ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2007, том 81, № 9, с. 1654-1659
OTHER PROBLEMS ^^^^^^^^^^^^ OF PHYSICAL CHEMISTRY
УДК 541.128
STUDY OF THE DISPERSITY OF IRON OXIDE AND IRON OXIDE-NOBLE METAL (Me = Pd, Pt) SUPPORTED SYSTEMS
© 2007 Z. P. Cherkezova-Zheleva*, M. G. Shopska*, J. B. Krstic* *, D. M. Jovanovic* *,
I. G. Mitov*, G. B. Kadinov*
*Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Block 11,1113 Sofia, Bulgaria **IChTM-Center of Catalysis, Njegoseva 12,11000 Belgrade, Republic of Serbia
E-mail: zzhel@ic.bas.bg
Abstract - Samples of one (Fe) and two-component (Fe-Pd and Fe-Pt) catalysts were prepared by incipient wetness impregnation of four different supports: TiO2 (anatase), y-Al2O3, activated carbon and diatomite. The chosen synthesis conditions resulted in formation of nanosized supported phase - iron oxide (in one-component samples) or iron oxide-noble metal (in two-component ones). Different agglomeration degree of this phase was obtained as a result of thermal treatment. Ultradisperse size of supported phase was kept in some samples, while a process of partial agglomeration occurred in the others giving rise to nearly bidisperse (ultra-and highdisperse) supported particles. The different texture of the used supports and their chemical composition are the reasons for different stability of nanosized supported phases. The samples were tested as heterogeneous catalysts in total benzene oxidation reaction.
The morphology and valence band structure of nanosized (1-10 nm) metal and metal oxide particles differ fundamentally from those of large particles in terms of short-range ordering [1-5]. Their catalytic activity and selectivity, based upon these new magnetic, optic, electric, and other properties, can be changed due to the critically small size. Thus an enhanced catalytic activity and selectivity in hydrocarbons oxidation is expected from the nanosized metal/metal oxide supported particles. As far as iron oxide based systems are concerned, the formation of highly active sites is associated with the formation of highly dispersed iron oxide because of the strong iron-support interaction [6]. Nowadays, the research interest is focused on deeper look insight the preparation techniques for highly active and selective catalysts, as well as into the way to stabilize this highly disperse and highly active nanosized phase.
On the other hand, the emission levels of volatile and semi-volatile organic compounds such as benzene, formaldehyde and polycyclic aromatic hydrocarbons in the air are under strict legislation and control in many countries [7, 8]. Therefore the complete hydrocarbon oxidation is of great importance for the environmental protection. The investigation of transition metal oxidenoble metal supported catalysts is a question of theoretical and practical interest, because of the expected syn-ergetic effect between the components leading to an improvement of their catalytic performance in the hydrocarbon oxidation due to formation of such nano-sized metal/metal oxide supported particles.
The main factors for the preparation of high disper-sity and stable active phase are the method and the conditions for precursor deposition, calcination, reduction, support pre-treatment, etc. The impregnation is a clas-
sical preparation method, but it has unused possibilities to prepare active phases in nanosized region. Because of its simplicity and accessibility the method is not in largely substituted by new techniques - like laser ablation, "ion assisted" preparation (sol/gel techniques, sol precursors, zeolite encaged metal particles, etc.).
The aim of the present study was to investigate the influence of support characteristics and thermal treatment on the active phase dispersity and its stability, in order to obtain different catalytic properties of supported active phase.
EXPERIMENTAL
Sample Preparation. Four different supports were used - TiO2 (anatase, BDH Chemical Ltd., England), Y-Al2O3 (type 20-1/83P, G-3, Poland), activated carbon (NORIT PKDA, The Netherlands) and diatomite. The used diatomite support is a natural substance (Baros evac, the "Kolubara" Coal Basin - field B, Laz-arevac, Serbia). The crude diatomite has relatively high humidity level and is preliminary ground, chemically (with an aqueous solution of HCl) and thermally (at 1073 K) treated in order to obtain an activated support before catalyst synthesis. The chemical composition (wt. %) of the diatomite after activation is: 93.07% SiO2, 3.87% Al2O3, 0.56% Fe2O3, 0.59% CaO, 0.80% MgO, 0.05% Na2O, 0.56% K2O.
The fraction of supports with particle size 0.63-0.8 mm is used. The catalysts were prepared by incipient wetness impregnation of the supports with aqueous solutions of Fe(NO3)3 ■ 9H2O and Pd(NO3)2 ■ 2H2O or (NH3)4PtCl2 ■ H2O, respectively. The samples were dried at 343 K in vacuum, heated in vacuum for 3 hours
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Table 1. Physical characteristics of supports
Parameters TiO2 Diatomite Y-AI2O3 AC
.Sbet, m2/g 11.9 16.8 303.9 923.4
Ft0t, cm3/g 0.054 0.102 0.618 0.750
Vmic, cm3/g 0.004 0.006 - 0.400
d0, nm UD 2-4 (33.6%) 3-6 (69.5%) <3 (47.0%)
4-6 (14.2%) 6-10 (21.4%) 3-6 (34.8%)
Notes: AC - activated carbon, d0 - predominant pore size, UD - uniform distribution.
at 493 K (as prepared samples) and calcined in air for 3 hours at 713 K (thermally treated samples). The metal loading was 8 wt. % Fe and 0.7 wt. % noble metal.
Support characterization. The total pore volume of all samples, were determined by nitrogen adsorption/desorption isotherms at 77 K using the Sorptomat-ic 1990 apparatus (ThermoQuest, CE Instruments). The specific surface area of samples sBET, was calculated according to Brunauer-Emmett-Teller method from the linear part of the nitrogen isotherms [9]. The pore size distribution for mesopores was calculated according to Barett-Joyned-Halenda (BJH) method [10, 11] from desorption branch of the isotherm. The micropore volume was calculated using Dubinin-Radushkevich equation [12].
Active phase composition. The chemical composition of the prepared systems was analyzed by atomic emission spectrometer (AES) with ICP, model 3410 (ARL, USA). The as prepared and calcinated samples were studied before and after catalytic tests by Moess-bauer spectroscopy (MS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).
MS measurements were carried out on Wissenschaftliche Elektronic GmbH instrument, operating in a constant acceleration mode (57Co/Cr source, a-Fe standard). The following parameters of hyperfine interactions of spectral components were determined by computer fitting: isomer shift (IS), quadrupole splitting (QS), effective hyperfine magnetic field (Heff), line widths (FWHM) and component relative weights (G).
XRD patterns were obtained on TUR M62 apparatus, HZG-4 goniometer with Bregg-Brentano geometry, CoKa radiation and Fe filter. JCPDF data base was used for the phase identification [13].
XPS study was performed on ESCALAB-Mkll spectrometer (VG Scientific), by unmonochromatized MgKa radiation (1253.6 eV). The total instrumental resolution was 1.5 eV (measured from the Ag5d5/2 line width). The energy scale was calibrated by the C1s line (285 eV).
Catalytic activity. The catalytic activity in total benzene oxidation was studied in a flow type glass reactor at atmospheric pressure in the temperature range 373-773 K.
Reaction mixture of C6H6, N2 and air (2.88-3.57 mol/h C6H6) at a total flow rate of 7.2 l/h (120 ml/min) was used. Catalyst loading was about 140 mg. The reaction products were analyzed by a gas chromatograph Varian Model 3700 equipped with TCD (Tfilament = 353 K, T = = 333 K) and FID (T = 453 K) and 2 m Porapak Q (0.150-0.180 mm, Riedel-de Haen AG D-3016 Seelze 1) column operating at 443 K. Nitrogen (30 ml/min) was used as carrier gas, whereas benzene (Merck, for spec-troscopy) was used for oxidation and calibration.
RESULTS AND DISCUSSION
The calculated values about sBET, the total pore volume and the amount of predominant pore diameters for the used supports are presented in Table 1. Presented data shows differences between used supports. TiO2 has smallest specific surface area, as well as smallest total pore volume of all tested supports. Compared to TiO2, diatomite has similar specific surface area, but the total pore volume and the pore size distribution are significantly different. It is obvious that these differences are due to the existence of meso- and macropores in the diatomite. y-Al2O3 is a typical mesoporous material. Up to 90% of total pore volume is in the mesoporous region and 80% of all pores have diameter in the range 3-10 nm. Activated carbon possess total pore volume comparable to y-Al2O3. All the three pore types (micro-, meso- and macropores) are present.
The metal loading obtained by AES was in the range of 5.5-6.5% for the iron and 0.65-0.7% for the noble metal (palladium or platinum) in the studied catalysts.
The XRD patterns of all as prepared and calcined alumina- and titania-supported samples showed the characteristic pattern of the carrier only. The supported metal-oxide phase was X-ray amorphous, because of the small crystallite size. There was a difference in the XRD patterns of the samples supported on diatomite and activated carbon only. The samples, which are thermally treated (Figs. 1b and 2c), exposed the patterns of the support (for comparison see Fig. 1a and 2a) and broadened lines of low intensity, belonging to a highly dispersed iron oxide phase. This is a hematite phase (PDf#72-0469) in the case of diatomite supported sam-
d, A
Fig. 1. XRD patterns of diatomite support (a) and 8% Fe/di-atomite (b) after calcination at 713 K.
ples (Fig. 1b) or hematite (PDF#72-0469) and magnetite (PDF#82-1533) on the activated carbon supported ones (Fig. 2c). According to the Scherrer equation [14] the average crystallite size of hematite was calculated about 12 nm in the diatomite supported samples (Fig. 1b).
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