Pd-promoted selective gas phase hydrogenation of p-chloronitrobenzene over alumina supported Au

Article abstract of DOI:10.1016/j.jcat.2008.12.019

The gas phase hydrogenation of p-chloronitrobenzene has been investigated over alumina supported Au (ca. 1%, w/w) prepared by deposition-precipitation with urea (DP) and impregnation in excess solvent (IMP). Both catalysts were 100% selective in terms of

Full text of DOI:10.1016/j.jcat.2008.12.019

Journal of Catalysis 262 (2009) 235–243
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Journal of Catalysis
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Pd-promoted selective gas phase hydrogenation of p-chloronitrobenzene over
alumina supported Au
a
a
b
b
b
Fernando Cárdenas-Lizana , Santiago Gómez-Quero , Antoine Hugon , Laurent Delannoy , Catherine Louis ,
a,∗
Mark A. Keane
a
Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, Scotland
Laboratoire de Réactivité de Surface, UMR 7609 CNRS, Université Pierre et Marie Curie-Paris 06, 4 Place Jussieu, 75252 Paris Cedex 05, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The gas phase hydrogenation of p-chloronitrobenzene has been investigated over alumina supported Au
Received 18 November 2008
Revised 22 December 2008
Accepted 23 December 2008
Available online 22 January 2009
(
(
ca. 1%, w/w) prepared by deposition-precipitation with urea (DP) and impregnation in excess solvent
IMP). Both catalysts were 100% selective in terms of –NO2 group reduction, resulting in the sole
formation of p-chloroaniline. Au–DP exhibited a smaller mean Au size (2.9 nm) compared with Au–IMP
4.5 nm) and delivered a higher (by a factor of 14) specific hydrogenation rate. Bimetallic Pd–Au/Al2O3
(
catalysts have also been prepared by DP and IMP with Au/Pd mol/mol = 8, 20 and 88. Catalyst activation
by temperature programmed reduction has been monitored and the activated catalysts characterized
in terms of H2 chemisorption, TEM analysis and DRIFTS. DRIFTS measurements using CO as a probe
molecule suggest the presence of bimetallic particles and surface Au–Pd interaction. A significant increase
(by up to a factor of 3) in activity was observed for Pd–Au/Al O3 (Au/Pd ꢀ 20) compared with Au/Al O
Keywords:
p-Chloronitrobenzene
Selective hydrogenation
p-Chloroaniline
Au/Al2O3
Pd–Au/Al2O3
DRIFTS analysis
TPR
2
2
3
where the exclusive conversion of p-chloronitrobenzene to p-chloroaniline was maintained. At a lower
ratio (Au/Pd = 8), nitrobenzene was produced as a result of a Pd catalyzed hydrodechlorination step.
Under the same reaction conditions, Au/Al2O3 +Pd/Al2O3 physical mixtures (Au/Pd = 20) delivered higher
reaction rates but with the formation of nitrobenzene and aniline, i.e. products of hydrodechlorination
and hydrogenation. We attribute the enhanced and exclusive production of p-chloroaniline over the
supported bimetallics to a surface Pd–Au synergism. Our results establish the viability of Pd-promotion
in the selective continuous gas phase catalytic hydrogenation of p-chloronitrobenzene over supported Au.
H2 chemisorption
©
2008 Elsevier Inc. All rights reserved.
1
. Introduction
with the hydrogenation of p-chloronitrobenzene [3,5], as shown in
Fig. 1. Path A involves hydrodechlorination to nitrobenzene, which
can be further hydrogenated to aniline via the nitrosobenzene in-
termediate. In Path B, the –NO2 group is initially hydrogenated
to yield the target product (p-chloroaniline), which can undergo
subsequent hydrodechlorination to aniline. Both mechanisms have
been reported for gas and/or liquid phase catalytic operation over
supported metals [3,6–13]. Dechlorination at high conversions is a
feature of reactions promoted by supported Ni [7,8], Pd [11,14], Pt
Halogenated aromatic amines, with multiple applications in the
manufacture of pesticides, herbicides, pigments, pharmaceuticals
and cosmetics, are commercially important target products [1]. Ex-
isting routes, notably the Bechamp process (Fe promoted reduction
in acid media), produce toxic by-products with low overall product
yields [2] and there is now a pressing demand for a cleaner tech-
nology. We have recently demonstrated that the gas phase hydro-
genation of p-chloronitrobenzene over Au supported on alumina
[
12,13,15], Ru [16], bi-metallic La–NiB [9], Pd–Ru [10], Pd–Pt [10],
[3,4] and titania [4] was 100% selective in terms of –NO2 group
reduction to p-chloroaniline over a prolonged (up to 80 h) time
on-stream. However, the level of p-chloronitrobenzene conversion
was appreciably lower than that delivered by supported Pd, which
generated nitrobenzene and aniline (non-selective hydrogenation)
as the principal products with a significant temporal loss of ac-
tivity [3]. Indeed, two possible reaction pathways are associated
Pt–M (M = Cr, Mn, Fe, Co, Ni and Cu) [17] and Pt–M (M = Sn, Pb,
Ge, Al, Zn) [18] catalysts. Xu et al. [19] have reported that the
incorporation of Sn4 in Pd/Al O3 served to limit the degree of
+
2
p-chloronitrobenzene dechlorination in batch liquid reaction and
4+
attributed this effect to interaction between Sn
and the –NO2
group that increased the polarity of the N=O bond and promoted
reduction. However, higher conversions of p-chloronitrobenzene
were accompanied by a decrease in selectivity to p-chloroaniline.
The main goal of our work is to direct the reaction along Path B,
Corresponding author.
*
E-mail address: m.a.keane@hw.ac.uk (M.A. Keane).
0
021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.12.019

236
F. Cárdenas-Lizana et al. / Journal of Catalysis 262 (2009) 235–243
Fig. 1. Reaction pathways associated with the hydrogen mediated conversion of p-chloronitrobenzene.
elevating the level of p-chloroaniline production without the for-
mation of aniline in continuous flow operation.
pared with Pd/Al2O3 which they attributed to Au/Pd interactions,
an effect that extended to unsupported Au/Pd nanoparticles [33].
With a view to elevating the catalytic activity of Au, we have
considered the possible role of Pd, with high activity in gas phase
Catalytic hydrogenation over supported Au has been pro-
posed to depend on the preparation method [20], metal particle
size/morphology [21] and support surface chemistry [22]. The op-
timum supported Au particle size is still a matter of some debate
but it is commonly accepted that, in order to achieve high activ-
ity, the presence of particles smaller than ca. 5 nm is crucial. An
increase in Au dispersion, with a consequent decrease in Au parti-
cle size, increases the proportion of surface low coordination sites
on which H2 can dissociate [23,24]. Nevertheless, there is some
evidence of a lower limit in terms of Au particle size (ca. 2 nm),
below which activity decreases due to quantum effects [25,26].
The ultimate degree of dispersion is dependent on catalyst synthe-
sis and Okumura et al. [27] obtained, in the case of (ca. 1%, w/w)
Au/Al2O3 prepared by deposition-precipitation (DP) and impregna-
tion (IMP), an order of magnitude difference in the mean Au di-
ameter (DP = 3 nm, IMP = 37 nm). Grisel et al. [28] also recorded
an appreciably smaller Au particle size (3–5 nm) by DP synthesis
when compared with IMP. Catalytic activity may also be enhanced
through the use of promoters, which can serve to modify the elec-
tronic properties of the active site [29]. In the case of Au catalysis,
the number of studies that have considered the use of promot-
ers in hydrogenation reactions is very limited but we can flag the
work of Gluhoi et al. [30,31] who demonstrated enhanced activ-
ity in NO reduction over Al2O3-supported combinations of Au with
CeOx and LiO2, ascribing the promotional effect of the metal oxides
to the formation of small and thermally stable Au particles. More-
over, Nutt et al. [32] reported enhanced activity in the hydrogen
mediated dechlorination of C2HCl3 over Au–Pd/Al2O3 when com-
–NO2 group reduction [3,5,34–36], as a promoter. In this pa-
per, we examine, for the first time, the catalytic action of Pd
promoted Au/Al2O3 (Au/Pd ꢀ 8) in the selective reduction of p-
chloronitrobenzene to p-chloroaniline. We also assess the feasibil-
ity of increasing hydrogenation rate, while maintaining exclusivity
in terms of –NO2 reduction, by controlling Au particle size dur-
ing catalyst preparation and compare IMP and DP as two distinct
synthesis routes.
2. Experimental
2.1. Catalyst preparation and activation
The monometallic Au and bimetallic Pd–Au catalysts were pre-
pared by deposition-precipitation (DP) with urea [37] and im-
pregnation (IMP) in excess solvent using Al2O3 (AluC Degussa,
2
−1
1
10 m g ) as support. In the case of the DP synthesis, a suspen-
3
sion of the support (3 g) in distilled water (100 cm ) was placed
in an ultrasound bath in order to disperse the support particles.
The suspension was then placed in a reactor, 190 cm3 of distilled
water was added and the mixture was heated to 353 K at which
point 6 cm3 of a solution of HAuCl4 (10 g dm−3), corresponding to
the desired Au loading (1%, w/w), was added with a ca. 100-fold
excess of urea. The pH of the suspension (initially ca. 2.5) pro-
gressively increased to reach ca. 7 after 16 h. The solids obtained
were separated by centrifugation, washed three times with deion-
ized water (with centrifugation between each washing) and dried

F. Cárdenas-Lizana et al. / Journal of Catalysis 262 (2009) 235–243
237
Table 1
Metal content, residual Cl content, Tmax and associated H2 consumption during TPR, H2 chemisorption values, metal particle size (dp ) and BET surface areas for the mono-
and bimetallic catalysts prepared by deposition/precipitation (DP) and impregnation (IMP).
Catalyst
Au
Au/Pd
Cl content
(ppm)
TPR
Tmax
TPR H2
consumption
(μmol g
H2
uptake
(μmol g
dp
(nm)
BET
(%, w/w)
area
−
1
−1
2
−1
(
K)
)
)
(m
g
)
catalyst
Au
catalyst
Au–DP
0.88
0.87
0.87
0.95
0.81
0.87
–
<200
<200
300
1381
9800
14,300
450
453
454 (434)b
491
62
62
68
75
62
68
33
47
2.9
112
97
a
Pd/Au–DP
Pd/Au–DP
Pd/Au–DP
Au–IMP
88
20
8
–
20
1.9
84 (62)b
109
2.0
a
100 (98)b
98
a
2.7
494
38
4.5
5.3
104
92 (95)
471 (444)c
30 (23)c
a
c
Pd/Au–IMP
a
Refers to total metal (Au and Pd) mean particle size.
b
c
Results for the physical mixture of Pd–DP + Au–DP (see Section 2).
Results for the physical mixture of Pd–IMP + Au–IMP (see Section 2).
−
under vacuum at 298 K for 12 h. For the bimetallic catalysts, an
adequate volume of a solution of PdCl2 (1 g dm ) was added to
3 K min 1. The effluent gas passed through a liquid N2 trap and
changes in H2 consumption were monitored by TCD with data ac-
quisition/manipulation using the TPR Win™ software. The reduced
samples were maintained at the final temperature for 2.5 h in a
−3
the gold solution to give nominal atomic Au/Pd ratios = 5, 10 and
2
+
amino species during
20. Because of the formation of soluble Pd
constant flow of H2, swept with 65 cm3 min
−1
N2 for 1.5 h, cooled
DP with urea, the experimentally determined (see Table 1) metal
loadings corresponded to Au/Pd ratios = 8, 20 and 88, respectively.
to room temperature and subjected to H2 chemisorption using a
pulse (10 μl) titration procedure. BET areas were recorded with a
30% (v/v) N2/He flow using pure N2 (99.9%) as internal standard. At
least 2 cycles of N2 adsorption–desorption in the flow mode were
employed to determine total surface area using the standard single
point method. BET surface areas and H2 uptake values were repro-
ducible to within ± 5%; the values quoted represent the mean. TEM
analysis was performed using a JEOL 100 CX II microscope where
the average metal particle sizes were established from a measure-
ment of at least 300 particles. The size limit for the detection of
gold particles on Al2O3 is ca. 1 nm. Infrared spectroscopy was per-
formed with an IFS 66V Bruker spectrometer using a DRIFTS cell.
The catalyst precursors (10–20 mg) were reduced in situ by TPR
(conditions as described above). The cell was then purged with
3
In the synthesis by IMP, Al2O3 (3 g) was dispersed in 25 cm
3
distilled water in a round bottomed flask (100 cm ), appropriate
−
3
volumes of HAuCl4 (10 g dm ) and (in the case of the bimetal-
−3
lic) PdCl2 (1 g dm ) were added to achieve a Au/Pd ratio equal
to 20 and the resulting slurry was vigorously stirred for 10 min.
Excess water was removed on a rotary evaporator at 353 K over
1
h and then dried under vacuum at 298 K for 12 h. Two ref-
erence monometallic Pd/Al2O3 samples were also prepared by DP
320 ppm Pd, <200 ppm Cl) and IMP (297 ppm Pd, 630 ppm Cl),
(
i.e. with a Pd loading corresponding to that in the Pd–Au samples
where Au/Pd = 20. In the case of Pd–IMP, 3 g Al2O3 were dispersed
in 29 cm3 distilled water, 1.25 cm PdCl2 (1 g dm ) solution were
added and the resulting slurry was stirred vigorously for 1 h. Ex-
cess water was removed on a rotary evaporator at 353 K over a 1 h
period and the sample was dried at 393 K for 12 h. In the prepara-
3
−3
3
−1
He at 298 K before introducing a flow (50 cm min ) of 1% (v/v)
CO in He. The spectrum recorded under He was used as reference
and the intensity of the spectrum under CO/He is expressed as
log(ICO/Iref).
3
tion of Pd–DP, 3 g Al2O3 were dispersed in 150 cm distilled water
at room temperature to which 1.25 cm of a PdCl2 (1 g dm ) solu-
tion was added with continuous stirring. Because of the formation
3
−3
of soluble Pd2 species when urea is used, Na2CO3 was employed
as the precipitating agent. The pH was increased by the dropwise
addition of a Na2CO3 (1 M) solution until it reached 10.5 and the
suspension was stirred for 1 h at room temperature; the solid was
recovered, washed and dried as above.
+
2.3. Catalysis procedure
Reactions were carried out under atmospheric pressure at
T = 393 K, in situ immediately after activation, in a continu-
ous fixed bed vertical glass reactor (l = 600 mm; i.d. = 15 mm).
The catalytic reactor and operating conditions to ensure negli-
gible heat/mass transport limitations, have been fully described
elsewhere [38] but some features, pertinent to this study, are
given below. A layer of borosilicate glass beads served as a pre-
heating zone, ensuring that the p-chloronitrobenzene was va-
porized and reached reaction temperature before contacting the
catalyst. Isothermal conditions (± 1 K) were ensured by diluting
the catalyst bed with ground glass (75 μm); the ground glass
was mixed thoroughly with catalyst before insertion into the
reactor. Reaction temperature was continuously monitored by a
thermocouple inserted in a thermowell within the catalyst bed.
The reactant was delivered to the reactor via a glass/Teflon air-
tight syringe and Teflon line using a microprocessor controlled
infusion pump (Model 100 kd Scientific) at a fixed calibrated
flow rate. A co-current flow of p-chloronitrobenzene and ultra
pure H2 (<1% v/v p-chloronitrobenzene/H2) was maintained at
After preparation, the samples were stored at 298 K under
vacuum (in the dark) in a desiccator. The metal loading was de-
termined (to within ± 2%) by inductively coupled plasma-optical
emission spectrometry (ICP-OES, Vista-PRO, Varian Inc.) from the
diluted extract of aqua regia. The Cl content was measured by sil-
ver potentiometric titration after combustion in a Schöniger flask.
Prior to use in catalysis, the samples (sieved into a batch of 75 μm
3
−1
−1
average diameter) were activated in 60 cm min
H2 at 3 K min
to 573 K (Au/Al2O3) or 773 K (Pd/Al2O3 and Pd–Au/Al2O3), which
was maintained for 2.5 h. After activation, the samples were passi-
vated in 1% (v/v) O2/He at room temperature for off-line analysis.
Physical mixtures of reduced/passivated Pd–DP + Au–DP and Pd–
IMP + Au–IMP (Au/Pd atomic ratio = 20) were also examined.
2.2. Catalyst characterization
4
−1
with an inlet p-chloronitrobenzene molar
BET surface area, temperature programmed reduction (TPR) and
a GHSV = 2 × 10
h
−1
H2 chemisorption were determined using the commercial CHEM-
BET 3000 (Quantachrome) unit. The samples were loaded into
a U-shaped Pyrex glass cell (10 cm × 3.76 mm i.d.) and heated
flow (F ) over the range 0.1–0.4 mmol h , where the H2 content
was up to 200 times in excess of the stoichiometric requirement
(PH2 = 0.92 atm), the flow rate of which was monitored using
a Humonics (Model 520) digital flowmeter. The molar metal (n)
to inlet p-chloronitrobenzene feed rate ratio spanned the range
3
−1
(Brooks mass flow controlled) 5% (v/v) H2/N2
in 17 cm min
to 573 K (Au/Al2O3) or 773 K (Pd/Al2O3 and Pd–Au/Al2O3) at

238
F. Cárdenas-Lizana et al. / Journal of Catalysis 262 (2009) 235–243
−
3
−3
h. In a series of blank tests, passage of p-
1
× 10 –14 × 10
chloronitrobenzene in a stream of H2 through the empty reactor
or over the support alone, i.e. in the absence of Pd, Au or Pd–Au,
did not result in any detectable conversion. The reactor effluent
was frozen in a liquid nitrogen trap for subsequent analysis, which
was made using a Perkin–Elmer Auto System XL gas chromato-
graph equipped with a programmed split/splitless injector and
a flame ionization detector, employing a DB-1 50 m × 0.20 mm
i.d., 0.33 μm film thickness capillary column (J&W Scientific), as
described elsewhere [39]. A chlorine (in the form of HCl) mass
balance was performed by passing the effluent gas through an
−4
−3
aqueous NaOH trap (7.0×10
mol dm , kept under constant ag-
itation at 400 rpm) with independent pH (Hanna HI Programmable
Printing pH Bench-Meter) analysis. Repeated (up to five separate)
catalytic runs with different samples from the same batch of cat-
alyst delivered product compositions that were reproducible to
within ± 8%. p-Chloronitrobenzene (Aldrich, ꢀ99.9%, w/w, purity)
and the 1-butanol solvent (Riedel-de Häen, ꢀ99.5%) were used as
supplied without further purification.
3
. Result and discussion
3.1. Catalyst characterization
3
.1.1. TPR, H2 chemisorption and TEM analysis
The BET surface areas of the activated catalysts (see Table 1)
2
−1
were close to that of the Al2O3 support (110 m g ). The TPR ac-
tivation profiles for Au/Al2O3 prepared by DP (Profile I) and IMP
(
Profile V) are presented in Fig. 2; the temperatures associated
with maximum H2 consumption (Tmax) are given in Table 1. Hy-
drogen consumption during TPR matched (to within ± 8%) that
required for the reduction of the supported Au precursors. The re-
duction profile for Au–DP is characterized by a single positive peak
at 450 K, which is consistent with the TPR results for Au/Al2O3
prepared by DP that have been reported in the literature [40–43].
3
+
The occurrence of a single TPR peak has been attributed to Au
reduction to Au0 [42,44]. TPR of our Au–IMP sample generated
a single Tmax at 494 K, which is in good agreement with the
work of Baatz and Prüße [41] and Gluhoi et al. [43]. Bus and
co-workers [45], in their analysis of Au/Al2O3 prepared by IMP
using time-resolved in situ XAS, established the presence of Al2O3-
Fig. 2. TPR profiles for: (I) Au–DP; (II) Pd/Au–DP (Au/Pd = 88); (III) Pd/Au–DP
(
Au/Pd = 20); (IV) Pd/Au–DP (Au/Pd = 8); (V) Au–IMP; (VI) Pd/Au–IMP (Au/Pd =
3
+
species in the precursor that
supported oxidic or hydroxidic Au
20); (VII) Au–IMP + Pd–IMP (Au/Pd = 20); (VIII) Au–DP + Pd–DP (Au/Pd = 20).
were reduced to Au0 in H2 at 440 K. The higher reduction temper-
ature recorded for Au–IMP compared with Au–DP may be related
to the greater residual Cl (see Table 1) associated with the for-
mer, which can affect the reducibility of Au(III) [45–49]. Moreover,
the mobility of the Cl containing precursor after impregnation is
known to induce Au sintering during activation [50–52]. This can
account for the larger mean Au particle size in the Au–IMP sample
reduction of both metals, suggesting some interaction between Au
and Pd species that modifies their respective reducibility. Hosseini
et al. [55], using co-deposited Au–Pd/TiO2 noted a shift in the re-
duction peak for PdO (with the introduction of Au) from 433 (100
Pd at%) to 373 K (33 Pd at%). In the TPR of Pd/Au–IMP (Profile
VI), the reduction peak appears at a lower temperature than that
recorded for Au–IMP. This response is the direct opposite of that
observed for the DP systems, possibly suggesting a different degree
of interaction between Au and Pd in the IMP sample, which may
be affected by the Cl content. The TPR profiles generated for both
physical mixtures (Profiles VII and VIII) exhibit a similar response
in that the onset of H2 consumption during reduction was shifted
to lower temperatures (with a lower Tmax, see Table 1) than that
observed for the Au/Al2O3 systems. The inclusion of Pd/Al2O3 in
the mixture must serve to facilitate the reduction of Au/Al2O3, pos-
sibly through the involvement of spillover hydrogen generated by
the Pd component.
(see Table 1). Representative TEM images are included in Fig. 3,
which serve to illustrate the nature of the metal dispersion in
these catalysts. It can be seen that the Au component is present
as discrete particles with a quasi-spherical morphology.
Taking the Pd/Au–DP samples (see Fig. 2, Profiles II, III and IV),
a progressive shift of the reduction peak to higher temperatures
was observed with increasing Pd content. In previous studies [3,5],
we have recorded a negative peak (H2 release at 340–380 K) dur-
ing the TPR of Pd/Al2O3 that we attributed to the decomposition
of β-Pd hydride formed by the absorption of H2 on zero valent Pd
when the H2 pressure exceeds 0.02 atm [53]. In the case of our
Pd/Au samples, there was no detectable H2 release but this can be
due to the low Pd content. In addition to H2 release/hydride de-
composition, Pd/Al2O3 has been reported to exhibit H2 consump-
tion where T > 573 K due to the reduction of oxidized Pd species
that interact strongly with the support [54]. The broad peaks ob-
served for Pd/Au–DP (453–491 K) may be the result of a composite
Hydrogen uptake (see Table 1) on both Au/Al2O3 samples was
low, which is consistent with reports [56,57] that have demon-
strated a limited capacity of Au to chemisorb hydrogen. This can
be linked to the high activation energy barrier required for the
dissociative adsorption of hydrogen on Au [45,58]. The incorpo-

F. Cárdenas-Lizana et al. / Journal of Catalysis 262 (2009) 235–243
239
Fig. 3. Representative TEM images of (I) Au–DP, (II) Au–IMP, (III) Pd/Au–DP (Au/Pd = 20) and (IV) Pd/Au–IMP (Au/Pd = 20).
ration (chemically or as a physical mixture) of Pd resulted in an
increase in H2 uptake. In the case of the DP samples (see Ta-
ble 1), it was proportional to the Pd content. The higher uptake on
Pd/Au–DP (Au/Pd = 20) compared with the corresponding physical
mixture suggests a surface Pd–Au synergy that serves to increase
the chemisorption capacity. In the case of the IMP system, the
inclusion of Pd did not result in an increase in H2 uptake. This re-
sponse may be due to the higher Cl content that acts to suppress
H2 chemisorption allied to the larger particle size observed by TEM
lated to bridged (two-fold bound, 2000–1895 cm 1) and three-fold
−
−1
bound carbonyl species (1920–1830 cm ) on metallic Pd [60,61].
The Au–DP sample (Profile III) shows an intense single band at
−1
2105 cm , which has been previously associated with CO lin-
early adsorbed on low coordination sites of metallic Au nanoparti-
cles [62–64]. The spectrum of Au–IMP (Profile IV) is characterized
−
1
by a broad band with a maximum at 2063 cm
and a weak
−1
broad band at 2127 cm . The presence of bands at wave num-
bers lower than those for CO linearly adsorbed on metallic Au
(around 2100 cm ) has been reported previously [65–69]. The
formation of bridged CO species can be discounted as they give
−1
(see Table 1). Indeed, it has been shown [47,59] that Cl and Br can
act to inhibit CO uptake on Au where CO also adsorbs on low coor-
dination surface sites. The monometallic Pd reference catalysts (DP
or IMP) did not generate any signal during TPR and did not exhibit
any measurable H2 chemisorption, which we attribute to the very
low Pd content resulting in experimental measurements that fell
below instrumental detector limits.
−1
rise to bands around 2000–1950 cm
the region 2050–2080 cm
[65,66]. Bands observed in
can be ascribed either to CO adsorbed
−1
on metallic sites at the support/particle interface [67,68] or to CO
δ−
adsorbed on negatively charged Au
of Au
sites [65,68]. The presence
δ−
in Au–IMP can result as a consequence of the high chlo-
ride content (Table 1). The residual chloride may be located either
on the alumina support in close proximity to the Au particles or
associated directly with the Au particles as reported in recent pa-
3
.1.2. DRIFTS analysis
DRIFTS analysis using CO as a probe molecule was performed
−
and the resultant spectra are given in Fig. 4. The spectra of the
carbonyl region for Pd–DP (Profile I) and Pd–IMP (Profile II) are
similar and can be divided into two regions, i.e. high (2200–
pers [47,70,71]. Computer modeling of Cl interactions with Au
−
0
(111) has revealed that Cl adsorption on Au should result in a
0.05 positive electric charge associated with the Cl atom while the
conjoined Au atom should develop a 0.190 negative electric charge
where the other surface Au atoms adopt a negative charge from
− −1
1
2
000 cm ) and low (2000–1800 cm ) wave numbers. The bands
at high wave numbers have been assigned in the literature [60] to
linearly adsorbed carbonyl species on different Pd crystal planes
−
1
0.05 to 0.08 [72]. The weak band at 2127 cm
has been previ-
−1
(2105–2070 cm ). Those at low wave numbers have been re-
ously assigned either to CO adsorbed on metallic Au at low CO

240
F. Cárdenas-Lizana et al. / Journal of Catalysis 262 (2009) 235–243
Fig. 4. DRIFTS spectra (under 1%, v/v, CO/He at 298 K) of (I) Pd–DP, (II) Pd–IMP, (III) Au–DP, (IV) Au–IMP, (V) Pd/Au–IMP (Au/Pd = 20), (VI) Pd/Au–DP (Au/Pd = 8) and
(VII) Pd/Au–DP (Au/Pd = 20).
δ+
creases in intensity of the band due to linear CO on Pd0 (with
the decrease in intensity of the band due to linear CO on Au )
coverage [20,60,73] or to CO-Au species [74–77]. At this juncture,
we cannot propose a definitive interpretation for this band but it
is important to note that the IR spectra of Au–IMP and Au–DP are
quite distinct. We associate these differences with the higher chlo-
ride content and the slightly larger Au particle size exhibited by
Au–IMP.
0
represent evidence for the formation of bimetallic particles. The
dilution of Pd in Au limits the extent of multi-bonded CO on Pd,
especially three-fold bridge CO formation and thus favors linear in-
teraction between CO and any isolated Pd0 sites. The spectra of our
Au–Pd samples also show bands at higher wave numbers (2110–
In the case of the three spectra (Profiles V–VII) for the bimetal-
lic catalysts, we also observed carbonyl bands at high (2150–
−1
2135 cm ), which were not systematically observed on Au–Pd
films [78,80] or Au–Pd clusters [79]. However, in the latter case,
a frequency shift of the band for CO linearly adsorbed on Au clus-
− −1
1
2
050 cm ) and low (2000–1900 cm ) wave numbers but the
spectra deviate significantly from that expected on the basis of the
sum of the monometallic Au and Pd contributions (combination
of Profiles I and III and Profiles II and IV). The bands at low wave
numbers can again be assigned to CO bridged species on Pd crystal
planes. However, the broad and poorly resolved feature that char-
acterized the monometallic Pd samples (Profiles I and II) is trans-
−1
ters (from 2109 to 2129 cm ) was noted with a decrease in CO
coverage. Accordingly, we tentatively assign the bands at 2110–
2135 cm to CO linearly adsorbed on Au0 sites in monometallic or
bimetallic particles. Our DRIFTS results indicate that, in the case of
the Pd–Au samples, the contribution of multi-bonded CO on Pd is
reduced with respect to the linearly bonded CO on Pd. A geometric
effect, due to the formation of bimetallic Pd–Au nanoparticles and
resulting in a decrease in the size and/or the number of Pd ensem-
bles required for multiple bonding (three-fold), can account for this
response [78,79,81]. This suggests that there is a larger proportion
of isolated singleton Pd sites in the Pd–Au/Al2O3 catalysts prepared
by both DP and IMP. It should be noted that the slight shift of the
band due to CO linearly adsorbed on Pd0 in the Au–Pd samples
has also been also observed by Goodman et al. [78] and attributed
to changes in dipole–dipole coupling due to variations in CO inter-
molecular distances rather than to chemical interaction mediated
by the substrate, i.e. an electronic effect.
−1
−1
formed into two resolved peaks at 1955 and 1935 cm
for the
bimetallic Pd–Au catalysts, i.e. at a higher wave number than that
which characterizes three-fold bound carbonyl species on Pd. These
observations suggest the absence of three-fold CO bridges and the
presence of only two-fold bridged species. At higher frequencies, a
is observed for all the Pd–Au catalysts at a
similar frequency to that of linear adsorbed CO on Pd but with a
−1
band at ∼2082 cm
−1
greater intensity. It should be noted that the band at 2105 cm
,
associated with CO linearly adsorbed on gold, exhibits an appre-
ciable decrease in intensity. By analogy with observations reported
for Au–Pd films [78] and Au–Pd clusters on silica films [79], the
disappearance of the three-fold bridge band and concomitant in-

F. Cárdenas-Lizana et al. / Journal of Catalysis 262 (2009) 235–243
241
−
1
−1
h ) and
Fig. 5. Variation of p-chloronitrobenzene fractional conversion (xp-CNB) with time-on-stream over (P) Au–IMP (p-chloronitrobenzene/Au = 74 molp-CNB mol
Au
−
1
−1
(
1) Au–DP (p-chloronitrobenzene/Au = 284 molp-CNB mol
h ). Inset: Pseudo-first order kinetic plot for the hydrogenation of p-chloronitrobenzene over Au–DP.
Au
3
.2. Catalytic response
Table 2
Pseudo-first order initial rate constants (k) for the hydrogenation of p-chloro-
nitrobenzene over mono- and bimetallic catalysts and monometallic physical mix-
tures with product selectivities at the same initial fractional conversion (x0 ∼ 0.12).
Hydrogenation of p-chloronitrobenzene over Au–DP and Au–
IMP generated exclusively the target product, p-chloroaniline, with
no evidence of any hydrodenitrogenation, hydrodechlorination or
ring reduction, in agreement with our earlier work [3,4]. The time-
on-stream profiles for the conversion of p-chloronitrobenzene over
both Au/Al2O3 catalysts are shown in Fig. 5 where it can be seen
that Au–DP delivered a consistently higher fractional conversion
Catalyst
Au/Pd
Rate constant
Product(s)
(% selectivity)
−1
(k, h
)
Au–DP
–
12
24
39
92
p-Chloroaniline (100)
p-Chloroaniline (100)
p-Chloroaniline (100)
p-Chloroaniline (80)
Nitrobenzene (20)
p-Chloroaniline (100)
p-Chloroaniline (100)
Nitrobenzene (80)
Aniline (20)
Pd/Au–DP
Pd/Au–DP
Pd/Au–DP
88
20
8
(
xp-CNB) than Au–IMP. The temporal variation of activity can be
Au–IMP
Pd/Au–IMP
Pd–IMP + Au–IMP
–
20
20
1
22
130
represented by [82]
(
xp-CNB − x0)
ꢀt
=
,
(
x
5
− x0)
(β + ꢀt)
h
Pd–DP + Au–DP
20
77
Nitrobenzene (45)
Aniline (55)
where x5 h represents fractional conversion after 5 h on-stream
and β is a time scale fitting parameter. Fit convergence yields val-
ues for x0, the initial fractional conversion, which is a measure of
initial activity. The applicability of pseudo-first order kinetics can
be tested using the following kinetic expression
preponderance of uncoordinated Au atoms associated with the ti-
tania support. Jia et al. [87] have recorded a maximum acetylene
hydrogenation over Au/Al2O3 bearing Au particles of 3 nm diam-
eter while Zanella et al. [24] have reported a significant increase
in crotonaldehyde hydrogenation turnover frequency (TOF) over
Au/TiO2 with a decrease in Au particle size from 8.7 to 1.7 nm. The
higher TOF exhibited by smaller Au particles may be associated
with an increased capacity to activate/dissociate hydrogen [23,24].
Stobi n´ ski et al. [88] proposed that the chemisorption of H2 is facil-
itated on Au surface atoms with low coordination number to form
a AuH2 complex. The lower activity associated with Au–IMP may
also be related to the higher residual Cl content (Table 1). Indeed,
we have demonstrated elsewhere [46] that the presence of chlo-
ride in Au/Al2O3 samples served to lower butadiene hydrogenation
activity regardless of Au particle size. DRIFTS analysis has indicated
that the presence of surface Cl species can induce strong modifica-
tions to the electronic state of the Au particles, which may affect
reactant adsorption/activation and impact on hydrogenation perfor-
mance. Indeed, there is evidence in the literature [47,49,70,72] that
halides can adsorb on metallic gold with a resultant suppression
of reactant adsorption. In short, the observed catalytic response
for the monometallic samples where Au–DP outperformed Au–IMP
can be attributed to a composite effect due to higher intrinsic ac-
tivity for smaller Au particles (Au–DP) and the deleterious effects
of surface Cl (Au–IMP).
ꢀ
ꢁ
n
F
ln(1 − x0)− = k
1
and is demonstrated in the inset to Fig. 5 for Au–DP. The extracted
pseudo-first order rate constants are given in Table 2. Convert-
ing the raw (experimental) rate constants to specific rates (per
Au surface area, based on the mean Au particle size obtained
from TEM), Au–DP delivered a rate (418 μmol m
over an order of magnitude higher than that recorded for Au–
−
2
−1
h
) that was
Au
−
2
−1
h ). This suggests that smaller Au particles
IMP (29 μmol m
Au
are intrinsically more active. Taking an overview of the available
literature, the role of Au particle size in hydrogenation reactions
appears to be dependent on the nature of the reaction [83], the
support [84] and even the catalyst preparation method [85]. Al-
though under optimum reaction conditions, Au particles as large as
2
0 nm can exhibit significant activity [86], the general trend sug-
gests increasing hydrogenation efficiency over smaller Au particles
(
ꢁ10 nm) [56]. Mohr et al. [21] have considered the hydrogena-
tion of acrolein to allyl alcohol over Au/TiO2 and Au/ZrO2 catalysts
which exhibited similar Au particle sizes. The Au/TiO2 catalyst was
approximately twice as active and more selective in terms of al-
lyl alcohol production, a response that was attributed to a greater

242
F. Cárdenas-Lizana et al. / Journal of Catalysis 262 (2009) 235–243
tion of both metals in the physical mixtures (leading to the for-
mation of nitrobenzene and aniline). There is, however, an upper
limit to the amount of Pd that can be incorporated in the Au sys-
tem (Au/Pd ꢁ 8), above which the selectivity of Au is diminished
by the catalytic action of Pd, resulting in hydrodechlorination with
the formation of nitrobenzene. The presence of bimetallic Au–Pd
particles is consistent with our TPR and DRIFTS measurements.
A feature of the IMP synthesis has been the incorporation of a
significant residual Cl content which impacts on the reducibility of
gold and induces metal particle sintering with an overall decrease
in hydrogenation activity. Our results demonstrate that Pd–Au syn-
thesis by DP with urea is an effective route for the production of
efficient supported bimetallic particles. More detailed characteri-
zation at the scale of individual metal nanoparticles is, however,
necessary to explicitly establish the bimetallic character of the sup-
ported particles. Advanced electron microscopy techniques, such as
high angle annular dark field (HAADF) or energy filtered transmis-
sion electron microscopy (EFTEM) could provide this level of anal-
ysis. This is far from straightforward as the low proportion of Pd
present renders detection very difficult. Nevertheless, the results
presented here demonstrate that the use of Pd as a promoter in Au
systems prepared by either co-deposition-precipitation and/or co-
impregnation results in the formation of bimetallic Au–Pd particles
that promote the exclusive hydrogenation of p-chloronitrobenzene
to p-chloroaniline. The promotional effect of Pd has resulted in a
significant increase in hydrogenation rate while still retaining the
selectivity due to Au. Future work will examine the applicability of
these bimetallic Au–Pd systems to other industrially relevant sub-
stituted aromatic hydrogenation processes with a consideration of
alternative promoters.
Fig. 6. p-Chloroaniline output (μmoles p-CANOUT) as a function of the number of
moles of p-chloronitrobenzene that had been processed (μmoles p-CNBIN) over
Au–DP (1) and Pd/Au–DP (2, Au/Pd = 20). Inset: Au–IMP (P) and Pd/Au–IMP
−2
−1
(
Q, Au/Pd = 20); inlet Au/p-chloronitrobenzene = 1 × 10
molmetal h mol
.
p-CNB
In earlier work [3], we reported that the incorporation of Au
with Pd via reductive deposition to generate Au–Pd/Al2O3 (where
Pd/Au = 10) did not influence p-chloronitrobenzene hydrogena-
tion activity or selectivity, which were equivalent to that delivered
by Pd/Al2O3, where aniline was the predominant product. In this
study, the inclusion of a low Pd content to Au/Al2O3 by DP and
IMP (Au/Pd > 8) was considered and the results are presented in
Fig. 6 and Table 2. The presence of Pd served to significantly raise
the hydrogenation rate constant, which increased with increasing
Pd content (Table 2). Where Au/Pd ꢀ 20, the reaction was 100% se-
lective in terms of p-chloroaniline production with no detectable
by-products. At a lower Au/Pd ratio, reaction over Pd–Au/Al2O3–DP
4
. Conclusions
The catalytic results presented in this study establish Pd pro-
motion of selective hydrogenation over supported Au, which can
be exploited as a cleaner sustainable route for the production of
p-chloroaniline. Both Au–DP and Au–IMP catalyze the exclusive
hydrogenation of p-chloronitrobenzene to p-chloroaniline where
Au–DP delivered a (greater than) order of magnitude higher spe-
cific hydrogenation rate, which we can link to smaller Au parti-
cle size and a lesser residual Cl content. The inclusion of Pd (at
Au/Pd ꢀ 20, IMP and DP) resulted in increased activity while re-
taining exclusivity to p-chloroaniline. At higher Pd contents, a fur-
ther increase in reaction rate was accompanied by the formation
of nitrobenzene as a result of a Pd promoted hydrodechlorination.
DRIFTS analysis using CO as a probe molecule has demonstrated a
decrease in the contribution due to multi-bonded CO on Pd rela-
tive to linearly bonded CO in the bimetallic samples, an effect that
we take to be diagnostic of Pd–Au surface interaction.
(
Au/Pd = 8) generated p-chloroaniline and nitrobenzene, the latter
resulting from a hydrodechlorination step (see Fig. 1), which is a
characteristic of catalysis by Pd. The combination of both Au/Al2O3
and Pd/Al2O3 (DP and IMP samples) as physical mixtures (Au/Pd =
2
0) resulted in the formation of nitrobenzene and aniline (compos-
ite hydrodechlorination/hydrogenation product). Such a response
demonstrates that, under these reaction conditions, Au/Al2O3 had
little effect on reaction selectivity, which was governed by the
Pd/Al2O3 component. This result is consistent with catalytic action
due to segregated Pd and Au particles where the catalytic prop-
erties of Pd dominate. At Au/Pd = 20, the rate constants delivered
by the physical mixtures were appreciably higher than that deliv-
ered by Pd–Au/Al2O3 (both DP and IMP). Allied to the quite dis-
tinct product composition (at an equivalent p-chloronitrobenzene
conversion, see Table 2), these results demonstrate that the con-
tribution of Pd and Au to the catalytic action of Pd–Au/Al2O3 is
not merely an additive effect and is the result of a surface Pd/Au
synergism. The p-chloroaniline produced (μmoles p-CANOUT) as a
function of the number of moles of p-chloronitrobenzene that had
been processed (μmoles p-CNBIN) is illustrated in Fig. 6 where it
can be seen that the inclusion of Pd ensured an appreciably higher
output of the target product for both DP and IMP bimetallics. These
results establish the viability of Pd promoted selective –NO2 reduc-
tion over supported Au.
Acknowledgments
The authors are grateful to Maria-Luz Pérez-Cuevas and Noémie
Perret for their contribution to the work and financial support
from EPSRC through Grant 0231 110525. CNRS and financial sup-
port from the World Gold Council are also acknowledged.
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