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Influence of temperature schedules on particle size and crystallinity of titania synthesized by vapor-phase oxidation route
Powder Technology 170 (2006) 135 – 142 www.elsevier.com/locate/powtec
Influence of temperature schedules on particle size and crystallinity of titania synthesized by vapor-phase oxidation route Zhi Wang ⁎, Zhangfu Yuan, E. Zhou Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, PR China Received 7 February 2006; received in revised form 14 August 2006; accepted 24 August 2006 Available online 5 September 2006
Abstract Crystalline TiO2 particles were produced in a tubular flow reactor by chemical vapor synthesis using titanium tetrachloride as a starting material in oxygen containing atmospheres. The dependence of particle size, morphology and crystalline phase of titania on temperature schedules including the reactor temperature, the oxygen preheated temperature and the product cooling measure were explored. It is found that there are two opposite effects of temperature on particle size and crystalline phase content. The particle size distribution, SEM and TEM of resulting powders show that the grain size is controlled by the relative magnitudes of the nucleation rate and growth rate, both of them being subject to the temperature schedules. XRD indicates that particles crystalline phase is predominately anatase and the rutile content increment is not consistent with temperature increase. Anatase titania can be converted to rutile by addition of crystal modifier AlCl3. The element analysis by EDS shows that Al enriches on the particle outer surface. © 2006 Elsevier B.V. All rights reserved. Keywords: Titania; Titanium tetrachloride; Crystallinity; Particle size; Gas phase route
1. Introduction Titanium dioxide is one of the most utilized particulate materials in the world, such as pigment, plastic, paper making, printing ink, and chemical fiber and rubber. Recent developments have led to the use of titania as a photocatalyst [1]. China imports tens of thousands of tons of high-grade rutile titania annually and this presents an increase trend in recent years. On the industrial scale, the titania pigment manufacture routes include the sulphate process and the chloride process. However, economic and environmental pressures are shifting the world balance of titanium dioxide production away from sulphate based manufacture towards the more cost effective and cleaner chloride route. This so-called chloride process has been reported by several researchers [2–5]. In this process, TiO2 and other metal oxides in the feed ore are reacted at high temperature with chlorine in the presence of carbon. The product gases are cooled after they leave the reaction zone, upon which solid contaminants precipitate and are ⁎ Corresponding author. Institute of Process Engineering, Chinese Academy of Sciences, Zhong Guan Cun, Haidian District, P.O. Box 353, Beijing 100080, PR China. Tel./fax: +86 10 62558489. E-mail address: [email protected] (Z. Wang). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.08.020
substantially removed. The liquid titanium tetrachloride is purified by distillation before again being vaporized and interacting with hot oxygen to form titanium dioxide and regenerate chlorine for recycle. The base pigment is wet milled, surface treated and redispersed before packaging [6]. The formation of TiO2 crystallites in the oxidation reactor is shown as the following overall reaction in the temperature range of 700–1400 °C [7]. TiCl4 þ O2 →TiO2 þ 2Cl2
ð1Þ
Table 1 compares the TiO2 rutile and anatase structures and their physical properties [8]. The phase content, size characteristics Table 1 TiO2 rutile and anatase structures and physical properties [8] Property
Rutile
Anatase
Crystal structure Space group Lattice spacing a//c (nm) Density (g/cm3) Refract. index, 550 nm Band gap (eV) Melting point (°C)
Tetragonal P42/mnm 0.459//0.296 4.25 2.75 3.05 1830–1850
Tetragonal I41/amd 0.378//0.951 3.895 2.54 3.25 Conv. to rutile
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contents as the reactor temperature increase was observed by Suyama and Kato [12], but Kobata et al. [13] reported that rutile weight fraction was at a maximum at 1000 °C and decreased rapidly at both lower and higher temperatures. Mezey certified that the presence of 0.01 to 10% of an aluminum-containing compound in the flame oxidation of TiCl4 led to the formation of rutile [14]. Machenzie concluded that oxygen vacancies promoted crystallite growth and the anatase to rutile transformation [15]. While some of the preparations of titania have been studied,
Fig. 1. Experimental apparatus for preparation of TiO2 by gas-phase route. 1— O2 steel cylinder; 2—Ar steel cylinder; 3—silica desiccant; 4—flow controller; 5—O2 preheater; 6—electric furnace; 7—reactor; 8—AlCl3 gasifier; 9—TiCl4 container; 10—heated water bath; 11—N2 steel cylinder; 12—cooling device; 13—chlorine absorber; 14—electric heating tape.
and morphology of the individual particles making up the powder often have a strong influence on properties for a variety of applications [9]. Since the pigmentary powder properties, e.g. grain size, phase content, are predominantly determined by the oxidation course, the TiCl4 oxidation is the key procedure in the chloride process. Though the chloride process is a mature technology, the product properties are not often in control in China and the fundamentals of dopant addition have not been understood. As for one particle evolution during oxidation process, apart from the reactor temperature, the temperature schedules also comprise the reactant preheated temperature and with or without product cooling measure. Akhtar et al. [7, 10] studied the effect of process variables on titania powder characteristics and found that increasing temperature, reactant concentration, and reactor residence time resulted in larger titania particles, but the spread of the particle size distribution (PSD) remained rather unaffected. Suyama and Kato [11] found that the particle size decreased with increasing reaction temperature and oxygen concentration and with decreasing TiCl4 concentration. A linear increase in rutile
Fig. 2. Effects of TF1 on particles median diameter at different TiCl4 conentrations.
Fig. 3. TEM morphology of particles at different TF1 (C[TiCl4] = 1.018 × 10− 3 mol/l).
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Fig. 4. Effects of TF1 on particle size distribution at different TiCl4 concentrations.
contradictions and different results may arise due to different experimental conditions [3,16,17]. Furthermore, the relative importance of the synthesis variables, especially the influence of temperature schedules on the grain size and crystalline phase of TiO2 is not well understood. Based on the above backgrounds and for better control of the characteristics of powders, we have established a vapor-phase route for preparation of titanium dioxide particles by oxidation of titanium tetrachloride vaporized with hot oxygen. This article presents an experimental investigation of the dependence of particle size and crystalline phase of titania on temperature schedules. Changes in the particle morphology and crystalline phase of titania with dopant were also examined. 2. Materials and methods 2.1. Titanium dioxide synthesis The schematic diagram of the apparatus for vapor-phase reaction of TiCl4 and O2 is shown in Fig. 1. It consisted of gas purifiers, reactant preheaters, a tubular flow reactor, a cooler and separator and an off-gas treatment unit. A 15- or 30-mm-ID
quartz tube (Reactor-1: 15 mm and Reactor-2: 30 mm in I.D., both 1500 mm in length) was used as the reactor that was heated by a horizontal electrical furnace (Tianjin Zhonghuan Furnace Ltd. Co.), whose isothermal zone is 500 mm within ± 15 °C in the usual experimental condition. Temperature was measured with a copper-constantan (K type) thermocouple. Since TiCl4 is sensitive to the presence of water, the O2 and Ar were dried through silica desiccant respectively. Prepurified dry Ar (99.99% pure) was used as a carrier gas and bubbled through a fritted glass outlet into a flask containing liquid TiCl4 (reagent grade) in a constant temperature water bath (90 ± 1 °C). This premixed stream of Ar and TiCl4 vapor is first mixed with preheated O2 stream and then flows into the reactor tube along with another Ar stream for dilution. The gas flow rate ranges from 1.5 to 3.5 l/min. O2 used in these experiments was between 4 and 8 times in excess of the stoichiometric amount. The reactants are present in dilute concentrations so the released heat of reaction is insufficient to appreciably affect reactor temperature. All lines transporting TiCl4 and AlCl3 were maintained at temperature high enough to prevent their condensation by electric heating tapes. Exiting from the isothermal zone of the reactor, the gas stream was cooled with room temperature
Fig. 5. Schematic of gas-to-particle conversion process.
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dry N2 (99.95% pure) in a quartz jacketed cooler at the rear of the reactor. The cooling gas quenched chemical reaction and inhibited particle growth by coagulation outside of the reaction zone. In addition, it reduces thermophoretic losses from the product stream onto the cool zone walls. The exhaust gas was neutralized with 1 M NaOH solution before exiting through the laboratory hood. The particles were collected by natural sedimentation into a box connected to the end of the reactor. 2.2. Titanium dioxide characterization The median diameter and PSD of products were obtained by a laser particle size analyzer BI-90 (Brook-haven Instrument Co.). The TEM and SEM images were performed on a JEM1200EX (JEOL Co.) and a JSM-6700F (JEOL Co.) fieldemission scanning electron microscope respectively, which were used to study the morphology of titania and validate the particle size measurements obtained from the laser particle size analyzer. The energy dispersive spectra (EDS) were conducted on a QX2000 (Link Co.). X-ray diffraction (XRD) was used to determine the phase composition using a D/MAX-RB X-ray powder diffractometer (X'Pert PRO MPD, using CuKα radiation). The weight fractions of the anatase and rutile phases in the samples were calculated from the relative intensities of the strongest peaks corresponding to anatase and rutile as described by Spurr and Myers with formula 2 [18]. M R =ðM A þ M R Þ ¼ 1=ð1 þ 1:26I A =I R Þ
all the products have log-normal distribution curves, which shift to smaller sizes with temperature in the range of 800–1000 °C, later on the peaks move positively slightly. This phenomenon agrees with the evolution of median diameter with temperature. In addition, the increase in TiCl4 concentration does not change the shape of size distribution. As illustrated in Fig. 5 [19], during powder production by gas phase processes, product molecules generated by decomposition of the precursor and chemical reactions form molecular clusters by collision of intermediate Ti-containing species with O-containing species, then the clusters become particles by nucleation/ surface reactions and/or uninhibited coagulation [20]. It is expected that chemical reaction and coagulation determine the characteristics of the product titania particle size distribution, which is a consequence of two different roles of nucleation and growth that are temperature dependence. In the first place, the continuous increase in temperature increases the supersaturation of titania in the system and a higher concentration of precursor monomers are obtained through a higher superficial reaction rate due to the increase of gas mean free path, no doubt, the extremely small particles in the early stages bring about smaller particles caused by an enhancement of nucleation rate [3]. In addition, allowed for the gas expansion in higher temperature, the mean
ð2Þ
where MA and MR are the phase fraction of anatase and rutile titania; IA and IR are their intensities of the strongest XRD peaks, respectively. 3. Results and discussion 3.1. Effects of reactor temperature on particle size and morphology When the O2 preheater temperature (TF2) is 1100 °C, the influence of reactor temperature (TF1) on the median diameter of titania is shown in Fig. 2. The experiments show that the particle sizes are fairly sensitive to temperature changes. With increase of reaction zone temperature, the products show a decrease in median diameter from 217 nm at 800 °C to 176 nm at 1000 °C then a prominent increase to 198 nm at 1100 °C when TiCl4 concentration is 1.018 × 10− 3 mol/l. In addition, it is observed that the median diameter of the particles increases with increasing TiCl4 vapor concentration (C[TiCl4]) at the same conditions. The TEM results in Fig. 3 indicate that at all temperatures investigated, the particle morphology remains dense polyhedral structures with well-defined edges, and individual primary particles can be easily distinguished, as has been shown by Akhtar [7] and Suyama [11,12]. The shape of the individual primary particles and the morphology of the aggregate at different temperature remain rather unaffected. However, we observe that the product size at 1000 °C is smallest, while the product size at 1100 °C is largest. This is in agreement with the above diameter results. It is observed from the PSD in Fig. 4 that
Fig. 6. Effects of TF2 (a) and cooling measure (b) on particle average diameter.
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residence time in the reactor is shorter although the effective reaction zone becomes slightly longer, and decreasing residence time leads to a decrease in the time for collision and results in smaller particles. On the other hand, when the temperature exceeds certain value, for example 1000 °C in this experiment, the particle size shows an increase trend. The higher collision frequency between two monomers causes forming clusters that are prone to coalescence and sintering at higher temperature, that is, increasing temperature promotes the occurrence of growth that brings large grain sizes [16]. In general, the particle size of the product is determined by the relative magnitudes of the nucleation rate and growth rate. The relative importance of these two opposing effects of temperature on powder size determines the trend of size evolution. Accordingly, the products reach minimums at 1000 °C, and the particle size will increase beyond that point. Similarly, it is not difficult to understand the fact that the increase in TiCl4 concentration causes particle size increase because the particle diameter is proportional to (C0 / N)1/3, where C0 is the concentration of metal halide and N is the number of nuclei. This implies that the larger the material concentration, the larger is the efficient collision frequency and the faster is the particle growth [21,22].
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3.2. Effects of preheating and cooling measures on particle size and morphology By virtue of chemical vapor deposition and chemical vapor condensation processes [23,24], we have adopted two measures to study the influence of the reactant preheater temperature and the product cooling treatment on titania properties. First, we increase the initial oxygen temperature by utilizing a controllable preheater; second, we blow nitrogen with flow rate of 5.0 l/min into a quartz jacketed cooler at the rear of the reactor, so the gas stream and entrained product particles from the reaction zone are rapidly cooled. When the reactor temperature (TF1) is 1000 °C and other conditions are fixed, the influence of oxygen preheater temperature (TF2) on median diameter of titania was shown in Fig. 6(a). It can be seen from Fig. 6(a) that, with increase of oxygen preheated temperature, the median diameters of products become smaller. As the reactants temperature in the mixing zone is very low, so the initial reaction was relatively more controlled by environment temperature. The increase in temperature of oxygen enhances the role of the mixing zone playing on final product. It seems that the increase of oxygen temperature increases of
Fig. 7. SEM morphology of particles at different TF2 (a–b) and with or without cooling (c–d).
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nucleation speed. The high concentration of very fine monomers in the initial stage makes the particle size smaller at the condition when the residence time is not long enough. Therefore, the preheating of reactants is a very important factor in controlling the nucleation and consequently particle sizes. In addition, when the TiCl4 concentration and the flow rate were kept constant, and the reactor tube inner diameter increases from 15 mm (Reactor1) to 30 mm (Reactor-2), the products from the larger reactor is bigger than those from the smaller one, which may be related to the residence time increases in larger reactor. At high residence times the particles produced have more time for coagulation. From the SEM results in Fig. 7 (a)–(b), we can find that the powders synthesized are composed of irregularly shaped aggregate particles. It is the finite coalescence rate that results in the formation of irregularly shaped aggregate particles [25]. By comparison, the product at TF2 1100 °C includes more fine grains. The effect of cooling measure on particle size can be found from Fig. 6(b), and the product size is reduced by the cooling gas through quenching chemical reaction and inhibiting particle growth at the outside of the reaction zone. Fig. 5 has illustrated that in the initial stage of the oxidation process nucleation was the main route for TiCl4 conversion, while growth or coalescence in the latter stage. With the cooling procedure, the time of particles in high temperature state reduces, which corresponds to their sintering energy decreases substantially so particles coagulation and coalescence can no longer be regarded as main part of particle growth. Combining with the nucleation and growth characteristics of oxidation process, we confirm that particle size can be adjusted by altering the grains temperature gradient experienced. The morphologies of products from cooling treatment are shown in Fig. 7(c)–(d). It indicates that the particles without cooling treatment are even in size and mostly plate-like or fractal-like shape, while the particles with cooling are non-uniform and “mosaic” aggregate with polyhedral shape. 3.3. Effects of process conditions on crystalline phase Crystalline TiO2 particles produced industrially are in the crystal form of anatase or rutile. TiO2 monomers generated by reaction (1) can form metastable anatase clusters by homogeneous nucleation. Then, the clusters grow by the heterogeneous condensation of TiO2 vapor and by the coagulation–fusion mechanism. Some of them become anatase particles, and others are transformed into rutile particles that are thermochemically stable [13]. The crystalline phases of products at different reactor temperatures are shown in Fig. 8(a) and products with or without nitrogen cooling treatment are shown in Fig. 8(b). The XRD results show that all the products were primarily anatase with some rutile, and the fraction of rutile at 1000–1100 °C increases compared with those of 800–900 °C, but this increment is not consistent with temperature increase. In contrast, almost the same rutile contents were observed for the titania powders with and without the cooling treatment. The main factors influencing rutile content are reaction temperature, crystal transformer and particle residence time. As for the temper-
Fig. 8. XRD of products at different TF1 (a), with cooling measure (b), and with AlCl3 additive (c).
ature, it has two opposite effects on crystal phase transform. On the one hand, the activation energy for transformation from anatase to rutile is 460 kJ/mol, so crystal conversion rate is presumed to increase with temperature increase [12], therefore, the rutile mass fraction increase with temperature. On the other hand, as above mentioned, homogeneous nucleation rate increasing with temperature causes the formation of lots of anatase clusters that coalesce to form more anatase crystals, although the rate of transformation of the particles of anatase into rutile increases. Furthermore, the high temperature reduces the defect
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those produced at 1100 °C [13]. Yeh et al. observed a significant increase in rutile content of titania particles by increasing the particle residence time [26,27]. Additives are routinely used in synthesis of particles to control the morphology and phase composition of product powders. Generally, with no presence of crystal conversion agent, crystal transforming rate is slow, so the product hardly finish the rutile transforming in short residence time. Then, we use the AlCl3 dopant (weight ratio of AlCl3 and TiCl4: 1%) to find its action on crystal conversion, and the crystalline phases of products with and without additives are shown in Fig. 8(c). It is clear that the anatase to rutile transformation rate was dramatically enhanced in the presence of AlCl3. Weight fractions of the anatase and rutile phases in the samples were calculated from the relative intensities of the strongest peaks corresponding to crystal planes b101N and b110N according to Eq. (2), and the results revealed that the rutile content of product with additive is 95%, while the product in the absence of additive is only 4%. Since the transformation involves contraction and co-operative movement of ions, creation of oxygen vacancies would be expected to enhance the transformation. The crystal ionic radius of Ti4+ is 0.61 Å while that of Al3+ is 0.53 Å for a coordination number of 6, Akhtar [7] found the substitutional incorporation of aluminum ions in titania lattice as Eq. (3) increase the oxygen vacancy concentration and, subsequently, enhances the phase transformation rate. Al2 O3 →2Al′Ti þ VP þ 3O Fig. 9. TEM morphology of particles with or without AlCl3 additive.
concentration in the TiO2 particles that strongly affected the transformation rate from anatase to rutile, consequently, slows the phase conversion rate. Kobata found that anatase particles produced at 900 °C are transformed to rutile more rapidly than
ð3Þ
Parallel results are that the AlCl3 additive also has influence on particles morphology as shown in Fig. 9. Different from the irregular surface of product without additive, introduction of AlCl3 resulted in more spherical particles (smooth surface). EDS analysis on the aggregate particles of Fig. 9 (b) shows that both aluminium and titanium are present (Fig. 10). The analysis
Fig. 10. EDS analysis of Al content at outer surface (a) and inner part (b) of TiO2 particle.
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on individual particles did not reveal any particles to be exclusively titania or alumina, indicating their co-precipitation during particle formation. The inset tables in Fig. 10 show the relative composition of Al and Ti in titania particles. Al element content (wt.%) on the outer surface of TiO2 particle is 1.0084%, and 0.6431% in the inner layer part. It seems that the AlCl3 plays as not only a crystal transforming agent but also a nucleating agent. Further studies on its mechanisms will be summarized in our next study. 4. Conclusions The synthesis of titania particles by the vapor-phase oxidation of TiCl4 was explored. The two opposite effects of temperature on particle size and crystalline phase content were found. The particle size exhibits a minimum with increasing reactor temperature, which can be modified by adjusting the temperature gradient through preheating and cooling measure. As the preheating temperature of oxygen increase or product temperature decrease, the median diameter of titania decrease. Increasing the TiCl4 concentration or the residence time leads to an increase in the particle size under the process conditions employed in this work. The titania particles were primarily anatase and that the transformation to rutile phase was accelerated by addition of crystal transforming agent AlCl3. The element Al concentrates on particle surface. Acknowledgment This research was supported by the China Postdoctoral Science Foundation (Grant No. 2003034200). References [1] D.F. Ollis, E. Pelizzetti, N. Serpone, Photocatalytic destruction of water contaminants, Environ. Sci. Technol. 25 (1991) 1522–1529. [2] R. Bandyopadhyaya, A.A. Lall, S.K. Friedlander, Aerosol dynamics and the synthesis of fine solid particles, Powder Technol. (139) (2004) 193–199. [3] L. Shi, C. Li, A. Chen, Morphology and structure of nanosized TiO2 particles synthesized by gas-phase reaction, Mater. Chem. Phys. 66 (2000) 51–57. [4] D.J. Hee, K. Seong-Kil, Controlled synthesis of titanium dioxide nanoparticles in a modified diffusion flame reactor, Mater. Res. Bull. 36 (2001) 627–637. [5] S.E. Pratsinis, S. Vemury, Particle formation in gases: a review, Powder Technol. 88 (1996) 267–273. [6] P.H. Dundua, M.L. Thorpe, Ionarc, N.H. Bow, Titanium dioxide production by plasma processing, Chem. Eng. Prog. 66 (1970) 66–71.
[7] M.K. Akhtar, Y. Xiong, S.E. Pratisinis, Vapor synthesis of titania powder by tetrachloride oxidation, AIChE J. 37 (1991) 1561–1570. [8] Petri Ahonen, Aerosol Production and Crystallization of Titanium Dioxide from Metal Alkoxide Droplets, VTT Publication, Finland, 2001, p. 11. [9] K. Wegner, W.J. Stark, S.E. Pratsinis, Flame-nozzle synthesis of nanoparticles with closely controlled size, morphology and crystallinity, Mater. Lett. 55 (2002) 318–321. [10] M.K. Akhtar, S.E. Pratsinis, V.R. Mastranglo, Vapor phase synthesis of Aldoped titania powders, J. Mater. Res. 5 (1994) 1241–1249. [11] Y. Suyama, A. Kato, TiO2 produced by vapor-phase oxygenolysis of TiCl4, J. Am. Ceram. Soc. 59 (1976) 146–149. [12] Y. Suyama, A. Kato, Effect of additives of the formation of TiO2 particles by vapor phase reaction, J. Am. Ceram. Soc. 68 (1985) 154–156. [13] A. Kobata, K. Kuaskabe, S. Morooka, Growth and transformation of TiO2 crystallites in aerosol reactor, AIChE J. 37 (1991) 347–359. [14] E.J. Mezey, in: C.F. Powell, J.H. Oxley, J.M. Blocher Jr. (Eds.), Vapor Deposition, John Wiley & Sons, New York, 1966, pp. 423–451. [15] K. Machenzie, The calcinations to titania: the effect of additive on the anatase-rutile transformation, Trans. J. Br. Ceram. Soc. 74 (1975) 29–34. [16] K. Nakaso, K. Okuyama, M. Shimada, S.E. Pratsinis, Effect of reaction temperature on CVD-made TiO2 primary particle diameter, Chem. Eng. Sci. 58 (2003) 3327–3335. [17] Y. Xiong, S.E. Pratsinis, Formation of agglomerate particles by coagulation and sintering-Part I. The evolution of the morphology of aerosol-made titania, silica and silica-doped titania powders, J. Aerosol Sci. 3 (1993) 301–313. [18] R.A. Supurr, H. Myers, Quantitative analysis of anatase-rutile mixture with an X-ray diffractometer, Anal. Chem. 29 (1957) 760–762. [19] A. Gurav, T. Kodas, T. Pluym, Y. Xiong, Aerosol processing of materials, Aerosol Sci. Tech. 19 (1993) 411–452. [20] Y. Xiong, S.E. Pratsinis, Gas phase production of particles in reactive turbulent flows, J. Aerosol Sci. 22 (1991) 637–642. [21] M.K. Akhtar, S.E. Pratsinis, Dopants in vapor-phase synthesis of titania, J. Am. Ceram. Soc. 75 (1992) 3408–3416. [22] M.K. Akhtar, S. Vemury, S.E. Pratsinis, Competition between TiCl4 hydrolysis and oxidation and its effect on product TiO2 powder, AIChE J. 40 (1994) 1183–1192. [23] J.H. Yu, S.Y. Kim, J.S. Lee, K.H. Ahn, In-situ observation of formation of nanosized TiO2 powder in chemical vapor condensation, Nanostruct. Mater. 12 (1999) 199–202. [24] W. Chang, G. Skandan, S.C. Danforth, Chemical vapor processing and application for nanostructured ceramic powders and whiskers, Nanostruct. Mater. 6 (1995) 321–324. [25] S. Tsantilis, S.E. Pratsinis, Evolution of primary and aggregate particle-size distribution by coagulation and sintering, AIChE J. 2 (2000) 407–415. [26] C.L. Yeh, S.H. Yeh, H.K. Ma, Flame synthesis of titania particles from titanium tetraisopropoxide in premixed flames, Powder Tech. 145 (2004) 1–9. [27] J. Hebrard, P. Nortier, M. Pijolat, M. Soustelle, Initial sintering of submicrometer titania anatase powder, J. Amer. Ceram. Soc. 73 (1990) 79–84.
Influence of temperature schedules on particle size and crystallinity of titania synthesized by vapor-phase oxidation route Zhi Wang ⁎, Zhangfu Yuan, E. Zhou Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, PR China Received 7 February 2006; received in revised form 14 August 2006; accepted 24 August 2006 Available online 5 September 2006
Abstract Crystalline TiO2 particles were produced in a tubular flow reactor by chemical vapor synthesis using titanium tetrachloride as a starting material in oxygen containing atmospheres. The dependence of particle size, morphology and crystalline phase of titania on temperature schedules including the reactor temperature, the oxygen preheated temperature and the product cooling measure were explored. It is found that there are two opposite effects of temperature on particle size and crystalline phase content. The particle size distribution, SEM and TEM of resulting powders show that the grain size is controlled by the relative magnitudes of the nucleation rate and growth rate, both of them being subject to the temperature schedules. XRD indicates that particles crystalline phase is predominately anatase and the rutile content increment is not consistent with temperature increase. Anatase titania can be converted to rutile by addition of crystal modifier AlCl3. The element analysis by EDS shows that Al enriches on the particle outer surface. © 2006 Elsevier B.V. All rights reserved. Keywords: Titania; Titanium tetrachloride; Crystallinity; Particle size; Gas phase route
1. Introduction Titanium dioxide is one of the most utilized particulate materials in the world, such as pigment, plastic, paper making, printing ink, and chemical fiber and rubber. Recent developments have led to the use of titania as a photocatalyst [1]. China imports tens of thousands of tons of high-grade rutile titania annually and this presents an increase trend in recent years. On the industrial scale, the titania pigment manufacture routes include the sulphate process and the chloride process. However, economic and environmental pressures are shifting the world balance of titanium dioxide production away from sulphate based manufacture towards the more cost effective and cleaner chloride route. This so-called chloride process has been reported by several researchers [2–5]. In this process, TiO2 and other metal oxides in the feed ore are reacted at high temperature with chlorine in the presence of carbon. The product gases are cooled after they leave the reaction zone, upon which solid contaminants precipitate and are ⁎ Corresponding author. Institute of Process Engineering, Chinese Academy of Sciences, Zhong Guan Cun, Haidian District, P.O. Box 353, Beijing 100080, PR China. Tel./fax: +86 10 62558489. E-mail address: [email protected] (Z. Wang). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.08.020
substantially removed. The liquid titanium tetrachloride is purified by distillation before again being vaporized and interacting with hot oxygen to form titanium dioxide and regenerate chlorine for recycle. The base pigment is wet milled, surface treated and redispersed before packaging [6]. The formation of TiO2 crystallites in the oxidation reactor is shown as the following overall reaction in the temperature range of 700–1400 °C [7]. TiCl4 þ O2 →TiO2 þ 2Cl2
ð1Þ
Table 1 compares the TiO2 rutile and anatase structures and their physical properties [8]. The phase content, size characteristics Table 1 TiO2 rutile and anatase structures and physical properties [8] Property
Rutile
Anatase
Crystal structure Space group Lattice spacing a//c (nm) Density (g/cm3) Refract. index, 550 nm Band gap (eV) Melting point (°C)
Tetragonal P42/mnm 0.459//0.296 4.25 2.75 3.05 1830–1850
Tetragonal I41/amd 0.378//0.951 3.895 2.54 3.25 Conv. to rutile
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contents as the reactor temperature increase was observed by Suyama and Kato [12], but Kobata et al. [13] reported that rutile weight fraction was at a maximum at 1000 °C and decreased rapidly at both lower and higher temperatures. Mezey certified that the presence of 0.01 to 10% of an aluminum-containing compound in the flame oxidation of TiCl4 led to the formation of rutile [14]. Machenzie concluded that oxygen vacancies promoted crystallite growth and the anatase to rutile transformation [15]. While some of the preparations of titania have been studied,
Fig. 1. Experimental apparatus for preparation of TiO2 by gas-phase route. 1— O2 steel cylinder; 2—Ar steel cylinder; 3—silica desiccant; 4—flow controller; 5—O2 preheater; 6—electric furnace; 7—reactor; 8—AlCl3 gasifier; 9—TiCl4 container; 10—heated water bath; 11—N2 steel cylinder; 12—cooling device; 13—chlorine absorber; 14—electric heating tape.
and morphology of the individual particles making up the powder often have a strong influence on properties for a variety of applications [9]. Since the pigmentary powder properties, e.g. grain size, phase content, are predominantly determined by the oxidation course, the TiCl4 oxidation is the key procedure in the chloride process. Though the chloride process is a mature technology, the product properties are not often in control in China and the fundamentals of dopant addition have not been understood. As for one particle evolution during oxidation process, apart from the reactor temperature, the temperature schedules also comprise the reactant preheated temperature and with or without product cooling measure. Akhtar et al. [7, 10] studied the effect of process variables on titania powder characteristics and found that increasing temperature, reactant concentration, and reactor residence time resulted in larger titania particles, but the spread of the particle size distribution (PSD) remained rather unaffected. Suyama and Kato [11] found that the particle size decreased with increasing reaction temperature and oxygen concentration and with decreasing TiCl4 concentration. A linear increase in rutile
Fig. 2. Effects of TF1 on particles median diameter at different TiCl4 conentrations.
Fig. 3. TEM morphology of particles at different TF1 (C[TiCl4] = 1.018 × 10− 3 mol/l).
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Fig. 4. Effects of TF1 on particle size distribution at different TiCl4 concentrations.
contradictions and different results may arise due to different experimental conditions [3,16,17]. Furthermore, the relative importance of the synthesis variables, especially the influence of temperature schedules on the grain size and crystalline phase of TiO2 is not well understood. Based on the above backgrounds and for better control of the characteristics of powders, we have established a vapor-phase route for preparation of titanium dioxide particles by oxidation of titanium tetrachloride vaporized with hot oxygen. This article presents an experimental investigation of the dependence of particle size and crystalline phase of titania on temperature schedules. Changes in the particle morphology and crystalline phase of titania with dopant were also examined. 2. Materials and methods 2.1. Titanium dioxide synthesis The schematic diagram of the apparatus for vapor-phase reaction of TiCl4 and O2 is shown in Fig. 1. It consisted of gas purifiers, reactant preheaters, a tubular flow reactor, a cooler and separator and an off-gas treatment unit. A 15- or 30-mm-ID
quartz tube (Reactor-1: 15 mm and Reactor-2: 30 mm in I.D., both 1500 mm in length) was used as the reactor that was heated by a horizontal electrical furnace (Tianjin Zhonghuan Furnace Ltd. Co.), whose isothermal zone is 500 mm within ± 15 °C in the usual experimental condition. Temperature was measured with a copper-constantan (K type) thermocouple. Since TiCl4 is sensitive to the presence of water, the O2 and Ar were dried through silica desiccant respectively. Prepurified dry Ar (99.99% pure) was used as a carrier gas and bubbled through a fritted glass outlet into a flask containing liquid TiCl4 (reagent grade) in a constant temperature water bath (90 ± 1 °C). This premixed stream of Ar and TiCl4 vapor is first mixed with preheated O2 stream and then flows into the reactor tube along with another Ar stream for dilution. The gas flow rate ranges from 1.5 to 3.5 l/min. O2 used in these experiments was between 4 and 8 times in excess of the stoichiometric amount. The reactants are present in dilute concentrations so the released heat of reaction is insufficient to appreciably affect reactor temperature. All lines transporting TiCl4 and AlCl3 were maintained at temperature high enough to prevent their condensation by electric heating tapes. Exiting from the isothermal zone of the reactor, the gas stream was cooled with room temperature
Fig. 5. Schematic of gas-to-particle conversion process.
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dry N2 (99.95% pure) in a quartz jacketed cooler at the rear of the reactor. The cooling gas quenched chemical reaction and inhibited particle growth by coagulation outside of the reaction zone. In addition, it reduces thermophoretic losses from the product stream onto the cool zone walls. The exhaust gas was neutralized with 1 M NaOH solution before exiting through the laboratory hood. The particles were collected by natural sedimentation into a box connected to the end of the reactor. 2.2. Titanium dioxide characterization The median diameter and PSD of products were obtained by a laser particle size analyzer BI-90 (Brook-haven Instrument Co.). The TEM and SEM images were performed on a JEM1200EX (JEOL Co.) and a JSM-6700F (JEOL Co.) fieldemission scanning electron microscope respectively, which were used to study the morphology of titania and validate the particle size measurements obtained from the laser particle size analyzer. The energy dispersive spectra (EDS) were conducted on a QX2000 (Link Co.). X-ray diffraction (XRD) was used to determine the phase composition using a D/MAX-RB X-ray powder diffractometer (X'Pert PRO MPD, using CuKα radiation). The weight fractions of the anatase and rutile phases in the samples were calculated from the relative intensities of the strongest peaks corresponding to anatase and rutile as described by Spurr and Myers with formula 2 [18]. M R =ðM A þ M R Þ ¼ 1=ð1 þ 1:26I A =I R Þ
all the products have log-normal distribution curves, which shift to smaller sizes with temperature in the range of 800–1000 °C, later on the peaks move positively slightly. This phenomenon agrees with the evolution of median diameter with temperature. In addition, the increase in TiCl4 concentration does not change the shape of size distribution. As illustrated in Fig. 5 [19], during powder production by gas phase processes, product molecules generated by decomposition of the precursor and chemical reactions form molecular clusters by collision of intermediate Ti-containing species with O-containing species, then the clusters become particles by nucleation/ surface reactions and/or uninhibited coagulation [20]. It is expected that chemical reaction and coagulation determine the characteristics of the product titania particle size distribution, which is a consequence of two different roles of nucleation and growth that are temperature dependence. In the first place, the continuous increase in temperature increases the supersaturation of titania in the system and a higher concentration of precursor monomers are obtained through a higher superficial reaction rate due to the increase of gas mean free path, no doubt, the extremely small particles in the early stages bring about smaller particles caused by an enhancement of nucleation rate [3]. In addition, allowed for the gas expansion in higher temperature, the mean
ð2Þ
where MA and MR are the phase fraction of anatase and rutile titania; IA and IR are their intensities of the strongest XRD peaks, respectively. 3. Results and discussion 3.1. Effects of reactor temperature on particle size and morphology When the O2 preheater temperature (TF2) is 1100 °C, the influence of reactor temperature (TF1) on the median diameter of titania is shown in Fig. 2. The experiments show that the particle sizes are fairly sensitive to temperature changes. With increase of reaction zone temperature, the products show a decrease in median diameter from 217 nm at 800 °C to 176 nm at 1000 °C then a prominent increase to 198 nm at 1100 °C when TiCl4 concentration is 1.018 × 10− 3 mol/l. In addition, it is observed that the median diameter of the particles increases with increasing TiCl4 vapor concentration (C[TiCl4]) at the same conditions. The TEM results in Fig. 3 indicate that at all temperatures investigated, the particle morphology remains dense polyhedral structures with well-defined edges, and individual primary particles can be easily distinguished, as has been shown by Akhtar [7] and Suyama [11,12]. The shape of the individual primary particles and the morphology of the aggregate at different temperature remain rather unaffected. However, we observe that the product size at 1000 °C is smallest, while the product size at 1100 °C is largest. This is in agreement with the above diameter results. It is observed from the PSD in Fig. 4 that
Fig. 6. Effects of TF2 (a) and cooling measure (b) on particle average diameter.
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residence time in the reactor is shorter although the effective reaction zone becomes slightly longer, and decreasing residence time leads to a decrease in the time for collision and results in smaller particles. On the other hand, when the temperature exceeds certain value, for example 1000 °C in this experiment, the particle size shows an increase trend. The higher collision frequency between two monomers causes forming clusters that are prone to coalescence and sintering at higher temperature, that is, increasing temperature promotes the occurrence of growth that brings large grain sizes [16]. In general, the particle size of the product is determined by the relative magnitudes of the nucleation rate and growth rate. The relative importance of these two opposing effects of temperature on powder size determines the trend of size evolution. Accordingly, the products reach minimums at 1000 °C, and the particle size will increase beyond that point. Similarly, it is not difficult to understand the fact that the increase in TiCl4 concentration causes particle size increase because the particle diameter is proportional to (C0 / N)1/3, where C0 is the concentration of metal halide and N is the number of nuclei. This implies that the larger the material concentration, the larger is the efficient collision frequency and the faster is the particle growth [21,22].
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3.2. Effects of preheating and cooling measures on particle size and morphology By virtue of chemical vapor deposition and chemical vapor condensation processes [23,24], we have adopted two measures to study the influence of the reactant preheater temperature and the product cooling treatment on titania properties. First, we increase the initial oxygen temperature by utilizing a controllable preheater; second, we blow nitrogen with flow rate of 5.0 l/min into a quartz jacketed cooler at the rear of the reactor, so the gas stream and entrained product particles from the reaction zone are rapidly cooled. When the reactor temperature (TF1) is 1000 °C and other conditions are fixed, the influence of oxygen preheater temperature (TF2) on median diameter of titania was shown in Fig. 6(a). It can be seen from Fig. 6(a) that, with increase of oxygen preheated temperature, the median diameters of products become smaller. As the reactants temperature in the mixing zone is very low, so the initial reaction was relatively more controlled by environment temperature. The increase in temperature of oxygen enhances the role of the mixing zone playing on final product. It seems that the increase of oxygen temperature increases of
Fig. 7. SEM morphology of particles at different TF2 (a–b) and with or without cooling (c–d).
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nucleation speed. The high concentration of very fine monomers in the initial stage makes the particle size smaller at the condition when the residence time is not long enough. Therefore, the preheating of reactants is a very important factor in controlling the nucleation and consequently particle sizes. In addition, when the TiCl4 concentration and the flow rate were kept constant, and the reactor tube inner diameter increases from 15 mm (Reactor1) to 30 mm (Reactor-2), the products from the larger reactor is bigger than those from the smaller one, which may be related to the residence time increases in larger reactor. At high residence times the particles produced have more time for coagulation. From the SEM results in Fig. 7 (a)–(b), we can find that the powders synthesized are composed of irregularly shaped aggregate particles. It is the finite coalescence rate that results in the formation of irregularly shaped aggregate particles [25]. By comparison, the product at TF2 1100 °C includes more fine grains. The effect of cooling measure on particle size can be found from Fig. 6(b), and the product size is reduced by the cooling gas through quenching chemical reaction and inhibiting particle growth at the outside of the reaction zone. Fig. 5 has illustrated that in the initial stage of the oxidation process nucleation was the main route for TiCl4 conversion, while growth or coalescence in the latter stage. With the cooling procedure, the time of particles in high temperature state reduces, which corresponds to their sintering energy decreases substantially so particles coagulation and coalescence can no longer be regarded as main part of particle growth. Combining with the nucleation and growth characteristics of oxidation process, we confirm that particle size can be adjusted by altering the grains temperature gradient experienced. The morphologies of products from cooling treatment are shown in Fig. 7(c)–(d). It indicates that the particles without cooling treatment are even in size and mostly plate-like or fractal-like shape, while the particles with cooling are non-uniform and “mosaic” aggregate with polyhedral shape. 3.3. Effects of process conditions on crystalline phase Crystalline TiO2 particles produced industrially are in the crystal form of anatase or rutile. TiO2 monomers generated by reaction (1) can form metastable anatase clusters by homogeneous nucleation. Then, the clusters grow by the heterogeneous condensation of TiO2 vapor and by the coagulation–fusion mechanism. Some of them become anatase particles, and others are transformed into rutile particles that are thermochemically stable [13]. The crystalline phases of products at different reactor temperatures are shown in Fig. 8(a) and products with or without nitrogen cooling treatment are shown in Fig. 8(b). The XRD results show that all the products were primarily anatase with some rutile, and the fraction of rutile at 1000–1100 °C increases compared with those of 800–900 °C, but this increment is not consistent with temperature increase. In contrast, almost the same rutile contents were observed for the titania powders with and without the cooling treatment. The main factors influencing rutile content are reaction temperature, crystal transformer and particle residence time. As for the temper-
Fig. 8. XRD of products at different TF1 (a), with cooling measure (b), and with AlCl3 additive (c).
ature, it has two opposite effects on crystal phase transform. On the one hand, the activation energy for transformation from anatase to rutile is 460 kJ/mol, so crystal conversion rate is presumed to increase with temperature increase [12], therefore, the rutile mass fraction increase with temperature. On the other hand, as above mentioned, homogeneous nucleation rate increasing with temperature causes the formation of lots of anatase clusters that coalesce to form more anatase crystals, although the rate of transformation of the particles of anatase into rutile increases. Furthermore, the high temperature reduces the defect
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those produced at 1100 °C [13]. Yeh et al. observed a significant increase in rutile content of titania particles by increasing the particle residence time [26,27]. Additives are routinely used in synthesis of particles to control the morphology and phase composition of product powders. Generally, with no presence of crystal conversion agent, crystal transforming rate is slow, so the product hardly finish the rutile transforming in short residence time. Then, we use the AlCl3 dopant (weight ratio of AlCl3 and TiCl4: 1%) to find its action on crystal conversion, and the crystalline phases of products with and without additives are shown in Fig. 8(c). It is clear that the anatase to rutile transformation rate was dramatically enhanced in the presence of AlCl3. Weight fractions of the anatase and rutile phases in the samples were calculated from the relative intensities of the strongest peaks corresponding to crystal planes b101N and b110N according to Eq. (2), and the results revealed that the rutile content of product with additive is 95%, while the product in the absence of additive is only 4%. Since the transformation involves contraction and co-operative movement of ions, creation of oxygen vacancies would be expected to enhance the transformation. The crystal ionic radius of Ti4+ is 0.61 Å while that of Al3+ is 0.53 Å for a coordination number of 6, Akhtar [7] found the substitutional incorporation of aluminum ions in titania lattice as Eq. (3) increase the oxygen vacancy concentration and, subsequently, enhances the phase transformation rate. Al2 O3 →2Al′Ti þ VP þ 3O Fig. 9. TEM morphology of particles with or without AlCl3 additive.
concentration in the TiO2 particles that strongly affected the transformation rate from anatase to rutile, consequently, slows the phase conversion rate. Kobata found that anatase particles produced at 900 °C are transformed to rutile more rapidly than
ð3Þ
Parallel results are that the AlCl3 additive also has influence on particles morphology as shown in Fig. 9. Different from the irregular surface of product without additive, introduction of AlCl3 resulted in more spherical particles (smooth surface). EDS analysis on the aggregate particles of Fig. 9 (b) shows that both aluminium and titanium are present (Fig. 10). The analysis
Fig. 10. EDS analysis of Al content at outer surface (a) and inner part (b) of TiO2 particle.
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on individual particles did not reveal any particles to be exclusively titania or alumina, indicating their co-precipitation during particle formation. The inset tables in Fig. 10 show the relative composition of Al and Ti in titania particles. Al element content (wt.%) on the outer surface of TiO2 particle is 1.0084%, and 0.6431% in the inner layer part. It seems that the AlCl3 plays as not only a crystal transforming agent but also a nucleating agent. Further studies on its mechanisms will be summarized in our next study. 4. Conclusions The synthesis of titania particles by the vapor-phase oxidation of TiCl4 was explored. The two opposite effects of temperature on particle size and crystalline phase content were found. The particle size exhibits a minimum with increasing reactor temperature, which can be modified by adjusting the temperature gradient through preheating and cooling measure. As the preheating temperature of oxygen increase or product temperature decrease, the median diameter of titania decrease. Increasing the TiCl4 concentration or the residence time leads to an increase in the particle size under the process conditions employed in this work. The titania particles were primarily anatase and that the transformation to rutile phase was accelerated by addition of crystal transforming agent AlCl3. The element Al concentrates on particle surface. Acknowledgment This research was supported by the China Postdoctoral Science Foundation (Grant No. 2003034200). References [1] D.F. Ollis, E. Pelizzetti, N. Serpone, Photocatalytic destruction of water contaminants, Environ. Sci. Technol. 25 (1991) 1522–1529. [2] R. Bandyopadhyaya, A.A. Lall, S.K. Friedlander, Aerosol dynamics and the synthesis of fine solid particles, Powder Technol. (139) (2004) 193–199. [3] L. Shi, C. Li, A. Chen, Morphology and structure of nanosized TiO2 particles synthesized by gas-phase reaction, Mater. Chem. Phys. 66 (2000) 51–57. [4] D.J. Hee, K. Seong-Kil, Controlled synthesis of titanium dioxide nanoparticles in a modified diffusion flame reactor, Mater. Res. Bull. 36 (2001) 627–637. [5] S.E. Pratsinis, S. Vemury, Particle formation in gases: a review, Powder Technol. 88 (1996) 267–273. [6] P.H. Dundua, M.L. Thorpe, Ionarc, N.H. Bow, Titanium dioxide production by plasma processing, Chem. Eng. Prog. 66 (1970) 66–71.
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