Promotional effect of palladium in Co-SiO2 core@shell nanocatalysts for selective liquid phase hydrogenation of chloronitroarenes

Article abstract of DOI:10.1016/j.jcat.2020.03.006

This study describes the synthesis of palladium-promoted Co@SiO₂ catalyst developed by electrostatic immobilization of Pd ionic precursor onto Co₃O₄ nanoparticles core, coated with a mesoporous SiO₂ shell. The oxidized Pd-Co₃O₄@SiO₂ (xPdCo-ox) and partially reduced Pd-Co@SiO₂ (xPdCo-red) nanocatalysts were used in the direct synthesis of chloro-arylamines from chloronitroarenes employing heterogeneous hydrogenation process. The effect of palladium content (xPdCo; x = 0.0, 0.5, 1.0 and 3.0 Pd wt%) in the Co₃O₄ core of the structures on catalytic performance for the hydrogenation of 4-chloronitrobenzene (CNB) to 4-chloroaniline (CAN) was systematically studied. It was found that the incorporation of palladium ionic precursor promotes both Co₃O₄ core nanoparticles flocculation and an increase in the mesoporous shell thickness in the Pd-promoted catalysts compared to the pristine Co-SiO₂ core-shell structure. The TPR, XRD, XPS and magnetic measurements results indicated that the palladium addition promoted the reduction of Co₃O₄ core during the isothermal H₂ treatment at 873 K rendering metallic Pd° and Co° species. The catalytic CNB hydrogenation experiments showed that the 0.5PdCo-red catalyst inhibited both the hydrodechlorination and intermediates accumulation reaching 99% yield in CAN compared to 1.0PdCo-red and 3.0PdCo-red catalysts which provided aniline as undesired product. Additionally, the 0.5PdCo-red catalyst was easily recycled with a moderate catalytic activity after five consecutive reaction cycles. Finally, the catalytic hydrogenation performance of the 0.5PdCo-red catalyst for different pharmaceutical substituted chloro-nitroarenes such as 1-(4-chlorophenoxy)-2-nitrobenzene, 2-chloro-1-((3-fluorobenzyl)oxy)-4-nitrobenzene and 2-chloro-5nitrobenzotrifluoride was also evaluated and revealed high activity (>99% at 3 h of reaction) and selectivity towards the desired chloro-arylmines production, highlighting the potential of this catalyst in these processes.

Full text of DOI:10.1016/j.jcat.2020.03.006

Journal of Catalysis 385 (2020) 224–237
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Promotional effect of palladium in Co-SiO core@shell nanocatalysts for
2
selective liquid phase hydrogenation of chloronitroarenes
a
a,
⇑
b
c
a,d,
⇑
Tatiana M. Bustamante , Cristian H. Campos , Marco A. Fraga , J.L.G. Fierro , Gina Pecchi
a
Universidad de Concepción, Departamento de Físico-Química, Facultad de Ciencias Químicas, Concepción, Chile
Instituto Nacional de Tecnologia/MCTIC, Rio de Janeiro, Brazil
Instituto de Catálisis y Petroleoquímica (ICP-CSIC), Grupo de Energía y Química Sostenible (EQS), c/Marie Curie s/n Cantoblanco, Madrid, Spain
Millenium Nuclei on Catalytic Processes towards Sustainable Chemistry (CSC), Chile
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
This study describes the synthesis of palladium-promoted Co@SiO2 catalyst developed by electrostatic
immobilization of Pd ionic precursor onto Co nanoparticles core, coated with a mesoporous SiO shell.
The oxidized Pd-Co @SiO (xPdCo-ox) and partially reduced Pd-Co@SiO (xPdCo-red) nanocatalysts
were used in the direct synthesis of chloro-arylamines from chloronitroarenes employing heterogeneous
hydrogenation process. The effect of palladium content (xPdCo; x = 0.0, 0.5, 1.0 and 3.0 Pd wt%) in the
Received 3 December 2019
Revised 10 February 2020
Accepted 5 March 2020
3
O
4
2
O
3 4
2
2
Co
CNB) to 4-chloroaniline (CAN) was systematically studied. It was found that the incorporation of palla-
dium ionic precursor promotes both Co core nanoparticles flocculation and an increase in the meso-
porous shell thickness in the Pd-promoted catalysts compared to the pristine Co-SiO core-shell
structure. The TPR, XRD, XPS and magnetic measurements results indicated that the palladium addition
promoted the reduction of Co core during the isothermal H treatment at 873 K rendering metallic Pd°
3 4
O core of the structures on catalytic performance for the hydrogenation of 4-chloronitrobenzene
Keywords:
Core@shell
Cobalt
Palladium
Hydrogenation
Chloronitroarenes
(
3 4
O
2
3
O
4
2
and Co° species. The catalytic CNB hydrogenation experiments showed that the 0.5PdCo-red catalyst
inhibited both the hydrodechlorination and intermediates accumulation reaching 99% yield in CAN com-
pared to 1.0PdCo-red and 3.0PdCo-red catalysts which provided aniline as undesired product.
Additionally, the 0.5PdCo-red catalyst was easily recycled with a moderate catalytic activity after five
consecutive reaction cycles. Finally, the catalytic hydrogenation performance of the 0.5PdCo-red catalyst
for different pharmaceutical substituted chloro-nitroarenes such as 1-(4-chlorophenoxy)-2-nitroben
zene, 2-chloro-1-((3-fluorobenzyl)oxy)-4-nitrobenzene and 2-chloro-5nitrobenzotrifluoride was also
evaluated and revealed high activity (>99% at 3 h of reaction) and selectivity towards the desired
chloro-arylmines production, highlighting the potential of this catalyst in these processes.
Ó 2020 Elsevier Inc. All rights reserved.
1
. Introduction
Functionalized arylamines have been used for long time as the
(treatment and prevention of cardiovascular diseases) [6,7], Sorafe-
nib (antineoplastic for kidney cancer treatment) [8], chloropro-
caine (anesthetic effect) [9] and others. The main synthetic
strategy reported is the catalytic reduction of ANO group by
heterogeneous hydrogenation employing H as reducing agent or
transfer hydrogenation using hydrogen donors like hydrazine, for-
mic acid, or NaBH [1,2,10]. Furthermore, compared with transfer
starting point of a wide range of building blocks products with dif-
ferent applications and they are largely synthetized by the selec-
tive reduction of their nitro-derivate precursors [1,2]. Among
them, halo-nitroarenes are taken as raw materials in the produc-
tion of halo-arylamines used in production of commercial drugs
such as Linezolid (antibiotic) [3], Loxapine (antiphsycotic) [4], Lap-
atinib (antineoplastic for breast cancer treatment) [5], Lixivaptan
2
2
4
hydrogenation, heterogeneous hydrogenation is preferred because
of their ease of isolation and recyclability.
The reported literature on hydrogenation of halo-nitroarenes
has focused on the use of chloro-nitroarenes as test molecules
2
operated at elevated H pressure in liquid phase, where heteroge-
neous supported noble metal catalysts are the main alternative
used in this process [2,10]. Despite the use of Au [11–13], Pt [14–
⇑
Corresponding authors at: Universidad de Concepción, Departamento de
Físico-Química, Facultad de Ciencias Químicas, Concepción, Chile (C.H. Campos);
Millenium Nuclei on Catalytic Processes towards Sustainable Chemistry (CSC), Chile
1
8], Ir [19,20], Pd [21–24] and Rh [25–27] as usual active phases
(
G. Pecchi).
E-mail addresses: ccampos@udec.cl (C.H. Campos), gpecchi@udec.cl (G. Pecchi).
for this reaction, all efforts have been focused on the production
https://doi.org/10.1016/j.jcat.2020.03.006
021-9517/Ó 2020 Elsevier Inc. All rights reserved.
0

T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
225
of catalysts based on non-noble metals due to the scarce natural
2. Experimental
abundance of noble metals and ensuing high cost [28,29].
Among non-noble metals, cobalt (Co) has been reported as low
cost benign alternative. Indeed metallic Co can act as a hydrogen
acceptor owing to the electron-deficient state of its surface, which
2.1. Materials
Co(NO ) ꢀ6H O, absolute ethanol, NH (25%) solution, tetraethy-
3
2
2
3
promotes both cleavage of H
attack to the absorbed ANO
2
molecule bonds and the nucleophilic
group by the chemisorbed hydrogen
lorthosilicate (TEOS) and cetyltrimethylammonium bromide
(CTAB) were purchased from Merck. Polyvinylpyrrolidone (PVP),
4-chloronitrobenzene (CNB) and 2-chloro-5-nitrobenzotrifluoride
were used as received from Sigma-Aldrich. The details of the mate-
rials and characterization of synthetized chloro-nitroarenes 1-(4-
chlorophenoxy)-2-nitrobenzene and 2-chloro-1-((3-fluorobenzyl)
oxy)-4-nitrobenzene are shown in the Supplementary Material.
Unless otherwise stated, all solvents and chemicals used were of
commercially available analytical grade and used without further
treatment. All gases (H , N and 5%O /N ) used for catalyst prepa-
2
on the metallic Co active phase [28,30].
A critical factor to improve the catalytic performance in the
liquid-phase hydrogenation of chloro-nitroarenes using metallic
Co as active phase is the ability to control the surface chemistry
of these materials to increase the activity, selectivity and opera-
tional stability. To this purpose, some recent reports in literature
are focused on generating Co-based core@shell nanocatalysts.
The metal based core@shell nanomaterials are defined as a single
catalytic unit formed from multifunctional components with syn-
ergistic properties among the components of the core, commonly
metallic and/or metal oxide, and the shell which could be organic
and/or inorganic nature [31]. Recently, Liu et al. has demonstrated
that the use of carbon as shell in CoAC core@shell nanocatalysts
improved the catalyst stability, avoiding the metallic Co oxidation
by CoAC interactions. It enhanced the chemoselective production
2
2
2
2
ration and reaction process were ultrahigh purity (>99%).
2.2. Catalysts synthesis
2
The synthesis of Pd-promoted Co-mSiO core@shell nanocata-
lysts were carried out following four sequential steps: the Co O
3 4
core synthesis by solvothermal treatment, deposition of Pd precur-
sor by the ion-exchange process, the shell synthesis employing a
Stöber method and calcination-reduction of the system to produce
the designed catalysts [40,41].
2
of substituted anilines during the hydrogenation of ANO group,
in the presence of other reducible organics moieties, which was
attributed to the presence of metallic Co covered by a carbon layer
as protecting agent [29]. Moreover, Chen et al. produced Co-based
core@shell catalysts using CoNC as core and N-doped porous car-
bon nanotubes as shell for the liquid phase hydrogenation of a
wide range of nitroarene molecules. They reported high yields over
those systems and a low production of undesired by-products were
detected [32]. Zhang et al. reported similar CoNCAC core@shell
catalyst, with carbonaceous mesoporous shell, which exhibited
high catalytic activity and chemoselectivity for hydrogenation of
halo-nitrobenzenes under mild reaction condition (2.0 mol% Co,
Firstly, Co
3
O
4
core was synthetized dissolving Co(NO
3
)
2
ꢀ6H
2
O
(
(
0.35 g) in absolute ethanol (40 mL) in presence of PVP
0.7080 g) as capping agent and the mixture was transferred to
a teflon-lined autoclave (80 mL) for treatment at 453 K for 4 h.
Then, Pd sorption was carried out following the next general pro-
cedure: the pH of 40 mL of Co
adjusted to 10.0 in a conical polypropylene tube, then the
required amount of Pd(NO was added and mechanically shaken
for 24 h at room temperature [42]. Next, the Pd doped Co -NPs
3 4
O core dispersion (0.01 M) was
3 2
)
3 4
O
1
.0 MPa H
as stabilizer-shell, the work reported by Kondeboina et al is the
first study in the application of Co-SiO core@shell catalysts using
mesoporous silica-shell (mSiO ) for the hydrogenation of nitroare-
nes [34]. The use of Co-mSiO catalysts to achieve high selectivity
2
, 353 K, <150 min) [33]. Despite of the use of carbon
were coated with TEOS using a Stöber method, followed by a
dried process overnight at 333 K and calcination at 773 K for
2
5
h
to produce PdO-Co
41,43]. The final step of the catalysts synthesis was a reduction
treatment defined by previous TPR experiments to obtain the
Pd-Co/CoO -mSiO . The oxidized PdO -Co -mSiO catalysts pre-
cursor was named as xPdCo-ox and the reduced in hydrogen at
3 4 2
O -mSiO core-shell nanostructures
2
[
2
in the conversion of halonitroarenes to produce halo-arylanilines
in liquid phase still represents a challenge as it comes to avoiding
the metallic Co oxidation.
x
2
x
O
3 4
2
8
0
73 K as xPdCo-red where x: 0.0, 0.5, 1.0 or 3.0 correspond to
.0 non-promoted catalysts (used as a control sample) and
On other Co-based core@shell catalytic applications, the Co-
core has been modified by the low addition of a noble metal as
dopant, in quantities lower than 5.0 wt% in the catalyst composi-
tion. Introduction of noble metals aims at preserving the Co metal-
lic nature rendering improved catalytic systems [35–38]. Among
them, palladium (Pd) arises as an excellent metal dopant mainly
due to its hydrogenation capacity, low cost and much higher abun-
dance than other noble-metals as Rh, Ir or Pt [35,39].
x = 0.5, 1.0 and 3.0 are the nominal Pd wt% as shown in Table 1.
For the characterization measurements, xPdCo-red catalysts were
stabilized by a passivation process with 5% O
bath at 203 K for 1 h [44].
2 2
/N in a cryostatic
2.3. Characterization
The study described herein reports the synthesis of Pd-
promoted Co-mSiO
immobilization of Pd ionic precursor onto Co
core, coated with mesoporous SiO shell. The oxidized
Pd-Co -SiO and partially reduced Pd-Co-SiO nanocatalysts
2
nanocatalyst developed by electrostatic
The Pd and Co atomic metal loading were determined by atomic
absorption spectrometry (AAS) using a Perkin Elmer instrument
model 3100. Samples were solubilized in a mixture of HCl and
3 4
O
nanoparticles
a
2
3
O
4
2
2
3
HNO acids and then appropriately diluted to get concentrations
were used in the direct synthesis of chloro-arylamines from
chloroniotroarenes employing heterogeneous hydrogenation pro-
cess. The effect of the palladium content (xPd-Co; x = 0.0, 0.5, 1.0
and 3.0 Pd wt%) in the structure and performance of the
catalyst was systematically studied. The hydrogenation of
within the calibration range of the instrument.
TPR was performed using a TPR/TPD 2900 Micromeritics system
equipped with a thermal conductivity detector (TCD). Two reduc-
tion profiles were recorded following our previously reported
methodology [45]. In the first run TPR, data were recorded for
4
-chloronitrobenzene (CNB) to 4-chloroaniline (CAN) used as test
2
0.050 g samples of xPdCo-ox using a 5 vol% H /Ar flow of
ꢁ1
reaction. The durability of the most active and selective Pd-
promoted catalyst was evaluated on consecutive reaction cycles.
This catalyst behavior was also investigated using other chloro-
nitroarenes with interest in the synthesis of pharmaceutical inter-
mediaries as Loxapine, Lapatinib, Sorenafib.
40 mL min and heating from room temperature to 1173 K at a
rate of 10°/min (see below). In the second TPR, 0.050 g samples
of calcined xPdCo-ox were first thermally reduced in the TPR/
TPD 2900 Micromeritics instrument using the same experimental
conditions as those employed for the preparation of xPdCo-red

2
26
T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
Table 1
Chemical composition (AAS), hydrogen consumption (H
2
-TPR), specific area (SBET), average pore size (dpore) and particle sizes (dTEM), coercivity (H
c
) and saturation magnetization
(M
s
) for the as-synthesized core@shell structures.
-TPR (mmol gꢁ1)
Reducibility degree (%)
AAS
S
BET (m2
g
ꢁ1
)
dpore (nm)
d
TEM (nm)
M
(emu gCo
ꢁ1
)
H
Samples
H
2
s
c
(Oe)
%
Co*
% Pd*
BJH
DFT
core
shell
0.0PdCo-ox
0.5PdCo-ox
1.0PdCo-ox
3.0PdCo-ox
0.0PdCo-red
0.5PdCo-red
1.0PdCo-red
3.0PdCo-red
3.5
3.7
4.1
4.9
0.22
0.09
0.05
0.04
–
–
–
–
93.6
97.6
98.8
99.1
17.0(20)
19.9(20)
19.8(20)
15.6(20)
19.2(20)
19.3(20)
20.6(20)
14.9(20)
—
758
789
763
743
764
736
760
736
2.9
3.8
2.9
3.7
2.2
2.0
2.1
2.0
2.7
2.8
2.6
2.7
2.7
2.7
2.6
2.7
21
23
39
46
16
14
32
35
16
17
32
38
18
19
31
36
–
–
–
–
–
–
–
–
0.4 (0.5)
1.1 (1.0)
2.6 (3.0)
—
0.4 (0.5)
1.1 (1.0)
2.4 (3.0)
111.9
85.4
65.9
258.8
380.4
264.1
316.8
115.8
*
Nominal values in parenthesis.
(
pure H
at 10°min ), then cooled until room temperature and heated at
73 K in a flow of Ar for 2 h, and subjected to TPR measure-
2
at 40 mL minꢁ1, heating from room temperature to 873 K
2.4. Catalytic activity
ꢁ1
3
The catalytic activity evaluation in the hydrogenation of chloro-
nitroarenes was performed in a ParrÒ Instruments Model 4561
autoclave at experimental conditions of catalyst weight and agita-
tion speed to ensure absence of any transport limitations, evalu-
ated by the Weisz-Prater parameter [44]. The reaction was
ments under the same conditions as the first TPR experiment.
The second TPR profiles are added in the supplementary mate-
rial. Thus, the first TPR data represented the complete reduction
of the Co species in the precursor materials, and the second TPR
data represented Co reduction in the prepared xPdCo-red cata-
lysts. The reducibility degree was calculated from the TPR data
as follows:
2
performed at 40 bar of H using 10 mg of catalyst and chloro-nitr
oarenes/(Co + Pd) mole ratio equal to 100 in a volume of 100 mL
of absolute ethanol and stirring was settled up to 750 rpm. To
ensure the correct catalytic evaluation, the AAS Co and Pd content
was employed to calculate the substrate quantity.
Total normalizated area of second TPR
reducibility degree ð%Þ ¼
ꢀ 100
ð1Þ
The reaction was monitored by taking non-invasive samples
Total normalizated area of first TPR
(
50-100uL) periodically during the reaction mixture at different
reaction times. The analyses were carried out using GC-FID instru-
ment (Hewlett Packard HP 4890D) and GC–MS (Perkin Elmer
Clarus 680 gas chromatograph coupled to a GCMS-SQ8T mass
spectrometer) both of them were equipped with a capillary column
HP-5. The total chloro-nitroarenes conversion and products selec-
tivity were calculated as:
In this expression, the peak areas were normalized by the total
experimental Co content determined using AAS characterization.
The specific area was calculated using the BET method from the
nitrogen adsorption isotherms obtained on a Micromeritics appa-
ratus Model TriStarII 3020 at 77 K. Pore-size distributions were
estimated using both the BJH and DFT methods, respectively.
The TEM Micrographs were obtained on NPs dispersed on amor-
phous carbon coated copper grids on a JEOL model JEM-1200 EX II
microscope. The High-resolution transmission electron microscopy
½
chloro ꢁ nitroareneꢂ ꢁ ½chloro ꢁ nitroareneꢂ
i
t
xchloroꢁnitroareneð%Þ ¼
ꢀ 100
ð2Þ
½
chloro ꢁ nitroareneꢂ
i
(
HRTEM) micrographs were collected on a JEOL-2010 F instrument
which is equipped with STEM accessory.
X-ray powder diffraction (XRD) patterns were obtained with
a1
nickel-filtered Cu K radiation (k = 1.5418 Å) using a Rigaku
½productꢂ
t
S
productð%Þ ¼
ꢀ 100
t
½chloro ꢁ nitroareneꢂ ꢁ ½chloro ꢁ nitroareneꢂ
i
ð3Þ
were
diffractometer and collected in the 2h range of 20-80°. Three types
of X-ray diffraction analysis were performed. The first for the
xPdCo-ox precursor catalysts was a conventional diffraction pat-
tern between 20 and 80°. A second analysis of XRD as a function
of the reduction temperature for 0.0PdCo-ox. Finally the diffraction
patterns of the xPdCo-red reduced catalysts to 873 K with a 5 h of
isotherm subsequently passivated to which a standard of crys-
talline silicon was added to calibrate the signals and to determine
possible shifts associated with the formation of alloys.
The pseudo first-order rate constants (kglobal
)
calculated by fitting the first-order rate constant equation for
ln(1-xchloro-nitroarene) values against t (min) with a linear fit, where,
chloro-nitroarene is the conversion a given time. The correlation
coefficients (R ) values for this linear fits, varied from 0.97 to
.99. The kglobal values were obtained with an accuracy of ±5%.
The experimental kglobal values are the average values for triplicate
experiments. The effect of temperature on the kglobal values were
studied by varying the temperature from 333 to 393 K.
x
2
0
The XPS measurements were performed using a VG Escalab
The turnover frequency (TOF) of the catalysts was calculated as:
2
00R electron spectrometer equipped with a hemispherical elec-
tron analyzer and Mg K (1253.6 eV) X-ray source. Prior to analysis,
the samples were degassed at 573 K for 1 h inside the pre-
treatment chamber of the spectrometer and reduced in situ at
a
ꢀ
ꢁ
ꢁ
1
mol of substrate hydrogenated
surface mol of metals ðCo þ PdÞ ꢀ timeðhÞ
TOF
h
¼
ð4Þ
8
73 K.
Magnetic characterization of the reduced and passivated
And the details of the method and calculations are included in
the supplementary material.
xPdCo-red catalysts were carried out using a Quantum Design
Dynacool Physical Properties Measurement System (PPMS),
equipped with a Vibrating Sample Magnetometer (VSM). Magneti-
zation curves were recorded at 298 K. Data are expressed in emu
per gram of cobalt. M was evaluated by extrapolating to infinite
S
field the experimental results obtained in the high field range
where the magnetization linearly increases with 1/H.
The operational stability of the catalyst with the best catalytic
performance was assessed by evaluating the catalytic performance
after consecutive reaction cycles. The catalyst was recovered by
magnetization after each reaction run, washed twice with absolute
ethanol to remove organic species that could be adsorbed on the
catalyst surface, dried in oven at 333 K and re-reduced in hydrogen
at 873 K for the next reaction cycle.

T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
227
3
. Results an discussion
2
The total H consumption estimated with a confidence level of
9
4% by deconvolution of the peaks is shown in Table 1. The larger
The TPR profiles of the xPdCo-ox pre-catalysts are shown in
Fig. 1. It can be seen for the 0.0Pd-Co-ox system two well defined
reduction peaks at ~523 K and 803 K attributed to the reduction of
3 4
Co O to CoO followed to the reduction to metallic cobalt [41,46]
confirmed by the XRD as a function of the reduction temperature
for 0.0PdCo-ox (Fig. S1a) were it is detected disappearance of
hydrogen consumption for the Pd promoted catalysts is attributed
to the easier reduction of oxidized cobalt due to the presence of the
noble metal. Therefore, to ensure the largest reduction of oxidized
cobalt species and increase the interaction between cobalt and pal-
ladium species during the reduction processes, the reduction of the
2
reduced xPdCo-red structures will be carried out using H flow of
ꢁ1
ꢁ1
Co
3
O
4
and appearance of CoO at 573 K. The addition effect of Pd
20 mL min at a rate of 5 degree min from ambient temperature
up to 873 K for 5 h. These results were corroborated by measure a
second TPR for the xPdCo-red catalysts (Fig. S2). Indeed the profiles
are characterized by a peak at around 353 K the intensity of which
decreases with the increase of Pd content. By comparing TPR of
xPdCo-ox with TPR of xPdCo-red, the observed shift of the reduc-
in the reduction of cobalt oxides is clearly seen in the TPR profile
of Fig. 1, with the shift towards lower temperature of the two
cobalt reduction peaks for 0.5PdCo-ox, 1.0PdCo-ox and 3.0PdCo-
ox structures. This behavior indicates a synergetic effect of Pd in
the reduction of cobalt oxide species being more significant as pal-
ladium Pd content increases. The easier reduction of Co
3
O
4
in pres-
tion peak to lower temperature and the H
confirms that the reduction treatment at 873 K nearly reduces the
surface of Co core.
The similarity of the experimental and nominal atomic wt% of
Co and Pd shown in Table 1 indicates non-metal loss during syn-
thesis and confirm the chemical composition of the xPdCo-ox
pre-catalysts and partially reduced xPdCo-red catalysts.
2
-consumption measured
ence of palladium can be explained by a nucleation/growth
autocatalytic model, considering that once a surface metallic nuclei
center is formed, it facilitates the reduction process of other oxi-
dized species by hydrogen spillover [47]. The nature of Pd ionic
species produced by calcination corresponds to Pd2 as detected
by XPS characterization (see below). We thus suggest that the
reduction processes seen in Fig. 1 proceed from the easier reduc-
tion of PdO to metallic palladium, which then promotes the activa-
tion and dissociation of hydrogen, enhancing the reducibility
cobalt species [35].
3 4
O
+
2
The adsorption-desorption N isotherms at 77 K for the xPdCo-
red catalysts are shown in Fig. 2, and the corresponding to the
xPdCo-ox in the supplementary material (Fig. S3). It can be seen
that all materials showed isotherms to type IVa (according with
the IUPAC classification [50]) indicative of mesoporous with a
H1-type abrupt hysteresis loop associated with a uniform and
cylindrical regular-form shape mesoporous [51–53]. The surface
area values (SBET) showed in Table 1 demonstrates that the reduc-
tion treatment do not change the mesoporous-shell nature. To a
better understand, the pore size distribution calculated by BJH
and DFT methods are included in the supplementary material
(Fig. S4). All the materials showed a narrow pore size distribution
with a maximum at 2.1 nm for BJH and 2.7 nm by DFT calculations.
These results do not represent significant textural differences
between the calcinated and reduced structures with a mean pore
diameter shown in Table 1 that support the availability of meso-
porosity in the xPdCo-red structures.
The expected decomposition of the Pd-hydride phase as a neg-
ative signal at 333 K [48] is only seen for the highest Pd content
(
(
3 wt%) (Fig. 1). Therefore the absence for the lower Pd content
0.5 and 1.0 wt%) can be explained that for nanosize of PdO it
occurs at lower temperature than the started reduction tempera-
ture [49]. The broad reduction peak at ~1023 K for the non-
doped 0.0PdCo-ox structure (Fig. 1) is attributed to the reduction
of surface CoO
results reported by Xie et al for Co
x
-SiO
2
interfacial species, in agreement with similar
@mSiO nanocomposites
3
O
4
2
employed as catalysts for Fischer–Tropsch synthesis [41].
To confirm the presence of a core@shell structure, it was
carried out sequences of TEM micrographs of the started oxidized
xPdCo-ox and xPdCo-red structures are shown in Fig. 3. The parti-
cle size was determined as the length of the segment joining the
two ends of the particle and passing through the mass center
Fig. 2. N
2
adsorption–desorption isotherms at 77 K of xPdCo-red catalysts (x = 0,
Fig. 1. TPR profiles of xPdCo-ox; x = 0, 0.5, 1.0, 3.0 precursors.
0.5, 1.0, 3.0).

2
28
T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
Fig. 3. TEM images (a) 0.0PdCo-ox (left), 0.0PdCo-red (right) and HR-TEM zoom (down); (b) 0.5PdCo-ox (left), 0.5PdCo-red (right) and HR-TEM zoom (down); (c) 1.0PdCo-ox
left), 1.0PdCo-red (right) and HR-TEM zoom (down); (d) 3.0PdCo-ox (left), 3.0PdCo-red (right) and HR-TEM zoom (down).
(
and the distribution of both xPdCo-ox and xPdCo-red structures
are resumed in Table 1. The well encapsulation of Co and Pd-
doped Co NPs used as cores with TEOS followed by a calcination
procedure at 773 K shown in Fig. 3a–d (right) indicates a success-
fully coating with silica depressing the grain growth of the Co
and PdO-Co nanocrystals [43] maintained after the calcination
procedure. It can be seen well defined core-shell xPdCo-ox struc-
tures with differences in morphology and size increase with the
increase of Pd loading. The homogeneous core size distribution in
Fig. 3a–b) corresponds to 0.0PdCo-ox and 0.5PdCo-ox precursor
but high shell thickness and irregular cores size for 1.0PdCo-ox
and 3.0PdCo-ox materials were detected in Fig. 3c–d). The similar
was carried out by electrostatic sorption and the dispersions were
adjusted until pH = 9.0 adding aqueous NH solution to provide
both negative surface charge on the Co -core (Co ZPC ~ 8.0
3
O
4
3
3
O
4
3
O
4
3 4
O
2
+
[54]) and the formation of Pd[NH
3
]
ion complex [55]. The
ꢁ
3
O
4
increase in Pd loading increased the incorporation of NO
3
from
ꢁ
2+
3
O
4
the metal precursor (mole ratio NO
concentration into dispersion media could provide flocculation
whereby Co NPs are held together leading heterogeneous core
3
/Pd = 2). The rise of nitrate
3 4
O
size bigger than pristine Co
3
O
4
-NPs. Moreover, the presence of
thicknes as shown in
ꢁ
NO
3
anion also affects the mesoporous SiO
2
Table 1. This behavior is in agreement with studies reported by
Kim et al in relation with the size’s increase of SiO micro- and
2
trend was identified in the particle size distribution of the Co
3
O
4
-
nano-particles in presence of electrolyte additives [56]. On this
way, the increase in the shell thickness and core-morphology con-
firms that the incorporation of electrolytes upper than 0.5 wt%
core dispersions as shown in (Fig. S5a–d) in the supplementary
material. The deposition of Pd2 species on the Co
+
3 4
O NPs surface

T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
229
with respect to the Co
3
O
4
-NPs suspension promotes the produc-
main effect after the reduction treatment at 873 K is the disappear-
ance of the CoO crystalline phases and appearance of metallic Co
for the 0.0PdCo-red and metallic Pd for the 3.0PdCo-red with a sys-
tematic increase of metallic Co with the increases of the Pd con-
tent. The detected crystalline phases and mean crystallite size
tion of higher and hetero-dispersed core@shell systems in com-
pares with the low-doped or non-doped Pd-Co core@shell
materials.
In Fig. 3a–d (left) the xPdCo-red structures formed after the
reduction process at 873 K show defined core@shell structures
with a smaller core and almost no changes in the thickness of
the silica compared to xPdCo-ox counterparts (Fig. 3a–d right).
The decrease in the core’s size could be associated to the reduction
dhkl calculated by the Debye-Scherrer equation, using the diffrac-
tion line at 2h 36.8° for Co , 33.9° for PdO, 42.5° and CoO are
3 4
O
listed in Table S1. The increase in the size of metallic Pd, no
changes in the size of CoO upon palladium content and the absence
of diffraction peaks attributed to a metallic palladium cobalt alloy
reported at 2h of 42.2 and 49.1° [57], which is in line with the HR-
TEM characterization.
The magnetic hysteresis curves for xPdCo-red catalysts are
depicted in Fig. 5 and the corresponding magnetic properties (co-
c s
ercivity H and saturation magnetization M ) are also listed in
of both PdO
x 3 4
and Co O oxidized species. The diameter decrease is
in line with the unit change in the crystal cell dimensions decrease
3
3
from Co
in ICSD patterns of Co
62-2435).
In addition, the magnified HR-TEM image in Fig. 3 a-d (zoom
3
O
4
(volume 135.42 Å ) to Co (10.91 Å ) as can be observed
3
O
4
cubic (98-017-3831) and Co cubic (98-
0
down) shows that metallic Co has a spherical shape in all the
xPdCo-red systems. Also, these catalysts showed a visible set of lat-
tice fringes of 0.204 nm, characteristics of the (1 1 1) plane of
metallic Co. The xPdCo-red catalysts with x > 0 revealed the fringes
with a d-spacing of 0.223 nm correspond to the (1 1 1) planes of
the metallic Pd. Furthermore, compositional line profiles are
included in the supplementary material (Fig. S5e–h). Co and Pd
across the catalysts show similar peaks and valleys evidencing
the formation of the homogeneous distribution of Pd onto the
Table 1.The observed saturation magnetization values for xPdCo-
red is between 65.9 and 115.8 emu/g, lower than that of the bulk
particles (163 emu gCo ) [58]. This behavior can be attributed to
both the non-magnetic silica coating and the small particle size
effect. It is known that the saturation magnetization value of
nanoparticles is smaller than that of corresponding bulk materials
ꢁ1
c
[59]. On the other hand, the coercivity (H ) of the structures dis-
plays a ferromagnetic behavior as it can be seen in Fig. 5 (inset)
and Table 1. The obtained values in Table 1, higher than the
reported for bulk metallic cobalt, are in line with those reported
elsewhere for nanometric cobalt particles [60]. Magnetic proper-
ties are known to be very sensitive to parameters such as the size,
surface states and compositions and we are currently investigating
the factors governing magnetism in our xPdCo-red catalyst.
The Co 2p3/2 and Pd 3d5/2 XPS spectra for the xPdCo-red are dis-
played in Fig. 6 while those for the xPdCo-ox counterparts are
shown in Fig. S6. Table 2 collects all Pd 3d5/2 and Co 2p3/2 binding
energies (BE) as well as their atomic surface ratio.
x
Co-CoO core surface.
The XRD patterns of the xPdCo-ox core@shell structures can be
found in the Supporting Information (Fig. S1b). At the bottom and
at the top of the figures in vertical lines were added the ICSD pat-
terns of Co
281). The non-doped palladium (0.0 PdO) solid show sharps
3 4
diffraction peaks indicative of presence only of crystalline Co O
3 4
O cubic (98-017-3831) and PdO tetragonal (98-002-
9
phase, whereas upon addition of palladium appearance of PdO
diffraction peaks in the bimetallic systems cannot be detected.
The absence of PdO diffractions could be attributed to the forma-
tion of highly dispersed species with crystal sizes lower than
The spectra of the Si 2p3/2 and O 1s indicates BE of 103.4 eV for
4
+
2ꢁ
Si and 532.8 eV for O, corresponding to surface Si and O species
associated to the SiO shell. With regard to the xPdCo-ox struc-
tures, the single doublet peaks at 780.0–795.7 indicate surface
5
.0 nm.
The XRD patterns of the xPdCo-red core@shell structures subse-
2
+
2,+3
2+
quently passivated and with addition of silicon crystalline standard
were showed in Fig. 4a) At the bottom and at the top of the figures
in vertical lines were added the ICSD patterns of Co cubic (98-062-
Co
3 4
associated to Co O and Pd corresponding to the single
doublet at 336.1–341.7 eV [12,14]. In the xPdCo-red (Fig. 6),
changes in both Co 2p3/2 and Pd 3d5/2 core-levels are clearly seen.
As for Co 2p3/2, the doublet can be deconvoluted into two compo-
nents, indicative of two cobalt surface species maintaining the
2
435), Si cubic (98-002-9288) and Pd cubic (98-006-4920). The
2
+
satellite peak attributed to oxidized (Co ) cobalt species. The
appearance of the new doublet at 778–792 ± 0.1 eV, lower BE than
+
2,+3
that attributed to Co
(780.0–795.7 eV) indicates a surface
reduction process to metallic Co°. Fortunately, the differences in
Fig. 4. XRD patterns with a Si crystalline standard of the (a) xPdCo-red; x = 0, 0.5,
.0, 3.0 reduced catalysts up to 600 °C for 5 h of isothermally treatment and
subsequently passivated, b) zoom for palladium and cobalt.
1
Fig. 5. Magnetic hysteresis loops for xPdCo-red; x = 0, 0.5, 1.0, 3.0 catalysts.

2
30
T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
Fig. 6. XPS spectra of Co 2p3/2 for (a) 0.0PdCo-red; (b) 0.5PdCo-red; (c) 1.0PdCo-red; (d) 3.0 PdCo-red and Pd 3d5/2 for (e) 1.0PdCo-red and (f) 3.0PdCo-red.
Table 2
Core level BE (eV) of Pd 3d5/2 and Co 2p3/2 and atomic surface ratio of xPdCo-ox precursors and xPdCo-red catalysts.
BE (eV)
Co/Si
Pd/Co*
2
+
+2,+3
Pd° Pd
Co° Co
0.0PdCo-ox
0.5PdCo-ox
1.0PdCo-ox
3.0PdCo-ox
0.0PdCo-red
0.5PdCo-red
1.0PdCo-red
3.0PdCo-red
— —
— 780.2 (1 0 0)
— 780.2 (1 0 0)
— 780.1 (1 0 0)
— 780.2 (1 0 0)
778.1 (58) 780.2 (42)
778.0 (68) 780.0 (32)
777.9 (70) 780.1 (30)
777.9 (75) 780.2 (25)
0.030
0.026
0.027
0.025
0.025
0.025
0.023
0.020
0
— traces
— 336.4
— 336.5
— —
traces —
335.0 —
335.0 —
– (0.003)
0.009 (0.006)
0.020 (0.018)
0
– (0.003)
0.010 (0.006)
0.021 (0.018)
—
: Not detected.
Nominal in parenthesis.
*
the BE of Co+2,+3 and Co° peaks allow differentiating the surface
contributions of the oxidized and reduced cobalt species as shown
in Table 2. As concerning palladium, the single doublet shifted
towards lower BE, indicative of total reduction of surface PdO into
confirms the proposed ionic-interaction during the Pd deposition
on the Co NPs surface. The entire palladium precursor was
deposited on the core surface and during the calcination treatment
at 773 K the doped systems do not form a PdO-Co solid solu-
3 4
O
3 4
O
metallic Pd. Due to remaining CoO
detected by TPR characterization, Co should be expected as the
dominant specie in the reduced bimetallic hetero-aggregates on
x
-SiO
2
interfacial species [41] as
tion, avoiding the formation of Pd-Co alloys during the reduction
process [36].
x+
+
2,+3
xPdCo-red systems. The largest amount of Co
surface species
3.1. Catalytic performance
(
42%) was found on the non-doped palladium (0.0) and the mono-
tonic decrease by gradually adding Pd supports the beneficial effect
of Pd in the reduction of cobalt oxides species (Table 2). In line with
XRD results, no shift in the BE of the surface metallic Co° 2p3/2 and
Pd° 3d5/2 core-levels allows to discard the formation of PdCo alloy
species. Therefore, the xPdCo-red with x = 0.5, 1.0 and 3.0 struc-
tures can be represented as a mixture of metallic Pd°, Co° and ionic
cobalt species. The surface Co/Si shown in Table 2 is noticeable
lower than the nominal (0.291), an expected result for a cobalt core
The experimental results of the catalytic hydrogenation of CNB
as test molecule on xPdCo-red catalysts are shown in Fig. 7. It can
be seen that CNB could be effectively hydrogenated by the pro-
posed catalysts and the variation of CNB conversion with the reac-
tion time fitted with a pseudo-first order kinetic model for the ln
(1-xCNB) vs time (Fig. 7 inset). The global pseudo-first order con-
stant and the kinetic parameters are shown in Table 3. In all exper-
iments, the hydrogen transfer from the ethanol was discarded as
no conversion was achieved in the absence of hydrogen as shown
in Table S2. The only reaction products detected were CAN, N-(4-
chlorophenyl)-hydroxylamine (CHY) and aniline (AN) in all the cat-
alytic experiments.
2
and SiO shell structure. The Pd/Co ratio somewhat larger than the
nominal one indicates a surface enrichment by palladium in the
core particles, which is in agreement with the formation of
hetero-aggregate on the core surface. On the same way, this result

T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
231
0
anamine, and/or CAC coupling reaction to produce 4-4 -
diaminobiphenyl were not detected. According to our results, only
two possible reaction pathways can be proposed (see Scheme 1).
The first one corresponds to the hydrogenation pathway that takes
place mediated by a nucleophilic attack to the ANO
2
group
produced by the chemisorbed hydrogen in the metallic active
phase, producing a consecutive formation of the nitroso-4-
chlorobenzene and N-(4-chloro-phenyl)-hydroxylamine interme-
diaries to give CAN as final product. The second one is the HDC
pathway that corresponds to the CACl hydrogenolysis by the
hydrogen electrophilic attack on the CACl bond providing AN as
final product. Despite the presence of CNB in the reaction media,
the scheme does not includes the HDC reaction on CNB to produce
2
nitrobenzene as by-product because the presence of ANO group
would have a net electron-withdrawing effect on the electron den-
sity and the aromatic ring would be deactivated towards an elec-
trophilic substitution as reported by Aramendia et al studies on
the catalytic hydrogenolysis of substituted chlorobenzene deri-
vates [61]. It is interesting to notice that all the Pd promoted cata-
lysts provided both an increase in velocity for the hydrogenation of
CNB and a decrease in the CAN yield with the sequence of 3.0PdCo-
red ꢃ 1.0PdCo-red > 0.5PdCo-red at low conversion levels. As the
Pd content increased, three effects were observed: (i) the increase
in the TOF value, (ii) the disappearance of the CHY intermediate
and (iii) the appearance of the hydrodehalogenation (HDC) product
AN, which was significant for the 3.0PdCo-red and 1.0PdCo-red cat-
alysts (see Table 3). Although the formation of AN was attributed
to the presence of metallic Pd as promoter, the reaction pathway
for HDC side reaction was examined in depth employing both
CNB and CAN as starting molecules.
Fig. 7. Total CNB conversion; (inset) pseudo first order adjustment for xPdCo-red;
x = 0, 0.5, 1.0, 3.0 catalysts.
The effect of adding Pd in the catalysts is revealed by the
increase in the TOFs values with the increase of Pd content. Indeed,
ꢁ1
the 0.0PdCo-red catalyst showed the lowest activity (TOF = 438 h
)
with CAN selectivity of 98% while 0.50PdCo-red system exhibited
In order to envisage the sequence in which these processes take
place, we also evaluated the catalytic activity of both xPdCo-ox and
xPdCo-red catalysts in the hydrogenation of both CNB and CAN as
shown in Figs. S7 and S8. In the CNB hydrogenation (Fig. S7a), the
0.0PdCo-ox catalyst showed a negligible activity (~5.0% of conver-
sion level at 6 h of reaction). All xPdCo-ox catalytic systems with
x > 0.0 showed to be active in the hydrogenation reaction and
the increase of Pd loading increased the velocity in the order
3.0PdCo-ox ꢃ 1.0PdCo-ox > 0.5PdCo-ox. These experiments sug-
ꢁ1
1
00% selectivity to CAN with higher activity (TOF = 593 h ). The
selectivity to CAN was enhanced to >99% either by incorporation
of Pd as a second metal along with Co until the 0.5 wt% as optimal
value. However, despite of the increase in the TOF values for the
1
.0PdCo-red and 3.0PdCo-red catalysts, the increase in the Pd load-
ing enhanced the production of AN as a byproduct.
Fig. 8 illustrates the selectivity to CAN and AN as a function of
the reaction time using the xPdCo-red catalytic systems. Over
0
.0Pdco-red catalyst a continuous production of CAN and a negligi-
gest a non-significant reduction of CoO core and the total Pd-
x
ble acumulation of CHY as a by-product were detected. However,
the Pd-promoted catalysts showed a different products distribu-
tion at which both CAN and AN products were detected. The
heterogeneous hydrogenation of chloro-nitroarenes using Pd as
active phase exhibits a complex kinetic behavior because the sub-
strate can follow complex reaction pathways and/or parallel reac-
tion as hydrodechlorination (HDC), arene hydrogenation and/or
diaryl coupling reaction (DAC) of halo-aren-amines formed, which
reduces the catalyst selectivity [61]. The AN production as a by-
product is a consequence of the hydrogenolysis reaction of the
CACl bond (HDC) mediated by the presence of Pd. In our
case, the arene moiety hydrogenation to produce 1-chloro-4-
nitrocyclohexane, 4-chlorocyclohexan-1-amine or cyclohex-
dopant reduction at the hydrogenation reaction conditions. The
2+
use of H to produce Pd-nanoparticles from reduction of Pd pre-
2
cursor has been reported even at room temperatures, which is in
line with the reaction conditions employed in our hydrogenation
experiments [62–64]. The same catalytic tests were carried out in
absence of H (using N inert gas at the same H pressure) with
2
2
2
0.5PdCo-ox, 1.0PdCo-ox and 3.0PdCo-ox catalysts and no reaction
was observed (Table S2). This behavior is in line with the conver-
sion profile showed for the xPdCo-ox with x > 0 because all sys-
tems showed an induction time previous to the production of
CNB to produce the CAN and AN as main reaction products
(Fig. S7a). However, the selectivity patterns showed a different
trend in comparison with the catalytic performance of xPdCo-red
Table 3
Time to reach 100% of conversion (t), global pseudo first order constant (kglobal), initial reaction rate (v
o
), turnover frequency (TOF) and selectivity towards CAN and AN xPdCo-red
catalysts. (H
2
40 bar, 10 mg of catalyst, substrate/catalyst (mol) ratio of 100, 50 mL of ethanol, 750 rpm and 373 K).
global, (minꢁ1·gꢁca1t
)
v
(molL ·min
ꢁ1
ꢁ1
)
TOFa (hꢁ1
)
Selectivity (%)b
CAN
Catalyst
t (min)
k
o
AN
CHY
0.0PdCo-red
0.5PdCo-red
1.0PdCo-red
3.0PdCo-red
180
120
90
0.352
0.424
0.666
2.146
0.009
0.014
0.025
0.074
438
593
1125
2566
96.4
98.8
95.3
89.4
–
–
4.7
10.6
3.1
1.2
–
60
–
a
Calculation at 40% conversion level.
Selectivity at 50% of conversion level.
b

2
32
T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
Fig. 8. Time-resolved selectivity to CAN(▲), AN(j) and CHY (d) in the hydrogenation of CNB over (a) 0.0PdCo-red, (b) 0.5PdCo-red, (c) 1.0PdCo-red and d)3.0PdCo-red
catalysts.
Scheme 1. Proposed reaction routes in the hydrogenation of CNB.
catalytic systems in the hydrogenation of CNB (Fig. S7b–d). The
xPdCo-ox catalyst showed a higher production of AN (Table S2)
than the xPdCo-red (Table 3) catalysts at the same conversion
level. This behavior is in agreement with the proposed reaction
pathway where the metallic Pd promotes the HDC of CAN to pro-
duce AN.
The main difference between xPdCo-ox and xPdCo-red catalysts
is the AN accumulation where xPdCo-ox catalysts showed a
decrease in the CAN production and an increase in the AN yield.
To support this discussion, the hydrogenation of CAN was carried
out over xPdCo-ox and xPdCo-red. The conversion curves are
shown in Fig. S8 and the global pseudo-first order constant and
the kinetic parameters are shown in Table S3. The results corrobo-
rate the understanding that the presence of both metallic Pd and
Co, at low Pd content, avoids the HDC of CAN. The use of an excess
of Pd as dopant under these reactions conditions enhances the HDC
reaction pathway producing AN as undesirable by-product.
Based on the above catalytic results and physicochemical char-
acterizations, we propose a reaction mechanism for the chemose-
lective hydrogenation of CNB on xPdCo-red catalysts. As shown
in Scheme 2, the increase of the Pd content on the surface of the
catalysts promoted the covering of Pd on the core surface. Accord-

T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
233
Scheme 2. Reaction mechanism for the chemoselective hydrogenation of CNB on xPdCo-red catalysts.
ing with the DFT calculation from the N
2
isotherms, the pore size
x 2
than that PdCo/CoO -TiO reported catalyst (entry 8). Nevertheless,
distribution reached 2.7 nm for all the tested catalysts. The
increase of the Pd loading increased the Pd mean particles size
reaching values upper than 5.0 nm for 3.0PdCo-red catalysts as
shown in the XRD characterization (see Fig. 4). In the case of both
the activity of the PdCo-red catalyst in spite of the presence of Pd
was much lower than which included a significant contribution of
the noble metals (entries 1, 2, 4, 7, and 10).
A comparative study, in which the temperature effect was eval-
uated in the catalytic hydrogenation of both CNB and CAN using
0.5PdCo-red as model catalyst, is shown in Fig. 9. The logarithm
of the kinetic rate pseudo-constants versus the reciprocal temper-
ature was plotted to obtain the Arrhenius curve and calculate the
apparent activation energies (Eaa). The increase in the conversion
level from 37% to 100% and the selectivity towards CAN from
79% to 97% by increasing the temperature from 333 K to 393 K
(Table 5) indicates that above 373 K the accumulation of the CHY
intermediate is not favored. The Eaa for the hydrogenation of CNB
0
.0PdCo-red and 0.50PdCo-red catalysts, CNB will be adsorbed
onto metallic Co preferentially through the ANO groups, which
will be reduced by the active H species formed on Co and/or Pd
surface. We suggest that the adsorption of CACl bond on the
.0PdCo-red and 0.50PdCo-red sample could be relatively weak.
However, in the case of 1.0PdCo-red and 3.0PdCo-red catalysts,
both ACl and ANO groups can have direct access to the surface
of Pd nanocrystals. As a consequence, both NO and ACl groups
could be hydrogenated by H , resulting in lower selectivity. This
2
0
0
0
2
0
2
2
ꢁ1
ꢁ1
behavior was confirmed by the use of xPdCo-ox catalysts. When
xPdCo-ox catalysts is used as catalytic systems for the CNB hydro-
genation, only the Pd0 hetero-aggregates are disposal as active
sites, at the reaction conditions, which are responsible for the
increase in the production of AN as main reaction product.
As can be seen from the above literature examples, the selectiv-
ity of non-noble metals based catalysts and chloroanilines was
enhanced by the incorporation of a noble metal [11,17,65,66].
is 27.8 kJ mol , lower than the 74.9 kJ mol calculated for the
hydrogenation of CAN at temperatures range. Despite the decrease
in the accumulation of CHY at temperatures above 353 K, an oppo-
site effect was observed in the CAN hydrogenation which is agree-
ment with the Eaa values for the 0.5PdCo-red catalyst. The HCD of
CAN to produce AN is favored at temperatures higher than 373 K,
which limited the use of this catalysts at higher temperatures.
Due to the improvement in the conversion level and highest
yields in the production of CAN detected by using 0.5PdCo-red cat-
alyst at 373 K, this system was selected to evaluate both opera-
tional stability and versatile application in the hydrogenation of
other chloro-nitroarenes used in the pharmaceutical industry. As
regarding recycles, the catalyst was recovered by magnetization
after each batch reaction and its catalytic performance, textural
properties and Pd loading were evaluated.
Here, we report the CAN selectivity enhancement over Co@SiO
2
core@shell catalyst in hydrogenation CNB by incorporating Pd to
form hetero-aggregates Pd-Co catalytic system. An evaluation of
the activity of various bimetallic non-noble/noble metal-based cat-
alysts reported in the literature with the 0.5PdCo-red system is
exhibited in Table 4. Catalysts containing Pd, Pt and Rh as second
metal in non-noble based catalysts (Co, Ni or Cu) were active for
the hydrogenation of CNB using H
2
as reducing agent. However,
The operational stability of the 0.50PdCo-red catalyst was eval-
uated through the recycling tests. A second cycle was measured
employing the used catalyst as shown in Fig. S9 in the supplemen-
tary material. The catalytic activity after the first reaction cycle
all the formulation considered a higher metal mole ratio in com-
pares with our catalytic system. Interestingly, our catalyst formu-
lation resulted in an enhancement in activity by almost 3 times

2
34
T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
Table 4
Comparison of various bimetallic catalysts in the reduction of CNB under different reaction conditions.
Metal mole ratioa
0.597
T (K)
393
Pressure (bar)
15
CAN selectivity (%)
> 99
TOF (h
2172
ꢁ1
)
Ref.
Entry
1
Catalyst
Metal Loading (wt%)
1.0 (Cu)
PtCu/CNT
[17]
1
.0 (Pt)
0.04 (Cu)
.30 (Pt)
10 (Ni)
.3 (Pt)
0.30 (Ni)
.57 (Pd)
0.24 (Ni)
.36 (Pd)
0.23 (Ni)
.36 (Pd)
0.04 (Ni)
.30 (Pt)
38 (Co)
.1 (Pd)
0.50 (Co)
.40 (Rh)
0.04 (Co)
.30 (Pt)
19 (Co)
.4 (Pd)
2
3
4
5
6
7
8
9
PtCu/TiO
2
0.043
10.0
303
373
383
303
303
303
353
318
303
373
1.0
10
97.4
100
94.6
68.5
82.7
97.0
100
100
96.6
100
1584
58
[65]
0
PtNi/TiO
2
[18]
0
PdNi-B/C
PdNi@SiO
0.291
0.368
0.366
0.040
210
30
8316a
1.2b
1.8b
2016
204b
812
[24]
0
2
1.0
1.0
1.0
20
[66]
0
Pd-NiO@SiO
PtNi/TiO
Pd-Co/CoO
Rh-Co/Al
PtCo/TiO
0.5PdCo-red
2
[66]
0
2
[65]
0
x
-TiO
2
[45]
0
2
O
3
0.729
0.040
26.3
10
[27]
0
1
1
0
1
2
1.0
40
1872
593
[65]
0
This work
0
a
Metal mole ratio = non-noble metal/noble metal.
The TOF was calculated by the data provided in the reference based on the consumption of CNB, conversion level and time to reach this value.
b
showed an important decrease reaching a conversion level 52.75
and 95.9% CAN selectivity at 360 min of reaction, respectively.
We attributed this deactivation to the oxidation/hydration of the
metallic Co on the core surface during the reaction advance in a
similar manner as was reported by our research group through
used catalyst to a regeneration treatment. The recycles were car-
ried out five times under the same reaction conditions. The conver-
sion of CNB and CAN yield after each run is shown in Fig. 10. It can
be seen only a slight decrease from the first to the second reaction
cycle, and then the conversion level remains unchanged up to the
fourth cycle. This behavior could be an evidence of leaching of
some active species or textural changes after the first use of the
x 2
the hydrogenation of CNB using PdCo/CoO -TiO catalysts at simi-
lar reactions conditions [45]. For this reason we have subjected the
Fig. 9. Apparent activation energies (Eaa) calculated for hydrogenation of (j) CNB
and (d) CAN over 0.5PdCo-red catalyst.
Fig. 10. Conversion and selectivity towards CAN on CNB hydrogenation for reused
0.5PdCo-red catalyst.
Table 5
Conversion and kinetic parameters for the hydrogenation of CNB on 0.5PdCo-red at different reaction temperature. (H
00, 50 mL of ethanol, 750 rpm, 3 h of reaction).
2
40 bar, 10 mg of catalyst, substrate/catalyst mole ratio of
Selectivity (%)
1
Temperature (K)
Conversion(%)
k
aap (min 1·gcat
ꢁ1
ꢁ
)
o
v , (molL ·min )
ꢁ1
ꢁ1
CAN
CHY
AN
3
3
3
3
33
53
73
93
37
56
100
100
0.108
0.194
0.424
0.464
0.003
0.006
0.014
0.015
79
96
97
98
16
2
0
4
1
3
3
0

T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
235
Table 6
Fresh and post-reaction characterization for 0.5PdCo-red catalyst.
BET (m2
g
ꢁ1
)
dpore (nm)
AAS
S
%
Co*
% Pd*
BJH
DFT
Fresh
After 5th cycle
19.3 (20)
19.2 (20)
0.4 (0.5)
0.4 (0.5)
636
316
3.6
6.8
2.7 ——
2.7 27
*
Nominal in parenthesis.
Table 7
Fine chemical applications of catalytic performance in nitroarenes hydrogenations on 0.5PdCo-red catalyst. (H
2
40 bar, 10 mg of catalyst, substrate/catalyst (mol) ratio of 100,
5
0 mL of ethanol, 750 rpm and 373 K).
Entry
1
Substrate
t (h)
7
X (%)
Selectivity (%)
96
>99
2
3
5
1
97
97
90
>99
0
.5PdCo-red core@shell structure. In order to evaluate this hypoth-
ization results showed the effective formation of such core@shell
2
+
esis, some characterization post-reaction after the 5th run were
carried out and the results are shown in Table 6. The characteriza-
tion results obtained for the fresh sample is also provided for the
sake of a straightforward comparison. It can be seen that Co and
Pd content remains unchanged after 5 runs whereas a dramatic
decrease in the surface area (SBET) and an increase in the pore
diameter indicate a collapse of the core@shell structure. It is
indeed evidenced by N
in Fig. S10. These results can be ascribed to the production of H
as a second product during the hydrogenation of ANO group.
According to the hydrogenation stoichiometry, water is produced
at mole ratio H O/CAN = 2; the presence of this quantity of H
at 373 K could provide the collapse of mesoporous SiO shell struc-
ture by hydrothermal reaction as was reported by [67,68]. Finally,
the conversion decrease at the last cycle could be attributed to the
sintering of 0.5PdCo-red catalyst, which limits the accessibility of
substrate molecules to the metallic sites on the core surface.
To test the general scope of the 0.5PdCo-red catalysts in the
hydrogenation of other nitroarenes of interest in pharmaceutical
industry [69,70], hydrogenation reactions of three substituted
nitroarenes were also tested, namely 1-(4-chlorophenoxy)-2-nitro
benzene (entry 1), 2-chloro-1-((3-fluorobenzyl)oxy)-4-nitroben
zene (entry 2) and 2-chloro-5nitrobenzotrifluoride (entry 3). The
results as well as the structural formula of different nitroarenes
are summarized in Table 7. 0.5PdCo-red catalyst was found to be
highly active in the hydrogenation reactions with almost total
selectivity to the respective anilines at different times of reaction.
The lower selectivity of the entry 2 molecule (90%) is attributed to
an C-O hydrogenolysis reaction catalyzed by metal palladium
nanoparticles at hydrogenation conditions [71].
type configuration using Pd doped Co
factant assisted Stöber’s methodology to coat SiO
3
O
4
-NPs dispersion and sur-
shell on the core
2
surface. The Pd incorporation up to 0.5 wt% promoted the forma-
tion of irregular core-shell materials attributed to the increase of
electrolytes concentration during the Stöber process. After
calcination-reduction processes the core@shell architecture was
preserved. The reduction treatment mainly provided metallic Pd
and metallic Co species. As a consequence of the presence of
metallic Co in core of these systems, magnetically recoverable
catalysts were obtained. The catalytic hydrogenation of
4-chloronitrobenzene to 4-chloroaniline as a reference reaction
showed that the intermediate accumulation were fully inhibited
and an increase in the reaction velocity for the Pd promoted cata-
lysts when compared to a non-doped catalyst. However, Pd load-
ings higher than 0.5 wt% favored the hydrodechlorination as an
undesired side reaction. This report demonstrates that the produc-
tion of aniline as a by-product proceeds by the hydrodechlorina-
tion of 4-chloroaniline mediated by the presence of metallic Pd
on the catalysts surface. The large resistance to leaching of the
2
isotherms and TEM micrographs depicted
2
O
2
2
2
O
2
2
0.5Pd-Co@SiO catalyst and large activity and chemoselectivity
towards the production other multi-functional chloro-
aminoarenes by catalytic hydrogenation reactions of their
chloronitroarenes reveals the potential of the Pd-Co@SiO
catalysts.
2
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
4
. Conclusions
Acknowledgements
Well-defined bimetallic Pd-Co core coated with mesoporous
SiO shell nanocatalysts were successfully prepared. The character-
2
The authors acknowledge CONICYT-ANID FONDECYT 1170083
(Chile), FONDECYT 11170095 (Chile), DIUC-VRID 217.022.028.OIN

2
36
T.M. Bustamante et al. / Journal of Catalysis 385 (2020) 224–237
Grants and Conicyt-Fondequip/PPMS/ EQM130086 (Chile). TM
Bustamante thanks to Universidad de Concepción and CONICYT-
ANID 21190739 for graduate fellowships. MA Fraga also thanks
CNPq (Brazil). The authors thank to Dr. J.N. Díaz de León of the
Universidad Nacional Autónoma de México, Centro de Nanocien-
cias y Nanotecnología, Ensenada, B.C., México for his support in
the HR-TEM analysis and Dr. Carlos Cruz of the Universidad Andres
Bello, Facultad de Ciencias Exactas, Departamento de Ciencias
Químicas, Santiago – Chile for his support in the VSM analysis.
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