Kinetics of the Titanium Isopropoxide Decomposition in Supercritical

The formation of TiO2 powder from Ti(O-iC3H7)4 dissolved in supercritical isopropyl alcohol has been studied. Kinetic data were obtained in a temperat...
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Ind. Eng. Chem. Res. 1996, 35, 2539-2545

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Kinetics of the Titanium Isopropoxide Decomposition in Supercritical Isopropyl Alcohol Vale´ rie Gourinchas Courtecuisse, Khay Chhor, Jean-Franc¸ ois Bocquet, and Claude Pommier*,† Laboratoire d’Inge´ nierie des Mate´ riaux et des Hautes Pressions, CNRS, Avenue J. B. Cle´ ment, 93430 Villetaneuse, France

The formation of TiO2 powder from Ti(O-iC3H7)4 dissolved in supercritical isopropyl alcohol has been studied. Kinetic data were obtained in a temperature range of 531-568 K. A reaction mechanism has been proposed, based on a hydrolytic decomposition of Ti(O-iC3H7)4 by water produced in a catalytic dehydration of the isopropyl alcohol used as solvent and where the limiting steps are the decomposition reactions of the formed titanium hydroxides. In accordance with experimental results, such a mechanism leads to a first order kinetics relative to the precursor concentration. The activation energy was determined equal to 113 ( 16 kJ‚mol-1, and an increase in the supercritical fluid density was found to decrease the overall reaction rate. The obtained results will be used to optimize the working of a continuous reactor producing submicronic TiO2 powder at a laboratory pilot scale. Introduction Because of various interesting physical and chemical properties, titanium dioxide has been extensively used, either as thin films or as submicronic powders, in a wide application range in electronic or optical devices as well as pigment or catalyst. In many processes, titanium alkoxides are used as precursors in order to get oxide with controlled purity and morphology. The involved reactions are essentially hydrolysis (as in sol-gel synthesis) and thermolysis (mainly in CVD experiments). We have recently synthesized submicronic TiO2 powders from thermal decomposition of titanium isopropoxide, Ti(O-iC3H7)4 (referred to hereafter as TTIP), in supercritical alcohol, at temperature and pressure around 573 K and 10 MPa, respectively (see Chhor et al., 1992). In such conditions, narrow-sized particles (20-60 nm) partly crystallized in the anatase structure and softly associated into spherical agglomerates (0.5-2 µm) are obtained. Similar reactions conducted in supercritical CO2-isopropyl alcohol mixtures lead to the formation of homogeneous TiO2 films on alumina substrate (Bocquet et al., 1994). Higher solute concentrations in the surrounding phase, as compared with CVD experiments, allow higher decomposition rate on the substrate. An anatase film with about 5 µm thickness can be formed in less than 10 min below 573 K. In such reactions, the influence of pressure is to favor crystallization of the formed solid. Moreover, in powder synthesis, pure dry particles can be recovered, without further washing and drying steps, by removing the solvent and byproducts, simply decompressing the system at a temperature above the alcohol critical point. We have recently developed a continuous process for TiO2 powder production at a laboratory scale. However, in order to model and optimize the reactor system, the knowledge of kinetic data on TTIP decomposition in supercritical isopropyl alcohol is needed. The aim of the present study is then to get this information and to determine a reaction mechanism for TiO2 formation under these conditions. Such a mechanism could then be compared to that occurring in a gas or liquid phase. †

E-mail address: [email protected].

Before presenting our results on the TTIP behavior in supercritical isopropyl alcohol, a brief review of previously reported kinetic studies in gas phase and liquid solution will be given. As mentioned above, a first way to transform the Ti(OR)4 precursor into titanium oxide is the hydrolysis of the four alkoxy groups. Do¨ring et al. (1992) have recently reported thin film formation by pulsed beam chemical vapor deposition from TTIP and water vapors. In powder synthesis, the main encountered problem in the reaction between alcoholic solutions of titanium alkoxide and water is the control of the polydispersity and agglomeration state of the formed particles. It can be overcome adding a dispersant, rapidly mixing the reactive solutions and controlling the aging of the mixture. Ogihara et al. (1989) and Nahass and Bowen (1988) have shown the industrial feasibility of such a process in a continuous plug-flow reactor. Amorphous TiO2 powders some hundreds of nanometers in diameter have also been obtained by Visca and Matijevic (1979) hydrolyzing aerosols of liquid titanium alkoxide. However, the sol-gel route, involving controlled hydrolysis and polycondensation reactions, is now recognized as the best solution process, producing homogeneous nanometric powders at room temperature. It has been applied to various ceramic oxides synthesis and particularly to TiO2 formation. Barringer and Bowen (1985) have proposed a reaction mechanism and kinetic rate laws when starting with Ti(OC2H5)4 as precursor. Kallala et al. (1993) studied the growth of oxopolymers formed in the condensation steps, starting with Ti(OnC4H9)4 under various experimental conditions. Although nanometric titania sols are first formed, dramatic increase in particle size occurs during heat treatment of the xerogel. Resulting TiO2 powders, first crystallized in the anatase structure and transformed into rutile at higher temperature, have diameters of tens to hundreds of nanometers (Xu et al., 1992; Koebrugge et al., 1993). Terabe et al. (1994) have recently shown the influence of the experimental synthesis conditions (pH, H2O/alkoxide ratio) on the number of unhydrolyzed alkoxy groups and, consequently, on the gel crystallization. Conventional drying of the gel can lead to dense powders; however, porous solids can be obtained by removing the solvent under supercritical conditions.

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2540 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996

Campbell and Na (1992) then prepared TiO2 aerogels with high surface area (up to about 200 m2‚g-1) containing mesopores (2-10 nm). Numerous studies have been reported on TTIP thermal decomposition. The overall reaction is usually accepted as

Ti(O-iC3H7)4 f TiO2 + 4C3H6 + 2H2O but, when available, conflicting results are given about kinetic rate laws and reaction mechanisms. Komiyama et al. (1984) and Shimogaki and Komiyama (1986) have shown that decomposition becomes detectable only above 623 K by using a clean glass reactor but has a significant rate at temperature as low as 523 K when catalyzed by previously deposited TiO2 on the reactor walls. Amorphous highly porous particles with surface area of about 300 m2‚g-1 were obtained below 593 K, and anatase powder (20-30 m2‚g-1) was recovered above 673 K. Temperatures in the range 673-723 K are reported by many authors as the lowest ones, allowing anatase formation in CVD experiments (Yokozawa et al., 1968; Okuyama et al., 1986). The rutile phase is obtained at higher temperatures (Sakurai and Watanabe, 1963), or it can be grown, in a narrow temperature range (673-723 K), on a previously formed anatase film (Takahashi et al., 1985). Kurtz and Gordon (1987) reported that TTIP pyrolysis is catalyzed by the presence of Al2O3, which is known to dehydrate alcohols at temperatures higher than 473 K. At 643 K, the rate of the catalyzed reaction is estimated to be about 10 times that of the pure thermal decomposition. Takahashi et al. (1985) also observed that decomposition in presence of isopropyl alcohol occurs at lower temperatures because it is assisted by water formed from this compound. Some kinetic results have been reported in studies on TiO2 powder synthesis. Kanai et al. (1985) first proposed an overall reaction order about 0.5. Okuyama et al. (1986, 1989) explained the obtained particle size distribution by a fast decomposition of the precursor molecules above 673 K and condensation of the formed TiO2 vapor into clusters and solid particles through a dominant brownian coagulation process. However, most kinetic studies have been CVD experiments. The apparent dependence of the film growth rate with TTIP partial pressure was found to be first order when the later parameter was varied by changing the temperature of the evaporator used as TTIP source but was 3/2 order when changing the ratio of the carrier gas flow through this evaporator; see Takahashi et al. (1985). A first mechanism was proposed by Siefering and Griffin (1990) in order to explain their CVD results on a copper substrate between 493 and 573 K, at low TTIP partial pressure (5-260 Pa). They observed a second order dependence at high temperature or at low pressure and zero order under the reverse conditions. They proposed a mechanism with three elementary steps: (i) activation of the gas phase TTIP molecule by collisional excitation with another TTIP molecule, (ii) adsorption of the activated species, and (iii) surface decomposition. According to the limiting step (i or iii) under the experimental conditions, the deposition reaction can be second or zero order. At low partial pressure ( 873 K) 0.5 1

ref b c

mass transport gas phase bimolecular collision surface reaction gas phase TTIP-N2 collision surface reaction TTIP diffusion TTIP adsorption surface decomposition gas phase reaction catalysis by TiO2 deposit coagulation of TiO2 vapor

d 20 35

e f

150

101

g h

i j

a Powder synthesis experiments. b Yokozawa et al. (1968). c Takahaschi et al. (1985). d Sladek and Herron (1972). e Kamata et al. (1990). Siefering and Griffin (1990) and Lai et al. (1991). g Chen and Derking (1993). h Lee and Kim (1993). i Kanai et al. (1985). j Okuyama et al. (1986, 1989).

f

drawal of small quantities of the reacting medium at various reaction times; the fluid is condensed, and the liquid is then analyzed. In order to ascertain that successive samplings have negligible influence on the system behavior, the total withdrawn volume was limited to less than 10% of the starting volume. The internal surface state of the reactor has been shown to have an important influence on the reaction rate so that special care has been devoted to keeping it as reproducible as possible from one experiment to the other. The reactor is washed with alcohol and wiped clean to eliminate excess of TiO2 powder formed in the preceding reaction. Only a small film of strongly held particles remains on the internal wall. Before the next run, the reactors is washed again and dried at around 373 K. The TTIP solution is introduced into the reactor and heated up to the study temperature T at 5 K‚min-1. Reaction at this temperature can be followed by either measuring the pressure increase inside the reactor or by analyzing successive samples withdrawn at regular time intervals. This last operation also allows one to keep the pressure at about a constant value during the experiment. Analysis is performed by IR spectrometry following the intensity of the absorption band at 1025 cm-1 attributed to the C-O stretching vibration in the alkoxy group bonded to a titanium atom. From a calibration curve, we deduced the overall O-iC3H7 concentration for various species in solution in the supercritical fluid. Errors on the TTIP or alkoxy group concentrations reported in the next section originate from various sources: (i) IR absorbance measurements, (ii) condensation of the supercritical phase during sampling, and (iii) small decrease of the reacting system content due to successive withdrawings. We estimate an overall relative error around (10%. Besides IR spectrometry (Perkin Elmer 580 spectrometer), other methods used for the study of Ti containing species in solution or for TiO2 powder characterization are Raman spectroscopy (Dilor XY multichannel spectrometer), X-ray diffraction (Siemens apparatus with Cu KR radiation), and scanning and transmission electronic microscopy (Cambridge 5360 and Philips EM 300, respectively).

Figure 1. Evolution of the pressure as a function of time (T ) 546 K, ci ) 0.595 mol‚L-1), for three experiments with various reactor surface state: (a) previously formed TiO2 powder left on the internal reactor wall; (b, c) scratched internal wall before reaction.

Isopropyl alcohol and TTIP used are commercial products (Prolabo and Aldrich Chemicals) with purity 99.7% and greater than 99%, respectively. TTIP was distillated under reduced pressure (600 Pa) at 368 K before use. Results As mentioned above, many authors reported unreproducible results in TTIP decomposition studies. Such behavior was also observed in our experiments when no special care was taken about the surface state of the reactor wall. An easy way to follow the reaction advancement in the closed system is to measure pressure change versus time. Figure 1 reports the experimental curves obtained at the same temperature, 546 K, starting with the same TTIP solution but with a different state of the reactor internal surface. It can be seen that reaction can already occur at a significant rate when the system reaches the set temperature, if TiO2 powder formed in the previous experiment was left on the internal reactor wall (Figure 1a). If this remaining deposited powder is tentatively eliminated by scratching the internal wall, a curve such as Figure 1b or 1c is obtained. The observed more or less pronounced latent

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2542 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996

Figure 2. Evolution of the isopropoxy group concentration as a function of time at different temperatures (ci ) 0.19 mol‚L-1): T ) (a) 531, (b) 546, (c) 556, and (d) 568 K. Figure 4. Evolution of the isopropoxy group concentration as a function of time at different TTIP densities (ci ) 0.45 mol‚L-1, T ) 536 K): F ) (a) 0.331, (b) 0.471, and (c) 0.544 g‚cm-3.

Figure 3. Evolution of the isopropoxy group concentration as a function of time at different starting TTIP concentrations (T ) 536 K): ci ) (a, () 0.595, (b, 4) 0.190, and (c, b) 0.100 mol‚L-1.

period can be related with a nonreproducibility of the wall surface state. The nature of this surface, metallic or covered with TiO2 particles, is then a very important factor on the reaction kinetics. This problem has been overcome by taking very special care when the reactor was prepared for successive runs, as mentioned in the above section. Reaction advancement with IR spectrometric measurements is determined in the following manner. The volumic molar concentration of OR alkyl groups in liquid at room temperature and at the supercritical fluid temperature are denoted [A] and [A]′; the densities of the two phases are FL and FSCF; the reactor volume is VR, and the liquid solution is VL. Then

FSCF VL ≈ [A] FL VR

[A]′ ) [A]

In the starting alcoholic liquid solution, the TTIP concentration ci corresponds to [A]i ) 4ci. In the supercritical fluid at time t and temperature T, concentration [A]t′ will be calculated from the measured value [A]t in the condensed liquid withdrawn. If no reaction would occur during heating from room temperature, at time t ) 0, [A]0′ ) [A]iVL/VR. Really this is not the case, and measurements lead to lower [A]0′ values. Figure 2 reports the reaction advancement at various temperatures, starting with 100 cm3 of TTIP solution (ci ) 0.19 mol‚L-1). The calculated [A]0′ value assuming no reaction during heating is 0.317 mol‚L-1 so that about 25-35% of the initial alkoxy groups have been decomposed before reaching temperature T. The reported curves [A]t′ ) f(t) are not linear so that a zero kinetic order is a priori ruled out for the overall reaction. It can also be noticed that, at 556 K and t ) 10 min, for

Figure 5. First order kinetics for TTIP thermal decomposition at different temperatures (ci ) 0.19 mol‚L-1): T ) (a) 531, (b) 536, and (c) 546 K. Table 2. Overall Reaction Rate Constant at Various Temperatures and Energy of Activation T (K)

k (min-1)

T (K)

k (min-1)

531 ( 2 536 ( 2 546 ( 2

0.076 ( 0.008 0.088 ( 0.009 0.219 ( 0.022

556 ( 2 568 ( 2

0.220 ( 0.022 0.405 ( 0.041

Ea ) 113 ( 16 kJ‚mol-1

example, about 10% of OR groups bonded to Ti atoms remain in solution. Figure 3 shows results obtained at 536 K, starting with various TTIP solutions in the range ci ) 0.1-0.6 mol‚L-1. In order to display all experimental data on the same figure, the curves obtained for the lowest ci values have been translated along the time axis. Such a curve shows a good reproducibility of the results in a rather large TTIP concentration range. The above experiments were conducted in a supercritical fluid with a density of FSCF ) 0.331 g‚cm-3. The influence of this parameter on the reaction rate can be investigated by changing the introduced volume VL of the starting liquid solution. The results obtained at 536 K are reported in Figure 4 and clearly show that the TTIP decomposition rate increases when the fluid density decreases. This can be caused either by changes in pressure, in isopropyl alcohol molar concentration, or in the physical properties of the reaction medium, such as viscosity, that influences the reactive species diffusivity. From these as-obtained experimental results, various kinetic orders can be tested. Best fits with linear variations are obtained by plotting ln [A]t′ ) f(t), in

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Figure 6. First order kinetics for TTIP thermal decomposition at different temperatures (ci ) 0.19 mol‚L-1): T ) (a) 556 and (b) 568 K.

Figure 7. Arrhenius plot for the kinetic constant k.

accordance with an overall first order reaction. The kinetic constants derived from Figures 5 and 6 are reported in Table 2. The Arrhenius plot (Figure 7) allows calculation of the activation energy: Ea ) 113 ( 16 kJ‚mol-1. The reported error takes into account a (2 K and about 10% errors on the measured temperature and calculated rate constants, respectively. Discussion In various studies on titanium and mainly zirconium alkoxides, some authors have shown that the presence of alcohol facilitates precursor transformation into metal oxide. Takahashi et al. (1985) did not find isopropyl alcohol as a byproduct from pure TTIP decomposition. This phenomenon was supposed to be due to a rapid dehydration under the reaction conditions. Bradley and Faktor (1959) emphasize the difficulty of obtaining “pure” alkoxide without any trace of the corresponding alcohol. Water formed in the dehydration of this alcohol can then initiate the hydrolysis reaction of the complex. They pointed out that traces of alcohol can also be due to the reaction between alkoxide molecules and water adsorbed on the glass reactor walls. In all cases, “thermal” decomposition of the alkoxide is in fact a chain reaction involving the dehydration of a first alcohol molecule, producing one water molecule which, by the hydrolysis reaction, gives two additional alcohol molecules. Although Bradley and Faktor (1959) essentially studied zirconium alkoxides, the occurrence of such an “hydrolytic” decomposition of titanium derivatives can be supposed but this has not been taken into account in the most recently reported papers on CVD experiments from TTIP; see Lai et al. (1991), Chen and Derking (1993), and Lee and Kim (1993). It is well-known that dehydration facility increases from primary to secondary and tertiary alcohols and

that the reaction is catalyzed by metal oxide surfaces, such as Al2O3, above about 473-523 K; see Knozinger (1968). Chhor et al. (1992) have shown that solid particle formation from TTIP in supercritical alcohol occurs above 523 K, a temperature lower than that observed for pure TTIP vapor decomposition. This allows us to assume that the first step in TiO2 formation from titanium alkoxide under our experimental conditions is the alcohol dehydration followed by hydrolysis reactions. This can also explain the problems encountered in obtaining reproductible results with different reactor internal surface states. In support of this hypothesis, we studied the Raman spectra of TTIP-isopropyl alcohol solutions, heated in a closed vessel up to 536 K. Figure 8 shows the time dependence of the Raman spectra recorded on a TTIP solution (0.5 mol‚L-1) in the frequency range 250-800 cm-1. A similar evolution of band intensities associated with Ti-OR, Ti-OH, or Ti-O-Ti functional groups was observed. The vibrational stretching modes were observed in the region of 500-700 cm-1; see Berglund et al. (1986) and Mudler et al. (1987). The corresponding intensities, with respect to that of the solvent bands (i.e. at 486 cm-1), decreased when the solution was left isothermally at 536 K. Conversely, the intensity ratios of two ν(Ti-O) associated lines, for example at 562 and 615 cm-1, remained nearly constant. Such a phenomenon is observed at a temperature as low as 513 K. These results indicate that hydrolysis equilibria are established in the system, at a given temperature. The following mechanism can then be proposed for TTIP transformation into titanium dioxide under our experimental conditions (R is the isopropoxy group, R′ is C3H6): K1, k1

ROH y\z R′ + H2O K2, k2

Ti(OR)4 + H2O y\z Ti(OR)3(OH) + ROH

(1) (2)

K3, k3

Ti(OR)3(OH) + H2O y\z Ti(OR)2(OH)2 + ROH (3) K4, k4

Ti(OR)2(OH)2 + H2O y\z Ti(OR)(OH)3 + ROH (4) k3′

Ti(OR)2(OH)2 98 TiO2 + 2ROH k4′

Ti(OR)(OH)3 98 TiO2 + H2O + ROH

(3′) (4′)

where ki (or ki′) and Ki are rate constants and equilibrium constants, respectively.

K2 )

K3 )

K4 )

[Ti(OR)3(OH)][ROH] [Ti(OR)4][H2O] [Ti(OR)2(OH)2][ROH] [Ti(OR)3(OH)][H2O] [Ti(OR)(OH)3][ROH] [Ti(OR)2(OH)2][H2O]

Kurtz and Gordon (1987) have previously shown that, around 623 K, the catalyzed initial formation of water molecules from reaction 1 is about 10 times the alkoxide thermolysis rate. Although no kinetic data were given,

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2544 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996

water concentrations can be considered constant and the kinetic law of the overall transformation can be written

d[A]′/dt ) -k[A]′

Figure 8. Time dependence of TTIP solution Raman spectra (ci ) 0.5 mol‚L-1, T ) 536 K): t ) (a) 0 and (b) 10 min.

Barringer and Bowen (1985) reported that the hydrolysis of the first alkoxy group is fast and the reaction rate decreases when an increasing number of OR radicals are hydrolyzed. As these last authors, we consider that the last step is very slow and we do not mention it in the above sequence. The partly hydrolyzed species are known to form linear oligomers at room temperature in liquid solution (Livage and Sanchez, 1992). In the above mechanism we only indicate the monomeric forms Ti(OR)4-x(OH)x, and the kinetic and equilibria constants associated with steps 2-4′ must be understood as mean values for a given hydrolysis ratio x. Furthermore, in our experimental conditions (higher temperature, lower fluid density), oligomer formation is unfavored. Finally the titanium hydroxides are unstable and transform into solid TiO2 via thermal decomposition reactions 3′ and 4′. It is known (Ishino and Minami, 1953) that the hydrolysis reactions are faster than the TiO2 formation reactions 3′ and 4′, which can be considered as the rate limiting steps for the process. Under such conditions, the disappearance rate of OR groups bonded to Ti atoms in various species in the supercritical solution can be expressed as

d[A]′ ) -2k3′[Ti(OR)2(OH)2] - k4′[Ti(OR)(OH)]3 (5) dt with

[A]′ ) 4[Ti(OR)4] + 3[Ti(OR)3(OH)] + 2[Ti(OR)2(OH)2] + [Ti(OR)(OH)3] (6) Using classical expressions of the equilibrium constants K2, K3, and K4, it follows

(

)

2k3′ [A]′ d[A]′ + k4′ )dt Z K4[H2O]

(7)

(9)

The reaction can then be considered as pseudo first order, in accordance with experimental results reported above. Furthermore, an increase in the fluid density has been shown to decrease the overall rate constant k. This can be explained either by the increase of the [ROH] value in eqs 7 and 8 or by a change in the steady state between various species in equilibrium. The alcohol dehydration (reaction 1) is effectively unfavored by the pressure increase associated with a higher fluid density. The activation energy determined from experimental results was found to be Ea ) 113 ( 16 kJ‚mol-1. It is difficult to compare this value to those reported by various authors under very different reaction conditions (see Table 1). However, the order of magnitude appears to be in agreement with those reported when a thermal decomposition reaction is the limiting step (Yokozawa et al., 1968; Lai et al., 1991; Siefering and Griffen, 1990; Chen and Derking, 1993). When oxygen is present, the Ea value is much more lower, down to 20-27 kJ‚mol-1. TiO2 formation is then favored by a faster elimination of the alkoxy group through oxidation reaction. Conclusions The present study on TiO2 powder formation from Ti(O-iC3H7)4 in supercritical isopropyl alcohol allowed the determination of reaction kinetic constants and the activation energy in a temperature range from about 533 to 573 K, at 10 MPa. We observed catalytic dehydration of the alcohol on the reactor walls and the formed oxide powder, thus explaining the difficulties encountered in obtaining reproductible results. The proposed mechanism is based on a hydrolytic decomposition of the alkoxide initiated by water formed in the above reaction 1. The derived kinetic order of the overall reaction was unity, in accordance with our experimental results. Such a mechanism also allows us to explain the influence of the fluid density on the reaction rate. We have developed a continuous reactor for TiO2 submicronic powder synthesis in the above conditions, at a laboratory pilot scale. The results reported here will be used to model this reactor and determine the best experimental conditions for complete reaction and high TiO2 yields. Acknowledgment The authors acknowledge Professor A. Vignes for helpful discussions on reaction mechanism. Literature Cited

with

Z) 4[ROH]3 + 3K2[H2O][ROH]2 + 2K2K3[H2O]2[ROH] K2K3K4[H2O]3 (8) As shown from the lack of strong time dependence of the Raman spectra, a steady state between relative concentrations of various species containing titanium can be assumed. Under these conditions, alcohol and

Barringer, E. A.; Bowen, H. K. High-purity, monodisperse TiO2 powders by hydrolysis of titanium tetraethoxide. 1. Synthesis and physical properties. Langmuir 1985, 1, 414-420. Berglund, K. A.; Tallant, D. R.; Dosch, R. G. Time-Resolved Raman Spectroscopy of Titanium Isopropoxide Hydrolysis Kinetics. In Science of Ceramic Chemical Processing; Hench, L. L., Ulrich, D. R., Eds.; Wiley: New York, 1986; pp 94-99. Bocquet, J.-F.; Chhor, K.; Pommier, C. A new TiO2 film deposition process in a supercritical fluid. Surf. Coat. Technol. 1994, 70, 73-78. Bradley, D. C.; Faktor, M. M. The pyrolysis of metal alkoxides, part 2-Kinetic studies on zirconium tetra-tert-amyloxide. Trans. Faraday Soc. 1959, 2117-2123.

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Received for review September 21, 1995 Accepted May 20, 1996X IE950584R

X Abstract published in Advance ACS Abstracts, July 1, 1996.