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Hydrolysis of TiCl4: Initial Steps in the Production of TiO2 Tsang-Hsiu Wang, Alejandra M. Navarrete-Lo´pez, Shenggang Li, and David A. Dixon* Department of Chemistry, The UniVersity of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487-0336
James L. Gole Schools of Physics and Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0430 ReceiVed: March 5, 2010; ReVised Manuscript ReceiVed: May 26, 2010
The hydrolysis of titanium tetrachloride (TiCl4) to produce titanium dioxide (TiO2) nanoparticles has been studied to provide insight into the mechanism for forming these nanoparticles. We provide calculations of the potential energy surfaces, the thermochemistry of the intermediates, and the reaction paths for the initial steps in the hydrolysis of TiCl4. We assess the role of the titanium oxychlorides (TixOyClz; x ) 2-4, y ) 1, 3-6, and z ) 2, 4, 6) and their viable reaction paths. Using transition-state theory and RRKM theory, we predicted rate constants including the effect of tunneling. Heats of formation at 0 and 298 K are predicted for TiCl4, TiCl3OH, TiOCl2, TiOClOH, TiCl2(OH)2, TiCl(OH)3, Ti(OH)4, and TiO2 using the CCSD(T) method with correlation consistent basis sets extrapolated to the complete basis set limit and compared with the available experimental data. Clustering energies and heats of formation are calculated for neutral clusters. The calculated heats of formation were used to study condensation reactions that eliminate HCl or H2O. The reaction energy is substantially endothermic if more than two HCl molecules are eliminated. The results show that the mechanisms leading to formation of TiO2 nanoparticles and larger ones are complicated and will have a strong dependence on the experimental conditions. Introduction Titanium dioxide (TiO2) is technologically one of the most important compounds formed by the group IVB transition-metal elements. It is widely used as a white pigment, catalyst support, and photocatalyst.1-4 At room temperature, bulk TiO2 exists in three phases: rutile, anatase, and brookite.1 TiO2 as a photocatalyst has been used for solar energy conversion and for the removal of organic pollutants from wastewater.5-8 It is wellestablished that anatase TiO2 has a higher photocatalytic activity than the rutile or brookite phases. For example, one of the most active commercial TiO2 photocatalysts, Degussa P25, is 60-80% anatase phase.9 The pure anatase phase is thermodynamically less stable than rutile at room temperature, and it can undergo thermal conversion into the rutile phase in the temperature range of 700-800 °C.10 The most important commercial route for the production of TiO2 nanoparticles and larger particles is based on the chloridebased process where purified TiCl4 is oxidized at high temperature (1200-1700 °C) and modest pressure (∼300 kPa), in an oxygen plasma or flame, as given by reaction 1a.11,12 Reaction 1b corresponds to the same reaction for the formation of (TiO2)n nanoparticles.
TiCl4 + O2 f TiO2(s) + 2Cl2
(1a)
rutile phase as otherwise the anatase phase is formed.13,14 Introduction of a small amount of water is important in the industrial process to initiate the above reaction. The aqueous hydrolysis of TiCl4, as indicated in reaction 2a, represents another process to produce nanostructured polycrystalline TiO2.15 This process can be run at room temperature. TiO2 nanoparticles have also been generated by the vapor-phase hydrolysis of TiCl4 at temperatures in the range of 360-550 °C with the TiCl4 present at 1% or lower by volume (reaction 2b). This process can yield anatase-phase particles because of the lower reaction temperature.16 Reaction 2c represents the formation of TiO2 nanoclusters in the gas-phase hydrolysis. 298 K
TiCl4(g) + 2H2O(aq) 98 TiO2(s,rutile) + 4HCl(aq)
(2a) 630-830 K
TiCl4(g) + 2H2O(g) 98 TiO2(s,rutile) + 4HCl(g)
(2b) nTiCl4(g) + 2nH2O(g) f (TiO2)n(g) + 4nHCl(g)
(2c) nTiCl4 + nO2 f (TiO2)n + 2nCl2
(1b)
In the industrial combustion process, AlCl3 is added in small quantities to the TiCl4 reactor feed to promote formation of the * To whom correspondence should be addressed. E-mail: dadixon@ bama.ua.edu.
TiO2 colloidal particles have been synthesized from titanium isopropoxide in aqueous acid where the acid leads to charging of the particles, which can enable the control of their growth.17 Nanocrystalline TiO2 films have also been synthesized by a sol-gel method using reverse micelles formed by Triton X-100 and water in cyclohexane with titanium isopropoxide as the
10.1021/jp102020h 2010 American Chemical Society Published on Web 06/24/2010
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TABLE 1: Reaction Enthalpy (∆H, kcal/mol) at 298 K for Reactions 1a and 1ba reaction reaction reaction reaction reaction
1b, 1b, 1b, 1b, 1a
n n n n
) ) ) )
1 2 3 4
∆H298K
∆H1500K
∆G298K
∆G1500K
114.2 105.4 86.6 66.7 -43.4
110.8 103.0 85.5 66.7 -41.5
103.7 96.6 77.8 65.1 -39.0
64.9 64.3 44.4 59.8 -23.0
a
Heats of formation for (TiO2)n from ref 19 and for TiCl4 (-182.4 kcal/mol) and TiO2(s,rutile) from ref 21.
TABLE 2: Reaction Enthalpy (∆H, kcal/mol) at 298 K for Reactions 2a and 2ba ∆H298K ∆H700K ∆H1500K ∆G298K ∆G700K ∆G1500K reaction reaction reaction reaction reaction reaction
2c, 2c, 2c, 2c, 2b 2ab
n n n n
) ) ) )
1 2 3 4
141.4 159.8 171.7 175.5 -16.2 -66.6
140.8 159.9 172.8 177.4 -15.2
139.4 160.3 174.8 181.2 -12.8
121.8 132.9 135.8 137.7 -20.9 -51.0
95.7 96.6 86.8 85.8 -27.8
44.8 24.1 -12.4 -20.6 -43.1
a Heats of formation for (TiO2)n from ref 19 and for TiCl4 (-182.4 kcal/mol) and TiO2(s,rutile) from ref 21 unless noted otherwise. b Heats of formation from ref 22. If aqueous TiCl4 is used, then ∆H ) 1.3 kcal/mol and the reaction is essentially thermoneutral.
reagent.18 An important issue is that both reactions 1b and 2c are endothermic to produce small (TiO2)n nanoparticles in the gas phase. Therefore, one must partially oxidize or hydrolyze the TiCl4 to build up an oxychloride/hydroxyoxychloride particle large enough so that the reaction thermodynamics begins to resemble that of the solid as evidenced in Table 1 for TiO2 clusters starting from Cl2 (reactions 1a and 1b) and in Table 2 starting from H2O.19-22 The enthalpy and free energy in Table 1 at 298 K show that, as the particle gets larger, the energy to form it decreases. At 1500 K, the structure of the individual particle begins to play a more important role as the presence of low-lying vibrational modes in the cluster can significantly contribute to the entropy contribution to the free energy. In Table 2, the addition of H2O leads to more exothermic (in terms of the free energy) reactions as the temperature increases, especially as the nanoparticle size gets larger, due to the release of more HCl particles in the gas phase. The hydrolysis reaction to form rutile is overall exothermic, independent of whether the process takes place with gas-phase reactants or in the aqueous phase. The gas-phase processes for the generation of TiO2 have been modeled in terms of the oxidation of TiClx radicals which then cluster to form (TiO2)n nanoparticles for large n.23 In another case, TiOCl2 has been suggested to be a key intermediate.24,25 West et al.26 have predicted the thermochemical parameters for a number of TiOxCly intermediates using density functional theory (DFT)27 with different functionals and with coupled cluster28 CCSD(T)29 calculations using modest basis sets. These data were then used in kinetic models to predict the formation of TiO2 nanoparticles.14,30,31 The major reactions they studied were based on the thermal decomposition of TiCl4 to from the TiCl3 and Cl radicals. The compounds that we studied are similar to those given by Kraft and co-workers14,30,31 with the addition of the presence of hydroxyl groups, and their compounds could also play a role in the hydrolysis process. Although the combustion approach to the synthesis of TiO2 particles has been used in industry for many years, the overall mechanism is still poorly understood. The goal of this work is to study the potential energy surface for the initial steps in the gas-phase hydrolysis of TiCl4, which may serve as an initiation
step in the combustion system and is relevant to the hydrolysis mechanism as well. The formation of the species described herein can be coupled with those based on the thermal decomposition of TiCl4 in further studies of the formation of TiO2 nanoparticles. For a number of species, the thermodynamics have been obtained at the coupled cluster CCSD(T) level at the complete basis set (CBS) limit using approaches developed in our group in collaboration with Washington State University.32 We have previously used such an approach to study the thermodynamic properties of TiO2 and other transition-metal oxide clusters.19 The focus of the current study is on the species formed by the reaction of H2O with TiCl4 and their subsequent clustering reactions. Computational Methods The potential energy surfaces were initially calculated at the density functional theory level with the B3LYP exchangecorrelation functional33,34 and the aug-cc-pVDz/aug-cc-pVDZPP basis set described below. Equilibrium geometries and harmonic vibrational frequencies were calculated at the secondorder Møller-Plesset perturbation theory (MP2) level for various structures on the potential energy surface using the Gaussian 03 program.35 These calculations were done with the aug-cc-pVnZ basis sets36 for H and O, the aug-cc-pV(n+d)Z basis sets for Cl,37 and the small core effective core potential (ECP) based aug-cc-pVnZ-PP basis sets19,38 for Ti with n ) D and T. (We denote this combination of basis sets as aVnZ.) For the hydrates and transition states, the geometries calculated at the MP2/aVTZ level were subsequently used in single-point CCSD(T) calculations with the aVDZ, aVTZ, and aVQZ basis sets. The geometries of TiCl4, TiCl3OH, TiOCl2, TiOClOH, and TiO2 were also optimized at the CCSD(T) level with the aVDZ and aVTZ basis sets. The geometry calculated at the CCSD(T)/ aVTZ level was then used in single-point CCSD(T)/aVQZ calculation. All of the CCSD(T) calculations were performed with the MOLPRO 2006.1 program.39 The CCSD(T) energies were then extrapolated to the CBS limit using a mixed Gaussian/ exponential formula (eq 3).40 n ) 2, 3, and 4 for aVDZ, aVTZ, and aVQZ, respectively.
E(n) ) ECBS + Be-(n-1) + Ce-(n-1)
2
(3)
To predict thermochemical properties to high accuracy, it is necessary to include additional corrections. For the complexes with one Ti atom, the core-valence correlation corrections (∆ECV) were obtained at the CCSD(T)-DK/aug-cc-pwCVTZDK level.38 For the calculation of transition-metal compound atomization energies, the core-valence calculations should be calculated at the CCSD(T)-DK level to achieve the best accuracy.19 In addition, we also account for relativistic effects in atoms and molecules. The first is the spin-orbit correction (∆ESO), which lowers the sum of the atomic energies (decreasing ∑D0) by replacing energies that correspond to an average over the available spin multiplets with energies for the lowest multiplets. This correction is required as most electronic structure codes produce only spin multiplet averaged wave functions. The experimental ground-state atomic spin-orbit corrections are ∆ESO(Ti) ) 0.64 kcal/mol, ∆ESO(Cl) ) 0.84 kcal/mol, and ∆ESO(O) ) 0.22 kcal/mol.41 The second correction is the scalar relativistic correction (∆ESR). As we already have such a correction for Ti through the use of the ECP, we evaluated ∆ESR for O and Cl as expectation values of the two dominant terms in the Breit-Pauli Hamiltonian, the mass-
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velocity and one-electron Darwin (MVD) corrections, at the configuration interaction singles and doubles (CISD) level with the aVTZ basis set at the MP2/aVTZ geometry. Our computed total atomization energy (∑D0) values were obtained with eq 4.
ΣD0 ) ∆ECBS + ∆EZPE + ∆ECV + ∆ESR + ∆ESO
(4) Given the known heats of formation at 0 K for the elements, ∆Hf,0K(Ti) ) 112.4 ( 0.7 kcal/mol,42 ∆Hf,0K(Cl) ) 28.59 kcal/ mol,21 ∆Hf,0K(O) ) 58.98 kcal/mol,21 and ∆Hf,0K(H) ) 51.63 kcal/mol,21 we can derive ∆Hf,0K values for the molecules under study. The heats of formation at 298 K can be obtained by following the procedures outlined by Curtiss et al.43 The temperature dependence of the enthalpy and the entropy calculations were done in the rigid-rotor, harmonic oscillator approximation with hindered rotors treated as a vibration.44 At higher temperatures, this may lead to modest errors in the thermodynamic properties due to the transition of a hindered rotor to a free rotor. The CCSD(T) method scales approximately as N7 with N basis functions, and large basis sets are required to reach the CBS limit. The approach described above for the CCSD(T)/ CBS calculations has been used to predict a wide range of thermodynamic properties to chemical accuracy.19,20,32,45 DFT has a much better scaling, scaling as N3 to N4 depending on the exchange-correlation functional, but is not as accurate as the CCSD(T)/CBS approach for the calculation of a broad range of thermodynamic properties including atomization energies. For small clusters with one Ti atom, accurate thermodynamic properties can be calculated using the CCSD(T)/CBS approach, which is more reliable than similar DFT-based approaches.19,20,46 In addition, the CCSD(T)/CBS values can be used to benchmark different DFT functionals in the future. We avoided the use of isodesmic and similar types of reaction energies as used previously because of issues with the experimental heats of formation of similar Ti compounds and because there are so few experimental values.47 For the larger clusters, we used the normalized clustering energy method that we have previously developed19,20 for the prediction of the heats of formation of transition-metal oxide clusters. All calculations were performed on a Xeon- and Opteronbased Penguin Computing Linux cluster in our group, the Itanium 2-based SGI Altix and the Opteron-based dense memory cluster Linux cluster at the Alabama Supercomputer Center, the Xeon-based Dell Linux cluster at The University of Alabama, and the Opteron-based Linux cluster at the Pacific Northwest National Laboratory. Results and Discussion Geometries and Frequencies. The calculated Ti-X (X ) Cl, O, OH) bond lengths at the CCSD(T)/aVTZ level for selected reactants, intermediates, and products are given in Table 3. They are compared with available experimental values. The electronic states and symmetry labels for these molecules are given in Table 3 and will not be repeated hereafter. The optimized molecular structures for these molecules together with those of the transition states are given in the potential energy surface plots (Figure 1). The total CCSD(T) energies, calculated harmonic frequencies compared with the available experimental values, and T1 diagnostics are given as Supporting Information. The calculated Ti-Cl bond distance of 2.186 Å at the CCSD(T)/aVTZ level in TiCl4 is slightly longer (
