J. Phys. Chem. C 2008, 112, 6469-6476
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Stability of Hydroxylated (1h11) and (1h01) Surfaces of Monoclinic Zirconia: A Combined Study by DFT and Infrared Spectroscopy Satu T. Korhonen,†,‡ Monica Calatayud,*,‡ and A. Outi I. Krause† Laboratory of Industrial Chemistry, Helsinki UniVersity of Technology, P.O. Box 6100, FI-02015 TKK, Finland, and Laboratoire de Chimie The´ orique, UniV. Paris 06, UMR 7616 CNRS, Paris F-75005, France ReceiVed: January 29, 2008; In Final Form: February 27, 2008
We report the structure and stability of hydroxylated (1h11) and (1h01) surfaces for a water coverage ranging from 0.25 to 1. For the (1h11) surface water dissociated at low coverage (θ ) 0.25), whereas both dissociated and molecular species coexisted at higher coverage. For the (1h01) surface molecular adsorption was only observed at the highest coverages (θ ) 0.75 and θ ) 1). The calculated adsorption energy for (1h11) ranged from -1.20 to -0.83 eV for θ ) 0.25 and θ ) 1, respectively. For the (1h01) surface the adsorption energies ranged from -1.50 to -1.21 eV for θ ) 0.25 and θ ) 1, respectively. The 1-, 2-, and 3-fold coordinated hydroxyl groups were present in our models. Their structure, energetics, and vibrational frequencies were calculated by ab initio techniques and agreed with in situ infrared spectroscopic measurements. The simultaneous presence of hydroxyl groups of different coordination was concluded from both theoretical and experimental results.
1. Introduction Zirconia has received considerable attention in the recent literature for applications ranging from catalysis to microelectronic gate dielectrics and to ceramic engineering. One of the main benefits of zirconia for many applications is its high thermal stability.1 Zirconia catalyzes e.g. the hydrogenation and isomerization of alkenes2 and the dehydrogenation of alkanes3 and is an active component in the methanol synthesis from CO/ H2 or CO2/H2.4 The versatile reactivity of zirconia originates among others from the amphoteric character of its hydroxyl groups.2 In the methanol synthesis the hydroxyl species of zirconia are responsible for the formation of formate or carbonate species that are important intermediates of the reaction.4 In the atomic layer deposition (ALD) technique, which can be used to prepare thin films or catalysts, the hydroxyl groups take part in grafting the gaseous precursors to the substrate surface.5,6 It has been suspected that the hydroxyl groups influence the film growth rate.6 Bulk zirconia exhibits three well-defined crystal phases, of which the monoclinic C2h5 phase (space group P21/c, stable below ∼1100 °C)7 is commonly used for catalytic applications. The monoclinic phase transforms into the tetragonal D4h15 (space group P42/nmc, stable below ∼2400 °C) and the cubic Oh5 (space group Fm3m, stable below the melting point of ∼2670 °C) phases with increasing temperature.7 Zirconia can be stabilized into the tetragonal or cubic phases at ambient temperatures by doping with e.g. yttrium (yttrium-stabilized zirconia, i.e., YSZ) or into the tetragonal phase by reducing the particle size to nanoscale (below ∼30 nm).8 The stabilized zirconias, in either tetragonal or cubic phases, are also often used for catalytic applications.9,10 For the most thermodynamically stable zirconia, * Corresponding author: e-mail
[email protected], Tel +33 1-4427-26-82, Fax +33 1-44-27-41-17. † Helsinki University of Technology. ‡ Univ. Paris 06.
the monoclinic phase, the Zr atoms are 7-fold coordinated, whereas in the bulk of the tetragonal and cubic structures the coordination is 8-fold.7 The most stable surfaces for monoclinic, tetragonal, and cubic zirconia are (1h11),11 (101),9 and (111),11 respectively. The bulk and surface properties of zirconia have been studied by many experimental techniques, such as X-ray diffraction (XRD),12,13 electron paramagnetic resonance (EPR),12,14 and Raman,3,12,13 infrared,3,4,15-17 and neutron scattering18,19 spectroscopy. Of these techniques, especially infrared spectroscopy has brought insights into the characterization of the hydroxyl groups. On the monoclinic zirconia the hydroxyl groups are mainly terminal and 3-fold coordinated,15-17 whereas on the tetragonal zirconia 2-fold coordinated hydroxyls are the predominant ones.2,15 On the amorphous zirconia roughly equal concentrations of all of the three types of hydroxyls are present.2 The higher acidity of the monoclinic zirconia compared with the tetragonal zirconia is mainly related to the higher concentration of hydroxyl groups on the monoclinic surface.2 In addition to the hydroxyl species, undercoordinated Zr4+ and O2- species and defects such as oxygen vacancies exist on the zirconia surface.15,20 The relative concentration of the surface species depends on the phase of zirconia, the particle size, and the preparation route (e.g., impurities and dopants). Therefore, the comparison of results from different characterization techniques and research groups may be difficult. Nonetheless, only few studies focused on the detailed atomic surface structure of the working catalyst have been published. To our knowledge, the only computational studies on the hydroxyl groups of monoclinic zirconia are by Iskandarova et al.21 for the (001) surface and by Ignatchenko et al.22 for the (1h11), (1h01), and (111) surfaces. Both groups observed that especially at low surface coverage of water the adsorption is dissociative. However, only few adsorption sites and water coverage were considered in these studies, and the detailed atomic structure of the hydroxylated surfaces under catalytic
10.1021/jp8008546 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008
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TABLE 1: Comparison of Calculated Bulk Unit Cell Parameters for Monoclinic Zirconia
Zra OIa OIIa
a (Å) b (Å) c (Å) β (deg) x y z x y z x y z
present work
calcd32
calcd20
exptl30,31
5.184 5.274 5.358 99.30 0.278 0.043 0.209 0.071 0.337 0.342 0.448 0.758 0.481
5.197 5.280 5.350 99.53 0.276 0.043 0.207 0.071 0.336 0.342 0.448 0.758 0.480
5.192 5.265 5.358 99.81 0.277 0.044 0.209 0.072 0.338 0.341 0.447 0.758 0.479
5.150 5.212 5.315 99.23 0.275 0.040 0.208 0.070 0.332 0.345 0.450 0.757 0.479
a The position of the atoms in the unit cell: ((x, y, z) and ((x,1/2-y, 1/2+z).7
reaction conditions is still lacking. The aim of our work was to investigate the hydroxylated surfaces of monoclinic zirconia on an atomic basis. The ultimate goal was to bring some understanding for the zirconia material under catalytic conditions. In the present work we report the results on the adsorption of water on the most stable (1h11) and (1h01) surfaces of monoclinic zirconia at coverage from θ ) 0.25 to 1. Statistical thermodynamics was applied for the analysis of the stability of such hydroxylated surfaces as a function of temperature and partial pressure of water. Recently similar studies have been performed for tetragonal zirconia by Hoffmann and Sauer10 and Eichler and Kresse,23 for monoclinic hafnia by Muckhopadhyay et al.,6 and for γ-alumina by Digne et al.24 In parallel, we report results by infrared spectroscopic measurements on a commercial zirconia catalyst. The experimental and theoretical results are compared, and the nature of the hydroxyl groups is discussed. 2. Experimental Section 2.1. Computational Details. DFT Calculations. The Perdew-Burke-Ernzerhof functional was used for all the calculations as implemented in the VASP code.25-27 The electron configurations [Kr]4d25s2 and [He]2s22p4 were used for the zirconium or the oxygen atoms, respectively. The core electrons were kept frozen and replaced by PAW-generated pseudopotentials,28,29 whereas the valence electrons were described with a plane wave basis set with the cutoff of 400 eV. The distance between k-points in the reciprocal space was 0.05 Å-1. This methodology was tested for the monoclinic bulk structure, leading to excellent agreement with previous experimental30,31 and theoretical20,32 results (see Table 1). The most stable (1h11) and (1h01) surfaces11 of the monoclinic zirconia were chosen for the calculations. A slab containing four ZrO2 layers was selected (see Figure 1), and this was repeated periodically in three dimensions. The (1 × 1) unit cell dimensions for the (1h11) surface were 7.446 × 6.793 Å2, and it contained four ZrO2 units per layer (total of 48 atoms). For the (1h01) surface a 2 × 1 unit cell was chosen to keep the number of surface Zr atoms constant for the different surfaces. Because of the bigger unit cell, a two-layer slab was used for this surface (also a total of 48 atoms). The unit cell dimensions were 8.618 × 8.340 Å2. A vacuum of 10 Å prevented the interaction between successive slabs. Water was adsorbed on one side of the slab and the full model was allowed to relax with the conjugate gradient algorithm until the difference in energy was smaller than 0.001 eV. The description of the surfaces is given below.
The vibrational frequencies of the hydroxylated systems were calculated by finite differences within the harmonic approximation. The Hessian matrix was then constructed and diagonalized. Note that the intensities cannot be calculated with the code used. Thermodynamic Analysis. The stability of the hydrated surfaces was studied with respect to both temperature and water vapor pressure by the formalism already applied to other hydroxylated metal oxide surfaces.6,10,23,24 We assume a thermodynamic equilibrium between the surface and the gas-phase water. In the method used (i) the surface free energies of a series of model surfaces were calculated by total energy calculations, (ii) the energies as a function of the water chemical potential were used to draw a phase diagram, and (iii) the chemical potentials were related to the temperature T and pressure p. This procedure reveals the most stable surface (lowest surface free energy) under external temperature and pressure conditions. The following working equation was used:
1 ∆γ ) (Esurf - Eref - NH2OEH2O - NH2O∆µH2O) A
(1)
where ∆γ is the difference in the surface free energies, A is the surface area of the unit cell, Esurf is the total energy of a hydrated model, Eref is the total energy of the reference model (relaxed bare surface), NH2O is the number of water molecules per unit cell (from 1 to 4), EH2O is the total energy of a water molecule, and ∆µH2O is the chemical potential of water. In the derivation of the equation the effect of entropy and changes in volume were assumed to be negligible. The chemical potential was related to a given temperature T and partial pressure of water pH2O by means of the ideal gas equation:
∆µH2O(T,pH2O) ) µ˜ H2O(T,p0) + kBT ln(pH2O/p0)
(2)
where p0 is the pressure of the reference state (1 atm ) 10-5 Pa), the µ˜ H2O(T,p0) term is tabulated in the thermochemical reference tables,33 and kB is the Boltzmann constant. 2.2. In Situ DRIFTS. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study the dehydroxylation of monoclinic zirconia (Mel Chemicals, EC0100, BET surface area 47 m2/g3). The equipment consisted of a Nicolet Nexus FTIR spectrometer equipped with a SpectraTech high-temperature and high-pressure reaction chamber (ZnSe windows). The sample was studied undiluted in powder form. The spectrum measured with an aluminum mirror (4 cm-1, 200 scans) was used as the background. The total gas flow of 10% O2/N2 (AGA: synthetic air 99.99% and N2 99.999% further purified by Oxisorb (Messer Griesheim GmbH)) was kept constant at 50 cm3/min. The spectrum of a sample exposed to room air prior to the measurement was recorded at 25 °C (4 cm-1, 100 scans). Thereafter, the dehydration was studied by heating the sample from room temperature to 580 °C. The spectra were recorded at 200, 400, and 580 °C (4 cm-1, 100 scans) during the heating. The experiment was continued by drying the sample at 580 °C for 2 h, after which the sample was cooled down to room temperature. The spectra were recorded at 580, 400, 200, and 25 °C (4 cm-1, 100 scans) during the cooling. 3. Results and Discussion 3.1. Monoclinic (1h11) and (1h01) Zirconia Surfaces. Figure 1(I) presents the relaxed (1h11) slab used in the calculations. The calculated surface energy for our relaxed slab was 1.224 J/m2. This was in good agreement with previous works (Christensen and Carter11 reported 1.246 J/m2 for the relaxed slab). The bulk coordination of Zr was 7-fold, and bulk O was
(1h11) and (1h01) Surfaces of Monoclinic Zirconia
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6471
Figure 1. (I) Relaxed slab for the (1h11) surface used in this work and (II) relaxed slab for the (1h01) surface used in this work. Side and top views are presented for both slabs. The Zr and O atoms are presented in yellow and red, respectively. The unit cell is presented by the gray lines. The underlying atoms have been removed from the top view for clarity. Atom labeling shall be used to refer to the specific atoms in the following of the text: Zr sites (numbers) and O sites (letters).
3- or 4-fold coordinated.7 For the relaxed slab four nonequivalent Zr atoms were present on the surface. These are marked in Figure 1(I) by numbers from 1 to 4. The Zr atoms 1, 3, and 4 were 6-fold coordinated, whereas the Zr(2) was 7-fold coordinated and was, therefore, expected to be unreactive. The surface O atoms are marked in Figure 1(I) by letters from A to G. The undercoordinated O atoms were atoms A (2-fold coordinated, pointing outward from the surface), C, and D (3-fold coordinated, pointing outward from the surface). Atoms B and E were 3-fold coordinated and planar as in the bulk, and atoms F and G were pointing downward and were 4-fold coordinated as in the bulk. Figure 1(II) presents the relaxed (1h01) slab used in the calculations. The calculated surface energy for the relaxed slab was 1.548 J/m2, also in agreement with previous results (1.512 J/m2 for a four-layer slab).11 Because of the doubled unit cell, symmetry within the cell was present. The undercoordinated atoms formed rows with grooves containing fully coordinated atoms between them. Two nonequivalent Zr atoms were present, marked in Figure 1(II) by 1 and 2. The Zr(3) atoms were 7-fold coordinated and lay deep in the grooves. Undercoordinated oxygen atoms are marked with the letters A, B1, and B2 in Figure 1(II). Of these, the O(A) were 2-fold coordinated and the B1 and B2 were 3-fold coordinated. The O atoms C and D were 3or 4-fold coordinated, respectively, as in the bulk and lay in the grooves. Because of the symmetry in the unit cell, fewer combinations of atoms for water adsorption were possible for this surface than for the (1h11) slab. 3.2. Adsorption of Water. The adsorption of one to four water molecules per unit cell was studied for the (1h11) and (1h01) surfaces of monoclinic zirconia. Since the unit cells contained four zirconium atoms, this corresponds to a coverage of θ ) 0.25, 0.50, 0.75, and 1. Both dissociative and molecular adsorption modes were considered for all coverages, and all possible combinations of adsorption sites were tested. A total of 45 structures were calculated for the (1h11) slab. Because of
the symmetry in the unit cell of the (1h01) slab, fewer combinations of atoms were tested for the water adsorption on this surface (35 structures). The adsorption at low coverage was dissociative, whereas at higher coverage combination of dissociative and molecular adsorption was present. The most stable structures contained hydrogen bonds whose number increased with increasing water coverage. Hydrated (1h11) Surfaces. Figures 2 and 4A display the most stable hydrated (1h11) surface models for the water coverage considered. Figure 4A also displays their adsorption energy as a function of coverage. Table 2 presents the adsorption energies for the different structures. The adsorption energies per water molecule were calculated as a difference between the hydroxylated slab and the reference units (relaxed bare slab and gasphase water). The negative values indicate exothermic processes. The adsorption of one molecule of water per unit cell was dissociative. The most stable slab contained a terminal hydroxyl on Zr(1) and a 2-fold hydroxyl on O(A), stabilized by a hydrogen bond (see Figures 2 and 4A). The calculated adsorption energy was -1.20 eV (1 eV ∼ 96.5 kJ/mol). A similar structure and an adsorption energy of -1.25 eV were reported by Ignatchenko et al.22 using a three-layer slab. In the two second best structures the terminal hydroxyl was coordinated to the Zr(3) or Zr(4) atoms, but no hydrogen bonding could occur due to the long distance between the hydroxyl groups, resulting in less exothermic adsorption energies (-0.91 eV). For comparison, the adsorption energy for a molecular adsorption of water on Zr(1) was -0.87 eV. Note that water adsorption on monoclinic hafnia is reported to be molecular (-1.11 eV6); this could be due to a lower acid-base character of the Hf4+O2- pairs of hafnia compared to those of zirconia. The adsorption of two water molecules per unit cell resulted in a combined dissociative and molecular adsorption (Figures 2 and 4A). In the most stable structure the dissociative adsorption occurred between the Zr(4) and O(A) and the molecularly adsorbed water was bound to Zr(1). The adsorption
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Figure 2. Most stable hydrated (1h11) surfaces, top view. The Zr and O atoms are presented in yellow and red, respectively, and the atoms belonging to the adsorbed water molecule are presented in dark blue. The lattice O atoms taking part in the dissociative adsorption are presented in light blue. The hydrogen bonds are presented by the light blue dashed line.
Figure 3. Most stable hydrated (1h01) surfaces, top view. See the figure caption for Figure 2 for more details.
energy was -1.05 eV. The differences in the adsorption energies for the different combinations of surface atoms for the dissociative and molecular adsorption were, however, minor, and the adsorption energy for the second best structure was only 0.07 eV less exothermic. A value of -1.20 eV has been reported for the (1h11) surface of monoclinic zirconia for a combined molecular and dissociative adsorption.22 Hafnia also presented a combined molecular dissociative mode with adsorption energy of -1.11 eV.6 For the (1h11) surface of monoclinic zirconia the adsorption energy of two fully dissociated water molecules was -0.90 eV per water molecule. A fully dissociative adsorption
was preferred for the (001) surfaces of monoclinic zirconia (-1.71 eV21) and hafnia (-1.54 eV6). The preference for fully dissociative adsorption on the (001) surface might be due to the lower stability, i.e., higher reactivity, of the surface in comparison with the (1h11) surface. Ushakov and Navrotsky34 reported experimental adsorption enthalpies between -1.1 and -1.8 eV for a 50% coverage of water on monoclinic zirconia in agreement with our results. The adsorption of three water molecules per unit cell again resulted in combined dissociative and molecular adsorption (Figures 2 and 4A). In the most stable structure two dissocia-
(1h11) and (1h01) Surfaces of Monoclinic Zirconia
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Figure 4. Adsorption energy per water molecule as a function of coverage (in eV): (A) for the (1h11) surface and (B) for the (1h01) surface. The figure also presents the structures of the most stable hydrated surfaces in a side view. See Figures 2 and 3 for more details on the surfaces.
TABLE 2: Adsorption Energies per Water Molecule (in eV) for the (1h11) and (1h01) Surfaces with the Water Coverage (NH2O) between 1 and 4, Calculated as Eads ) [Eslab - Ebare NEH2O]/Na coverage (NH2Ob)
adsorption energy (Eads/NH2O) (eV)
adsorption mode
1 2 3 4
1h11 -1.20 -1.05 -0.93 -0.83
D D+M D+M D+M
1 2 3 4
1h01 -1.50 -1.45 -1.21 -1.21
D D D+M D+M
aThe negative values indicate an exothermic reaction. The total energies for the bare relaxed (1h11) and (1h01) surfaces and the water molecule were -451.96, -446.40, and -14.22 eV, respectively. D ) dissociative, M ) molecular. b Per unit cell.
tively adsorbed water molecules formed hydroxyl groups on Zr(3), Zr(4), O(A), and O(D), resulting in the formation of two terminal, one 2-fold coordinated, and one 3-fold coordinated hydroxyl groups. The molecularly adsorbed water was coordinated to Zr(1). The adsorption energy of water for this structure was -0.93 eV. Other structures were close in energy, always involving dissociated and molecular water. A similar adsorption energy and the combination of dissociative and molecular
adsorption was observed for the (1h11) surface of monoclinic hafnia (-0.95 eV).6 The addition of a fourth water molecule took place on top of the first hydroxylated layer, and it was stabilized by hydrogen bonds (Figures 2 and 4A). The calculated adsorption energy per water molecule was -0.83 eV. All the structures considered were very close in energy, -0.79 eV per water molecule. On monoclinic hafnia a value of -0.89 eV has been reported for a combination of dissociative and molecular adsorption.6 Figure 4A shows the adsorption energies calculated for the most stable slabs. A decrease in adsorption energy with increasing coverage was observed. The same behavior was found for similar systems using theoretical6,10,24 and experimental34 methods. Hydrated (1h01) Surfaces. Water adsorption for the (1h01) surface was always more exothermic than for the (1h11) surface. For the most stable hydrated structures the hydroxyls were always coordinated to the atoms with the lowest coordination number, i.e., Zr(1) and O(A). Similar to the (1h11) surface, hydrogen bonding stabilized the structures further, and the number of hydrogen bonds increased with increasing water coverage. Figures 3 and 4B present the most stable hydrated (1h01) models for the coverage considered. Figure 4B also displays the adsorption energy as a function of coverage. Table 2 presents the adsorption energies for the different structures. The adsorption of one water molecule was dissociative. The most exothermic adsorption energy was found for a structure where the surface became slightly modified by the water adsorption. In this structure (see Figures 3 and 4B) two terminal hydroxyls were bound to neighboring Zr(1) and Zr(2) atoms. The hydroxyls were further stabilized by a hydrogen bond. One of these terminal hydroxyls was originally a 2-fold coordinated O(A). The calculated adsorption energy was -1.50 eV. In the second best structure (-1.44 eV) the water was also dissociatively adsorbed on Zr(1) and O(A), but for this structure the O(A) stayed intact as a 2-fold coordinated O. The small difference in the adsorption energies for the two structures suggested that the O(A) was only weakly bound to the 6-fold coordinated Zr(2) and that the two structures most likely were in equilibrium. Molecular adsorption of water (-0.88 eV for water bound to Zr(1)) was less favorable than dissociative adsorption. In contrast to the (1h11) surface, the adsorption of two water molecules on the (1h01) surface was dissociative. Two studied structures had the same adsorption energy of -1.45 eV per water molecule. Figures 3 and 4B present only one of these for the sake of brevity. In the presented structure the dissociative adsorption occurred between the Zr(1) and O(A) atoms. Similar to the case of the adsorption of one water molecule, the O(A) was weakly bound to the Zr(2). Thus, the adsorption of two water molecules also led to the formation of a structure containing three terminal (two Zr(1) and Zr(2)) and one 2-fold (O(A)) hydroxyls (-1.45 eV, not shown). A value of -1.57 eV was previously reported22 for the dissociative adsorption of water on the same surface. The adsorption energy for the combination of dissociative and molecular adsorption was -1.17 eV. The difference in the adsorption energies (0.28 eV) clearly showed the preference for dissociative adsorption on this surface. Similar to the (1h11) surface, adsorption of three water molecules was a combination of dissociative and molecular adsorption. For the most stable structure (Figures 3 and 4B) the dissociative adsorption occurred between the two Zr(1) and O(A) pairs, similar to the adsorption of two water molecules. The molecularly adsorbed water was bound to Zr(2) and further
6474 J. Phys. Chem. C, Vol. 112, No. 16, 2008 stabilized by hydrogen bonding to the terminal hydroxyl on Zr(1). However, fully dissociative adsorption was only 0.03 eV less exothermic. It is, therefore, likely that both adsorption modes, the combination of dissociative and molecular and fully dissociative, coexist. In the fully dissociated structure the hydroxyl groups were again bound to the Zr(1)-O(A) pairs, and the third water was dissociated between Zr(2) and O(B2). For the adsorption of four molecules of water a combination of molecular and dissociative adsorption was observed. In contrast to the (1h11) surface, no second layer of water was formed for this surface due to the presence of four undercoordinated Zr atoms. The adsorption energy for the most stable structure was -1.21 eV. This structure (Figures 3 and 4B) was similar to the structure of fully dissociative adsorption of three water molecules per unit cell described above. The molecularly adsorbed water was bound to the second Zr(2). The acid-base character of the Zr(2)-O(B2) pair was, however, lower than that for the Zr(1)-O(A), and the second most stable structure (-1.15 eV) contained two molecularly adsorbed water molecules on the two Zr(2) atoms and the dissociation occurred between the Zr(1)-O(A). All studied structures were, however, close in energy, and the calculated adsorption energy for fully dissociative adsorption was -1.06 eV. For comparison, Ignatchenko et al.22 reported the value of -1.31 eV for the combined dissociative and molecular adsorption on this surface. Figure 4B presents the adsorption energies for the most stable structures as a function of coverage. Similar to the results for the (1h11) surface, the adsorption energy decreased with increasing water coverage. However, the adsorption of water was more exothermic for the (1h01) surface at all studied coverage than for the (1h11) surface. Surface Phase Diagrams. Figure 5 presents the free energies of the most stable hydrated structures for the (1h11) and (1h01) surfaces as a function of chemical potential of water calculated with eq 1. Such a diagram allows the comparison of the slabs containing different numbers of adsorbates. The most negative ∆γ values indicate the predominance of a slab for a given chemical potential. At low water potentials (left side) the surfaces were bare. As the water potential increased, the hydroxylated slabs were stabilized. Thus, for the (1h11) surface (Figure 5A) between ∆µ ) -1.2 and -0.9 eV the slab with water coverage of θ ) 0.25 was stabilized. The half-covered (θ ) 0.5) slab was stable for chemical potentials between -0.9 and -0.7 eV. Higher coverages were stabilized with increasing chemical potential of water. For the (1h01) surface (Figure 5B) the behavior was slightly different. The hydrated slabs were stabilized at lower water potentials (starting from ∆µ ) -1.5). In addition, the phase diagram suggested that the θ ) 0.25 coverage was not formed, but the surface was immediately half-covered. Similar phenomenon took place at higher values of the chemical potential of water, and the fully covered surface was more favored than the θ ) 0.75. Thus, only the coverages θ ) 0.5 and 1 are expected to be stable for the (1h01) surface. The relation of the chemical potential of water with external temperature and pressure is given by eq 2, assuming an ideal behavior of the water gas phase. If we consider atmospheric pressure, the temperature of desorption of water from the (1h11) and (1h01) surfaces is calculated to be ∼360 and ∼480 °C, respectively. However, hydroxyls are known to exist on the zirconia surface even after evacuation at 500 °C.16 According to our infrared results, the hydroxyls are stable at atmospheric pressure even at ∼600 °C (see below). This disagreement could arise from the presence of other exposed planes and defects in
Korhonen et al.
Figure 5. Difference in surface free energy as a function of the chemical potential of water: (A) for the (1h11) surface and (B) for the (1h01) surface.
the real catalysts, responsible for more stable hydroxyl groups. Thus, a more reactive plane would lead to further dissociation of water and hydroxyl groups more strongly bound and to an increased desorption temperature. 3.3. In Situ DRIFTS and Computed Vibrational Frequencies. The computed hydrated structures were compared with experimentally observed hydroxyls by calculating the vibrational frequencies for the different hydroxyl groups and molecularly adsorbed water molecules. Figure 6 presents the in situ DRIFT spectra recorded during the heating (A) and cooling (B) of the monoclinic sample powder. During the heating of the sample water was observed to desorb from the surface. The spectrum recorded at room temperature before the heating showed a band at 3684 cm-1 and a broad strong feature centered at ∼3200 cm-1. In addition, a band at 1620 cm-1 (δ(OH)) was observed (not shown). A similar spectrum has been described19 for monoclinic zirconia with two layers of molecular water on top of the surface hydroxyl groups. Flushing the sample for prolonged time at room temperature had only little effect on the spectrum (not shown). The broad feature at ∼3200 cm-1 disappeared by heating to 200 °C, suggesting that physisorbed water desorbed. The remaining bands originated from surface bound hydroxyl groups. By prolonged vacuum treatment similar desorption was observed at room temperature.17 The spectra were only little affected by heating the sample further to 580 °C. According to these results, monoclinic zirconia exposed to room air would be covered by hydroxyl groups and few monolayers of water, the latter disappearing while heating to 200 °C. The highest water coverage considered in our calculations was 1, most likely too low to describe the surface of the
(1h11) and (1h01) Surfaces of Monoclinic Zirconia
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6475 low-intensity band assigned to the 2-fold coordinated hydroxyls in the in situ DRIFT spectra might originate from small amounts of tetragonal zirconia in the commercial catalyst. However, the most stable hydroxyl groups observed by theoretical methods by us and by Ignatchenko et al.22 for the (1h11) and (1h01) surfaces of monoclinic zirconia are the 2-fold coordinated hydroxyl groups. The disagreement is suspected to originate from the presence of other exposed planes and defects in the real catalysts, responsible for the increased stability of the 3-fold coordinated hydroxyls. However, it should also be noted that that the presence of hydroxyl pairs might result in frequencies in the region of 3675 cm-1 due to the weakening of the OH group by hydrogen bonding. 4. Conclusions
Figure 6. In situ DRIFT spectra for monoclinic zirconia. (A) Heating from room temperature to 580 °C and (B) cooling down to room temperature. The colored bars indicate the ranges of the computed frequencies for the 1-, 2-, and 3-fold coordinated hydroxyls.
sample that had been exposed to room air. However, the fact that our most stable structures presented molecular water in interaction with hydroxyl groups is fully consistent with this picture. For instance, for the (1h11) surface at the coverage of θ ) 1 (Figures 2 and 4A) molecular water was adsorbed on top of the hydroxyl covered surface, and it is believed to resemble the surfaces with higher water coverages. Higher temperatures would stabilize hydroxylated surfaces with decreasing number of molecular water in accordance with our models. More details could be observed in the spectra while cooling (Figure 6B) the sample to room temperature. This was suspected to originate from the temperature induced broadening of the bands at high measurement temperatures. At room temperature after the heating treatment three main bands were observed at 3774, 3738, and 3675 cm-1 assigned to the OH stretches of terminal, 2-fold coordinated (bibridged), and 3-fold coordinated (tribridged) hydroxyl groups, respectively.2,3,15,16 According to this assignment, the samples contained hydroxyl groups of different coordination in agreement with the calculated structures. The vibrations for the most representative structures were computed at the harmonic level in order to compare with the experiment. The calculated values were in good agreement with the experimental ones: terminal hydroxyls were found to vibrate between 3822 and 3743 cm-1, bibridged hydroxyls in the range of 3755-3568 cm-1, and tribridged hydroxyls between 3647 and 3498 cm-1. In some structures the hydroxyl groups were found to form strongly hydrogen-bonded pairs, and the corresponding vibrations were shifted toward lower wavenumbers by 100-500 cm-1. The molecularly adsorbed water was calculated to vibrate in the range of 3760-3120 cm-1, in agreement with the experimental results (see Figure 6A). Previous calculations at the Hartree-Fock level report vibrations of 4200-3800 cm-1 for hydroxyls on tetragonal zirconia.35 The presence of terminal and 3-fold coordinated hydroxyls is commonly reported for monoclinic zirconia,15-17 whereas the 2-fold coordinated hydroxyls have been reported to exist on the tetragonal surface.2,15 In this respect, the presence of the
The adsorption of water (θ ) 0.25-1) was studied for the two most stable surfaces of monoclinic zirconia. At low coverage (θ ) 0.25) the water adsorbed dissociatively. At higher coverage both dissociative and molecular adsorption occurred and the structures were stabilized by hydrogen bonding. The adsorption energy calculated per adsorbed water molecule became less exothermic with increasing water coverage: for the (1h11) surface -1.20 eV for θ ) 0.25 and -0.83 eV for θ ) 1 and for the (1h01) surface -1.50 eV for θ ) 0.25 and -1.21 eV for θ ) 1. The more exothermic adsorption of water on the (1h01) surface compared with the (1h11) was in agreement with the lower stability, i.e., higher reactivity of the (1h01) surface. The simultaneous presence of hydroxyl groups of different coordination was concluded from DFT and DRIFTS results. Calculated vibrational frequencies were in good agreement with the experimentally observed ones. Acknowledgment. We gratefully acknowledge the financial support provided by the Academy of Finland and the aid provided by the COST action D36 in organizing a researcher exchange between TKK and Paris6. Computational facilities by IDRIS, CINES, and CCRE are acknowledged. We thank D. Costa and C. Minot for stimulating discussions. References and Notes (1) Damyanova, S.; Petrov, L.; Centeno, M. A.; Grange, P. Appl. Catal., A 2002, 224, 271. (2) Jung, K. T.; Bell, A. T. J. Mol. Catal. A 2000, 163, 27. (3) Korhonen, S. T.; Airaksinen, S. M. K.; Ban˜ares, M. A.; Krause, A. O. I. Appl. Catal., A 2007, 333, 30. (4) Jung, K.-D.; Bell, A. T. J. Catal. 2000, 193, 207. (5) Puurunen, R. L. Chem. Vap. Deposition 2005, 11, 79. (6) Muckhopadhyay, A. B.; Sanz, J. Fdez.; Musgrave, C. B. Chem. Mater. 2006, 18, 3397. (7) Jomard, G.; Petit, T.; Pasturel, A.; Magaud, L.; Kresse, G.; Hafner, J. Phys. ReV. B 1999, 59, 4044. (8) Ferna´ndez-Garcı´a, M.; Martı´nez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Chem. ReV. 2004, 104, 4063. (9) Hofmann, A.; Clarks, S. J.; Oppel, M.; Hanhdorf, I. Phys. Chem. Chem. Phys. 2002, 4, 3500. (10) Hofmann, A.; Sauer, J. J. Phys. Chem. B 2004, 108, 14652. (11) Christensen, A.; Carter, E. A. Phys. ReV. B 1998, 58, 8050. (12) Matta, J.; Lamonier, J. -F.; Abi-Aad, E.; Zhilinskaya, E. A.; Aboukaı¨s, A. Phys. Chem. Chem. Phys. 1999, 1, 4975. (13) Li, C.; Li, M. J. Raman Spectrosc. 2002, 33, 301. (14) Anpo, M.; Che, M.; Fubini, B.; Garrone, E.; Giamello, E.; Paganini, M. C. Top. Catal. 1999, 8, 189. (15) Bachiller-Baeza, B.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Langmuir 1998, 14, 3556. (16) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K. J. Chem. Soc., Faraday Trans. 1997, 93, 169. (17) Cerrato, G.; Bordiga, S.; Barbera, S.; Morterra, C. Appl. Surf. Sci. 1997, 115, 53. (18) Loong, C. -K.; Richardson, J. W. Jr.; Ozawa, M. J. Catal. 1995, 157, 636. (19) Mamontov, E. J. Chem. Phys. 2004, 121, 9087.
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