22
textural properties [23]. These features enable a narrow particle
size distribution for the active phase and a high catalytic efficiency
and selectivity, as recently reported by Duan et al. for the hydro-
genation of nitrobenzene with Pd entrapped into SBA-15 [22,24].
chemical stability, large specific surface area, eco-friendly proper-
ties, and high surface functionality [25,26]. Additionally, TiO2NTs
provide a suitable framework for the efficient dispersion of the
metal active phase [27,28]. Based on this background, the use of
TiO2NTs as support for Au nanoparticles appears as a valuable
strategy to improve the catalytic performance of this metal in
hydrogenation reactions of nitrobenzenes, as studied in the present
report.
In this work, TiO2NTs were prepared by hydrothermal synthe-
sis using an alkali treatment of TiO2 nanoparticles. This material
was used to support Au nanoparticles synthetized by deposition-
precipitation method. The Au-TiO2NT system was tested as
catalyst for the selective hydrogenation of a series para-substituted
nitrobenzenes using semi-batch conditions in ethanol liquid phase
[5]. Compounds under study contain electron-withdrawing and
electron-donating groups in the p-position relative to the nitro
moiety, aimed at examining the role of electronic effects on the
catalytic performance of the Au-TiO2NT system. Kinetic data (log
k) was modeled using a multilinear regression equation, in which
the Hammet’s sigma constants () of the p-substituents, and the
DFT-calculated solvation energy of the reactants in ethanol were
employed as predictor variables.
the mixture, the system was heated to reflux at 80 ◦C under stir-
ring until the change of color from yellow to purple. The adsorbed
anionic complex [AuCl2(OH)2]− was decomposed under flowing
hydrogen forming Au0. The solid was filtered and dried for 2 h at
100 ◦C, and labeled as Au-TiO2NT.
2.4. Materials characterization
The characterization of TiO2NT and Au-TiO2NT systems
was conducted using the following techniques. N2 adsorption-
desorption isotherms were carried out in an ASAP 2010
Micromeritics apparatus. Pore-size distributions were calculated
from the N2 adsorption branch using the Barrett-Joyner-Halenda
model (BJH). X-ray diffraction (XRD) was performed on a Rigaku
X-ray Geigerflex diffractometer using a Ni filter and Cu K ␣ radia-
tion within 2−80◦ 2 range. HRTEM was performed using a Philips
CM200 model high resolution electron microscope with energy dis-
persive analyzer and digital camera coupled to a high speed TVIPS
FastScan F-114 model of 1024 × 1024 pixels and 12 bits. Metal
particles were counted, and the surface area-weighted mean Au
diameter (dp) was calculated using the following equation:
ꢀ
inidi3
ꢀ
dp
=
(1)
inidi2
UV–vis spectra of diffuse reflectance of solid state was studied
in the range of 200–900 nm on a Varian Cary 3 UV–vis spectropho-
tometer equipped with an area of 150 mm in diameter covered
with poly integration tetra-fluoroethylene (PTFE). The dust samples
were mounted in a quartz cell, which provided a sample thickness
greater than 3 mm and thus guaranteed “infinite” sample thickness.
Chemical analysis was performed by inductively coupled plasma-
mass spectrometry on an ICP-MS Spectrometer Perkin Elmer Elas
6000. Photoelectron spectra (XPS) were recorded using an Escalab
200R spectrometer provided with a hemispherical analyzer, oper-
ated in a constant pass energy mode and Mg K␣ X-ray radiation
(hꢁ = 1253.6 eV) operated at 10 mA and 12 kV.
2. Experimental
2.1. Reagents
All reagents were purchase and used as received. The syn-
thesis of TiO2NTs was performed using titanium oxide anatase
nanoparticles (nanopower, particle size < 25 nm, BET surface
area 45–55 m2 g−1), sodium hydroxide (NaOH, Merck) and HNO3
(Merck, 65%). Gold(III) chloride trihydrate (Sigma-Aldrich 99.9%),
nitrobenzene (Ph-NO2), p-nitrotoluene (p-CH3), p-nitrophenol
(p-OH, Merck), p-nitroaniline (p-NH2, Merck), p-dinitrobenzene
(p-NO2, Sigma-Aldrich), p-nitrobenzonitrile (p-CN, Fluka), p-
nitroanisol (p-OCH3, Sigma-Aldrich), p-nitrobenzaldehyde (p-CHO)
and p-chloronitrobenzene (p-Cl, Merck), absolute ethanol (Merck),
2.5. Liquid phase hydrogenation of nitrobenzenes
The catalytic assays of nitrobenzenes hydrogenation were per-
formed in a stainless steel (150 mL) Parr-type semi-batch reactor at
a 25 ◦C using 0.100 g of catalyst and 0.02 mol·L−1 of nitrobenzene
as model molecule in ethanol, under 40 bar hydrogen pressure. All
found in a catalytic test established using the Madon and Boudart
approach [30]. No catalytic activity for pure TiO2NT was confirmed
in a blank test. Pseudo-first-order kinetic constants (k) were calcu-
lated as reported in previous studies [31]. Analytical quantifications
for reactants and products were analyzed by gas chromatography
using a GC–MS Shimadzu GCMS-QP5050 with a capillary column
-Dex 225 (Supelco).
2.2. TiO2NT synthesis
The synthesis of TiO2NTs was performed using a hydrothermal
process [29]. For this purpose, 2.5 g of the TiO2 nanoparticles was
dispersed in 125 mL of 10 mol·L−1 NaOH. The mixture was placed
into a hydrothermal reactor at 110 ◦C for 24 h with constant agita-
tion. After the hydrothermal treatment the solid was washed with
distilled water and put in contact with a 0.10 mol·L−1 solution of
HNO3 overnight at room temperature. The mixture was washed
and filtered with distilled water and the obtained nanotubes were
dried at room temperature for the night and then at 100 ◦C for 12 h.
2.6. Solvation energy calculations
Quantum chemical computational calculations were employed
to estimate the solvation energy of the series of nitrobenzenes
culations were computed under the frame of Density Functional
Theory (DFT) using the Gaussian 09 program. The hybrid B3LYP
functional was employed, which includes Beckeı´s 3-parameter
nonlocal-exchange functional [32] with the correlation functional
of Lee et al. [33]. The basis set employed for the calculations
was 6–311G(2p,2d). No symmetry constrains were applied dur-
ing geometry optimization procedures. Solute solvation energies
100 ◦C
Gold nanoparticles synthesis was performed using a deposition-
precipitation method [14]. The appropriate amount of TiO2NT was
placed in a round bottom flask together with 50 mL of water. Subse-
quently, the required amount of HAuCl4 for 1.0% wt. Au was added
together with a stoichiometric amount of NaOH (0.10 mol L−1).
After connecting a flux of H2 to the round bottom flask containing