Article pubs.acs.org/JPCC
Hydrolysis of ZrCl4 and HfCl4: The Initial Steps in the HighTemperature Oxidation of Metal Chlorides to Produce ZrO2 and HfO2 Zongtang Fang and David A. Dixon* Department of Chemistry, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487-0336, United States S Supporting Information *
ABSTRACT: The gas-phase hydrolysis of MCl4 (M = Zr, Hf) to produce the initial particles on the way to zirconia and hafnia nanoparticles has been studied with electronic structure theory. The potential energy surfaces, the themochemistry of the reaction species, and the reaction paths for the initial steps of MCl4 reacting with H2O have been calculated. The hydrolysis of MCl4 at higher temperatures begins with the formation of oxychlorohydroxides followed by the elimination of HCl instead of the direct production of MOCl2 and HCl or MO2 and HCl due to the substantial endothermicities associated with the formation of gas-phase MO2. The structural properties and heats of formation of the reactants and products are consistent with the available experimental results. A number of metal oxychlorides (oxychlorohydroxides) intermediate clusters have been studied to assess their role in the production of MO2 nanoparticles. The calculated clustering reaction energies of those intermediates are highly exothermic, so they could be readily formed in the hydrolysis process. These intermediate clusters can be formed exothermically from metal oxychlorohydroxides by the elimination of one HCl or H2O molecule. Our calculations show that the mechanisms leading to the formation of MO2 nanoparticles are complicated and are accompanied by the potential production of a wide range of intermediates, as found for the production of TiO2 particles from the high-temperature oxidation of TiCl4.
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INTRODUCTION Zirconia (ZrO2) and hafnia (HfO2), two important group IVB transition metal dioxides, are widely used either as catalyst supports or as photocatalysts due to their ability to capture UV light.1−3 Zirconia-based solid acid catalysts have been used for the synthesis of organic compounds by the petrochemical industry.4 In addition, solid MO2 (M = Zr, Hf) have much higher dielectric constants than SiO2, so they are being considered as replacements for SiO2 as the gate insulator in metal oxide semiconductors.5,6 At room temperature, bulk ZrO2 and HfO2 are stable as the monoclinic phase. The other two high-temperature phases, tetragonal and cubic, can be stabilized in the form of cation-doped metal oxides or as nanoparticles. The bulk band gaps, 5.8 eV for ZrO2 and 5.9 eV for HfO2,7 are much higher than those of small (ZrO2)nand (HfO2)n nanoclusters, which fall in the range 1.5−4.0 eV depending on their structures,8 so the nanoclusters are predicted to be more efficient as visible light photocatalysts than the bulk oxides. Studies of the reactions of ZrCl4 (a solid at room temperature) in aqueous solution have a long history and show that the structures generated by solution hydrolysis can be complicated and depend on a range of solution factors.9,10 The classic method to synthesize ZrO2 nanoparticles11−14 from zirconium tetrachloride (ZrCl4) is similar to the solution-phase production of TiO2 particles.15,16 Purified ZrCl4 from ZrO2 ore (baddeleyite) can be hydrolyzed with water or oxidized to produce solid zirconium dioxide by the following global reactions 1 and 2 (with M = Zr):12−14 © 2013 American Chemical Society
MCl4 + 2H 2O → MO2 + 4HCl
(1)
MCl4 + O2 → MO2 + 2Cl 2
(2)
The temperature for forming ZrO2 nanoparticles by the hydrolysis method was reported to require at least 270−300 °C.13 The synthesized solid was determined to be tetragonal after heating at 530 °C. A metastable tetragonal ZrO2 phase and its transition to the stable monoclinic phase were found in the oxidation process.14 Doping cations, for example, Y3+ and Ce4+, have been introduced to stabilize the tetragonal phase via the hydrolysis process.12 Nanoscale ZrO2 also can be formed by nonhydrolytic approaches such as sol−gel methods17 or flame spray processes.18 Both of those methods require organic zirconia precursors, such as the tetra-n-proproxide and tetraisopropoxide MR4 complexes. However, ZrCl4 is cheaper than these organometallic precursors and is thus widely used in industry. ZrOCl2 also has been used as precursor to make ZrO2 nanoparticles.19,20 Hydrous zirconia particles were formed by boiling a solution containing ZrOCl2 at 100 °C for 95 h.19 Xray diffraction analysis showed that the dried zirconia particles were monoclinic with dimensions of 3.0−4.0 nm. The formation of zirconia sol−gels has been accomplished by force hydrolysis from zirconyl oxynitrate (ZrO(NO3)2) and zirconyl oxychloride (ZrOCl2) under acid conditions20 at 102 °C for 24 h. The synthesized hydrated zirconia nanoparticles Received: July 3, 2012 Revised: March 6, 2013 Published: March 8, 2013 7459
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B3LYP functional was chosen for compatibility with the previous work on TiO2 formation32 and because it has been shown to work reasonably well for the type of ionic bonding present in these metal clusters.8,40 The aug-cc-pVDZ basis set41 was used for H, O, and Cl, and the relativistic effective core potential (RECP) aug-cc-pVDZ-PP basis sets42 were used for Zr and Hf. Second-order Møller−Plesset perturbation theory (MP2)37 was used to in some cases to optimize the geometries for structures on the DFT PES as well as for single point calculations at the DFT geometries. These calculations were done with the aug-cc-pVTZ basis sets41 for H and O, the augcc-pV(T+d)Z basis sets for Cl,43 and the aug-cc-pVTZ-PP basis sets for the metal. (We denote these combinations of basis sets as aX for X = D and T) The DFT and MP2 calculations were carried out with the Gaussian 09 program package.44 For the hydrates and transition states, the geometries optimized at the MP2/aT level were subsequently used in single point CCSD(T)33−35 calculations with the aD, aT, and aQ basis sets. The geometries of MCl4, MCl3(OH), M(O)Cl2, M(O)Cl(OH), and MO2 were also optimized at the CCSD(T) level with the aD and aT basis sets. The geometries calculated at the CCSD(T)/aT level were then used in single point CCSD(T)/aQ calculations. The CCSD(T) energies with X = D, T, and Q were extrapolated to the complete basis set (CBS) limit using a mixed Gaussian/exponential formula.45 Following the method developed by us for the prediction of the thermodynamic properties of the transition metal oxides,40 core−valence (CV) correlation corrections for the 1s2 electrons on O and (n − 1)s2(n − 1)p6 electrons for Zr (n = 4) and Hf (n = 5) were calculated at the CCSD(T) level with the aug-ccpwCVTZ basis set for O and the aug-cc-pwCVTZ-PP basis set for Zr and Hf; these combined basis sets will be denoted as awCVTZ. Scalar relativistic (SR) corrections were calculated as the expectation values of the mass velocity and Darwin operators (MVD) from the Breit−Pauli Hamiltonian for the CISD (configuration interaction with single and double excitations) wave function46 with the aT basis set. The pseudopotential corrections were calculated with the second-order Douglas− Kroll−Hess Hamiltonian47 and the all-electron aug-ccpwcVTZ-DK basis set;42,48,49 these basis sets will be collectively denoted as awCVTZ-DK. For Hf, the 4f electrons are also correlated by including additional high angular momentum functions (2f2g1h), as the energies of these 4f orbitals are less negative than those of the metal 5s and 5p orbitals and close to the energies of the 2s orbital for O, so there is substantial mixing between the 4f metal orbitals and 2s O orbitals. For Zr, the 4d electrons are also correlated as the energies of these electrons are close to the 2s and 2p orbitals for Cl. The CV corrections, SR effect, and pseudopotential correction calculations for the molecules of MCl4, MCl3(OH), M(O)Cl2, M(O)Cl(OH), and MO2 were performed using the geometries optimized at the CCSD(T)/aT level. All CCSD(T) calculations were performed with the MOLPRO 2010.1 program.50 In order to calculate the total atomization energy (TAE) values, the spin−orbit corrections (ΔESO) for the atoms were taken from the experiment.51 The ground-state atomic spin− orbit corrections are ΔESO(Hf) = 7.84, ΔESO(Zr) = 2.06, ΔESO(Cl) = 0.84, and ΔESO(O) = 0.22 kcal/mol. Following our previous work,40 the TAE is calculated as
were determined to be spherical with diameters of 7−10 nm and in the monoclinic phase. The synthesis of HfO2 nanoparticles has been less studied even with the interest in HfO2 for use in microelectronic devices. In order to replace SiO2 as the gate insulator, HfO2 needs to be grown as a film with a controlled thickness. Atomic layer deposition (ALD) is widely used to deposit such thin films.21,22 The majority of the studies of HfO2 film growth are from the reaction of HfCl4 with H2O in the gas phase under reduced pressure (see reaction 1 with M = Hf).23 The reaction of HfCl4 (a solid at room temperature) with O2 was reported to grow thin hafnia films under atmospheric pressure (reaction 2 with M = Hf).24,25 The hydrolysis of hafnium oxychloride (HfOCl2) in methanol for the synthesis of HfO2 nanoparticles was first reported in 2000,26 and the resulting nanocrystals are in the monoclinic phase. A sol−gel method was used to produce nanoscale HfO2 particles from the organic precursor, hafnium isopropoxide.27 Flame spray processes are becoming more widely used for the synthesis of metal oxides, for example, vanadia-based and other catalysts, with novel compositions and surface properties.28−30 However, there is usually little available information about the properties of the intermediates formed in the hightemperature system. It is well established in industry that water needs to be added to TiCl4 (a liquid at room temperature) as an initiator in the high-temperature gas-phase flame oxidation of TiCl4 to produce TiO2 nanoparticles.15,16,31 We have previously studied the gas-phase hydrolysis of TiCl432 as the initiation step in the production of (TiO2)n nanoparticle using electronic structure calculations at the coupled cluster theory [CCSD(T)], 33−36 Møller−Plesset perturbation theory (MP2),37 and density functional theory (DFT) with the B3LYP functional38,39 levels. Our results showed, consistent with those of Green and co-workers,31 that the gas-phase process has to proceed through various oxychloride or oxychlorohydroxide intermediates as it is not possible to directly produce gas-phase (TiO2)n nanoparticles until they are large enough to have a bulklike heat of formation. Our goal in the current work is to examine the possibility that H2O could be used to initiate the formation of ZrO 2 and HfO 2 nanoparticles under similar high-temperature oxidation conditions to those used to produce TiO2 particles starting from the metal chloride. In the current work, we extend our prior studies on the initial hydrolysis mechanisms of TiCl4 to a study of the potential energy surfaces (PESs) for the hydrolysis of MCl4 and MOCl2 for M = Hf and Zr to provide insights into the potential production of MO2 nanoparticles using a gas-phase process similar to that used for TiO2 production. Even though the solution-phase hydrolysis of ZrCl4 to produce ZrO2 particles has been widely used for many years, there is little if any thermochemical data for various intermediates. The current work provides reliable predictions of the thermodynamic properties of these intermediates for the first time in addition to the information on the PESs, which can lead to a better understanding of how to improve the synthesis of these oxides especially using flame processes.
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COMPUTATIONAL METHODS The PESs were calculated using the same computational methods as in the TiCl4 study.32 Equilibrium geometries and harmonic frequencies were first calculated at the DFT level with the B3LYP exchange-correlation functional.38,39 The 7460
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Table 1. Calculated M−X (M = Zr, Hf; X = Cl, O, OH) Bond Lengths (Å) at the CCSD(T)/aT Level and/or MP2/aT Level M = Zr molecule
a
state
method
ZrCl
A1/Td
CCSD(T)/aT MP2/aT expt CCSD(T)/aT CCSD(T)/aT MP2/aT CCSD(T)/aT CCSD(T)/aT MP2/aT MP2/aT MP2/aT MP2/aT
2.346 2.338 2.328(5)a 2.415 2.441 2.439
MCl4
1
MOCl2 MOCl(OH)
1
MO2 MCl3(OH)
1
MCl2(OH)2 MCl(OH)3c M(OH)4
1
A1/C2v A/C1
1
A1/C2v A1/C3v
1
A1/C2v A1/C3v 1 A/S4 1
ZrO
1.760 1.771 1.775 1.799
2.365 2.358 2.380 2.401
M = Hf ZrOH
HfCl
1.995 1.993
2.333 2.303 2.316(5)b 2.390 2.413 2.378
1.918 1.912 1.926 1.941 1.960
2.350 2.321 2.343 2.363
HfO
HfOH
1.760 1.772 1.756 1.795
1.967 1.933 1.898 1.868 1.883 1.898 1.914
Reference 58. bReference 59. cThe state for ZrCl(OH)3 predicted at the MP2/aT level is 1A/C3.
energetics. Both the water complexation energies and the reaction barriers predicted at the MP2/aT level are close to those at the CCSD(T)/CBS level with energy differences less than 2 kcal/mol. For these reactions, the performance of MP2 is better than B3LYP DFT for the description of these Lewis acid donor−acceptor interactions and the hydrogen transfer reaction barriers.
ΣD0,0K = ΔECBS + ΔE PP,Corr + ΔESR + ΔECV + ΔESO + ΔEZPE
(3)
where ΔEPP,Corr is the error of the valence electronic energy calculated with RECP basis sets.40 By using the heats of formation at 0 K for the elements ΔHf,0K(Hf) = 147.7 ± 1.5 kcal/mol, ΔHf,0K(Zr) = 145.5 ± 2.0 kcal/mol,52 ΔHf,0K(Cl) = 28.59 kcal/mol, ΔHf,0K(O) = 58.98 kcal/mol, and ΔHf,0K(H) = 51.63 kcal/mol,53 we can derive ΔHf,0K values. The heats of formation at 298 K can then be calculated using the approach described by Curtiss et al.54 All of the calculations were performed on local Xeon and Opteron based Penguin Computing clusters in our group, a Xeon based Dell Linux cluster at the University of Alabama, the Opteron and Xeon based Dense Memory Cluster (DMC) and Itanium 2 based SGI Altix systems at the Alabama Supercomputer Center, and the Opteron based HP Linux cluster at the Molecular Science Computing Facility at Pacific Northwest National Laboratory. Molecular visualization was done using the AGUI graphics program from the AMPAC program package.55 Basis Set Dependence and DFT Performance. The relative energies calculated at the CCSD(T) level as well as those at the B3LYP DFT and MP2 levels are shown in the Supporting Information. The dependence of the relative energies on the basis set at the CCSD(T) level is also shown in the Supporting Information. The relative energies obtained at the CBS limit are close to those obtained with the aX basis sets for X = D, T, and Q, with energy differences generally less than 2 kcal/mol, so a reasonable PES can be calculated with a modest basis set. In our previous studies of the hydrolysis of TiCl4,32 B3LYP/aD predicted complexation energies for H2O to TiO2 that were smaller than the CCSD(T)/CBS values. This is the same result as found in the current study for the H2O dative bonds to ZrO2 and HfO2. This is similar to issues found with DFT with other dative bonds.56,57 The water complexation energies of ZrCl4, ZrCl3(OH), ZrCl2(OH)2, and ZrCl(OH)3 calculated at the B3LYP/aD level are lower than those at the CCSD(T)/CBS level for ΔECBS + ΔECV + ΔEZPE by 4.9, 4.8, 3.3, and 3.0 kcal/mol, respectively. For M = Hf, those energy differences are 5.3, 4.8, 4.1, and 3.6 kcal/mol. In addition, the reaction barriers for some hydrogen transfer reactions are underestimated by DFT. Thus, DFT/B3LYP provides only qualitatively correct predictions of the reaction
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RESULTS AND DISCUSSION Geometries and Frequencies. The M−X (X = Cl, O, OH) bond lengths predicted at the CCSD(T)/aT and/or MP2/aT levels for selected reactants and products are given in Table 1. Existing computational and experimental values are compared with our predictions. The optimized molecular structures at the CCSD(T)/aT or MP2/aT levels are given in the PESs (Figures 1 and 2) for M = Zr. The optimized molecular structures for M = Hf, the cartesian coordinates for the optimized structures, CCSD(T) energies and MP2/aT energies, calculated harmonic frequencies at the MP2/aT level, and T1 diagnostics are given in the Supporting Information. The calculated ZrCl bond distance at the CCSD(T)/aT level in ZrCl4 is predicted to be slightly longer than the gasphase experimental value from electron diffraction by 0.018 Å.58 At the MP2/aT level, the ZrCl bond distance in ZrCl4 is predicted to be ∼0.01 Å shorter than that predicted at the CCSD(T)/aT level. When two ZrCl single bonds are replaced with one ZrO bond to form ZrOCl2, the ZrCl bonds lengthen by 0.07 Å. The ZrCl bond distances in the zirconium oxychlorohydroxides are longer than that in ZrCl4 and follow the order ZrCl(OH)3 > ZrCl2(OH)2 > ZrCl3(OH). Replacing one of the remaining Cl atom with an OH group leads to a small lengthening of the ZrCl bond by ∼0.02 Å. The ZrO bond distances at the CCSD(T)/aT level increase by 0.04 Å from ZrOCl2 to ZrOCl(OH) to ZrO2. The ZrOH bond distances in the zirconium oxychlorohydroxides follow the same order as that of the ZrCl bond and are shorter than those in Zr(OH)4 and ZrOCl(OH). The predicted HfCl bond distance in HfCl4 level at the CCSD(T)/aT level is 0.017 Å longer than the experimental value.59 The HfCl bond distances follow the order HfCl4 < HfOCl2 < HfOCl(OH), as found for M = Zr. At the MP2/aT level, the HfCl bond distance is predicted to be ∼0.03 Å shorter than that predicted at the CCSD(T)/aT level. As found for M = Zr, replacing the Cl atom in HfCl4 with an OH group 7461
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Figure 2. PES (kcal/mol) for the reaction MOCl2 + H2O → MO2 + 2HCl. Relative energies are determined from heats of formation in black for M = Zr and in dark blue for M = Hf; see the Supporting Information for the structures for M = Hf.
compounds due to the smaller ionic radius of Hf4+, as the metal in these compounds is in the formal +IV oxidation state.60 As discussed in further detail below, we also studied the MxOyClz(OH)w clusters as potential intermediate species. Our previous work on the production of TiO2 nanoclusters32 found that it is possible to generate a number of intermediates from the initial hydrolysis reactions of the clusters. Figures 3 and 4 show the MxOyClz(OH)w structures for M = Zr and Hf at the B3LYP/aD level. In M2O2Cl4, the M−Cl bond length is predicted to be 2.401 Å for M = Zr and 2.356 Å for M = Hf. The M−Cl bond is elongated when the Cl is substituted by an OH group. As for the species derived from MCl4, the more the Cl atoms are substituted by OH, the longer the remaining M− Cl bonds will be. The M−OH bond lengths follow the order of M2O2(OH)4 > M2O2Cl(OH)3 > M2O2Cl2(OH)2. There is not much difference in the M−Cl and M−OH bond distances for the three isomers of M2O2Cl2(OH)2. For M2OClz(OH)w (z + w = 6), the fewer Cl atoms in the molecule, the longer the M− Cl and M−OH bonds will be. The M−Cl bond distances in M2O3Cl2 fall into the range predicted for M2OClz(OH)w and M2O2Clz(OH)w. For the M3OyClz and M4O6Cl4 clusters, the M−Cl bond distances fall in the same range. The M−O bonds for all of the studied clusters are predicted to be 1.9−2.0 Å depending on the structure except for the metal terminal oxygen bond in the species of M2O3Cl2. The Zr−O and Hf−O bond distances are calculated to be 1.764 and 1.762 Å, respectively, in M2O3Cl2, close to the metal terminal oxygen bond lengths of the (MO2)n (n = 1−4) nanoclusters.8 Atomization Energies and Heats of Formation. The different energy contributions to the TAEs and the heats of formations at 0 and 298 K are shown in Tables 2 and 3. The CV corrections are found to generally increase the TAEs by up to 6.8 kcal/mol for M = Zr and 5.6 kcal/mol for M = Hf. The scalar relativistic corrections for the compounds are usually smaller than the CV corrections and decrease the TAEs by 0.4−1.7 kcal/mol. The pseudopotential corrections are predicted to increase the TAEs by 2.5−4.5 kcal/mol for M = Zr, whereas for M = Hf, this correction decreases the TAEs by 3.5−7.5 kcal/mol. The T1 diagnostic can be used to estimate the role of multireference character in the wave function for the CCSD calculation.61 The values for the T1 diagnostics are
Figure 1. PESs (kcal/mol) for the reactions (a) MCl4 + H2O → MOCl2 + 2HCl; (b) MCl3(OH) + H2O → MCl2(OH)2 + HCl; (c) MCl2(OH)2 + H2O → MCl(OH)3 + HCl; and (d) MCl(OH)3 + H2O → M(OH)4 + HCl. Relative energies are determined from heats of formation in black for M = Zr and in dark blue for M = Hf; light blue = Zr, red = O, green = Cl, and gray = H; see the Supporting Information for the structures for M = Hf.
elongates the remaining HfCl bond distances and the more Cl atoms that are replaced, the longer the HfCl bond distance. The HfO bond distances follow the order HfOCl2 < HfOCl(OH) < HfO2. The HfOH bond distances in the hafnium oxychlorohydroxides follow the same order as for M = Zr. The bond distances in many of the Hf compounds are slightly shorter than the corresponding ones in the Zr 7462
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Figure 3. Optimized molecular structures for (cis) Zr2O2Cl2(OH)2, (trans) Zr2O2Cl2(OH)2, (iso) Zr2O2Cl2(OH)2, Zr2O2Cl4, Zr2O2Cl(OH)3, Zr2O2(OH)4, Zr2OCl2(OH)4, Zr2OCl4(OH)2, Zr2OCl5(OH), Zr2OCl6, Zr2O(OH)6, Zr2O3Cl2, Zr3O3Cl6, Zr3O5Cl2, Zr3O4Cl4, and Zr4O6Cl4. Bond lengths are given in Å at the B3LYP/aD level. Light blue = Zr, red = O, green = Cl, and gray = H.
The calculated ΔHf298(ZrCl4) is in good agreement with the experimental value, −207.9 ± 0.5 kcal/mol at 298 K, considering the size of the ±2 kcal/mol error bar for the atomic heat of formation of Zr.53 The calculated ΔHf298(ZrO2)
generally small (
