Pd3Sn nanoparticles on TiO2 and ZnO supports as catalysts for semi-hydrogenation: Synthesis and catalytic performance

Article abstract of DOI:10.1016/j.apcata.2017.07.005

The two catalysts Pd₃Sn/TiO₂ and Pd₃Sn/ZnO were prepared via a one-pot procedure based on the “polyol method” with the addition of a capping agent (polyvinylpyrrolidone) to control the particle size distribution. The same procedure was used to prepare Pd/TiO₂ and Pd/ZnO for comparison. All four catalysts showed high activity and selectivity for the selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) in the liquid phase under identical conditions. However, Pd₃Sn/TiO₂ and Pd₃Sn/ZnO show selectivities to alkene significantly higher than that of the Pd catalysts. Specifically, the selectivity increases from 96.4% to 97.4% on TiO₂ support, and 96.2% to 97.6% on ZnO support, at 90% conversion. Transition electron microscopy shows nanoparticles evenly dispersed on the support, with mean particle sizes as low as 4.1 (±0.8) nm when Sn is incorporated into the catalyst. Unsupported Pd₃Sn was prepared using the same method and characterised by powder X-Ray diffraction followed by the Rietveld refinement. Pd₃Sn was found to be single-phase and isostructural to Pd metal with a face centred cubic unit cell.

Full text of DOI:10.1016/j.apcata.2017.07.005

AppliedCatalysisA,General544(2017)40–45
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Pd3Sn nanoparticles on TiO2 and ZnO supports as catalysts for semi-
hydrogenation: Synthesis and catalytic performance
Shaun K. Johnstona, Nikolay Cherkasova,b, Elena Pérez-Barradoa,1, Atte Ahoc, Dmitry Y. Murzinc,
Alex O. Ibhadond,⁎, M. Grazia Francesconia,⁎
a
School of Mathematics and Physical Sciences, Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
School of Engineering, University of Warwick, Coventry CV4 7AL, United Kingdom
Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University, Biskopsgatan 8, FI-20500 Turku/Åbo, Finland
School of Engineering and Computer Science, Chemical Engineering, University of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
b
c
d
A R T I C L E I N F O
A B S T R A C T
Keywords:
Alkyne
Alkene
Bimetallic
Catalysis
Palladium
Selective hydrogenation
Acetylene
The two catalysts Pd3Sn/TiO2 and Pd3Sn/ZnO were prepared via a one-pot procedure based on the “polyol
method” with the addition of a capping agent (polyvinylpyrrolidone) to control the particle size distribution. The
same procedure was used to prepare Pd/TiO2 and Pd/ZnO for comparison. All four catalysts showed high ac-
tivity and selectivity for the selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol
(MBE) in the liquid phase under identical conditions. However, Pd3Sn/TiO2 and Pd3Sn/ZnO show selectivities to
alkene significantly higher than that of the Pd catalysts. Specifically, the selectivity increases from 96.4% to
97.4% on TiO2 support, and 96.2% to 97.6% on ZnO support, at 90% conversion. Transition electron microscopy
shows nanoparticles evenly dispersed on the support, with mean particle sizes as low as 4.1 ( 0.8) nm when Sn
is incorporated into the catalyst. Unsupported Pd3Sn was prepared using the same method and characterised by
powder X-Ray diffraction followed by the Rietveld refinement. Pd3Sn was found to be single-phase and iso-
structural to Pd metal with a face centred cubic unit cell.
1. Introduction
active and more selective than other platinum-group metals [5], but
their selectivity towards alkene products is still limited due to over-
Highly selective, active and environmentally-friendly catalysts are
of prime importance for many areas of chemistry. In particular, the
production of fragrances, pharmaceuticals, vitamins and agrochemicals
relies on catalysts developed more than 60 years ago and requires
significant improvement in terms of both environmental footprint and
performance. An example of these catalytic reactions is selective hy-
drogenation, which involves, for example, the hydrogenation of a
carbon-carbon triple bond to a double bond (semi-hydrogenation),
avoiding over-hydrogenation to a single bond. Semi-hydrogenation is
one of the critical steps in the synthesis of vitamins A and E, the fra-
grance compound linalool and other important compounds [1].
The semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-me-
thyl-3-buten-2-ol (MBE) (Fig. 1) is an important starting reaction in
industry [2] and is widely used as a model substrate to screen for ac-
tivity and selectivity of catalyst materials in semihydrogenation reac-
tions [3,4].
hydrogenation to alkanes and other side reactions such as dimerization
and oligomerization reactions [6]. To overcome this, a co-metal is often
added to Pd to increase the selectivity. The co-metal can be added to the
Pd catalyst as a surface poison, as is the case in the widely used Lindlar
catalyst [7], in which Pd/CaCO3 is modified by the addition of Pb.
However, the use of lead for catalyst modification is subject to in-
creasing environmental and product poisoning concerns [8].
Alternatively, a second metal (co-metal) can be incorporated to
form a Pd-M alloy, which can take the form of core-shell, or a random
or ordered alloy, depending on the synthetic procedure employed [9].
The crystal structure of any resulting compounds formed between the
two metals is often overlooked but can be very important. For example,
work carried out on intermetallic compounds formed between Pd and
Ga has shown that, depending on the stoichiometry and crystal struc-
ture of the compound formed between Pd and Ga, drastically different
selectivities are achieved in gas phase acetylene hydrogenation [10].
Therefore, achieving phase purity, rather than mixtures of the various
Palladium-based catalysts in semi-hydrogenation reactions are
⁎
Corresponding authors.
E-mail addresses: A.O.Ibhadon@hull.ac.uk (A.O. Ibhadon), M.G.Francesconi@hull.ac.uk (M.G. Francesconi).
Present address: School of Chemical and Physical Sciences, Keele University, Staffordshire, ST5 5BG, United Kingdom.
1
http://dx.doi.org/10.1016/j.apcata.2017.07.005
Received 9 May 2017; Received in revised form 20 June 2017; Accepted 5 July 2017
Availableonline08July2017
0926-860X/©2017ElsevierB.V.Allrightsreserved.

S.K. Johnston et al.
AppliedCatalysisA,General544(2017)40–45
in the catalytic performance compared to Pd/TiO2 and Pd/ZnO cata-
lysts in the semi-hydrogenation of MBY.
2. Experimental
2.1. Materials
The metal precursors used in the catalyst preparation were palla-
dium(II) acetylacetonate (99%, Aldrich) and tin(II) acetyl acetate
(99.9%, Aldrich). Zinc oxide (99.9%, 200 mesh powder, Alfa Aesar) and
titanium(IV) oxide, rutile phase (powder, < 5 μm, ≥99.9%, Aldrich)
was used as support and polyvinylpirrolidone (PVP) (average mol. Wt.
40,000 g mol−1, Sigma-Aldrich) was used as a capping agent. Acetone
(laboratory reagent grade, Fischer Chemical), ethanediol (laboratory
reagent grade, Fischer Chemical), n-hexane (GPR grade, VWR
Chemicals) and deionised water were used as solvents without any pre-
treatment. For the hydrogenation reactions, butan-1-ol (analytical re-
agent grade, Fisher Scientific) was used as internal standard for GC
analysis. All chemicals were used as purchased.
Fig. 1. Reaction scheme of the hydrogenation of MBY to MBA. (MBY = 2-methyl-3-
butyn-2-ol, MBE = 2-methyl-3-butene-2-ol and MBA = 2-methylbutan-2-ol.) For a se-
lective semi-hydrogenation, steps (b) and (c) should be avoided.
active compounds present throughout a catalyst, can also be considered
of vital importance since different compositions formed between two
metals can have different catalytic properties.
Many Pd-based alloy (bimetallic) materials have been tested in the
literature as selective hydrogenation catalysts, utilising co-metals such
as Ag [11,12], Au [13–15], Bi [8,16–18], Co [19], Cu [20,21], Fe
[22,23], Ni [23,24], Sn [25], Zn [26–29], Zr [30]. When forming an
alloy with the second metal, selectivity can be enhanced by two main
factors. Firstly, an increase in selectivity can occur due to changes in the
electronic structure of Pd brought on by the presence of an additional
metal, which can change the relative adsorption energies of alkyne and
alkene bonds. This can result in a more favourable adsorption, and
hence, the hydrogenation of alkyne, while disfavouring the adsorption
and hydrogenation of alkene species [31]. Secondly, alloy formation
can reduce the number and size of active site ensembles due to dilution
of Pd [9,32]. This can inhibit secondary reactions that involve different
functional groups, or neighbouring reactants to be adsorbed in close
proximity to each other. Geometric effects should, however, have less
impact on the semi-hydrogenation of acetylene alcohols as only one
functional group is available for hydrogenation, although it may assist
in preventing unwanted dimerisation reactions from taking place.
The co-metal of interest in this work is Sn, because Sn is non-toxic
and is known to form a range of compounds with Pd. Known Pd-Sn
binary alloys include PdSn4, PdSn3, PdSn2, α- and β-Pd3Sn2, PdSn,
Pd2Sn and Pd3Sn. We have focused our studies on Pd3Sn, which is
isostructural to Pd and hence makes for a better comparison to the
catalytic performance of monometallic Pd.
Catalysts containing Pd-Sn alloys have already been used for elec-
trooxidation [33,34], hydrogen peroxide synthesis [35], water deni-
tration [36–38], and in the selective hydrogenations of unsaturated
aldehydes [39,40], hexa- and butadienes [41–43], and gas phase
acetylene [44]. In many of these reactions, high selectivity is achieved
and attributed to the suppression of the adsorption of certain functional
groups onto the catalyst due to the altered electronic structure, or to the
formation of Pd-Sn bonds by the reduction of active site ensembles. The
change in electronic structure of Pd on alloying with Sn has been shown
by photoemission data to involve a valence charge transfer from Sn to
Pd, and is most prominent when the alloys Pd2Sn or Pd3Sn are formed
[45]. This charge transfer effect has also been shown to suppress the
adsorption of C]C specifically, compared to Pd alone [40]. Further-
more, work carried out on compounds such as PdGa has shown the
benefit of isolated Pd sites (ensemble effect) in ordered alloys and in-
termetallic compounds in favouring the successive, stepwise hydro-
genation of the acetylene bond rather than full hydrogenation to alkane
[10,46–48].
2.2. Catalyst preparation
The method used for the preparation of the catalysts was adapted
from a polyol method used by Cable and Shark [49] for the preparation
of Pt3Sn.
Firstly, appropriate stoichiometric amounts of the metal precursors,
palladium acetylacetonoate and tin acetate, each in 100 mL ethylene
glycol, were combined into the reaction vessel and either ZnO or TiO2
support was added alongside PVP as capping agent (10 monomer mol%
of total Pd and Sn). The amount of the support used was calculated to
achieve 2 wt% Pd loading. The mixture was stirred under nitrogen gas
flow at room temperature for at least 30 min to displace air before
heating at reflux (469 K) for 1 h. The mixture was cooled and the re-
sulting brown/grey solid was obtained from the reaction mixture by
centrifugation at 3500 rpm, washing alternately with acetone and
deionised water (3 × 30 mL each) and dried at 353 K in air for 1 h to
yield a grey powder. The catalysts are stored in a vacuum desiccator
until use.
Unsupported Pd3Sn for structural characterisation was prepared
using the same method with exclusion of the support and PVP.
2.3. Catalyst characterisation
Powder X-ray diffraction (PXRD) was carried out using
a
PANalytical Empyrean X-ray diffractometer using monochromatic Cu
Kα1 radiation and line PIXcel detector, scanning in the 2θ range of
20–120°, step size 0.105° and step time 12,000 s. Results were recorded
using the instrument’s built-in software. Rietveld refinement was per-
formed using GSAS software with the EXPGUI interface [50].
Samples for transmission electron microscopy (TEM) were dispersed
in ethanol using an ultrasonic bath and a drop of dispersion was de-
posited onto carbon-coated copper grids. Images were obtained using a
Gatan Ultrascan 4000 digital camera with Digital Micrograph software,
attached to
a Jeol 2010 TEM instrument running at 200 kV.
Nanoparticle size distributions were obtained studying 3–5 TEM mi-
crophotographs obtained from different areas of the catalyst sample,
measuring the diameter of a total of 200–400 individual nanoparticles
using Gatan Digital Micrograph software.
However, in the current literature on Pd-Sn catalysts, there has been
no attempt to achieve purity of the catalytically active Pd-Sn phase,
hence mixtures of Pd/Sn monometallic and/or bimetallic phases are
present in the catalyst [33,34,37–40,44]. This means that it is unclear
which phases are responsible for any improved catalytic performance
compared to monometallic Pd.
The aim of this work was to study the Pd-Sn alloy Pd3Sn and assess
the purity and crystal structure of the material. Furthermore, Pd3Sn/
TiO2 and Pd3Sn/ZnO catalysts were prepared and showed enhancement
The total uptake of CO was measured with
a Micromeritics
AutoChem 2910 apparatus. Prior to a measurement, the catalyst was
reduced in hydrogen with the following programme: flushing with
helium at room temperature for 10 min, reduction in hydrogen at 573 K
(10 K/min) for 120 min, flushing with helium at 573 K for 30 min and
then returned to ambient temperature.
Palladium and tin content of the catalysts were determined using a
Perkin Elmer Optima 5300DV emission inductively coupled plasma
41

S.K. Johnston et al.
AppliedCatalysisA,General544(2017)40–45
Fig. 2. TEM images and particle size distributions of (from left to right) Pd/TiO2, Pd3Sn/TiO2, Pd/ZnO, and Pd3Sn/ZnO.
CMBE + CMBA
(ICP) spectrometer. The samples were dissolved in the solution of HF/
HCl/HNO3 in a 1/1/3 ratio, heated at 473 K for 10 min using micro-
wave digestion system CEM MARS Xpress Plus, followed by the addi-
tion of the boric acid saturated aqueous solution to complex excess HF,
and the vessels were heated again at 453 K for 10 min. The solutions
were diluted with deionised water and analysed.
XMBY
=
CMBY + CMBE + CMBA
(1)
(2)
100 × CMXBE
SMBE,x
=
CMXBY + CMXBE + CMXBA
The activity of the catalysts was characterised per mol. of Pd (A)
using the average MBY consumption rate in 0–80% MBY conversion
80
interval as in Eq. (3), where
X
is the MBY conversion close to 80%
MBY
2.4. Hydrogenation reaction
80
occurred during the reaction time of tM80BY , C
is the initial MBY
MBY
concentration, Vsol is the solution volume, mcat is the catalyst mass taken
with Pd content of ωPd, and MPd is the molar mass of Pd.
For the hydrogenation procedure, freshly-prepared catalysts (be-
tween 5 and 50 mg, depending on the activity of the catalyst) and a
hexane solution (100 mL) containing 0.741 g (10 mmol) of butan-1-ol
used as an internal standard were transferred into a 250 mL 3-neck
round bottom flask fitted with a silicone rubber septum, thermometer
and reflux condenser. A Schlenk line was connected through the top of
the condenser to enable alternation between nitrogen gas, hydrogen
gas, and vacuum. The contents of the flask were stirred at 1100 rpm and
heated to 308 K. Air from the reactor and the solvent was removed
purging the reactor 3 times with nitrogen gas – the pressure was slowly
decreased until the solvent started to boil, and the reactor was filled
with nitrogen gas quickly. Nitrogen gas was then similarly substituted
by hydrogen gas, purging the reactor 3 times. The contents were left
stirring under hydrogen atmosphere (ambient pressure) for approxi-
mately 30 min to ensure the solvent and catalyst were saturated with
hydrogen, and that no leak was present in the system (the hydrogen
consumption was measured by a 300 mL gas burette). The reaction was
started by quickly injecting 1.00 g MBY using a syringe through the
septum. Aliquots of approximately 150 μL were taken at regular inter-
vals with the increased sampling rate when the reaction approached
100% conversion (estimated using hydrogen consumption). Analysis of
the products was performed using a Varian 430 gas chromatograph
equipped with a 30 m Stabilwax® capillary column (Restek). A series of
experiments with various stirring rates and catalyst amounts confirmed
the absence of external mass transfer limitations, while the calculation
of the Weisz-Prater number for the most active catalyst (0.021 and 0.13
for organic and H2 species, respectively, ≪6) demonstrated that in-
ternal mass transfer has a negligible effect on observed kinetics [51].
Conversion of MBY (XMBY) and selectivity to MBE (SMBE,x) at dif-
ferent levels of conversion were calculated using Eqs. (1)–(2), where
CMBY, CMBE, CMBA and Cx are the concentrations of MBY, MBE, MBA and
the concentrations at x% MBY conversion, respectively. These formulae
imply that no components other than MBY, MBE and MBA were ob-
tained, which was confirmed by the carbon balance for all reactions of
80
1 − X
CM0 BY
mcat ωPd
V
sol MPd
MBY
A =
80
t
(3)
MBY
Turn-over frequency (TOF) was used to characterise the average
activity of the catalyst normalised per active site. The TOF was calcu-
lated using Eq. (4), where nCOuptake is the total molar CO uptake de-
termined by CO chemisorption studies.
80
1 − X
CM0 BY
nCOuptake
V
sol
MBY
TOF =
80
t
(4)
MBY
3. Results and discussion
We adapted the polyol method used by Cable and Shark [49] by
performing the reduction of Pd or the co-reduction of Pd and Sn pre-
cursors in polyol with the inclusion of TiO2 or ZnO supports along with
the capping agent PVP to control the size distribution of the Pd and
Pd3Sn nanoparticles. The solvent, ethylene glycol, causes the co-re-
duction of each precursor in the presence of support and the relatively
high boiling point (470 K) causes annealing, which induces the for-
mation of the Pd3Sn alloy phase.
The two supports used, TiO2 and ZnO were chosen as two common,
commercially available materials that are well known for their use as
support materials for Pd-based catalysts for hydrogenation reactions
[4,52–56]. When supported ZnO, Pd has been shown to offer good
catalytic performance in semi-hydrogenation reactions, with selectiv-
ities towards MBE reaching around 95% for ZnO supports at 99% MBY
conversion [55,56]. Pd nanoparticles supported on TiO2 were also re-
ported with the selectivity in the range of 60–85% [3,57,58].
TEM micrographs and particle size distributions of each catalyst are
shown in Fig. 2. Nanoparticles can be seen on each support material and
no nanoparticles greater than 16 nm are observed. The Pd percentages
obtained from ICP analysis show close to 2 wt% Pd in each catalyst.
100
2%.
42

S.K. Johnston et al.
AppliedCatalysisA,General544(2017)40–45
Table 1
throughout [62]. As additional Sn-containing phases are not detected
by PXRD, all of the Sn detected by ICP analysis must be incorporated
within a Pd3Sn phase. In this lattice, the Pd and Sn atoms are randomly
distributed on the same crystallographic site, hence, the material can be
categorised as a random nanoalloy [9]. We have also studied the PXRD
patterns for the supported catalysts. However, only diffraction peaks
belonging to the support were present and it was impossible to observe
any peaks corresponding to Pd or Pd3Sn due to low particle size
(compared to the size of the particles of the support) and low percen-
tage of Pd and Pd3Sn compared to the support (∼2 wt%).
The unit cell parameter and the Pd-Pd(Sn) distance were calculated
via Rietveld refinement using the GSAS software with the EXPGUI in-
terface [50] and found to be a = 3.949(3) Å, and Pd-Pd(Sn)
= 2.7926(13) Å (Rwp = 0.098; Rp = 0.074, see Supporting informa-
tion) in agreement with the Pd3Sn model [64].
Recently, Yan et al. [66,67] demonstrated that mechanical strain in
catalysts such as Pt/Ni/Cu thin film on a Ti substrate and WC on a
polymeric substrate significantly affects the catalytic activity. We con-
sidered the possibility of a similar effect in the catalysts studied in this
work, however, such an effect is unlikely, as our catalysts are single-
phase metal or alloy nanoparticles dispersed on micro sized particles of
an oxide support rather than heterogeneous layers.
Comparison of textural properties, elemental analysis and catalyst performance of Pd/
TiO2, Pd3Sn/TiO2, Pd/ZnO, and Pd3Sn/ZnO. SMBE 90 and SMBE,100 refer to selectivity to-
,
wards MBE at the onset of conversion of 90 and 100%, respectively. A is the average
reaction rate in molMBY molPd−1 s−1
.
Catalyst
Pd/TiO2 Pd3Sn/TiO2 Pd/ZnO Pd3Sn/ZnO
SMBE,90 (%)
SMBE,100 (%)
Mean Pd/Pd3Sn particle size
(nm)
Particle size standard deviation
ICP Pd%
96.4
93.7
6.8
97.4
95.8
5.6
96.2
93.7
5.5
97.6
96.1
4.1
2.0
1.73
n/a
0.42
10.8
16.1
4.3
1.4
1.97
2.9
0.22
2.63
10.9
3.8
1.1
1.95
n/a
0.88
0.07
3.3
0.8
1.58
2.8
0.06
0.29
4.9
ICP Pd/Sn ratio
A (s−1
)
CO uptake (μmol g−1
)
)
a
b
CO uptake (μmol g−1
TOF (s−1
)
49.6
1.9
a
Catalysts reduced at 573 K in H2.
Catalysts dried at 373 K in air.
b
Therefore, the vast majority of the Pd used in the synthesis procedure is
found in the catalyst with very little Pd lost during or after the synth-
esis.
The CO uptake for each catalyst is shown in Table 1 and tells us how
much Pd is available for chemisorption at the catalyst surface. Initially,
the CO uptake was measured after performing a pre-reduction treat-
ment in which the catalysts were heated to 573 K in H2 to remove any
residual PVP at the catalyst surface, which is known to be an important
factor for CO uptake [68]. However, the CO uptakes were found to be
very low compared to those analysed without reduction. This is most
apparent when ZnO is used as support, which might be due to a strong
metal-support interaction (SMSI) between Pd and ZnO as a result of the
reduction pre-treatment applied before CO uptake measurements were
carried out, as observed by Ramos-Fernández et al. for the CO uptake of
Pt/ZnO [69]. Hence, the calculations for TOF (turnover frequency)
were done using CO chemisorption data obtained without any pre-re-
duction treatment being carried out on the catalysts.
To assess their catalytic performance, the catalysts were tested in
batch, liquid phase MBY hydrogenation reactions under identical con-
ditions. The reaction profiles using each catalyst are shown in Fig. 4. In
all cases, the reaction proceeds by MBY, after which only MBA is pro-
duced. This reaction behaviour is expected for Pd-based catalysts as
alkynes (MBY) are known to be preferentially adsorbed to the surface of
Pd compared to alkene bonds (MBE) [5,6]. The relative formation of
MBA compared to MBE is, however, notably lower at all stages of the
reaction in the Pd3Sn catalysts compared to Pd catalysts on the same
supports, indicating a lower affinity for alkene adsorption on Pd3Sn
compared to Pd.
Pd3Sn mean particle size and size distributions (Table 1) are notably
lower in Pd3Sn catalysts than Pd when prepared on the same support in
agreement with observations made by Esmaeili et al. [44]. Further-
more, the particle size is lower for both Pd and Pd3Sn catalysts with
ZnO support compared to TiO2 support. The lowest mean particle size
(4.1 nm) and size distribution is observed in Pd3Sn/ZnO. This low
particle size may have an effect on the activity of this catalyst compared
to the other catalysts, because activity towards semi-hydrogenation has
been known to increase sharply above a certain particle size in some
cases, due to reaction with the support [59].
PXRD and ICP analyses were carried out on Pd3Sn nanoparticles
prepared in the absence of support in order to study the active com-
ponent of the Pd3Sn catalysts. The PXRD pattern (Fig. 3) was indexed
using a face-centred cubic unit cell, in agreement with previous reports
on Pd3Sn [60–65]. Since the PXRD pattern for single-phase Pd is very
similar to that of Pd3Sn, the possibility of overlapping peaks from any
pure Pd present in the material cannot be excluded using PXRD alone.
However, additional phases, such as SnO2, Sn metal, or other Pd-Sn
compounds that are often found in other works on Pd-Sn materials
[37,38,40,44] are not detected. ICP analysis on the same material gave
a Pd/Sn ratio of 2.8/1, which is around the lower limit of the Pd3Sn-
phase solid solution reported, the composition of which ranges between
Pd2.85Sn and Pd3.26Sn, showing a fcc unit cell (space group Fm m)
3
The selectivity towards MBE is measured at both 90 and 100% MBY
conversion (SMBE,90 and SMBE,100 in Table 1) but for the purposes of
discussion, only SMBE,90 is used for a more reliable comparison between
catalysts, since the MBA production begins to accelerate slightly before
100% MBY conversion. SMBE,90 for each catalyst is shown in Table 1.
The catalysts containing Pd3Sn are ≥1% more selective at 90% MBY
conversion than the corresponding Pd catalyst on the same support. The
difference in SMBE,90 between Pd/TiO2 and Pd/ZnO, and between
Pd3Sn/TiO2 and Pd3Sn/ZnO is only 0.2%. Therefore in this case, the
support cannot be considered the main factor in enhancing the se-
lectivity, which can be attributed to the addition of Sn.
Catalytic activity normalised per mole of Pd (A) and per active site
(TOF) for all catalysts is also listed in Table 1. Activity per mole of Pd in
the Pd3Sn catalysts is lower in all cases compared to the monometallic
Pd catalysts, probably due to dilution of Pd with Sn. When measured
per active site (TOF) the TiO2-supported catalysts are comparable with
a slight decrease in TOF from 4.3 s−1 (for Pd/TiO2) to 3.8 s−1 (Pd3Sn/
TiO2). However, TOF for Pd/ZnO (49.6 s−1) is an order of magnitude
higher than that of any other catalyst, including Pd3Sn/ZnO (1.9 s−1).
Fig. 3. PXRD pattern of unsupported Pd3Sn.
43

S.K. Johnston et al.
AppliedCatalysisA,General544(2017)40–45
Fig. 4. Concentrations of MBY, MBE, and MBA over
time (seconds-gram of catalyst) during the semi-hy-
drogenation reaction at 308 K, 1 atm. H2 pressure,
0.12 M solution of MBY in hexane. The catalysts are:
(a) Pd/TiO2, (b) Pd3Sn/TiO2, (c) Pd/ZnO and (d)
Pd3Sn/ZnO.
The TOF values for the MBY hydrogenation in the range between 1
and 4 s−1 are typical for Pd supported on TiO2 [28,57], SiO2 [17] and
ZnO [4] nanowire supports. The strong metal-support interactions ob-
served on titania-supported catlaysts, which may increase their activity
by a factor of 10, were not observed because we have not performed
high-temperature (> 500 °C) reduction [53]. Interestingly, the notable
increase in TOF for the Pd/ZnO catalysts observed in the current work
was also reported previously to be an order of 60 s−1, which is in good
agreement with our results [55,70]. However, although selectivity is
enhanced, Both A and TOF are much lower for Pd3Sn catalysts com-
pared to Pd when ZnO support is used. It is difficult to establish what
effect the addition of a second metal has on the interaction between Pd
and ZnO support, therefore further work is required to understand the
interaction between Pd3Sn alloy and ZnO support, compared to that
between Pd and ZnO.
The mean particle sizes for each catalyst are also listed in Table 1
alongside the values for catalytic activity and selectivity. When the Pd/
ZnO and Pd/TiO2 catalysts are compared, the particle size increases
from 5.5 to 6.8 nm SMBE,90 shows little change. However, the activity
decreases dramatically. Comparison between Pd3Sn/ZnO and Pd3Sn/
TiO2 shows that the mean particle size increases from 4.1 to 5.6 nm and
once again SMBE,90 exhibits little change, however, in contrast to the Pd
based catalysts, the activity dramatically increases. Therefore, no ob-
vious correlation can be seen which relates particle size with catalytic
performance.
bond in MBY as observed by Hammoudeh and Mahmoud in the selec-
tive hydrogenation of cinnamaldehyde [40], leading to very little MBE
hydrogenation until almost full conversion of MBY (Fig. 4).
4. Conclusions
Pd/TiO2, Pd/ZnO and the novel catalysts Pd3Sn/TiO2 and Pd3Sn/
ZnO were prepared with a one-pot synthesis based on the “polyol
method” with the addition of a capping agent (PVP) to control the
particle size distribution. All these catalysts showed high activity and
selectivity in the semi-hydrogenation of MBY to MBE. However, the
new catalysts, in which the catalytically active compound is the Pd3Sn
alloy, are more selective by ≥1%, at as low as 90% conversion, com-
pared to the Pd-only based catalysts with the same support. The highest
selectivity towards MBE is 97.6% using Pd3Sn/ZnO. The origin of the
enhanced selectivity in Pd3Sn based catalysts should be further ex-
plored, although a plausible explanation based on concomitant factors
was offered.
TEM shows nanoparticles on support with Pd3Sn showing lower
mean particle size and less broad size distribution. PXRD analyses of
unsupported Pd3Sn showed that Pd3Sn is single phase, and is iso-
structural to Pd metal with Pd and Sn atoms randomly distributed on
the same crystallographic position.
Acknowledgements
As there is no evidence of Pd/Sn ordering in the FCC lattice of
Pd3Sn, with the distribution of Pd and Sn being random, the enhanced
selectivity in Pd3Sn catalysts may not be entirely attributed to the de-
struction of Pd ensembles as some literature has suggested for other Pd-
Sn catalysts [41,43,44,71]. In fact, there is a 3/1 = Pd/Sn ratio,
therefore a large number of Pd ensembles may be intact, hence not
achieving a substantial isolation of active sites. Instead, the enhanced
selectivity may also be due to a charge transfer from Sn to Pd, sup-
pressing the adsorption of the C]C bond in MBE relative to the C^C
This work was carried out under an EU funded FP 7 project:
Microwave, Acoustic and Plasma Synthesis, Project Number FP70NMP-
2012-SMALL-6.
We wish to acknowledge the use of the EPSRC funded National
Chemical Database Service hosted by the Royal Society of Chemistry,
UK.
We are grateful to Ann Lowry and Bob Knight for TEM and ICP
analyses, respectively.
44

S.K. Johnston et al.
AppliedCatalysisA,General544(2017)40–45
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
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