Synthesis of a hybrid Pd0/Pd-carbide/carbon catalyst material with high selectivity for hydrogenation reactions

Article abstract of DOI:10.1016/j.jcat.2020.06.036

We present a highly selective and active Pd carbon catalyst prepared by an easy hydrothermal synthesis method. This synthetic procedure allows the stabilization under mild conditions of interstitial carbon atoms on the surface of a Pd⁰ carbon catalyst. The so formed Pd carbide phase appears on the upper surface layers of the Pd carbon catalyst, as demonstrated by X-ray photoelectron depth profile analysis using variable synchrotron X-ray energies. The presence of carbon in the palladium carbide species modifies the electronic state of surface Pd atoms, resulting in more electron positive Pd species (Pd^δ+). This influences the adsorption of reactants and reaction intermediates during the hydrogenation of alkynes, dienes and imines, resulting in high selectivities at practically 100percent conversion.

Full text of DOI:10.1016/j.jcat.2020.06.036

Journal Pre-proofs
Synthesis of a hybrid Pd0 / Pd-carbide / carbon catalyst material with high se-
lectivity for hydrogenation reactions
Andrea Garcia-Ortiz, Juan Diego Vidal, Sara Iborra, Maria José Climent,
Jorge Cored, Daniel Ruano, Virginia Pérez-Dieste, Patricia Concepción,
Avelino Corma
PII:
S0021-9517(20)30269-4
https://doi.org/10.1016/j.jcat.2020.06.036
YJCAT 13807
DOI:
Reference:
To appear in:
Journal of Catalysis
Received Date:
Revised Date:
Accepted Date:
2 April 2020
25 June 2020
28 June 2020
Please cite this article as: A. Garcia-Ortiz, J. Diego Vidal, S. Iborra, M. José Climent, J. Cored, D. Ruano, V.
Pérez-Dieste, P. Concepción, A. Corma, Synthesis of a hybrid Pd0 / Pd-carbide / carbon catalyst material with
high selectivity for hydrogenation reactions, Journal of Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.
2020.06.036
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Synthesis of a hybrid Pd0 / Pd-carbide / carbon catalyst material with
high selectivity for hydrogenation reactions
Andrea Garcia-Ortiza, Juan Diego Vidala, Sara Iborraa, Maria José Climenta, Jorge Coreda,
Daniel Ruanoa,b, Virginia Pérez-Diesteb, Patricia Concepcióna,*, Avelino Cormaa,*
a Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de Investigaciones
Científicas (UPV-CSIC), Avenida de los Naranjos s/n, 46022 Valencia, Spain
b ALBA synchrotron light source, Carrer de la Llum 2-26, 08290 Cerdanyola del Vallès, Spain
ABSTRACT
We present a highly selective and active Pd carbon catalyst prepared by an easy hydrothermal
synthesis method. This synthetic procedure allows the stabilization under mild conditions of
interstitial carbon atoms on the surface of a Pd0 carbon catalyst. The so formed Pd carbide phase
appears on the upper surface layers of the Pd carbon catalyst, as demonstrated by X-ray
photoelectron depth profile analysis using variable synchrotron X-ray energies. The presence of
carbon in the palladium carbide species modifies the electronic state of surface Pd atoms,
resulting in more electron positive Pd species (Pd+). This influences the adsorption of reactants
and reaction intermediates during the hydrogenation of alkynes, dienes and imines, resulting in
high selectivities at practically 100% conversion.
Keywords
Palladium carbide, selective hydrogenation, reductive amination, synchrotron XPS,
hydrothermal synthesis
1

1. Introduction
Currently, selective hydrogenation of conjugated double carbon-carbon bonds in dienes and
trivalent carbon-carbon bonds such as alkynes to achieve partial hydrogenation products is a
1-2
highly desirable and challenging process in the field of chemical production.
A variety of
heterogeneous and homogeneous catalysts based on transition metals for the partial
hydrogenation of dienes and alkynes have been reported, being Pd the most widely used. In
fact, it is observed that Pd-based catalysts can selectively hydrogenate triple into double C-C
bonds. On this base, it is possible to remove acetylene from ethylene streams for the
manufacture of polymer grade ethylene. The high selectivity of Pd to hydrogenate mixtures of
alkyne and alkene hydrocarbons has been explained on the base of the weaker adsorption
energy of the alkene with respect to the alkyne on Pd. [3] Recent spectroscopic studies based on
4,5
operando X-ray absorption spectroscopy
combined with X-ray powder diffraction and
theoretical simulation of the X-ray absorption near-edge structure (XANES) spectra using a
6
[7]
Monte-Carlo approach, as well as near ambient X-ray photoelectron spectroscopy, have
shown the tendency of Pd to form carbides and hydrides under reaction conditions, which
markedly affects the adsorption of olefins and alkynes, influencing accordingly the selectivity to
the alkene. It has been experimentally and theoretically found [8] that bulk dissolved hydrogen
in Pd, resulting in the formation of β-hydride phase, is considerably more energetic than the
hydrogen adsorbed on the surface and can emerge from the bulk to the surface, enhancing total
[9]
hydrogenation of acetylene. On the other hand, when no subsurface hydrogen exists, the
hydrogen on the surface of the metallic nanoparticles is more selective toward partial
[10]
hydrogenation of acetylene to ethylene. By studying binding energy maps, Tescher et al.
proposed that the distribution of hydrogen in Pd is affected by the presence of dissolved carbon
species within the metal nanoparticles, decreasing the population of hydrogen in the subsurface
region. In this case the binding energy of hydrogen on the surface is strongly reduced and its
population decreases leading to the selective hydrogenation of the alkyne into the
2

corresponding alkene. [11] These carbon species are formed in situ during the reaction from the
fragmentation of carbon containing feed molecules, being their amount strongly dependent on
the reaction conditions such as reaction time, pressure and temperature. [4-6,11,12] The formation
of metal-C species in the absence of carbon species in the feed have been also detected by
dispersing small metal clusters onto a carbon support, resulting in a high degree of metal-carbon
13
interaction, specifically at defects sites of the support
with the diffusion of carbonaceous
species from the support into the Pd lattice. 13,14 While the incorporation of carbon into the Pd
lattice has been proven to be beneficial for catalyst selectivity, the controlled synthesis of
palladium based catalysts containing interstitial carbon atoms has been scarcely reported. To
the best of our knowledge, there are only few reports dealing with the efficient incorporation
of C atoms in Pd catalysts. Between these studies, Okitsu et al., 15 reported a sonochemical
reduction of a palladium salt in the presence of an organic additive. However, the radical nature
of the synthesis results in important compositional changes of the final material depending on
the synthesis conditions, hampering the scalability of the method. In another approach Guo et
al, 16 described a method for the synthesis of palladium carbide (PdCx) nanocubes, starting from
Pd nanocube seeds, glucose and oleylamine stirred at 200 C for a certain time. The method
requires the initial synthesis of Pd nanocubes using a surfactant mediated method and the use
of amines, which are not desirable from an environmental perspective. However, it has been
shown the possibility to control the C/Pd atomic ratio by simply adjusting the reaction time. On
the other hand, Beltzung et al. 17 reported a KOH activated high temperature (900 C) pyrolysis
procedure of a polyacrylonitrile polymer containing Pd nanoparticles, resulting in partial
carburization of the Pd nanoparticles. The method uses harsh conditions and has drawbacks of
accessibility of the reactants to the active sites. On the basis of these outcomes, the possibility
to employ a sustainable method using mild conditions for the synthesis of Pd catalysts
containing dissolved carbon atoms would be very interesting from a point of view of eco-
efficiency. On the other hand, the effect of carburization on the catalytic performance of Pd
3

catalysts in other reactions than alkyne semihydrogenation has not been yet investigated, being
matter of interest. Thus, the goal of our work is to state an efficient and facile approach of
effective incorporation of carbon atoms in the Pd lattice and study their influence on the
catalytic behaviour of highly demanding selective hydrogenation reactions.
The hydrothermal synthesis methodology has been successfully employed for the
18
synthesis of carbon materials
and mono and bimetallic carbon coated catalysts, with
19-22
applications in many catalytic processes,
however in none of these studies metal
carburization has been reported. Recently, we demonstrated the formation of ruthenium
carbide species stabilized in the upper surface layers of a ruthenium carbon catalyst, using a
23
hydrothermal approach.
Inspired on this finding and based on the high affinity of Pd to
carbon, we tried to use the same approach for the synthesis of Pd carbide containing catalysts.
The herein employed hydrothermal synthesis procedure involves an aqueous solution of a
palladium salt (i.e. Pd(NO3)2), an organic compound as carbon source (i.e Na2EDTA) and a water-
methanol mixture, heat-treated under autogenous pressure at 200 C for 24 h. This synthesis
method is very easy, reproducible and cost-efficient in comparison to the previous methods
found in the literature for the controlled synthesis of palladium based catalysts containing
interstitial carbon atoms. The method can also be applied using glucose instead of Na2EDTA as
carbon source, and water as solvent, rendering as a “green” alternative to the actual synthesis
procedure.
2. Materials and methods
2.1. Synthesis of PdHT catalyst
The PdHT-EDTA catalyst was prepared through a hydrothermal process using Pd(NO3)2 as the
metal precursor and Na2EDTA as organic compound. The pH of the synthesis gel was 13. In detail,
3.50 g Pd(NO3)2, 1.8 g Na2EDTA and 0.38 g NaOH were dissolved in 8.1 mL H2O and 4 mL MeOH,
resulting in a black suspension. The solution was transferred into a 35 mL stainless steel
4

autoclave and heated at 200 C for 24 h. After cooling to room temperature, the solid was
precipitated, filtered and washed with deionized water and acetone several times followed by
air-drying at 25 C.
The PdHT-Glucose catalyst. 120 mg of glucose (Aldrich) was dissolved in 7 mL of deionized water
and stirred at room temperature for 0.5 h. Then 0.1 g of RuO2 (Aldrich, 99.9%, particle size 32
nm determined by XRD) is added and mixed under ultra-sonication (Branson 3510 operating at
40 Hz) for 0.5 h, obtaining a black suspension. The so obtained suspension was transferred into
a Teflon-coated stainless steel autoclave of 12.5 mL and heated at 175 °C for 18 h under static
conditions. After cooling, the content of the autoclave was filtrated under vacuum conditions,
recovering a black solid. The solid was washed five times, first with water and later with acetone.
Then it was dried in an oven at 60 °C for 12 h.
2.2. Characterization methods
Power X-ray diffraction patterns (XRD) were collected using a Panalytical CUBIX diffractometer
with a monochromatic Cu Kα radiation (=0.15406 nm) at 45 kV and 40 mA. The angle (2) was
measured in a scan range of 2.00–90.038 in steps of 0.04018 with a counting time of 34.92 s.
The crystal size was calculated from the main peaks (40.3, 46.9, 68.3, 82.3, 86.8; 2θ) using the
Scherrer equation and assuming a shape factor 0.9. TEM images of the catalyst were taken with
a JEOL JEM 2100F electron microscope operating at 200 kV. Before being transferred into the
TEM chamber, the samples were deposited onto a carbon-coated copper grid and then quickly
moved into the vacuum evaporator. Raman spectra were obtained with a “Reflex” Renishaw
spectrometer, equipped with an Olympus microscope and a CCD detector. Spectra were
acquired using a 514 nm laser. The laser power on the sample was 10 mW, taken 10 acquisitions
for each spectra. Synchrotron X-ray Photoelectron Spectroscopy experiments were performed
at the NAPP end-station of CIRCE BL24 at the ALBA Synchrotron Light Source Facility. The data
were acquired with a Phoibos 150 NAP electron energy analyser, pass energy of 10 eV and
5

beamline exit slit of 20 μm. The beam spot size at the NAPP sample position was 100×30 (HxV)
µm2. Incident photon energies of 500, 700, 950 and 1386 eV for Pd 3d were used, allowing to
probe sample depths between 1.9 nm and 4.7 nm. The probing depth was obtained according
[24]
to the inelastic mean free path (IMFP) data.
The sample (80-100 mg) was pelletized and
mounted onto the sample holder using a resistive button heater for sample heating and a K-type
thermocouple in direct contact with the sample for temperature monitoring. In the in situ H2
reduction experiments, 1 mbar H2 was dosed into the analysis chamber using leak valve, and the
temperature raised from 25 to 250 C. Shirley type background and Lorentzian type curves have
been used in the spectra fitting. The surface area of palladium in Pd catalysts was estimated
from CO adsorption using the double isotherm method on a Quantachrome Autosorb-1C
equipment. Prior to adsorption, the samples, except the PdHT, (300 mg with a pellet size of 0.8-
1.1 mm) were reduced in situ in flowing pure hydrogen (25 mL/min) at the same reduction
temperature applied before catalysis, that is 100 C for 1 h (10 C/min rate). The PdHT sample
was treated in a helium flow at 100 °C (2 h). After activation, the samples were degassed at
1333x10-3 Pa for 2 h at 100 C, and then the temperature lowered at 25 C (1 hour for cooling
down the sample to adsorption temperature). Then, pure CO was admitted and the first
adsorption isotherm (i.e. the total CO uptake) was measured. After evacuation at 25 C, the
second isotherm (i.e. the reversible CO uptake) was taken. The amount of chemisorbed CO was
obtained by subtracting the two isotherms. The pressure range studied was 0.5-11 x 104 Pa. The
dispersion of Pd (D %) was calculated from the amount of irreversibly adsorbed CO assuming a
[25]
stoichiometry of Pd/CO=1 as was used by other authors.
The mean Pd0 diameter (d) was
determined from chemisorption data assuming spherical geometry for the metal particle
according to the procedure described by Anderson. Equations used for metal dispersion and
metal diameter determination are shown below.
Nm* Fs * M *100
D (%) =
*100
L
6

Nm: chemisorption uptake expressed in mol of CO per gram of sample. FS: adsorption
stoichiometry. M: molecular weight of the supported metal. L: percent loading of the supported
metal.
L*6
d =
ASA* Z *100
d: mean metal diameter (m). Z: density of the supported metal (g/m3). ASA: active surface area
(m2/gsample) calculated from the following equation:
ASA=Nm*Fs*Am*NA
Am: cross-sectional area occupied by each active surface atom. NA: Avogadro’s number.
Temperature programmed desorption (TPD) of phenylacetylene and styrene adsorbed on PdHT
and Pd/C samples were done using a quartz reactor connected to a Balzer mass spectrometer
(QMG 220M1). Phenylacetylene and styrene were adsorbed on the sample by wet
impregnation, followed by drying in vacuum at 25 C. For the TPD experiments, the sample was
submitted to a flow of argon and the temperature increased from 25 C up to 200 C at a rate
of 2 C/min.
2.3. Catalytic studies
Phenylacetylene reduction: The reactions were carried out in a schlenk reactor. The catalyst (5
mg) was placed under vacuum at room temperature. Then, a solution of 1 mmol of
phenylacetylene in 10 mL of ethanol was added. The system was purged with H2 and the reaction
was left under 1 bar of H2 using a balloon. The reaction was followed by taking samples at regular
periods, that were analyzed by gas chromatography using a flame ionization detector and a
capillary column (HP5, 30 m x 0.25 mm x 0.25 mm), using dodecane as external standard.
Hydrogenation of 1,5-cyclooctadiene: The hydrogenation of 1,5-cyclooctadiene was performed
in a stainless steel high-pressure reactor. 1,5-Cyclooctadiene (2 mmol), 4 mL of n-octane and
7

catalyst (14 mg) were introduced into the reactor, purged with N2 and H2, pressurized with H2
o
and finally heated at 50 C under stirring (1000 rpm). The reaction was followed by taking
samples at regular periods that were analyzed by gas chromatography using a flame ionization
detector and a capillary column (HP5, 30 m x 0.25 mm x 0.25 mm), using dodecane as external
standard. The identification of the products was performed by using a GC-MS spectrometer.
Reductive Amination of Furfural: The reaction was performed in a stainless steel high-pressure
reactor. Furfural (2 mmol), aniline (2 mmol) in 2 mL of TFT, and the catalyst (20 mg) were
o
introduced into the reactor, purged with N2 and H2, and then heated at 100 C under stirring
(1000 rpm). Finally, the reactor was pressurized at 4 bar of H2 (pressurization was set as t=0).
The products were analyzed on Agilent GC-7980A gas chromatograph equipped with a capillary
column HP-5 (30 m × 0.25 μm × 0.25 mm) and FID detector. The identification of the products
was performed by using a GC-MS spectrometer.
In all cases the initial reaction rates r0 [mol·h-1] were calculated for the corresponding
compounds at conversions below 20%.
3. Results and discussion
3.1. Synthesis of Pd samples
The samples have been prepared under autogenous pressure in a stainless steel autoclave
heated in an oven at 180-200 C for 18-24 h. The synthesis gels contains a palladium salt, an
organic compound and water and/or methanol solution. Details are given in the experimental
section. The as prepared samples are labelled as PdHT-EDTA (when using Na2EDTA as carbon
source) and PdHT-Glucose (when using glucose as carbon source). The metal loading in both
samples is 20 wt % Pd, according to ICP analysis.
3.2. Spectroscopic characterization of the as prepared PdHT sample
8

Independent on the carbon source used in the synthesis, TEM, EDS and XRD of the as prepared
PdHT samples show the presence of Pd0 particles with an average size of 27 nm surrounded by
a carbon matrix (Figure 1, and Figures S1, S2a).
Figure 1. (a) TEM image of PdHT-EDTA sample, (b) Palladium EDS and (c) Carbon EDS analysis of
the PdHT sample.
Raman spectra show two main bands at 1575 and 1371 cm-1 corresponding to graphitic sp2
carbon and disordered type graphitic carbonaceous species respectively, together with a small
component at 1500 cm-1 corresponding to amorphous carbon species (Figure S3). [26] In addition,
very weak bands at around 767 and 685 cm-1 corresponding to PdOx. 27 In contrast to the results
obtained using bulk type characterization tools where only Pd0 and a small amount of PdOx are
detected, high-resolution synchrotron photoelectron spectroscopy working under high surface
sensitivity conditions (i.e. 500 eV x-ray excitation energy) shows the presence of an additional
9

palladium phase beside Pd0 and PdOx, ascribed to palladium carbide (PdC). Indeed, three
components at 335.0, 335.5 and 336.0 eV BE are clearly detected using 500 eV excitation energy
(Figure 2a) contributing 33%, 60% and 7% respectively to the total peak intensity in the PdHT-
EDTA sample. These species correspond to Pd0, palladium carbide (PdC) and PdOx respectively.
[7,28] To verify that PdC species are located on the upper layers of the Pd particle, depth profile
analysis using variable synchrotron X-ray excitation energies was performed and the
corresponding Pd 3d5/2 spectra are displayed in Figure 2. The concentration profile of the PdC
phase among different atomic layers is given in Table 1. This result confirms the effective
incorporation of C atoms into the uppermost surface layers of a bulk Pd metal catalyst prepared
under the hydrothermal synthesis conditions described above. Similar profile of interstitial
carbon is observed in the PdHT-Glucose sample when glucose is used as carbon source (Fig. S4
and Table S1). The effective stabilization of the PdC phase represents an interesting way to
modulate the surface electronic properties of bulk metal catalysts and, accordingly, as will be
shown below, their reactivity.
Pd3d3/2
Pd3d5/2
Pd0
PdC
335.5
PdOx
336.0
335.0
1.9 nm
2.8 nm
3.6 nm
4.7 nm
342
340
338
336
334
B.E (eV)
10

Figure 2. Pd 3d XPS spectra of the as prepared PdHT-EDTA sample acquired under UHV at
variable X-ray excitation energy (h) corresponding to different sample depth: h=500 eV (1.9
nm, black), h=700 eV (2.8 nm, red), h=950 eV (3.6 nm, blue), h=1386 eV (4.7 nm,green).
Table 1 Sampling depth profile of the as prepared PdHT-EDTA catalyst obtained from
synchrotron XPS analysis using variable X-ray energies.
X-ray energy
(eV)
Sample depth
Pd/Ca
PdC (%)/Pd0 (%)/PdOx (%)
(nm)
1.9
500
12.4 / 87.6
23 / 77
60.3 / 33.1 / 6.6
57.5 / 31.6 / 10.8
59.7 /40.3 / 0
700
2.8
950
3.6
32.5 / 67.5
38.8 / 61.2
1386
4.7
43.2 / 56.8 / 0
a) Pd/C: Palladium/Carbon surface composition (mol %) obtained from the area of the Pd
3d and C 1s core levels, corrected by their respective cross section value and the
intensity of the incident X-ray radiation.
3.3. Catalytic performance of the PdHT sample
In order to study the catalytic effect of a PdC phase present in the upper layers of the PdHT
catalyst, we carried out various hydrogenation reactions with different substrates. Given results
are referred to the PdHT-EDTA sample, while similar behaviour has been observed with the
PdHT–Glucose sample (see Table S2). The results are compared with those obtained with a
commercial 5 wt % Pd/C sample where no dissolved carbon species (i.e. PdC phase) are detected
in the Synchrotron XPS spectra acquired at low X-ray excitation energy (700 eV) (Figure S5) and
with literature data.
11

3.3.1. Selective hydrogenation of phenylacetylene
Pd cat,
rt, H2
+
3
2
1
Scheme 1. Hydrogenation of Phenylacetylene
Table 2 Results of hydrogenation of phenylacetylene.a
Time
(min)
15
Conv. 1
(%)
Yield 2 Yield 3 Selec.2
Entry
1b
Catalyst
Pd/C
TOF (h-1) / (s-1)c
1800 / 0.5
(%)
22.7
55
(%)
0.7
20
49
0
(%)
97
23.4
75
45
73
60
99
51
52
15
80
80
100
90
2
PdHT-EDTA
24120 / 6.7
20
100
90
10
[a] 1 mmol phenylacetylene (1), room temperature, balloon H2, 5 mg cat, 10 mL EtOH.
[b] 5% Pd/C from Sigma-Aldrich.
[c] TOF are calculated as initial reaction rate of disappearance of 1 divided by Pd
(µmol/g) obtained by chemisorption of CO.
The hydrogenation of phenylacetylene (Scheme 1) was performed at room temperature in H2 at
atmospheric pressure. The results given in Table 2 and Figures S6 and S7 show much enhanced
activity and selectivity to the alkene with the PdHT-EDTA sample that was hydrothermally
prepared and contains PdC species. Moreover, when it is compared to the literature 29-33 (Table
S3), the PdHT-EDTA catalyst maintains high selectivity (90%) at 100% conversion, with reaction
rates (2·10-4 mol·g-1·s-1) one order superior to most of the catalysts reported in the literature
(10-5 mol·g-1·s-1). Only a recent study reported by Peng et al 30 using an ionic liquid (IL) micro
12

phase and highly dispersed Pd nanoparticles inside a porous MOF shows similar activity than the
PdHT-EDTA catalyst. However, the use of IL has main drawbacks such as difficulties in recycling
and purification, hindering industrial applicability.
Achieving high selectivity and activity is still a challenge, being important for energy efficient
processes. Many strategies have been developed in order to improve catalyst performance
29, 30
31
based on novel and complex synthetic strategies:
formation of single metal alloys,
selective deposition of metal or metal oxides on the surface of the Pd particles, 32 and formation
of intermetallic compounds with isolation of Pd active sites in the crystalline structure. 33 Our
results show high activity and selectivity in the hydrogenation of phenylacethylene ascribed to
the presence of palladium carbide species, where the presence of carbon atoms modifies the
electronic state of surface Pd atoms, resulting in more electropositive Pd species (Pdδ+), as
34
shown by XPS. This influences the adsorption of reactant and/or intermediates
and,
accordingly, the activity and selectivity. Indeed, studying the adsorption-desorption of
phenylacetylene and styrene on the PdHT-EDTA and Pd/C samples (Figure S8) a much higher
adsorption of both reactants is observed on the Pd/C sample. In addition, as previously reported,
11
the presence of carbon species inhibits hydrogen (H) bulk diffusion enhancing accordingly
the selectivity to phenylethylene.
Hydrogenation of phenylacetylene is a standard reaction where the role of dissolved carbon has
been widely accepted. Most of the studies refer to Pd particle sizes <6 nm (see table S3) where
the effect of the particle size in the activity and selectivity to styrene has resulted in controversial
conclusions. 3,35 In a recent work Murzin et al. 36 reported a positive effect on the selectivity to
styrene at increasing the Pd particle size in a Pd/Al2O3 catalyst, which could influence the
catalytic performance of the PdHT-EDTA compared to the Pd/C sample. Thus, in order to
disentangle the effect of the carbide phase from the particle size, other reactions for which
activity and selectivity is disfavoured at increased particle size are selected. This is the case of
13

the selective hydrogenation of 1,5-cyclooctadiene to 1,5-cyclooctene (Scheme 2) 37-39 and the
reductive amination of furfural (Scheme 3) 40,41
.
3.3.2. Selective hydrogenation of cyclooctadiene
The selective hydrogenation of 1,5-cyclooctadiene into cyclooctene is a reaction of interest since
cyclooctene is an important industrial intermediate for manufacturing special polymers used as
42
modifiers in rubbers and thermoplastics.
When using Pd catalysts, the selectivity to
monoalkene considerably decreases when conversion of the dialkene increases. However, with
the PdHT-EDTA catalyst, that contains surface carbon species, it has been found (see results in
Table 3 and Figures S9, S10 and S11) not only a higher intrinsic catalytic activity of Pd (i.e. TOF)
than Pd/C and Pd black for the hydrogenation of 1,5-cyclooctadiene, but a much better
selectivity for the formation of the mono-olefins.
Pd cat,
+
50ºC, H2
6
5
4
Scheme 2. Hydrogenation of 1,5-cyclooctadiene
Table 3 Hydrogenation of 1,5-cyclooctanediene with different Pd catalysts.a
Time Conv. 4 Yield 5 Yield 6 Selec. 5
Entry
Catalyst
TOF (h-1) / (s-1)c
12960 / 3.6
6120 / 1.7
(min)
20
(%)
80
(%)
80
(%)
-
(%)
100
100
98
PdHT-
EDTA
1
45
100
80
100
78
-
6
2
2b
Pd/C
10
100
76
24
76
14

120
180
85
77
78
8
90
78
3
Pd black
485 / 0.13
100
22
[a] 2 mmol 1,5-cylooctadiene (1), 4mL n-octane, 14 mg cat, 10 bar H2.
[b] 5% Pd/C from Sigma-Aldrich.
[c] TOF are calculated as initial reaction rate of disappearance of 4 divided by Pd
(µmol/g) obtained by chemisorption of CO.
Remarkably, 100% selectivity to cyclooctene at 100% conversion is achieved on the PdHT-EDTA
sample. These values are higher to those of the literature 37,43,44 (Table S4) when comparing Pd
based catalysts, showing also a higher intrinsic activity of the Pd surface site. Some reports have
shown high selectivity, using gold based catalyst 45 in the hydrogenation of dienes, while their
activity is lower than Pd based catalysts.
3.3.3. Reductive amination of furfural with aniline
In the case of the reductive amination of furfural with aniline, the aniline acts as a nucleophile
that attacks the carbonyl group of the HMF forming imine 7, which is subsequently
hydrogenated to the corresponding secondary amine. Notice that the production of secondary
furfurylamines by reductive amination of furanic aldehydes with amines and hydrogen as
reducing agent in presence of metal catalysts is not usually a selective process due to the
46-48
competitive hydrogenation of the furanic ring.
In this reaction, the formation of the
intermediate imine (7) (Scheme 3) is a very fast process, being the imine hydrogenation the
controlling step of the process.
O
O
NH2
N
O
HN
HN
O
O
Pd cat
100ºC, H2
+
+
H
7
9
8
Scheme 3. Reductive amination of furfural with aniline
15

Table 4 Reductive amination of furfural with aniline.a
Time
(min)
10
Conv. 7
(%)
Yield 8
(%)
Yield 9
(%)
Selec. 8
(%)
Entry
1
Catalyst
TOF (h-1) / (s-1)c
5061 / 1.4
53
53
0
100
PdHT-EDTA
30
30
100
26
99
26
57
87
1
0
99
100
100
87
2b
Pd/C
45
57
0
533 / 0.14
180
100
13
[a] 2 mmol furfural, 2 mmol aniline , 20 mg cat, 2 mL TFT, 100 °C, 4 bar H2.
[b] 5% Pd/C from Sigma-Aldrich
[c] TOF are calculated as initial reaction rate of disappearance of 7 divided by Pd (µmol/g)
obtained by chemisorption of CO.
As it can be observed in Table 4 the TOF for the hydrogenation of the imine 7 using PdHT-EDTA
catalyst is ten times higher than for Pd/C while the selectivity to furfurylamine 8 using the PdHT-
EDTA catalyst is 100% (Figures S12 and S13). These values are superior to those of the literature
25,40,48 (Table S5). In a recent work 40 we reported 100% selectivity at 100% conversion on a 1
wt % Pd/C catalyst prepared by wet impregnation of Pd(acac)2 on carbon (Norit ROW 0.8mm).
In that case, deposition of carbon species onto Pd(111) facets have been determined
experimentally, being responsible of the 100 % selectivity. However the intrinsic activity of that
Pd catalyst is markedly lower (TOF of 0.4 s-1) than the PdHT-EDTA catalyst (TOF of 1.4 s-1) while
100 % selectivity is maintained.
The positive effect of the PdC surface layer in the PdHT-EDTA catalyst has been further
demonstrated using the ex situ H2 reduced PdHT-EDTA (Figure S11 and S14). H2 reduction results
16

[49,50]
in the destabilization of the Pd carbide and its transformation into a Pd-Hydride phase
keeping the particle size unaltered (see Fig. S2b). Indeed, we confirmed by synchrotron XPS
studies (see detailed description of the procedure and results in the supplementary part and
Figure S15) that the PdHT-EDTA is transformed in a less selective catalyst when the PdC phase
disappears. In the case of 1,5-cyclooctadiene the selectivity to cyclooctene was decreased from
100% to 80% at 100% conversion after ex situ reduction at 150 oC (Figure S10). Similarly, in the
case of the reductive amination of furfural, after the ex situ reduction of the PdHT-EDTA catalyst,
the selectivity to the secondary amine 8 decreased from 100% to 90% at 100% conversion
(Figure S14). It is interesting to point out that in both examples, the selectivity achieved with the
ex situ reduced PdHT-EDTA was similar to those obtained with the commercial catalyst.
Interestingly, while PdC species in the PdHT-EDTA catalyst are unstable in the presence of H2
during ex situ sample reduction, the catalyst remains quite stable when working in liquid phase
under H2 atmosphere. Indeed, the PdHT-EDTA catalyst is fully reusable maintaining both activity
and selectivity, at least after three catalytic cycles (see Figure 3). In opposite, an 87% activity
drop is observed with the commercial Pd/C sample already after the second reuse (see Figure
4). The high stability of the PdHT-EDTA sample is remarkable since deactivation is an important
drawback in the selective hydrogenation of alkynes and dienes, leading to a short life of the
catalyst. 50
17

100
80
60
40
20
0
1st Use
2nd Use
3rd Use
Conversion (%)
Selectivity (%)
Figure 3. Results of 1,5-cyclooctadiene reduction using PdHT-EDTA catalyst. Reaction conditions:
1,5-cyclooctadiene (2.08 mmol), n-octanol (4 mL), 14 mg PdHT, 50 °C, 5 bar H2, 1 h reaction time.
100
80
60
40
20
0
1st Use
2nd Use
2nd Use
(Soxhlet)
Conversion (%)
Selectivity (%)
Figure 4. Results of 1,5-cyclooctadiene reduction using 5% Pd/C catalyst. Reaction conditions:
1,5-cycloctadiene (2.08 mmol), n-octanol (4 mL), 14 mg 5% Pd/C, 50 °C, 5 bar H2, 1 h reaction
time.
In this type of reactions, deactivation is usually caused by the deposition of oligomers and
polymeric material on the catalyst surface as a result of the condensation of unsaturated
compounds. In fact, thermogravimetric analysis (TG) shows strong adsorption of organic species
18

(15%) on the used Pd/C catalysts. Thus, a stronger adsorption can be inferred on the
commercial Pd/C sample, exhibiting Pd0 surface sites. This is in line with the higher adsorption
of reactants and intermediate species observed previously in the temperature programmed
desorption experiments of Fig. S8 on Pd/C sample.
4. Conclusions
We present a highly selective and active hydrogenation Pd catalyst composed by a palladium
carbide phase stabilized on the upper surface layers of a Pd carbon catalyst. The catalyst
synthesis method comprises a hydrothermal process at 200 C, starting from a palladium
precursor (i.e. Pd(NO3)2), an organic compound (i.e. Na2EDTA or glucose) and a water-methanol
solution. The easy and mild conditions of this synthetic procedure contrast with those reported
in the literature, where scalability and efficiency are main drawbacks. XPS depth profile analysis
using synchrotron radiation at variable X-ray excitation energy shows an enrichment of PdC on
the upper layers of the catalyst, independently of the organic precursor used in the
hydrothermal synthesis. In detail, at a sample thickness of 1.9 nm 56-60 % Pd carbide is
observed, and around 66-43 at % a depth of 4.7 nm. The presence of surface Pd carbide modifies
the electronic properties of the Pd resulting in Pdδ+ species. This has a strong positive effect on
activity and selectivity for a variety of demanding selective hydrogenation reactions as, for
instance, the hydrogenation of phenylacetylene, hydrogenation of 1,5-cyclooctadiene and the
reductive amination of furfural with aniline. In all cases, selectivities close to 100% at almost full
conversion are achieved with higher intrinsic activity per surface Pd atom to that reported in the
literature. The positive effect of the PdC species on the catalytic performance of the PdHT
catalyst has been demonstrated by ex situ reduction of the sample, that results in the
destabilization of the PdC phase with formation of Pd-hydride and Pd0, and the corresponding
19

decrease in selectivity. PdHT catalysts, in contrary to Pd/C, are quite stable under reaction
conditions, and no loss of activity or selectivity has been found after several catalyst reuses.
Appendix A. Supplementary material
Figure S1. Images TEM of PdHT-EDTA at different zoom
Figure S2a. XRD of PdHT-EDTA sample
Figure S2b. XRD of the ex situ reduced PdHT-EDTA sample
Figure S3. Raman spectra of the as prepared PdHT-EDTA sample
Figure S4. Pd 3d XPS spectra of the as prepared PdHT-Glucose sample acquired under UHV at
variable X-ray excitation energy (h) corresponding to different sample depth
Figure S5. Pd 3d XPS spectra of the commercial 5% Pd/C sample (Sigma Aldrich) acquired at 700
eV X-ray excitation energy
Figure S6. Kinetic curve of reduction reaction of phenylacetylene with PdHT-EDTA
Figure S7. Kinetic curve of reduction reaction of phenylacetylene with commercial 5% Pd/C
Figure S8. Temperature desorption of phenylacetylene and styrene adsorbed on PdHT-EDTA and
commercial 5% Pd/C samples
Figure S9. Kinetic curve of reduction reaction of 1,5-cyclooctadiene with PdHT-EDTA
Figure S10. Kinetic curve of reduction reaction of 1,5-cyclooctadiene with commercial 5% Pd/C
Figure S11. Kinetic curve of reduction reaction of 1,5-cyclooctadiene with ex situ reduced PdHT-
EDTA
Figure S12. Kinetic curve of reductive amination of furfural with aniline with PdHT-EDTA
20

Figure S13. Kinetic curve of reductive amination of furfural with aniline with commercial 5% Pd/C
Figure S14. Kinetic curve of amination reduction reaction of furfural and aniline with ex situ
reduced PdHT-EDTA
Figure S15. Pd 3d XPS spectra of the as prepared PdHT-EDTA sample acquired at 500 eV X-ray
excitation energy and in the presence of 1 mbar H2 at 175 C and 250 C
Table S1. Sampling depth profile of the as prepared PdHT-Glucose catalyst obtained from
synchrotron XPS analysis using variable X-ray energies
Table S2. Catalytic activity of PdHT-Glucose sample in the a) reductive amination of furfural with
aniline, b) selective hydrogenation of cyclooctadiene
Table S3. Hydrogenation of phenylacetylene (PhA) to styrene using Pd based catalysts
Table S4. Hydrogenation of cyclooctadiene (COD) to cyclooctene (COE) on Pd based
heterogeneous catalysts
Table S5. Reductive amination of furfural with aniline on Pd based catalysts
Acknowledgments
Funding: The research leading to these results has received funding from the Spanish Ministry
of Science, Innovation and Universities through “Severo Ochoa” Excellence Programme (SEV-
2016-0683) and the PGC2018-097277-B-100 project. The authors also thank the Microscopy
Service of UPV for kind help on measurements. A. García-Ortiz thanks “Severo Ochoa”
Programme (SEV-2016-0683) for a predoctoral fellowship. J. Cored thanks the Spanish
Government (MINECO) for a “Severo Ochoa” grant (BES-2015-075748). D. R. thanks European
Research Council project SYNCATMATCH (671093). The NAP-XPS experiments were performed
at the NAPP branch of the CIRCE beamline at the ALBA Synchrotron with the collaboration of
21

ALBA staff. Author contributions: A.C. conceived the project and contributed in the production
of the manuscript. S.I, M.J.C and P.C. directed the study and wrote the manuscript. P.C. did the
Raman and participated together with D.R., V.P.D and J.C. in the XPS measurements at ALBA
Synchrotron. A.G.O. did the synthesis of the samples, the TEM analysis and the catalytic study.
All authors participated in the discussion of the results. Competing interest: The authors declare
no competing financial interest. Data and materials availability: all data are presented in the
paper and/or supplementary materials.
AUTHOR INFORMATION
Corresponding authors
*pconcepc@upvnet.upv.es
*acorma@itq.upv.es
ORCID
Patricia Concepción 0000-0003-2058-3103
Avelino Corma 0000-0002-2232-3527
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
29