J.J. Martínez et al.
MolecularCatalysis465(2019)54–60
strong metal support interaction (SMSI) effect which occurs at low re-
duction temperatures [18–20]
removed before arriving the TCD.
X-ray photoelectron spectroscopy (XPS) analysis was performed
using a Thermo Scientific Spectrometer Escalab XI 250, equipped with
an aluminum anode (1486.8 eV) and monochromator system X-ray
analyzer coupled with a power step 25 eV and a step size of 0.05 eV.
The pressure in the analysis chamber was 6.3 × 10−9 MPa. To correct
the charging effect of the sample in the X-ray spectrum, all binding
energies were referenced to the C 1 s line at 284.6 eV. The XPS spectra
were analyzed using Avantage software.
2. Experimental
2.1. Materials
All reactive grade solvents and reagents were purchased from dif-
ferent companies and used without further purification; m-DNB (Merck,
975%), m-NA (Aldrich, 98%), m-PDA (Merck, 99%). acetone (J. T.
Baker, 99%) and ethanol (J. T. Baker, 99%). The precursors used were
rhodium (III) acetylacetonate (Aldrich, 97%), Zirconium(IV) oxynitrate
hydrate (Aldrich, 99.9%), and SiO2 (Grace Davison), TiO2 (Evonik,
Aeroxide P-25), Al2O3 (Alúmina S.A), as supports were commercial
available.
The nature of the acid sites was studied by pyridine adsorption
followed by FTIR (Pyr-FTIR). Infrared spectra were collected using a
Nicolet Thermo iS50 equipment and in-situ adsorption of pyridine was
carried out in a diffuse reflectance cell (Harrick, Praying Mantis). The
samples were reduced at 300 °C under hydrogen flow (50 mL/min) for
1 h and then gradually cooled down to 50 °C in helium flow (30 mL/
min). A reference spectrum of the solid was collected. Then pyridine
adsorption took place for 1 h. After adsorption, pyridine was gradually
removed by evacuation with an helium flow (30 mL/min) at 373 °C and
the spectrum was recorded. Spectra were collected from 120 scans with
2.2. Catalyst preparation
Tetragonal ZrO2 was prepared dissolving 66.06 g of ZrO(NO3)2 and
60 g of CO(NH3)2 in 90 mL H2O and then added to 900 mL of H2O.
Subsequently, 30 mL of an aqueous NH3.H2O solution was added
dropwise and the mixture was refluxed for 5 h. The obtained solid was
washed for 3 times with hot deionized H2O in the centrifuge, dried in a
rotary evaporator at 100 °C for 12 h and finally crushed.
The supports (ZrO2, Al2O3 and TiO2) were sieved to mesh size 100
(150 μm) and subsequently calcined at 400 °C for 4 h using a heating
ramp of 5 °C/min. Catalysts with a nominal 1 wt% Rh content were
prepared by impregnation of each support with the appropriate amount
of rhodium (III) acetylacetonate precursor. In a typical experiment,
2.97 g of support and 0.1203 g of rhodium (III) acetylacetonate in
acetone, were mixed and stirred at 150 rpm for 40 min at room tem-
perature [21]. The resulting solid was dried at 50 °C under vacuum and
then calcined at 400 °C for 2 h with a heating rate of 2 °C/min in air
flow of 50 mL /min. The materials were labelled as Rh/TiO2, Rh/Al2O3
and Rh/ZrO2.
a resolution of 2 cm−1
.
2.4. Catalytic experiments
The catalytic hydrogenation of m-DNB (Fig. 1) in liquid phase was
carried out in a batch type reactor at 50 °C and a pressure of H2 of
200 psi. The catalyst was reduced ex-situ at 300 °C in H2 flow (30 mL/
min) and loaded carefully in the reactor already containing 40 mL of m-
DNB solution in ethanol 0.02 M purged with helium. The reactor was
then sealed and H2 was loaded to the desired pressure. Typically, 0.2 g
of catalyst was used, and the reaction was monitored for 3 h.
The reaction progress was followed by gas chromatography in a
Varian 3800 equipment using a RTX-5 column working with the fol-
lowing heating program: 80 °C for 1 min, then 10 °C/min up to 300 °C
and finally hold time of 3 min. The CG injector and FID detector tem-
peratures were 250 and 300 °C, respectively. Under these conditions the
elution time was 15.8 min, 16.6 min and 13.5 min for m-DNB, m-NA and
m-PDA, respectively.
2.3. Characterization methods
Rh content in the samples was determined by energy dispersive X-
ray spectroscopy (EDS) using an Oxford-ISIS detector coupled to a
scanning electron microscope (JEOL JSM-5900-LV). The average metal
loading was reported in grams of metal per gram of sample after ana-
lysis of 25 different areas of each sample.
Surface area was determined by N2 adsorption at −196 °C in a
Micromeritics ASAP 2020 equipment. The calcined samples were pre-
viously degassed at 200 °C.
3. Results and discussion
Table 1 displays results about the Rh content as determined by EDS,
the dispersion (D) expressed as H/Rh molar ratio as obtained from H2-
chemisorption isotherms and the mean particle size (ds) obtained by
TEM. As observed the nominal Rh loading (1 wt%) was obtained in
catalysts supported on TiO2 and Al2O3, while a slightly lower content
(0.8 wt%) was observed in the Rh/ZrO2 catalyst. This difference could
be related to sampling for collecting the EDS spectra. As pH of the
impregnating solution is close to the PZC of ZrO2 (pH = 5) it is ex-
pected a better interaction of the precursor with the surface of TiO2 and
Al2O3 than with ZrO2. Heterogeneity of the resulting distribution is also
confirmed by TEM.
Rh dispersion was determined by H2 adsorption using
a
Micromeritics ASAP 2020 chemisorption analyzer at 35 °C in the pres-
sure range of 0.0001 to 0.5 MPa. For these experiments, the catalysts
were reduced in-situ at 300 °C under hydrogen flow for 2 h. Then, the
samples were evacuated at the reduction temperature for 30 min and
cooled to 35 °C. At this temperature, the catalysts were further evac-
uated for 2 h followed by analysis. Rh dispersion was determined using
the dual isotherm method to estimate total and reversible chemisorp-
tion uptakes. The H/Rh molar ratio was assumed as equal to 1.5 [22].
Particle size distribution and average particle size was determined
by transmission electron microscopy in a JEM 2010 F analytical mi-
croscope equipped with HAADF detector. At least 150 particles were
measured to obtain the mean particle size < ds > defined as,
ds = Σni di3/Σnidi2, ni corresponding to the number of particles with a
diameter di.
Regarding metal dispersion determined by H2-chemisorption, Rh/
ZrO2 shows the lowest one which results in the highest particle size
value as confirmed by TEM measurements. A very good agreement
The reduction properties of the catalysts were studied by tempera-
ture programmed reduction with hydrogen (TPR-H2) using an ISRI RIG-
100 unit. The calcined sample was reduced using a 5%H2/N2 gas
mixture flow (30 mL/min) and a heating rate of 10 °C/min from room
temperature to 600 °C. The H2O produced by the reduction was
Fig. 1. Hydrogenation of m-dinitrobenzene to m-PDA scheme.
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