Selective gas phase hydrogenation of p-chloronitrobenzene over Pd catalysts: Role of the support

Article abstract of DOI:10.1021/cs4001943

The gas phase (1 atm, 453 K) hydrogenation of p-chloronitrobenzene (p-CNB) over a series of laboratory-synthesized and commercial Pd (1-10% wt) supported on activated carbon (AC) and non-reducible (SiO₂ and Al ₂O₃) and reducible (ZnO) oxides has been examined. Reaction over these catalysts generated the target p-chloroaniline (p-CAN) (via selective hydrogenation) and nitrobenzene (NB)/aniline (AN) as a result of a combined hydrodechlorination/hydrogenation. A range of Pd nanoparticles with mean sizes 2.4-12.6 nm (from HRTEM and H₂/CO chemisorption) were generated. Both the p-CNB transformation rate and H₂ chemisorption increased with decreasing Pd size. Residual Mo (from the stabilizer used in the synthesis of Pd colloids) suppressed activity, but this was circumvented by the use of poly(N-vinyl-2-pyrrolidone) (PVP). Pd/AC generated p-CAN and AN as principal products, Pd on SiO₂ and Al₂O₃ exhibited hydrodechlorination character generating AN and NB, and Pd/ZnO promoted the sole formation of p-CAN at all levels of conversion. Reaction selectivity is linked to Pd electron density with the formation of Pd ^δ+ on AC and the occurrence of Pd^δ- on SiO₂ and Al₂O₃. Reaction exclusivity to p-CAN over Pd/ZnO is attributed to the formation of PdZn alloy (demonstrated by XPS), which selectively activates the -NO₂ group. This is the first report that demonstrates 100% selectivity for p-CNB → p-CAN over supported Pd.

Full text of DOI:10.1021/cs4001943

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Article
Selective Gas Phase Hydrogenation of p-
Chloronitrobenzene over Pd Catalysts: Role of the Support
Fernando Cárdenas-Lizana, Yufen Hao, Micaela Crespo-Quesada, Igor
Yuranov, Xiaodong Wang, Mark Andrew Keane, and Lioubov Kiwi-Minsker
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs4001943 • Publication Date (Web): 02 May 2013
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Selective Gas Phase Hydrogenation of
p-Chloronitrobenzene over Pd Catalysts:
Role of the Support
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Fernando Cárdenas-Lizanaa, Yufen Haob, Micaela Crespo-Quesadaa, Igor Yuranova,
Xiaodong Wangb, Mark A. Keaneb* and Lioubov Kiwi-Minskera*
aGroup of Catalytic Reaction Engineering, Ecole Polytechnique Fédérale de Lausanne
(GGRC-ISIC-EPFL), Lausanne CH-1015, Switzerland
bChemical Engineering, School of Engineering and Physical Sciences,
Heriot-Watt University, Edinburgh EH14 4AS, Scotland
*corresponding authors:
tel.: +44(0)131 451 4719, e-mail: M.A.Keane@hw.ac.uk
tel.: +41(0) 21 693 3182, e-mail: Lioubov.kiwi-minsker@epfl.ch
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Abstract
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The gas phase (1 atm, 453 K) hydrogenation of p-chloronitrobenzene (p-CNB) over a
series of laboratory synthesised and commercial Pd (1-10 % wt.) supported on activated
carbon (AC), non-reducible (SiO2 and Al2O3) and reducible (ZnO) oxides has been examined.
Reaction over these catalysts generated the target p-chloroaniline (p-CAN) (via selective
hydrogenation) and nitrobenzene (NB)/aniline (AN) as a result of a combined
hydrodechlorination/hydrogenation. A range of Pd nanoparticles with mean sizes 2.4-12.6 nm
(from HRTEM and H2/CO chemisorption) was generated. The p-CNB transformation rate and
H2 chemisorption both increased with decreasing Pd size. Residual Mo (from the stabilizer
used in the synthesis of Pd colloids) suppressed activity but this was circumvented by the use
of poly N-vinyl-2-pyrrolidone (PVP). Pd/AC generated p-CAN and AN as principal products,
Pd on SiO2 and Al2O3 exhibited hydrodechlorination character generating AN and NB, while
Pd/ZnO promoted the sole formation of p-CAN at all levels of conversion. Reaction
selectivity is linked to Pd electron density with the formation of Pdδ+ on AC and the
occurrence of Pdδ- on SiO2 and Al2O3. Reaction exclusivity to p-CAN over Pd/ZnO is
attributed to the formation of PdZn alloy (demonstrated by XPS), which selectively activates
the –NO2 group. This is the first report that demonstrates 100% selectivity for p-CNB → p-
CAN over supported Pd.
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Keywords: Supported Pd; p-chloronitrobenzene hydrogenation; p-chloroaniline; PdZn alloy;
support effects; PVP-Pd nanoparticles.
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1. Introduction
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Functionalised anilines are key intermediates in the synthesis of fine chemicals,
herbicides, pesticides, dyes and pigments (1). Amine production via nitroarene hydrogenation
has been promoted using a range of (carbon (2-5), Al2O3 (6-10), SiO2 (11-13), TiO2 (14-17),
CeO2 (18), Fe2O3 (18-20), Fe3O4 (19), SnO2 (21), CeO2 (18,22) and polymer (23)) supported
metal (Pd (2-7,16,24), Pt (11,20,23,25,26), Ru (21,26-28), Rh (29), Ni (8,10,14,30), Cu (31),
Ag (13,32) and Au (33,34)) catalysts. Taking the hydrogenation of p-chloronitrobenzene (p-
CNB), undesired C-Cl scission (Scheme 1) is difficult to circumvent, particularly at high
conversions (33,35). To date, p-CNB hydrogenation has focused on liquid phase batch
reactions at high H2 pressure (up to 60 atm) (30). A move from batch to continuous operation
has been identified (36) as a priority for sustainable production in the pharmaceutical and fine
chemical sectors. We have demonstrated (34) exclusive –NO2 reduction (but at low reaction
rate) in the continuous gas phase hydrogenation of substituted nitroarenes over supported Au.
In examining the role of Pd as promoter to increase activity, selective p-chloroaniline (p-
CAN) production was achieved for Au/Pd ≥ 20 (9). Increasing the Pd content delivered higher
rates but with the formation of nitrobenzene (NB). This is consistent with the literature on gas
(16) and liquid (7,35) phase operation that has demonstrated non-selective CNB
hydrogenation over supported Pd, generating aniline (AN) (7,16,35), NB (7,16,35) and azo-
compounds (16) as by-products.
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Catalytic activity/selectivity in nitro-group reduction can be governed by the electronic
(18,22) and crystallographic (15) character of the metal phase that is, in turn, is influenced by
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interactions with the carrier (12,37). Support acid-base properties can impact on metal
dispersion with electron transfer resulting in the formation of partially charged nano-scale
particles (38,39). Sangeetha et al. (40) observed higher activity in the hydrogenation of NB
over Pd/hydrotalcite relative to Pd/MgO and Pd/-Al2O3, which they linked to enhanced Pd
dispersion due to support basicity. Higher CAN selectivity (from CNB) has also been
achieved with the incorporation of metal cations (Cr3+/Cr2+ (41) and Sn4+ (7)) on supported Pd
and associated with enhanced polarization of the N=O bond (7). When reducible oxides are
used as carriers, partial reduction of the support can lead to alloy formation (42). Recent work
has demonstrated selective hydrogenation of functionalized alkynes over PdZn alloy (43,44).
The formation of a PdZn phase, established by high-resolution X-ray diffraction/absorption
spectroscopy, has been shown to enhance selectivity in the hydrogenation of 1-pentyne to
pentenes (relative to Pd/SiO2) (45) and acetonitrile to ethylamine (compared with Pd black)
(46). Use of supported Pd catalysts to achieve high selectivity in the conversion of p-CNB to
p-CAN represents a challenge. We have set out to identify the critical variable(s) that control
–NO2 group reduction selectivity by examining a range of (commercial and laboratory
synthesised) catalysts with varying Pd content, support and method of preparation. We
compare the catalytic action of bulk (unsupported) Pd with that of Pd supported on three
distinct carriers; (i) activated carbon (AC); (ii) non-reducible oxides (SiO2 and Al2O3); (iii)
ZnO.
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2. Experimental
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2.1. Catalyst Preparation and Activation
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The AC support (905 m2 g-1) was obtained from NORIT (UK) and subjected to a
demineralization treatment (1 M HNO3 under continuous stirring at 500 rpm for 7 days) to
remove any residual metal that could contribute to the hydrogenation process. The AC
support was thoroughly washed with deionized water (until pH of the wash water approached
7) and oven-dried at 383 K for 12 h. The oxide supports, SiO2 (Cab-O-Sil M-5, Cabot), Al2O3
(Puralox, Condea Vista Co.) and ZnO (Aldrich) were used as received. A series of (AC, SiO2
and Al2O3) supported Pd catalyst were prepared by standard impregnation (with Pd(NO3)2 as
a precursor dissolved in water at 363 K) and denoted as Pd/AC-I, Pd/SiO2 and Pd/Al2O3-III.
The impregnated samples were oven dried at 393 K for 12 h. In addition, Pd/Al2O3-I, Pd/ZnO
and Pd/ZnO-PVP were prepared by deposition of ex-situ synthesised monodispersed Pd0
nanoparticles. In the case of Pd/Al2O3-I and Pd/ZnO, an aqueous solution of PdCl2 (Fluka,
>99 %) and Na2MoO4·H2O (Fluka, >99 %) (Pd/Mo mol ratio = 1) was heated at 368 K (under
continuous stirring, 500 rpm) until complete evaporation. The solid residue was dissolved in
water and contacted (at ambient temperature) with a continuous flow of H2 (100 cm3 min-1)
for 30 min. This procedure has been demonstrated (47) to result in the formation of uniform
Pd0 nanoparticles stabilised by molybdate anions. The ex-situ synthesis of monodispersed
poly N-vinyl-2-pyrrolidone (PVP, Aldrich) stabilized Pd nanoparticles followed the method
described by Lim et al. (48). A known mass (0.277 g) of PVP and ascorbic acid (0.156 g,
99%, Aldrich), which acts as reducing agent, were dissolved in 15 cm3 of water at 368-371 K.
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An aqueous solution of PdCl2 (0.088 g, 60% Pd, Aldrich) and NaCl (0.058 g, 99.5%, Fluka)
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was added under constant agitation (ca. 500 rpm) with an instantaneous change in color (from
brown to black), indicative of Pd reduction and colloid formation (48). The colloidal solution
was kept under agitation at 368-371 K for 3 h, cooled and diluted with 75 cm3 acetone and
left for 12 h. The colorless liquid was decanted and the black viscous residue dissolved to
give an homogeneous and stable Pd(PVP) colloidal solution. Palladium deposition was
achieved via adsorption where the support (ca. 2 g) was immersed and stirred in the Pd
colloidal solution for ca. 2 h, the slurry filtered and dried in air at ambient temperature. Two
commercial supported catalysts (Pd/AC-II and Pd/Al2O3-II, Aldrich) and bulk PdO (Aldrich)
were also examined. Prior to use, the catalysts were sieved into a batch of 75 μm average
diameter and reduced in 60 cm3 min-1 H2 at 10 K min-1 to 573 K, which was maintained for 1
h. Samples for off-line characterisation were passivated in 1% v/v O2/He at ambient
temperature.
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2.2. Catalyst Characterisation
The Pd content was measured by inductively coupled plasma-optical emission
spectrometry (ICP-OES, Vista-PRO, Varian Inc.) from the diluted extract in HF. Temperature
programmed reduction (TPR), H2 and CO chemisorption and BET surface area were
determined using the commercial CHEM-BET 3000 (Quantachrome) unit. The samples were
loaded into a U-shaped Quartz cell (3.76 mm i.d.) and heated in 17 cm3 min-1 (Brooks mass
flow controlled) 5% v/v H2/N2 at 10 K min-1 to 573±1 K. The effluent gas passed through a
liquid N2 trap and H2 consumption monitored by a thermal conductivity detector (TCD) with
data acquisition/manipulation using the TPR WinTM software. The reduced samples were
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maintained at 573 K until the signal returned to baseline, swept with 65 cm3 min-1 N2 for 1.5
h, cooled to ambient temperature and subjected to H2 (or CO) chemisorption using a pulse (10
µl) titration procedure. In a series of blank tests, chemisorption measurements on each support
(AC, Al2O3, SiO2 and ZnO) did not result in any detectable uptake. BET areas were recorded
with a 30% v/v N2/He flow using pure N2 (99.9%) as internal standard. At least two cycles of
N2 adsorption-desorption in the flow mode were employed to determine total surface area
using the standard single point method. BET surface area, H2 and CO uptake were
reproducible to within ±5 %; the values quoted represent the mean.
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X-ray photoelectron spectroscopy (XPS) analyses were conducted on an Axis Ultra
instrument (Kratos Analytical) under ultra-high vacuum conditions (<10-8 Torr) using a
monochromatic Al K X-ray source (1486.6 eV). The source power was maintained at 150 W
and the emitted photoelectrons were sampled from a 750×350 m2 area at a take-off angle =
90°. The analyser pass energy was 80 eV for survey spectra (0–1000 eV) and 40 eV for high
resolution spectra (Pd 3d5/2 and Pd 3d3/2). The adventitious carbon 1s peak was calibrated at
284.5 eV and used as an internal standard to compensate for any charging effects. Spectra
curve fitting and quantification were performed with the Casa XPS software, using relative
sensitivity factors provided by Kratos. Palladium particle morphology and size were
determined by transmission electron microscopy analysis; JEOL JEM 2011 HRTEM unit with
a UTW energy dispersive X-ray detector (Oxford Instruments) operated at an accelerating
voltage of 200 kV using Gatan Digital Micrograph 3.4 for data acquisition/manipulation. The
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samples for analysis were crushed and deposited (dry) on a holey carbon/Cu grid (300 Mesh).
Mean Pd size was based on a count of up to 800 individual Pd particles.
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2.3. Hydrogenation of p-Chloronitrobenzene (p-CNB)
2.3.1. Catalytic System
The hydrogenation of p-CNB (Sigma-Aldrich, purity ≥99 %) as a solution in ethanol
(Sigma-Aldrich, ≥99 %) was carried out immediately after catalyst activation in situ, under
atmospheric pressure at 453 K in a fixed bed vertical glass reactor (i.d. = 15 mm). The
operating conditions ensured negligible heat/mass transport constraints. A layer of
borosilicate glass beads served as preheating zone where the p-CNB reactant was vaporized
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 reaction
temperature was continuously monitored using a thermocouple inserted in a thermowell
within the catalytic bed. p-CNB was delivered at a fixed calibrated flow rate to the reactor via
a glass/teflon air-tight syringe and teflon line using a microprocessor controlled infusion
pump (Model 100 kd Scientific). A co-current flow of p-CNB and ultra pure (BOC, >99.99
%) H2 (<1 % v/v p-CNB in H2) was maintained at GHSV = 2×104 h-1 with an inlet flow (F)
over the range 2×10-4 – 7×10-4 mol h-1. The H2 content was up to 110 times in excess of the
stoichiometric requirement for hydrogenation to p-CAN and the flow rate was monitored
using a Humonics (Model 520) digital flowmeter. The molar Pd (n) to F ratio spanned the
range 1×10-3 – 9×10-3 h. In a series of blank tests, passage of p-CNB in a stream of H2 through
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the empty reactor or over the support alone did not result in any detectable conversion. The
reactor effluent was frozen in a liquid nitrogen trap for subsequent analysis.
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2.3.2. Analytical Method and Activity/Selectivity Measurements
The composition of the reactant/product(s) mixtures was determined using a Perkin-
Elmer Auto System XL chromatograph equipped with a programmed split/splitless injector and
a flame ionization detector, employing a DB-1 capillary column (i.d. = 0.33 mm, length = 30
m, film thickness = 0.20 μm). Data acquisition and manipulation were performed using the
TotalChrom Workstation Version 6.1.2 (for Windows) chromatography data system and the
overall reactant/product molar fractions (xi) determined from detailed calibration plots (not
shown). Conversion (Xp-CNB) was obtained from
[p CNB]in [p CNB]out
Xp  CNB

[p CNB]in
where selectivity with respect to p-CAN was calculated as
[p CAN]out
Sp  CAN (%) 
100
[p CNB]in [p CNB]out
Repeated reactions with different samples from the same batch of the catalyst delivered
conversion/selectivity values that were reproducible to within ±7 %.
3. Results and Discussion
3.1. Catalyst Characterisation
Characterisation results for the series of Pd catalysts studied are presented in Tables 1
and 2. Commercial and laboratory synthesized samples exhibiting a range of Pd loading (0.9-
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10.2 % w/w) and BET area (2-875 m2 g-1) have been used in this work to promote p-CNB
hydrogenation. We must stress that a very broad set of Pd catalysts were chosen in order to
establish critical characteristics that determine activity and selectivity.
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3.1.1. Palladium Particle Size
Representative temperature programmed reduction (TPR) profiles (for PdO, Pd/AC-II,
Pd/Al2O3-II and Pd/ZnO) are presented in Figure 1. Each profile is characterised by the
appearance of a negative peak at 343-377 K (Table 2) that can be attributed to Pd hydride
decomposition. Ambient temperature H2 absorption is known to generate β-phase Pd hydride
when H2 partial pressure exceeds 0.02 atm (49,50); PH2 during TPR in this work = 0.05 atm.
The hydride is thermally unstable and decomposes during thermal treatment to release H2
over the temperature range 342-386 K (51-53). Hydride composition, presented in Table 2 as
a H/Pd ratio, is known to depend on Pd particle size (10,54) where H/Pd decreases with
decreasing size (increasing metal dispersion) (51) to approach zero for Pd size <2.5 nm
(51,55). Bulk Pd, as our benchmark, is characterised by the highest H/Pd (0.67), a result in
good agreement with values (0.66-0.73) reported in the literature (54,56). Considering the
supported catalysts, the commercial Pd/Al2O3-II exhibited the lowest H/Pd (0.05) while the
remaining systems generated ratios in the range 0.16-0.36, characteristic of Pd particles of 3-
24 nm (56,57). The positive signal at higher temperature (Tmax = 573 K) for Pd/ZnO (Figure 1
(IV)) has been observed previously and linked to: (i) secondary reduction of an oxidised Pd
component or reduction of ZnO (or ZnCl2) at the metal/support interface (58); (ii)
simultaneous reduction of Pd2+ and Zn2+ resulting in the formation of PdZn particles (59).
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The mean Pd particle sizes determined from TEM and (CO and H2) chemisorption are
given in Table 2. Consistent values for all three measurements were obtained in the case of
Pd/Al2O3-II (2.2  0.2 nm), which agree with the low associated H/Pd from TPR analysis.
However, there are inconsistencies in the results generated with each technique for the other
supported Pd samples. We should note that there is a dearth of published studies recording Pd
particle sizes using more than one analytical methodology. Taking an overview of the
published work on supported metals in general, both agreement (4,51,60-62) and
disagreement (52,62-64) have resulted from the estimation of metal particle size using gas
chemisorption and TEM analysis. The discrepancy can be linked to limitations associated
with these techniques. In the case of chemisorption, a source of error can result from a partial
(or total) active phase occlusion by residual precursor species post-activation that serve to
suppress uptake, leading to an over-estimation of metal particle size (63,64). CO
chemisorption delivered a range of Pd sizes that do not correlate directly with Pd hydride
composition and deviate significantly from values obtained from H2 uptake and TEM. This
may be the result of applying a fixed Pd/CO adsorption stoichiometry (52,65) (surface metal
atoms/CO molecules chemisorbed = 2), as has been the standard approach for carbon and
oxide supported Pd systems (66,67). However, CO can adsorb on Pd in both linear (Pd:CO =
1:1) (68) and bridged (Pd:CO = 2:1) (68,69) forms and this appears to depend on the nature of
the support (70) and Pd size (71). Hydrogen chemisorption is commonly employed to
estimate Pd dispersion (63,72) where a dissociative adsorption (Pd:H stoichiometry = 1:1) is
applied. Prelazzi et al. (73), in reviewing the available literature, have identified instances of
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good agreement between H2 uptake (taking Pd:H = 1:1) and TEM data. In this study, Pd sizes
obtained from H2 chemisorption differ from the TEM derived values but both methods
delivered an equivalent trend in terms of increasing (or decreasing) size across the group of
catalysts. Particle size measurements from H2 chemisorption can be compromised due to
hydrogen spillover, i.e. dissociative chemisorption on metal sites with transport to the support
surface (74), which results in an apparent higher Pd dispersion (under-estimation of metal
size) due to additional H2 consumption. Indeed, there is evidence in the literature for
hydrogen spillover under ambient conditions for carbon (75) and oxide (76) supported Pd. We
should flag Pd/AC-I that exhibits an unfeasibly large Pd size (129.0 nm) based on H2
chemisorption. This can result from occlusion of Pd surface sites by amorphous carbon
associated with the support, as noted elsewhere (64), leading to inhibited H2 uptake.
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Electron microscopy as an imaging technique provides a particle size distribution from
which the mean can be determined. There are certain limitations in the application of this
method, as highlighted in the review of Matyi et al. (65) and recently published analysis by
Liu (77). This approach provides a two dimensional imaging of three dimensional
nanoparticulate structures where the occurrence of irregular morphologies, as a result of
metal-support interactions, presents problems in terms of consistent diameter measurement.
Accurate diameter estimation can also be compromised by poor contrast between the support
and metal(s) phases while apparatus detection limit (sizes below 1-2 nm (78)) can also lead to
inaccurate sizes. Representative TEM micrographs and associated Pd particle size
distributions on carbonaceous, reducible and non-reducible oxide carriers are presented in
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Figure 2. The supported metal phase exhibits a pseudo-spherical morphology with particles
predominantly <10 nm. It is critical in TEM analysis that the particle population used for size
estimation is representative of the entire ensemble of crystallites. Taking Pd/C-II as an
example, the standard deviation of the mean (m) is presented as a function of Pd particle
count (inset to Pd particle size distribution in Figure 2 (I)), following the approach reported
previously (78). It can be seen that m is sensitive to the total number of Pd particles counted
and m invariance (at counts >200) ensures that the size distribution is truly representative.
Invariance of the mean applies to all the dTEM values recorded in Table 2. While the
combination of techniques used in this study should be complementary, our results suggest
that the chemisorption measurements (particularly CO) do not give a reliable measure of Pd
size. TEM analysis delivers the most valid Pd size information where particle counting is
conducted with statistical rigour. Hydrogen uptake capacity is, nonetheless, an important
consideration in H2 mediated catalysis. The correlation between H2 uptake (at ambient
temperature) and mean Pd size (from TEM) is presented in Figure 3 (I), where there is a clear
increase in H2 chemisorption capacity with decreasing Pd size (12.6→2.4 nm). This tendency
is in agreement with published literature (51,57) showing enhanced H2 chemisorption on
smaller Pd particles.
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3.1.2. Palladium Electronic Characteristics
In order to gain insight into the electronic character of the Pd phase, XPS analysis over
the Pd 3d binding energy (BE) region was conducted. XPS profiles for three representative
catalysts (Pd/AC-II (I), Pd/Al2O3-II (II) and Pd/ZnO (III)) are shown in Figure 4. It is known
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that the support can impact on the electronic properties of the metallic phase via metal-
support interactions (79), which are more pronounced for Pd particles at the nano-scale.
Taking bulk Pd as benchmark, the core level Pd 3d5/2 BE (= 335.3 eV) is in good agreement
with the value (335.2 ± 0.2 eV) reported by Briggs and Seah (80) for Pd0. In order to
explicitly probe modification(s) to Pd electron density due to particle size, we present the
relationship between dTEM and Pd 3d5/2 BE in Figure 5. The BE values exhibited a measurable
increase (334.7→334.9 eV) with decreasing mean Pd size (6.4→2.4 nm) for the Al2O3
system, as has been noted elsewhere (81). Moreover, the BE showed a marked dependence on
the support. Palladium on AC exhibits a Pd 3d5/2 BE that is ca. 0.6 eV higher than bulk Pd,
suggesting electron transfer from Pd to the carbon support resulting in a partial positively
charged metal phase (Pdδ+). Jiang and co-workers (82) observed a positive displacement (ca.
1.1 eV) for carbon supported Pd (3-12 nm) when compared with bulk Pd that they ascribed to
metal-support interactions. Ramos et al. (83) reported a shift to higher BE for Pd/C that was
attributed to electronic transfer associated with residual surface Cl. Gómez-Sainero et al. (84)
have also demonstrated the occurrence of electron-deficient Pd (3.8-10.5 nm) on a carbon
carrier. They linked this to the presence of surface species (e.g. HCl and NOx) generated
during catalyst preparation/activation that serve to modify the electron density of the Pd sites
with the formation of supported Pdn+. The presence of Pdδ- on Al2O3 and SiO2 follows from
the observed lower (by 0.4-0.6 eV) BE (relative to bulk Pd), as shown in Figure 5. Oxidic
supports have been found to influence the electronic properties of supported transition metals
where a change in the electron density of Ru (2-10 nm) on MgO, Al2O3 and SiO2 has been
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attributed to interaction with surface OH groups (85). The Pd 3d5/2 spectrum for Pd/ZnO
(Figure 4 (III)) suggests the presence of two distinct surface Pd species. The main component
shows a BE (335.0 eV) close to the reference for zero valent Pd (80) while a secondary
component exhibits a signal shifted to higher BE (by 0.7 eV) and can be attributed to the
occurrence of PdZn (58). The formation of an intermetallic PdZn alloy phase after H2
treatment at T ≥ 373 K has been demonstrated by HRXRD and XAS (45) but the mechanism
is still a matter of some debate. Hong et al. (76) have proposed the growth of a thin PdZn
alloy phase at the metal-support interface resulting in a strong anchoring of the metal that
serves to inhibit sintering. Alternatively, reversible migration and transformation of ZnO (to
Zn) on Pd can result in the partial/total coverage of Pd clusters and the ultimate formation of a
PdZn outerlayer (58).
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3.2. Correlation of Catalyst Characteristics with Catalytic Performance
3.2.1. Activity
The catalytic response of supported metal nanoparticles in hydrogenation reactions can
be governed by electronic (86) and/or geometric (87,88) considerations. A series of
experiments were conducted to investigate the effect of particle size on catalyst performance
where an increase in activity with decreasing Pd size (12.6→2.4 nm) is apparent from the
entries in Figure 3 (II). The results suggest that the nature of the carrier (carbon vs. oxide) or
catalyst source (laboratory synthesized or commercial) does not impact significantly on rate,
which is determined by Pd size. Indeed, similar activity was obtained for catalysts with an
equivalent mean Pd size on different supports (Pd/AC-I (11.7 nm) and Pd/SiO2 (12.6 nm))
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while, taking a common support (Pd/AC-I (11.7 nm) and Pd/AC-II (4.4 nm)), a four-fold
higher activity was recorded for the catalyst bearing smaller Pd particles. This response has
also been observed for gas phase p-CNB hydrogenation over supported Au (22) but we
provide here the first report of Pd size effects in this reaction. We should note that the
opposite trend has been reported for batch liquid phase p-CNB hydrogenation where higher
activities were recorded for Pt/Al2O3 with lower metal dispersion and ascribed to stabilization
of the negatively charged reaction transition state on larger Pt particles (89). A salient feature
of the data generated in this study is the match of activity dependence on Pd size with that of
H2 chemisorption (Figure 3 (I)). Increased rate can be attributed to greater surface hydrogen,
which is a feature of enhanced Pd dispersion. We should flag Pd/Al2O3-I and Pd/ZnO,
synthesised by deposition of (Na2MoO4·H2O) stabilized monodispersed Pd nanoparticles
prepared ex situ, that deviate from the general trend in delivering lower activities. This can
result from residual surface stabilizer post-activation that occludes active sites and inhibits
rate, as has been reported for the hydrogenation of acetylene (90). Indeed, XPS analysis has
established the presence of surface Mo species (1% w/w) in activated Pd/Al2O3-I and
Pd/ZnO that is consistent with an earlier report of a Pd nanoparticulate catalyst prepared using
Na2MoO4·H2O as stabilizer (43). Zina and Ghorbel (91) have demonstrated that inclusion of
Mo in zeolite supported Pd suppressed 1,3-butadiene hydrogenation activity, which was
linked to the formation of PdnMom clusters. It should, however, be noted that H2
chemisorption on Pd/Al2O3-I and Pd/ZnO was consistent with the general trend line shown in
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Figure 3 (I). This suggests that any Mo remaining from the stabilizer does not impact on H2
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3.2.2. Selectivity
The main reaction pathways in the hydrogenation of p-CNB are identified in Scheme 1
where nitro group reduction (step I) generates the target amine (p-CAN). Dechlorination of p-
CNB (step II) results in the formation of NB, which can undergo further hydrogenation (step
III) or dechlorination of p-CAN (step IV) to give, in both cases, AN. Reaction over the Pd
catalysts in this study generated p-CAN, NB and/or AN in varying proportions. In such a
parallel/sequential reaction mechanism, selectivity is only meaningful at an equivalent level
of fractional conversion (Xp-CNB), which we set at 0.2 in Table 1. Bulk Pd was employed as
benchmark, serving as an index against which Pd particle size and Pd-support effects can be
evaluated. Reaction over unsupported Pd generated all three products with the principal
formation of AN (composite hydrodechlorination and hydrogenation). Taking an overview of
the catalytic response exhibited by the supported catalysts, three groups emerge, i.e. Pd on
carbon, non-reducible oxides and ZnO, which are separated by the dashed lines in Tables 1
and 2. Carbon supported Pd (Pd/AC) promoted hydrogenation and hydrodechlorination steps
with p-CAN selectivity that deviated from that observed for bulk Pd. Palladium on non-
reducible Al2O3 and SiO2 did not generate any detectable p-CAN but exhibited
hydrodechlorination character (to produce NB and AN). In marked contrast, Pd/ZnO was
100% selective to p-CAN. The latter is a very significant finding as, to the best of our
knowledge, there have been no reports demonstrating reaction exclusivity to p-CAN in the
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gas phase hydrogenation of p-CNB over supported Pd.
Given the distinct selectivity
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response for the three groupings, one catalyst (commercial Pd/AC-II and Pd/Al2O3-II and
laboratory synthesised Pd/ZnO) was chosen from each group for further comprehensive
catalyst testing; the relationships between conversion (Xp-CNB) and selectivity (Si) are shown in
Figure 6. At low Xp-CNB (<0.05), p-CAN was the principal product over bulk Pd (Figure 6 (I))
with the preferential formation of AN at higher conversions and NB as a by-product. This
suggests that Pd activates both −NO2 and −Cl functionalities and is in agreement with
published literature showing that dechlorination (92) and nitro-group reduction (41) are
promoted over unsupported Pd. The switch in selectivity to C–Cl bond scission and AN
formation can be linked to the electron-donating character of the –NH2 function (relative to
electron-withdrawing −Cl and −NO2), which can induce electron transfer via the benzene
ring, increasing C−Cl polarity (93,94) and facilitating hydrogenolytic attack by dissociated H.
In the case of Pd/AC-II (Figure 6 (II)), p-CAN and AN were the principal products with trace
formation of NB. XPS measurements are consistent with the formation of Pdδ+ in Pd/AC. The
nitro group exhibits a higher relative electronegativity than the para-substitued Cl where the
ring inductive and conjugative effects serve to increase the electron density of –NO2 (93) with
activation at electron-deficient Pd to generate p-CAN, as reported elsewhere (95,96). In
common with bulk Pd, a decrease in selectivity to p-CAN with increased AN is in evidence at
higher Xp-CNB (>0.3), indicating a sequential mechanism via steps (I) and (IV) (Scheme 1).
Functionalised chlorobenzenes can interact with Pdδ+ through the Cl substituent with a
weakening of the C-Cl (97), facilitating hydrogenolysis to AN. In contrast, NB and AN were
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the only products generated over Pd/Al2O3-II (Figure 6 (III)), where an increase in conversion
(>0.3) was accompanied by higher AN selectivity, suggesting that dechlorination preceded
NB hydrogenation, i.e. reaction via steps (II) and (III) in Scheme 1. Both −NO2 and –Cl
functions can reduce the electron density of the benzene ring, favouring interaction with Pdδ-
in Pd/Al2O3-II, where both substituents are activated. The ring carbon bonded to Cl is more
susceptible to hydrogenolytic attack as it bears the lowest electron density of all the carbons
in the ring (93), leading to NB formation with further hydrogenation to AN.
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Reaction over Pd/ZnO (Figure 6 (IV)) generated p-CAN as the sole product, regardless
of conversion, i.e. exclusive reaction via path (I) in Scheme 1. The Pd/ZnO catalyst is
characterized by a Pd 3d5/2 core level BE close to that of the reference Pd (Figure 5). As both
hydrodechlorination and hydrogenation were promoted over bulk Pd, a similar product
distribution should be expected for Pd/ZnO. However, the formation of PdZn alloy,
demonstrated by XPS (Figure 4 (III)), must influence p-CNB adsorption and surface reaction.
Sárkány et al. (58) reported the complete suppression of butane formation in the
hydrogenation of 1,3-butadiene due to the formation of a PdZn intermetallic phase and
attributed this to weaker diene/surface interaction. Iwasa et al. (46) recorded enhanced
selectivity to primary amine in acetonitrile hydrogenation over Pd/ZnO, ascribing this to
differences in substrate and/or intermediate adsorption strength relative to other supported Pd
catalysts. XPS analysis has established the formation of zero valent Pd with a smaller (34 %
w/w) PdZn component. We envisage H2 dissociation on Pd0 to generate surface reactive
hydrogen where p-CNB interaction with the PdZn phase selectively activates the –NO2
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functionality. Indeed, reaction of p-CAN over Pd/ZnO delivered AN as the sole product
where the rate (1.6 molp-CAN h-1 molPd-1) was appreciable lower than that (13.5 molp-CAN h-1
molPd-1) recorded for p-CNB as reactant. This is consistent with exclusivity to p-CAN where
path (IV) (in Scheme 1) does not contribute to the overall process over Pd/ZnO.
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3.2.3. Role of the Stabilizer in Determining Pd/ZnO Performance
In order to establish a possible effect of the stabilizer on the catalytic response exhibited
by the catalysts synthesised using the colloid route with Na2MoO4·H2O, an additional ZnO
supported Pd was prepared, employing PVP as stabilizer (see Experimental section), which
we label Pd/ZnO-PVP. The as-prepared sample was calcined at 873 K as there is evidence in
the literature for the removal of capping polymers at T  550 K (98). There was no detectable
(on the basis of XPS analysis, not shown) surface nitrogen associated with the calcined
sample, indicating effective PVP removal. Subsequent TPR activation, following the same
procedure applied to Pd/ZnO (see Experimental section), generated an equivalent XPS
response (profile not shown) with a principal Pd0 (BE = 335.0 eV) and secondary PdZn (31%
w/w, BE = 335.7 eV) component. Palladium particle size distribution from TEM analysis
delivered a larger mean size (11.4 nm) for Pd-PVP /ZnO relative to Pd/ZnO (6.5 nm) that can
be attributed to sintering during the calcination step. Reduction of p-CNB over Pd-PVP/ZnO
resulted in the sole formation of the target p-CAN. Reaction exclusivity in nitro-group
reduction for both ZnO supported Pd catalysts can be explicitly attributed to the presence of a
PdZn alloy phase. Pd-PVP/ZnO delivered a greater hydrogenation rate than Pd/ZnO (Figure 3
(II)) although the former exhibited larger Pd particle size. This confirms rate inhibition by
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residual Mo in the case of Pd/ZnO. The correlation of H2 chemisorption (Figure 3 (I)) and
activity (Figure 3 (II)) with Pd size for Pd-PVP /ZnO coincides with the general trend. Our
results demonstrate enhanced hydrogenation activity for smaller Pd nanoparticles irrespective
of the support, a rate decrease due to the presence of residual (Mo-containing) stabilizer and
exclusive selectivity to p-CAN associated with PdZn alloy formation.
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4. Conclusions
We have established that the support (AC, non-reducible (SiO2, Al2O3) and reducible
(ZnO) oxides) can influence the catalytic performance of Pd in the gas phase hydrogenation
of p-CNB. Characterisation of Pd size by TEM, H2 and CO chemisorption has been assessed
where the former is shown to give the most reliable results when particle counting is
statistically robust. Chemisorption leads to erroneous values arising from (i) the application of
an exclusive adsorption stoichiometry factor, (ii) possible Pd site occlusion and/or (iii)
contribution due to spillover. Transformation rate and surface hydrogen concentration both
increase with decreasing mean Pd size (from 12.6 to 2.4 nm). A lower rate was recorded for
particles prepared by a colloidal deposition technique, which is linked to site blocking by
residual Mo from the stabilizer (Na2MoO4·H2O). Use of PVP as stabilizer (Pd-PVP/ZnO),
that is removed by thermal treatment, circumvents Pd poisoning but the calcination step
induces Pd agglomeration, which impacts on H2 uptake and hydrogenation rate. Bulk Pd, used
as a benchmark, promoted hydrodechlorination and hydrogenation to generate p-CAN, AN
and NB. XPS analysis has demonstrated electron transfer from Pd to AC with the generation
of Pdδ+ that activates both –Cl and –NO2 functions with preferential p-CAN formation at low
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p-CNB conversion and increased selectivity to AN in subsequent dechlorination at higher
conversions. Palladium supported on Al2O3 and SiO2 was non-selective to p-CAN, exhibiting
hydrodechlorination character with preferential NB formation at lower conversion and AN as
major product at higher conversion. This can be accounted for in terms of repulsion between –
NO2 and –Cl and surface Pdδ- leading to adsorption via the benzene ring, which facilitates
hydrogen attack of both substituents. In complete contrast, Pd/ZnO (and Pd-PVP/ZnO)
delivered p-CAN as the sole product without further dechlorination, which is linked to the
formation of a PdZn alloy that serves to exclusively activate the nitro functional group.
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Acknowledgments
The authors acknowledged financial support from the Swiss National Science
Foundation (Grant 200020-132522/1) and the Overseas Research Student Award Scheme
(ORSAS).
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Table 1: Catalyst source or preparation method (for laboratory synthesised samples), Pd loading, BET
surface area and hydrogenation performance in terms of product selectivities at a common fractional
p-CNB conversion (Xp-CNB = 0.2).
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60
SXp-CNB = 0.2
(%)
Catalyst
Source/
Peparation
Pd loading BET area
(% w/w)
Catalyst
(m2 g-1)
p-CAN
AN
NB
Pda
Commercial
-
2
29
74
38
0
55
0
16
26
10
10
22
62
32
0
Prepared by
impregnation
Pd/AC-Ib
Pd/AC-II
5.5
10.2
7.7
0.9
1.2
6.9
4.7
875
826
178
157
160
173
8
Commercial
52
90
78
38
68
0
Prepared by
impregnation
b
Pd/SiO2
Prepared by
deposition
Pd/Al2O3-Ib
Pd/Al2O3-II
Pd/Al2O3-IIIb
Pd/ZnOb
0
Commercial
0
Prepared by
impregnation
0
Prepared by
deposition
100
agenerated by reduction of PdO; blaboratory synthesised
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Table 2. Temperature and H/Pd ratio associated with Pd hydride decomposition and Pd nanoparticle
8
9
size obtained from TEM (dTEM), CO (dCO) and H2 (dH ) chemisorption.
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2
Pd hydride
Pd nanoparticle size
Catalyst
dH
dTEM
dCO
T
(K)
H/Pd
2
(mol mol-1)
(nm)
(nm)
(nm)
Pd
377
365
373
362
343
355
377
356
0.67
-
-
-
Pd/AC-I
0.29
0.16
0.36
0.25
0.05
0.33
0.22
11.7
4.4
12.6
6.4
2.4
9.6
6.5
8.2
129.0
4.7
Pd/AC-II
Pd/SiO2
19.9
36.7
28.8
2.3
9.4
Pd/Al2O3-I
Pd/Al2O3-II
Pd/Al2O3-III
Pd/ZnO
4.1
2.0
49.0
14.6
25.0
4.0
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