Preparation of a ternary Pd-Rh-P amorphous alloy and its catalytic performance in selective hydrogenation of alkynes

Article abstract of DOI:10.1039/c4cy00338a

Palladium-rhodium-phosphorus amorphous alloy nanoparticles (～5.2 nm) were prepared via a facile one-pot synthesis method, exhibiting excellent catalytic behaviour in selective hydrogenation of alkynes under mild conditions. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c4cy00338a

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Preparation of a ternary Pd–Rh–P amorphous alloy
and its catalytic performance in selective
hydrogenation of alkynes†
Cite this: Catal. Sci. Technol., 2014,
4, 1920
Received 18th March 2014,
Accepted 2nd May 2014
*
*
Mengrui Ren, Changming Li, Jiale Chen, Min Wei and Shuxian Shi
DOI: 10.1039/c4cy00338a
www.rsc.org/catalysis
Palladium–rhodium–phosphorus amorphous alloy nanoparticles
(~5.2 nm) were prepared via a facile one-pot synthesis method,
exhibiting excellent catalytic behaviour in selective hydrogenation
of alkynes under mild conditions.
and Rh(acac)3, employing trioctylphosphine (TOP) as the
stabilizer. The XRD pattern and HRTEM image confirm
the amorphous structure feature of the alloy NPs with a mean
size of 5.2 0.4 nm, and the aberration-corrected scanning
transmission electron microscopy with energy dispersive X-ray
mapping-scan (STEM-EDS) further demonstrates the uniform
distribution of Pd, Rh and P elements in each amorphous
particle. The catalytic performance of PdRhP amorphous
alloy NPs was investigated in the chemoselective hydrogena-
tion of various alkynes. Interestingly, high selectivity toward
various alkenes was observed in a few hours with a high con-
version (up to 100%) under quite mild reaction conditions
(atmospheric pressure and 35 °C) which is superior to their
single counterparts, showing promising applications in catalysis.
Uniform alloy NPs with an average size of 5.2 0.4 nm
were successfully synthesized in high yield (the detailed
experimental procedure is described in the ESI†) and are
arranged regularly just like a decorative pattern (Fig. 1a). No
distinct reflection corresponding to crystalline phases can be
observed in the XRD pattern aside from one broad peak at
Alloy catalysts have recently attracted considerable attention
in heterogeneous catalysis1–3 because the incorporation of a
second metal is an effective approach for tailoring electronic
and geometric properties and thus optimizing the catalytic
performance. Especially, amorphous alloys,4,5 as a relatively
young class of materials with short-range order and long-range
disorder, have shown promising catalytic performance in
many hydrogenation reactions (e.g., selective hydrogenation
of diene, selective hydrogenation of benzene,6 hydrogenation
of sulfolene7 and hydrodesulphurization of dibenzothiophene8).
In addition, Yamashita et al.9 reported that amorphous alloys
have much higher catalytic activity than alloys in the crystal-
lized state, resulting from the active site number and the
electronic state of surface metals. However, previous studies
mainly focused on monometallic alloys with non-metallic ele-
ments (e.g., Ni–B amorphous alloy6), and there are very few
reports7,8 on bimetallic amorphous catalysts. The reason can
be explained as follows: two completely fused metal elements
in a phase diagram (e.g., Pd and Pt10,11) are apt to form a
crystallized but not amorphous alloy while in the case of two
incompletely- or non-fused metal elements (e.g., Pd and Rh),
it's rather difficult to obtain amorphous alloys based on con-
ventional methods. Taking into account the unique merits
of amorphous alloys, fabricating bimetallic amorphous alloys
with promising behaviour in heterogeneous catalysis is of
vital importance and remains a great challenge.
Herein, we report the synthesis of monodispersed PdRhP
amorphous alloy nanoparticles (NPs) via co-reduction of Pd(acac)2
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing, PR China. E-mail: weimin@mail.buct.edu.cn,
shisx@mail.buct.edu.cn; Fax: +86 10 64425385
Fig. 1 (a) The TEM image of the alloy NPs, (b) the HRTEM image of the
amorphous alloy NPs (inset: enlarged HRTEM image), (c) the XRD
pattern and (d) the EDX spectrum of the alloy NPs.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00338a
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2θ ~ 45° (Fig. 1c), which is rather different from that of single
crystalline Pd or Rh NPs (Fig. S1†). This indicates the forma-
tion of an amorphous alloy structure. Moreover, despite the
signals of Pd and Rh elements both being captured (Fig. 1d),
no lattice fringe image can be detected for the alloy NPs
via the HRTEM technology (Fig. 1b, inset). Both the typical
features above confirm the amorphous state of the alloy NPs.
In order to study the distribution of these two metal elements,
high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) was used to carry out further anal-
ysis (Fig. 2a). The energy dispersive X-ray (EDX) mapping-scan
recorded through the single nanoparticle in the red square
frame (Fig. 2a) shows a similar element distribution of Pd
and Rh (Fig. 2b). The line-scan spectra of Pd and Rh which
were collected through the center of an individual nanoparti-
cle (marked by the red line in Fig. 2a) also show rather similar
behavior of Pd and Rh, demonstrating homogeneous disper-
sion of these two species in an individual particle. This
was further verified by detecting more than ten nanoparticles
(Fig. 2c).
Moreover, it is worth noting that a strong P element signal
was also detected in the NPs (Fig. 1d), suggesting that P may
participate in the formation of the amorphous alloy structure.
We further performed elemental analysis of the amorphous
alloy and the results are listed in Table S1.† The results dem-
onstrate the existence of P with a molar percentage of 27%.
This was further confirmed by the HAADF-STEM-EDS tech-
nique (Fig. S2†), and a uniform inter-distribution of Pd, Rh, P
can be observed in all of the amorphous nanoparticles by the
EDX-mapping scan. Fig. S3† displays the XRD patterns of
alloy materials synthesized with various P/metal ratios, and it
was found that the amorphous phase can only form at a high
level of nominal P/metal ratio (14/1, Fig. S3A†). Therefore, it
can be concluded that the as-synthesized amorphous alloy is
a ternary system (denoted as Pd34Rh39P27).
The semihydrogenation of alkynes plays a key role in
industry because alkyne impurities in the olefin feedstocks
will lead to the deactivation of the polymerization catalyst.12,13
Pd is regarded as the best active component in this reaction.
As a result, the selective hydrogenation of alkynes was employed
as the model reaction to evaluate the catalytic performance
of the amorphous alloy. For comparison, pristine Rh and
pristine Pd NPs with close sizes (~5.2 nm) were prepared by
regulating the temperature and the amount of the capping
agent using the same method (Fig. 3). Commercial Rh/C and
Pd/C were also employed as contrast catalysts. The concentra-
tion of each catalyst was determined so as to ensure the same
noble metal content of all the catalyst samples. The hydroge-
nation of a model alkyne, phenylacetylene, was carried out at
quite low temperature (35 °C) and atmospheric pressure.
Fig. 4a shows the composition variation of the reaction mix-
ture over the alloy amorphous catalyst at different points in
time. The content of phenylacetylene decreased gradually in
the whole reaction time range (3 h), while the styrene content
increased rapidly at first and reached a maximum (100% con-
version with 82% selectivity) at 2.5 h, followed by a sharp
decrease from 2.5 to 3.0 h (Fig. 4a), exhibiting the well-known
behaviour of consecutive reactions. The content of ethylbenzene,
the complete hydrogenation product, exhibits a slow increase
(0–2.5 h) and a fast enhancement (2.5–3.0 h). The best yield
(82%) of styrene can be achieved at 2.5 h (Fig. 4b).
In contrast, single Pd NPs and Pd/C show rather high
catalytic activity: phenylacetylene completely transforms to
ethylbenzene quickly in less than one hour and a half
(Fig. 4c). The Pd NPs show a slightly higher selectivity than
Pd/C at various conversion levels, but both of them have lower
selectivity than Pd34Rh39P27 NPs (Fig. 4d). For Rh catalysts,
both the Rh NPs and Rh/C samples display rather low conver-
sion for the whole reaction duration (0–3.0 h); moreover, the
selectivity dramatically decreased with the increase of conver-
sion (0–20%). For comparison, the Lindlar Pd–Pb/CaCO3 cata-
lyst was also evaluated (Fig. 4d), displaying a maximum olefin
yield of 86.7%, close to that of the previous report.14 The
Pd34Rh39P27 catalyst in this work shows a close maximum
yield to that of the Lindlar Pd–Pb/CaCO3 one, indicating a
Fig. 2 (a) The HAADF-STEM image of the alloy NPs, (b) the EDX map-
ping image and line scanning results of an individual alloy particle,
and (c) the mapping image of a few alloy NPs supported on SiO2.
Green: Pd–L; yellow: Rh–L.
Fig. 3 TEM images of (a) Pd–Rh–P, (b) Rh, and (c) Pd NPs with the
corresponding size distributions.
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No obvious change in the mixture composition was observed
except for a rather slight increase in the phenylacetylene
conversion which might be due to the residual catalyst in
the filtrate. This suggests that the alloy NPs are responsible
for the catalytic activity. The cycle ability of alloy NPs was
also explored by employing the selective hydrogenation of
phenylacetylene. The catalyst was separated by centrifugation,
washed and reused without further treatment. After five recy-
cles, the conversion of phenylacetylene still remained 100%
and no obvious change in the product composition was found
(Fig. S5†). From the viewpoint of practical applications,
supporting nano-catalysts on a carrier is highly significant for
improving stability, separation and recycling.20 This is under
investigation in our lab.
Furthermore, the ternary amorphous alloy also exhibits
excellent catalytic performance in selective hydrogenation
of other alkynes, and the results are given in Table 1. For
4-nitroethynylbenzene hydrogenation, a high selectivity of
94.7% (at 44.4% conversion) for the olefin was obtained over
the Pd34Rh39P27 alloy, which was maintained at a high level
(99.2% selectivity) even at 100% conversion. However, a rela-
tively low selectivity or quite low activity was observed over
single Pd (81.5% selectivity at 45.9% conversion) or Rh cata-
lyst, respectively. In the case of 4-bromostyrene hydrogena-
tion, the Pd34Rh39P27 NPs display rather satisfactory catalytic
behavior both at low and high conversions (95.5% selectivity
at 38.5% conversion; 91.9% selectivity at 100% conversion)
compared with the monometallic Pd or Rh catalyst. Hydro-
genation of tolane was also investigated. The Pd34Rh39P27
(72.9% selectivity at 40.1% conversion) and pristine Pd
(69.4% selectivity at 51.7% conversion) catalysts revealed
rather similar catalytic behavior in this reaction while the Rh/C
sample shows the highest selectivity for (Z)-stilbene (78.3%)
but at rather low conversion (25.2%). These results demon-
strate that Pd34Rh39P27 displays greatly enhanced catalytic
performance in selective hydrogenation of various alkynes
compared with the pristine Pd or Rh catalyst.
Fig. 4 (a) Product distribution of phenylacetylene hydrogenation as a
function of reaction time over the bimetallic amorphous catalyst.
(b) The conversion of phenylacetylene and selectivity for styrene in
the reaction time range 0–3.0 h. (c) The phenylacetylene conversion
as a function of time over various catalysts, with a metal amount of
0.16 μmol. (d) The styrene selectivity vs. phenylacetylene conversion
over various catalysts with the following metal contents: Pd34Rh39P27
:
0.16 μmol; Pd: 0.08 μmol; Rh: 0.16 μmol; Pd/C: 0.08 μmol; Rh/C:
0.16 μmol; and Pd–Pb/CaCO3: 0.08 μmol.
similar bimetallic reaction mechanism for the improved cata-
lytic performance.15,16 Therefore, it is concluded that the
PdRhP amorphous alloy exhibits significantly superior activity
and selectivity to their single counterparts (pristine Pd or Rh
catalyst).
Since some previous studies17–19 showed that the high
catalytic activity of nanoparticles might be due to molecular
species leaching from the nanoparticles, we conducted a fil-
tration test to investigate this issue. The hydrogenation of
phenylacetylene catalyzed by alloy NPs was terminated after
1 h at a conversion of 81% (Fig. S4†), then the H2 pressure
was released and the nanoparticles were removed by centrifu-
gation. Subsequently, the filtrate was placed into the flask to
restart the reaction under the same conditions for one hour.
Based on the results above, it can be seen that the pristine
Pd catalyst shows limited selectivity, in accordance with the
Table 1 Selective hydrogenation reaction of various alkynes
Substrates
Catalyst
Time (h)
Con. (%)
Sel.a (%)
Con. (%)
Sel.b (%)
Con. (%)
Sel.c (%)
Pd34Rh39P27
4
100
78.6
44.4
100
86.3
45.9
100
76.9
38.7
0
99.2
98.6
94.7
0
73.7
81.5
0
83.9
87.3
0
100
91.9
93.3
95.5
84.7
80.5
87.2
88.9
95.7
93.2
81.4
99.3
73.5
40.1
99.4
82.8
51.7
99.4
76.5
47.7
25.2
54.9 cis
77.9 cis
72.9 cis
57.1 cis
61.8 cis
69.4 cis
55.2 cis
66.4 cis
74.2 cis
78.3 cis
2.5
1.5
4
2.5
1.5
4
2.5
1.5
Rh/C
67.2
38.5
100
83.1
39.9
62.5
48.7
26.8
12.6
Pd
Pd/C
Rh/C
a
b
c
Reaction conditions: dioxane (1 ml), 1 bar H2, 35 °C. Selectivity for 1-ethenyl-4-nitrobenzene. Selectivity for 4-bromostyrene. Selectivity for
(Z)-stilbene.
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previous reports.21,22 This can be possibly attributed to the for-
mation of a hydride phase which is active in over-hydrogenation
reactions.23 The employment of a second element is the main
strategy to improve performance of Pd catalysts (e.g., Pd–Ga,24
Pd–Cu,25 Pd–Ni12 and Pd–Si26). The presence of other metals
or metalloids would modify the geometric effect and electron
density of Pd atom, influencing the adsorption behavior of
reactants and intermediates.27
In this work, Rh and P elements in the PdRhP amorphous
alloy may function in a similar way (e.g., the geometric and
electronic modification effect of Rh and P elements12,24,28) to
improve the hydrogenation selectivity for alkynes. On the
other hand, the inherent nature of the amorphous alloy may
also have a unique influence on the enhanced selectivity.
From a theoretical point of view, the metalloid element in an
amorphous alloy could change the coordination environment
of an active metal (e.g., in the NiB system29), and the short-
range order between neighboring metal atoms would influ-
ence the adsorption of reactants.30 Therefore, the improved
catalytic performance of PdRhP can be attributed to the
intrinsic nature of the amorphous alloy with a modified
geometric/electronic structure, which will be studied in our
future work.
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preparation of ternary PdRhP amorphous alloy NPs with a
uniform particle size. The combination of Pd and Rh with
the presence of P shows excellent catalytic performance in
hydrogenation of several alkynes under mild reaction condi-
tions compared with the pristine Rh or Pd catalyst. This
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This work was supported by the 973 Program (grant no.
2011CBA00504), the National Natural Science Foundation of
China (NSFC), the Scientific Fund from Beijing Municipal
Commission of Education (20111001002) and the Funda-
mental Research Funds for the Central Universities (ZD 1303).
M. Wei particularly appreciates the financial aid from the
China National Funds for Distinguished Young Scientists of
the NSFC.
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