of Pd or Pt–Ni P/SiO is partly attributed to the increase the phosphorous precursor is uncontrollable while the forma-
in the number of active sites. tion of Pd nanoparticles depends on Ni P reducing the Pd
Fig. 5 shows the XRD patterns of noble metal–Ni P/SiO . The precursor. As shown in Fig. 6, the distribution density of Pd is
2
P/SiO
2
. This is roughly consistent with the activity
HAADF-STEM mapping demonstrates the distributions of
2
2
2
2
2
XRD peaks of Ni P(111), Pd(111), Pt(111) and Ru(111) are 40.714 mainly consistent with that of Ni P. This result further veries
2
2
(
PDF#74-1385), 40.365 (PDF#87-0645), 39.796 (PDF#87-0646) that a controllable distribution of noble metal nanoparticles is
ꢁ
and 40.772 (PDF#88-2333), respectively. The intensity of the achieved and the intimate contact structure between the noble
2 2 2
Ni P(111) diffraction peak is relatively weak, whereas the noble metal and Ni P is formed by Ni P reducing the noble metal
metal peak becomes prominent with the addition of noble precursor. Combined with the CO chemisorption and XRD
metal. This implies that the generation of a noble metal nano- results, we speculate that the controllable distribution of the
particle is at the expense of Ni
2
P. The XRD patterns show that noble metal nanoparticle results in the intimate contact
the Ni P(111) peak shis toward lower angles for Pd(111) and between noble metal and Ni P, further leading to interactions
2
2
Pt(111) with the addition of the noble metal, but not for between both of them and the increment in the number of
Ru–Ni P/SiO2 with its weak diffraction. This result indicates active sites, compared with 10% Ni P/SiO .
2
2
2
that Pd (0.179 nm) or Pt (0.183 nm) with their larger atomic radii
Fig. 7 shows the XPS spectra of the different pure noble metal
is doped into the Ni
or Pt leads to lattice distortion of Ni
2
P (Ni (0.124 nm)), or that the addition of Pd and composite catalysts. It has been noted that the signal to
20
2
P. Furthermore, we noise ratio is similar for the three couples of samples contain-
believe that an interaction between the noble metals and Ni
formed.
2
P is ing Pd, Pt and Ru, respectively, and that the intensity differ-
ences between the SiO and Ni P/SiO supported samples are in
2
2
2
the same direction. This implies that noble metal–Ni P/SiO2
2
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2 2
Fig. 5 XRD diffraction patterns of the different noble metal–Ni P/SiO
catalysts.
Fig. 4 Phenol conversion and product selectivities of 1% Pd–10%
Ni P/SiO and the physical mixture of Pd/SiO and Ni P/SiO .
2 2 2 2 2
2 2
Fig. 6 HAADF-STEM of 1.0% Pd–10% Ni P/SiO .
might have a stronger signal in the XPS spectra than the pure
noble metal catalysts. In addition, Table S2† shows the surface
atomic ratios with respect to Si, O and the noble metal of these
catalysts. A comparison also showed that the composite cata-
lysts have higher loading ratios of the noble metal on the
catalysts. The XPS signal of the supported metal particles shis
to a higher binding energy with decreasing particle size.
Meanwhile, the electrons transferring from Ni P to the noble
21
2
surface than the SiO supported catalyst. This result might be
2
metal render the latter electron rich and move the corre-
sponding signal toward a lower binding energy. Therefore, the
Pd spectrum peak shiing toward the lower binding energy in
Fig. 7 is a compromise, resulting from the smaller noble metal
particles and electron transferring. However, this electron
related to Ni P reducing the noble metal precursor into nano-
2
particles. The existence of reductant Ni
generation of more noble metal particles. On the other hand,
the spectrum peak of Pd in 1% Pd–10% Ni P/SiO shis toward
2
P contributes to the
2
2
a lower binding energy compared to the pure noble metal
Table 1 Textural and structural properties of different catalysts
Pore size
(nm)
Pore volume
BET surface
area (m g
CO uptake
3
ꢀ1
2
ꢀ1
ꢀ1
Catalyst
(cm g
)
)
(mmol g
)
SiO
2
–sol–gel
0% Ni P/SiO
% Pt–10% Ni
% Ru–10% Ni
% Pd–10% Ni
9.30
10.03
11.60
11.49
15.62
0.98
0.90
0.85
0.88
1.00
421.00
238.55
293.63
307.39
257.12
—
1
1
1
1
2
2
92.25
109.51
90.77
121.32
2
P/SiO
2
2
P/SiO
2
2
P/SiO
2
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2 2
Fig. 7 XPS spectra of the different noble metal–Ni P/SiO catalysts.
transferring is likely the reason for the high catalytic perfor- selectivity as well as a higher conversion than Ni P/SiO . That is,
2
2
2
2
mance of Pd–Ni
2
P/SiO
2
. As proposed by Ueckert et al., a metal the synergistic effect of Pt and Ni
P/SiO
because Ru is prone to oxidation
promotes the hydrogenation of the aromatic ring. Moreover, again. Ru that has been oxidized and fewer active sites on 1%
since the lowest unoccupied molecular orbital (LUMO) of C–O is Ru–10% Ni P/SiO account for a lower catalytic performance
antibonding and surfaces that are able to transfer electron than of the other composite catalysts.
density to this orbital facilitate the dissociation of the C–O The electronegativity of an element represents the capability
bond, the increased electron density on Pd accounts for the of an atom to draw an electron to itself. A larger electronega-
higher hydrodeoxygenation catalytic activity of Pd–Ni P/SiO tivity means that this atom has a greater ability to draw elec-
For product selectivity, cyclohexanol increases while cyclohex- trons. For Ni
anone decreases over the noble metal–Ni P/SiO catalysts due to 1). Ni has the lower electronegativity (1.91) while the value for
the improvement in the hydrogenation capacity in comparison phosphorus is 2.19. There is a small electron transfer from Ni to
with 10% Ni P/SiO at 453 K. As mentioned above, the syner- P in nickel phosphide. In contrast, the electronegativities of Pd,
2
P on the catalytic performance
site with a high electron density favors the formation of a p back is less prominent than Pd–Ni
2
2
. Ru exists in the form of
4
+
2 2
bond between the aromatic ring and metal sites, which Ru in 1% Ru–10% Ni P/SiO
2
2
23
2
2
.
d+
dꢀ
2
P, Ni (0 < d < 2) covalently bonds with P (0 < d <
2
2
2
2
gistic effect of the hydrodeoxygenation from Pd and the deox- Pt and Ru are 2.20, 2.28 and 2.20, respectively. A noble metal in
ygenation and carbonyl hydrogenation from Ni P results in the contact with Ni P probably accepts electrons from nickel due to
2
2
promotion of the catalytic performance of the noble metal–Ni2P the larger electronegativity of the noble metal.
catalysts. Herein, this effect is mainly due to the interaction CO-FTIR spectroscopy was also performed to further verify
between Ni P and the noble metal and the electron transfer the electronic contribution of the supported catalysts (Fig. 8).
from Ni P to Pd on Pd–Ni
rises from 453 K to 493 K, the main product changes from higher wavenumbers, 2094, 2083 and 2079 cm
cyclohexanol to cyclohexane because of the intensifying deoxy- addition of Pd, Pt and Ru, respectively. According to the Bly-
2
ꢀ
1
2
2 2
P/SiO . As the reaction temperature The FTIR spectra of adsorbed CO shis from 2077 cm to
ꢀ
1
with the
24
genation at higher temperatures. In contrast, although elec- holder model, this shi toward higher frequencies is indica-
trons also transfer from Ni P to Pt on the noble metal–Ni P tive of the strengthening of the CO bond due to decreased
2
2
11
catalysts, 1% Pt–10% Ni P/SiO exhibits a lower catalytic donation of the metal d-electrons into the CO p* antibonding
2
2
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2 2 2 2 2 2 2 2
Fig. 8 Infrared spectra of adsorbed CO on 10% Ni P/SiO , 1% Pd–10% Ni P/SiO , 1% Pt–10% Ni P/SiO and 1% Ru–10% Ni P/SiO .
25
orbital by back-electron transferring. Since the presence of Pd number by CO chemisorption. Utilizing CO-FTIR for quanti-
gives rise to the electron donation from Ni P to Pd, electron cation is inaccurate due to the effects of the loading of catalysts
2
enrichment on Pd and electron deciency on Ni P occur, as also and the roughness of the tested surface. In comparison with
2
reported by the XPS data. Thus, metal d-electron transfer into CO-FTIR, CO chemisorption is a typical method to determine
the CO p* antibonding orbital decreases and the stretching the active sites of catalysts. Therefore, herein CO-FTIR is per-
vibration of the adsorbed CO shis toward higher frequencies. formed to only conrm the electron transfer from Ni P to the
2
In addition, the vibration frequency of CO adsorbed on the noble metal on the catalyst surface.
ꢀ1
catalysts between 2077 and 2094 cm also indicates that most
of the CO is linearly adsorbed on noble metal–Ni P/SiO , in the key factors inuencing the catalyst activity and the selec-
The evaluation and characterization results suggested that
2
2
agreement with the assumption for the determination of active tivities toward different products were the increased number of
sites in CO chemisorption. However, we also notice that the active sites and the electron transfer from Ni P to Pd upon
2
intensity change of the CO stretching vibration with the addi- addition of the noble metal as well as the appropriate reaction
tion of the noble metal is not consistent with the active site temperature and time. Furthermore, the increment of the active
2 2
Fig. 9 A possible deoxygenation scheme for noble metal–Ni P/SiO .
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sites and the electron transfer are from the reduction of the
noble metal precursor on Ni P/SiO and the interaction between
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Ni P and the noble metal. Based on experimental results, a
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possible deoxygenation scheme over noble metal–Ni P/SiO is
2
2
2
also suggested as follows (Fig. 9). Pure Ni P has a higher
selectivity for cyclohexanone and cyclohexanol. In contrast,
cyclohexanone can continue to hydrogenate into cyclohexanol
over noble metal–Ni
due to the synergistic effect of Ni
2
P/SiO
2
under the same reaction conditions
P and Pd. When the reaction
2
´
´
temperature further increases or the hydrodeoxygenation time
is prolonged, cyclohexanol can continue to convert into cyclo-
hexane over noble metal–Ni P/SiO . In terms of the Ni P catalyst
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. Conclusions
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0% Ni
a uniform distribution of Ni
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2
P/SiO
2
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P nanoparticles and a relatively
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2
2
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at 453 K to cyclohexane at 493 K on Ni
Ni P/SiO . In addition, 1% Pd–10% Ni
2
P/SiO
P/SiO
2
or noble metal–
presents higher
S. J. A. Figueroa, F. G. Requejo and F. C. Vicentini, J.
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2
2
2
2
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2
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2
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180.
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15791.
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Acknowledgements
2
This project was supported nancially by the National Natural
Science Foundation of China (Grant No. 21476188, 21106118).
We would like to appreciate Xing Zhang’s contribution to the
preparation of the manuscript.
2
2
2
3 V. M. L. Whiffen and K. J. Smith, Energy Fuels, 2010, 24, 4728.
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80396 | RSC Adv., 2015, 5, 80388–80396
This journal is © The Royal Society of Chemistry 2015