ARTICLE pubs.acs.org/JPCC
Density Functional Theory Study of Carboxylic Acids Adsorption and Enolization on Monoclinic Zirconia Surfaces Alexey V. Ignatchenko* Energy & Environmental Research Center, University of North Dakota, 15 North 23rd Street, Stop 9018, Grand Forks, North Dakota 58202-9018, United States
bS Supporting Information ABSTRACT: Computational modeling is a valuable tool for understanding atomic-level catalytic processes on metal oxides. Carboxylic acid adsorption and enolization with varied degrees of R branching on monoclinic zirconia's most important surfaces, (111), (111), and (101), have been studied by the density functional theory (DFT). Carboxylates on zirconia (111) and (111) surfaces are preferentially stabilized in the bidentate bridging mode, with OH bond dissociation and hydrogen bonding to a 2-fold coordinated (2-fc) lattice oxygen. Carboxylic R hydrogen abstraction by another 2-fc lattice oxygen results in enolization of adsorbed carboxylates most readily on the (111) surface of monoclinic zirconia with activation energy ∼25 kcal/mol, which is not sensitive to acid branching. Enolization on the (111) surface requires higher activation energy, 2933 kcal/mol depending on acid branching. This study demonstrates the origin of an important intermediate in the carboxylic acid ketonization mechanism—often named “surface ketene”.
1. INTRODUCTION Zirconia (ZrO2) has become an increasingly popular catalyst support and a catalyst itself for a variety of industrial processes in recent decades.1,2 It is a useful ceramic material for thin-film coatings,3,4 sensors,5,6 and fuel cells.7,8 The stability of zirconia surfaces and their atomic structure has been studied both experimentally9 and computationally.10 Three of the most stable surfaces of monoclinic zirconia according to DFT computational studies10 are (111), (101), and (111). A lowsurface energy found computationally for monoclinic zirconia surfaces signifies a stable termination, so the surface reconstruction is not reported in either theoretical or experimental studies. Unreconstructed monoclinic zirconia surfaces are further stabilized upon interaction with small molecules, such as water,11,12 alcohols,13,14 and carboxylic acids.15 Interaction with carboxylic acids serves as a probe for surface basicity and reactivity.16,17 Atomic scale details of carboxylic acid interaction with a catalyst surface are critical in our understanding of such important catalytic processes as esterification of acids18,19 and decarboxylation to ketones20,21 and to hydrocarbons in the production of biofuels from fatty acids.22 A growing number of research studies have been recently devoted to computational modeling of carboxylic acid behavior on metal oxide surfaces. Adsorption of formic and acetic acids has been extensively studied by computations on anatase and rutile surfaces of titania.2331 Carboxyl group oxygen atoms bind to the surface titanium atoms in either a monodentate or a bidentate mode, which is sensitive to the structure of the surface. It has been shown that formic acid,23,24,26,27,29,30 acetic acid,26 and r 2011 American Chemical Society
alkyl-substituted carboxylic acids31 are dissociated and adsorbed in the bidentate bridging mode on the anatase (001)29 surface and on the rutile (110)23,24,27,30,31 and (011)26 surfaces. When adsorbed on the anatase (101) surface, formic acid28,32 and substituted carboxylic acids25 are preferentially stabilized in the monodentate form without dissociation but with a hydrogen bonding to the lattice oxygen. By contrast, computational studies of carboxylic acid adsorption on zirconia surfaces are rather scarce,33 and a systematic study is needed. The location of adsorption sites and the form of adsorption, either dissociated or molecular, with a bidentate or a monodentate binding can be estimated by computations. Dissociation of carboxylates on the surface may be ultimately associated with their dehydration and further transformations. Carboxylic acids other than formic acid could also undergo R hydrogen abstraction by basic sites on the surface leading to the enolization of carboxylates. Experimental evidence for the enolization of surface carboxylates comes from the observation of their R proton exchange for deuterium.3437 Exchange of R hydrogen atoms under basic conditions mechanistically can take place only through the enolization of carboxylates followed by protonation of the enolized fragment, as depicted in Scheme 1. The enolized form of carboxylate resembles the structure of the so-called “surface ketene” proposed as one of the possible intermediates in the mechanism of carboxylic acid ketonization.34,3840 Received: April 11, 2011 Revised: July 1, 2011 Published: July 13, 2011 16012
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However, the details of the surface ketene formation and structure have not been discussed. It is not clear how the enolized carboxylate could be formed and at what energy cost. To estimate the likelihood of the carboxylate enolization, the first principle computational approach is applied in the current study of carboxylic acid behavior on monoclinic zirconia surfaces.
were used. SCF density convergence, optimization energy convergence, and gradient convergence were set to 0.00001, 0.00002, and 0.004 au, respectively. The same parameters were utilized for energy minimization with acetic, propionic, or isobutyric acids and their enolized forms on the surface. Transition states were found by combining linear and quadratic synchronous transit methods with conjugate gradient refinements as described in the original work.44 Frequency calculations have confirmed that only one imaginary frequency exists. The geometry of each transition state was refined through its optimization. The transition state location on the reaction path coordinate is described in fractions from 0 (reactant) to 1 (product).
2. COMPUTATIONAL METHOD Density functional theory (DFT) calculations were performed using Accelrys Inc.’s DMol3 software.41 Electron exchange and correlation were described by the generalized gradient approximation (GGA) based on the work of Perdew et al.42 The bulk structure was built based on initial lattice parameters published for monoclinic zirconia43 with P21/c symmetry, and then it was adjusted by computing and minimizing its energy. The minimum energy obtained for the lattice length parameters increased by 3.2%, resulting in a = 0.52156 nm, b = 0.52412 nm, and c = 0.53918 nm. To model surfaces, periodic slab geometry was used. Previously used models10,11 were adopted. The (101) surface was cleaved to a depth of ∼16 Å, while (111) and (111) surfaces were cleaved to a depth of ∼12 Å. A supercell was constructed of 16 zirconium atoms and 32 oxygen atoms for (111), (101), and (111) monoclinic zirconia surfaces. A vacuum slab of 20 Å in thickness was built above the surface and found sufficient by comparing the total energy and bond lengths of adsorbed acetic acid at vacuum thickness values of 10, 15, and 20 Å until their change became negligible. The top three surface layers, consisting of 12 ZrO2 units, were allowed to relax, while the bottom layer of four ZrO2 was constrained. Energy minimization was performed for the obtained supercell by density functional calculations using the double numerical plus polarization basis set. Core electron treatment included all electron relativistic options. All calculations were performed spin restricted. A real space cutoff of 4.6 Å and a k-point sampling spacing of 0.05 Å1
3. RESULTS AND DISCUSSION 3.1. Surface Energies. The surface energy, Esurf, for clean unrelaxed surfaces was calculated by a common equation29
Esurf ¼
ðEslab N Ebulk Þ 2A
ð1Þ
where Eslab denotes the total energy of the unrelaxed two-sided supercell; N is the number of stoichiometric ZrO2 units in the supercell; Ebulk is the total energy of the single ZrO2 unit in the bulk; and A is the surface area of one side of the two-sided supercell, which is doubled in the denominator due to having two sides of the supercell with equal areas. As the result of the geometry optimization of each supercell, three layers of the top side surface were allowed to relax, while atoms of the bottom side layer remained fixed in their bulk positions. Formula 1 in that case can not be used since it would give the value of the surface energy, (E0surf + Esurf)/2, averaged between that for the top relaxed side, E0surf, and the bottom unrelaxed side, Esurf. Due to the symmetry of the supercell, surfaces on the top and bottom sides were identical before the relaxation. Assuming the surface energy on the unrelaxed side has not changed, and the total energy change is completely due to the relaxation of the top side, the corrected surface energy for the relaxed surface can be calculated by the (trivially) derived formula 2a
Scheme 1. H/D Exchange of r Hydrogen Atoms in Surface Carboxylates through Their Enolization
0
0
Esurf ¼
ðEslab N Ebulk Þ Esurf A
ð2aÞ
Table 1. Comparison of Calculated Surface Energies for Monoclinic Zirconia Surfaces, mJ/m2 empty surface
a
surface
unrelaxed
relaxed
average
relaxed, lit. data
(111)
1430
1013
1222
1246a a
1224b
with AcOH adsorbed
AcOH conc., nm2
899
2.21
(111)
1424
1174
1299
1537
--
1038
1.97
(101)
1866
1359
1612
1512a
1548b
1153
2.79
Ref 10. b Ref 12.
Table 2. Calculated Absolute Energy and Type of AcOH Exothermic Adsorption on Monoclinic Zirconia Surfaces entry
surface
coverage, %
molecular or dissociative adsorption
type of adsorption
adsorption energy, kcal/mol (eV)
figure
1
(111)
25
dissociative, AcO and H
bidentate bridging
32.2 (1.40)
2a
2
(111)
25
dissociative, AcO and H
bidentate bridging
43.3 (1.88)
1, 3a
3 4
(101) (101)
50 50
dissociative, Ac and OH dissociative, AcO and H
bidentate chelating bidentate bridging
45.7 (1.98) 45.3 (1.97)
4a 4b
5
(101)
50
dissociative, AcO and H
bidentate chelating
46.4 (2.01)
4c
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25.0 (1.09)
32.9 (1.43)
32.6 (1.41)
H, CH3
CH3, CH3
16014
43.9 (1.90)
CH3, CH3
26.8 (1.16)
26.8 (1.16)
26.0 (1.13)
32.7 (1.42)
2.0 (0.09)
1.1 (0.05)
0.1 (0.005)
17.8 (0.77)
15.7 (0.68)
14.4 (0.62)
8.3 (0.36)
6.1 (0.26)
4.8 (0.21)
kcal/mol (eV)
activation energy,
reverse reaction
0.75
0.77
0.87
0.65
0.67
0.67
0.61
0.62
0.65
(product)
(reactant) to 1
scale from 0
location on a
transition state
lattice
d
c
b
d
c
b
d
c
b
d
c
b
d
c
b
d
c
b
d
c
b
d
c
b
d
c
b
1.168 1.007
2.890
1.016
1.155
2.857
1.036
1.115
1.013 2.924
1.206
3.877
1.012
1.206
3.157
1.008
3.459 1.206
0.973
1.196
3.170
0.976
1.189
3.043
1.180 0.980
3.026
adsorbate OH
1.556 2.053
1.108
1.987
1.558
1.105
1.806
1.594
2.866 1.097
1.523
1.108
2.376
1.506
1.103
2.316
1.097 1.455
2.881
1.515
1.109
2.787
1.520
1.106
1.526 2.554
1.103
HC1
1.412 1.379
1.521
1.377
1.407
1.512
1.385
1.405
1.363 1.503
1.428
1.521
1.409
1.421
1.512
1.420
1.503 1.443
1.362
1.424
1.521
1.359
1.417
1.509
1.415 1.360
1.502
1.320 1.347
1.271
1.338
1.314
1.269
1.325
1.313
1.382 1.272
1.322
1.271
1.343
1.322
1.276
1.335
1.272 1.309
1.364
1.332
1.284
1.363
1.333
1.285
1.331 1.357
1.284
1.344 1.364
1.299
1.362
1.345
1.301
1.351
1.340
1.363 1.297
1.323
1.299
1.326
1.320
1.292
1.318
1.297 1.308
1.367
1.324
1.279
1.361
1.322
1.279
1.322 1.357
1.280
2.143 2.060
2.268
2.069
2.145
2.275
2.086
2.124
2.089 2.262
2.075
2.268
2.191
2.078
2.239
2.216
2.262 2.127
2.043
2.117
2.254
2.046
2.120
2.258
2.121 2.057
2.261
2.141 2.105
2.242
2.109
2.139
2.232
2.134
2.158
2.087 2.255
2.207
2.242
2.175
2.204
2.269
2.184
2.255 2.215
2.114
2.188
2.313
2.121
2.186
2.315
2.184 2.122
2.336
C1C2 C2O1 C2O2 O1Zr1 O2Zr2
12.42 4.46
0.50
7.24
15.07
2.86
7.04
10.54
4.92 115.82
35.18
0.50
19.88
33.10
66.26
29.89
115.82 46.39
2.71
28.01
40.44
3.89
26.79
37.24
29.49 8.09
39.66
angle
torsion
12.90 5.64
25.32 10.10
10.12
25.20
5.03
47.89
50.75
50.84
14.76
21.80
51.35
19.52
30.06
12.27
8.13 169.91
27.69 3.21 54.09
7.48
0.88 129.16
19.00 51.35
78.47 21.39
11.71
14.07
15.82 12.22
169.91 38.80
54.09 7.59
4.14
15.50 1.43
12.52
11.77
15.02
3.30
19.52
17.72
27.79
12.38 14.72
17.11 22.81
12.65
22.14
17.52
0.59
angles
torsion angle
torsion
R1C1C2O1 R2C1C2O2 sum of
a Hydrogen abstraction by 2-fold coordinated lattice oxygen. b Surface carboxylate. c Optimized transition state. d Enolized form of the surface carboxylate. e Hydrogen abstraction by 3-fold coordinated lattice oxygen.
44.4 (1.92)
43.3 (1.88)
43.9 (1.90)
31.1 (1.35)
43.8 (1.90)
CH3, H
(111)e H, H
CH3, CH3
H, CH3
29.4 (1.28)
43.3 (1.88)
25.3 (1.10)
25.0 (1.08)
32.2 (1.40)
kcal/mol (eV)
energy,
mol (eV)
energy, kcal/
H, H
R1, R2
(111)a H, H
(111)
a
surface
activation
acid adsorption
enolization
bond distances, Å
Table 3. Geometry for the Enolization of Substituted Carboxylic Acids on Monoclinic ZrO2 Surfaces According to Scheme 2 and Energy Absolute Values
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Figure 1. Acetic acid adsorption on the (111) surface of monoclinic zirconia with H dissociation; acetate adsorbed in bidentate bridging mode; H moves to the 2-fc lattice oxygen.
or after transformation 0
Esurf ¼
0
0
Eslab þ ΔEslab N Ebulk 2A
ð2bÞ
where E0slab denotes the total energy of the two-sided supercell after relaxation of one side as described in the Computational Methods section, and ΔE0slab = E0slab Eslab. The surface energies calculated in this work by GGA (Table 1) are in a generally good agreement with the literature data reported for calculations within the local density approximation (LDA) to DFT.10 The values obtained by GGA relative to LDA are 9% and 6% lower for (111) and (111) surfaces, respectively, but 9% higher for the (101) surface. Because of that, the rank of monoclinic zirconia surfaces stability determined by Christensen et al.10 is slightly changed in the present work by placing the (111) surface as the second most stable one after the (111) surface. A variation between the GGA and LDA data is a known problem; its origin has been previously discussed in detail.10 Adsorption of carboxylic acids on the top side of the supercell further stabilizes surfaces by additionally decreasing the surface energy by the value ΔEsurf calculated by formula 3. 0
ΔEsurf ¼
ðEslab00 Eslab m EAcOH Þ A
ð3Þ
where EAcOH denotes the gas-phase energy of AcOH and E00slab is the total energy of the slab with m molecules of AcOH adsorbed on the top, relaxed, side. Surface energies resulting from the adsorption of one molecule of AcOH are shown in Table 1. The order of the monoclinic zirconia surfaces stability with approximately the same concentration of AcOH on the surface (Table 1) remains the same as for the clean surfaces. Adsorption energy expressed per surface area depends, of course, on the surface coverage and the chemical potential of the adsorbed molecules, which is varied with temperature and pressure. Such information is important in determining the stability of surfaces with adsorbed molecules. However, it is outside the scope of the current study, which focuses primarily on the possibility of carboxylic acid enolization on the surface. 3.2. Adsorption of Carboxylic Acids. Calculated adsorption energies of acetic, propionic, and isobutyric acids on the most stable zirconia surfaces are shown in Tables 2 and 3. Only the strongest adsorption sites are shown. The same supercell models for (111), (101), and (111) monoclinic zirconia surfaces previously studied for water adsorption11 have been used
Figure 2. Enolization of acetate on the (111) surface of monoclinic zirconia. (a) Acetate adsorption in bidentate bridging mode; the dissociated H is attached to the 2-fc lattice oxygen. (b) Optimized transition state. (c) Enolized acetate.
in the current work. It has been found that the relative strength of different adsorption sites on the studied surfaces does not depend on the adsorbate. The same rank of surface sites is ascertained by comparing adsorption energies of either acetic acid or water. Details on the structure and location of the strongest adsorption sites on monoclinic zirconia, with water as an example, were described earlier.11,12 Preference for the dissociative form of carboxylic acid adsorption over the molecular form on all zirconia surfaces studied has been determined by DFT computations. The classification of carboxylic acids adsorption as suggested for the formic acid adsorption by Vittadini et al.28 is adopted in the present work. It is found that carboxylic acids should dissociate on all zirconia surfaces preferentially by breaking the HO bond. On monoclinic zirconia (111) and (111) surfaces, the strongest adsorption of acetic acid as well as its homologues is achieved through the bidentate bridging configuration of AcO (Table 2, Figures 1, 2a, 3a) on the same sites as for water.11 On the (101) zirconia surface, the preference for the dissociation of the OH bond (entry 5, Table 2) over the CO bond (entry 3, Table 2) is very small, only 0.7 kcal/mol. One of the most stable configurations of the resulting AcO involves bidentate chelating as shown in Figure 4c, while the bidentate bridging one (Figure 4b) is almost equally probable (entries 4 and 5 in Table 2). Energetically very close to both of them is the dissociation of AcOH to Ac and OH on two adjacent Zr atoms, which becomes possible as a result of the stabilization of the acetyl group by lattice oxygen with the formation of a chelate-like structure (Figure 4a and entry 3, in Table 2). Consequently, a carboxylic acid adsorbed on the zirconia (101) surface can be stabilized in several possible forms, which could potentially lead to different reaction pathways. Adsorption of carboxylic acids on zirconia closely resembles that found on rutile (110), (011), and anatase (001) surfaces.2331 In conclusion, the general trend for the interaction of carboxylic acids with most of the titania and zirconia surfaces is the dissociation of the OH bond and the 16015
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Figure 3. Top view of the catalytic site of acetate enolization on the (111) surface of monoclinic zirconia. (a) Acetate adsorption in the bidentate bridging mode; the dissociated H is attached to the 2-fc lattice oxygen. (b) Optimized transition state of the enolization by the 2-fc lattice oxygen. (c) Product of the enolization by the 2-fc lattice oxygen. (d) Optimized transition state of the enolization by the 3-fc lattice oxygen. (e) Product of the enolization by the 3-fc lattice oxygen.
Figure 4. Acetic acid adsorption on the (101) surface of monoclinic zirconia. (a) OH dissociation and stabilization of the resulted acyl group in the bidentate chelating mode via addition to a lattice oxygen atom. (b) Bidentate bridging mode with H dissociation. (c) Bidentate chelating mode with H dissociation.
bidentate bridging mode of adsorption. The molecular adsorption of carboxylic acids on the anatase (101) surface looks rather as an exception. 3.3. Enolization of Carboxylic Acids. The focus of the present study is the possibility of enolization of carboxylic acids on the surface. Estimation of the activation energy and enthalpy of enolization may help to ascertain whether this process could be a part of the ketonization mechanism. Using (111) and (111) zirconia surfaces as an example, it has been found that the lattice oxygen can readily abstract the hydrogen atom in the R position to the carboxylic group (Scheme 2). Acetic, propionic, and isobutyric acids were studied, representing an increasingly higher degree of R branching. The energy barrier for enolization appears to be low, only 25 kcal/mol and 2933 kcal/mol on (111) and (111) zirconia surfaces, respectively (Table 3). The structure of the enolized form of acetate and the transition state to its formation are shown in Figures 2 and 3.
Hydrogen abstraction on the (111) monoclinic zirconia surface is found to proceed by involving a 2-fold coordinated (2-fc) lattice oxygen (Figure 2b,c). On the (111) zirconia surface, there are two possibilities found, hydrogen abstraction by either 2-fc (Figure 3b,c) or 3-fc lattice oxygen (Figure 3d,e). The geometry of the adsorbed carboxylate favors the abstraction by 3-fc oxygen because of a shorter distance between the lattice oxygen and carboxylate R hydrogen. The energy barrier is low, 2627 kcal/mol, and close to that on the (111) surface, 25 kcal/mol. However, the energy level of the resulting enolization product and the protonated 3-fc oxygen (Figure 3e) is quite high. The reverse reaction has an extremely low energy barrier, 0.12.0 kcal/mol. Therefore, the equilibrium must be significantly shifted toward the starting carboxylate. The enolized product derived from isobutyric acid is slightly more stable because of the well-known effect of the steric hindrance in the kinetic protonation of enolized structures,45 in this case caused by two methyl groups. 16016
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The Journal of Physical Chemistry C Scheme 2. Enolization of Carboxylates on the Zirconia Surface through the Hydrogen Abstraction Transition State
In contrast, hydrogen abstraction by the 2-fc oxygen on the (111) surface generates a low energy product (Figure 3c) with a relatively high barrier for the reverse reaction, 1418 kcal/mol (Table 3). A critically important contribution to the stability of the enolization product in the latter case is made by hydrogen bonding between the newly formed surface OH group and one of the carboxylate oxygen atoms (Figure 3c). However, the deprotonation step by 2-fc oxygen requires 2933 kcal/mol activation energy (Table 3), which makes it less favorable. As a result, none of the enolization pathways on the (111) surface can compete with the low-energy process on the (111) surface. Consequently, enolization should preferentially occur on the (111) surface of the monoclinic zirconia catalyst whenever it is available. Fortunately, it also is the most stable surface and, therefore, the most important one for catalytic applications. Enolization of carboxylic acids on the (111) zirconia surface has 48 kcal/mol higher activation energy and may possibly occur at higher temperatures. It is found that branching at the R position of a carboxylic acid has a very low effect on the energy barrier for enolization on the (111) surface by 2-fc oxygen and on the (111) surface by 3-fc oxygen. By contrast, enolization on the (111) surface by 2-fc oxygen is sensitive to the steric effect, especially with the introduction of the second methyl group (Table 3). All studied deprotonation reactions exhibit a late transition state (Table 3), having the R carbon near the sp2 geometry. It is similar to the well-known sp2-like geometry of the transition state characteristic for the enol protonation to ketones,45 which is a reverse reaction to enolization. The geometry of the developing doublebond center in the enolization reaction becomes increasingly flat as the reaction progresses, which is indicated by decreasing the sum of torsion angles, defined in Table 3, from the starting carboxylate to the transition state and further to the enolized product. A preliminary computational modeling of the next possible steps in the ketonization mechanism has been done to provide an indication for the most favorable reaction pathway. It has been found that arrangement of two carboxylic acids on the same metal atom on the surface is highly unfavorable, with the energy penalty in excess of 90 kcal/mol. Even if the metal atom has a double coordinative unsaturation, a significant steric repulsion between two alkyl groups does exist and results in the energy rise. The concept of the double acid arrangement on the same metal atom and the related requirement for the double coordinative unsaturation of the metal atom in the ketonization mechanism has been widely accepted in the literature37,4648 but may be in need of critical revision. Computational study of a complete ketonization mechanism on the (111) surface of monoclinic zirconia is currently in progress.
4. CONCLUSIONS DFT computations indicate that carboxylic acids are adsorbed on monoclinic zirconia (111) and (111) surfaces in the bidentate
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bridging mode with the OH bond dissociation. Hydrogen preferentially moves to the 2-fold coordinated lattice oxygen. On the (101) surface, three forms of acetic acid adsorption are similar in energy: (1) bidentate chelating with OH bond dissociation; (2) bidentate bridging, also with the OH bond dissociation; and (3) bidentate chelating with dissociation to AcO and OH. Enolization of carboxylic acids on monoclinic zirconia (111) and (111) surfaces proceeds with a relatively low activation energy barrier, which is independent of acid branching on the (111) surface and increases on the (111) surface with the increase of acid branching at the R position. The 2-fold coordinated lattice oxygen on the most stable, (111) surface of monoclinic zirconia is the preferred basic site for hydrogen abstraction and enolization of adsorbed carboxylates.
’ ASSOCIATED CONTENT
bS
Supporting Information. Cartesian Coordinate Files and Crystallographic Information Files for the structures in all figures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (701) 777-5052. Fax: (701) 777-5181. E-mail:
[email protected].
’ ACKNOWLEDGMENT This paper is dedicated to Howard E Zimmerman, Emeritus Professor of Chemistry at the University of Wisconsin—Madison, for his outstanding contributions on reaction mechanisms. The author sincerely thanks Evguenii I. Kozliak (University of North Dakota, Department of Chemistry) for his critical comments and Ted R. Aulich (University of North Dakota, Energy & Environmental Research Center) for his support in obtaining the Materials Studio computational package. Financial support of this research by the U.S. Army Corps of Engineers Construction Engineering Research Laboratory is greatly acknowledged. ’ REFERENCES (1) Reddy, B.; Khan, A. Recent Advances on TiO2-ZrO2 Mixed Oxides as Catalysts and Catalyst Supports. Catal. Rev. - Sci. Eng. 2005, 47, 257–296. (2) Yamaguchi, T. Application of ZrO2 as a Catalyst and a Catalyst Support. Catal. Today 1994, 20, 199–217. (3) Ryshkewitch, E.; Richerson, D. W. Oxide Ceramics; Academic Press: Orlando, 1985. (4) Hannink, R. H.; Kelly, P. M.; Muddle, B. C. Transformation Toughening in Zirconia-Containing Ceramics. J. Am. Ceram. Soc. 2000, 83, 461–487. (5) Benammar, M. Design and Assembly of Miniature Zirconia Oxygen Sensors. Sens. J., IEEE 2004, 4, 3–8. (6) Satsuma, A.; Shimizu, K.; Hattori, T.; Nishiyama, H.; Kakimoto, S.; Sugaya, S.; Yokoi, H. Polytungstate Clusters on Zirconia as a Sensing Material for a Selective Ammonia Gas Sensor. Sens. Actuators, B 2007, 123, 757–762. (7) Isaacs, H. S. Zirconia Fuel Cells and Electrolyzers. Adv. Ceram. 1981, 3, 406–418. (8) Singhal, S. C. Recent Progress in Zirconia-Based Fuel Cells for Power Generation. Sci. Technol. Zirconia V, [Int. Conf.], 5th 1993, 631–651. 16017
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