Gas phase hydrogenation of nitroarenes: A comparison of the catalytic action of titania supported gold and silver

Article abstract of DOI:10.1016/j.molcata.2010.04.006

We have examined the catalytic action of 1 mol% Au/TiO₂ and Ag/TiO₂ in the continuous gas phase hydrogenation of a series of para-substituted (-H, -OH, -OCH₃, -CH₃, -Cl and -NO₂) nitroarenes. Both catalysts promoted exclusive -NO₂ group reduction, resulting in the sole formation of the corresponding amino-compound. The catalysts have been characterized in terms of temperature-programmed reduction (TPR), H₂ chemisorption, BET area, diffuse reflectance UV-vis, X-ray diffraction and HRTEM measurements. The formation of zero valent Au and Ag is established post-TPR. Au/TiO₂ showed a narrower metal particle size (1-10 nm) distribution than Ag/TiO₂ (1-15 nm) but both catalysts exhibited a similar surface area weighted mean size (6-7 nm). A time-invariant nitroarene conversion has been established where Au/TiO₂ delivered higher specific hydrogenation rates. We associate this response to an enhanced reactant activation on Au/TiO₂ to generate a negatively charged intermediate, consistent with a nucleophilic mechanism. The presence of electron-withdrawing substituents is shown to enhance -NO₂ reduction rate. This effect is quantified in terms of the Hammett relationship where a linear correlation between the substituent constant (σ_i) and rate is established and a higher reaction constant (ρ) was recorded for Au/TiO₂ (0.93) relative to Ag/TiO₂ (0.22). The data generated provide the first direct comparison of the catalytic action of supported Au and Ag in the hydrogenation of substituted nitroarenes and establish the viability of both catalysts to promote selective -NO₂ group reduction.

Full text of DOI:10.1016/j.molcata.2010.04.006

Journal of Molecular Catalysis A: Chemical 326 (2010) 48–54
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Gas phase hydrogenation of nitroarenes: A comparison of the catalytic action of
titania supported gold and silver
Fernando Cárdenas-Lizanaa, Zahara M. de Pedrob, Santiago Gómez-Queroa, Mark A. Keanea,∗
a Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, United Kingdom
b Ingeniería Química, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
We have examined the catalytic action of 1 mol% Au/TiO2 and Ag/TiO2 in the continuous gas phase hydro-
genation of a series of para-substituted (–H, –OH, –OCH3, –CH3, –Cl and –NO2) nitroarenes. Both catalysts
promoted exclusive –NO2 group reduction, resulting in the sole formation of the corresponding amino-
compound. The catalysts have been characterized in terms of temperature-programmed reduction (TPR),
Received 12 January 2010
Received in revised form 15 March 2010
Accepted 11 April 2010
Available online 18 April 2010
H2 chemisorption, BET area, diffuse reflectance UV–vis, X-ray diffraction and HRTEM measurements. The
formation of zero valent Au and Ag is established post-TPR. Au/TiO2 showed a narrower metal parti-
cle size (1–10 nm) distribution than Ag/TiO2 (1–15 nm) but both catalysts exhibited a similar surface
area weighted mean size (6–7 nm). A time-invariant nitroarene conversion has been established where
Au/TiO2 delivered higher specific hydrogenation rates. We associate this response to an enhanced reac-
tant activation on Au/TiO2 to generate a negatively charged intermediate, consistent with a nucleophilic
mechanism. The presence of electron-withdrawing substituents is shown to enhance –NO2 reduction
rate. This effect is quantified in terms of the Hammett relationship where a linear correlation between
the substituent constant (ꢀi) and rate is established and a higher reaction constant (ꢁ) was recorded for
Au/TiO2 (0.93) relative to Ag/TiO2 (0.22). The data generated provide the first direct comparison of the
catalytic action of supported Au and Ag in the hydrogenation of substituted nitroarenes and establish the
viability of both catalysts to promote selective –NO2 group reduction.
Keywords:
Au/TiO2
Ag/TiO2
Selective hydrogenation
Nitroarenes
Hammett equation
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
variations in catalyst preparation [9,14] and the incorporation
of a second metal [15,16]. Gold and (to a lesser extent) silver
The catalytic hydrogenation of poly-functional nitro-
compounds to the corresponding amino-product is of commercial
importance in the manufacture of a diversity of herbicides, dyes
and pharmaceuticals [1]. Exclusivity in terms of –NO2 group
reduction is difficult to achieve in the presence of other reactive
substituents (e.g. –Cl, –CH3 and/or –OH), as has been noted for
the hydrogenation of a range of aliphatic [2] and aromatic [3]
nitro-compounds in both gas [4,5] and liquid [6,7] phase opera-
tion. The conventional synthesis route for amino-derivates, using
stoichiometric amounts of Fe in acid media (Béchamp process) is
no longer sustainable due to the production of Fe/FeO sludge waste
and low selectivites/product yields [8]. The catalytic (liquid phase)
alternative using standard transition metals (e.g. Ni [9], Pd [10],
and/or Pt [7]) also exhibits limitations in terms of undesirable toxic
secondary reaction products, i.e. azo- [11] and/or azoxy-derivates
[12]. Various attempts have been reported to enhance selectivity
to the target amino-compound, through the use of additives [13],
have been successfully used in the hydrogenation of CO2 [17],
NOx [18,19], alkenes [20] and ␣,␤-unsaturated aldehydes [21,22].
Moreover, recent studies of batch liquid phase nitroarene reduc-
tion over supported Au [14,23] and Ag [24–26] show potential
in terms of high selectivity to amino-compound formation. We
have previously demonstrated exclusive -NO2 group reduction in
the hydrogenation of chloro- [4,27–29] and dinitro-benzene [30]
over supported Au in continuous gas phase operation. To the best
of our knowledge, the catalytic action of supported Ag in the gas
phase hydrogenation of nitro-compounds has not been reported
in the literature. In this study, we compare the catalytic perfor-
mance of (titania) supported Ag and Au in the hydrogenation of a
series of substituted nitro-compounds to commercially important
amino-derivates.
In general, supported Au [17,31,32] and/or Ag [17,21,33] exhibit
lower hydrogenation activities relative to typical transition metal
catalysts, e.g. supported Ni [31], Pd [33], Pt [32] or Rh [21]. There is
evidence [22,34–36] of a structure sensitive response in the case
of Au where increased efficiency is attributed to smaller parti-
cles (≤10 nm). Theoretical calculations have demonstrated a high
energy barrier for H2 dissociation on group IB metals [37] and, in
∗
Corresponding author. Tel.: +44 0131 4514719.
E-mail address: M.A.Keane@hw.ac.uk (M.A. Keane).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.04.006

F. Cárdenas-Lizana et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 48–54
49
the case of Au catalysts, H2 activation has been associated with step,
edge and corner sites on small Au particles [38]. While we could not
find any study that explicitly compares the catalytic performance
of Au with Ag in nitroarene reduction, we can flag related reports
dealing with hydrogen-mediated reactions. Deng et al. [39] stud-
ied the hydrogenation of anthracene with NaBH4 as reducing agent
over (unsupported) Ag and Au nano-particles and achieved higher
rates over the former for particles below 4 nm, a response that they
associated with a greater capacity of Ag to generate hydrogen for
reaction. Słoczyn´ ski et al. [17] considered M/(ZnO·ZrO2) (M = Cu,
Au and Ag; mole ratio ZnO/ZrO2 = 3) and reported a similar yield of
methanol (from CO2) over the Au and Ag catalysts that was lower
than that for Cu/(ZnO·ZrO2), attributing this response to a low sta-
bilization of the Ag+ and Au+ ions under reaction conditions. In
contrast, Wambach et al. [40] observed the exclusive formation of
methanol over Ag/ZrO2 while methane, methanol and CO were gen-
erated over Au/ZrO2. We provide, in this report, a direct comparison
of titania supported Au and Ag (at 1 mol% loading) in the selective
hydrogenation of a series of para-substituted nitroarenes where the
catalytic response has been correlated to critical characterization
measurements.
JCPDS-ICDD reference standards, i.e. anatase (21-1272), rutile (21-
1276), Au (04-0784) and Ag (04-0783). Diffuse reflectance UV–vis
(DRS UV–vis) measurements were conducted using a PerkinElmer
Lambda 35 UV-vis Spectrometer with BaSO4 powder as reference;
absorption profiles were calculated from the reflectance data using
the Kubelka–Munk function. Transmission electron microscopy
analysis was conducted using a JEOL JEM 2011 HRTEM unit with a
UTW energy dispersive X-ray detector (EDX) (Oxford Instruments)
operated at an accelerating voltage of 200 kV, employing Gatan Dig-
italMicrograph 3.4 for data acquisition/manipulation. Samples for
analysis were prepared by dispersion in acetone and deposited on a
holey carbon/Cu grid (300 mesh). Up to 600 individual metal parti-
cles were counted for each catalyst and the surface area-weighted
metal diameter (dTEM) was calculated from
ꢀ
nidi3
i
dTEM
=
(1)
ꢀ
nidi2
i
where ni is the number of particles of diameter di. The size limit for
the detection of metal particles on TiO2 is ca. 1 nm.
2. Experimental
2.1. Materials and catalyst preparation
2.3. Catalytic procedure
The TiO2 (Degussa, P25) support was used as received. Two
(1 mol%) TiO2 supported Au and Ag catalyst precursors were
prepared by standard impregnation where adequate volumes of
HAuCl4 (Aldrich, 25 × 10−3 g cm−3, pH = 2) and AgNO3 (Riedel-de
Haën, 1 × 10−3 g cm−3) solutions were added to 5 g of support. The
slurry was heated (2 K min−1) to 353 K and maintained under con-
stant agitation (600 rpm) in a He purge. The solid residue was dried
in a flow of He at 383 K for 3 h and stored under He in the dark
at 277 K. Prior to use in catalysis, the samples (sieved into a batch
of 75 ␮m average diameter) were activated in 60 cm3 min−1 H2 at
2 K min−1 to 603 1 K, which was maintained for 2.5 h. After acti-
vation, the samples were cooled and passivated in 1% (v/v) O2/He
at 298 K for off-line analysis.
Reactions were carried out under atmospheric pressure, in situ
immediately after activation, in a fixed bed vertical continuous
glass reactor (l = 600 mm, i.d. = 15 mm) at T = 473 K. The catalytic
reactor, and operating conditions to ensure negligible heat/mass
transport limitations, have been fully described elsewhere [41] but
some features, pertinent to this study, are given below. A pre-
heating zone (layer of borosilicate glass balls) ensured that the
nitroarene reactant was vaporized and reached reaction temper-
ature before contacting the catalyst. Isothermal conditions ( 1 K)
were maintained by thoroughly mixing the catalyst with ground
glass (75 ␮m) before insertion into the reactor. The tempera-
ture was continuously monitored by a thermocouple inserted in
a thermowell within the catalyst bed. Butanolic solutions of the
nitroarene reactants were delivered, in a co-current flow of H2, via
a glass/teflon air-tight syringe and a teflon line, using a micropro-
cessor controlled infusion pump (Model 100 kd Scientific) at a fixed
2.2. Characterization analyses
Temperature-programmed reduction (TPR) response, BET area
and H2 chemisorption measurements were recorded using the
commercial CHEMBET 3000 (Quantachrome Instrument) unit
with data acquisition/manipulation using the TPR WinTM soft-
ware. The samples were loaded into a U-shaped Pyrex glass cell
(100 mm × 3.76 mm i.d.) and heated in 17 cm3 min−1 (Brooks mass
flow controlled) 5% (v/v) H2/N2 to 603 K at 2 K min−1. The effluent
gas passed through a liquid N2 trap and changes in H2 consump-
tion were monitored by TCD (thermal conductivity detector). The
reduced samples were swept with 65 cm3 min−1 N2 for 1.5 h, cooled
to room temperature and subjected to H2 chemisorption using a
pulse (10–50 ␮l) titration procedure. Hydrogen pulse introduction
was repeated until the signal area was constant, indicating surface
saturation. BET areas were recorded with a 30% (v/v) N2/He flow;
pure N2 (99.9%) served as the internal standard. At least two cycles
of N2 adsorption–desorption in the flow mode were used to deter-
mine total surface area using the standard single point method.
BET surface area and H2 uptake values were reproducible to within
5% and the values quoted in this paper represent the mean. Pow-
der X-ray diffractograms were recorded on a Bruker/Siemens D500
incident X-ray diffractometer using Cu K␣ radiation. The samples
were scanned at a rate of 0.02◦ step−1 over the range 20◦ ≤ 2ꢂ ≤ 90◦
(scan time = 5 s step−1). Diffractograms were identified using the
calibrated flow rate, with an inlet –NO2 molar flow (F
2 ) over
–NO
the range 0.01–0.11 mmol
2 h−1, where the molar metal to inlet
–NO
molar –NO2 feed rate ratio spanned the range 2 × 10−2–27 × 10−2 h.
The H2 content was at least 150 times in excess of the stoichio-
metric requirement, the flow rate of which was monitored using
a Humonics (Model 520) digital flowmeter; GHSV = 2 × 104 h−1. In
a series of blank tests, passage of each nitroarene in a stream of
H2 through the empty reactor or over the support alone, i.e. in
the absence of Au or Ag, did not result in any detectable con-
version. The reactor effluent was frozen in a liquid nitrogen trap
for subsequent analysis, which was made using a PerkinElmer
Auto System XL gas chromatograph equipped with a programmed
split/splitless injector and a flame ionization detector, employ-
ing a DB-1 50 m × 0.20 mm i.d., 0.33 ␮m film thickness capillary
column (J&W Scientific), as described elsewhere [42]. The reac-
tants: nitrobenzene, p-nitrophenol, p-nitroanisole, p-nitrotoluene,
p-chloronitrobenzene and p-dinitrobenzene (Aldrich, ≥98%) and
the solvent (1-butanol: Riedel-de Haën) were used as supplied
without further purification. Repeated catalytic runs with different
samples from the same batch of catalyst delivered product com-
positions that were reproducible to within 6%. Hydrogenation
activity is expressed in terms of the degree of nitro-group reduction

50
F. Cárdenas-Lizana et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 48–54
Table 1
BET surface area, hydrogen consumed (theoretical and experimentally determined) during activation by TPR, H2 chemisorption values, DRS UV–vis characteristics, metal
particle size (surface area weighted mean and range), specific metal surface area and pseudo-first order rate constants for the hydrogenation of nitrobenzene (to aniline)
associated with Au/TiO2 and Ag/TiO2.
Catalyst
Au/TiO2
Ag/TiO2
BET area (m2 g−1
)
47
193
195
15
48
130
136
5
TPR H2 consumption/theoretical (␮mol g−1
)
TPR H2 consumption/experimental (␮mol g−1
)
H2 chemisorption (␮mol g−1
)
metal
DRS UV–vis Amax (nm)
dTEM (nm)
568
450
6.1
53
1–10
7.3
82
1–15
−1
Smetal (m2
g
)
a
metal metal
Metal size range (nm)
k473 K (k4ꢀ 73 K
)
2b (15 × 10−5
)
1b (12 × 10−5
)
b
c
c
c
a
Smetal = 6/(ꢁmetal × dTEM) where ꢁAu = 18.88 g cm−3; ꢁAg = 10.50 g cm−3
.
−1
Units = mol
Units = mol
mol
m−2
h−1
.
b
c
–NO
2
metal
−1
h
.
–NO
2
metal
(x
)
over, the peak at 27.4◦ is diagnostic of a tetragonal rutile content
(JCPDS-ICDD 21-1276). This XRD response is consistent with a mix-
ture of anatase (80% volume fraction) and rutile forms of TiO2 in
agreement with the reported [56] Degussa, P25 composition. It is
important to note that only weak signals (overlapping with the
(0 0 4) anatase peak) due to metallic Au and Ag (2ꢂ = 38.1◦) were
distinguishable, a result that suggests a highly dispersed metallic
phase.
–NO
2
[–NH2]out
[–NO2]in
x
=
(2)
–NO
2
where the subscripts in and out refer to the inlet and outlet streams.
3. Results and discussion
In the case of titania supported metals, a migration of Ti sub-
oxide species to occlude the metal particles can occur at elevated
temperature (≥773 K) and has been described in terms of an encap-
sulation or decoration [57]. We observed a similar effect in our TEM
analysis after prolonged electron beam irradiation. This is illus-
trated for Au/TiO2 in Fig. 2. In both images, an encapsulation of
Au nano-particles is in evidence. Indeed, in image (II) atomic layers
of the support can be seen (indicated by arrows) surrounding an
isolated Au particle. The diffractogram patterns for a selected area
of the support and an Au particle are shown in images (IA) and (IB),
respectively. In both cases, the d-spacing obtained (ca. 0.35 nm)
are close to that associated with the characteristic (1 0 1) main
plane of TiO2-anatase (JCPDS-ICDD 21-1272). Metal encapsulation
as a result of analytical TEM measurements has been previously
demonstrated for Au/CeO2 [58] but we provide here, for the first
time, evidence of such an effect in the case of Au/TiO2. In order to
circumvent this response, all images of the catalyst surface were
taken at short beam exposure times where there was no evidence
of metal particle occlusion. Representative TEM images (I and II)
and metal particle size distributions (based on TEM measurements,
V) of reduced/passivated Au/TiO2 (A) and Ag/TiO2 (B) are given
in Fig. 3. The TEM images of both catalysts show well-dispersed
nano-scale pseudo-spheroidal metal particles. The diffractogram
patterns for individual Au (AIII) and Ag (BIII) particles and the
TiO2 support (AIV and BIV) are included as insets in Fig. 3. The
diffractograms for the support are consistent with anatase. The
spacings (0.23 nm) between the planes in the atomic lattice for
both metal particles are characteristic of the (1 1 1) plane of metal-
lic Au (JCPDS-ICDD 04-0784) and Ag (JCPDS-ICDD 04-0783). The
metal particles associated with Ag/TiO2 present a bimodal size dis-
tribution while Au/TiO2 is characterized by a narrower size range.
However, both systems present a similar surface area weighted
mean particle size (6–7 nm, see Table 1). There is an appreciable
component (65% Au/TiO2 and 45% Ag/TiO2) of metal particles with
diameters <5 nm, the particle size proposed to be crucial for cat-
alytic activity in hydrogen-mediated reactions over Au [22,35] and
Ag [39]. Taken as a whole, the characterization results demonstrate
precursor reduction to the metallic form post-TPR where the Ag and
Au particles exhibit a relatively narrow size distribution and similar
mean size.
3.1. Catalyst characterization
The results of the BET surface area, room temperature H2
chemisorption, DRS UV–vis measurements along with the mean
metal particle sizes (and range of values) obtained from TEM anal-
ysis are given in Table 1. In terms of TPR analysis, the experimentally
determined hydrogen consumption is also provided and can be
compared with the predicted (or theoretical) values based on the
precursor loading. The TPR profiles generated for Au/TiO2 (AI) and
Ag/TiO2 (BI) are shown in Fig. 1. In the case of Au/TiO2, hydrogen
consumption matched (within 1%) that required for the reduction
of HAuCl4 to the metallic form. This result agrees with reports in
the literature where the reduction of Au3+ to Au0 at T ≤ 503 K has
been proposed [27,43–45] for oxide (Al2O3 and/or TiO2) supported
Au prepared by impregnation. Hydrogen consumption during TPR
in the case of Ag/TiO2 is consistent with a two-step reduction
mechanism, i.e. AgNO3 → Ag2O → Ag0, as has been suggested for Ag
supported on TiO2 [46], Al2O3 [47], SiO2 [48] and carbon [49]. Bog-
danchikova et al. [50] examined an Ag/Al2O3 precursor prepared
by impregnation with AgNO3 and established (by XRD) that Ag0
was not present in the precursor, demonstrating metallic Ag forma-
tion post-TPR to 673 K. The room temperature H2 chemisorption
values recorded for both catalysts were close to the instrument
detection limits but the uptake on Ag/TiO2 was measurably lower.
These values are in keeping with the low capacity of Au [4,37]
and Ag [51,52] for H2 uptake due to the filled d-band [35,53].
The DRS UV–vis spectra for the passivated/reduced samples are
shown in Fig. 1. The spectrum for Au/TiO2 (AII) exhibits an absorp-
tion band at 568 nm that is characteristic of Au nanoclusters on
a titania substrate [54]. The spectrum for Ag/TiO2 (BII) presents
an absorption band at 450 nm, which can be linked to the pres-
ence of Ag0. Indeed, wavelengths >390 nm have been attributed
elsewhere [24,50] to the presence of metallic Ag particles. Jia et
al. [55] have associated a band at 420 nm to absorbance by Ag
nano-particles, where a displacement to higher wavelengths was
ascribed to an increase in particle size (from 6 nm to 18 nm). The
XRD profiles for passivated/reduced Au/TiO2 (AIII) and Ag/TiO2
(BIII) are also presented in Fig. 1. Both systems exhibit signals at
2ꢂ = 25.3◦, 37.8◦ and 48.1◦ corresponding to the (1 0 1), (0 0 4) and
(2 0 0) planes of tetragonal anatase (JCPDS-ICDD 21-1272). More-

F. Cárdenas-Lizana et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 48–54
51
Fig. 1. Au/TiO2 (A) and Ag/TiO2 (B) TPR profiles (I), DRS UV–vis spectra (II) and XRD patterns (III). Note: XRD peak assignments based on JCPDS-ICDD reference data: (ꢀ)
anatase (21-1272); (ꢁ) rutile (21-1276); (᭹) Au (04-0784) and (ꢁ) Ag (04-0783).
Fig. 2. Representative high-resolution TEM images of passivated/reduced (I and II) Au/TiO2 with diffractrogam patterns (IA and IB) for the selected (dashed) areas. Note:
Arrows in image (II) indicate TiOx layer covering an isolated Au particle.

52
F. Cárdenas-Lizana et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 48–54
Fig. 3. Representative TEM images (I and II) and metal particle size distributions (V) of passivated/reduced Au/TiO2 (A) and Ag/TiO2 (B). Note: The diffractogram patterns of
isolated metal particles (III) and TiO2 support (IV) are included as insets.
3.2. Hydrogenation of nitrobenzene
has been reported and ascribed to the deleterious effect of H2O as
by-product [59,69], metal leaching [68] and coking [5,67,69–71].
Catalyst deactivation is also a documented feature of hydrogena-
tion reactions over supported Au [34,72–76] and/or Ag [19,24,52]
and has been linked to metal sintering [24,34,52,74], leaching [24]
and carbon deposition [19,34,72,75,76].
Catalyst performance was quantified using pseudo-first order
kinetics based on the mass balance under continuous flow condi-
tions where
In order to compare the catalytic response of Au vs. Ag in gas
phase hydrogenation, we selected nitrobenzene as a reference reac-
tant. Both catalysts promoted the exclusive formation of aniline
with no evidence of hydrodenitrogenation and/or aromatic ring
reduction, i.e. exclusive –NO2 group hydrogenation. This result
alone is significant in that aniline is an important chemical used as
an additive for rubber production and in the manufacture of sev-
eral dyes, pigments, pesticides and herbicides [1,59]. About 85% of
global aniline production draws on catalytic nitrobenzene hydro-
genation [60] over supported Pt [60,61] or Pd [62,63] catalysts in
batch liquid operation where the formation of toxic by-products,
e.g. nitrosobenzene [64], azobenzene [61], azoxybenzene [60,61,65]
and/or phenylhydroxylamine [66], is still a major drawback asso-
ciated with commercial processes. Furthermore, a time-invariant
conversion was observed for both Au/TiO2 and Ag/TiO2, as shown
in Fig. 4(I). A temporal loss of activity in the gas phase hydrogena-
tion of nitroarenes over supported Pd [5,59,67–69] and Cu [70,71]
ꢁ
ꢂ
ꢃ
ꢄ
1
n
ln
= k ×
(3)
1 − x
F
–NO
–NO
2
2
F
is the total –NO2 inlet molar flow, n the moles of metal
–NO
2
in the catalyst bed: (n/F
2 ) has the physical meaning of con-
–NO
tact time. The extracted pseudo-first order rate constants (k, units:
mol
−1
mol
h
−1) are given in Table 1. We have previously
–NO
metal
2
demonstrated the applicability of this approach for the hydro-
genation of nitroarenes over supported Au catalysts [4,27–30] but
establish here that it also applies to data generated for supported

F. Cárdenas-Lizana et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 48–54
53
Fig. 5. Hammett plot for the selective –NO2 group reduction of para-substituted
nitroarenes over Au/TiO2 (᭹, solid line) and Ag/TiO2 (ꢁ, dashed line) at T = 473 K.
Fig. 4. (I) Variation of nitrobenzene fractional conversion (x
) to aniline
–NO
with time-on-stream over Au/TiO2 (᭹) and Ag/TiO2 (ꢁ) (metal/nit2robenzene =
7 × 10−2 molmetal h mol−1 ). (II) Pseudo-first order kinetic plot for reaction over
–NO
Au/TiO2 (᭹, solid line) and2Ag/TiO2 (ꢁ, dashed line) at T = 473 K.
reference) benzene derivate (k0) according to
ꢅ
ꢆ
ki
log
= ꢁ × ꢀi
(5)
k0
Ag (see Fig. 4(II)). A direct comparison of the performance of both
catalysts is only meaningful in terms of specific activities, i.e. per m2
of exposed metal. The Au and Ag metal surface areas were obtained
from
The ꢁ term (reaction constant) is an estimation of the charge devel-
opment during the course of the reaction and provides a measure
of the susceptibility of the system to substituent electronic effects
[80,81]. In a nucleophilic attack, the reaction rate is enhanced by
electron-withdrawing substituents and ꢁ > 0, while ꢁ values close
to 0 are indicative of a partially charged transition state [81]. The
ꢀi factor is an empirical parameter (values used in this study were
taken from [79,81]) that is dependent on the substituent electron
donating/acceptor character [81]. This equation was initially con-
ceived for homogeneous systems where good linear correlations
have been established for the hydrogen treatment of substituted
acetophenones [82], acetamidoacrylic acid derivatives [78] and
aromatic nitro-compounds [83]. The involvement of adsorption
phenomena [84], the inhomogeneous distribution/nature of active
sites and the greater contribution of steric effects has limited the
applicability of the Hammett expression in heterogeneous cat-
alytic systems [85]. Nevertheless, there have been some studies
where it has been successfully employed in the gas [86] and liquid
[87–92] phase hydrogen-mediated treatment of chlorobenzenes
[86], acetophenones [87] and (of direct relevance to this study)
nitroaromatics [88–92] over solid catalysts. We provide, in this
report, the first example of its application to the gas phase hydro-
genation of substituted nitroarenes. The fit of the experimental rate
data to the Hammett relationship is presented in Fig. 5. Both cat-
alysts generated positive reaction constants (ꢁ), consistent with a
nucleophilic reaction mechanism. The higher ꢁ value generated for
Au/TiO2 (0.93) compared with Ag/TiO2 (0.22) indicates a greater
dependence of rate on the electronic character of the substituent
and is diagnostic of a higher charge development in the reac-
tant → intermediate step [81]. This, in turn, can account for the
measurably higher specific activities observed for Au/TiO2. Our ꢁ
values are comparable to those reported for liquid phase opera-
tion (MgFeO, 0.690 [90], iron oxides 0.546 [91] and Pt/SiO2–AlPO4
0.1–2.0 [92]), suggesting a common reaction mechanism for the
formation of amines promoted by heterogeneous catalyst systems.
6
−1
Smetal (m2
g
metal) =
(4)
metal
ꢁmetal × dTEM
where ꢁmetal is the metal density and dTEM is the surface area
weighted mean particle size as measured by TEM. The specific
pseudo-first order rate constants are included in Table 1 where it
can be seen that Au delivered a measurably higher (in excess of the
associated experimental error) specific hydrogenation rate when
compared with Ag. We can attribute this response to a less effec-
tive activation of nitrobenzene and/or hydrogen by Ag/TiO2 relative
to Au/TiO2. Indeed, Boccuzzi et al. [46] studied the water gas shift
reaction over Ag/TiO2 and Au/TiO2 and ascribed the lack of activity
in the case of the former to a limited capacity to activate CO.
3.3. Effect of para-substituents
The hydrogenation of nitroarenes has been proposed to proceed
via a nucleophilic mechanism [4,7], where a weak nucleophilic
agent (hydrogen) attacks the activated –NO2 group with the forma-
tion of a negatively charged intermediate. In the previous section,
we have noted that aniline was the only product in the hydrotreat-
ment of nitrobenzene over both Au and Ag. In the hydrogenation
of substituted nitrobenzenes bearing –OH, –OCH3, –CH3, –Cl and
–NO2 in the para-position, both catalysts were again 100% selective
in generating the corresponding amine product. The higher activity
observed for Au/TiO2 (relative to Ag/TiO2) in the case of nitroben-
zene also extended to all the substituted nitrobenzenes where, for
both catalysts, the following activity sequence was established:
p-dinitrobenzene > p-chloronitrobenzene > nitrobenzene > p-
nitrotoluene > p-nitroanisole > p-nitrophenol. In order to assess
the dependence of rate on the nature of the para-substituent, we
have applied the Hammett correlation. The Hammett equation
evaluates the effect of a (meta- or para-) substituent on reaction
kinetics and can be used to predict rate and equilibrium constants
without prior experimental determination [77,78]. Furthermore,
it is a useful tool for the elucidation of reaction mechanisms [79].
In this approach, the rate constants for the substituted reactants
(ki) can be related to that obtained for the non-substituted (or
4. Conclusions
1 mol% Au and Ag on TiO2 catalysts were synthesized by
impregnation (with HAuCl4 and AgNO3) to deliver (post-TPR) a
well-dispersed metallic phase, characterized by metal particles in
the overall range 1–15 nm and surface area weighted mean values

54
F. Cárdenas-Lizana et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 48–54
of 6–7 nm. Hydrogen consumption during TPR is consistent with
Au3+ precursor reduction to Au0 while AgNO3 undergoes a step-
wise reduction to Ag0 via Ag2O. Both catalysts promoted exclusive
and time-invariant –NO2 reduction for a range of para-substituted
(–H, –OH, –O–CH3, –CH3, –Cl and –NO2) nitrobenzenes. The reac-
tion proceeds via a nucleophilic mechanism where the presence
of electron-withdrawing ring substituents served to elevate rate
as demonstrated by the linear Hammett relationship. The higher
reaction constant (ꢁ) obtained for Au/TiO2 (0.93) when compared
with Ag/TiO2 (0.22) indicates a greater dependency of rate on the
electronic character of the substituent and we associate this with a
more effective reactant activation to generate a negatively charged
intermediate leading to a higher specific rate. We provide, in this
report, the first direct comparison of the catalytic action of (TiO2)
supported Au and Ag in gas phase nitroarene hydrogenation and
establish the viability of both catalytic systems to promote the
sustainable (continuous and clean) production of a range of com-
mercially important amino-compounds.
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Acknowledgments
The authors are grateful to Dr. W. Zhou and Mr. R. Blackley
for their contribution to the TEM analysis. We acknowledge L.-
A.J. Wong Fo Sue and E. Díaz for their involvement in this project.
This work was financially supported by EPSRC through Grant 0231
110525. EPSRC support for free access to the TEM/SEM facility at
the University of St Andrews is also acknowledged.
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