1. Introduction E. ILLER, D. WAWSZCZAK, L. GÓRSKI - PDF


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A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S Volume Issue 2 DOI: /v E. ILLER, D. WAWSZCZAK, L. GÓRSKI STRUCTURE AND PROPERTIES OF WO 3 -ZrO 2 GEL COMPOSITES AS POTENTIAL CATALYSTS FOR HYDROCARBONS ISOMERIZATION PROCESS STRUKTURA I WŁAŚCIWOŚCI KOMPOZYTÓW ŻELOWYCH WO 3 -ZrO 2 JAKO POTENCJALNYCH KATALIZATORÓW W PROCESIE IZOMERYZACJI WĘGLOWODORÓW Tungsten oxide dispersed on zirconia in WO 3 -ZrO 2 system found application as strong acidic catalyst in hydrocarbons isomerization process. Specific method for synthesis of gel metal oxide composites based on Complex Sol-Gel Process (CSGP) were elaborated in Institute of Nuclear Chemistry and Technology, Warsaw. Structural investigations of WO 3 -ZrO 2 gel composites thermally treated at 500 C, 650 C and 800 C using X-ray diffraction (XRD) and scanning electron microscopic (SEM) methods were carried out. The gel composites annealed at 500 C appears as amorphous material, in the composites thermally treated at 650 C beginning of crystalline forms are observed. The gel composites annealed at 800 C are polycrystalline. Both oxides exist as separate crystalline phases, ZrO 2 in the high symmetry forms: cubic and tetragonal, WO 3 in the monoclinic form. Keywords: WO 3 -ZrO 2 gel composites, structure investigations, hydrocarbons isomerisation process, XRD, thermal analysis, SEM Tlenek wolframu dyspergowany w tlenku cyrkonu znalazł zastosowanie jako silnie kwaśny katalizator w procesie izomeryzacji węglowodorów. Zaproponowano specyficzną metodę syntezy kompozytów żelowych tlenków metali bazującą na opracowanym w Instytucie Chemii i Techniki Jądrowej w Warszawie Kompleksowym Procesie Zol-Żel. Badania strukturalne kompozytów żelowych WO 3 -ZrO 2 poddanych termicznej obróbce w temperaturach 500 C, 650 C, 800 C. prowadzono stosując metody dyfrakcji promieniowania rentgenowskiego (XRD) oraz skaningowej mikroskopii elektronowej (SEM). Żele wygrzewane w temperaturze 500 C okazały się materiałem amorficznym, w kompozytach wygrzewanych w temperaturze 650 C zaobserwowano pojawienie się form krystalicznych. Natomiast żele wygrzewane w temperaturze 800 C posiadały strukturę polikrystaliczną. Oba tlenki występują jako oddzielne fazy krystaliczne, ZrO 2 w postaci faz regularnej i tetragonalnej oraz WO 3 w postaci fazy jednoskośnej. 1. Introduction The tungsten oxide-zirconium oxide composites have been recognized as catalysts with strong acid properties [1] and they have found applications as catalysts in the process of isomerization of parafines. The advantage of these compounds with comparison to previously used sulfates is that they do not lose dopant during heat treatment and deactivation extent. is smaller in this case. In accessible scientific literature the authors stress that strong acid properties is connected with the formation of WO 3 micro-areas. This hypothesis has been confirmed by several experimental observations: such as the increase in activity with the percentage contents of tungsten, the need of compounds heating to the temperature where sintering begins to become evident, as well as observation in the Raman spectrum the lines corresponding to the clusters of WO 3. D.G.Barton et al. [2] reported that, in the tungsten oxide zirconium oxide composite the isolated atoms of tungsten are in slightly deformed octahedral positions and that is no tetrahedral tungsten in this case J.M Parera et al. [3] present catalysts based on tungsten oxide on zirconia support which are prepared starting from solution of ammonium metatungstate at ph = 6 or more. Tungsten, in the form of single layer on support, exists in tetrahedral positions but sometimes appears in octahedral positions which become dominant when amount of tungsten deposited increases. Sohn and INSTITUTE OF ATOMIC ENERGY POLATOM, OTWOCK ŚWIERK, POLAND INSTITUTE OF NUCLEAR CHEMISTRY AND TECHNOLOGY, WARSZAWA, 16 DORODNA STR., POLAND 444 J. Park [4] prepared WO 3 -ZrO 2 catalysts containing only 5% of tungsten with a Raman line at 935cm 1 attributed to positions of tetrahedral types. However this line disappears and was replaced by other which is attributed to the octahedral tungsten with the tungsten content increasing up to 13%. In the presented paper for preparation of WO 3 -ZrO 2 gel composites the Complex Sol-Gel Process elaborated in Institute of Nuclear Chemistry and Technology has been proposed [5,6]. Investigation of structure and properties of synthesized composites is the main purpose of presented work. 2. Experimental procedure 2.1. Chemicals for synthesis of gel metal-oxide composites The following reagents have been used for synthesis of gel composites: Ammonium tungsten oxide pentahydrate % obtained from Alfa Aesar, Zirconyl nitrate hydrate 99.99% obtained from Sigma-Aldrich, Ascorbic acid, pharmaceutic grade from Teakea Europe GmbH, Hamburg, TEOS-Tetroethylosilane 98% p.a from Fluka Synthesis The synthesis of zirconium-tungstate WO 3 -ZrO 2 composites with different molar ratio of metal oxides 1:1, 1:2 and 3:2 were prepared by CSGP method [5]. The initial stage of the process is preparation of ammonium tungstate ascorbate sol next the separately prepared zirconyl, titanium sols are added gradually to the reaction mixture. When the gelation is over, the gel are thermally treated at temperature indicated by thermal analysis. Flow-chart of synthesis is presented in Fig. 1. Fig. 1. Flow-chart of WO 3 -ZrO 2 composite synthesis 445 Fig. 2. Thermal analysis of WO 3 -ZrO 2 composites, TG and DTA curves The gels were annealed accordingly to the temperatures indicated by thermal analysis; 500 C, 650 C, 800 C. Weight loss curves (TG) were determined by thermogravimetry using Paulik-Paulik Erdey system with MOM Derivatograph. The composites samples of 100 mg weight are heated from room temperature up to 1000 C with the rate 10 C/min in flowing air. The TG and DTA curves for investigated samples have been shown in Fig. 2. The TG and DTA curves of WO 3 -ZrO 2 samples indicates for few stages of weight loss. First correspond to desorption of the water molecules adsorbed on particles surface, removal of nitrates, ammonium, and methyl groups as well as creation of W-O bonds. The peaks in temperature range C indicates on running of exothermic reactions, with taking part of above mentioned reagents Next exothermic reactions we can observe in temperature range C, connected with transition gel composites from amorphous form to crystalline phase. It is worthwhile to remark the slight displacement of these last peaks in DTA curve in the dependence on the molar ratio WO 3 to ZrO 2 in investigated samples. X- ray diffraction study The samples of composites were analyzed by X- ray diffraction technique using a Rigaku Miniflex diffractometer with Cu-Kα radiation (tube output voltage 30kV,current 15mA) and scanning range from 2Θ =3 to 2Θ =90 (step 0.02 and rate 2 /min). In the diffraction patterns of composites annealed at 500 C there is rather no peaks, whereas in the X-ray diffraction patterns WO 3 -ZrO 2 (1:1,1:2, 3:2) composites annealed at 650 C, the some group of peaks occurring above background level are observed. The gels annealed at 800 C are fully polycrystalline. Both oxides exist as separate crystalline phase:. ZrO 2 in a high symmetry forms cubic and tetragonal, WO 3 in monoclinic form [6]. The X-ray diffraction patterns of WO 3 -ZrO 2 (1:2) composite annealed at the above temperatures have been shown in Fig. 3. Due to X-ray scattering dependence on atomic number of studied elements WO 3 peaks are generally stronger than that of ZrO 2. Therefore composite with proportion WO 3 ZrO 2 (1:2) is specially chosen in order to obtain similar strongest peaks intensities for both oxides which may be illustrated in Fig. 3 for sample annealed at 800 C. Particular peaks may be identified with attached JCPDS standards. For estimation of crystallite dimensions in particular crystalline phases forming above composites the Scherrer equation was applied D hkl = kλ360/(βcosθ2π) where D hkl average crystallite dimensions perpendicular to the reflecting plane, λ wave length, k Scherrer constant, θ diffraction angle, β diffraction peak half-width Calculations were done for λ =1.542Ŕ (Cu anode) and k=0.89. The obtained data has been presented in Table 1. TABLE 1 Dimensions of crystallites in WO 3 -ZrO 2 gel composites annealed at 800 C 2θ θ cosθ β D hkl WO 3 -ZrO 2 3:2 WO 3 23,36 11,68 0,9793 0,20 D 002 =401Å ZrO 2 30,44 15,22 0,9649 0,55 D 101 =148Å 1:1 WO 3 23,34 11,87 0,9786 0,28 D 002 =287Å ZrO 2 30,46 15,23 0,9649 0,58 D 101 =140Å 1:2 WO 3 23,38 11,69 0,9793 0,20 D 002 =401Å ZrO 2 30,42 15,21 0,9650 0,50 D 101 =163Å Fig. 3. X-ray diffraction patterns of WO 3 -ZrO C, 650 C, 800 C gels annealed at As can be seen the crystallite sizes of both phases differ each other. ZrO 2 crystallite sizes are considerably smaller than that of WO 3. Between ZrO 2 polymorphic forms tetragonal form shows smallest crystallite sizes. But values obtained for WO 3 may be connected with agglomeration of some nanocrystals. Important parameter of catalytic materials characterization, which will be used as a powder is specific surface. In the Table 2 the change of WO 3 -ZrO 2 specific surface data with increasing temperature of thermal treatment have been presented. We can observe the increasing of specific surface values of WO 3 -ZrO 2 (1:2) composite with annealed temperature. The samples of thermal treatment at 800 C achieved the value of specific surface near to 28 m 2 /g. 447 TABLE 2 Specific surface data of WO3-ZrO2 (1:2) composite Number of sample Parameters of synthesis Specific surface [m 2 /g] C / 24 h 0, C / 2 h 11, C / 5 h 8, C / 5 h o C / 5 h 8, C / 5 h 13, C / 5 h 15, C / 5 h 27,7 Increase of specific surface values of WO 3 -ZrO 2 composites with growth of temperature of theirs thermal treatment can be also documented by means of SEM micrographs (Fig. 4). Surface changes directed to small crystallites forming of precise sizes may be observed especially for samples annealed at 800 C. Some agglomerates are still remaining. The adhesion degree of catalytic materials to different surfaces and porosity degree obtained layers allows us to determination of active surface of catalyst. The several trials of adhesive spread of WO 3 -ZrO 2 gels on nickel, titanium and glass plates were performed. The results of experiments for Ni substrate are shown on SEM micrographs (Fig. 5). The central part of image from Fig. 5a is visible at higher magnification in Fig. 5b. Fig. 4. SEM images of WO 3 - ZrO 2 (1:2) gels, (a) 110 C, (b) 200 C, (c)500 C, (d) 800 C 448 Fig. 5. The surface layer of WO 3 -ZrO 2 gel composites on nickel: a) x 1 000, b) x Discussion of results Composites obtained by the CSGP method appear as amorphous or polycrystalline in the dependence on annealing temperature. Chemical reactions during gel-sol process are finished after gel annealing with crystalline phases forming. The results of XRD studies show that low temperature annealing gives amorphous phases with only short range order. The beginning of crystalline phase forming is observed in composites annealed at 650 C. Composites annealed at 800 C are polycrystalline. These X-ray results are confirmed by TG and DTA curves. Both oxides exist as separate crystalline phases. However WO 3 shows stabilising effect to higher symmetry ZrO 2 forms which normally not exist at ambient temperature. This situation is similar to that encountered in Al 2 O 3 ZrO 2 composites [7]. Common phase of tungstate type reported by Lambrecht et al. form in other conditions [8]. Planned experiments with long time gels annealing may explain whether such phases may appear in our materials. Crystallites dimensions counted by Scherrer formula show the sizes difference in both oxides. Tetragonal ZrO 2 has smaller crystallite sizes than other crystal forms of this compound. One can suppose that WO 3 crystallites are agglomerates of smaller nano crystallites which may be the cause of above mentioned catalytic effects. Forming of crystalline phases is accompanied by microstructure changes observed by SEM method. The specific surface values are higher for crystalline phases. Scanning transmission electron microscopy methods (STEM) with higher resolving power would be useful to detect morphological changes in nano scale connected with catalytic effects. On the basis of obtained results the following conclusions may be written 1. The degree of crystallinity for studied composite depends on annealing temperature. Polycrystalline structure is observed after annealing at 800 C. WO 3 exists in monoclinic form, ZrO 2 is mainly tetragonal with some amount of cubic phase. 2. Crystallite ZrO 2 sizes are smaller than that of WO The appearance of crystalline phases is accompanied by microstructure changes. 4. The carried out investigations proved, that tested selective properties of WO 3 -ZrO 2 gel composites, synthesized by means of CSGP, indicate proper features as potential catalysts for hydrocarbons isomerization process. The described method of WO 3 -ZrO 2 composites synthesis can be used for preparation of these potential catalysts. This work was supported by Polish National Center for Research and Development within Project NR REFERENCES [1] H. H i n o, K. A r a t a, J. Chem. Soc. Chem. Commun (1987). [2] D.G. B a r t o n et al. J. Phys. Chem. B 103, (1999). [3] J.M. P a r e r a et al. Applied Catalysis A, General, Elsevier Science, Amsterdam 163, 2, (1997). [4] S o h n, J o n g R a c k et al. Langmuir 14, 21, (1998). [5] A. D e p t u ł a, W. Ł a d a et al. Polish Patent no [6] E. I l l e r, A. D e p t u ł a, J.J. M i l c z a r e k, L. G ó r s k i, Proceedings of the 11 th European Powder Diffraction Conference, p. 129, Warsaw. [7] L. G ó r s k i, A. P a w ł o w s k i, Z. W i ś n i e w s k i, Archiv. of Metallurgy and Materials 55, (1), (2010). [8] R. L a m b r e c h t, M.S. A s t a c h o v, J. Mater. Chem. 7(9), (1997). Received: 20 November 2011.
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