Selective hydrogenation of phenylacetylene over bimetallic Pd-Cu/Al2O3 and Pd-Zn/Al2O3 catalysts

Article abstract of DOI:10.1016/j.cattod.2015.08.018

Bimetallic Pd-Cu/Al₂O₃ and Pd-Zn/Al₂O₃ catalysts with various Cu/Pd and Zn/Pd molar ratios were prepared and applied to the selective hydrogenation of phenylacetylene to styrene. The structure properties of the

Full text of DOI:10.1016/j.cattod.2015.08.018

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Selective hydrogenation of phenylacetylene over bimetallic
Pd–Cu/Al2O3 and Pd–Zn/Al2O3 catalysts
Zhanqi Wanga, Lei Yanga, Rui Zhangb, Li Lic, Zhenmin Chenga, Zhiming Zhoua,∗
a State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
b Engineering Research Center of Large Scale Reactor Engineering and Technology, Ministry of Education, East China University of Science and Technology,
Shanghai 200237, China
c Chemical Engineering Research Center, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Bimetallic Pd–Cu/Al2O3 and Pd–Zn/Al2O3 catalysts with various Cu/Pd and Zn/Pd molar ratios were pre-
pared and applied to the selective hydrogenation of phenylacetylene to styrene. The structure properties
of the bimetallic catalysts were characterized by inductively coupled plasma–optical emission spec-
troscopy (ICP–OES), CO chemisorption, high-resolution transmission electron microscopy (HRTEM) and
X-ray photoelectron spectroscopy (XPS). Different from Pd–Cu/Al2O3 where the interaction between Pd
and Cu only generated the geometric effect, both geometric and electronic effects were observed for
Pd–Zn/Al2O3. The activity and selectivity of the bimetallic catalysts can be tuned by varying the Cu/Pd
and Zn/Pd molar ratios, which were essentially determined by the geometric and electronic effects of
the catalysts. Although both Pd–Cu/Al2O3 and Pd–Zn/Al2O3 exhibited improved selectivity to styrene
compared to monometallic Pd/Al2O3, there is a difference in the activity and selectivity between the two
types of bimetallic catalysts at the same Cu/Pd and Zn/Pd molar ratio, which is mainly caused by the
different modification degrees of Cu and Zn on Pd. Among all the catalysts prepared in this work, the
Pd–Zn/Al2O3 with a Zn/Pd molar ratio of 6 displayed the best selectivity to styrene, being 86.3% at 99.5%
phenylacetylene conversion.
Received 19 May 2015
Received in revised form 10 August 2015
Accepted 12 August 2015
Available online xxx
Keywords:
Selective hydrogenation
Phenylacetylene
Pd–Cu/Al2O3
Pd–Zn/Al2O3
Activity
Selectivity
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
ascribed the increased selectivity to the electronic effect when CuO
was present in the catalyst or the geometric effect for the Pd–Cu
Selective hydrogenation of phenylacetylene is an important
reaction in styrene polymerization, aiming at removing the small
amount of phenylacetylene in the styrene monomer [1–3]. At
present, supported Pd catalysts are most widely used for this reac-
tion, but a sharp decrease in the selectivity to styrene usually occurs
over these catalysts when conversion of phenylacetylene is as high
as 95% [4–6]. It has been found that the addition of a second metal to
Pd can effectively increase the catalyst selectivity to styrene owing
to electronic and geometric effects, and the commonly used second
metal includes Ag [7], Cu [8–10], Ni [11] and Zn [12,13].
Pd–Cu bimetallic supported catalysts are widely applied to
selective hydrogenation of unsaturated organic compounds such
as acetylene [14], 1,3-butadiene [15], phenylacetylene [8–10] and
alkynol [16]. When it comes to phenylacetylene, Guczi et al. [8] pre-
pared Pd–Cu/pumice catalysts and reported selectivity to styrene
of 83–92% when conversion of phenylacetylene was 95%. They
system. Boucher et al. [9] designed and synthesized nanoparticle
Pd–Cu catalysts based on model single atom alloy surfaces, where
Pd atoms acted as active sites for H2 uptake, dissociation, and
spillover onto the Cu surface on which the selective hydrogenation
of phenylacetylene took place. The developed Pd–Cu alloy cata-
lysts exhibited a marked increase in the selectivity to styrene when
compared to Pd/Al2O3, which was mainly due to the H2 spillover
on the Cu surface. Very recently, we prepared a Pd–Cu/Al2O3 cat-
alyst that demonstrated 95% selectivity to styrene at a conversion
of phenylacetylene of 90%, and around 82% selectivity even at 99%
conversion [10].
Pd–Zn bimetallic supported catalysts also find applications in
selective hydrogenation reactions, and the PdZn alloy is normally
formed by reducing Pd/ZnO at an increased temperature [13,17,18].
Tew et al. [17] made use of in situ high-resolution X-ray diffraction
and X-ray absorption spectroscopy to characterize the forma-
tion of a PdZn alloy from a Pd/ZnO catalyst, and found that the
alloy formation started at 373 K and its amount increased with
the reduction temperature. Compared to Pd/SiO2, the developed
Pd–Zn/ZnO catalyst showed a higher selectivity to pentenes in
∗
Corresponding author. Tel.: +86 21 64252230; fax: +86 21 64253528.
E-mail address: zmzhou@ecust.edu.cn (Z. Zhou).
http://dx.doi.org/10.1016/j.cattod.2015.08.018
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Z. Wang, et al., Selective hydrogenation of phenylacetylene over bimetallic Pd–Cu/Al2O3 and
Pd–Zn/Al2O3 catalysts, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.018

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the selective hydrogenation of 1-pentyne. Similarly, Yoshida et al.
[13] reported that the alloyed Pd–Zn/ZnO catalyst obtained by
reduction of Pd/ZnO at 773 K was more selective to the hydrogena-
tion of phenylacetylene to styrene by comparison with Pd/ZnO.
Unfortunately, ZnO is not an ideal catalyst support due to some
disadvantageous properties, such as low surface area, high den-
sity and poor crush strength [19]. Thus, other support materials are
used to prepare Pd–Zn catalysts. For instance, Mashkovsky et al.
prepared Pd–Zn/Al2O3 [20] and Pd–Zn/C [21], which displayed high
selectivity to ethylene in the selective hydrogenation of acetylene.
However, to the best of our knowledge, no other supported Pd–Zn
catalysts except Pd–Zn/ZnO have been reported for the selective
hydrogenation of phenylacetylene.
The above literature survey clearly shows that Pd–Cu and Pd–Zn
bimetallic supported catalysts have high selectivity to alkenes in
alkyne hydrogenation, but which one is better in the selective
hydrogenation of phenylacetylene? It is very difficult to make a
comparison based on the data reported in the literature because
of the difference in preparation method, metal loading, reaction
condition, etc. In this work, a series of bimetallic Pd–Cu/Al2O3
and Pd–Zn/Al2O3 catalysts is prepared by a two–step technique
and applied to the selective hydrogenation of phenylacetylene
to styrene. The effects of Cu/Pd and Zn/Pd molar ratios on the
activity and selectivity of the catalyst are investigated, and the
catalytic performance of the two types of catalysts is compared.
Furthermore, the possible reasons for the difference in the activ-
ity and selectivity between the two catalysts are discussed in
detail.
Zn loading in Cu/Al2O3 or Zn/Al2O3 was 1.8 wt.%, which was equal
to that in PdCu6/Al2O3 or PdZn6/Al2O3, respectively.
2.3. Catalyst characterization
The actual metal loading of catalyst was determined by induc-
tively coupled plasma–optical emission spectroscopy (ICP–OES,
Varian 710ES). The amount of Pd active sites was calculated from
the CO chemisorption (Micromeritics AutoChem 2920) by assum-
ing a stoichiometry of one CO molecule per Pd atom on the surface.
Before measurements, the catalyst was reduced in flowing H2 (10%
H2/Ar, 100 mL min−1) at 673 K for 2 h, followed by switching to He
(100 mL min−1) at 673 K for 20 min to remove adsorbed H2. When
the sample was cooled to 308 K under a He flow, CO pulses were
injected and the signal was monitored by a thermal conductivity
detector. The micro-morphology of catalyst and the average Pd par-
ticle size were analyzed by high-resolution transmission electron
microscopy (HRTEM, JEOLJEM-2100). The samples for HRTEM were
dispersed in ethanol using ultrasound for 30 min, after which a
droplet of the suspension was placed on a carbon-coated copper
grid. The average Pd particle size was determined by measuring
at least 100 particles. The catalyst surface was examined by X-
ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB
250Xi) measurement. The XPS analysis was performed using Al
K␣ (hv = 1486.6 eV) as the excitation source and a pass energy of
40 eV. The binding energy calibration was based on the C1s peak at
284.8 eV. Before HRTEM and XPS analysis the catalyst sample was
reduced at 673 K in 50% H2/N2 for 2 h.
2.4. Catalyst test
2. Experimental
The catalysts were evaluated in the selective hydrogenation of
phenylacetylene using a 300 mL glass reactor equipped with a mag-
netic stirrer (Parr 5100). First, 90 g of absolute ethanol (solvent), 5 g
of phenylacetylene (reactant), 5 g of n-octane (internal standard)
and 0.15 g of reduced catalyst (reduced at 673 K in 50% H2/N2 for
2 h with a heating rate of 5 K min−1) were charged into the reac-
tor, and the solution was heated to 313 K under N2 atmosphere
with mild stirring. Next, the reactor was purged five times with
H2 in the absence of stirring, and then the reaction started under
313 K and 0.1 MPa with vigorous stirring at 1000 rpm. The reaction
product discharged from the reactor was analyzed by gas chro-
matograph (HP-6890) equipped with a DB-Wax capillary column
(30 m × 0.32 mm × 0.5 ␮m) and a flame ionization detector.
The selectivity to styrene is defined as the ratio of the number
of moles of styrene produced to that of phenylacetylene consumed
in the reaction. The catalyst activity is expressed in terms of the
initial rate (Rini), which is the number of moles of phenylacetylene
converted per gram Pd per second during the initial period, and the
turnover frequency (TOF), which is defined as the number of pheny-
lacetylene molecules converted per second per surface Pd site that
is determined by CO chemisorption measurements. It should be
noted that the adsorption of CO on Cu in Pd–Cu/Al2O3 and on Zn
in Pd–Zn/Al2O3 is not taken into account since these metals hardly
adsorb CO [22,23], and our results (Table 1) also confirmed this
point.
2.1. Materials and reagents
␥-Al2O3 (99.9%, average particle size of 40 ␮m) and pheny-
lacetylene (>98%) were obtained from Alfa Aesar. PdCl2 (Pd ≥59.5%),
HCl (36–38%), Cu(NO3)2·3H2O (≥99%), Zn(NO3)2·6H2O (≥99%),
NaBH4 (≥98%), polyvinylpyrrolidone (PVP, weight-average molec-
ular weight of 58,000), absolute ethanol (≥99.7%) and n-octane
(≥98%) were purchased from Sinopharm Chemical Reagent Co., Ltd.
All chemicals were used as received without further purification.
2.2. Catalyst preparation
The Pd/Al2O3 catalyst was prepared by impregnating colloidal
Pd nanoparticles (NPs) into ␥-Al2O3 [4,10]. First, about 0.426 g of
PVP, 24 mL of H2PdCl4 (2 mmol L−1) and 66 mL of deionized water
were added into a beaker and stirred vigorously for 30 min, and
then 10 mL of NaBH4 (9.6 mmol L−1) was rapidly added into the
above solution and stirred for 3 h for the formation of colloidal Pd
NPs. Next, 1 g of ␥-Al2O3 was added into the above colloidal solu-
tion and continuously stirred for 48 h at room temperature. Finally,
the solid was filtered, washed successively with absolute ethanol
and deionized water for several times, dried at 383 K for 12 h, and
calcined at 723 K in air for 3 h with a heating rate of 2 K min−1
The developed Pd/Al2O3 catalyst had a theoretical Pd loading of
0.5 wt.%.
.
The bimetallic Pd–Cu/Al2O3 or Pd–Zn/Al2O3 catalyst was pre-
pared by incipient wetness impregnation of the above Pd/Al2O3
catalyst with Cu(NO3)2 or Zn(NO3)2 solution with different concen-
tration [10], followed successively by drying at room temperature
for 6 h and at 383 K for 12 h, and calcining in air at 723 K for 3 h.
The resulting catalyst is denoted by PdCux/Al2O3 or PdZnx/Al2O3,
where x represents the molar ratio of Cu or Zn to Pd.
3. Results and discussion
3.1. Physico-chemical properties of catalyst
Table 1 summarizes the actual metal loading and CO uptake of
the catalysts. The Cu or Zn loading of all the catalysts is very close to
the theoretical loading, but the Pd loading is lower than the theo-
retical value (0.5 wt.%). The reason lies mainly in the different way
of introducing Pd and Cu or Zn used in this study. The Pd metal
In addition, Cu/Al2O3 and Zn/Al2O3 were prepared by incipient
wetness impregnation and used as reference catalysts. The Cu or
Please cite this article in press as: Z. Wang, et al., Selective hydrogenation of phenylacetylene over bimetallic Pd–Cu/Al2O3 and
Pd–Zn/Al2O3 catalysts, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.018

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3
Table 1
particle C, however, shows a lattice spacing of 0.194 nm that corre-
sponds to the (2 0 0) plane of fcc Pd [30], suggesting that not all Pd
particles are alloyed with Zn. In our previous study on Pd–Cu/Al2O3
[10], we also observed individual Pd particles besides PdCu alloy.
The electronic properties of the metal species on the cata-
lyst surface are investigated by XPS. Fig. 3 shows the XPS spectra
for Pd 3d of Pd/Al2O3, PdCu0.6/Al2O3 and PdCu6/Al2O3. Although
all the catalysts are reduced ex situ at 673 K before XPS analy-
sis, the PdO species is still present (Pd 3d5/2 at 336.4 eV) [31],
probably due to oxidation by air during the sample transfer.
The monometallic Pd/Al2O3 shows characteristic doublet peaks
assigned to metallic Pd0 (Pd 3d5/2 at 334.9 eV) [9,32], which is
unchanged for PdCu0.6/Al2O3 but shifted to a higher binding energy
for PdCu6/Al2O3 (Pd 3d5/2 at 335.4 eV). What is the reason for the
different binding energy of Pd 3d5/2? The XPS Pd 3d binding energy
shift is normally ascribed to the electronic effect of Cu on Pd (Mech-
anism I) [33–35], but some researchers [14,25,36] hold that this
electronic effect is non-existent and the energy shift is caused
by CuO that decreases the electron density of Pd (Mechanism II)
[8,9,37]. We consider that Mechanism II is suitable for PdCux/Al2O3,
because: (1) both PdCu0.6/Al2O3 and PdCu6/Al2O3 contain CuO (Cu
2p3/2 at 934.6 eV), in addition to metallic Cu0 (Cu 2p3/2 at 932.8 eV),
and a satellite peak at 943.5 eV for PdCu6/Al2O3 confirms the pres-
ence of surface CuO [9,37], as shown in Fig. 4. Similar to PdO, CuO
on PdCux/Al2O3 is formed through oxidation of Cu by air exposure
before XPS measurements; (2) PdCu0.6/Al2O3 and Pd/Al2O3 have
the same Pd 3d binding energy, indicating that there is no charge
transfer between Cu and Pd; and thus (3) the Pd 3d binding energy
shift for PdCu6/Al2O3 is induced by CuO. It should be noted that
PdCu0.6/Al2O3 also contains CuO, but its content is low, thus having
less effect on the electronic properties of Pd.
Fig. 5 compares the XPS spectra for Pd 3d of PdZn0.6/Al2O3 and
PdZn6/Al2O3. Both catalysts contain Pd (Pd 3d5/2 at 334.9 eV [9,32]
for PdZn0.6/Al2O3 at 335.3 eV [38] for PdZn6/Al2O3) and PdO (Pd
3d5/2 at 336.4 eV) just like PdCux/Al2O3. The Pd 3d5/2 binding energy
of PdZn6/Al2O3 shifts by 0.4 eV relative to Pd/Al2O3, indicating the
formation of PdZn alloy. It has been widely accepted that the higher
energy shift of the Pd 3d5/2 peak in alloyed PdZn catalysts is caused
by the charge transfer from Pd to Zn, which in turn increases the
binding energy of the core and valence levels of Pd [17,26,39]. The
binding energy of the Pd 3d5/2 of PdZn0.6/Al2O3 keeps the same
value with respect to Pd/Al2O3, primarily owing to the poor alloying
of Pd and Zn in this catalyst. The formation mechanism of the PdZn
alloy will be discussed in detail in the following section.
Actual metal loading and CO uptake of catalyst.
Catalyst
Metal loading (wt.%)
CO uptake (␮mol g−1
)
Pd
Cu or Zna
Pd/Al2O3
0.46
0.43
0.44
0.40
0.42
0.43
0.42
0.40
0.43
–
–
7.32
5.80
4.26
2.65
1.01
6.50
5.65
4.04
4.13
0
PdCu0.2/Al2O3
PdCu0.4/Al2O3
PdCu0.6/Al2O3
PdCu6/Al2O3
PdZn0.6/Al2O3
PdZn2/Al2O3
PdZn6/Al2O3
PdZn10/Al2O3
Cu/Al2O3
0.07 (0.06)
0.11 (0.12)
0.19 (0.18)
1.92 (1.80)
0.18 (0.19)
0.60 (0.60)
1.82 (1.80)
2.96 (3.00)
1.81 (1.80)
1.84 (1.80)
Zn/Al2O3
–
0
a
The measured and theoretical Cu or Zn loading are given, respectively, by the
values outside and inside the brackets.
is anchored on Al2O3 by impregnating with Pd NPs colloidal solu-
tion, by which not all Pd NPs can enter into the support and a small
amount of free Pd NPs is left in the solution. On the contrary, the
Cu or Zn metal is introduced into the catalyst by incipient wet-
ness impregnation of Cu or Zn precursors onto Pd/Al2O3, which
can ensure that all Cu or Zn are retained in the catalyst [10].
As listed in the last column in Table 1, the amount of
chemisorbed CO, corresponding to the number of Pd atoms exposed
on the catalyst surface, is strongly dependent not only on the type
of the second metal (Cu or Zn) but also on the Cu/Pd or Zn/Pd molar
ratio. For the Pd–Cu/Al2O3 catalysts, the CO uptake decreases with
increasing the Cu/Pd ratio, implying the formation of PdCu alloy,
which weakens the interaction between CO and Pd particles. This
result is in accordance with that reported by Sitthisa et al. [24] using
Pd–Cu/SiO2 catalysts. Note that the CO uptake for Cu/Al2O3 is zero,
which was also observed by Jeroro et al. [25], who found that CO
hardly bound to sites containing Cu atoms. The reason is that Cu has
complete d orbitals, which make it very difficult for the d orbitals to
accept electrons from the CO molecule to form the ␴ bond [22]. As
regards Pd–Zn/Al2O3, a similar pattern is observed, i.e., an increase
in the Zn/Pd ratio lowers the CO uptake of the catalysts, which
is probably due to the weakened Pd(4d)-CO(2␲) bonding interac-
tions derived from the electronic perturbations induced by Zn on
Pd [26]. However, the CO uptake of PdZn6/Al2O3 is similar to that
of PdZn10/Al2O3, indicating that the amount of PdZn alloy or the
modification degree of Zn on Pd hardly changes when the Zn/Pd
molar ratio increases to a certain value. In addition, Zn/Al2O3 has
a CO uptake of zero, which is reasonable considering that neither
metallic Zn [27] nor ZnO [28] adsorbs CO at room temperature. A
comparison between PdCu0.6/Al2O3 and PdZn0.6/Al2O3 or between
PdCu6/Al2O3 and PdZn6/Al2O3 clearly shows that the CO uptake of
Pd–Zn/Al2O3 is higher than that of Pd–Cu/Al2O3 at the same Zn/Pd
and Cu/Pd molar ratios, demonstrating that the modification degree
of Cu on Pd is greater than that of Zn.
Fig. 1 provides the representative HRTEM images and the metal
particle size distribution of Pd NPs and several catalysts. The aver-
age Pd particle size of the Pd/Al2O3 catalyst (Fig. 1C) is larger than
that of the colloidal Pd NPs (Fig. 1A and B), which is mostly caused
by the growth of Pd particles in the calcination process. As for the
catalysts, the metal particles are evenly distributed over the alu-
mina support and exhibit a narrow size distribution. Moreover,
compared to Pd/Al2O3 without addition of Cu or Zn, Pd–Cu/Al2O3
(Fig. 1D) and Pd–Zn/Al2O3 (Fig. 1E and F) possess a larger average
metal particle size, implying the formation of PdCu and PdZn alloys.
Fig. 2 further gives a high-magnification view of PdZn6/Al2O3. Metal
particles A and B display a lattice spacing of 0.206 nm (determined
by the FFT) that belongs to the (2 0 0) plane of the ␤1-tetragonal
Pd–Zn phase [21,29], indicating the formation of PdZn alloy. Metal
3.2. Selective hydrogenation of phenylacetylene
Figs. 6 and 7 compare the activity and selectivity of different
bimetallic PdCux/Al2O3 and PdZnx/Al2O3 catalysts. Let us first dis-
cuss the PdCux/Al2O3 catalysts. As shown in Fig. 6, both initial rate
(Rini) and TOF decrease with increasing the Cu/Pd molar ratio, and
when the Cu/Pd ratio is increased up to 6, the initial rate of catalyst
becomes very small, being only 0.5 mmol gPd−1 s−1. The Rini and
TOF of PdCu6/Al2O3 are about, respectively, 1/40 and 1/7 of that
of Pd/Al2O3. The activity variation with the Cu/Pd ratio is mainly
attributed to the dilution of Pd active sites by Cu [8], which is ulti-
mately derived from the presence of PdCu alloy and the possible
partial coverage of the Pd surface by Cu.
The selectivity to styrene at almost complete conversion of
phenylacetylene (>99%) increases with increasing the Cu/Pd ratio
up to 0.6, being about 31% for Pd/Al2O3, 60% for PdCu0.2/Al2O3, 71%
for PdCu0.4/Al2O3 and 82% for PdCu0.6/Al2O3, as shown in Fig. 7.
The improved selectivity to styrene over the alloyed Pd–Cu/Al2O3
catalysts can be ascribed to the geometric effect rather than
the electronic effect, since the aforementioned XPS analysis has
demonstrated the absence of charge transfer between metallic Cu
Please cite this article in press as: Z. Wang, et al., Selective hydrogenation of phenylacetylene over bimetallic Pd–Cu/Al2O3 and
Pd–Zn/Al2O3 catalysts, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.018

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Fig. 1. HRTEM images and the corresponding histograms of metal particle size distribution of (A and B) Pd NPs, (C) Pd/Al2O3, (D) PdCu0.6/Al2O3, (E) PdZn0.6/Al2O3 and (F)
PdZn6/Al2O3.
Fig. 2. HRTEM images and fast Fourier transformation (FFT) images of PdZn6/Al2O3. (A)–(C) represent different metal particles.
and Pd. The geometric effect originated from the PdCu alloy can
reduce the number of surface active sites (as confirmed by the
CO uptake listed in Table 1), which is favorable for inhibiting the
further hydrogenation of alkenes to alkanes [8,40]. Nevertheless,
with regards to PdCu6/Al2O3, the selectivity to styrene decreases
by comparison with other PdCux/Al2O3 catalysts. At about 24%
conversion of phenylacetylene, PdCu6/Al2O3 shows 92% selectiv-
ity to styrene, but other catalysts give about 98% selectivity. Note
that the experiment with PdCu6/Al2O3 was not continued to com-
plete conversion of phenylacetylene (it was carried out for 10 h),
because the activity of PdCu6/Al2O3 is so low that conversion of
phenylacetylene is only 24% after 10 h of reaction. The reason for
Please cite this article in press as: Z. Wang, et al., Selective hydrogenation of phenylacetylene over bimetallic Pd–Cu/Al2O3 and
Pd–Zn/Al2O3 catalysts, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.018

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5
Pd 3d
Pd 3d
335.4 eV
335.3 eV
336.4 eV
336.4 eV
PdCu6/Al2O3
PdZn6/Al2O3
PdZn0.6/Al2O3
334.9 eV
334.9 eV
PdCu0.6/Al2O3
Pd/Al2O3
350
345
340
335
330
325
350
345
340
335
330
325
Binding Energy / eV
Binding Energy / eV
Fig. 5. Comparison of Pd 3d XPS spectra of PdZn0.6/Al2O3 and PdZn6/Al2O3.
Fig. 3. Comparison of Pd 3d XPS spectra of Pd/Al2O3, PdCu0.6/Al2O3 and
PdCu6/Al2O3.
25
20
15
10
5
20
15
10
5
Rini
TOF
PdCux/Al2O3
Cu 2p
932.8 eV
934.6 eV
0
0
PdCu6/Al2O3
0.2
0.6
6
0.4
0
25
20
15
10
5
20
15
10
5
943.5 eV
PdZnx/Al2O3
0
0
0.6
2
0
6
10
Zn/Pd or Cu/Pd moral ratio (x)
PdCu0.6/Al2O3
Fig. 6. Effect of Cu/Pd and Zn/Pd moral ratios on the activity of PdCux/Al2O3 and
PdZnx/Al2O3.
100
Pd/Al2O3
80
60
40
20
80
60
40
20
PdCu0.2/Al2O3
PdCu0.4/Al2O3
PdCu0.6/Al2O3
PdCu6/Al2O3
965 960 955 950 945 940 935 930 925
Binding Energy / eV
Fig. 4. Comparison of Cu 2p XPS spectra of PdCu0.6/Al2O3 and PdCu6/Al2O3.
the decreased selectivity of PdCu6/Al2O3 probably lies in that most
Pd active sites are covered by Cu so that this catalyst behaves simi-
larly to a monometallic Cu/Al2O3 catalyst. Unlike Pd, Cu is much less
active for the dissociation and chemisorption of H2 [41], and thus
gives rise to a much lower activity and selectivity of PdCu6/Al2O3
under the experimental condition used here.
Pd/Al2O3
PdZn0.6/Al2O3
PdZn2/Al2O3
PdZn6/Al2O3
PdZn10/Al2O3
Next, we will discuss the PdZnx/Al2O3 catalysts. As shown in
Figs. 6 and 7, variations of activity and selectivity of PdZnx/Al2O3
with the Pd/Zn molar ratio are similar to those of PdCux/Al2O3. An
exceptional and interesting phenomenon is that PdZn6/Al2O3 and
PdZn10/Al2O3 have similar activities in terms of both Rini and TOF
as well as similar selectivity to styrene. In particular, PdZn6/Al2O3
displays the best selectivity to styrene, with 88.1% selectivity at
0
20
40
60
80
100
Conversion of phenylacetylene / %
Fig. 7. Effect of Cu/Pd and Zn/Pd moral ratios on the selectivity of PdCux/Al2O3 and
PdZnx/Al2O3.
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a conversion of phenylacetylene of 96.0% and 86.3% selectivity at
99.5% conversion.
Conversion of PA
Selectivity to ST
100
80
60
40
20
0
Different from PdCux/Al2O3, both electronic and geometric
effects contribute to the increased selectivity to styrene over
PdZnx/Al2O3. The presence of charge transfer from Pd to Zn has
been evidenced by XPS analysis, which has a stronger effect on
the adsorption of alkenes than of alkynes [13,17,42]. This can facil-
itate desorption of styrene and result in increased selectivity to
styrene. The geometric effect is undoubtedly present in the alloyed
PdZnx/Al2O3 catalysts, and its positive influence on the selectiv-
ity to styrene can be explained by the same reason as mentioned
above for PdCux/Al2O3. In order to explain the similar activity and
selectivity of PdZn6/Al2O3 and PdZn10/Al2O3, we turn to the forma-
tion mechanism of PdZn alloy, which can be simply expressed as
spillover of dissociated H2 from Pd (formed by reduction of PdO) to
the Pd–ZnO interface to reduce ZnO followed by formation of PdZn
alloy [17,43–48]. Apparently, the amount of PdZn alloy depends
not only on the reduction temperature but also on the amount of
Pd contacted with ZnO, since once these Pd are completely con-
verted to PdZn alloy, hydrogen spillover will stop and, accordingly,
the alloy formation process will cease [46]. This explains the simi-
lar performance of PdZn6/Al2O3 and PdZn10/Al2O3, because almost
all Pd engaged in the Pd–ZnO interface is alloyed at a Zn/Pd molar
ratio of 6 and a further increase in the Zn/Pd ratio cannot increase
the amount of PdZn alloy any more. As for PdZn0.6/Al2O3, only few
PdZn alloys are formed due to the limited amount of ZnO and, as
a result, the Pd 3d binding energy shift of this catalyst relative to
Pd/Al2O3 is not observed by XPS.
Finally, PdCux/Al2O3 is compared with PdZnx/Al2O3 with the
same x. As presented in Figs 6 and 7, PdCu0.6/Al2O3 has a
lower activity but higher selectivity to styrene compared to
PdZn0.6/Al2O3, while both activity and selectivity of PdCu6/Al2O3
are lower than those of PdZn6/Al2O3. The difference in the activ-
ity and selectivity between the two types of catalysts is closely
related to the modification degree of Cu or Zn on Pd. The CO
chemisorption measurements have shown that the modification
degree of Cu on Pd is higher than that of Zn, and therefore, the
activity of PdCux/Al2O3 is lower than that of PdZnx/Al2O3. The
selectivity of PdCux/Al2O3 should accordingly be higher than that
of PdZnx/Al2O3 and this is exactly what has been observed for
PdCu0.6/Al2O3 and PdZn0.6/Al2O3. However, it is not obeyed by
PdCu6/Al2O3 and PdZn6/Al2O3. This is because, on the one hand,
the behavior of PdCu6/Al2O3 is similar to that of Cu/Al2O3, and on
5
4
1
2
3
Cycle of reaction
Fig. 9. Reusability of PdZn6/Al2O3 in 5 consecutive cycles of reaction.
the other hand, the maximum modification degree of Zn on Pd is
achieved with PdZn6/Al2O3, both factors in favor of increasing the
selectivity of PdZn6/Al2O3. As shown in Fig. 7, among all the cata-
lysts prepared in this work, PdZn6/Al2O3 has the highest selectivity
to styrene at almost complete conversion of phenylacetylene Fig. 8
presents the concentration profiles of phenylacetylene, styrene and
ethylbenzene versus reaction time over PdZn6/Al2O3.
In order to evaluate the reusability of PdZn6/Al2O3, five consec-
utive cycles were carried out and the reaction time in each cycle
was fixed at 100 min. As shown in Fig. 9, both activity and selectiv-
ity of PdZn6/Al2O3 in 5 cycles are maintained, with a selectivity of
about 89% at a phenylacetylene conversion of around 92%. Further-
more, the metal loadings of Pd and Zn on the used PdZn6/Al2O3
after 5 cycles remain unchanged. These results indicate the good
stability of the bimetallic PdZn6/Al2O3 catalyst, which is expected
to a promising candidate for the selective hydrogenation of pheny-
lacetylene.
4. Conclusions
A series of Pd–Cu/Al2O3 and Pd–Zn/Al2O3 bimetallic catalysts
were prepared and characterized by various techniques, includ-
ing ICP–OES, HRTEM, CO chemisorption and XPS. CO chemisorption
and HRTEM analysis demonstrated the formation of PdCu and PdZn
alloys for Pd–Cu/Al2O3 and Pd–Zn/Al2O3, respectively. XPS mea-
surements revealed the absence of charge transfer between Cu and
Pd and the presence of charge transfer from Pd to Zn. Compared
to monometallic Pd/Al2O3, Pd–Cu/Al2O3 and Pd–Zn/Al2O3 showed
decreased activity but increased selectivity to styrene, mainly
due to the geometric effect of Pd–Cu/Al2O3 and the electronic
and geometric effects of Pd–Zn/Al2O3. The experimental results
obtained using different catalysts with varying Cu/Pd and Zn/Pd
molar ratios showed that PdCu0.6/Al2O3 (with a Cu/Pd molar ratio
of 0.6) had selectivity to styrene higher than that of PdCu6/Al2O3,
whereas PdZn0.6/Al2O3 displayed a lower selectivity compared
to PdZn6/Al2O3. This difference was mostly due to the different
modification degrees of Cu and Zn on Pd. A comparison between
bimetallic Pd–Cu and Pd–Zn catalysts showed that PdCu6/Al2O3
had the best selectivity to styrene, with 88.1% selectivity at 96.0%
phenylacetylene conversion and 86.3% selectivity at 99.5% conver-
sion.
0.5
phenylacetylene
styrene
ethylbenzene
0.4
0.3
0.2
0.1
0.0
0
20
40
60
80 100 120 140
Acknowledgements
Time / min
Financial supports from the National Natural Science Founda-
tion of China (21276076), the Program for New Century Excellent
Fig. 8. Concentration profiles of various species versus reaction time over
PdZn6/Al2O3.
Please cite this article in press as: Z. Wang, et al., Selective hydrogenation of phenylacetylene over bimetallic Pd–Cu/Al2O3 and
Pd–Zn/Al2O3 catalysts, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.018

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7
Talents in University (NCET-13-0801), the Fundamental Research
Funds for the Central Universities (222201313011) and the “111”
project (B08021) are gratefully acknowledged.
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Pd–Zn/Al2O3 catalysts, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.018