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Kinetic study of titanium tetraisopropoxide decomposition in supercritical isopropanol
High Pressure Chemical Engineering Ph. Rudolfvon Rohr and Ch. Trepp (Editors) 9 1996 Elsevier Science B.V. All rights reserved.
K i n e t i c S t u d y of T i t a n i u m Supercritical Isopropanol
Tetraisopropoxide
133
Decomposition
in
V. Gourinchas-Courtecuisse, K. Chhor, J.F. Bocquet, C. Pommier Laboratoire d'Ing6nierie des Mat6riaux et des Hautes Pressions, C.N.R.S., Universit6 Paris XIII, Avenue Jean-Baptiste C16ment, 93430- Villetaneuse, France
Decomposition of Ti(O-iC 3H7)4 dissolved in supercritical isopropanol leads to the formation of titanium oxide. The reaction is studied in the temperature range 531 to 568 K under 10 MPa and a mechanism is proposed. The obtained kinetic results are further used to optimize a continuous reactor producing submicronic TiO2 powder at a laboratory pilot scale.
1. INTRODUCTION We recently used titanium isopropoxide, Ti(O-iC3H7)4 (referred to hereafter as TTIP) as a precursor for TiO2 submicronic powder synthesis or thin film deposition [1,2]. Reactions were performed in supercritical isopropanol or alcohol-CO2 mixtures. Advantages of such processes were the recover of partially crystallized powders without further washing and drying steps removing solvent and by-products simply by decompressing the system at temperature above the alcohol critical point. Furthermore, homogeneous TiO2 films, of about 5 ~m thickness, can be formed on an alumina substrate in less than 10 minutes, i.e. much more rapidly than by conventional CVD methods. In order to model and optimize the behaviour of a continuous system for TiO2 powder production on a laboratory pilot scale, we need kinetic data forthe involved reaction. The aim of this study is then to get such informations on TTIP decomposition in supercritical isopropanol. The TTIP thermal decomposition has been studied by various authors. Although the usually accepted overall reaction can be written as Ti(O-iC3H7)4 --~ TiO2 + 4C3H6+2H20 conflicting results about kinetic rate laws and mechanisms have been reported.. These discrepancies appear to originate in the different experimental conditions in which the various studies were performed. TTIP decomposition becomes detectable above about 623 K in a clean glass reactor; while it has a significant rate at temperatures as low as 523 K when the
134 reactor walls are covered with previously formed TiO2 [3]. The presence of alumina, which is known to catalyse alcohol dehydration at temperature higher than 473 K, has been shown to greatly influence the alkoxide transformation [4,5]. Various overall reaction orders have been reported from kinetic studies. The order 0.5 was proposed in TiO2 powder synthesis [6] while values between 0 and 2 were found from CVD experiments in which TTIP partial pressure and temperature are measured[4, 7-10]. A reaction mechanism was first proposed by Siefering et al. [7] in order to explain their experimental results. The three elementary steps were successively : (i) activation of a TrIP molecule by coUisional excitation with another one in the gas phase, (ii) adsorption of the activated species, (iii) surface decomposition. Depending on the conditions, the limiting step, as well as the overall reaction order, can change. Few authors reported measurements on activation energy. From these studies, two sets of values can be distinguished : around 100 to 150 kJ.mole -1 [8,9] or around 20 or 27 kJ. mole -1 [11,12]. In the latter case, a rather high oxygen content in the gas phase of the CVD system has been shown to favor the film formation. The addition of water in CVD experiments also increases the film growth rate [12,13].
2. EXPERIMENTAL The reactor used is a closed 240 cm 3 stainless steel vessel fitted with temperature and pressure measurement sensors and with a device allowing the withdrawal of small samples at regular time intervals. The temperature was regulated within + 2 K and the pressure was fixed at around 10 MPa. A TTIP solution in isopropanol, with concentration between 0.1 and 0.6 mole.1-1 was first introduced into the system and heated up to the study temperature (531 to 568 K) at 5 K.min q. At this temperature, the reaction was followed analysing successive w i t h d r a w n samples. The concentration of alkoxy groups bonded to a titanium atom and remaining in solution was determined by IR spectrometry by measuring the intensity of the absorption band at 1025 cm -1 attributed to C-O stretching vibration. The overall error in concentration is estimated to be around + 10 %. Isopropanol (critical point Tc = 508 K, Pc = 4.7 MPa) has a purity higher than 99 %. TTIP was distilled under reduced pressure at 368 K before being used. In preliminary experiments, it was shown that the internal surface state of the reactor has an important influence on the reaction rate. Therefore, special care was taken to keep it as reproductible as possible from one experiment to the other by leaving a small film of strongly held TiO2 particles on the internal walls.
3. RESULTS As mentionned above, reaction advancement was followed from IR spectrometric measurements. In so far as the total withdrawn sample volume can be neglected compared with the initial one, the volumic molar concentrations of OR alkyl groups
135 in the liquid at room temperature and in the supercritical reaction medium, at temperature T (noted [A] and [A]' respectively) can be related by [A]' = [A] PSCF,, = [A]VL PL VR -where PSCF and PL are densities of the supercrifical fluid and liquid respectively, VR is the reactor volume, V L isthe introduced liquid solution. In the initial TTIP alcoholic solution of concentration Ci, the alkoxy group concentration is [A]i = 4Ci. If no reaction occurs during heating up to the study temperature TR, and before data collection is started (at time t=0), the above equations lead to [A]'~ = 4Ci. VL . This is really not the case and the measured
VR values are 20 to 35 % lower. !
Experimental [A]t values can be fitted as various functions of time in order to determine an overall kinetic reaction order. For the five investigated reaction temperatures in the range 531 to 568 K, the best linear fit is obtained plotting ln[A]t = f(t), in accordance with a first order reaction. The kinetic constants derived from curves in figures 1 and 2 are reported in table 1. Figure 3 shows an Arrhenius plot allowing calculation of the activation energy. Taking into account the previously estimated errors in TR and [A], the value Ea = 113 + 16 kJ.mole -1 can be proposed. Such a value is close to that reported when a thermal decomposition reaction is the limiting step [7,8]. t
4. DISCUSSION As pointed out above, many studies on TTIP "decomposition" emphasize the influence of water formed in a dehydration reaction of alcohol present either as solvent or as traces. Such a chain mechanism involving a first alcohol molecule producing one water molecule which, by the hydrolysis reaction, gives two additional alcohol molecules has been shown by Bradley and al. studying zirconium alkoxides [11]. Curiously, such a "hydrolytic" decomposition of titanium derivatives has not been taken into account in the most recently reported studies on CVD experiments from TrIP [8-10]. We have previously s h o w n that solid particle formation from TTIP in supercritical alcohol occurs at lower temperature than for pure vapor decomposition [1]. This allows us to assume that the first step in TiO2 formation from titanium alkoxide under our experimental conditions is alcohol dehydration followed by hydrolysis reactions. In support of this hypothesis, we studied Raman spectra recorded on samples withdrawn at various moments from the reaction medium. We shown that the intensities associated with Ti-OR, Ti-OH and Ti-O-Ti vibrational stretching bands decrease with time, but that the intensity ratios of two v(Ti-O) lines remain nearly
136
Figure 1 9First order kinetics plots at temperature 531 K (a), 536 K (b), 546 K (c).
Figure 2- First order kinetics plots at temperature 556 K (a), 568 K (b).
137
Table 1 Rate constants and activation energy T(K) 531 • 2 536 + 2 546 + 2 556 + 2 568 + 2 Ea = 113 +
k(min-1) 0.076 • 0.008 0.088 + 0.009 0.219 + 0.022 0.220 + 0.022 0.405 + 0.041 16 kJ. mole -1 10~/T (K) Figure 3" Arrhenius plot for the kinetic constants k.
constant. These results indicate that hydrolysis equilibria are established in the system at a given temperature. The following mechanism can then be proposed for transformation of TTIP into TiO2 under our experimental conditions (R is the isopropyl group, R' is C3H6) : Kl, kl ROH -~ R'+ H 2 0 (1) K2,k2 -~ Ti(OR)a(OH + ROH
Ti(OR)4 + H 2 0
(2)
6--
Ti(OR)3(OH + H 2 0
K3,k3 -~ Ti(OR)2(OH)2 + ROH
(3)
Ti(OR)2(OH)2 + I-I20
K4,k4 -~ Ti(OR)(OH)3 + ROH
(4)
Ti(OR)2(OH)2
TiO2 + 2 ROH
Ti(OR)(OH)3
~ ~
TiO2 + ROH + H 2 0
(5) (6)
where ki (or k' i) and Ki are rate constants and equilibrium constants respectively. It has been previously shown [5] that around 613 K, reaction (1) catalysed by A1203 is about 10 times faster than the alkoxide thermolysis. It is also well known [12] that the rate of successive hydrolyses decreases when the number of already reacted radicals increases. Therefore, we only mention the first 3 steps in the above sequence. Furthermore hydrolysis reactions are known to be faster than dehydration and dealcoholation of intermediate Ti(OR)4-x(OH)x species [13]. In the proposed mechanism, reactions (3') and (4') can then be considered as the limiting steps and the disappearance rate of OR groups bonded to Ti atom in the supercritical solution can be expressed as: d[A]_____~' = -2k'3[Ti(OR)2 (OH)2 ] - k4[Ti(OR)(OH)3 ] dt !
138 Using classical expressions of the equilibrium constants K2, K3, K4, it follows : d[A]' _ dt - -
[
]
2k 3 ' [A'] K4[H20 ] + k4" Z
where Z depends on K2, K 3, K4 and is a function of [H20] and [ROH]. A steady state between relative concentrations of various species containing titanium can be assumed as shown from the Raman study, so that water and isopropanol concentrations can be considered as constants. The kinetic law of the t
overall reaction can then be written d[A] =-k[A]'. The reaction is then found to be dt first order as observed from experimental data.
CONCLUSION The present study on TiO2 powder formation from Ti(O-iC3H7)4 in supercritical isopropanol has allowed the determination of reaction kinetic constants and activation energy in a temperature range from 531 to 568 K at 10 MPa. The proposed mechanism is based on a hydrolytic decomposition of the alkoxide initiated by water formed in alcohol dehydration catalysed by reactor walls. The derived reaction kinetic order is unity in accordance with experimental results. Such a mechanism also explains that special cares must be taken about the internal surface state of the reactor in order to obtain reproducible results.
REFERENCES
1. K. Chhor, J.F. Bocquet and C. Pommier, Mater. Chem. Phys. 32 (1992) 249. 2. J.F. Bocquet, K. Chhor and C. Pommier, Surface and Coatings Technol. 70 (1994) 73. 3. Y. Shimogaki and H. Komiyama, Chem. Letters (1986) 267. 4. Y. Takahaschi, H. Suzuki and N. Nasu, J. Chem. Soc., Farad. Trans. I, 81 (1985) 3117. 5. S.A. Kurtz and R.G. Gordon, Thin Solid Films 147 (1987) 167. 6. T. Kanai, H. Komiyama and H. Inoue, Kagaku Kogaku Ronbunshu 11 (1985) 317. 7. K.L. Siefering and G.L. Griffin, J. Electrochem. Soc. 137 (1990) 814. 8. W.G. Lai and G.L. Griffin, P. Vincenzini (eds), Performance Ceramic Films and Coatings, 151, Elsevier, 1991. 9. Z. Chen and A. Derking, J. Mater. Chem. 3 (1993) 1137. 10. H.Y. Lee and H.G. Kim, Thin Solid Films 229 (1993) 187. 11. D.C. Bradley and M.M. Faktor, Trans. Farad. Soc. (1959) 2117. 12. E.A. Barringer and H.K. Bowen, Langmuir I (1985) 414. 13. T. Ishino and S. Minami, Tech. Rep. Osaka Univ. 3 (1953) 357.
K i n e t i c S t u d y of T i t a n i u m Supercritical Isopropanol
Tetraisopropoxide
133
Decomposition
in
V. Gourinchas-Courtecuisse, K. Chhor, J.F. Bocquet, C. Pommier Laboratoire d'Ing6nierie des Mat6riaux et des Hautes Pressions, C.N.R.S., Universit6 Paris XIII, Avenue Jean-Baptiste C16ment, 93430- Villetaneuse, France
Decomposition of Ti(O-iC 3H7)4 dissolved in supercritical isopropanol leads to the formation of titanium oxide. The reaction is studied in the temperature range 531 to 568 K under 10 MPa and a mechanism is proposed. The obtained kinetic results are further used to optimize a continuous reactor producing submicronic TiO2 powder at a laboratory pilot scale.
1. INTRODUCTION We recently used titanium isopropoxide, Ti(O-iC3H7)4 (referred to hereafter as TTIP) as a precursor for TiO2 submicronic powder synthesis or thin film deposition [1,2]. Reactions were performed in supercritical isopropanol or alcohol-CO2 mixtures. Advantages of such processes were the recover of partially crystallized powders without further washing and drying steps removing solvent and by-products simply by decompressing the system at temperature above the alcohol critical point. Furthermore, homogeneous TiO2 films, of about 5 ~m thickness, can be formed on an alumina substrate in less than 10 minutes, i.e. much more rapidly than by conventional CVD methods. In order to model and optimize the behaviour of a continuous system for TiO2 powder production on a laboratory pilot scale, we need kinetic data forthe involved reaction. The aim of this study is then to get such informations on TTIP decomposition in supercritical isopropanol. The TTIP thermal decomposition has been studied by various authors. Although the usually accepted overall reaction can be written as Ti(O-iC3H7)4 --~ TiO2 + 4C3H6+2H20 conflicting results about kinetic rate laws and mechanisms have been reported.. These discrepancies appear to originate in the different experimental conditions in which the various studies were performed. TTIP decomposition becomes detectable above about 623 K in a clean glass reactor; while it has a significant rate at temperatures as low as 523 K when the
134 reactor walls are covered with previously formed TiO2 [3]. The presence of alumina, which is known to catalyse alcohol dehydration at temperature higher than 473 K, has been shown to greatly influence the alkoxide transformation [4,5]. Various overall reaction orders have been reported from kinetic studies. The order 0.5 was proposed in TiO2 powder synthesis [6] while values between 0 and 2 were found from CVD experiments in which TTIP partial pressure and temperature are measured[4, 7-10]. A reaction mechanism was first proposed by Siefering et al. [7] in order to explain their experimental results. The three elementary steps were successively : (i) activation of a TrIP molecule by coUisional excitation with another one in the gas phase, (ii) adsorption of the activated species, (iii) surface decomposition. Depending on the conditions, the limiting step, as well as the overall reaction order, can change. Few authors reported measurements on activation energy. From these studies, two sets of values can be distinguished : around 100 to 150 kJ.mole -1 [8,9] or around 20 or 27 kJ. mole -1 [11,12]. In the latter case, a rather high oxygen content in the gas phase of the CVD system has been shown to favor the film formation. The addition of water in CVD experiments also increases the film growth rate [12,13].
2. EXPERIMENTAL The reactor used is a closed 240 cm 3 stainless steel vessel fitted with temperature and pressure measurement sensors and with a device allowing the withdrawal of small samples at regular time intervals. The temperature was regulated within + 2 K and the pressure was fixed at around 10 MPa. A TTIP solution in isopropanol, with concentration between 0.1 and 0.6 mole.1-1 was first introduced into the system and heated up to the study temperature (531 to 568 K) at 5 K.min q. At this temperature, the reaction was followed analysing successive w i t h d r a w n samples. The concentration of alkoxy groups bonded to a titanium atom and remaining in solution was determined by IR spectrometry by measuring the intensity of the absorption band at 1025 cm -1 attributed to C-O stretching vibration. The overall error in concentration is estimated to be around + 10 %. Isopropanol (critical point Tc = 508 K, Pc = 4.7 MPa) has a purity higher than 99 %. TTIP was distilled under reduced pressure at 368 K before being used. In preliminary experiments, it was shown that the internal surface state of the reactor has an important influence on the reaction rate. Therefore, special care was taken to keep it as reproductible as possible from one experiment to the other by leaving a small film of strongly held TiO2 particles on the internal walls.
3. RESULTS As mentionned above, reaction advancement was followed from IR spectrometric measurements. In so far as the total withdrawn sample volume can be neglected compared with the initial one, the volumic molar concentrations of OR alkyl groups
135 in the liquid at room temperature and in the supercritical reaction medium, at temperature T (noted [A] and [A]' respectively) can be related by [A]' = [A] PSCF,, = [A]VL PL VR -where PSCF and PL are densities of the supercrifical fluid and liquid respectively, VR is the reactor volume, V L isthe introduced liquid solution. In the initial TTIP alcoholic solution of concentration Ci, the alkoxy group concentration is [A]i = 4Ci. If no reaction occurs during heating up to the study temperature TR, and before data collection is started (at time t=0), the above equations lead to [A]'~ = 4Ci. VL . This is really not the case and the measured
VR values are 20 to 35 % lower. !
Experimental [A]t values can be fitted as various functions of time in order to determine an overall kinetic reaction order. For the five investigated reaction temperatures in the range 531 to 568 K, the best linear fit is obtained plotting ln[A]t = f(t), in accordance with a first order reaction. The kinetic constants derived from curves in figures 1 and 2 are reported in table 1. Figure 3 shows an Arrhenius plot allowing calculation of the activation energy. Taking into account the previously estimated errors in TR and [A], the value Ea = 113 + 16 kJ.mole -1 can be proposed. Such a value is close to that reported when a thermal decomposition reaction is the limiting step [7,8]. t
4. DISCUSSION As pointed out above, many studies on TTIP "decomposition" emphasize the influence of water formed in a dehydration reaction of alcohol present either as solvent or as traces. Such a chain mechanism involving a first alcohol molecule producing one water molecule which, by the hydrolysis reaction, gives two additional alcohol molecules has been shown by Bradley and al. studying zirconium alkoxides [11]. Curiously, such a "hydrolytic" decomposition of titanium derivatives has not been taken into account in the most recently reported studies on CVD experiments from TrIP [8-10]. We have previously s h o w n that solid particle formation from TTIP in supercritical alcohol occurs at lower temperature than for pure vapor decomposition [1]. This allows us to assume that the first step in TiO2 formation from titanium alkoxide under our experimental conditions is alcohol dehydration followed by hydrolysis reactions. In support of this hypothesis, we studied Raman spectra recorded on samples withdrawn at various moments from the reaction medium. We shown that the intensities associated with Ti-OR, Ti-OH and Ti-O-Ti vibrational stretching bands decrease with time, but that the intensity ratios of two v(Ti-O) lines remain nearly
136
Figure 1 9First order kinetics plots at temperature 531 K (a), 536 K (b), 546 K (c).
Figure 2- First order kinetics plots at temperature 556 K (a), 568 K (b).
137
Table 1 Rate constants and activation energy T(K) 531 • 2 536 + 2 546 + 2 556 + 2 568 + 2 Ea = 113 +
k(min-1) 0.076 • 0.008 0.088 + 0.009 0.219 + 0.022 0.220 + 0.022 0.405 + 0.041 16 kJ. mole -1 10~/T (K) Figure 3" Arrhenius plot for the kinetic constants k.
constant. These results indicate that hydrolysis equilibria are established in the system at a given temperature. The following mechanism can then be proposed for transformation of TTIP into TiO2 under our experimental conditions (R is the isopropyl group, R' is C3H6) : Kl, kl ROH -~ R'+ H 2 0 (1) K2,k2 -~ Ti(OR)a(OH + ROH
Ti(OR)4 + H 2 0
(2)
6--
Ti(OR)3(OH + H 2 0
K3,k3 -~ Ti(OR)2(OH)2 + ROH
(3)
Ti(OR)2(OH)2 + I-I20
K4,k4 -~ Ti(OR)(OH)3 + ROH
(4)
Ti(OR)2(OH)2
TiO2 + 2 ROH
Ti(OR)(OH)3
~ ~
TiO2 + ROH + H 2 0
(5) (6)
where ki (or k' i) and Ki are rate constants and equilibrium constants respectively. It has been previously shown [5] that around 613 K, reaction (1) catalysed by A1203 is about 10 times faster than the alkoxide thermolysis. It is also well known [12] that the rate of successive hydrolyses decreases when the number of already reacted radicals increases. Therefore, we only mention the first 3 steps in the above sequence. Furthermore hydrolysis reactions are known to be faster than dehydration and dealcoholation of intermediate Ti(OR)4-x(OH)x species [13]. In the proposed mechanism, reactions (3') and (4') can then be considered as the limiting steps and the disappearance rate of OR groups bonded to Ti atom in the supercritical solution can be expressed as: d[A]_____~' = -2k'3[Ti(OR)2 (OH)2 ] - k4[Ti(OR)(OH)3 ] dt !
138 Using classical expressions of the equilibrium constants K2, K3, K4, it follows : d[A]' _ dt - -
[
]
2k 3 ' [A'] K4[H20 ] + k4" Z
where Z depends on K2, K 3, K4 and is a function of [H20] and [ROH]. A steady state between relative concentrations of various species containing titanium can be assumed as shown from the Raman study, so that water and isopropanol concentrations can be considered as constants. The kinetic law of the t
overall reaction can then be written d[A] =-k[A]'. The reaction is then found to be dt first order as observed from experimental data.
CONCLUSION The present study on TiO2 powder formation from Ti(O-iC3H7)4 in supercritical isopropanol has allowed the determination of reaction kinetic constants and activation energy in a temperature range from 531 to 568 K at 10 MPa. The proposed mechanism is based on a hydrolytic decomposition of the alkoxide initiated by water formed in alcohol dehydration catalysed by reactor walls. The derived reaction kinetic order is unity in accordance with experimental results. Such a mechanism also explains that special cares must be taken about the internal surface state of the reactor in order to obtain reproducible results.
REFERENCES
1. K. Chhor, J.F. Bocquet and C. Pommier, Mater. Chem. Phys. 32 (1992) 249. 2. J.F. Bocquet, K. Chhor and C. Pommier, Surface and Coatings Technol. 70 (1994) 73. 3. Y. Shimogaki and H. Komiyama, Chem. Letters (1986) 267. 4. Y. Takahaschi, H. Suzuki and N. Nasu, J. Chem. Soc., Farad. Trans. I, 81 (1985) 3117. 5. S.A. Kurtz and R.G. Gordon, Thin Solid Films 147 (1987) 167. 6. T. Kanai, H. Komiyama and H. Inoue, Kagaku Kogaku Ronbunshu 11 (1985) 317. 7. K.L. Siefering and G.L. Griffin, J. Electrochem. Soc. 137 (1990) 814. 8. W.G. Lai and G.L. Griffin, P. Vincenzini (eds), Performance Ceramic Films and Coatings, 151, Elsevier, 1991. 9. Z. Chen and A. Derking, J. Mater. Chem. 3 (1993) 1137. 10. H.Y. Lee and H.G. Kim, Thin Solid Films 229 (1993) 187. 11. D.C. Bradley and M.M. Faktor, Trans. Farad. Soc. (1959) 2117. 12. E.A. Barringer and H.K. Bowen, Langmuir I (1985) 414. 13. T. Ishino and S. Minami, Tech. Rep. Osaka Univ. 3 (1953) 357.