The Effect of Temperature on the Formation of Titanium Dioxide Structures on γ-al 2 O 3 Surface

Transcription

1 ISSN , Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 9, pp Pleiades Publishing, Ltd., Original Russian Text A.A. Malkov, O.M. Ishchenko, Yu.M. Koshtyal, K.L. Vasill eva, A.M. Abyzov, A.A. Malygin, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 9, pp INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY The Effect of Temperature on the Formation of Titanium Dioxide Structures on γ-al 2 O 3 Surface A. A. Malkov, O. M. Ishchenko, Yu. M. Koshtyal, K. L. Vasill eva, A. M. Abyzov, and A. A. Malygin St. Petersburg State Technological Institute, St. Petersburg, Russia Received February 5, 2010 Abstract The effect of calcination temperature of hydrated gamma-aluminum oxide on the phase formation of surface structures and the content of titanium in them in the course of successive treatment of the initial matrix with TiCl 4 and Н 2 О was determined. DOI: /S Control of chemical and phase composition of target products at the atomic and molecular levels is the promising direction in synthesis of new materials. Titanium oxide structures on porous materials (SiO 2, MgO, Al 2 O 3, etc.) are practically important solid-phase supports, catalysts, sorbents, fillers of composition materials, etc. [1, 2]. The chemical and phase composition of the synthesized surface compounds essentially affect functional properties of the system as a whole [3]. The molecular layering (ML) is one of the rapidly developing methods in synthesis of surface oxide structures. As was shown previously, the chemical nature of initial matrix and the temperature modes at different ML stages essentially affect the phase formation of a titanium dioxide nanolayer [4 8]. In this study, the temperature conditions for preparation of a porous aluminum dioxide by ML method were developed and the physicochemical and phase composition of products of its multiple interactions with titanium tetrachloride and water vapors were determined. As initial matrix was used γ-al 2 O 3 of the АА 1.5/3 brand (Axens). The volume of mesopores was 0.56 cm 3 g 1 and their average radius, 3.4 nm. We used in the experiments a mm fraction prepared by grinding initial granules in a corundum mortar. The ground sample was washed with distilled water and dried at 150 С. Two sets of samples (nti, n = 1 4) were prepared by successive treatment of γ-al 2 O 3 with TiCl 4 and Н 2 О vapors from 1 to 4 times. In the first set (АА300/200nTi), the initial matrix was heat-treated at Т о = 300 С and the following stage of the TiCl 4 chemisorption and desorption of physically adsorbed gaseous reaction products was performed at 200 С. Then, the sample was subjected to vapor hydrolysis in the temperature range 200 (at initial stage) 300 С with following drying of the product at 300 С. The second sample series (АА500/200nTi) was synthesized at different T o value (Т о = 500 С). The compositions synthesized were studied by the adsorption, chemicoanalytical, X-ray diffraction (XDA), and spectroscopic methods after each cycle of the treatment with TiCl 4 and Н 2 О vapors. The temperature of pretreatment of initial γ-al 2 O 3 samples was chosen based on necessity of removing of most of physically adsorbed and coordination water. To choose the pretreatment temperature for initial matrix, the samples were studied by differential thermal analysis (DTA) in air [Q-1500 MOM derivatograph (Hungary), linear heating from 1300 C at a step of 10 deg min 1, reference α-al 2 O 3 ] prior to the synthesis. The content of titanium and chlorine in the products synthesized was determined by the known methods of quantitative chemical analysis [9, 10]. The specific surface area was calculated by substituting the measured values of the gas volume adsorbed on the sample at four partial pressures (SORBI N.4.1 apparatus) into the BET equation. The X-ray diffraction patterns were recorded on a Difray DNR-401 diffractometer (CuK α radiation, 1520

2 THE EFFECT OF TEMPERATURE 1521 λ = nm) at 2θ = 20 o 60. The qualitative composition of crystalline phases was determined by comparing reflection maxima with the ICDD datafile. The content of the crystalline phases was determined by the external standard method of quantitative X-ray phase analysis [11] from the most intense reflections (d/n (101) = nm for anatase and d/n (110) = nm for rutile). The electronic diffuse reflectance spectra (EDRS) were recorded on a Specord M-40 spectrometer equipped with an attachment with an integrating sphere for reflectance measurements relative to an optical reference (MgO) in the UV and visible spectral regions ( nm). The previously performed study [12] showed that the integral spectrum gives no way of reliably identifying multiphase objects. Therefore, a hardware differentiation of the spectra with following graphic decomposition was used in this study for the case when two types of titanium dioxide structures differing in the coordination environments were present simultaneously. The DTA and DTG curves have two endothermic peaks at 130 and 484 C and two peaks of mass loss at 137 and 489 C corresponding to them (Fig. 1). The first endothermic effect is due to removal of physically adsorbed water and the second one, due to removal of coordination water. Two temperatures of treatment of the initial matrix (300 and 500 С), at which physically adsorbed and coordination water are removed, respecttively, were chosen based on the data obtained. By the data of DTA, the content of the coordination and structural water in the aluminum oxide matrices heattreated at 300 and 500 С is 9.56 and 5.51 μmol m 2. The obtained values agree with the literature data on gamma-aluminum oxide [13 15]. The specific surface areas of the initial samples heat-treated at 300 and 500 С are collected in the table. The differences between them may be ascribed to the fact that hydration of initial matrix at stage of washing with water proceeds with the formation of boehmite, which is stable at 300 С and has larger surface area than the transition aluminum oxide phases formed from it in subsequent cycles [16]. As the number of cycles of the TiCl 4 and Н 2 О treatment of the initial matrix increases from 1 to 4, the titanium content in the products synthesized increases and the specific surface area decreases for both sample series (see the table). As known, the latter dependence is associated with gradual growth (filling) of the aluminum oxide pores with the titanium-containing endo ΔT DTA TG DTG T, C Sample S sp, m 2 g 1 Concentration, mmol g 1 Ti АА300/200nTi Cl Δm, mg Fig. 1. Thermograms of initial γ-al 2 O 3. (Δm) mass loss, (Т) temperature. structures [17]. The large increase in the titanium content at each ML cycle in АА300/200nTi samples is associated with larger specific surface area of the initial sample and larger concentration of hydroxyl groups per unit surface in the initial and modified samples. Note also, a part of chloride groups, not substituted for hydroxyls after the stage of vapor hydrolysis at the considered temperature, remain in Specific surface area and the content of titanium and chloride ions by data of the chemical analysis for γ-al 2 O 3 samples before and after treatment with TiCl 4 and H 2 O TiО 2, a wt % Initial 301 1Ti Ti Ti Ti АА500/200nTi Initial 242 1Ti Ti Ti Ti a Calculated based on titanium content.

3 1522 MALKOV et al. (a) (b) λ, nm λ, nm Fig. 2. Differential EDR spectra of (a) АА300/200nTi and (b) AA500/200nTi samples after 1, 2, 3, and 4 ML cycles. (λ) Wavelength. АА300/200nTi samples. This is in line with the regularities obtained previously in studying the hydrolytic decomposition of titanium tetrachloride [18]. The diffraction patterns of the АА300/200nTi samples show the crystalline phase to be formed even after the first cycle. Then, in the course of successive treatments with TiCl 4 and Н 2 О vapors, the reflection intensity corresponding to the (101) anatase increases. The content of the crystalline phase, as calculated by the external standard method [11], showed that the weight fraction of anatase in the composition of the АА300/200nTi samples synthesized by successive treatment during 1, 2, 3, and 4 cycles increases to 0.6, 1.4, 1.6, and 2.4%, respectively, in the course of assembly of the titanium dioxide structures [11]. Comparison of the concentrations of the crystalline phase, as determined by X-ray diffraction, with the titanium content found by chemical analysis showed that the anatase fraction is 9 13% of the total content of the assembled titanium oxide structures. For a set АА500/200nTi (n = 1 3), only a decrease in the intensity of the γ-al 2 O 3 reflections is observed. This shows that the fraction of the initial matrix in the composition of a modified sample decreases with assembling oxide structures. Reflection corresponding to the anatase modification appears on the diffraction pattern only after 4th ML cycle (sample with n = 4). In this case, the fraction of the crystalline phase is 4% of the total content of the titanium dioxide structures. Comparison of the DTA data for two sets under consideration shows that, apparently, the structural water remained on the surface after hydration of gamma aluminum oxide and its subsequent heat-treatment at 300 С participates in the hydrolysis of titanium tetrachloride molecules and assists nucleation of the anatase crystalline phase even after the first cycle of the treatment with TiCl 4 and H 2 O vapors (set АА300/200nTi). The thermal anneal of the aluminum oxide matrix at 500 С, leading to removal of both physically adsorbed and coordination water, ensures, in the course of its treatment with TiCl 4 vapor, proceeding of the reactions between hydroxyls on the surface [6, 14, 17] m(=al OH) + TiCl 4 (=Al O ) m TiCl 4 m + mhcl. (1) and subsequent hydrolysis of the titanium oxochloride groups being formed (=Al O ) m TiCl 4 m + (4 m)h 2 O (=Al O ) m Ti(OH) 4 m + (4 m)hcl. (2) In a set АА500/200nTi the crystalline phase is recorded on the X-ray diffraction patterns only after 4 ML cycles, with its content 0.6 wt % TiO 2 (about 4% of the total TiO 2 content). Thus, under the conditions considered, the temperature modes of the preparation of the initial matrix modified in the course of successive treatment with TiCl 4 and H 2 O vapors affect the amount of assembled titanium dioxide structures and their ratio in the crystalline and amorphous state. As known, the phase transformations on the surface should lead to changes in the coordination state of elements in the composition of synthesized products.

4 THE EFFECT OF TEMPERATURE 1523 The electronic diffuse reflectance spectra (EDRS) of the modified samples are presented in Fig. 2. The charge-transfer bands were used to determine the coordination state of titanium in the composition of surface oxide structures. On the diffuse reflectance spectra, the absorption band edge is around 400 nm for all samples of the АА300/200nTi series, which is typical of anatase titanium dioxide. The spectrum of the 1Ti sample (after single treatment with TiCl 4 and Н 2 О vapor) has a clearly resolved shoulder at 340 nm, whose intensity considerably decreases during subsequent modification. The presence of the shoulder in this spectral region was found in the spectra of aluminum titanate [20] and products of gamma aluminum oxide modification with titanium dioxide surface structures [21, 22] and accounted for by formation of Al O Ti bonds. Hardware differentiation of the spectra of АА300/200nTi samples treated during one cycle yields curves with two maxima at 329 and 395 nm (Fig. 2а). As noted, the first maximum is likely to correspond to Al O Ti bond, whereas the second maximum confirms presence of titanium in the composition of octahedral polyhedron TiO 6 of the anatase-like bulk phase [12, 21, 22]. In the course of the further synthesis, the first maximum shifts toward long-wave region and the area of the second maximum increases with its position unchanged. In a sample synthesized after 4 cycles, the first maximum transforms into a shoulder. Probably, the above character of change of the first derivatives of EDRS is due to the increase in the relative content of the Ti O Ti and Al O Ti bonds in the course of assembling titanium oxide structures. At the same time, it may arise from the increase in the content of the crystalline phase in the composition of synthesized samples in the course of successive ML cycles [17, 23]. After modification, the absorption band edge in the integral spectra of the АА500/200nTi samples shifts from 370 to 400 nm on going from 1Ti to 4Ti and the whole absorption spectrum shifts toward longer wavelengths. Differentiation of the obtained dependences yields curve with only one maximum at 327 nm for 1Ti sample, which gradually decreases toward long-wavelength region to 350, 360, and 390 nm on going from 2Ti to 3Ti and 4Ti. Evidently, as in the case of АА300/200nTi samples, the shift of the maximum on the first derivative curve of the electronic spectrum is due to increasing amount of Ti O Ti bonds in the course of assembling titanium dioxide structures and, correspondingly, due to larger contribution into the charge-transfer spectrum of the energy transition from oxygen ions affected by the field of similar titanium ions [23]. Graphical decomposition of the obtained first derivative curves for samples 1Ti 3Ti did not reveal contribution of each predicted component into the total dependence. At the same time, for 4Ti sample decomposition of the same curve onto Gaussian contours revealed two components peaking at 360 and 390 nm, respectively. If, like for АА300/200nTi samples, the maximum at 390 nm characterizes presence of structures, in which the titanium coordination with oxygen is close to the anatase-like structure, then the maximum at 360 nm may indicate that titanium in the oxide structures is surrounded by less than 6 oxygen ions [20, 24]. The absence of reflection of the crystalline titanium dioxide for 1Ti 3Ti samples of АА500/200nTi series and presence of reflection due to (101) anatase for 4Tu sample is in favor of the above conclusion. CONCLUSIONS (1) In the course of successive treatment of gamma aluminum oxide, which was preliminarily hydrated and then annealed at 300 and 500 С with TiCl 4 and H 2 O vapors, a crystalline phase of anatase titanium dioxide is formed even after the first molecular layering cycle at 300 С, whereas at 500 С it is formed solely after four successive cycles. (2) In the course of the synthesis, a titanium oxygen polyhedron in the octahedral coordination is formed. (3) The stable chemical bond Al O Ti is formed in interaction of titanium tetrachloride molecules with hydroxy groups of aluminum oxide. ACKNOWLEDGMENTS The study was partially supported by the Russian Foundation for Basic Research (projects nos , , and ) and the Federal Target Program Scientific and Pedagogical Specialists of Innovation Russia for the period REFERENCES 1. Malygin, A.A., Soros. Obrazov. Zh., 1998, no. 7, p Malygin, А.А., Мalkov, А.А., and Dubrovenskii, S.D., in: Adsorption on New and Modified Inorganic Sorbents, Dabrowski, A. and Tertykh, V.A., Eds., Amsterdam: Elsevier, 1996, vol. 99, pp

5 1524 MALKOV et al. 3. Rubinshtein, A.M. and Kulikov, S.G., Izv. Akad. Nauk SSSR. Ser. Khim., 1951, no. 2, pp Malkov, A.A., Malygin, A.A., and Sosnov, E.A., Mezhvuz. sbornik trudov Napravlennyi sintez tverdykh veshchestv (Intercollegiate Collection of Works Controllable Synthesis of Solids, St. Petersburg: S-. Peterb. Gos. Univ., 1992, issue 3, pp Leboda, R., Gun ko, V.M., Marciniak, M., et al., J. Colloid Interface Sci., 1999, vol. 218, pp Malkov, A.A., Malygin, A.A., and Petrova, L.I., Zh. Prikl. Khim., 1991, vol. 64, no. 7, pp Puurunen, R.L., Chem. Vap. Deposition, 2005, vol. 11, no. 2, pp Aarik, J., Aidla, A., Mandar, H., et al., J. Crystal Growth, 2002, vol. 242, nos. 1 2, pp Babko, A.N. and Pilipenko, A.G., Kolorimetricheskii analiz (Colorimetric Analysis), Moscow: Khimiya, Charlot, G., Quantitative Inorganic Analysis, New York: Wiley, Abyzov, A.M., Rentgenodifraktsionnyi analiz polikristallicheskikh veshchestv na minidifraktometre Difrey: Uchebnoe posobie (X-Ray Diffraction Analysis on a Difray Minidiffractometer. Training Guide), St. Petersburg: SPb. Tekhnol. Inst (Tekhn. Univ.), Vasil eva, K.L., Ishchenko, O.M., Zakharova, N.V., et al., Zh. Prikl. Khim., 2009, vol. 82, no. 5, pp Kiselev, V.F., Poverkhnostnye yavleniya v poluprovodnikakh i dielektrikakh (Surface Phenomena in Semiconductors and Insulators), Moscow: Nauka, Petrova, L.I., Malkov, A.A., and Malygin, A.A., Zh. Prikl. Khim., 1986, vol. 59, no. 10, pp Vlaev, L., Damyanov, D., and Mohamed, M., Coll. Surfaces, 1989, vol. 36, no. 4, pp Lippens, B.C. and Steggerda, J.J., Physical and Chemical Aspects of Adsorbents and Catalysts, London: Academic Press, Foger, K. and Anderson, J.R., Appl. Catal., 1986, vol. 23, pp Shchegrov, A.N. and Vil nyanskii, Ya.E., Dokl. Akad. Nauk SSSR, 1961, vol. 140, no. 3, pp Pak, V.N. and Ventov, N.G., Zh. Fiz. Khim., 1975, vol. 49, no. 10, pp Winkler, E.R., Sarver, J.F., and Cutler, I.B., J. Am. Ceram. Soc., 1966, vol. 49, no. 12, pp Malkov, A.A., Khimiya poverkhnosti i nanotekhnologiya vysokoorganizovannykh veshchestv: Sbornik nauchnykh trudov (Surface Chemistry and Nanotechnology of Highly Organized Substances. Coll. of Research Works), St. Petersburg: S.-Peterb. Gos. Tekhnol. Inst. (Tekhn. Univ.), 2007, pp Malkov, A.A., Zakharova, N.V., Morozov, S.A., and Malygin, A. A., Izv. S.-Peterb Gos. Teknol. Univ (Tekhn. Univ), 2007, no. 1(27), pp Khodakovskaya, R.Ya., Khimiya titansoderzhashchikh stekol i sitallov (Chemistry of Titanium-containing Glasses and Glass-Ceramics), Moscow: Khimiya, Gao, X., Bare, S.R., Fierro, J.L.G., et al., J. Phys. Chem. B, 1998, vol. 102, pp