I. Cuauhtémoc et al. / Catalysis Today 166 (2011) 180–187
181
Our aim was to find out relations between the structure, chemi-
cal state of the catalysts and the activity and selectivity towards
mineralization.
referenced to the C 1s line of adventitious carbon at 284.6 eV. The
samples were placed on a thin sheet of indium and then analyzed.
In order to control the sample charge in all the experiments, an
electron flood gun was used. No additional treatment was applied
to the samples prior to these measurements.
2. Experimental
The FTIR-CO adsorption spectra were obtained at room tem-
perature by using a FTIR Nicolet-Magna 560 apparatus, with a
resolution of 2 cm−1 and 100 scanners. The samples pressed in
thin wafers were placed in a Pyrex glass cell, equipped with CaF2
windows, coupled to a vacuum system and gas lines supplied. The
samples were maintained under vacuum (10−6 Torr) at 400 ◦C for
30 min. Then, the cell was cooled to room temperature and the CO
(PRAXAIR UHP 100%) admission of 20 Torr was carried out. The CO
excess was evacuated during 30 min, after this the CO adsorbed
FTIR spectra were recorded.
The TPO measurements were carried out in a CHEMBET-3000
apparatus using a thermal conductivity detector (TCD), and 0.1 g
of catalyst. In these experiments the flow rate of 5%O2/95%He mix-
ture was 10 mL/min and the heating rate was 10 ◦C/min. Finally, the
spectra were recorded from room temperature to 500 ◦C.
2.1. Catalysts
The ␥-Al2O3 support was obtained by calcination of the
boehmite Catapal B (CONDEA, high purity 99.999%, 74% AlOOH, 26%
H2O) under air flow (3.6 L/min) at 650 ◦C for 12 h. The ␥-alumina-
ceria supports containing 1, 5 and 20 Ce wt%, were prepared by
adding the appropriated amounts of aqueous solution contain-
ing Ce(NO3)3·6H2O (Strem Chemicals, 99.9%) to the aluminum
boehmite Catapal B. Afterwards, the impregnated boehmite was
kept under stirring in a rotary evaporator for 4 h. The water excess
was evaporated under vacuum at 60 ◦C and then the solid was dried
completely in an oven at 120 ◦C for 12 h. Finally, the ␥-Al2O3–CeO2
supports were obtained by annealing the samples under air flow at
650 ◦C for 24 h.
The Rh supported catalysts were prepared by wet impregna-
tion of ␥-Al2O3 and ␥-Al2O3–Ce adding the appropriated amounts
of an aqueous solution containing RhCl3·3H2O (Strem Chemicals
99.99%) to obtain a nominal concentration of 1 wt% of Rh. The
Rh–Sn catalysts were prepared by the coimpregnation of ␥-Al2O3
and ␥-Al2O3–CeO2 supports by adding the appropriated amounts
of aqueous solutions containing RhCl3·3H2O or SnCl4·H2O (Strem
Chemicals 99.99%) to obtain a nominal concentration of 1 wt% of Rh
and 1.15 wt%Sn (molar ratio Rh/Sn = 1). The impregnated catalysts
were dried at 120 ◦C in an oven for 12 h and then calcined under air
flow (3.6 L/h) at 500 ◦C for 4 h. Finally, the catalysts were reduced at
500 ◦C under hydrogen flow (3.6 L/h) for 5 h and stored until char-
acterization. The final percentage of Rh and Sn on the catalysts was
obtained by Inductively Coupled Plasma Atomic Emission Spec-
trometry (ICP-AES). The catalysts containing CeO2 were labeled
as Rh/ACeX and Rh–Sn/ACeX, where A indicates alumina, X the Ce
amount for each sample, X = 1, 5, and 20 wt%.
2.3. Catalytic tests
Experiments were carried out by using a 300 mL stainless steel
autoclave (Parr Instrument Co Ltd., IL, USA) equipped with a valve
for sampling and a magnetic driven stirrer (set at 1000 rpm).
Oxygen (10 bar) was used as oxidant source. The reaction was
carried out as follows: 150 mL of an aqueous of TAME solution
at a concentration of 227 ppm (corresponding to 160 ppm of C)
was placed in a glass vessel to avoid the contact of the solu-
tion with the reactor and then 1 g/L of the catalyst was charged
into the reactor. The reactor was firstly purged with nitrogen for
15 min and then heated at 120 ◦C. Afterwards, an oxygen pressure of
10 bar was introduced in the reactor and under continuous stirring
(1000 rpm) the reaction was initiated. Previous calibration showed
that under such conditions the reaction rate was not controlled
by the diffusion of oxygen into the liquid phase. The evolution of
the reaction was followed by performing the analysis of aliquots at
intervals of 10 min per 1 h. The samples were collected by using
the sampling valve, which is equipped with a microspore glass
filter to prevent the catalyst loss. No leaching of rhodium or alu-
mina was detected. These catalytic systems showed stability in the
TAME oxidizing reaction under the CWAO conditions used in this
work.
2.2. Characterization methods
The specific surface areas were determined by nitrogen adsorp-
tion in a Quantachrome CHEMBET3000. Before performing the
adsorption, the calcined supports were treated at 400 ◦C for 1 h
under helium flow.
The samples were analyzed by GC with a FID detector equipped
with a capillary column (DBWAX 30 m × 0.53 mm id, 1.0 m). A
temperature ramp was set up in order to separate the pollutant
and their intermediates during the reaction. The analysis of total
organic carbon (TOC) was performed by using a 5000TOC Shimadzu
Analyzer, which was previously calibrated to obtain concentrations
in the range of 0–300 ppm of TOC. TOC = after at 1 h of reaction and
TOCo = value at t = 0.
Temperature programmed reduction (TPR) of the catalysts was
made in a CHEMBET-3000 (QUANTACHROME Co) equipment. The
experiments were carried out using 0.1 g of reduced catalyst. The
sample was treated under nitrogen (10 mL/min) at 400 ◦C for 1 h,
with a heating rate of 10 ◦C/min. After this, the sample was cooled
down at room temperature and then a flow of 5% H2/95% N2 was
passed through the sample. The TPR profiles were registered using a
heating rate of 10 ◦C/min up to 500 ◦C with a rate flow of 10 mL/min.
High angle annular dark field (HAADF) scanning transmission
electron microscopy (STEM) analysis of the samples annealed and
reduced at 500 ◦C was performed in a JEM-2200FS, transmission
electron microscope with an accelerating voltage of 200 kV. The
microscope is equipped with a Schottky-type field emission gun
and an ultra high resolution (UHR) configuration (Cs = 0.5 mm; Cc
1.1 mm; point-to-point resolution, 0.19 nm) and in-column omega-
type energy filter.
3. Results and discussion
It was observed that the addition of cerium nitrate to boehmite
leads to a diminution of the BET specific surface area of the supports
from 266 m2 g−1 for A to 232, 206 and 139 m2 g−1 for the ACe1, ACe5
Ce. This drop in the specific surface area increases with the Ce load,
showing that the cerium oxide modifies the textural properties of
the ␥-Al2O3.
The XPS analysis was carried out by using a THERMO VG
ESCALAB 250 spectrometer equipped with an aluminum anode
(energy of 1486.8 eV) and an X-ray system monochromator. The
X-ray source was powered at 15 kV and 7.5 mA. In order to correct
the effect of charge in the XPS spectra, all the binding energies were
In Table 1 are reported the load of metals for mono metallic
and bimetallic catalysts which are around 0.79–0.9 wt% for Rh and