Palladium-bismuth intermetallic and surface-poisoned catalysts for the semi-hydrogenation of 2-methyl-3-butyn-2-ol

Article abstract of DOI:10.1016/j.apcata.2015.02.038

The effects of poisoning of Pd catalysts with Bi and annealing in a polyol (ethylene glycol) were studied on the semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY). An increase in the Pd:Bi ratio from 7 to 1 in the Bi-poisoned catalysts decreased the hydrogenation activity due to blocking of active sites, but increased maximum alkene yield from 91.5% for the Pd catalyst to 94-96% for all Bi-poisoned Pd catalysts, by decreasing the adsorption energy of alkene molecules and suppressing the formation of β-hydride phase. Annealing of the catalysts induced the formation of intermetallic phases and decreased its activity due to sintering of the catalytic particles and low activity of intermetallic compounds. Langmuir-Hinshelwood kinetic modelling of the experimental data showed that poisoning of Pd with Bi changed the relative adsorption constants of organic species suggesting ligand effects at high Bi content.

Full text of DOI:10.1016/j.apcata.2015.02.038

Applied Catalysis A: General 497 (2015) 22–30
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Palladium–bismuth intermetallic and surface-poisoned catalysts for
the semi-hydrogenation of 2-methyl-3-butyn-2-ol
Nikolay Cherkasova, Alex O. Ibhadona,∗, Alan J. McCueb, James A. Andersonb,
Shaun K. Johnstona
a Catalysis and Reactor Engineering Research Group, Department of Chemistry and School of Biological, Biomedical and Environmental Sciences,
University of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
b Surface Chemistry and Catalysis Group, Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
The effects of poisoning of Pd catalysts with Bi and annealing in a polyol (ethylene glycol) were studied on
the semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY). An increase in the Pd:Bi ratio from 7 to 1 in the
Bi-poisoned catalysts decreased the hydrogenation activity due to blocking of active sites, but increased
maximum alkene yield from 91.5% for the Pd catalyst to 94–96% for all Bi-poisoned Pd catalysts, by
decreasing the adsorption energy of alkene molecules and suppressing the formation of ␤-hydride phase.
Annealing of the catalysts induced the formation of intermetallic phases and decreased its activity due to
sintering of the catalytic particles and low activity of intermetallic compounds. Langmuir–Hinshelwood
kinetic modelling of the experimental data showed that poisoning of Pd with Bi changed the relative
adsorption constants of organic species suggesting ligand effects at high Bi content.
Received 1 December 2014
Received in revised form 4 February 2015
Accepted 20 February 2015
Available online 27 February 2015
Keywords:
Heterogeneous catalysis
Alkynol
2-Methyl-3-butyn-2-ol
Hydrogenation
Semi-hydrogenation
Palladium
© 2015 Elsevier B.V. All rights reserved.
Bismuth
1. Introduction
to increase selectivity by competing with alkene molecules for
adsorption sites [7,8]. Lindlar catalyst, an industrial standard for
Semi-hydrogenation or selective hydrogenation reactions aim
to transform triple-bonded molecules into double bonded ones, and
constitute critical steps in the synthesis of a vast array of fine chemi-
cals including vitamins, fragrances, drugs and many other products
with a worldwide production of more than 1000 t a year [1,2].
However, traditional hydrogenation catalysts including supported
nanoparticles of Pd and Pt usually lead to low catalyst selectiv-
ity. To overcome the problem, the catalysts are usually modified
with reversible and irreversible adsorbates which introduce ligand
(electronic) effects that change the relative adsorption energies and
selectively block the active sites responsible for side-reactions [3,4].
The catalyst modification is performed by surface poisoning
and alloying. For example, active sites of Pd in Pd–Ag alloys
are isolated, which decreases the rate of oligomerization, while
silver induces ligand effects changing the adsorption energies
facilitating desorption of alkenes [5,6]. Often, carbon monox-
ide (a reversible adsorbate) is added to the acetylene feedstock
semi-hydrogenation, employs lead as an irreversible adsorbate and
quinoline as a reversible adsorbate. Metals such as Cu have been
extensively studied as irreversible adsorbates [9,10] while sulphur-
and nitrogen-containing polymer modifiers combine the func-
tions of reversible and irreversible adsorbates [11,12]. However,
the increase in selectivity brought about by modifiers is usually
counterbalanced by the decrease in activity [2,10,13], so indus-
trial semi-hydrogenation catalysts should combine an optimal
level of catalyst poisoning to improve selectivity while minimising
decrease in activity. Moreover, other considerations such as cata-
lyst price, the environmental footprint of catalyst synthesis and the
overall economy of the catalytic process should be considered. In
this respect, Lindlar catalyst is still the main catalyst used by indus-
try because of its high activity, selectivity and ease of preparation.
However, there are a number of problems associated with the use of
Lindlar catalysts including the toxicity of lead compounds and that
an additional separation step is required when quinoline is used.
In order to address these problems, bimetallic Pd-based cat-
alysts are being studied to improve selectivity and to mitigate
environmental problems associated with the use of lead in Lind-
lar catalyst. Highly selective Pd–Zn alloys were shown to form in
∗
Corresponding author. Tel.: +44 1723 357241.
E-mail address: a.o.ibhadon@hull.ac.uk (A.O. Ibhadon).
http://dx.doi.org/10.1016/j.apcata.2015.02.038
0926-860X/© 2015 Elsevier B.V. All rights reserved.

N. Cherkasov et al. / Applied Catalysis A: General 497 (2015) 22–30
23
Pd/ZnO catalysts during hydrogenation [14–17] while many other
bimetallic systems have been studied and theoretically evaluated
[4,18]. However, from an industrial point of view, the synthesis
of an industrial semi-hydrogenation catalyst should be scalable,
hence many catalytic systems that require expensive metallorganic
precursors for their synthesis and very carefully controlled reac-
tion conditions cannot compete with Lindlar catalyst. A rather
recent trend in catalysis is the study of the intermetallic com-
pounds, which can be defined as electrically conductive compounds
of metals, but the most interesting properties for catalytic appli-
cations might be precisely defined structural order and electronic
properties different from that of initial metals [19]. For example,
palladium–gallium intermetallic compounds have been reported to
be very selective in acetylene hydrogenation [20]; however, recent
data show that Pd2Ga intermetallic compounds oxidise quickly in
liquid-phase hydrogenation even under carefully controlled labo-
ratory conditions due to the high oxygen affinity of Ga [21].
Therefore, in an attempt to identify promising catalysts, consid-
ering their performance and the ease of synthesis, we have confined
our attention in this study to easily reducible heavy metals. Lead,
the adsorbate in Lindlar catalyst, has two neighbours in the periodic
table: thallium and bismuth. Palladium-based catalysts modified
with either one of these metals showed very high selectivity in
hydrodechlorination reactions [22], but considering the extremely
high toxicity of thallium, palladium–thallium catalysts cannot be
considered as a promising alternative to Lindlar catalyst. In con-
trast to lead, the toxicity of bismuth is significantly lower [23–25],
which makes it a promising catalyst for industrial applications in
the semi-hydrogenation in the synthesis of fine chemicals.
Palladium–bismuth systems have been studied in oxidative ace-
toxylation reactions and Pd3Bi intermetallic compounds have been
found to be the most selective [26,27]. Promotion of Pd with Bi
was found to increase the selectivity to the partial oxidation of glu-
cose to gluconic acid [28–30]. Similarly, in hydrogenation reactions,
intermetallic compounds of Pd and Bi were identified as promising
highly selective catalysts [31–33]. Anderson and co-workers stud-
ied a Pd catalyst selectively poisoned with Bi and demonstrated the
preferential adsorption of Bi on step sites, which had little effect
on 1-hexyne hydrogenation, but decreased the rates of subsequent
1-hexene isomerisation reactions [34,35]. To the best of our knowl-
edge, intermetallic compounds of palladium and bismuth have not
been tested in semi-hydrogenation reactions. These catalysts could
potentially combine the advantages of Pd–Ga intermetallics with
low toxicity and ease of synthesis of bismuth-modified catalysts.
The aim of the study was to investigate the effects of Bi-
poisoning and intermetallic formation of Pd catalysts on the
semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY), an impor-
tant industrial intermediate used in the synthesis of fine chemicals
and vitamins [1,36,37].
of a 20 mM Bi(NO3)3 solution in 2% acetic acid was added under
stirring to obtain the catalysts with Pd/Bi ratios of 1/1, 3/1 and
7/1 assuming that all Bi was reduced from the solution. The dis-
persion was stirred for 16 h at 80 ◦C in hydrogen atmosphere. The
catalysts were centrifuged, washed with water (2 × 40 mL), acetone
(3 × 30 mL), and dried in a rotary evaporator at 80 ◦C. After cooling
of the samples in the rotary evaporator to room temperature, the
vacuum pump was turned off, slowly admitting air to the catalyst
over a period of about 2 h to passivate the surface. The catalysts
were packed into vials and kept under a nitrogen atmosphere to
prevent further oxidation. The surface-poisoned catalysts obtained
were named Pd7Bi/SiO2, Pd3Bi/SiO2 and Pd1Bi/SiO2 to reflect the
nominal Pd:Bi ratio.
Each Bi-poisoned catalyst was annealed in ethylene glycol to
induce the formation of intermetallic phases adapting the polyol
method [38]. The catalyst (400 mg) was placed into a round-bottom
flask and 50.0 mL of ethylene glycol (99%, Fisher Scientific) was
added under stirring. Air from the flask was displaced by a flow
of nitrogen for 5 min; the slurry was heated, refluxed at 196 ◦C for
20 min, and cooled in nitrogen flow. The catalysts obtained were
centrifuged, washed, passivated as per Bi-poisoned Pd catalysts,
and named as Pd1Bi(t)/SiO2, Pd3Bi(t)/SiO2 and Pd7Bi(t)/SiO2.
Unsupported nanoparticles of Pd–Bi intermetallic compounds
were obtained using the polyol method annealing Pd and Bi precur-
sors in the presence of polyvinylpyrrolidone (PVP, Sigma–Aldrich)
[38]. Pd(Oac)2 (50 mg) and Bi(NO3)3·3H2O (the amount was calcu-
lated to obtain Pd:Bi molar ratios of 1:1 and 1:2) were dissolved
in 75 mL of ethylene glycol under sonication, PVP (250 mg) was
slowly added under stirring. After 30 min, NaBH4 (300 mg, 98%,
Sigma–Aldrich) was added and the solution was heated and
refluxed under nitrogen flow for 20 min. When the slurry was
cooled, the dispersion was diluted with 50 mL methanol, cen-
trifuged, washed with water (2 × 30 mL), methanol (2 × 30 mL),
acetone (2 × 30 mL) and left in 20 mL of acetone. The disper-
sion obtained was sonicated for 10 min and 15 mL were used
to impregnate silica (800 mg) giving raise to PdBi(im)/SiO2 and
PdBi2(im)/SiO2 catalysts.
Lindlar catalyst (Pd poisoned with Pb supported on CaCO3) was
purchased from Aldrich and used as received without the addition
of quinoline.
2.2. Characterisation
Powder X-ray diffraction (PXRD) measurements were per-
formed using an Empyrean X-ray diffractometer equipped with a
monochromatic K␣-Cu X-ray source and a PIXcel linear detector.
The scanning was performed in 2ꢀ range of 20–85◦, step length
0.0390◦, and step time 25 min. Silica-supported catalysts were
packed into conventional powder holders, while the dispersion of
the intermetallic nanoparticles in acetone was dropwise added on
a silicon zero-background sample holder.
2. Experimental
Scanning electron microscopy (SEM) study was performed on a
Zeiss EVO 60 instrument equipped with energy-dispersive X-ray
spectrometer (EDX) Oxford Instruments Inca System 350 under
the pressure of 10−2 Pa and electron acceleration voltage of 20 kV.
Catalyst powder was applied on carbon adhesive mats and carbon-
coated before the study.
Transmission electron microscopy (TEM) study was performed
using a Jeol 2010 instrument equipped with an energy-dispersive
X-ray spectrometer (EDX) produced by Oxford Instruments. For the
study, the materials were dispersed in ethanol under sonication
and a few droplets of the dispersion were applied on carbon-coated
copper grids. TEM study was performed from 5–8 regions for every
sample to obtain representative data.
2.1. Catalyst preparation
Pd/SiO2 catalyst with the Pd nominal loading of 5% was prepared
by dissolving palladium (II) acetate (Pd(Oac)2, 98%, Aldrich) in the
calculated amount of toluene (99%, VWR chemicals) to fill the pores
of amorphous fumed silica (Alfa Aesar, BET specific surface area of
200 m2 g−1). The slurry was dried in a rotary evaporator, calcined in
a tube furnace at 400 ◦C for 2 h and reduced in hydrogen at 150 ◦C
for 1 h. Amorphous silica and high Pd loading were deliberately
chosen to allow for analysis by X-ray diffraction.
The catalyst obtained was poisoned with Bi. Pd/SiO2 catalyst
(about 700 mg) was placed into a 50 mL flask and 2% acetic acid
(20 mL) in distilled water was added. Then the calculated aliquot
Elemental analysis was performed using a Perkin Elmer Optima
5300DV emission inductively coupled plasma spectrometer. The

24
N. Cherkasov et al. / Applied Catalysis A: General 497 (2015) 22–30
samples were weighed and dissolved in the mixture of HF (1 mL),
HCl (1 mL) and HNO3 (3 mL) in Teflon digestion vessels using a
microwave digestion system (CEM MARS Xpress Plus). After heat-
ing at 200 ◦C for 10 min and cooling, saturated boric acid (5 mL) was
added to each and then sealed and reheated at 180 ◦C for 10 min,
to complex excess HF before dilution and analysis.
Carbon monoxide uptake and temperature-programmed reduc-
Fig. 1. Scheme of hydrogenation reactions of MBY.
tion (TPR) was performed for selected catalysts using
a CE
Instruments TPDRO 1100 equipped with a TCD detector. Sample
was placed in a quartz reactor tube and reduced in a flow of 5%
H2/N2 (40 mL min−1). During the reduction step, the sample was
heated from room temperature to 100 ◦C at 10 ◦C min−1 and held
at that temperature for 1 h. The sample was then cooled to 35 ◦C
before CO was pulsed (0.285 mL loop size) into a He flow until
constant peak area were obtained (i.e., no further uptake was appar-
ent). Turn-over frequency (TOF) was calculated using an average
reaction rate (Aaverage) up to about 80% MBY conversion assum-
ing a Pd:CO ratio of 1.5 since bridging species generally contribute
significantly to the observed uptake for Pd samples [34].
steps between the adsorbed species. Only MBY, MBE and MBA were
included into the model, because no other products were identified
and the carbon balance was always 100 2% (Fig. 2c). The hydrogen
adsorption constant was considered to be low under the experi-
mental conditions, and relative (rather than absolute) adsorption
constants Q1 = KMBE/KMBY, Q2 = KMBA/KMBY were used due to strong
adsorption of organic species [41]. The reaction rates obtained (as
designated in Fig. 1) are presented in Eqs. (1)–(3), where k1∗, k2∗, k3∗
are the apparent rate constants, CMBY, CMBE, CMBA are concentra-
tions of MBY, MBE and MBA, respectively.
k1∗CMBY
r1
r2
r3
=
=
=
(1)
(2)
(3)
2
2.3. Catalyst testing
(Q1CMBE1 + CMBY + Q2CMBA
)
k2∗CMBE
A freshly prepared catalyst was transferred into a double-
neck flask, containing hexane (15 mL, 98%, Fisher Scientific) and
butanol-1 (15 ␮L, 99%, Fisher Chemical) as an internal standard.
The solution was degassed by bubbling nitrogen (99.999%, CK gas)
for 20 min while stirring at 1100 rpm. Afterwards, the system was
flushed twice with hydrogen (99.999%, CK gas), evacuating until the
residual pressure reached about 40 mbar following by filling with
hydrogen to ambient pressure. The system was left for 30 min under
stirring at 50 ◦C to achieve equilibrium and to ensure the absence
of leaks, which was done by monitoring the hydrogen level using
a 25 mL gas burette. The reaction flask was kept in a water bath
at 50 0.1 ◦C. To test the most active catalysts (Pd/SiO2, Lindlar,
Pd7Bi/SiO2, Pd7Bi(t)/SiO2), 5 mg of the catalyst was added; 40 mg
was used for medium-activity catalysts (Pd3Bi/SiO2, Pd3Bi(t)/SiO2)
and 100–250 mg for other catalysts to perform the reaction within
1–4 h.
Hydrogenation reaction was started by injecting a 25% MBY
(260 mg, 98%, Sigma–Aldrich) solution in hexane. During the reac-
tion, aliquots (80 ␮L) were extracted with the increased sampling
rate close to 100% MBY conversion (monitored using hydrogen
consumption). The extracted samples were analysed on a Varian
430 gas chromatograph (GC) fitted with a flame ionisation detec-
tor, an autosampler and a 30 m Stabiwax capillary column. The
absence of pore mass transfer limitations was confirmed calcu-
lating Weisz–Prater number for MBY and dissolved hydrogen, as
described by Vannice [39]. The numbers for the fastest Pd/SiO2 cat-
alyst were several orders of magnitude lower than the boundary
value of 6 (0.001 and 0.008 for hydrogen and MBY, respectively),
which shows that pore transfer limitations do not apply. The
absence of external mass transfer limitations was confirmed study-
ing the effect of stirring speed on the hydrogen consumption rate,
which did not increase at the stirring speed above 500 rpm.
2
(Q1CMBE + CMBY + Q2CMBA
k3∗CMBY
)
)
2
(Q1CMBE + CMBY + Q2CMBA
The reaction rates presented were integrated in Matlab using
an ode15s solver which is efficient for stiff systems of differ-
ential equations. The model parameters were optimised using
the Levenberg–Marquardt algorithm [42] to minimise the objec-
tive function S presented in Eq. (4), where Cexp and Cmodelled
are experimental and modelled concentrations, ωi are statistical
weights. The statistical weights were calculated using experimental
standard deviations ꢁexp,i determined for the analytical technique
used, which were 2% of the determined value or 0.3 mol m−3
,
whichever is larger [43]. Confidence intervals (90% probability) of
the model parameters were determined using the Monte-Carlo
method, which takes into account the covariation of the model
parameters [44].
ꢀ
1
S =
ωi(Cexp,i − Cmod,i)2; ωi =
.
(4)
ꢁe2xp,i
i
3. Results and discussion
3.1. Hydrogenation on Pd/SiO2 and Lindlar catalysts
Fig. 2a and b presents a comparison of MBY hydrogenation using
monometallic palladium (Pd/SiO2) and Lindlar catalysts. The con-
centration profiles of both catalysts were similar and consisted of
two consecutive stages: hydrogenation of MBY to MBE and MBE to
MBA (Fig. 1). In the presence of MBY, the over-hydrogenation to
MBA was comparatively low, which is traditionally related to the
strong adsorption of alkyne molecules on a Pd surface [2,4,45,46].
Carbon balance for these systems (Fig. 2c) was 100 2%, showing
that products other than MBY, MBE and MBA were not formed in
significant quantities.
However, the behaviour of the catalysts was not identical. The
difference was most obvious in the derivative plots which show the
apparent reaction rates (Fig. 2a and b, top). The Pd/SiO2 catalyst
showed a higher absolute reaction rates compared to the Lindlar
catalyst. These effects have also been observed by other authors
for various substrate molecules [13,34,47,48] and can be explained
by a combination of two factors. Firstly, catalyst support had a
significant effect on the dimensions of Pd nanoparticles formed:
2.4. Kinetic modelling
Fig. 1 shows the scheme of the MBY hydrogenation, where
along with the reaction of interest, MBY to 2-methyl-3-buten-2-ol
(MBE) hydrogenation, further hydrogenation to 2-methyl-2-
butanol (MBA) and direct full hydrogenation of MBY to MBA are
possible [17].
The Langmuir–Hinshelwood model was adopted from the works
of Singh and Vannice [40] and Duca et al. [41], considering
quasi-equilibrium competitive adsorption of organic and hydro-
gen species on the catalyst surface followed by rate-determining

N. Cherkasov et al. / Applied Catalysis A: General 497 (2015) 22–30
25
Fig. 2. Concentration profiles of MBY hydrogenation on the (a) Pd/SiO2 and (b) Lindlar catalysts with dots representing experimental data and lines – Langmuir–Hinshelwood
model (see Section 3.4). Corresponding derivative plots above the concentration profiles show the apparent reaction rates (the unit is mol m−3 s−1 gcat−1). (c) Carbon balances
for hydrogenation on Pd/SiO2 and Lindlar catalysts.
high-surface area silica (SBET 200 m2 g−1) stabilised Pd nanoparti-
cles 7.4 2.2 nm in diameter according to the TEM study (Fig. 3a
and b), while Pd nanoparticles on CaCO3 (SBET 3.3 m2 g−1) formed
agglomerates up to 0.1–1 ␮m (Fig. 3c). Secondly, poisoning with
Pb further decreased the activity of Pd catalysts due to blocking of
some Pd active sites [13,34].
Another notable difference between the catalysts was in the
temporal behaviour of MBY and MBE hydrogenation rates, which
were almost constant in the Pd/SiO2 catalyst (Fig. 2a, top). Con-
versely, in the Lindlar catalyst, the rates accelerated near the point
of maximum MBE yield (Fig. 2b, top), demonstrating the suppres-
sion of the alkene adsorption on Pd surface in the presence of Pb due
to ligand effects [3]. More importantly, MBA formation rate at the
initial reaction stages was significant in the Pd/SiO2 catalyst lead-
ing to a lower maximum MBE yield of 91.4% in comparison to 94.5%
for the Lindlar catalyst. As a result, these catalysts demonstrate
a compromise between higher activity that can be achieved on
monometallic Pd catalysts (Pd/SiO2) and higher selectivity demon-
strated by the Lindlar catalyst [21,49].
simple, Bi was selected as a poisoning agent. A number of detailed
studies showed that Bi preferentially adsorbs on step and edge
sites of Pt and terrace sites are occupied afterwards [50–52].
Furthermore, Anderson et al. [34] confirmed using infrared (IR)
spectroscopy that Bi poisons preferentially step and edge sited
of Pd catalysts, the sites that preferentially over-hydrogenate
MBY as found by Crespo-Quesada et al. [53]. Hence, the rate
of MBE hydrogenation was expected to decrease considerably
because of the incorporation of Bi, so a series of Bi-poisoned
Pd catalysts were prepared from the same batch of Pd/SiO2
catalyst.
TEM study of the catalysts showed that the diameter of the metal
nanoparticles did not change significantly on the introduction of
Bi. EDX study of individual particles showed the presence of Pd
and Bi, which confirms adsorption of Bi on Pd nanoparticles and
no purely Bi particles were identified. IR study of chemisorbed CO
on the catalysts was attempted in the transmission and diffused
reflectance modes; however, the absorbance was very high to allow
for spectra acquisition. Nevertheless, thermodynamic preference
of Bi atoms to adsorb on step and edge sites of platinum-group
metals, which was shown both experimentally and theoretically
[4,34,50,51], the same method of Bi introduction used by Anderson
et al. [34] and in the current work confirms that Bi was adsorbed
on step and edge sites.
3.2. Hydrogenation on Bi-poisoned Pd/SiO2 catalysts
In order to find more environmentally benign semi-
hydrogenation catalysts while keeping the synthesis procedure
Fig. 3. Representative (a) TEM image of Pd/SiO2 catalyst and (b) corresponding Pd nanoparticle size distribution. (c) SEM image of the Lindlar catalyst in the backscattered
electron mode which highlights Pd–Pb particles.

26
N. Cherkasov et al. / Applied Catalysis A: General 497 (2015) 22–30
Fig. 4. (a) Representative TEM image of Pd1Bi/SiO2 catalyst and (b) corresponding particle size distribution. (c) PXRD patterns of initial and Bi-poisoned catalysts.
Table 1
formation rate at the initial reaction stages were low, allowing
maximum MBE yield of 94–96% (similar to the Lindlar catalyst).
The apparent reaction rates on the Bi-poisoned catalysts decreased
compared to Pd/SiO2, which was expected considering that the
number of active sites decreased with the Bi poisoning. Moreover,
the poisoning had different effect on the ratio of the apparent MBY
and MBE consumption rates. The derivative plots in Fig. 5a–c (top)
show that on Pd7Bi/SiO2, the apparent rate of MBE hydrogenation
was higher than that of MBY, while on Pd1Bi/SiO2, MBE hydrogena-
tion was almost suppressed.
Elemental analysis of the catalysts studied.
Catalyst
Pd (wt%)
Bi (wt%)
Pd/Bi (mol)
Pd/SiO2
Lindlar
5.11
4.22
4.74
4.51
4.51
4.77
5.60
4.52
0.92
0.68
0
–
–
0.69 (Pb)
1.31
2.95
5.03
1.30
2.89
4.71
1.68
2.52
Pd7Bi/SiO2
Pd3Bi/SiO2
Pd1Bi/SiO2
Pd7Bi(t)/SiO2
Pd3Bi(t)/SiO2
Pd1Bi(t)/SiO2
PdBi(im)/SiO2
PdBi2(im)/SiO2
7.13
3.01
1.77
7.22
3.14
1.89
1.08
0.53
3.3. Hydrogenation on the annealed Bi-poisoned Pd/SiO2
catalysts
The dimensions of the nanoparticles in the Pd1Bi/SiO2 catalyst,
7.2 2.1 nm (Fig. 4a and b), were similar to that found in the Pd/SiO2
catalyst. PXRD study (Fig. 4c) showed very similar patterns with
the reflexes attributed to Pd nanoparticles of 5.0 0.2 nm in diam-
eter according to Scherrer equation. Interestingly, Bi content (5.0%)
observed in Pd1Bi/SiO2 (Table 1) was much higher than can be
expected based only on the surface poisoning, because only 1.26%
of Bi was needed to achieve monolayer coverage of surface Pd sites.
This indicates either (a) the deposition of Bi nanoparticles on the
support material, or (b) multilayer adsorption on the surface of
Pd particles. Multilayer adsorption on the Pd surface should have
increased the average particle diameter from 7.4 to 9.6 nm, which
is not supported by TEM studies (Fig. 4b). As such, deposition of
Bi on the support material is likely, but if Bi particles of 5 nm or
larger in diameter were formed they should have been apparent
from PXRD measurements due to very high intensity of Bi (inten-
sity of Bi reflexes is 5 times as high as that for Pd). Given that PXRD
patterns show no evidence of reflections associated with Bi, any
monometallic Bi particles are expected to be small in size. This
may be supported by a small shift in the particle size distribution
observed by TEM (Figs. 3b and 4b). After Bi addition, there is a small
increase in the number of particles observed that are smaller than
4 nm in size, although, the change is small and should not be con-
sidered as conclusive. Although the formation of Bi nanoparticles
cannot be entirely ruled out, we believe that Bi adsorbed on Pd
nanoparticles, preferentially on edge sites as demonstrated by DFT
calculations, IR-studies for Pd–Bi and electrochemical studies Pt–Bi
system [4,34,50,51].
Intermetallic compounds are usually obtained by melting
metals in an inert atmosphere [54–57], however very high temper-
atures are required (about 1000–2000 ◦C) resulting in the particles
showing low activity due to low dispersity [20,21]. A polyol method
overcomes this problem and implies the annealing of metal precur-
sors at much lower temperatures (200–300 ◦C) in polyol solvents
such as glycerol, ethylene or tetraethylene glycols, which provide
reducing environment, facilitate fusion of metal nanoparticles and
act as capping agents [38,58,59].
Palladium and bismuth form several intermetallic phases that
are stable below 200 ◦C with the chemical formulae PdBi2, PdBi,
Pd2Pd5, Pd3Bi [55–57]; and recent data show that Pd does not alloy
with Bi [56]. According to Heise et al. [58], intermetallic phases of
Pd and Bi are formed at 170–240 ◦C on microwave-assisted polyol
synthesis, while similar PtBi intermetallic compounds form on
brief annealing of Pt–Bi nanoparticles at 220 ◦C [38]. Because inter-
metallic Pd–Bi compounds were demonstrated to be promising
catalysts for various oxidation reactions [26–30], the Bi-poisoned
Pd catalysts obtained were refluxed in ethylene glycol to induce the
formation of intermetallic compounds. PXRD data (Fig. 6a) show
that on annealing, intermetallic compound Pd5Bi2 was formed in
the Pd1Bi(t)/SiO2 catalyst. A TEM study of the annealed catalysts
showed that metal nanoparticles formed chain-like agglomerates
(Fig. 6b and c) and led to an increase in nanoparticles dimen-
sions, being 9.9 3.6 nm for Pd7Bi(t)/SiO2 catalyst, and 15 8 nm
for Pd3Bi(t)/SiO2 and Pd1Bi(t)/SiO2 catalysts. EDX study of individ-
ual particles confirmed the presence of Pd and Bi. So the significant
increase in the nanoparticle dimensions observed for the poisoned
catalysts was likely caused by the increased mobility due to intro-
duction of low-melting Bi (bulk phase melting point is 271 ◦C [55]).
The concentration profiles of MBY hydrogenation using the
annealed catalysts are presented in Fig. 7a–c. Similar to the
Fig. 5 shows the concentration profiles of MBY hydrogenation on
Bi-poisoned Pd catalysts. The derivative plots (Fig. 5, top) demon-
strate that with the increase in Bi poisoning, the reaction rates
decreased by almost 50 times comparing Pd7Bi/SiO2 and Pd1Bi/SiO2
catalysts. More importantly, for all Bi poisoned catalysts, MBA

N. Cherkasov et al. / Applied Catalysis A: General 497 (2015) 22–30
27
Fig. 5. Concentration profiles of MBY hydrogenation on (a) Pd7Bi/SiO2, (b) Pd3Bi/SiO2, (c) Pd1Bi/SiO2 catalysts with dots representing experimental data (the same designations
as in Fig. 2) and lines – Langmuir–Hinshelwood model (see Section 3.4). Corresponding derivative plots above the concentration profiles show the apparent reaction rates
(the unit is mol m−3 s−1 gcat−1).
Fig. 6. (a) PXRD patterns of Bi-poisoned Pd catalysts annealed in ethylene glycol at 196 ◦C for 20 min. Representative TEM microphotographs of the (b) Pd7Bi(t)/SiO2 and (c)
Pd3Bi(t)/SiO2 catalysts.
Fig. 7. Concentration profiles of MBY hydrogenation on (a) Pd7Bi(t)/SiO2, (b) Pd3Bi(t)/SiO2 and (c) Pd1Bi(t)/SiO2 catalysts annealed in ethylene glycol at 196 ◦C for 20 min
with dots representing experimental data (the same designations as in Fig. 2) and lines – Langmuir–Hinshelwood model (see Section 3.4). Corresponding derivative plots
above the concentration profiles show the apparent reaction rates (the unit is mol m−3 s−1 gcat−1).

28
N. Cherkasov et al. / Applied Catalysis A: General 497 (2015) 22–30
Table 2
CO chemisorption and TPR data, average reaction rate per mol of Pd (Aaverage) and TOF.
Aaverage (min−1
)
CO uptake (␮mol g−1
)
TOF (s−1
)
H2 releasea (␮mol g−1
)
Pd/SiO2
35
7.1
0.41
0.12
60.3
23.9
6.6
3.03
1.56
0.32
–
254
62.3
n.d.
n.d.
Pd7Bi(t)/SiO2
Pd1Bi/SiO2
PdBi(im)/SiO2
n.d.
n.d. = not detected.
a
Release of hydrogen as a result of palladium hydride decomposition during the TPR.
initial Bi-poisoned Pd catalysts, the activity of the annealed
catalysts decreased at higher Bi content, being 2–3 times less
active than the corresponding Bi-poisoned catalysts, which was
expected considering the observed increase in the dimensions of
the nanoparticles. Furthermore, the relative apparent reaction rates
of MBY and MBE consumption for the annealed catalysts changed
with Bi poisoning. For the Pd7Bi(t)/SiO2 and Pd3Bi(t)/SiO2 catalysts,
MBE consumption rate was higher than the MBY consumption,
while the intermetallic-containing Pd1Bi(t)/SiO2 catalyst showed
very low apparent rate of MBE consumption. However, in com-
parison with the Pd/SiO2 catalyst, the apparent MBY consumption
rate for the Pd1Bi(t)/SiO2 catalyst was more than 100 times slower
(derivative plots in Figs. 2a and 7c).
The synthesis of intermetallic catalysts, PdBi2(im)/SiO2 and
PdBi(im)/SiO2 was attempted using the polyol method leading
to the formation of a mixture of several intermetallic phases
(PdBi2, PdBi, and Pd5Bi2). Interestingly, the PdBi2(im)/SiO2 catalyst
showed a very unusual behaviour for Pd based catalysts, prefer-
entially forming MBA (selectivity of 75%), rather than MBE, even at
low MBY conversion (Fig. S1 in the Supplementary Data). However,
the observed activity of the catalysts was 500–2000 times lower
in comparison to Pd/SiO2 hence little promise of hydrogenation
application.
Moreover, the k2 constant was very low for the Pd1Bi(t)/SiO2 cata-
lyst, showing almost full suppression of the MBE hydrogenation
rate. The rate constant k3 of direct MBY to MBA hydrogenation
(Fig. 8c) was at least one order of magnitude lower than the k2 con-
stant with very wide confidence intervals. Very low values of k3
show that the direct MBY over-hydrogenation rate was very low,
while wide confidence intervals show that very similar concentra-
tion profiles can be modelled with a wide range of k3 parameter
values. As a result, MBY hydrogenation on the studied catalysts can
be accurately modelled considering only two stepwise hydrogena-
tion stages, not taking into account the direct over-hydrogenation
stage.
The relative adsorption constants obtained are of more inter-
est because many theoretical works have shown that adsorption
factors play an important role in improving semi-hydrogenation
selectivity of Pd-based bimetallic catalysts [3,4,14,60]. The relative
adsorption constant Q1 (Fig. 8d), was similar for all studied cata-
lysts, showing that MBE adsorption constant was 5–15 times lower
than the adsorption constant of MBY. As a result, the reaction rates
accelerated at low MBY concentrations (Eq. (5)), because a higher
fraction of the catalyst surface was free to adsorb hydrogen and
MBE species [41]. Interestingly, undoped Pd/SiO2 catalyst showed
the highest value of Q1 constant, and as a result, the lowest acceler-
ation of the reaction rates at high MBY conversion (Fig. 2) and the
lowest MBE selectivity.
3.4. Kinetic modelling
In contrast, Q2 and Q3 constants, which show the ratio of MBA to
MBY and MBA to MBE adsorption constants, respectively, signifi-
cantly increased at high Bi poisoning (Fig. 8e and f). For the catalysts
Pd1Bi/SiO2, Pd3Bi(t)/SiO2 and Pd1Bi(t)/SiO2, Q3 value was higher
than 1, decreasing the MBE hydrogenation rate as shown in Eq. (6)
in the presence of MBA. The intermetallic PdBi(im)/SiO2 had a Q3
value higher than 100, suggesting that once some MBA was formed
as a result of MBE hydrogenation, it blocked the catalyst surface
and suppressed further MBE hydrogenation (Fig. 7c).
Fig. 9a shows the correlation between the apparent rate
constants k1 and k2, which is linear for almost all the catalysts
except PdBi2(im)/SiO2, i.e. Bi poisoning decreased both k1 and k2
constants almost to the same extent. Hence, the observed change in
the relative apparent reaction rates of MBE and MBY consumption
for different Bi-poisoned and annealed catalysts can be explained
by the change of adsorption constants. For example, Q1 is squared
in Eq. (6), showing very high impact of small variations in the rela-
tive adsorption constants on the reaction rates. More surprising is
the correlation of Q2 with the change of k1 constant (Fig. 9b). The
relative adsorption constant Q2 was almost identical for Pd/SiO2,
Lindlar and the catalysts with low Bi content. For the catalysts
Pd1Bi/SiO2, Pd1Bi(t)/SiO2, and intermetallic-containing catalysts,
the Q2 constant increased with the decrease in k1 constant, which
suggests strong ligand effects of Bi on Pd nanoparticles.
The effects of Bi poisoning and annealing was compared study-
ing the apparent reaction rate and adsorption constants derived
by kinetic modelling as described in Section 2.4. For better under-
standing of the model parameters, Eqs. (1) and (2) can be simplified
by neglecting MBA and MBY concentrations during the predom-
inant MBY and MBE hydrogenation stages giving rise to Eqs. (5)
and (6). High values of the apparent rate constants are responsi-
ble for the increase in the corresponding reaction rates, while high
relative adsorption constants Q1 and (Q3 = Q2/Q1) decrease the reac-
tion rates due to saturation of the active sites with the reacting
molecules and displacement of hydrogen species from the catalyst
surface.
k1∗CMBY
r1
≈
≈
(5)
2
(CMBY + Q1CMBE1
)
k2∗CMBEQ1−2
r2
(6)
2
(CMBE + Q2/Q1CMBA
)
The solid lines in Figs. 2, 5 and 7 show the modelled concentra-
tion profiles, which are in good agreement with the experimental
data. The resulting kinetic parameters obtained by the fitting are
presented in Fig. 8. The apparent rate constant k1 (Fig. 8a) of the
MBY to MBE hydrogenation reaction decreased with the increasing
Bi content for Bi-poisoned and the annealed Bi-poisoned catalysts.
The annealing step decreased the k1 constant further by 2.5–1.5
times possibly due to the sintering of the nanoparticles as observed
by TEM. The apparent rate constant k2 (Fig. 8b) of MBE to MBA
hydrogenation changed similarly to k1 constant, decreasing for the
catalysts with higher Bi content and for the annealed catalysts.
To test the possibility of strong ligand effects of Bi on Pd particles,
CO chemisorption studies were performed for the selected catalysts
to compare the average MBY consumption rates normalised per
mole of Pd and per active site (Table 2). The absolute reaction rate
decreased by almost 85 times compared Pd/SiO2 and Pd1Bi/SiO2
catalysts, while CO total uptake decreased only by about 10 times. It

N. Cherkasov et al. / Applied Catalysis A: General 497 (2015) 22–30
29
Fig. 8. Langmuir–Hinshelwood model parameters obtained by fitting the experimental data with 90% confidence intervals calculated using the Monte-Carlo method [44].
Fig. 9. The correlation between (a) the apparent rate constants k1 and k2, and (b) k1 and Q2 for all studied catalysts.
shows that the TOF for these catalysts decreased by almost an order
of magnitude due to the introduction of Bi. So significant change in
TOF indicates ligand effects of Bi on Pd, which led to the change in
the hydrogen dissociative adsorption energy – the effect demon-
strated for Pd, Pt, Ni-based bimetallic systems by density functional
theory calculations [61,62].
Interestingly, TPR data (Table 1) showed that Bi-poisoning has
a dramatic effect on the formation of palladium hydride, which
decreased by 75% comparing Pd/SiO2 and Pd7Bi(t)/SiO2 catalysts.
For gas-phase hydrogenation, the formation of sub-surface ␤-
hydride phase was identified as one of the reasons for non-selective
hydrogenation [63,64]. A study for Lindlar catalyst, showed that in
liquid-phase hydrogenation the poisoning with lead decreased the
hydride phase formation [65]. Similarly, the hydrogen consumption
data obtained in this work for Bi-poisoned catalysts suggest that Bi
suppresses the formation of hydride phase, which, along with the
poisoning of edge sites, can be a reason for increased selectivity of
Bi-poisoned catalysts.
4. Conclusions
Bi-poisoned Pd catalysts and annealed Bi-poisoned Pd cata-
lysts were compared in the liquid-phase semi-hydrogenation of
2-methyl-3-butyn-2-ol. The poisoning of Pd with Bi increased
the maximum alkene yield from 91.5% for Pd/SiO2 to 94–96% for
Bi-poisoned catalysts, which is comparable to the current indus-
trial standard – Lindlar catalyst (94.5%). At a lower Bi content
(Pd:Bi ratio above 3) the increased alkene selectivity and decreased
activity is likely associated with the adsorption of Bi on the low-
selective step and edge sites of Pd nanoparticles [4,34,50,51,53].
At higher Bi content in addition to blocking non-selective Pd
sites, Bi induced strong ligand effects and significantly decreased
adsorption energy of alkene species. The annealing in ethylene
glycol at 196 ◦C led to the sintering of nanoparticles and induced
the formation of Pd5Bi2 intermetallic compound, which demon-
strated MBY hydrogenation activity about 2 orders of magnitude
lower than the Pd catalyst and almost completely suppressed the

30
N. Cherkasov et al. / Applied Catalysis A: General 497 (2015) 22–30
alkene hydrogenation rate. Other intermetallic compounds were
even less active. However, increasing Bi content and annealing
notably increased the adsorption constant of alkane molecules,
suppressing alkene hydrogenation. Therefore, results from his
study suggest that intermetallic Pd–Bi compounds cannot be con-
sidered as promising catalysts for hydrogenation due to very low
activity and high price. However, Bi-poisoned Pd catalysts combine
the ease of preparation, high activity and alkene selectivity compa-
rable to that of Lindlar catalyst. An important advantage of Pd–Bi
catalyst is significantly lower toxicity of Bi in compared to Pb.
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