M. Li et al. / Applied Catalysis A: General 531 (2017) 52–59
53
Fig. 1. Reaction scheme for the reductive coupling of nitrobenzene with benzaldehyde.
ity in –NO2 reduction [23]. Moreover, control over contact time
in continuous operation can govern conversion and reaction rate
resulted in elevated conversion in benzonitrile → benzylamine [25]
and furfural → 2-methylfuran [26]. In this study, we consider these
factors and examine the performance of Au on a series of oxides
(TiO2, Fe2O3, Al2O3, ZrO2 and MgO). Supported Pd is used commer-
cially in nitrobenzene hydrogenation [27]. As selective nitroarene
adsorption via the –NO2 function on TiO2 and at the metal-TiO2
interface serves to enhance amine production rate [28], we have
adopted Pd/TiO2 as a benchmark.
tachrome Instrument) unit equipped with a thermal conductivity
detector (TCD) for continuous monitoring of gas composition and
the TPR WinTM software for data acquisition/manipulation. Sam-
ples were loaded into a U-shaped Pyrex glass cell (3.76 mm i.d.)
and heated in 17 cm3 min−1 (Brooks mass flow controlled) 5% v/v
H2/N2 at 2–5 K min−1 to 423–603 K, which was maintained until
the signal returned to baseline. The activated sample was swept
with 65 cm3 min−1 N2 for 1.5 h, cooled to reaction temperature
(413 K) and subjected to H2 pulse (10 l) titration. In blank tests,
there was no measurable H2 uptake on the support alone. Oxygen
chemisorption post-TPR was employed to determine the extent
of support reduction, where samples were reduced as described
above, swept with 65 cm3 min−1 He for 1.5 h, cooled to 413 K
with pulse (50 l) O2 titration. It has been demonstrated that
Au contribution to total O2 adsorbed is negligible [33]. Nitrogen
adsorption-desorption isotherms were obtained using the com-
mercial Micromeritics Gemini 2390p system. Prior to analysis, the
samples were outgassed at 423 K for 1 h in N2. Total specific sur-
face area (SSA) was obtained using the standard BET method. Metal
particle morphology (size and shape) was examined by scanning
transmission electron microscopy (STEM, JEOL 2200FS), employing
Gatan Digital Micrograph 1.82 for data acquisition/manipulation.
Samples for analysis were prepared by dispersion in acetone and
deposited on a holey carbon/Cu grid (300 Mesh). The surface area
weighted mean metal size (dSTEM) was based on a count of at least
2. Experimental
The supports used were commercial (TiO2 (P25, Degussa), Al2O3
(Puralox, Condea Vista) and MgO (Sigma-Aldrich)) or synthe-
sised (Fe2O3 and ZrO2) following procedures described elsewhere
[29,30]. Gold on MgO was prepared by impregnation of MgO (10 g,
Sigma Aldrich, >99%) with aqueous HAuCl4 (5 × 10−2 M, 50 cm3,
Sigma-Aldrich, 99%). The slurry was heated (at 2 K min−1) to 353 K
under vigorous stirring (600 rpm) and maintained in a He purge
for 5 h. Gold on TiO2, Fe2O3, Al2O3 and ZrO2 was synthesised
by deposition-precipitation with urea (Riedel-de Haën, 99%) as
and HAuCl4 (3 − 7 × 10−3 M, 400 cm3) was added to the support
(10–30 g). The suspension was stirred (600 rpm) and heated (at
2 K min−1) to 353 K, where the pH progressively increased to 6–8
after 3–4 h as a result of urea decomposition [31]. The solid obtained
in 45 cm3 min−1 He at 373 K for 5 h. A supported Pd benchmark was
prepared by precipitation of Pd(NO3)2 (4 × 10−3 M, 300 cm3, Sigma-
Aldrich, 99%) on TiO2 (10 g), adding aqueous Na2CO3 (2 M, Riedel-de
Haën, 99%) dropwise until pH >10 [32]. The slurry was heated (at
2 K min−1) to 353 K and maintained for 4 h. The solid was separated
by filtration, washed with distilled water and dried under vacuum
at 333 K for 12 h. The catalyst precursors were sieved (ATM fine test
sieves) to mean particle diameter = 75 m. Samples were activated
in 60 cm3 min−1 H2 at 2–5 K min−1 to 423–603 K and passivated at
ambient temperature in 1% v/v O2/He for ex situ characterisation.
300 particles according to
ꢀ
nidi3
i
dSTEM
=
(1)
ꢀ
nidi2
i
where ni is the number of particles of diameter di.
Catalyst testing was carried out at atmospheric pressure and
413 K in situ after activation in a continuous flow fixed bed tubu-
lar reactor (i.d. = 15 mm). The catalytic reactor has been described
elsewhere [29,30,34] but features pertinent to this study are given
below. A layer of borosilicate glass beads served as preheating
zone where the reactant was vaporised and reached reaction
temperature before contacting the catalyst bed. Isothermal con-
ditions ( 1 K) were maintained by diluting the catalyst bed with
ground glass (75 m), which was mixed thoroughly with the cat-
alyst before loading into the reactor. Reaction temperature was
continuously monitored by a thermocouple inserted in a ther-
mowell within the catalyst bed. The reactant(s) (benzaldehyde
and/or nitrobenzene) was(were) delivered as an ethanolic solu-
tion to the reactor via a glass/teflon air-tight syringe and teflon
2.2. Catalyst characterisation
Metal content was measured by atomic absorption spectroscopy
using a Shimadzu AA-6650 spectrometer with an air-acetylene
flame from the diluted extract in aqua regia (25% v/v HNO3/HCl).
Temperature programmed reduction (TPR) and H2 chemisorption
were conducted on the commercial CHEM-BET 3000 (Quan-