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P. Castillo-Villalón, J. Ramírez / Journal of Catalysis 268 (2009) 39–48
catalyst. This means that the HDS activity of Ru/Al2O3 catalysts
increases with the sulfur content of the active phase during reac-
tion (higher S/Ru ratio) [19].
It is most likely that the HDS activity of ruthenium sulfide will
be affected by the use of a semiconductor support such as TiO2,
although this case has not been studied in the past.
2.2.2. Dibenzothiophene (DBT) and 4,6-dimethyl-dibenzothiophene
(4,6-DMDBT) hydrodesulfurization
The catalytic tests were performed in a 350 ml Parr batch reac-
tor. The reaction products were analyzed in a Varian CP-3800 chro-
matograph. Prior to the reaction test, the catalyst was sulfided at
atmospheric pressure for 2 h with a 15 ml/min H2S(15 vol%)/N2
stream at 573 or 873 K, and was transferred to the batch reactor
in argon atmosphere to avoid contact with air. The activity was
measured at 593 K and 1300 psia for 9 h.
The objectives of the present work are to (a) establish the char-
acteristics and catalytic behavior of ruthenium sulfide particles
supported on a semiconductor oxide such as TiO2; (b) identify
the type of electronic interactions between TiO2 and the supported
ruthenium sulfided phase; and (c) determine the consequences of
the electronic interactions on the HDS activity of the catalysts. To
this end, Ru/TiO2 catalysts sulfided at different temperatures
(573–973 K) were studied by EPR, UV–Visible–NIR DRS, tempera-
ture-programed reduction of sulfided samples (TPR-S), Z-contrast
electron microscopy, and XRD. The catalysts were tested in the
HDS of thiophene, dibenzothiophene, and 4,6-dimethyl-dibenzo-
thiophene. Thiophene was chosen because this molecule can reach
without impediments the active sites and consequently, the
changes in the characteristics of the catalysts can be easily related
to the observed changes in activity.
2.3. Characterizations
2.3.1. Temperature-programed reduction (TPR-S)
The experiments were carried out in a flow system equipped
with a microreactor coupled to a Varian Cary 50 UV–Visible spec-
trometer to measure the evolution of H2S at fixed wavelength and
to a Gow-Mac Thermal Conductivity Detector (TCD) to calculate
the amount of H2 consumed during the sulfide reduction.
The TPR-S experiments were carried out with Ru/TiO2 either
freshly sulfided or used in the catalytic tests. In the first case the
catalysts were sulfided in situ; in the second case, the catalysts
used in the thiophene HDS experiment were immediately trans-
ferred to the TPR-S microreactor in argon atmosphere. For the
reduction, the catalyst was heated in a 25 ml/min stream of
H2(70 vol%)/Ar at a constant rate of 10 K/min from room tempera-
ture to 1273 K. The reactor outlet stream was monitored by UV–
Visible at 200 nm, and after removing H2S in a trap, by TCD. As
the traces obtained by TCD and UV–Visible showed similar behav-
ior, only the H2S evolution is reported here.
2. Experimental
2.1. Catalyst preparation
A Ru/TiO2 catalyst with nominal Ru content of 2.1 ruthenium
atoms per square nanometer of TiO2, equivalent to 1.78 wt% Ru,
was prepared using RuCl3ꢁxH2O (Aldrich) as a precursor. TiO2 De-
gussa P-25 with surface area of 52 m2/g and pore volume of
0.9 cm3/g was used as support.
2.3.2. X-ray diffraction
To prepare the catalyst the precursor salt was dissolved in 2.5
times the volume of water needed to obtain incipient wetness
(2.5 ꢂ 0.9 ml/g TiO2). The solution was maintained under stirring
for 12 h in N2, the titania powder was added and the suspension
was stirred for another 12 h. The catalyst was dried first in air flow
at room temperature to eliminate the excess liquid and then in an
oven at 383 K for 24 h. The solid was stored in a vacuum desiccator
and was used without further drying for the experiments.
An alumina-supported catalyst Ru/Al2O3 with 2.1 ruthenium
atoms per square nanometer of Al2O3 (equivalent to 7 wt% Ru)
was also prepared with the procedure described above.
The X-ray diffractograms of sulfided samples were registered
with a Phillips 1050/25 diffractometer using Cu K
a radiation
(k = 1.5418 Å) and a goniometer speed of 1°(2h) minꢃ1. The sulfid-
ed samples were transferred from the reactor to the diffractometer
in argon atmosphere.
2.3.3. DRS-UV–Visible–NIR spectroscopy
The spectra of freshly sulfided catalysts were taken with a Cary
500 Varian spectrometer equipped with a diffuse reflectance
sphere. The sulfided samples were transferred directly from the
reactor to the sample holder in argon atmosphere.
For thiophene, dibenzothiophene and 4,6-dimethyl-dibenzo-
thiophene HDS, TPR-S, XRD, UV–Visible–NIR DRS, and EPR experi-
ments the catalysts were sulfided for 2 h at different sulfidation
temperatures (573, 673, 773, or 873 K) in
H2S(15 vol%)/N2 stream.
a 15 ml/min
2.3.4. Electron paramagnetic resonance (EPR)
The EPR experiments were carried out with freshly sulfided
samples and with samples after thiophene HDS tests in a Bruker
ELEXSYS-E500 spectrometer (X Band) at room temperature, 150,
and 220 K. Only the spectra taken at 150 K are reported here. The
sulfided sample was transferred to an EPR tube previously filled
with argon; the tube was sealed and placed in the sample
compartment.
2.2. Catalytic tests
2.2.1. Thiophene hydrodesulfurization
The catalytic tests were performed at atmospheric pressure in a
continuous flow microreactor. The reaction products were
analyzed by on-line gas chromatography (5890 Series II Hewlett
Packard gas chromatograph). Prior to the catalytic tests the cata-
lyst (100 mg) was sulfided in situ for 2 h with a 15 ml/min
H2S(15 vol%)/N2 stream at 573, 673, 773, or 873 K. After sulfida-
tion, a 20 ml/min stream of hydrogen saturated with thiophene
at 275 K was contacted with the catalyst. Initially, the catalyst
was maintained at a reaction temperature of 633 K until the con-
version remained constant (ꢀ15 h), followed by measurements of
thiophene conversion at different temperatures (from 593 to
493 K and back to 633 K).
2.3.5. Z-contrast electron microscopy
After 15 h under HDS reaction at 633 K, the sulfided catalysts
were analyzed by Z-contrast in a Jeol JEM 2200 FS electron micro-
scope. To avoid contact with air, the catalyst powder was trans-
ferred directly from the reactor filled with N2 to a vial filled with
n-heptane. One drop of the suspension catalyst–n-heptane was
placed in a copper grid with carbon lacey, and, after evaporation
at ambient conditions, the sample was introduced in the
microscope.