Langmuir 1999, 15, 3993-3997
3993
Interaction of D2O with CeO2(001) Investigated by Temperature-Programmed Desorption and X-ray Photoelectron Spectroscopy G. S. Herman,*,† Y. J. Kim,‡ S. A. Chambers,† and C. H. F. Peden† Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, and Department of Chemical Technology, Taejon National University of Technology, Taejon, South Korea Received February 1, 1999. In Final Form: March 29, 1999 The interaction of D2O with the CeO2(001) surface was studied with temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). It was found with TPD that D2O desorption occurs in three states with temperatures of 152, 200, and 275 K, which are defined as multilayer D2O, weakly bound surface D2O, and hydroxyl recombination, respectively. O 1s XPS measurements for high D2O exposures, where multilayer water desorption was observed in the TPD, resulted in emission from only the substrate and surface hydroxyls. This is likely due to a nonwetting behavior of D2O on this surface with the formation of nanosized clusters. An analysis of the O1s XPS data indicates that the surface has a hydroxyl coverage of 0.9 monolayers for large water exposures at 85 K. This is consistent with a model in which the polar CeO2(001) surface can be stabilized by a reduction of the dipole in the top layer by the formation of a full monolayer of hydroxyls.
Introduction The influence of ceria (CeO2) on the chemistry in the automotive three-way exhaust catalyst is substantial. Primarily, the oxygen storage properties of ceria allow the catalyst to operate over wider air-to-fuel ratios in automotive applications. However, the effect that ceria has regarding CO oxidation and the water-gas-shift reactions on precious metals can be substantial. In the automobile exhaust stream CO, H2, and hydrocarbons can reduce ceria, while O2, H2O, and NO can oxidized ceria. A recent study has found that water can reoxidize reduced ceria; however, this interaction with water results in a significant reduction in the oxygen storage capacity of the material.1 The water-gas-shift reaction oxidizes CO to CO2 and provides H2 for the reduction of NOx. These two reactions are very important in automotive exhaust catalysis. It has been suggested that the water-gas-shift reaction occurs by a mechanism in which the noble metaladsorbed CO was oxidized by ceria and ceria was then oxidized by water.2 The reduction of ceria by hydrogen occurs by a mechanism in which oxygen is removed at the surface as water, and it is the reduced form of ceria (CeO2-x) that can subsequently be reoxidized by water. Further studies on the interaction of water with ceria surfaces may provide insight into these proposed mechanisms. Theoretically, the CeO2(001) surface is unstable due to the polar nature of the fluorite crystal structure with alternating planes of oxygen and cerium.3 This structure results in a dipole moment perpendicular to the surface for the bulk terminated surface.3 A recent low-energy ion scattering investigation suggested that the CeO2(001) surface was terminated by both oxygen and cerium planes * Corresponding Author. Phone: (509) 376-5220. Fax: (509) 3765106. E-mail:
[email protected]. † Pacific Northwest National Laboratory. ‡ Taejon National University of Technology. (1) Padeste, C.; Cant, N. W.; Trimm, D. L. Catal. Lett. 1993, 18, 305. (2) Bunluesin, T.; Gorte, R. J.; Graham, G. W. Appl. Catal. B 1998, 15, 107. (3) Sayle, T. X. T.; Parker, S. C.; Catlow, C. R. A. Surf. Sci. 1994, 316, 329.
in approximately a 50:50 ratio.4 However, direct recoil spectroscopy results suggested that the CeO2(001) surface was terminated by an oxygen plane with defect densities of a few percent.5 Recent mass spectroscopy of recoiled ions results indicated that the CeO2(001) surface is terminated by an oxygen plane with 50% of the oxygen removed.6 This structural model is in good agreement with one proposed in a prior theoretical study.7 It is the goal of this study to characterize the interaction of water with a high-quality single-crystal CeO2(001) surface. We have used temperature-programmed desorption (TPD) to investigate the desorption states of water and X-ray photoelectron spectroscopy (XPS) to determine the chemical state of the adsorbed water and the substrate. Experimental Section The studies were performed in an ultrahigh vacuum (UHV) system that contains a hemispherical analyzer and a nonmonochromotized X-ray source for XPS, a single pass cylindrical mirror analyzer for Auger electron spectroscopy (AES), a quadrupole mass spectrometer for TPD, optics for low-energy electron diffraction (LEED), and a sputter gun for sample cleaning. This system also has a sample holder that allowed sample temperatures to be varied from 80 to 1600 K by liquid nitrogen cooling and resistive heating. The sample was mounted to a molybdenum plate that can be resistively heated. Temperatures were monitored with a thermocouple that was spot welded to a thin tantalum ring (0.01 in. thick) that was press fitted to the surface of the sample. Based on the position of the water multilayer desorption peak, the temperatures reported here should be close to the actual sample temperature. The high-quality CeO2(001) sample was grown by plasmaassisted molecular beam epitaxy (MBE) on a SrTiO3(001) substrate in a separate UHV system.5 An extensive characterization of the sample was presented elsewhere.5 The substrate diameter was 10 mm with a thickness of 1 mm, and the CeO2(001) (4) Overbury, S. H.; Huntley, D. R.; Mullins, D. R.; Ailey, K. S.; Radulovic, P. V. J. Vac. Sci. Technol. A 1997, 15, 1647. (5) Kim, Y. J.; Gao, Y.; Herman, G. S.; Thevuthasan, S.; Jiang, W.; McCready, D. E.; Chambers, S. A. J. Vac. Sci. Technol. A., in press. (6) Herman, G. S. Phys. Rev. B., in press. (7) Conesa, J. C. Surf. Sci. 1995, 339, 337.
10.1021/la990094u CCC: $18.00 © 1999 American Chemical Society Published on Web 04/30/1999
3994 Langmuir, Vol. 15, No. 11, 1999
Figure 1. Structural models for the nonpolar reconstructions of the CeO2(001) surfaces. The cerium, oxygen, and hydrogen ions are represented by gray, white, and black circles, respectively. The oxygen ions in the third layer are shown as smaller circles with respect to those in the first layer. Model a is a vacancy model with a 1/2 ML of oxygen removed from the topmost layer. Model b is a hydroxylated surface. film was 500 Å thick. The sample was transferred from the MBE system in air and cleaned and ordered by several argon bombardment and annealing cycles. Before performing the experiments, the sample was annealed at 925 K in an oxygen background of 1 × 10-7 Torr and cooled to 575 K before evacuating the oxygen. This preparation resulted in a clean surface as judged by XPS and AES. LEED indicates that the surface was well ordered with a reasonable spot size (not sharp) and a (1 × 1) pattern. The AES and LEED measurements were performed with the sample held between 400 and 500 K. Below this temperature range, AES and LEED measurements could not be performed due to sample charging. The D2O used in these experiments was degassed by repeated cycles of freeze-pump-thaw. Exposures were performed by moving the sample to within 1 mm of a directional doser while the sample was kept at 85 K. A valve was then opened for a certain period of time after which the water was pumped out of the gas manifold and the sample was moved away from the doser. Attempts were made to calibrate the doser to background exposures, but significant amounts of water desorbed from the sample holder when not using the directional doser, preventing an accurate comparison. A cone with an opening diameter of 4.75 mm mounted to the end of the mass spectrometer helped to reduce desorption signals from other parts of the sample holder. The TPD spectra were obtained with a linear ramp rate of 2 K/s. The XPS measurements were performed using Al KR X-rays and an analyzer pass energy of 35.75 eV. The electrons were collected at an emission angle of 48° with respect to the surface. Charging of the sample was compensated by shifting the spectra so that the substrate Ce4+ 3d5/2 shake-down photoemission peak was at 883.2 eV.1
Results and Discussion The termination of the clean and hydroxylated CeO2(001) surfaces is shown in parts a and b of Figure 1, respectively. The cerium, oxygen, and hydrogen ions are represented by gray, white, and black circles, respectively. In Figure 1a, the oxygen ions in the third layer are shown as smaller circles with respect to those in the first layer. The clean surface has 50% of the top layer oxygen removed to eliminate the dipole moment at the surface, as has been previously proposed.6,7 Another method of removing the surface dipole is replacing the oxygen ions in the top layer
Herman et al.
Figure 2. Temperature-programmed desorption spectra from various exposure times of D2O on the CeO2(001) surface at 85 K.
with hydroxyls. In both cases this can be understood by considering the stacking sequence of the material.8 The bulk stacking sequence along the [001] direction can be viewed as 1 ML of oxygen and 1 ML of cerium (1 ML O/1 ML Ce), or as 1/2 ML of oxygen, 1 ML of cerium and 1/2 ML of oxygen (1/2 ML O/1 ML Ce/1/2 ML O). For the former, the stacking sequence itself is polar. For the later, the stacking sequence is nonpolar; however, the surface must have 50% of the oxygen removed (Figure 1a) or 1 ML of hydrogen ions adsorbed (Figure 1b) to prevent a surface dipole moment. Figure 2 shows the D2O TPD data for various exposures of D2O on the CeO2(001) surface held at 85 K. At low exposures, a broad feature at ∼275 K continues to grow with increasing exposures. After a 30 s exposure, a new feature at 200 K appears. Increasing the exposure time to 75 s results in a third state at 170 K, which shifts to 152 K for longer exposures. It appears that the states at 200 and 275 K quickly saturate and that the increase in height for higher exposures was due to the tail on the 152 K state. No signal was observed for D2 desorption for any of the TPD results. The TPD data were collected up to 950 K, and the data were flat and featureless above 400 K. The sample was not reoxidized after each run, and this may account for the presence of some thermally induced surface defects. It has been shown previously that ceria can become reduced by heating above 773 K in a vacuum.9,10 However, the TPD results were very reproducible, suggesting that the substrate surface is not changing during the experimental series. A plot of the water peak area versus water exposure (not shown) gives a straight line that has a y-intercept slightly above zero. The linear behavior indicates that water does not irreversibly decompose on the surface or lead to the production of other desorption products. The nonzero y-intercept was likely due to background adsorption of (8) Barbieri, A.; Weiss, W.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1994, 302, 259. (9) Soria, J.; Martinez-Arias, A.; Conesa, J. J. Chem. Soc., Faraday Trans. 1995, 91, 1669. (10) Pfau, A.; Schierbaum, K. D. Surf. Sci. 1994, 321, 71.
Interaction of D2O with CeO2(001)
Langmuir, Vol. 15, No. 11, 1999 3995
Figure 3. Ce 3d X-ray photoelectron spectrum from the CeO2(001) surface held at 85 K before exposing to D2O.
D2O on the surface during the period after cooling the sample and then heating for the TPD. Similarities in the TPD results are evident when they are compared to other oxygen-terminated surfaces. For example, on the oxygenterminated surface of ZnO(000-1)-O TPD results indicate that water binds weakly to the exposed oxygen anions, with all water desorbing below 250 K11 or with a small quantity desorbing above 300 K from a more defective surface.12 Similar water TPD measurements on FeO thin films grown on platinum indicate that water was weakly bound with desorption occurring below 225 K.13,14 These thin films were determined to be oxygen-terminated FeO(111)-like in a later X-ray photoelectron diffraction study.15 On the basis of prior water TPD studies on oxide surfaces, we assign the features at 152, 200, and 275 K to multilayer water, weakly bound surface water, and recombination of hydroxyls, respectively. X-ray photoelectron spectroscopy results for the Ce 3d emission are presented in Figure 3 for the sample at 85 K. The Ce 3d spectra exhibit features due to shake-up and shake-down processes that have been investigated in great detail.10,16,17 The features due to the Ce3+ initial state are labeled as vl and ul for the 3d5/2 and 3d3/2 states, respectively. The features due to the Ce4+ initial state are labeled as v, vll, and vlll and u, ull, and ulll, where the superscript corresponds to different final states and the v and u correspond to the 3d5/2 and 3d3/2 states, respectively. The binding energies for these features, determined from peak fitting the spectrum in Figure 3 with a nonlinear Shirley type background and asymmetric Doniach-Sunjic contributions to the Gaussian-Lorenzian (G-L), were Eb ) 883.2 (v), 885.5 (vl), 889.1 (vll), 898.9 (vlll), 901.7 (u), 906.2 (ul), 908.4 (ull), and 917.3 eV (ulll). These values are close to those determined previously,1 but there are likely to be errors on the order of (0.5 eV due to the degree of peak overlap and the large number of variables required to fit the data. After exposure of the sample to D2O for 220 s at 85 K (not shown) there was a reduction in Ce 3d
Figure 4. O 1s X-ray photoelectron spectra from the CeO2(001) surface held at 85 K before exposing to D2O (lower) and after exposing to D2O for 220 s (upper).
intensity and an energy shift due to band-bending, but otherwise the Ce 3d spectrum was identical in relative intensity of the features labeled in Figure 3. We have used the method of Shyu et al.16 to obtain quantitative information on the Ce4+/Ce3+ ratio. The authors found that the peak area of the ulll feature divided by the total intensity of the Ce 3d spectrum was linearly related to the Ce4+/(Ce4+ + Ce3+) ratio. For our data, we found that the value of ulll/(Σ(ui + vi)) was 0.135 which corresponds to 100% Ce4+.16 In Figure 4 are the XPS results for O 1s emission with the sample at 85 K before (lower) and after (upper) a 220 s D2O exposure. The open circles are the experimental data, the solid line running through the open circles is the fit to the data, and the solid lines with the solid circles are the G-L peaks. The background for the fitting was a Shirley type. The O1s peak for the clean surface was fit with a single G-L peak with a Gaussian width of 1.60 eV and with a 76% Gaussian component. The binding energy for the substrate O 1s was found to be 530.1 eV, which is the same value as obtained by Pfau et al.,10 but shifted from the values of 530.4, 529.6, and 528.8 eV determined by Mullins et al.,18 Praline et al.,19 and Paparazzo et al.,20 respectively. After a 220 s D2O exposure at 85 K we find that there was a shoulder at slightly higher binding energy for the O 1s XPS spectrum. Using the same fitting parameters as used for the clean surface, we find that it was necessary to include two peaks at 530.1 and 531.6 eV that are due to emission from lattice oxygen and hydroxyls, respectively. This assignment was based on literature values for O 1s emission from hydroxyls on oxide surfaces which vary between 531.2 and 531.6 eV,21-23 and literature values for O 1s emission from water on oxide surfaces which vary between 533.0 and 534.7 eV.21-24 If we use the
(11) Zhang, R.; Ludviksson, A.; Campbell, C. T. Surf. Sci. 1993, 289, 1. (12) Zwicker, J.; Jacobi, K. Surf. Sci. 1983, 131, 179. (13) Vurens, G. H.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1988, 201, 129. (14) Vurens, G. H.; Maurice, V.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1992, 268, 170. (15) Kim, Y. J.; Westphal, C.; Ynzunza, R. X.; Galloway, H. C.; Salmeron, M.; Van Hove, M. A.; Fadley, C. S. Phys. Rev. B 1997, 55, 13448. (16) Shyu, J. Z.; Optto, K.; Watkins, W. L. H.; Graham, G. W.; Belitz, R. K.; Gandhi, H. S. J. Catal. 1988, 114, 23. (17) Fujimori, A. Phys. Rev. B 1983, 28, 2281.
(18) Mullins, D. R.; Overbury, S. H.; Huntley, D. R. Surf. Sci. 1998, 409, 307. (19) Praline, G.; Koel, B. E.; Hance, R. L.; Lee, H.-I.; White, J. M. J. Electron Spectrosc. Relat. Phenom. 1980, 21, 17. (20) Paparazzo, E.; Ingo, G. M.; Zacchetti, N. J. Vac. Sci. Technol. A 1991, 9, 1416. (21) Clarke, N. S.; Hall, P. G. Langmuir 1991, 7, 678. (22) Junta-Rosso, J. L.; Hochella, M. F. Geochim. Cosmochim. Acta 1996, 60, 305. (23) Herman, G. S.; McDaniel, E. P.; Joyce, S. A. J. Electron Spectrosc. Relat. Phenom., in press.
3996 Langmuir, Vol. 15, No. 11, 1999
literature values for the ceria lattice O 1s binding energy,10,18-20 this results in an energy range for this new feature from 530.3 to 531.9 eV, which is still consistent with emission from hydroxyls and not water. For sputtered ceria surfaces at room temperature, an additional O 1s peak at 532.5 eV10 and 533.5 eV19 has been observed. It was suggested that this shifted component of the O 1s was due to hydroxyls bound to Ce3+ defects.10,19 The interaction between hydroxyls and Ce3+ or Ce4+ sites may account for the different binding energies observed between the present and prior studies. At 85 K, the intensity ratio of the hydroxyl to lattice oxygen for our data was 12.9%. After heating the sample to 203 K we find that the ratio was reduced to 10.2%, with identical binding energies after correcting for band bending. At 203 K, all of the molecular water should have desorbed from the surface based on the TPD data. If this O 1s feature was due to water, then a much larger reduction in the relative water to substrate O 1s intensity ratio should be observed after heating the sample. This result confirms that the O 1s feature at 531.6 eV was due to hydroxyls at the surface and not water. An estimation of the surface coverage for hydroxyls can be performed assuming a nonattenuating overlayer.25 Due to the nearly identical energy of the substrate and hydroxyl O 1s emission and identical differential cross sections, the nonattenuating overlayer equation can be simplified to
SOH ) (IOH/Ilattice)(Λe/d) sin θ where SOH is the fractional coverage of hydroxyls, IOH and Ilattice are the O 1s intensities from the hydroxyls and the lattice, respectively, Λe is the attenuation length for O 1s electrons in the substrate, d is the interlayer spacing for the oxygen planes, and θ is the emission angle with respect to the surface. This equation can be reduced to
SOH ) 6.87(IOH/Ilattice) where d equals 2.705 Å, Λe equals 25 Å,20 and θ equals 48°. The error bars on this type of estimate can be on the order of (35%.24 This analysis results in an initial hydroxyl coverage of 0.89 ( 0.31 monolayers (ML) at 85 K and with 0.70 ( 0.25 ML of the hydroxyls remaining on the surface after heating to 203 K. The formation of a hydroxyl on the surface requires dissociation of water to form OD- and D+. The D+ then reacts with a surface oxygen anion forming a second hydroxyl. Therefore, this suggests that 0.45 ( 0.16 ML of water dissociates on the surface at 85 K. The error in this value is likely to be large since the attenuation lengths for ceria have not been experimentally determined and the assumption of a nonattenuating overlayer may not necessarily be correct. However, the 0.9 ML coverage estimate is in good agreement with the structural model suggested in Figure 1b. The formation of the hydroxylated surface can be described by a mechanism in which water binds to the site above the third layer oxygen ion (i.e., the oxygen vacancy) shown in Figure 1a. The water then transfers a proton to the lattice oxygen ion at the surface due to the close proximity between the water and lattice oxygen ion (2.7 Å). This mechanism would then result in the model shown in Figure 1b. For high D2O exposures, TPD indicates that there should be a large concentration of water at the surface due to the observation of the multilayer peak. Comparing the area (24) Hugenschmidt, M. B.; Gamble, L.; Campbell, C. T. Surf. Sci. 1994, 302, 329. (25) Fadley, C. S. Prog. Surf. Sci. 1984, 16, 275.
Herman et al.
Figure 5. O 1s X-ray photoelectron spectra from the CeO2(001) (upper) and Fe3O4(111) (lower) surfaces held at 85 K after background dosing with 3 langmuir of D2O.
of the desorption feature at 275 K to the total area for the 195 s exposure in Figure 2 suggests that O 1s emission from water should be the dominant feature observed in XPS after the lattice oxygen. However, there was no indication of an O 1s component due to water in the XPS data. Several different experiments were performed to determine why the O 1s signal from water was not observed. In Figure 5 are O 1s XPS results for CeO2(001) (upper) and Fe3O4(111) (lower) surfaces held at 85 K and background dosed with 3 langmuir of D2O (300 s at PD2O ) 1 × 10-8 Torr). The experimental data are shown as solid lines, and the individual G-L components to the fit are shown as solid lines through closed circles. Each G-L component is labeled as to the species that contributes to the intensity. The O 1s XPS data for the CeO2(001) surface, shown in the upper portion of Figure 5, were nearly identical to the upper spectrum in Figure 4 (i.e., with no apparent molecular water). An identical 3 langmuir water dose on the Fe3O4(111) surface, shown in the lower portion of Figure 5, resulted in a O 1s component for water with a binding energy of 533.2 eV and a water to substrate O 1s ratio of ∼40%.23 On the basis of our XPS data, the interaction of water with ceria was very different than iron oxide. Furthermore, to determine if water desorption occurs due to the interaction of X-rays, secondary electrons, or local heating we gave the sample a large water dose, performed the XPS measurement, and then performed a TPD. Again, the XPS gave no indication of water, while the TPD gave a large multilayer peak at 150 K approximately 3 times as intense as the largest dose in Figure 2. This experiment suggests that water was stable on the surface even while taking the XPS data. Finally, we took the O 1s XPS data over a 30 eV range in case the water and substrate are electrically isolated from one another (i.e., the band bending observed for the ceria does not affect the position of the water O 1s photoemission peak). We saw no additional features in the spectra after adsorbing water on the surface, other than the presence of hydroxyls. A possible explanation for the lack of the water signal in the XPS data is that the CeO2(001) surface is hydrophobic and results in the formation of three-dimensional islands of water. For the hydroxylated surface, shown in Figure 1b, the nearest-neighbor hydroxyl-hydroxyl distance is 2.7 Å. This value is close to 2.76 Å, which is the nearest-neighbor oxygen bond distance in ice at 90 K;26 (26) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: New York, 1969.
Interaction of D2O with CeO2(001)
however, ice does not form in a unit cell that is commensurate with the CeO2(001) lattice. A comparison between the interaction of water with hexagonal metal surfaces found a strong dependence with the ice-substrate lattice mismatch and water desorption temperature.27 For example, Au(111) surfaces have been found to have a weak interaction with water in which water binds more strongly to itself than the gold substrate resulting in the formation of three-dimensional islands.28 It has been further suggested that water on Au(111) surfaces forms threedimensional spherelike nanoclusters.29 If water does form three-dimensional islands on the CeO2(001) surface, then it would be expected that the water O 1s signal would be reduced due to attenuation for emission from inside the island. The lack of any signal in the XPS due to water suggests that the islands are large compared to the attenuation length of the O 1s photoelectrons. Estimates on the size of the island would be on the order of several hundred angstroms for no water to be detected in the O 1s. Prior XPS experiments performed on oxidized Zn(0001) surfaces indicated that for a 15 langmuir H2O exposure at 77 K the O 1s emission from water was ∼14% of the total intensity.30 For ceria metal, water exposures above 5 langmuir, at 120 K, are required before an oxygen component for adsorbed water was observed in the XPS.31 In both cases, these exposures are large enough that multilayer water should be present in TPD experiments;27 however, TPD was not performed.30,31 It is possible that the formation of nanosized clusters on zinc and cerium surfaces may explain why the presence of water in the O 1s XPS was so low even for such high exposures. Further investigations are required to determine the extent of the interaction of water on the CeO2(001) surface and the formation of nanosized water clusters. On the basis of Ce 3d XPS results we have found that the ceria sample was fully oxidized. However, TPD and (27) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211 and references therein. (28) Kay, B. D.; Lykke, K. R.; Creighton, J. R.; Ward, S. J. J. Chem. Phys. 1989, 91, 5120. (29) Smith, R. S.; Huang, C.; Wong, E. K. L.; Kay, B. D. Surf. Sci. 1996, 367, L13. (30) Au, C. T.; Roberts, M. W.; Zhu, A. R. Surf. Sci. 1982, 115, L117. (31) Koel, B. E.; Praline, G.; Lee, H.-I.; White, J. M.; Hance, R. L. J. Electron Spectrosc. Relat. Phenom. 1980, 21, 31.
Langmuir, Vol. 15, No. 11, 1999 3997
O 1s XPS results indicate the presence of hydroxyls at the surface, which we assign to being bound at oxygen vacancy sites. On the basis of the linear behavior of the water desorption intensity with water exposure, we find that the water does not heal the oxygen vacancy sites after heating the surface. These results are in good agreement with prior studies that indicated that water does not fully oxidize ceria metal26 or reoxidize reduced ceria.1 Our results suggest that a significant reduction of the ceria is required before promotion of the water-gas-shift reaction can occur. For the CeO2(001) surface this can be explained by a model in which the adsorption and desorption of water requires final-state surface terminations that have a zero dipole perpendicular to the surface. Conclusions In conclusion, we have found that there is initially a strong interaction for 1/2 a ML of D2O with the CeO2(001) surface. It was found with TPD that D2O desorption occurs in three states with temperatures of 152, 200, and 275 K, which are defined as multilayer D2O, weakly bound surface D2O, and hydroxyl recombination, respectively. Ce 3d XPS measurements suggest that the ceria surfaces are stoichiometric CeO2. Furthermore, it was found that even for high D2O exposures, where multilayer water desorption was observed in the TPD, only O 1s emission from the substrate and hydroxyl oxygen are observed. This is likely due to a nonwetting behavior of D2O on this surface with the formation of water nanoclusters. Acknowledgment. Pacific Northwest National Laboratory (PNNL) is operated for the U.S. Department of Energy (DOE) by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. This work was supported in part by the U.S. DOE, Environmental Technology Partnership InitiativesVehicles of the Future (PNGV) program. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. DOE Office of Biological and Environmental Research and located at PNNL. LA990094U
