catalysts doped with transition metals by sol

CATALYSTS DOPED WITH TRANSITION METALS BY SOL-GEL AND USED IN
TRICHLOROETHYLENE COMBUSTION
Javier Rivera De la Rosa1,2* , Ma. Aracely Hernández Ramírez1, Carlos J. Lucio Ortiz1 and Emmanuel E. Villarreal España1
1
Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, Av. Universidad s/n, Cd. Universitaria, San
Nicolás de los Garza, N. L. C.P. 64450, (México).
2
Centro de Innovación, Investigación y Desarrollo en Ingeniería y Tecnología (CIIDIT), Km 10 de la nueva carretera al
Aeropuerto Internacional de Monterrey, PIIT Monterrey, C.P. 66600, Apodaca, N. L., (México)
* [email protected]
Abstract: Two oxide catalysts doped with La and Fe were synthesized by sol-gel method. ZrO2 and Al2O3 were characterized by
X-ray diffraction (XRD). Textural properties of the catalysts were studied through N 2 sorption isotherms measured at 77 K.
Scanning electronic microscopy (SEM) images showed how the doping agents caused different crystals aggregation modes
during the xerogel annealing process and transmition electronic microscopy (TEM) and XRD studies reveled that crystallite size
was affected by the doping. The catalytic tests were conducted using a fix bed tubular reactor and monolithic reactor. The La-Fe
doped ZrO2 resisted the high concentration of trichloroethylene flow in the corresponding catalytic test due to the presence of the
dominant tetragonal zirconia phase. The Fe doped Al2O3 catalyst showed better catalytic activity dues mainly to the higher
specific surface areas (239 m2/g).
I. INTRODUCTION
The sol–gel method has attracted considerable attention for the preparation of metallic catalysts, since the
constituents are mixed in an atomic scale and this generates a uniform distribution of active metals upon the
support [1]. The crystalline phase obtained by the sol–gel method depends on the starting metallic precursor.
The samples obtained this way were thoroughly characterized with the purpose of evaluating and comparing
them as useful catalysts for the combustion of trichloroethylene. The importance of studying the synthesis of
new catalysts for destruction of trichloroethylene molecule is that it belongs to the group of chlorinated
volatile organic compounds (VOC) that can be see a simulation of air pollution by industrial gas emissions.
II. EXPERIMENTAL
For the ZrO2 synthesis; zirconium butoxide was mixed with 1-butanol. Glacial acetic acid was added under
magnetic stirring until reaching pH 3. Acetate solutions of the doping metals were then added dropwise to the
butoxide-butanol mixture; the La and Fe doping species represented a 0.5 wt. % of the ZrO2 mass. The
solution was maintained under refluxing conditions at 60 °C until a gel was formed. Xerogels were then
obtained by treating the gel samples at 70 °C followed by calcination in an electrical furnace at 800 °C for 8 h.
For the Al2O3 synthesis, (Al(NO3)3•9H2O) was dissolved in water and NH4OH 8 M solution was added under
magnetic stirring until reaching pH ~ 9.5. Fe(NO3)3 solution was added dropwise to the colloidal alumina; the
doping iron represented a 0.5 wt. % of the Al 2O3 mass. The solution was maintained under refluxing
conditions at 25 °C until a gel was formed. Xerogels were then obtained by heating the gel samples at 67°C
followed by calcination in an electrical furnace at 600 °C for 8 h. All sample powders were characterized by
FT-IR, TGA, DTA, XRD, SEM and TEM techniques; the microstructure of pores was determined from N 2
adsorption isotherms The catalytic activity was tested using a steady state tubular micro-reactor inserted in a
tubular furnace equipped with a temperature controller, employing mixtures of trichloroethylene in air (3200
ppm by volume for alumina samples and 1% for zirconia samples) and the spatial time was of 500 hr -1. Table 1
shows the characteristics of the different catalysts synthesized.
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Table 1 Oxide catalysts characteristics.ABET: Specific area by BET method
Oxide
compound
%
wt
Fe
% wt
La
Z
ZrO2
0.0
ZLF
ZrO2
A
AF
Sample
Crystalline
Phase
ABET
m2 g-1
0.0
Tetragonal,
monoclinic
5.1
0.5
0.5
Tetragonal
1.5
Al2O3
0.0
0.0
Cubic
254
Al2O3
0.5
0.0
Cubic
239
III. RESULTS
Table 1 shows the crystalline phase identified by XRD patterns analysis. The dopping agents cause tetragonal
phase stability in ZLF sample. For AF sample the dopping does not cause appreciable change of the phase
(cubic -alumina). The DTA analysis let to see how the doping displaced the temperature transformation peaks
to tetragonal phase from 455 to 460 C; in case of Z and ZLF samples. For alumina samples, the corresponding
peaks to crystalline transformation were not displaced for the doping.
Figure 1 (a) shows the XRD patterns for pure zirconia (Z) and La-Fe doped zirconia (ZLF) catalysts. It can be
seen that the tetragonal phase is the dominant one. The development of the tetragonal phase by the presence of
doping agents could be due to a decrease in the surface free energy of ZrO2 or to the creation of anionic
vacancies [2].
(a)
(b)
ZLF
A.U.
Tetragonal
Monoclinic
Z
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2
Figure 1. DRX patterns of Z and ZLF samples calcined at 800 C (a) and DRX patters of AF sample calcined at different temperatures.
In Figure 1(b) are showed the AF XRD patterns at different temperatures of calcination, It can be seen that at
200 ºC are still present the precursors of sol gel synthesis. At 400 ºC is starting to define the cubic phase of
alumina ( -alumina), and at 600 ºC it is well formed. For pure alumina calcined at 600 ºC the cubic array was
the only identified crystalline phase by XRD.
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Table 2 shows the crystallite size calculated from XRD patterns by Scherrer equation.
Table 1 Crystallite size by two techniques. Tetragonal phase (t)
TEM
Scherrer equation
Sample
Images
nm
Size range in nm
Z
40.1 (t)
2-40
ZLF
36.5 (t)
2-40
A
12.89
2-14
AF
9.73
3-10
Nanoparticle size ranges observed in TEM images (not showed here) are reported in Table 2. It can be seen
that doping affect to obtained a minor size in the crystallite size. It can be related higher values of specific are
for alumina samples than ZrO2 samples. SEM images (not showed here) demonstrate how doping agents cause
effect in the aggregation mode of both particles of oxide compounds.
Figure 2 (a) shows the conversion as function of temperature in the combustion of trichloroethylene at 1 % in
air using Z and ZLF catalysts. The conversion curve observed in blank run (with silica) and due to
homogeneous combustion is shown for comparison (thermal labelled curve), to evidence the contribution of
the catalysts. Z catalyst does not present catalytic activity at 200°C and ZLF shows 9% of conversion at the
same temperature. Catalyst doped with La and Fe continues with excellent catalytic performance at high
temperatures. In this way the ZLF catalyst reaches almost 100% of conversion at 500°C meanwhile Z catalyst
reaches 67% of conversion and the thermal combustion only achieves 50% at the same temperature. In the
combustion test of A and AF catatlys (Fig. 2 b), it is observed that at 600 ºC the 100% of conversion for both
catalysts is reached. It is distinguished in thermal combustion (the reactor charged with inert silica) the
conversion of trichloroethylene begins upper 200 ºC, nevertheless for A catalyst the conversion at the same
temperature is 9 % and for AF catalyst is 21 %. It can be state that both synthesized catalyst, pure and doped
alumina, exhibit catalytic activity, depending on the surface reaction taking place at a range from 50 at 200 ºC.
100
Conversione %
80
Thermal
Z
ZLF
60
40
20
0
150 200 250 300 350 400 450 500 550 600 650
Temperature (°C)
Fig. 2. Combustion of trichloroethylene at 1 % in air for Z and ZLF catalysts (a). Combustion of trichloroethylene at 3200 ppm in air for A
and AF catalysts, thermal combustion is showed at the same conditions.
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The specific surface areas of the ZrO2 catalysts range from 1 to 5 m2 g-1 and the pore size from 10 to 60 nm
approximately (analysis from N2 adsorption isotherms not showed here), this means that low surface area have
a minor contribution on the surface reaction of catalytic combustion of trichloroethylene; on the other hand,
the relatively large cavity sizes can facilitate the transport of reactants and products through the porous
structure of ZrO2 catalysts; which is related to percolation and permeability characteristics of the porous
materials. The main crystalline phase stabilized for ZLF was the tetragonal for zirconia support that seems to
have a positive effect in the combustion tests. For alumina oxide catalysts the catalytic activity performance
can be explained mainly for the high values of specific surface areas.
IV. CONCLUSIONS
It has been prepared and characterized samples of zirconia and alumina substrates doped with La, and Fe. The
doping agents caused a minor crystallite size compared with undoped oxides and different aggregation mode.
The catalytic performance of pure ZrO2 and ZrO2 doped with La and Fe can be attributed at their morphologic
and porous characteristics. Tetragonal crystalline phase of ZrO2 has a positive effect. The high specific area of
pure and doped with Fe alumina catalysts improves the catalytic tests, but the dopping of 0.5% of Fe has a
positive effect in the activity even presents lower specific area than undoped sample.
References
1.
T. Lopez, M. Alvarez, R. Gómez, D.H. Aguilar, P. Quintana J. Sol–Gel Sci. Technol. 33, 93 (2005).
2.
J. A. Navio, M.C. Hidalgo, G. Colon, S.G. Botta, M.I. Litter, , Langmuir 17, 202 (2001).
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