Liquid-phase hydrogenation of benzaldehyde over Pd-Ru/C catalysts: Synergistic effect between supported metals

Article abstract of DOI:10.1016/j.cattod.2016.07.022

Bimetallic Pd-Ru/C catalysts prepared with different carbon supports were shown to be much more active in the liquid-phase hydrogenation of benzaldehyde in comparison with their monometallic analogs. In the case of systems based on nanoglobular carbon, th

Full text of DOI:10.1016/j.cattod.2016.07.022

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Liquid-phase hydrogenation of benzaldehyde over Pd-Ru/C catalysts:
Synergistic effect between supported metals
a,b,∗
, Olga B. Belskayaa,c, Tatyana I. Gulyaevaa,
Roman M. Mironenko
a
c,d
d
Mikhail V. Trenikhin , Alexander I. Nizovskii , Alexander V. Kalinkin ,
d
a
a,c,e
Valerii I. Bukhtiyarov , Alexander V. Lavrenov , Vladimir A. Likholobov
a
Institute of Hydrocarbons Processing, Siberian Branch of the Russian Academy of Sciences, Neftezavodskaya st., 54, 644040, Omsk, Russia
Dostoevsky Omsk State University, Mira ave., 55a, 644077, Omsk, Russia
Omsk State Technical University, Mira ave., 11, 644050, Omsk, Russia
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Lavrentieva ave., 5, 630090, Novosibirsk, Russia
Omsk Scientific Center, Siberian Branch of the Russian Academy of Sciences, Marksa ave., 15, 644024, Omsk, Russia
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Bimetallic Pd-Ru/C catalysts prepared with different carbon supports were shown to be much more active
in the liquid-phase hydrogenation of benzaldehyde in comparison with their monometallic analogs. In
the case of systems based on nanoglobular carbon, the synergistic effect manifested itself as an increased
selectivity for benzyl alcohol (86–90%) at a complete conversion of benzaldehyde, whereas the catalysts
containing carbon nanotubes were highly active in hydrogenolysis of the C O bonds, which caused an
increased yield of toluene (up to 56%). The synergistic effect between palladium and ruthenium in the
reaction under study can be related to the changes in the electronic and dispersion state of the supported
metals upon interaction with each other and formation of Pd-Ru alloy. In terms of the electronic factor,
the enhanced catalytic performance of bimetallic systems can be a result of facilitation of electrophilic
activation of the C O bond in benzaldehyde owing to an increased fraction of the electron-deficient
palladium species. The nature of carbon support was shown to affect the electronic state of supported
metals and thus catalytic properties of bimetallic Pd-Ru/C systems.
Received 11 November 2015
Received in revised form 15 June 2016
Accepted 27 July 2016
Available online xxx
Keywords:
Benzaldehyde
Catalytic hydrogenation
Bimetallic catalysts
Carbon supports
Synergistic effect
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
zyl alcohol is not always high owing to hydrodeoxygenation into
toluene [5,6,9,19]. The problem of selective reduction of the car-
bonyl group to the alcohol one (especially if the C C bonds are
present in a substrate molecule) is commonly solved with the use
of platinum or ruthenium catalysts [3,11–13,20,21]. The latter cat-
alysts are most attractive due to a relatively low cost of ruthenium
compared to other noble metals [22]. However, the application
of ruthenium catalysts for hydrogenation of aromatic aldehydes
or ketones does not ensure a complete conversion of the initial
compounds [13,14], and the hydrogenation of aromatic ring often
occurs, which decreases the yield of the target alcohol [10,20,23].
At the same time, the development of supported bimetallic
catalysts is still of interest because the introduction of the sec-
ond, less expensive metal makes it possible to lower the catalyst
cost and retain or even improve the catalytic properties [24–28].
Bimetallic composites often demonstrate the synergistic effect,
i.e. the activity (or selectivity) of a bimetallic catalyst is greater
than the sum of activities (or selectivities) of its monometal-
lic analogs. The synergistic effect was found, in particular, when
studying the palladium-ruthenium catalysts in hydrogenation of
The catalytic hydrogenation of benzaldehyde is used in industry
for the production of benzyl alcohol, which is an important inter-
mediate in the synthesis of vitamins, pharmaceuticals, pesticides,
fragrances, and various esters [1–4]. Much attention is given now
to the development of efficient catalysts for selective hydrogena-
tion of benzaldehyde to benzyl alcohol. Various supported catalysts
containing platinum [1–3], palladium [4–9], ruthenium [10–14]
and nickel [15–18] were proposed to perform this reaction under
mild conditions. Commercial palladium catalysts demonstrate high
activity and retain their properties after multiple uses in protic sol-
vents. The main drawback of these catalysts in hydrogenation of
benzaldehyde is related to the fact that their selectivity for ben-
∗ Corresponding author. Present address: Institute of Hydrocarbons Processing,
Siberian Branch of the Russian Academy of Sciences, Neftezavodskaya st 54, 644040
Omsk, Russia.
E-mail addresses: ch-mrm@mail.ru, chemistmrm@gmail.com (R.M. Mironenko).
http://dx.doi.org/10.1016/j.cattod.2016.07.022
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
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1
-hexene [29], benzene [30], o-xylene [31], 1,4-butanediol [32,33],
2.2. Catalyst characterization
phenol [33,34], anisole [33], cinnamaldehyde [35], benzoic acid
[
36], dimethyl terephthalate [37], 2-methyl-2-nitropropane [38], p-
Palladium and ruthenium contents in the catalysts were esti-
mated by optical emission spectrometry with inductively coupled
plasma (ICP-OES) on a Varian 710-ES spectrometer. The measure-
ments were made after the dissolution of catalyst samples in a
mixture of nitric and hydrochloric (or perchloric) acids.
chloronitrobenzene [39], pyridine [33], and glucose [40]. However,
the Pd-Ru composites remain poorly investigated among numerous
bimetallic catalysts [25,27].
In the synthesis of a catalyst with specified activity and selectiv-
ity, of key importance are not only the nature of a metal component
and its state in the catalyst but also features of the support. The
application of carbon materials as supports in the synthesis of
hydrogenation catalysts has some advantages. Carbon supports
have a high specific surface area, a developed pore space that
ensures a transfer of reactants and reaction products, controllable
chemical properties of the surface, and chemical inertness, espe-
cially in strong acids and bases [41–46]. Ding et al. [12] proposed the
mesoporous carbon-based supported catalysts, which are highly
active in hydrogenation of benzaldehyde and provide a nearly 100%
selectivity for benzyl alcohol. Nevertheless, revealing the effect of
the carbon support features (pore structure, chemical composition
and surface morphology) on the formation, chemical state, size and
catalytic properties of metal sites remains a topical problem.
The present work deals with the study of Pd-Ru/C catalysts pre-
pared with the use of different carbon supports in the liquid-phase
hydrogenation of benzaldehyde. Multi-wall carbon nanotubes
After deposition of the precursors on the supports and drying
in air at room temperature, the samples were examined by H -TPR
2
using an AutoChem II 2920 (Micromeritics) chemisorption analyzer
equipped with a thermal conductivity detector (TCD). The H -TPR
2
experiments were performed with a mixture of 10% H and argon
2
3
−1
(the flow rate 25 cm min ) in a temperature range of 308–673 K
−
1
at a heating rate of 10 K min
.
The dispersion of a supported metal in the catalysts was esti-
mated by pulse chemisorption of CO after the reduction in flowing
hydrogen at 523 K and cooling in an inert gas to room temperature.
A mixture of 10% CO and helium was fed into the flow of inert car-
rier gas (helium) by pulses at equal time intervals. The batching was
carried out at room temperature until the TCD signal became con-
stant. The dispersion was calculated taking into account the average
adsorption stoichiometry: CO:Pd = 0.5 and CO:Ru = 1 [6,47–49].
TEM images of the catalysts pre-reduced at 523 K were taken
by a JEM-2100 (JEOL) instrument using an accelerating voltage of
200 kV. The samples were prepared by dispersing the catalysts in
ethanol and spraying the suspension on a perforated carbon-coated
copper grid. The energy dispersive X-ray analysis (EDX) was per-
formed with an INCA 250 (Oxford Instruments) spectrometer. TEM
images were analyzed using DigitalMicrograph (Gatan) software.
XPS measurements of the catalysts were carried out on a
(
CNTs) and nanoglobular carbon (NGC), which were characterized
in our earlier works [9,14,47], served as the supports. The chosen
carbon materials differ fundamentally in the preparation method,
structure, and such physicochemical characteristics as texture and
acid-base properties of the surface. It was shown that the bimetallic
Pd-Ru/C catalysts exhibited a substantially enhanced hydrogena-
tion activity compared with the monometallic analogs (Pd/C and
Ru/C). To reveal the reason of synergistic effect between palladium
and ruthenium in the tested reaction, the samples were charac-
−
7
SPECS spectrometer at a residual pressure of 7 × 10 Pa in the
analyzer chamber. Before the measurements, the samples were
reduced in a hydrogen flow at 523 K. The spectra were recorded
using monochromatic AlK␣ radiation (hꢀ = 1486.7 eV). Preliminar-
ily, the binding energy (BE) scale of the spectrometer was calibrated
against signals obtained from metallic gold and copper, Au 4f7/2
terized by temperature-programmed reduction (H -TPR), pulsed
chemisorption of CO, transmission electron microscopy (TEM), and
X-ray photoelectron spectroscopy (XPS).
2
(84.0 eV) and Cu 2p3/2 (932.6 eV). The spectra were calibrated
with respect to C 1s line of the carbon support (284.6 eV). The
measurement data were processed using the XPSPEAK software.
The Gaussian-Lorentzian function with the fixed value of Gaus-
sian/Lorentzian ratio (Gaussian 30%) was used for deconvolution
of the spectra. During the fitting procedure, the parameters of each
component such as intensity, full width at half maximum and peak
position were varied within a reasonable range [50].
2
. Experimental
2.1. Catalyst preparation
2.3. Catalytic measurements
The catalysts were synthesized using CNTs Baytubes® C 150 HP
purchased from Bayer MaterialScience AG (Leverkusen, Germany)
and NGC represented by conductive carbon black P278-E (analog
to N472) and supplied by Experimental Technology Department of
Institute of Hydrocarbons Processing (Omsk, Russia). Prior to exper-
iments, the carbon supports were dried overnight in air at 393 K to
remove the adsorbed water.
Bimetallic Pd-Ru catalysts with the total content of supported
metals close to 1.5 wt.% were obtained by incipient wetness
impregnation of the carbon supports. To prepare the impregnating
solutions, palladium (II) chloride and ruthenium (IV) hydrox-
ytrichloride (both pure grade, JSC “Aurat”) were dissolved in
concentrated hydrochloric acid (Pd to HCl and Ru to HCl molar
ratios of 1:2 and 1:3, respectively) and diluted with water to
a desired concentration. Prior to impregnation, the Pd- and Ru-
containing solutions were mixed so as to reach the Ru:Pd molar
ratios of 1 and 2.5 in the resulting samples. The corresponding sam-
ples are denoted hereinafter as 1Pd-1Ru/C and 2Pd-5Ru/C. Data on
their composition are listed in Table 1. After the impregnation, the
samples were dried in air at room temperature.
Pretreatment of the catalyst samples prior to catalytic experi-
ments included drying in argon flow at 423 K for 0.5 h and reduction
in flowing hydrogen at 523 K for 2 h.
The liquid-phase hydrogenation of benzaldehyde (98%, Acros
Organics) in the presence of the synthesized catalysts was stud-
ied using a 180 cm3 steel autoclave. A 500-mg catalyst sample was
3
placed into the autoclave with 30 cm of ethanol (95%). Air compo-
nents were removed from the catalyst pore space by pre-reduction
with hydrogen at a temperature of 353 K and a pressure of 0.5 MPa
for 1 h. The reaction mixture was heated to a specified temperature
by circulation of heated water through the external jacket. To stir
the mixture, the autoclave was mounted on a shaker.
After pre-reduction, 5.0 cm3 of benzaldehyde and 65 cm3 of
ethanol were loaded into the autoclave. The hydrogenation was
carried out for 5 h at a temperature of 313 K and a hydrogen pres-
sure of 0.5 MPa under vigorous stirring of the reaction mixture,
which excluded external diffusion limitations. The reaction prod-
ucts were identified by gas chromatography-mass spectrometry
(
GC–MS) on an Agilent 5973N/6890N instrument. The quantitative
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Table 1
Designation and composition of bimetallic catalysts.
Sample
Pd content (wt.%)a
Ru content (wt.%)a
Total content of metals (wt.%)
Ru:Pd molar ratio
1Pd-1Ru/NGC
2Pd-5Ru/NGC
1Pd-1Ru/CNTs
2Pd-5Ru/CNTs
0.81 ± 0.05
0.48 ± 0.01
0.85 ± 0.02
0.49 ± 0.01
0.76 ± 0.05
1.15 ± 0.07
0.80 ± 0.05
1.22 ± 0.02
1.57
1.63
1.65
1.71
1.0
2.5
1.0
2.6
a
According to optical emission spectrometry with inductively coupled plasma (ICP-OES).
composition of the liquid phase in the autoclave was determined
every hour using gas chromatography (GC) on a Hewlett Packard
5
890 Series II chromatograph equipped with a capillary column
HP-PONA (50 m × 0.20 mm, Agilent Technologies) and a flame ion-
ization detector.
3
. Results and discussion
Pd-Ru/NGC and Pd-Ru/CNTs catalysts with the total content of
supported metals 1.6–1.7 wt.% and the Ru:Pd molar ratio of 1.0 or
.5 (Table 1) were studied in the hydrogenation of benzaldehyde in
2
ethanol medium at a temperature of 313 K and hydrogen pressure
of 0.5 MPa. Results of the catalytic measurements were compared
with the data obtained under identical conditions for monometal-
lic catalysts Pd/CNTs, Pd/NGC, Ru/CNTs and Ru/NGC containing
1
.5 wt.% of palladium or ruthenium. Besides, the catalytic behav-
ior of monometallic samples (1Pd/C, 1Ru/C) with a metal loading
close to a loading of one of the metal in 1Pd-1Ru/C samples (about
Fig. 1. The H2-TPR profiles of supported chloride precursors in the samples: Pd/NGC
(1), Pd/CNTs (2), Ru/NGC (3), Ru/CNTs (4), Pd-Ru/NGC (5), Pd-Ru/CNTs (6). Prior to
measurements, the samples were dried in air at room temperature.
1
wt.%) was taken into account.
All the samples of bimetallic catalysts showed high activity
in the tested reaction, and a complete conversion of benzalde-
hyde was already observed 1 h after the onset of the reaction
respectively, 4 h after the beginning of the experiment (Fig. S1a).
Under the same conditions, the 2Pd-5Ru/NGC catalyst showed a
higher selectivity for benzyl alcohol, whose mole fraction reached
0.35, whereas the fraction of toluene decreased to 0.64 (Fig. S1b).
Under the specified conditions of the reaction (313 K, 1 h),
CNTs-based bimetallic catalysts showed an increased activity in
hydrodeoxygenation, which is comparable with the activity of
Pd/NGC sample (Table 2). As a result, the selectivity for benzyl alco-
hol was lower as compared to NGC-based catalysts: about 42 and
57% for samples 1Pd-1Ru/CNTs and 2Pd-5Ru/CNTs, respectively,
and the yield of toluene reached 56%.
(Table 2). Under the same reaction conditions, only the Pd/NGC
sample showed a comparable activity (benzaldehyde conversion
of 99%), whereas Pd/CNTs, Ru/CNTs and Ru/NGC catalysts gave a
conversion of benzaldehyde equal to about 32, 40 and 43%, respec-
tively.
Under the indicated conditions of catalytic hydrogenation, the
products of benzaldehyde transformations were benzyl alcohol,
toluene, (ethoxymethyl)benzene and (diethoxymethyl) benzene,
which are formed according to Scheme 1.
Joint transformations of benzaldehyde or benzyl alco-
In the presence of Pd-Ru/CNTs catalysts, the degree of
hydrodeoxygenation was even higher in comparison with bimetal-
lic NGC-supported samples: as the reaction time increased, the
fraction of toluene in the products reached 100% (Fig. S1c and d).
Therewith, an increase of the Ru:Pd ratio in the bimetallic catalyst
ensured a pronounced growth of selectivity for benzyl alcohol only
at the beginning of the experiment.
The catalytic properties of 1Pd-1Ru/C catalysts were addition-
ally compared to those of monometallic 1Pd/C and 1Ru/C catalysts
containing almost the same amount of Pd and Ru (1 wt.%) as
bimetallic samples (Table S1). Under the same reaction conditions,
the bimetallic catalysts were superior in hydrogenation activity, i.e.
a conversion of benzaldehyde over 1Pd-1Ru/C catalysts is greater
than the sum of conversions over 1Pd/C and 1Ru/C. As compared
to monometallic catalysts, 1Pd-1Ru/NGC sample demonstrated an
increased yield of benzyl alcohol, whereas 1Pd-1Ru/CNTs catalyst
was highly active in hydrogenolysis of the C O bonds, which pro-
duced a high yield of toluene.
hol with
a
solvent yielding (ethoxymethyl)benzene and
(
diethoxymethyl)benzene may involve acid sites of the sup-
port [5,9,14] or electron-deficient species of the supported
metal [51,52]. It should be noted that in the case of bimetallic
catalysts the transformation sequence benzaldehyde → benzyl
alcohol → toluene prevailed, while the formation of side ethers
was virtually suppressed: the yield of (ethoxymethyl)benzene did
not exceed 3% and (diethoxymethyl)benzene was not detected
among the products (Table 2).
As follows from analysis of catalytic measurements (Table 2),
in distinction to monometallic NGC-supported samples, bimetallic
catalysts prepared with the same support were highly selective for
benzyl alcohol (86–90%) under the chosen conditions of the reac-
tion (313 K, 1 h). In the process, hydrogenolysis of the C O bonds
was much less pronounced in comparison with the Pd/NGC sample;
as a result, the toluene yield was not high (up to 10%).
In the presence of Pd-Ru/NGC catalysts, extending the reaction
time increased the fraction of toluene in the products up to 90% (Fig.
S1a and b in the Supplementary materials). At the same time, an
increase in the Ru:Pd ratio in the NGC-supported bimetallic system
made it possible to diminish the degree of hydrodeoxygenation and
raise the yield of benzyl alcohol. Thus, owing to hydrogenation of
benzaldehyde in the presence of 1Pd-1Ru/NGC sample, mole frac-
tions of benzyl alcohol and toluene were equal to 0.06 and 0.93,
To elucidate the causes of the increased hydrogenation activity
of bimetallic catalysts, the formation of metallic sites was stud-
ied by H -TPR. The dispersion state of formed metal nanoparticles
2
(NPs) was examined by pulsed chemisorption of CO and TEM.
The H -TPR profiles of monometallic Pd/C samples show a sin-
2
gle hydrogen consumption peak corresponding to the reduction
of supported palladium chloride complexes (Fig. 1, curves 1 and 2).
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Table 2
a
Catalytic properties of Pd/C, Ru/C and Pd-Ru/C samples in the liquid-phase hydrogenation of benzaldehyde under the hydrogen pressure . Prior to experiments, the catalysts
were dried in argon at 423 K for 0.5 h and reduced in flowing hydrogen at 523 K for 2 h.
Catalystb
Dc
d (nm)d
X (%)e
S (%)f
BA
T
EMB
DMB
Pd/NGC
0.16
0.79
0.69
0.62
0.36
0.54
0.43
0.30
n.d.
∼1
∼1
1.3
2.3
1.6
1.6
1.8
99.3
100
100
43.1
31.6
100
100
39.6
41.9
86.3
89.6
43.7
82.8
41.5
57.4
80.6
52.0
10.2
7.6
0.2
0.2
56.0
39.7
0.6
4.9
2.8
2.2
0
0.9
2.1
2.2
0
0
0
0
49.9
12.6
0
0
18.0
1
2
Pd-1Ru/NGC
Pd-5Ru/NGC
Ru/NGC
Pd/CNTs
1
2
Pd-1Ru/CNTs
Pd-5Ru/CNTs
Ru/CNTs
a
Reaction conditions: temperature of 313 K, pressure of 0.5 MPa, 1 h, 95 cm3 of ethanol, 5.0 cm3 of C6
The total content of supported metals in all catalysts is constant and close to 1.5 wt.%.
The metal dispersion determined by pulsed chemisorption of CO after H2-TPR. The calculation used the following adsorption stoichiometry: CO:Pd = 0.5, CO:Ru = 1
Н
5CНO, 500 mg of catalyst.
b
c
[
6,47–49].
d
The average size of supported metal particles estimated by TEM.
Conversion of benzaldehyde according to GC.
Selectivities for benzyl alcohol (BA), toluene (T), (ethoxymethyl)benzene (EMB) and (diethoxymethyl)benzene (DMB) according to GC.
e
f
Scheme 1. Reaction pathways of benzaldehyde hydrogenation in ethanol medium.
Therewith, a precursor in Pd/NGC sample is reduced under more
severe conditions (a temperature maximum at 414 K), which leads
to the formation of NPs with a lower dispersion as compared with
Pd/CNTs (Table 2). According to TEM measurements [47], the pal-
ladium NPs in Pd/CNTs sample have the average size of 2.3 nm
ladium samples. The supported palladium compounds are likely
to accelerate the reduction of ruthenium precursor; as a result,
the high-temperature peak (near 450 K) typical of monometallic
ruthenium catalysts is shifted toward low-temperature region and
overlaps with the peak corresponding to the reduction of palladium
compounds. A simultaneous reduction in the case of supported
palladium-ruthenium systems is confirmed by the data reported
in [34,36,37] and indicates the mutual interaction of palladium
and ruthenium compounds and the formation of bimetallic NPs. As
shown by a comparison of CO chemisorption data for the samples
prepared with the same support, the dispersion of NPs in bimetallic
samples is noticeably higher in comparison with the corresponding
monometallic systems (Table 2). As can be seen from TEM images
of Pd-Ru/C samples (Fig. 2), the bimetallic NPs have a spherical
shape, and the average particle size is somewhat less than that
of monometallic systems regardless of the nature of carbon sup-
port (Table 2). So, the results of TEM measurements are rather in
agreement with CO chemisorption data.
(Table 2, Fig. S2a). Unfortunately, it was not possible to accurately
determine the average particle size of Pd in the Pd/NGC sample
because of very blurred edges of the NPs in TEM images.
At least two hydrogen consumption regions can be distin-
guished on the H -TPR profiles of ruthenium catalysts (Fig. 1, curves
2
3
and 4); they may correspond to the reduction of supported
ruthenium compounds having different composition (the hydrol-
ysis products) or degree of their interaction with the support. The
reductions occur nearly identically in Ru/NGC and Ru/CNTs that
may be due to a close composition of the surface complexes in these
samples. However, in distinction from Ru/CNTs, the Ru/NGC sample
has a higher fraction of species which are more stable to reduction,
i.e., stronger bound to the support surface, since the temperature
maxima of H -TPR peaks are shifted toward high temperatures.
An increased dispersion caused by the interaction in the sup-
ported Pd-Ru system can be a reason of the observed synergistic
effect in benzaldehyde hydrogenation because, firstly, a decrease
in the size of metal NPs increases the fraction of active sites acces-
sible to reactants and, secondly, the presence of the second metal
prevents aggregation of the NPs during the catalyst pretreatment
and in the course of reaction [34,36,37]. Noteworthy is the fact
that dispersion of the NPs in bimetallic catalysts is affected by the
nature of the carbon support. The Pd-Ru/CNTs samples have a much
lower dispersion than NGC-based bimetallic catalysts. In the lat-
ter case, there are well-dispersed NPs with very homogeneous size
distribution and average size of about 1 nm (Fig. 2a and b, Table 2).
2
Assessment of the dispersion by pulsed CO chemisorption showed
that the reduction of a precursor in the Ru/NGC sample results
in the formation of ruthenium NPs with a higher dispersion in
comparison with Ru/CNTs (Table 2). According to microscopic mea-
surements [14], Ru/NGC sample contains ruthenium NPs with a
denser location and better dispersion as compared to Ru/CNTs that
is in agreement with CO chemisorption data. The average particle
sizes are determined to be around 1.8 and 1.3 nm for Ru/CNTs and
Ru/NGC, respectively (Table 2, Fig. S2b and c).
The H -TPR profiles of bimetallic samples (Fig. 1, curves 5 and
2
6
) have a single hydrogen consumption region (the temperature
maximum near 400 K) and resemble the H -TPR curves of pal-
2
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Fig. 2. Representative TEM images of Pd-Ru bimetallic samples at different magnifications: 1Pd-1Ru/NGC (a), 2Pd-5Ru/NGC (b), 1Pd-1Ru/CNTs (c), 2Pd-5Ru/CNTs (d). The
particle size distribution histograms are also presented. Prior to measurements, the samples were reduced in flowing hydrogen at 523 K.
To clarify the question of whether the Pd-Ru alloy is formed
upon interaction of metals, high resolution TEM (HRTEM) images
and EDX spectra were additionally obtained. As shown by HRTEM
analysis (Fig. 3a), reduced 1Pd-1Ru/CNTs sample contains NPs with
a d-spacing of 0.217 nm. This value does not correspond to any lat-
tice spacings of palladium or ruthenium and very close to that of
Pd-Ru alloy (0.219 nm [53]). The EDX spectrum (Fig. 3b) recorded
from the region shown in Fig. 3a includes signals corresponding to
palladium and ruthenium. In very rare cases (Fig. S3), the Pd NPs
with a d-spacing of 0.225 nm were observed in HRTEM images of
the bimetallic sample. However, numerous micrographs and EDX
spectra (as shown in Fig. 3) confirm that supported NPs are mostly
bimetallic.
The palladium-ruthenium interaction in bimetallic systems was
also revealed by XPS, which was employed to investigate the elec-
tronic state of supported palladium and ruthenium in the reduced
catalysts. The Pd 3d spectra recorded for mono- and bimetallic
catalysts show a typical Pd 3d5/2–Pd 3d3/2 doublet consisting of
two overlapping components (Fig. 4), which indicates that the
supported palladium has two electronic states. The positions of
the signals with lower BE values (335.9-336.4 eV in the 3d5/2
region, Table 3) are close to those observed for metallic palladium
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Fig. 3. HRTEM image of 1Pd-1Ru/CNTs sample (a) and EDX spectrum recorded from the region shown in micrograph (b).
Fig. 4. Pd 3d spectra of Pd/C (1) and 1Pd-1Ru/C (2) catalysts prepared with the use of NGC (a) and CNTs (b). Prior to measurements, the samples were reduced in flowing
hydrogen at 523 K.
Table 3
interaction of metals during joint reduction of supported precur-
sors (see Fig. 1) and the formation of Pd-Ru alloy (see Fig. 3). The
Results of XPS analysis of Pd/C, Ru/C and 1Pd-1Ru/C catalysts.
Sample
BEofPd3d5/2 (eV) BE of Ru 3p3/2(eV)
Atomic ratios
direction of electron transfer (from palladium to ruthenium) is pos-
sibly related to a greater surface electronegativity of ruthenium in
comparison with palladium [58]. This effect is contrary to that seen
in bulk, possibly as a result of the anisotropic character of a surface
that modifies the relative electronegativities of the metal atoms.
Thus, the synergistic effect between palladium and ruthenium in
hydrogenation of benzaldehyde can be caused by the mutual inter-
action of supported metals and the formation of alloy, which (1)
increases the dispersion and thus the fraction of accessible active
sites on which hydrogen adsorption is facilitated, (2) changes the
electronic state of metals and therefore promotes the activation of
hydrogen and substrate molecules. In terms of the electronic factor,
it is somewhat difficult to unequivocally establish a participation
of palladium and ruthenium species as active sites in the tested
reaction. The most plausible mechanism of promoting action of
the second metal involves the formation of so-called mixed sites
where both metal components participate in the catalytic trans-
formations, but one of them is not fully reduced in the running
conditions of the reaction [59,60]. In this case, the promotion effect
is due to the presence of electron-deficient species which enhance
the polarization of C O bond and activation of hydrogen. The elec-
tron transfer from palladium to a more electronegative ruthenium
increases the fraction of electron-deficient palladium species in the
catalysts, which serve as the electrophilic sites of adsorption and
activation of the C O bond in the benzaldehyde molecule (Fig. 6).
The most possible ways of electrophilic activation of the carbonyl
Pd0
Pdı+
Ru␦+
Pd :Pd
0
␦+
Pd:C
Pd/NGC
Pd/CNTs
335.9
336.2
336.3
336.4
–
337.2
337.9
338.0
338.2
–
–
–
1.3
1.0
1.1
0.7
–
0.0016
0.0031
0.0007
0.0021
–
1
1
Pd-1Ru/NGC
Pd-1Ru/CNTs
462.5
462.2
462.7
462.5
Ru/NGC
Ru/CNTs
–
–
–
–
(
(
BE = 335.7 ± 0.3 eV [54,55]), while the signals with higher BE values
337.2-338.2 eV in the 3d region, Table 3) can be attributed to the
5
/2
ı+
electron-deficient palladium species Pd (BE = 337.9 ± 0.3 eV for
PdCl [54,55]). Since the signals from ruthenium in the Ru 3d region
2
overlap with the C 1s line from the support, Ru 3p3/2 peak was
employed to estimate the electronic state of ruthenium in mono-
and bimetallic samples [36,56,57].
As shown by analysis of XPS spectra, an obvious shift (by
.2–0.8 eV) of the maxima of Pd and Pd lines toward high BE
values and a decrease in the Pd :Pd atomic ratio are observed
for 1Pd-1Ru/C with respect to Pd/C catalyst based on the same
support (Fig. 4, Table 3). At the same time, a negative shift (0.2
or 0.3 eV) of Ru 3p3/2 peak occurs for 1Pd-1Ru/C with respect to
monometallic analog Ru/C (Fig. 5, Table 3). In the case of 1Pd-
0
␦+
0
0
␦+
1
Ru/CNTs catalyst, the Ru 3p3/2 signal can be assigned to metallic
ruthenium (BE = 462.2 eV [57]). Such changes in the electronic state
of supported palladium and ruthenium in bimetallic Pd-Ru system
have been observed earlier [34,36] and can be caused by the mutual
1
2
group on these sites may involve the ␩ -(O) and ␩ -(C,O) modes
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Fig. 5. Ru 3p3/2 spectra of Ru/C (1) and 1Pd-1Ru/C (2) catalysts prepared with the use of NGC (a) and CNTs (b). Prior to measurements, the samples were reduced in flowing
hydrogen at 523 K.
4. Conclusions
Thus, bimetallic Pd-Ru/C catalysts synthesized with different
carbon supports were shown to be much more active in the liquid-
phase hydrogenation of benzaldehyde than their monometallic
analogs. In the case of NGC-based systems, the synergistic effect
shows up as an increased selectivity for the formation of ben-
zyl alcohol (86–90%) at a complete conversion of benzaldehyde,
whereas the catalysts prepared with the use of CNTs were highly
active in hydrogenolysis of the C O bonds, which caused an
increased yield of toluene (up to 56%). The data obtained for the
catalysts by H -TPR, pulsed CO chemisorption, TEM and XPS sug-
2
Fig. 6. The proposed mechanism of promoting action of the second metal in bimetal-
lic Pd-Ru/C catalysts and the preferential modes of benzaldehyde activation on their
surface.
gest that the synergistic effect between palladium and ruthenium
in the tested reaction can be related to changes in the electronic and
dispersion states of supported metals upon interaction with each
other and formation of Pd-Ru alloy. In terms of the electronic factor,
the enhanced catalytic performance of bimetallic systems can be
caused by facilitation of electrophilic activation of the C O bond in
benzaldehyde owing to an increased fraction of electron-deficient
palladium species. The nature of carbon support was shown to
affect the electronic state of supported metals and thus catalytic
properties of bimetallic Pd-Ru/C systems. Obviously, this effect may
be related to the physicochemical features of each carbon support
influencing the interaction in bimetallic system and requires more
detailed investigation.
in which the C O bond is weakened, thus facilitating a subsequent
addition of chemisorbed hydrogen [61].
The differences in the catalytic properties of bimetallic samples
synthesized with different supports can also be related to changes
in the electronic state of supported metals. An increased activity of
the CNTs-containing catalysts in hydrodeoxygenation into toluene
(
Table 2, Fig. S1, Table S1) is apparently caused by a stronger inter-
0
␦+
action in the Pd-Ru system, which decreases the Pd :Pd ratio
(
Table 3). Besides, due to electron transfer from palladium to ruthe-
Acknowledgements
0
nium, the latter is present in the 1Pd-1Ru/CNTs catalyst as Ru . It
is known that activation of the hydrogen molecule and cleavage
of the H H bond are less hindered if the metallic sites have an
increased number of accessible electrons at the external d-orbital
The authors thank O.V. Maevskaya for participation in the syn-
thesis of catalysts, T.V. Kireeva and A.V. Shilova for the analysis of
the synthesized samples by ICP-OES, O.V. Ivashchenko for charac-
terization of catalysts by TEM and E.N. Kudrya for identification of
hydrogenation products by GC–MS. Characterization of catalysts
and identification of reaction products by GC–MS were performed
using equipment of the Omsk Regional Center of Collective Usage,
Siberian Branch of the Russian Academy of Sciences. The study was
financially supported by the Russian Foundation for Basic Research
[
62]. So, the preferential sites of hydrogen adsorption and activa-
tion are represented most likely by the electron-rich supported
ruthenium species whose fraction can increase with raising the
Ru:Pd ratio. Although CNTs-based bimetallic catalysts have a much
lower dispersion than NGC-based samples, the adsorption ability
of metal sites in Pd-Ru/CNTs catalysts with respect to reactants can
be increased due to the higher fraction of Pd␦ and Ru . This leads
to a deeper hydrogenation and increase in the toluene yield. The
described effect may be related to the features of CNTs affecting the
interaction in bimetallic system (composition of the surface func-
tional cover and electrical conductivity) and requires more detailed
investigation.
The mechanism considered above can explain the observed syn-
ergistic effect and is corroborated by some works on selective
hydrogenation of cinnamaldehyde in the presence of supported
bimetallic systems Ru-Sn [63], Rh-Sn [64], Pd-Sn [65], Pd-Ru [35],
Pt-Sn [61], Pt-Fe [66] and Pt-Ru [67].
+
0
(Project No. 12-03-00153-a).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.07.
0
22.
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Please cite this article in press as: R.M. Mironenko, et al., Liquid-phase hydrogenation of benzaldehyde over Pd-Ru/C catalysts: Synergistic
effect between supported metals, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.07.022

G Model
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effect between supported metals, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.07.022