Pd0.01Ru0.01Ce0.98O2-δ: A highly active and selective catalyst for the liquid phase hydrogenation of p-chloronitrobenzene under ambient conditions

Article abstract of DOI:10.1016/j.molcata.2013.04.018

Nanostructured bimetal ion substituted ceria, Pd_0.01Ru _0.01Ce_0.98O_2-δ (PdRuC2), prepared for the first time by a novel solution combustion synthesis and characterized employing XRD, BET, HRTEM and XPS has been shown to be very active and selective than the monometal ion substituted analogue Pd_0.02Ce_0.98O _2-δ (PdC2), whereas Ru_0.02Ce_0.98O _2-δ (RuC2) is inactive towards liquid phase hydrogenation of p-chloronitrobenzene to p-chloroaniline under ambient conditions. Structural studies show metal ion substituted ceria as the predominant phase. The hydrogenation over PdRuC2 is completed beyond 75 min with 100% selectivity. Conversely, PdC2 hydrogenates ~40% of p-chloronitrobenzene with 82% selectivity. Increase of temperature from 35 C to 80 C showed a little higher activity of PdRuC2 but with a lower selectivity. The as-prepared and aged forms of PdRuC2 showed similar activity, whereas PdRuC2 heat-treated at 500 C increased the conversion and the 800 C heated catalyst reduced it (both ~2%) indicating high thermal stability. Maximum hydrogenation activity has been observed in ethanol as compared to methanol and butanol. The PdRuC2 catalyst also shows excellent hydrogenation activity towards o-, m-chloronitrobenzene and nitrobenzene. The enhancement of activity and selectivity of Pd in presence of Ru in the PdRu bimetal ionic catalyst for the hydrogenation reaction has been attributed to involvement of remarkable Ru⁴⁺-promotion in Pd_0.01Ru _0.01Ce_0.98O_2-δ.

Full text of DOI:10.1016/j.molcata.2013.04.018

Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Pd0.01Ru0.01Ce0.98
hydrogenation of p-chloronitrobenzene under ambient conditions
O
: A highly active and selective catalyst for the liquid phase
2−ı
Rajib Mistria,1, Jordi Llorca , Bidhan Chandra Ray , Arup Gayen
b
a
a,∗
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Institut de Tècniques Energètiques and Centre for Research in Nanoengineering, Universitat Politècnica de Catalunya, 08028 Barcelona, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanostructured bimetal ion substituted ceria, Pd0.01Ru0.01Ce0.98
O
2−ı
(PdRuC2), prepared for the first
Received 21 February 2013
Accepted 17 April 2013
Available online 25 April 2013
time by a novel solution combustion synthesis and characterized employing XRD, BET, HRTEM and
XPS has been shown to be very active and selective than the monometal ion substituted analogue
Pd0.02Ce0.98O2−ı (PdC2), whereas Ru0.02Ce0.98O2−ı (RuC2) is inactive towards liquid phase hydrogenation
of p-chloronitrobenzene to p-chloroaniline under ambient conditions. Structural studies show metal
ion substituted ceria as the predominant phase. The hydrogenation over PdRuC2 is completed beyond
Keywords:
PdRu
Ceria
7
5 min with 100% selectivity. Conversely, PdC2 hydrogenates ∼40% of p-chloronitrobenzene with 82%
◦
◦
selectivity. Increase of temperature from 35 C to 80 C showed a little higher activity of PdRuC2 but
Ionic substitution
Chloronitrobenzene hydrogenation
with a lower selectivity. The as-prepared and aged forms of PdRuC2 showed similar activity, whereas
4+
PdRuC2 heat-treated at 500 ◦C increased the conversion and the 800 ◦C heated catalyst reduced it (both
Ru -promotion
∼
2%) indicating high thermal stability. Maximum hydrogenation activity has been observed in ethanol
as compared to methanol and butanol. The PdRuC2 catalyst also shows excellent hydrogenation activity
towards o-, m-chloronitrobenzene and nitrobenzene. The enhancement of activity and selectivity of Pd
in presence of Ru in the PdRu bimetal ionic catalyst for the hydrogenation reaction has been attributed
4
+
to involvement of remarkable Ru -promotion in Pd0.01Ru0.01Ce0.98O2−ı.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
their catalytic activities are poor. Supported Pd catalysts, though
show better activity, the selectivity is low. For base metal sup-
ported catalysts, use of foreign substances like NaOAC, pyridine,
quinoline has also been reported to suppress the dechlorination
reaction [26].
There has been ever growing interest in synthesizing uniform
bimetallic nanoparticles as new catalysts for the organic reaction
[24,27]. The suitable geometric and/or electronic effects of blend-
ing foreign metal atoms into metal structure allow the bimetallic
metal nanoparticles with tunable catalytic properties. This has spe-
cial importance in any new catalytic process where high activity
and ultra high selectivity of product in environmental conscious
manner are urgently required.
Chloroanilines (CANs) are important intermediates in industry
for synthesis of herbicides, pesticides, pigments, pharmaceuticals,
cosmetics and commercially important organic fine chemicals [1,2].
The main synthesis routes to these chloroanilines involve selective
hydrogenation of the corresponding nitro compounds over metal
catalysts due to its lower impact on environment [3,4]. It is known
that the catalytic hydrogenation of p-chloronitrobenzene (p-CNB)
involves hydrodehalogenation to some extent, if right choices of
catalysts are not used in the reaction medium [5]. Hence, design-
ing a suitable catalyst system for the hydrogenation reaction is an
important proposition. Several types of supported metal catalysts,
like Pt [6–9], Ru–M (M = Sn, Pb and Ge) [10], Pt–M (M = Cr, Mn, Fe, Co,
Ni and Cu) [11–13], Ag [14], Ni [15,16], Au [17], Pd [1,3,18], Pd–Au
In this work we show that Pd0.01Ru0.01Ce0.98
a single step solution combustion method, has a much higher
selective hydrogenation activity than the monometal ion sub-
O
, prepared by
2−ı
[
19], Ru [20], Rh [21], Pd–Ru [22,23], Pd–Pt [24] and Au–Pt [25] have
been used with varying degree of success. Among these formula-
tions, noble metal-based catalysts are primarily used in most of
the chemical industries. Although supported Ru, Rh are selective,
stituted analogue Pd0.02Ce0.98
O
(whereas the Ru0.02Ce0.98O
2−ı 2−ı
is inactive) towards p-CNB to p-CAN at room temperature and
atmospheric pressure. The effect of changing various parameters
on this reaction has also been investigated. A detailed analysis
of the microstructure of the catalyst in its various forms sup-
plemented with the XPS and XRD findings have enabled us to
correlate the activity-selectivity patterns with the structure of the
catalysts.
∗ Corresponding author. Tel.: +91 33 2457 2767; fax: +91 33 2414 6223.
E-mail addresses: agayenju@yahoo.com, gayen.arup@gmail.com (A. Gayen).
Present address: Achhruram Memorial College, Jhalda 723 202, India.
1
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.04.018

1
12
R. Mistri et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
Table 1
Nomenclature, nominal composition and surface area of different catalysts.
damage of the samples was observed under prolonged electron
beam exposure. Samples were dispersed in alcohol in an ultra-
sonic bath, and a drop of supernatant suspension was poured
onto a holey carbon-coated grid. Images were not filtered or
treated by means of digital processing, and they correspond to raw
data.
Surface area (m2
g
−1
)
Catalyst nomenclature
Nominal composition
PdC1
PdC2
RuC1
RuC2
PdRuC1
PdRuC2
PdRuC2HT500
PdRuC2HT800
PdRuC2aged
PdRuC3
Pd0.01Ce0.99O2−ı
–
15.4
–
11.9
–
13.6
13.5
8.4
14.4
–
Pd0.02Ce0.98O2−ı
Ru0.01Ce0.99O2−ı
Ru0.02Ce0.98O2−ı
Surface characterization was done with X-ray photoelectron
spectroscopy (XPS) on a SPECS system equipped with an Al anode
XR50 source operating at 150 mW and a Phoibos 150 MCD-9 detec-
Pd0.005Ru0.005Ce0.99O2−ı
Pd0.01Ru0.01Ce0.98O2−ı
Pd0.01Ru0.01Ce0.98O2−ı
Pd0.01Ru0.01Ce0.98O2−ı
Pd0.01Ru0.01Ce0.98O2−ı
Pd0.015Ru0.015Ce0.98O2−ı
−
7
tor. The pressurein the analysischamberwas always below 10 Pa.
The area analysed was about 2 mm × 2 mm. The pass energy of the
hemispherical analyser was set at 25 eV and the energy step was
set at 0.1 eV. Charge stabilization was achieved by using a SPECS
Flood Gun FG 15/40. The sample powders were pressed to self-
consistent disks. The following sequence of spectra was recorded:
survey spectrum, C 1s, Ce 3d, Pd 3d, Ru 3p and C 1s again to check
for charge stability as a function of time and the absence of degra-
dation of the sample during the analyses. Data processing was
performed with the CasaXPS programme (Casa Software Ltd., UK).
The binding energy (BE) values were referred to the C 1s peak at
2
. Experimental
2.1. Preparation of catalyst
We have synthesized the catalysts by single step solution
combustion method in an open muffle furnace kept in a fume
hood with exhaust by the combustion of the corresponding metal
salts with oxalyldihydrazide [C H N O (ODH)] as the fuel. Oxa-
2
84.8 eV. Atomic fractions (%) were calculated using peak areas nor-
2
6
4
2
malized on the basis of acquisition parameters after background
subtraction, experimental sensitivity factors and transmission fac-
tors provided by the manufacturer.
lyl dihydrazide was prepared by the dropwise addition of diethyl
oxalate (C H N O , Sisco Research Laboratories Pvt. Ltd., 99%) to
2
6
4
2
ice-cooled aqueous solution of hydrazine hydrate (N H ·2H O,
2
4
2
Qualizens Fine Chemicals, 99%) as reported in [28]. The preparation
involves combustion of the metal salts
of Pd0.01Ru0.01Ce0.98
O
2−ı
2.3. Catalytic test
(
NH ) Ce(NO ) , RuCl ·3H O, PdCl with ODH, taken in a molar
4
2
3
6
3
2
2
ratio 0.98:0.01:0.01:2.352, at the temperature of ignition of the
redox mixture. In a typical preparation, 5 g of (NH ) Ce(NO ) (Loba
To test the catalytic activity we have fabricated a simple reac-
tion setup in our laboratory. In this setup, the catalyst, reactant
and liquid solvent are taken in a 250 mL three-necked round bot-
tom flask. The flask is fixed in an oil bath placed on a magnetic
stirrer that helps to achieve homogeneity of the reaction mix-
ture. A heating coil, connected to a temperature controller and
powered with a variac can be used for heating purpose when
required. The gas lines are made up of silicone rubber. The react-
4
2
3 6
Chemie, 99%), 1.64 mL 1% PdCl (Spectrochem Pvt. Ltd., 99.9%) solu-
2
tion, 2.435 mL 1% RuCl ·3H O (Arora Matthey Pvt. Ltd., 40% Ru)
3
2
solution and 2.5877 g of ODH are dissolved in ∼30 mL of double dis-
tilled water in a borosilicate dish. The solution is then transferred to
a preheated muffle furnace kept in a fume hood with exhaust and
◦
maintained at ∼350 C. Initially the solution boils with frothing and
foaming followed by complete dehydration when the surface gets
ignited and burns with a flame yielding a voluminous solid product
within a minute.
ing (purging) gasses [H2 (N )] were fed to the system using proper
2
gas regulators via a “T” connector which flows through a bub-
bler of paraffin oil before entry to the reactor. This “T” connector
allows flow of the reacting gas or purging/carrier gas or both. The
gas outlet also passes through a similar bubbler. An upper end
sealed bulb water condenser is connected to the flask for conden-
sation of reaction mixture. Pure N2 was used to purge the reactor
before performing any catalytic test. All the gases were obtained
from Indian Refrigeration Stores, Kolkata and were of UHP quality
We have also prepared Pd0.01Ce0.99
O
,
Pd0.02Ce0.98
and Pd0.005Ru0.005Ce0.99O
2−ı
O
,
2−ı
2−ı
Ru0.01Ce0.99
O
, O
Ru0.02Ce0.98
2−ı 2−ı
catalysts
in
a
similar
manner.
The
as
prepared
◦
Pd0.01Ru
Ce0.98
O
has also been heated in air at 500 C
0.01
◦
2−ı
and 800 C for 36 h and named as heat-treated (HT) catalysts,
HT500 and HT800, respectively. All the catalysts studied here are
listed in Table 1. PdRu with higher loading (3 at.%, 1.5 at.% each of
Pd and Ru, catalyst PdRuC3 in Table 1) was also prepared in order
to decide the best catalyst composition.
(
>99.9%).
In a typical catalytic run, the catalyst in its powder form and
00 mL EtOH (Changshu Yangyuan Chemical, China, 99.9%) are
1
taken in a 250 mL three-necked round bottom flask and purged
with N2 (about 125 mL/min for 2 min) more than five times (total
2
.2. Characterization of catalysts
∼10 min) under constant stirring (rpm = 900) to get an inert reac-
tion atmosphere. Then the gas flow was switched over to pure
hydrogen flow (30 mL/min). After 30 min of hydrogen flow, 1 g
p-CNB (BDH Chemicals Pvt. Ltd., England, 99.3%) dissolved in min-
imum volume of ethanol (∼5 mL) was injected into the reaction
medium. The reaction compositions were analysed by taking small
amounts of liquid reaction mixture at different time intervals and
analysed using Waters Alliance 2695 Separations Module equipped
with built-in auto samplers (Waters, Milford, MA, USA) liquid chro-
matography equipped with Quattro micro API triple-quadrupole
mass spectrometer (Micromass, Manchester, UK) fitted with a
Z-spray electrospray interface in positive ion mode detector. A
The synthesized materials have been characterized by XRD, BET,
HRTEM and XPS. X-ray powder diffraction patterns were collected
in a Philips PW3040/60 X’pert PRO diffractometer (equipped with
an X’celerator detector) operated at 40 kV and 40 mA, using Ni-
filtered Cu-K␣ radiation in the 2ꢀ range 20–100 with a step size of
0
◦
◦
.02 and a counting time of 80 s per step.
The BET surface areas were measured in a TriStar3000 surface
area analyser (Micromeritics). Before each measurement, the sam-
ples were degassed at 150 C in vacuum for 60 min.
Microstructural characterization by high resolution trans-
mission electron microscopy (HRTEM) was performed at an
accelerating voltage of 200 kV in a JEOL 2010F instrument equipped
with a field emission source. The point-to-point resolution was
◦
5
.0 ␮m reverse phase C18 Polaris column (50 mm × 4.6 mm) [Varian
Inc., Lake Forest, CA, USA] is used for the separation of prod-
ucts and was determined by quantitative calculation of the area
percentage.
0
.19 nm, and the resolution between lines was 0.14 nm. The
magnification was calibrated against a Si standard. No induced

R. Mistri et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
113
Fig. 1. Powder XRD patterns of (a) PdC2, (b) RuC2, (c) PdRuC2, (d) PdRuC2HT800
and (e) PdRuC2aged catalysts.
Fig. 2. Normalized rates of p-CNB hydrogenation as a function of catalyst amount for
the PdRuC2 catalyst after (a) 60 min, (b) 75 min and (c) 90 min. Reaction condition:
◦
1
g p-CNB and 100 mL EtOH at 35 C.
3
. Results and discussion
PdRuC1 catalyst (having overall 1 at.% metal component, 0.5 at.%
each of Pd and Ru) although show a lower conversion than PdC2
after 120 min of hydrogenation, it shows higher activity as well as
selectivity (70% and 98%, respectively) when the reaction time was
increased from 120 min to 180 min. Maximum activity is achieved
at a metal loading of overall 2 at.% on ceria (1 at.% each of Pd and
Ru). So we choose PdRuC2 as the catalyst for further investigations.
Higher loading of PdRu (overall 3 at.%, 1.5 at.% each of Pd and Ru)
in ceria gave a high conversion (99%) but a lower selectivity (88%)
because of aniline formation (11%) and hence will not be discussed
further.
3
.1. XRD studies
Powder XRD patterns of different catalysts are shown in Fig. 1.
All the diffraction lines can be indexed to the fluorite structure
of Ceria (Fm3m) and diffraction lines due to Pd or Ru metal or
their oxides could not be detected even with a very slow scan (not
◦
shown) performed in the range 2ꢀ = 25–50 . So, under highly oxi-
dizing preparation condition, metal oxides are not observed. We
can speculate that the metal ion is probably substituted for Ce4
in CeO2 matrix in line with our previous reports in these kinds of
systems [29].
+
3.3.2. Effect of mass transfer influences on the hydrogenation
3.2. BET studies
After screening of the mono and bimetal ionic catalysts, a
number of preliminary tests were performed over the PdRuC2 cat-
alyst to investigate possible influences caused by mass transfer
phenomena. We used the catalyst in its power (finely divided)
forms and used a sufficiently high rpm to reduce/eliminate any
mass transfer influence in the reaction. It is to be noted that any
change in the quantity of catalyst used will affect the observed rate
of reaction even when the chemical reaction is limiting. That is why
we employed a very simple procedure to understand whether the
hydrogenation is being run under mass transfer limitations. The
procedure involves determining the normalized rate (rate per unit
mass of the catalyst) which should not change much (lies within
± 5%) when the catalyst amount is halved [30]. Fig. 2 incorporates
the normalized rates at three different time intervals. For a reac-
tion time of 60 min, the deviation of normalized rate using 500 mg
catalyst (∼46%) is far greater than the allowed deviation (± 5%). But
the deviation of this rate reduces appreciably when the reaction
time is increased to 75 min (∼15% deviation) and it falls within the
limit after 90 min of hydrogenation. Hence the mass transfer effect
decreases appreciably with the increase of reaction time beyond
75 min and it did not significantly influence the global reaction rate
after 90 min of reaction time. Thus, we conclude that the conver-
sions and TOFs reported in Table 2 corresponding to reaction time
≥75 min using 500 mg of catalyst is only slightly affected by mass
transfer.
The surface areas of the catalysts are low, fall in the
2
−1
range 12–15 m g
(see Table 1) and follows the order
RuC2 < PdRuC2 < PdC2. There occurs sintering of PdRuC2 catalyst
◦
on thermal treatment at 800 C, the surface area decreases by about
2
−1
2
−1
3
8% (from 13.6 m g in as-prepared form to ∼8 m g in HT800
◦
form). Thermal treatment at 500 C and ageing in the reaction atmo-
sphere (for 6 h) has no appreciable effect on the surface area of
PdRuC2 catalyst. For these rather low surface area materials, the
activity patterns of PdRuC2 in its as-prepared, heat-treated and
aged forms are similar as will be shown latter indicating no appre-
ciable role of surface area in deciding the activity of the different
forms of this catalyst. This point to the structure of PdRuC2 as the
responsible factor to decide the activity behaviour.
3
3
.3. Activity studies
.3.1. Screening of catalysts
Hydrogenation activities of p-CNB to p-CAN in Pd, Ru and
PdRu catalysts at room temperature and atmospheric pressure are
incorporated in Table 2. The PdRuC2 catalyst shows much higher
reactivity and selectivity than the other catalysts. Using ethanol
as solvent, p-chlorophenylhydroxylamine (p-ClPhNHOH) was pro-
duced as the byproduct (just about 1%) in this process. With PdRuC2
as catalyst, both reactivity and selectivity to p-CAN are 100% after
5 min of reaction. However the reactivity is less than 40% in other
catalysts and selectivity is also below 82% after 120 min barring
RuC1 and RuC2 that are inactive. It is to be noted that even the
Now, the possible effect of intraparticle diffusion could not be
completely ruled out and so we also studied the influence of catalyst
particle size on the reaction rate. For this purpose, we carried out
the hydrogenation reaction using as synthesized powder catalyst
7

1
14
R. Mistri et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
Table 2
p-CNB hydrogenation activities of different catalysts.
2
a
−1
)
Catalyst
Reaction time (min)
Conversion (%)
Products (%)
A
Selectivity of A (%)
TOF × 10 (s
B
PdC1
PdC2
RuC1
RuC2
PdRuC1
PdRuC2
PdRuC3
120
120
120
120
120
75
17.3
39.7
1
12.5
32.5
–
4.8
7.2
1
72.2
81.8
–
0.04
0.59
0.03
0.015
1 (2)
1
–
1
–
33.3
100
98.9
29.3
100
86.9
3.9
–
11.0
88.1
100
87.9
2.43 (4.83)
0.99 (1.98)
b
120
◦
Catalyst: 0.5 g; EtOH: 100 mL; temperature: 35 C; reagent: 1 g p-CNB; A: p-CAN; B: p-ClPhNHOH.
a
Calculated considering the total moles of Pd and Ru in the initial nominal composition (the values in the parenthesis correspond to TOF considering Pd-only contribution
since Ru is essentially inactive in the hydrogenation).
b
Aniline.
◦
and also different mesh sized catalysts keeping all other reaction
parameters constant. The findings are shown in Fig. 3. It can be seen
that with larger catalyst particle (smaller mesh size) the hydrogena-
tion activity is lower and the as-synthesized powder catalyst shows
a higher conversion compared to the larger catalyst particles. This
indicates the presence of some liquid/solid mass transfer limita-
tion on the reaction with larger sized catalyst particles [30]. The
as-synthesized catalysts are fine powders with which these diffu-
sion limitations are minimal or negligible and these have been used
throughout our study.
temperature of 80 C as compared to the activity at RT. Since the
catalyst is already very active at RT, the increase of temperature
does not affect the activity significantly in the studied temperature
range. But it is to be noted that the selectivity is lower at this high
temperature because of formation of about 2% aniline after 120 min
of reaction. These results are understandable since higher temper-
atures usually lead to faster reactions, but they may also activate
other bonds such as C Cl bond to form aniline.
3.3.4. Effect of solvent
Fig. 5 shows the hydrogenation activity of PdRuC2 in three sol-
vents. The activity and selectivity is maximum in ethanol (complete
conversion with 100% selectivity after 75 min of reaction). The con-
version decreases to ∼72% in methanol and further below to 48%
in butanol with more than 85% selectivity. The solvent plays an
important role in controlling the substrate catalyst interaction. It is
preliminarily adsorbed on the catalysts surface. More polar solvent
is firmly adsorbed on the catalyst surface and hence disfavours the
p-CNB adsorption decreasing the hydrogenation activity. In addi-
tion to this, the steric effect of absorbed solvent can also decrease
the substrate adsorptions and this in turn decreases the hydrogena-
tion activity. There is usually a competition between the solvent
polarity and steric effect. Now, the solvent polarity increases in the
order methanol > ethanol > butanol and the steric effect increases
in the reverse order. In case of ethanol both are moderate, so
it shows the highest activity. Also, the main reaction product p-
CAN and the byproduct p-ClPhNHOH through which the reaction
proceeds are more soluble in polar solvents. For a highly polar sol-
vent like MeOH, the selectivity decreases due to more solubility of
3
.3.3. Effect of temperature and reaction time
Reaction conditions have been reported to have some influences
on the hydrogenation rate and the selectivity in the hydrogena-
tion of halonitrobenzenes over supported metal catalysts [31,32].
We also carried out hydrogenation at temperatures higher than RT
◦
(
limited to 80 C since this is the b.p. of the solvent ethanol used in
this study) using a lower amount (200 mg) of catalyst keeping all
the other reaction parameters constant at different time intervals
and this is incorporated in Fig. 4. Note that the conversion increases
more or less linearly with time up to 105 min with 100% selec-
tivity. The four temperatures studied here can be categorized into
two ranges. Between 35 and 40 C, the activity is similar and then
the conversion increases by 3–5% and remains similar in the tem-
perature range 60–80 C. After 75 min, the p-CNB conversion was
◦
◦
about 35% in the first temperature range and then it increased to
about 40% in the second temperature range. Thus, we note that
the PdRuC2 catalyst shows a little higher activity at the highest
Fig. 3. Time variation of p-CNB conversions over PdRuC2 catalyst in (a) powder
form, (b) 85–100 mesh size and (c) 60–85 mesh size particles. Reaction condition:
Fig. 4. Hydrogenation of p-CNB as a function of time over PdRuC2 at different tem-
peratures. Reaction condition: 1 g p-CNB, 0.2 g catalyst and 100 mL EtOH at 35 C.
◦
◦
1
g p-CNB, 0.2 g catalyst and 100 mL EtOH at 35 C.

R. Mistri et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
115
◦
Fig. 5. Effect of solvent on the hydrogenation of p-CNB over PdRuC2 at 35 C. Reac-
tion condition: 1 g p-CNB, 0.5 g catalyst, 100 mL solvent and for 75 min reaction.
Fig. 6. Time variation of p-CNB conversion over (a) as-prepared (AP), (b) HT500, (c)
HT800 and (d) aged forms of PdRuC2 catalyst. Reaction condition: 1 g p-CNB, 0.2 g
◦
p-ClPhNHOH which can leave the catalyst surface more easily
before conversion to p-CAN. On the other hand, in a solvent of com-
paratively lower polarity, the product desorption will be lower and
so the selectivity will be higher. In butanol, although less polar than
ethanol, the selectivity decreases due to more pronounced steric
effect of the absorbed solvent on p-ClPhNHOH which now leaves
the catalyst surface more easily before conversion to p-CAN. Hence,
a moderately polar and bulky solvent like ethanol shows higher
activity and selectivity for this hydrogenation reaction.
catalyst and 100 mL EtOH at 35 C.
more reactant conversion because of more availability of catalyst
surface increasing the overall conversion. This also enhances the
possibility of conversion of the intermediate to the product p-CAN
before going into the solution leading to increased selectivity.
3
.3.6. Effect of thermal treatment and ageing
We have also investigated the effect of thermal treatment
◦ ◦
at 500 C and 800 C for 36 h) on the stability of PdRuC2 and
(
3.3.5. Effect of reagent amount
subsequently investigated their activity. Fig. 6 compares the hydro-
genation activity of as-prepared, heat-treated and aged PdRuC2
catalysts at different time intervals. The activity pattern follows the
order: HT500 > AP ≈ aged > HT800. Interestingly, the as-prepared
and aged (in the reaction atmosphere for 6 h) catalysts showed
almost the same activity at all time intervals. The by-product is
formed only to a little extent up to a reaction time of 75 min, beyond
which p-CAN is the only hydrogenation product. Thermal treat-
ment at 500 C has a little positive effect (conversion increases by
about 2%) on the activity pattern of as-prepared catalyst. But heat-
ing of the catalyst at 800 C for 36 h reduces the activity by just about
–3%. Thus the PdRuC2 catalyst is essentially stable up to 800 C. It
is to be kept in mind that this slight loss of activity at the high tem-
perature of 800 C is not of concern for our present study because
the as-prepared catalyst is already very active at room temperature
and the catalyst heat-treated at 500 C is slightly more active. The
nearly similar activities of heat-treated catalysts indicate that no
appreciable loss of Ru due to evaporation happen even at 800 C.
This is very important because Ru-based catalysts have this inher-
ent problem and it is interesting to efficiently incorporate Ru in the
catalyst in order to maintain the activity.
From the foregoing results it becomes obvious that the hydro-
genation activity in the combustion synthesized catalysts is only
due to palladium. Thus, ruthenium in the bimetal ion substituted
catalyst (PdRuC2) acts as a remarkable promoter to increase the
activity of palladium with 100% selectivity. It is also to be noted
that the activity remains unaltered for consecutive cycles. The
essentially unaltered hydrogenation activity of as-prepared, heat-
treated, recycled and aged PdRuC2 catalyst must then be attributed
to the unchanged structure of the catalyst.
Table 3 lists the hydrogenation activities of PdRuC2 catalyst
using a varied amount of p-CNB after different time intervals under
the same reaction conditions. Complete conversion is achieved
for a reagent amount of 1 g with 100% selectivity after 75 min of
reaction. Doubling the reagent amount to 2 g decreased the conver-
sion to about 73% and is associated with a decrease of selectivity
(
∼94%) because of the formation of byproduct after 120 min of reac-
tion. Continuation of hydrogenation for further one hour results in
an increase of both the activity and selectivity. When the p-CNB
amount was increased by five times, the conversion was between
◦
◦
5
0 and 60%. No further increase of byproduct formation was noted
◦
2
again suggesting very good selectivity (>92%) of the catalyst.
Thus, under the chosen reaction conditions, hydrogenation for
◦
7
5 min can completely convert a maximum of 1 g of p-CNB to
p-CAN. Further increase of p-CNB amount results in a lower cat-
alyst/reagent ratio and thus a decreased conversion as well as
selectivity even after longer duration of reaction. This is under-
standable since not all the reactant molecules can get adsorbed on
the catalyst surface for reaction and the intermediate p-ClPhNHOH
also can escape the surface before conversion to p-CAN decreasing
the selectivity. When a major portion of p-CNB is converted after
◦
◦
1
20 min, further increase of reaction time to 180 min allows some
Table 3
Effect of reagent amount on p-CNB hydrogenation over PdRuC2 catalyst.
Amount of
p-CNB (g)
Reaction
time (min)
Conversion (%)
Products (%)
Selectivity
of A (%)
A
B
–
1
2
75
100
100
100
120
73.2
84.8
69.1
83.2
4.1
1.6
94.4
98.1
3
.3.7. Hydrogenation of o- and m-CNB and nitrobenzene
It is of great interest to see if the PdRuC2 catalyst can be a general
catalytic system for the hydrogenation of other chloronitroben-
zenes and the parent compound nitrobenzene. Hydrogenation
activity of o-CNB, p-CNB and nitrobenzene (NB) was thus tested
180
5
120
52.6
58.8
48.5
55.6
4.1
3.2
92.3
94.6
180
◦
Reaction condition: 0.5 g catalyst, 100 mL EtOH and 35 C.

1
16
R. Mistri et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
Table 4
superstructure formation, since neither extra spots appear in the FT
images nor strips or clustered spots. Thus, it appears that M–O–Ce
superstructure formation is restricted to Pd only. In Fig. 7(g) several
Ru particles of 2–4 nm in size are shown. Spots at 2.34 A are due to
Ru (1 0 0) planes.
Hydrogenation activities of PdRuC2 towards other chloronitrobenzenes and
nitrobenzene.
2
a
−1
)
Reagent
p-CNB
o-CNB
Reaction time (min) Conversion (%) TOF × 10 (s
˚
75
100
4.83
A general view of as-prepared PdRuC2 is depicted in Fig. 7(h).
A lattice fringe view is shown in Fig. 7(i), where several particles
of 2–4 nm are identified. Given their dimension, these particles are
likely ruthenium. Fig. 7(j) corresponds to another part of the sample
in which small particles of about 1 nm are well distributed over the
support. Given their similarity to the particles present in sample
PdC2, these particles in sample PdRuC2 are likely palladium. Inter-
estingly, the FT image of the ceria support exhibit several spots
clustered around the nominal pure CeO2 values, thus suggesting
the existence again of a Pd–O–Ce structure. A comprehensive image
is provided in Fig. 7(k). In this image, individual Ru and Pd parti-
cles as well as Pd–O–Ce superstructure are identified. The FT image
corresponds to the Pd–O–Ce support, whereas an individual Ru par-
ticle showing (0 0 2) fringes at 2.14 A is indicated along with an
individual Pd cluster, too small for exhibiting lattice fringes.
Thus, in PdC2, Pd is encountered both in individual Pd entities
of about 1 nm in diameter and in a surface Pd–O–Ce superstruc-
ture. In RuC2, Ru is encountered only in individual Ru crystallites
of about 2–4 nm in diameter. In PdRuC2, the Pd–O–Ce superstruc-
ture is maintained along with individual Pd and Ru particles of
ca. 1 and 2–4 nm, respectively. Note that ordering of the super-
structure originates extra spots in HRTEM because the symmetry
order decreases, but the absence of such extra spots does not elim-
75
20
98
100
4.73
–
1
1
m-CNB
NB
75
20
96
100
4.64
–
75
20
80
86.4
93.2
100
5.34
–
–
1
1
◦
Reaction condition: 1 g p-CNB, 0.5 g catalyst, 100 mL EtOH and 35 C.
a
Calculated considering Pd-only contribution.
over this catalyst and the results are summarized in Table 4. As
expected the o- and m-derivatives show similar activity (o- and
m-CNB gives ∼98% and ∼96% of the corresponding aniline after
˚
7
5 min of reaction) with more than 99% selectivity and for the
parent compound NB, a relatively lower activity is observed. Never-
theless, the NB to aniline (AN) conversion was 86% after 75 min that
increased to 93% after 120 min and hydrogenation was complete
beyond 180 min of reaction with 100% selectivity. Thus, PdRuC2
has the potential to be used as a general hydrogenation catalyst for
these types of organic molecules.
Table 2 also lists the TOF values for various catalysts calculated
considering the total moles of metal component taken in the prepa-
ration. As because Ru has been found to be inactive towards the
reaction and it is the Pd component that contributes to the activity,
we have calculated the TOF values of the bimetal ionic catalysts in
reference to Pd-only and those are also incorporated in table. The
TOF over PdRuC2 is almost four times that over PdC2 considering
total metal (Pd and Ru both) and it is more than eight times if we
consider Pd-only value. Although the TOF reported by Liu et al. for
this hydrogenation on PVP-Ru/Pd colloids is higher than our cal-
culated TOFs, the selectivity obtained by them is much less [22].
The TOF’s are similar for all the three chloronitrobenzenes, namely
inate metal ion incorporation in the CeO lattice. If incorporation is
2
random there will be no extra spots. So, random Ru–O–Ce phase
is likely there for Ru, whereas supercell structure is present in
Pd–O–Ce. No evidence for alloy formation between Pd and Ru is
encountered, although the possibility of surface alloy cannot be
completely ruled out by HRTEM measurements over these rather
small metallic particles.
The heated sample PdRuC2HT500 is similar to the sample
PdRuC2 (see Fig. 8(a)–(c)). The only difference is that Ru parti-
cles maintain their size after heating (2–4 nm), but Pd particles
experience a size growth, from about 1 nm in sample PdRuC2 up
to 2–3 nm in sample PdRuC2HT500. This is clear from particle
size distribution measurements over Fig. 8(a)–(c) corresponding to
sample PdRuC2HT500. Upon sintering, Pd particles develop crystal-
lographic facets, as shown in Fig. 8(c) for an individual Pd crystallite
oriented along the [1 1 0] crystallographic direction. In Fig. 8(b),
the FT image corresponding to the support particle indicates that
the existence of the Pd–O–Ce superstructure is maintained after
heating.
The sample aged in the reaction atmosphere for 6 h,
PdRuC2aged, is indistinguishable from the fresh sample PdRuC2
(Fig. 8(d)–(f)). In Fig. 8(d) several Ru particles of 2–4 nm in diam-
eter are well dispersed over the support particles. Pd entities are
also present in a highly dispersed form of about 1 nm, as shown
in Fig. 8(e). A FT image is enclosed corresponding to the support
particle, where spots at 2.9–3.2 A˚ are likely to originate from the
existence of the Pd–O–Ce phase. In Fig. 8(f) several Ru particles are
depicted and identified on the basis of their lattice fringes.
−2
−2
−2 −1
s , respectively for the p-
4
.83 × 10 , 4.73 × 10 and 4.64 × 10
,
o- and m-CNB after 75 min of hydrogenation (see Table 4). For the
−2
−1
s was obtained.
hydrogenation of NB, a TOF of 5.34 × 10
3.4. Microstructural studies
A low magnification view of PdC2 (see Fig. 7(a)) shows it to be
comprised by small (5–8 nm) and large (∼50 nm) ceria crystallites.
Numerous, randomly oriented ceria crystallites are visible and their
electron diffraction pattern (inset in Fig. 7(b)) shows the expected
CeO rings with spacing at 3.12, 2.71, 1.91, 1.63 and 1.56 A˚ ascribed
2
to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystallographic planes,
respectively. A lattice-fringe image of PdC2 sample (Fig. 7(c)) shows
tiny palladium particles with a mean particle size of about 1 nm.
In addition, the support shows evidence of structural distortion.
FT image of the support (Fig. 7(d)) shows multiple spots clustered
around the crystallographic nominal values, which points to the
formation of Pd–O–Ce phase [29]. Moreover, additional spots at
5
.7 A˚ are indicative of Pd–O–Ce superstructure formation (as dis-
cussed in [29]). In Fig. 7(d), lattice fringes of Pd (1 1 1) at 2.2 A˚ are
3.5. XPS studies
recognized in various particles, which likely have an epitaxial rela-
tionship with the support particle.
Sample RuC2 also contains both large and small CeO2 crystal-
lites, as shown in Fig. 7(e). Ruthenium particles in the range of
XPS of Pd(3d) and Ru(3p) regions of fresh PdRuC2 are shown in
Fig. 9. The Pd(3d5/2,3/2) peaks at 338 and 343.2 eV can be attributed
to Pd in 2+ oxidation state [33]. Similarly, Ru(3p3/2,1/2) peaks at
363.3 and 486.4 eV are due to Ru4+ ions in ceria [34]. In each spectra,
the lower energy shoulder (negligibly smaller for Pd) indicates sig-
nature of metallic components in line with the HRTEM findings. The
Ce(3d) spectra (not included) with characteristic satellite features
2
3
–4 nm are well dispersed over the support (Fig. 7(f)). Spots at
.12 A˚ due to CeO2 (1 1 1) planes are aligned with spots at 2.14 A˚
of Ru (0 0 2) crystallographic planes, suggesting epitaxial growth.
It merits to be highlighted that in this sample there is no Ru–O–Ce

R. Mistri et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
117
Fig. 7. Transmission electron microscopic images of as-prepared samples of (a)–(d) PdC2, (e)–(g) RuC2 and (h)–(k) PdRuC2.
correspond to cerium in 4+ oxidation state. For the fresh sample,
the atomic composition of the surface is: 30.7% Ce, 65.6% O, 1.8%
Pd and 1.9% Ru. The ratio Ce/O corresponds to that expected for
clearly indicating surface segregation of Pd and Ru mostly as ions on
CeO2 crystallites surface. XPS study thus confirms ionic dispersion
of Pd and Ru on the surface of CeO2 crystallites. Also, the atomic
ratio Pd/Ru is close to one as taken in the preparation of the cat-
alyst, indicating that no preferred segregation of any of the two
metals occurs. It is not easy to quantify metallic particles against
oxidized ones because air exposure might oxidize their surface
CeO , and the high values of Pd/Ce and Ru/Ce (about 0.06 for both)
2
indicate a good dispersion of the metal component. In particular,
there is about 6 times enhancement in surface concentration of
Pd and Ru in the PdRuC2 catalyst than that of bulk concentration,

1
18
R. Mistri et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
Fig. 8. Transmission electron microscopic images of (a)–(c) PdRuC2HT500 and (d)–(f) PdRuC2aged.
(
1–2 atomic layers), which would be detected by XPS but not by
HRTEM.
The catalytic studies have shown that the HT500 catalyst is lit-
the slightly higher activity of the HT500 catalyst as compared to the
as-prepared and aged catalysts. This has some resemblance with
the findings by Krillov and Ryndin who found that a higher disper-
sion of smaller Pd particles having a lower electron density shows
lower hydrogenation activity [35]. Good hydrogenation properties
tle more active than the as-prepared and aged catalysts, whereas
the activity of HT800 catalyst is a bit lower. Our thorough micro-
structural study shows no major differences in the microstructure
except that Pd particles are very small in size (about 1 nm) in the
as-prepared and aged PdRuC2 catalysts whereas the Pd particle size
increases to 2–3 nm in the HT500 catalyst. There is neither any dif-
ference in the surface area of these three forms of PdRuC2 catalyst.
It seems therefore that the smaller Pd crystallites do not participate
in the hydrogenation whereas when they grow on heating to about
have also been obtained on large Pt particles for Pt/Al O3 catalysts
2
by Coq et al. [6] and for bimetallic PtSn/nylon catalysts by Galvagno
et al. [36] and poorly dispersed Ru-catalysts by Tijani et al. [10].
The predominant role towards hydrogenation is still played by the
ionic palladium species which is the major Pd-species present in
the combustion synthesized catalysts. We are not very concerned
with the HT800 catalyst because such high temperature stability is
not a prerequisite for the present study. We just wanted to look into
the stability of the catalyst and its influence on the hydrogenation
activity pattern. The finding is promising since there is just a mere
loss of activity of the HT800 catalyst compared to the as-prepared
one. We attribute this loss to originate from (a) loss of surface area
and (b) there may be some loss of the ruthenium component on
2
–3 nm as in the HT500 catalyst, there is some active participation
of these crystallites in the hydrogenation reaction as is evident from
◦
prolonged heating at 800 C.
There are a number of studies wherein chloronitrobenzene
hydrogenation activities have been shown to be promoted in pres-
ence of a second metal component. Han et al. has shown that
the CNB hydrogenation over Pt/ZrO2 is improved with the intro-
duction of rare earths with the maximum using praseodymium
and they attributed this to an electronic effect by the rare earths
which activate the nitrogen–oxygen bond [37]. They also found
such a promotional effect when a transition metal is incorporated
in Pt/TiO2 catalyst, the highest effect was observed in presence of
iron [11]. Coq et al. showed that suboxide TiOx species promotes
the properties of Pt particles in Pt/Al O3 catalyst [9]. A SMSI effect
2
between Ni-particles and TiOx species produced on reduction has
been explained to be responsible for the best performance over
the Ni/TiO2 catalyst by Xiong et al. [38]. Xu et al. have shown
that Sn4+ played an excellent promoting role over PVP-Pd/␥-Al O3
2
for both the hydrogenation activity of p-CNB and the selectiv-
Fig. 9. XPS of (a) Pd(3d) and (b) Ru(3p) region in as-prepared PdRuC2 catalyst.

R. Mistri et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 111–119
119
ity of forming p-CAN [39]. Vishwanathan et al. have reported the
effect of alkali metal promotion in alumina-supported palladium
catalysts where both the stability during on-stream activity tests
and resistance to coke formation is improved over the promoted
catalyst with respect to the unpromoted one [40]. Liu et al. inves-
(iv) The PdRuC2 catalyst has also been shown to be an effec-
tive catalytic formulation for the hydrogenation of other
chloronitrobenzenes, namely o- and m-chloronitrobenzene
and nitrobenzene. It is expected that this catalyst has the
potential to act as a general catalyst for several other nitroben-
zene derivatives of concomitant interest.
tigated the metal ions Fe3 , Ni and Co supported on Pt colloids,
+
2+
2+
which displayed different modification roles for the hydrogena-
2
+
tion reactions [41]. Whereas Co increased both the activity and
Acknowledgements
3
+
selectivity, Fe decreased the activity of Pt/PS (PS = poly styrene-
co-divinylbenzene) catalyst and Ni2 increased the selectivity but
decreased the activity. Liu et al. have reported a significant role
of Fe3+ towards PVP-Ru/Pd and that of Co towards PVP-Ru/Pt
+
RM thanks CSIR for a research fellowship. Financial support from
DST, Government of India and UGC-CAS, Department of Chemistry,
Jadavpur University is gratefully acknowledged. JLl is grateful to
ICREA Academia program (Generalitat de Catalunya).
2+
[
22]. Thus, a positive role towards CNB hydrogenation is played
by either an alkali metal ion or a transition metal ion or rare
earths.
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[
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[
[
[
(
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[
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(
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(
(
role towards hydrogenation of p-CNB without dechlorination
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+