CeO2 thin films supported on α-Al2O3(0 0 0 1) and YSZ(1 0 0)

Surface Science 476 (2001) 9±21

www.elsevier.nl/locate/susc

Reaction of NO on CeO2 and Rh/CeO2 thin ®lms supported on a-Al2O3(0 0 0 1) and YSZ(1 0 0) R.M. Ferrizz a, T. Egami b, G.S. Wong a, J.M. Vohs a,* b

a Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA

Received 21 August 2000; accepted for publication 4 December 2000

Abstract The adsorption and reaction of NO on ceria-based model catalysts was studied using a combination of temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Speci®c systems investigated included: CeO2 (1 1 1), CeO2 /a-Al2 O3 (0 0 0 1), CeO2 /YSZ(1 0 0), Rh/a-Al2 O3 (0 0 0 1), Rh/CeO2 /a-Al2 O3 (0 0 0 1), and Rh/CeO2 / YSZ(1 0 0). The results of this study show that NO does not adsorb on fully oxidized CeO2 surfaces, while on partially reduced CeO2 surfaces NO adsorbs and dissociates. The reaction of NO on Rh supported on a ceria thin ®lm was found to be similar to that for reaction on Rh/a-Al2 O3 (0 0 0 1) and Rh single crystals as long as the surface of the ceria ®lm was fully oxidized. For Rh supported on partially reduced CeO2 , adsorbed oxygen atoms, formed via dissociation of NO, migrated from the Rh to the ceria resulting in oxidation of the surface of the oxide ®lm. The results of this study also demonstrate that interactions at the CeO2 ±YSZ(1 0 0) interface in¯uence the extent of reduction of the ceria ®lm, its thermal stability, and oxygen ion transport properties. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Cerium; Zirconium; Aluminum oxide; Rhodium; Nitrogen oxides; Thermal desorption; X-ray photoelectron spectroscopy

1. Introduction Ceria plays an important role in automotive emissions control catalysts, where it is used to dampen out changes in the partial pressure of oxygen [1±8]. This function relies on the fact that cerium has multiple stable oxidation states and can easily release oxygen under reducing conditions and take up oxygen under oxidizing conditions. In

* Corresponding author. Tel.: +1-215-8986318; fax: +1-2155732093. E-mail address: [email protected] (J.M. Vohs).

current automotive catalyst formulations the redox properties of ceria are enhanced by mixing with zirconia [8±12]. Although the bene®cial effects of adding zirconia are well documented, the mechanism by which zirconia enhances the oxygen storage properties of ceria is still not well understood. In order to provide a better understanding of this phenomenon we have been investigating the reactivity of model systems consisting of thin ceria ®lms supported on single crystal surfaces of yttria-stabilized zirconia (YSZ) and a-Al2 O3 [8,13± 17]. Our previous temperature programmed desorption (TPD) studies of the reaction of CO and C2 H4 on Rh supported on CeO2 /YSZ(1 0 0) and

0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 1 1 0 - 9

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R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

CeO2 /a-Al2 O3 (0 0 0 1) have demonstrated that the underlying support has a signi®cant impact on both the oxidation activity and the reducibility of the ceria layer [14,16,17]. For example, it was found that carbon atoms deposited on the Rh particles via dehydrogenation of adsorbed C2 H4 could be oxidized to CO using oxygen provided by reduction of the ceria support [16]. The temperature at which this reaction takes place is a function of the extent of reduction of the ceria. For Rh on oxidized ceria ®lms, oxidation of surface carbon occurs near 575 K, while on highly reduced ceria this temperature increases to 700 K. Using this probe reaction it was shown that CeO2 ®lms supported on a-Al2 O3 (0 0 0 1) are thermally stable to temperatures in excess of 900 K, while those supported on YSZ(1 0 0) become signi®cantly reduced upon heating to this temperature [17]. Our previous studies have focussed exclusively on characterizing the in¯uence of the YSZ support on the oxidation activity of the ceria thin ®lms. In the work reported here we have extended these studies to include the reduction of NO on both the ceria ®lms and on Rh supported on the ceria ®lms. It should be noted that the interaction of NO with epitaxial CeO2 ®lms on SrTiO3 (1 0 0) and Ru(0 0 0 1) has been previously reported by Overbury et al. [18,19]. In those studies it was demonstrated, based on TPD and soft X-ray photoemission spectroscopy (SXPS) data, that binding of NO to a ceria surface is strongly dependent on the oxidation state of the Ce cations. NO was found not to adsorb on fully oxidized CeO2 surfaces at temperatures above 150 K. In contrast, partially reduced ceria surfaces were active for the adsorption and dissociation of NO. Stable NO species were also formed on these surfaces. In the present study we have used TPD of NO in combination with XPS to assess the oxidation state of surface cerium cations on CeO2 (1 1 1) and ceria thin ®lms supported on both YSZ(1 0 0) and a-Al2 O3 (0 0 0 1). The reaction of NO on Rh supported on the ceria thin ®lms was also investigated. The results of this study provide additional insight into the in¯uence of the underlying oxide support on the extent of reduction and thermal stability of the ceria ®lms.

2. Experimental methods Growth of the ceria thin ®lms, Rh deposition, and the TPD experiments were conducted in a single ultra-high vacuum surface analysis system. This system has a background pressure of 2  10 10 Torr and is equipped with a mass spectrometer (UTI), cylindrical mirror electron energy analyzer (Omicron), ion sputter gun (Physical Electronics), electron gun, quartz crystal ®lm thickness monitor (Maxtek), and metal deposition sources. X-ray photoelectron spectroscopy (XPS) was performed in a separate Physical Electronics 560 XPS analysis system which was equipped with a doublepass cylindrical mirror analyzer and a Mg(Ka) X-ray source. The background pressure in the XPS chamber was 1  10 9 Torr. Ceria thin ®lm samples characterized by XPS were grown in the TPD chamber, then transferred in air to the XPS chamber. The experimental procedures used in this study were similar to those in our previous investigations of the reactivity of ceria thin ®lms [14,16,17]. The a-Al2 O3 (0 0 0 1), YSZ(1 0 0), and CeO2 (1 1 1) substrates were each cleaned via sputtering with 2 keV Ar‡ ions followed by annealing at 800 K for 60 min. This procedure was repeated until the surface was free from impurities as determined by Auger electron spectroscopy (AES). Ceria ®lms were grown on the Al2 O3 (0 0 0 1) and YSZ(1 0 0) substrates by vapor depositing cerium metal in the presence of 1  10 7 Torr of O2 while maintaining a sample temperature of 450 K. The evaporative cerium source consisted of a small tantalum boat ®lled with cerium metal, which could be heated by electron bombardment. Ceria ®lms, 10 monolayers  in thickness, as determined using a quartz (40 A) crystal ®lm thickness monitor, were used in this study. Previous surface X-ray scattering measurements and TEM studies have shown that vapordeposited ceria ®lms are epitaxial on YSZ(1 0 0) with a (1 0 0) surface orientation, while they are polycrystalline on a-Al2 O3 (0 0 0 1) [15,20]. After deposition of the ceria layer, the sample was annealed at 450 K in 1  10 7 Torr of O2 for 15 min to insure complete oxidation of the ceria. In a few experiments, a partially reduced ceria single crystal or partially reduced ceria ®lms were used. These

R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

were produced by either depositing cerium in 10 9 Torr of O2 , or by lightly sputtering with 0.5 keV Ar‡ ions for 15 min. Rhodium metal was deposited on the oxide substrates using an evaporative source consisting of a small Rh wire (99.8%) wrapped around a tungsten ®lament that could be resistively heated. An equivalent Rh coverage of two monolayers was used in this study. The Rh ®lm was annealed at 600 K after deposition to facilitate Rh particle formation. Previous studies indicate that Rh forms three-dimensional particles on a-Al2 O3 and CeO2 substrates [13,21]. An AES spectrum was taken for each model catalyst to con®rm ceria thin ®lm growth and Rh particle deposition. N15 O (99%) , CO (99.9%), and C2 H4 (99.5%) were all obtained from Matheson and used without further puri®cation. Isotopically labeled NO was used in order to allow the mass spectrometer to distinguish between N2 and CO which is present in the background gas in the vacuum chamber. Saturation exposures of the gaseous reactants at 300 K and a heating rate of 4 K/s were used in all TPD experiments.

11

(a)

NO N2

(b)

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N2

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(d)

NO

N2 (x5)

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Temperature (K)

3.1. NO TPD-ceria thin ®lms

Fig. 1. TPD spectra from N15 O-dosed CeO2 /a-Al2 O3 (0 0 0 1) samples. The ceria ®lm for sample (a) was grown in 10 7 Torr O2 , while for sample (b) it was grown in 10 9 Torr O2 . Sample (c) corresponds to sample (b) after one NO TPD run and the ceria ®lm in sample (d) was grown in 10 7 Torr O2 and then lightly sputtered with Ar‡ .

3.1.1. CeO2 (1 1 1) and CeO2 /a-Al2 O3 (0 0 0 1) TPD curves for NO and N2 obtained following exposure of three di€erent CeO2 /a-Al2 O3 (0 0 0 1) samples to NO at 300 K are displayed in Fig. 1. Spectra (a) in this ®gure were obtained from a freshly prepared, fully oxidized CeO2 thin ®lm. For this sample, both the NO and N2 desorption spectra are ¯at, indicating that NO does not adsorb at 300 K. Identical results were obtained from the annealed CeO2 (1 1 1) sample. A NO TPD experiment was also performed after annealing the CeO2 /a-Al2 O3 (0 0 0 1) sample at 1100 K and again NO adsorption was not observed. The lack of NO adsorption on CeO2 (1 1 1) and the fully oxidized CeO2 /a-Al2 O3 (0 0 0 1) sample is consistent with that obtained previously by Overbury et al. [18], who studied the interaction of NO with (0 0 1) oriented CeO2 thin ®lms supported on SrTiO3 -

(0 0 1). In that study it was found that NO does not adsorb on a fully oxidized CeO2 (0 0 1) surface at temperatures as low as 160 K. Spectra (b) and (c) in Fig. 1 were obtained from a reduced CeO2 x /a-Al2 O3 (0 0 0 1) sample which was produced by growing the ceria ®lm in 10 9 Torr O2 rather than the standard 10 7 Torr of O2 . In the ®rst NO TPD experiment with this sample (Fig. 1(b)), two N2 desorption peaks centered at 325 and 360 K were observed. Since NO desorption was not detected, essentially all of the adsorbed NO dissociated and desorbed as N2 . This result is also similar to that reported by Overbury et al. for CeO2 (0 0 1) thin ®lms that had been partially reduced by sputtering with Ar‡ [18,19]. In the second NO TPD run with this CeO2 x /aAl2 O3 (0 0 0 1) sample (Fig. 1(c)), the NO and N2

3. Results

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R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

desorption curves were ¯at indicating that the oxygen provided by NO dissociation during the ®rst TPD run was sucient to fully oxidize the ceria surface. NO TPD experiments were also performed using CeO2 (1 1 1) and CeO2 /a-Al2 O3 (0 0 0 1) samples that had been sputtered with 0.5 keV Ar‡ ions for 15 min. We have previously shown that sputtering preferentially removes oxygen and results in partial reduction of the surface [21]. The TPD results for sputtered CeO2 /a-Al2 O3 (0 0 0 1) are presented in part (d) of Fig. 1. The results obtained from sputtered CeO2 (1 1 1) were similar. Following exposure of the sputtered CeO2 x /a-Al2 O3 (0 0 0 1) sample to NO, only a small amount of N2 , which desorbed in two overlapping peaks centered at roughly 475 and 625 K, was detected during the TPD run. In subsequent TPD runs on both the sputtered CeO2 (1 1 1) and CeO2 /a-Al2 O3 (0 0 0 1) samples, no NO or N2 desorption was detected, once again indicating that the ceria surfaces were oxidized via oxygen provided by NO dissociation. 3.1.2. CeO2 /YSZ(1 0 0) The NO TPD results for CeO2 /YSZ(1 0 0) samples were somewhat di€erent than those from CeO2 /a-Al2 O3 (0 0 0 1). The results of a series of TPD experiments for NO-dosed CeO2 /YSZ(1 0 0) are displayed in Fig. 2. Curve one in this ®gure was obtained following exposure of a freshly prepared CeO2 /YSZ(1 0 0) sample to NO at 300 K, while curves two through seven were obtained in subsequent runs. Note that in run one the sample was heated to only 600 K and in runs two and three it was heated to 750 K. In all subsequent runs the sample was heated to 900 K. The only desorbing species detected in each TPD run in the series was N2 . The initial run (Fig. 2, run 1) with the freshly prepared CeO2 /YSZ(1 0 0) sample exhibits two distinct N2 desorption peaks centered at 360 and 400 K. A smaller, less well resolved peak is also present near 460 K. This result is similar to that obtained from the partially reduced CeO2 x /aAl2 O3 (0 0 0 1) sample, suggesting that the ceria ®lm on the YSZ(1 0 0) support was not fully oxidized. This result is somewhat surprising given that the conditions used to grow the CeO2 ®lm on YSZ-

1

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7

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800

900

1000

Temperature (K) Fig. 2. Series of m/e 30 (N2 ) TPD spectra obtained from N15 Odosed CeO2 /YSZ(1 0 0). The ceria ®lm was grown immediately freshly prior to run 1.

(1 0 0) were identical to those used to grow fully oxidized CeO2 ®lms on a-Al2 O3 (0 0 0 1). As described above, NO adsorption was not observed on the fully oxidized CeO2 /a-Al2 O3 (0 0 0 1) sample. Thus, identical ®lm growth conditions produce slightly reduced CeO2 ®lms on YSZ(1 0 0) and fully oxidized CeO2 ®lms on a-Al2 O3 (0 0 0 1). The results obtained in subsequent NO TPD runs were also unexpected. During the ®rst TPD run oxygen atoms were deposited on the ceria ®lm via dissociation of the adsorbed NO. As was the case for CeO2 x /a-Al2 O3 (0 0 0 1), one would expect the ceria ®lm to become increasingly oxidized with each successive NO TPD cycle. This, however, was not what was observed for the CeO2 /YSZ(1 0 0) sample. The results of the second NO TPD run in the series were nearly identical to those of the ®rst and again indicated that the surface of the ®lm was partially reduced. Heating the ®lm to higher temperatures also appeared to in¯uence the extent of

R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

reduction of the YSZ(1 0 0)-supported CeO2 ®lm. In run three, the ®rst after the sample had been heated to 750 K, the desorption feature centered at 460 K is more pronounced in the N2 TPD spectrum. The intensity of this peak increases further in run 4. As shown by the data for runs 5±7, heating to 900 K had an even more dramatic e€ect on the NO TPD results. The N2 desorption spectra for these runs were nearly identical and contained two broad peaks centered at 470 and 610 K. This spectrum is similar to that obtained from the sputtered CeO2 x /a-Al2 O3 (0 0 0 1) sample, suggesting that the surface contained a high number of oxygen vacancies. Note, however, that these peaks were not observed for CeO2 /a-Al2 O3 (0 0 0 1) samples that had previously been annealed at 1100 K. This result demonstrates that interactions at the ceria±YSZ interface in¯uence the thermal stability of the ceria thin ®lm. Insight into the identity of the surface species that give rise to the various N2 desorption peaks in the TPD spectra can be obtained by comparison to the previous SXPS study by Overbury et al. of the interaction of NO with oxidized and reduced ceria surfaces [19]. In that study four distinct N(1s) peaks centered at 402.6, 400.5, 398.6, and 396.5 eV were observed for reduced ceria surfaces dosed with NO at 300 K. By comparison to the XPS spectra of nitrogen containing inorganic compounds, these peaks were assigned to NO , Na , Nb , and N3 , respectively. NO is produced by charge transfer from the surface to molecularly adsorbed NO and N3 is a surface nitride species. Presumably, the nitride species is formed on highly reduced portions of the surface. The Na and Nb peaks were relatively small and assigned to anionic, atomic N species adsorbed at defect sites. Following NO adsorption on reduced CeO2 surfaces at 300 K, the peaks for NO and N3 tended to dominate the spectra. Heating the sample to 350 K, resulted in the disappearance of the NO peak and a decrease in the intensity of the Na and Nb peaks. The N3 peak persisted up to temperatures in excess of 500 K. Based on these SXPS results, the two low-temperature N2 desorption peaks in the TPD spectra of the NO-dosed CeO2 /YSZ(1 0 0) sample can be assigned to decomposition of adsorbed NO and the high-temperature peaks can

13

be assigned to decomposition of the surface nitride species. 3.2. XPS-CeO2 /a-Al2 O3 (0 0 0 1) YSZ(1 0 0)

and

CeO2 /

XPS was used to assess the extent of reduction of the ceria thin ®lms as a function of pre-treatment conditions. The ceria thin ®lm samples characterized by XPS were grown in the TPD analysis chamber. They were then removed from the TPD system and loaded into the XPS analysis chamber. Thus, these samples were exposed to air prior to analysis. C(1s) spectra indicated that the surfaces of the air-exposed ceria ®lms were covered with carbon. Ce(3d) XPS spectra as a function of temperature for a CeO2 ®lm on the a-Al2 O3 (0 0 0 1) support are displayed in Fig. 3. Spectrum (a) in this ®gure corresponds to a freshly gown ceria ®lm at 300 K. Since this sample was exposed to atmospheric conditions while being transferred to the XPS analysis system, the ceria ®lm should be highly oxidized. Indeed, this spectrum is consistent with that reported previously for nearly stoichiometric CeO2 [22±25]. Note that the spectrum is rather complex and can be resolved into four separate doublets. The peaks labeled u have been assigned to 3d3=2 spin±orbit states, and those labeled v are the corresponding 3d5=2 states [23,25]. The u000 /v000 doublet is due to the primary photoemission process. The u/v and u00 /v00 doublets are shakedown features resulting from transfer of one or two electrons from a ®lled O(2p) orbital to an empty Ce(4f ) orbital. The u0 /v0 doublet has been assigned to photoemission from Ce3‡ cations. Since the XPS spectrum of Ce2 O3 also contains several shakedown features, the Ce(3d) spectrum of partially reduced CeO2 contains a myriad of peaks. It is therefore very dicult to quantify the ratio of Ce3‡ to Ce4‡ in a cerium oxide sample using XPS. Note, however, that the u000 peak is speci®c to Ce4‡ and the intensity of this peak can be used as a qualitative measure of the extent of reduction. As shown in spectrum (b) of Fig. 3, annealing the CeO2 /a-Al2 O3 (0 0 0 1) sample to 750 K produced several changes to the Ce(3d) spectrum, the most noticeable being a decrease in the intensity of

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R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

v

v''' u u'''

v' u'

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(b)

(c)

930

920

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900

890

880

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860

Binding Energy (eV) Fig. 3. Series of photoelectron spectra for a CeO2 /aAl2 O3 (0 0 0 1) sample at (a) 300 K and following heating to (b) 750 K and (c) 900 K. A Shirley type background [32] has been subtracted from the spectra presented in this ®gure.

Fig. 4. Series of photoelectron spectra for a CeO2 /YSZ(1 0 0) sample at (a) 300 K and following heating to (b) 450 K, (c) 600 K, (d) 750 K, and (e) 900 K. A Shirley type background [32] has been subtracted from the spectra presented in this ®gure.

the u000 peak relative to the other lower energy peaks. A further decrease in the intensity of this peak occurred after annealing the sample to 900 K (spectrum (c), Fig. 3). An analogous set of XPS data for a CeO2 /YSZ(1 0 0) sample is displayed in Fig. 4. Note that for this sample the u000 peak undergoes an even more pronounced decrease in intensity with increasing annealing temperature. These results indicate that the ceria ®lms on both the a-Al2 O3 (0 0 0 1) and YSZ(1 0 0) supports undergo some reduction upon heating to 900 K. In Fig. 5 the ratio of the area of the u000 peak to the total area of all of the Ce(3d) peaks is plotted as a function of temperature for both the CeO2 /aAl2 O3 (0 0 0 1) and CeO2 /YSZ(1 0 0) samples. As noted above, a decrease in this ratio corresponds to reduction of a portion of the cerium cations in the ®lm from ‡4 to ‡3. The data for the freshly grown ®lms at 300 K indicate that the CeO2 ®lm

on a-Al2 O3 (0 0 0 1) is slightly more oxidized than that on YSZ(1 0 0). Heating the CeO2 /YSZ(1 0 0) sample to 600 K produced only a small change in peak area ratio, suggesting that the ®lm is stable up to this temperature. Heating to 750 K, however, caused signi®cant reduction of the ceria ®lm and after heating to 900 K the YSZ(1 0 0)-supported ceria ®lm was highly reduced. The data in Fig. 5 also show that annealing the CeO2 /aAl2 O3 (0 0 0 1) sample to high temperature results in reduction of the ceria ®lm. The extent of reduction, however, is less than that observed for the CeO2 /YSZ(1 0 0) sample. Since the samples analyzed by XPS were exposed to air and their surfaces were covered with carbon, it is dicult to make direct comparisons between the XPS and TPD results. It has previously been shown that carbon adsorbed on CeO2 surfaces can react with lattice oxygen at tempera-

R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

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0.12 (a)

0.11

Area Ratio (u'''/Ce3d)

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Temperature (K) 000

Fig. 5. Plot of u /Ce(3d) area ratio vs. temperature for CeO2 /aAl2 O3 (0 0 0 1) ( ) and CeO2 /YSZ(1 0 0) (e).

tures above 550 K forming gaseous CO and CO2 [13±17,21]. Thus, the adsorbed carbon species may act as a reductant upon heating the CeO2 ®lms. The XPS results do suggest, however, that the YSZ(1 0 0)-supported CeO2 ®lms are less thermally stable than those on a-Al2 O3 (0 0 0 1). This conclusion is consistent with the NO TPD results presented above. 3.3. NO TPD-supported Rh 3.3.1. Rh/a-Al2 O3 (0 0 0 1) In order to provide a base case for comparison, TPD spectra for the reaction of NO on Rh supported on a-Al2 O3 (0 0 0 1) were collected. TPD spectra obtained following a saturation dose of NO on a freshly prepared Rh/a-Al2 O3 (0 0 0 1) sample are presented in Fig. 6(a). The NO desorption spectra contain a single NO desorption peak centered at 475 K and a much broader N2

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Temperature (K) Fig. 6. Series of TPD spectra obtained from a N15 O-dosed Rh/ a-Al2 O3 (0 0 0 1) sample. Spectra (a) are from the initial run with a freshly prepared sample, while spectra (b) are from a subsequent run.

peak centered at 650 K. Based on a peak area analysis, the conversion of NO to N2 is estimated to be 74% which is consistent with that obtained for high coverages of NO on Rh single crystal surfaces [26±29]. Oxygen desorption was not observed since the maximum temperature in the TPD run was 900 K, which is less than that required for oxygen desorption from a Rh single crystal. The results of a subsequent NO TPD run on the same sample are presented in Fig. 6(b). In this experiment two NO desorption states were observed, a large peak centered at 460 K and a much smaller and broader peak centered at 660 K. Relative to the fresh sample, the total amount of NO and N2 that desorbed decreased by approximately 75%, and the conversion of NO to N2 dropped to 47%. These changes can be attributed to the presence of adsorbed oxygen atoms produced

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R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

during the ®rst NO TPD experiment and to agglomeration of the Rh particles upon heating. Previous studies have shown that Rh particles supported on CeO2 thin ®lms and single crystal surfaces formed by vapor deposition of Rh, undergo a signi®cant amount of agglomeration upon heating above 750 K [16,21]. Thus, the total Rh surface area is less in the second NO TPD run than in the ®rst. It is has been shown that adsorbed O atoms hinders both NO adsorption and dissociation due to site blocking and repulsive interactions [29]. The high-temperature NO desorption state can be assigned to recombination of adsorbed N and O atoms. This peak is also observed during NO TPD on oxygen pre-covered Rh single crystal surfaces [29]. These results demonstrate that the reaction of NO on the Rh/a-Al2 O3 (0 0 0 1) sample is similar to that reported for Rh single crystal surfaces. Thus, the a-Al2 O3 (0 0 0 1) support is essentially inert and does not in¯uence the reactivity of the Rh.

NO

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N2 1 2 3 7 10

300

3.3.2. Rh/CeO2 /a-Al2 O3 (0 0 0 1) Fig. 7 presents results from a series of NO TPD runs obtained from a freshly prepared Rh/CeO2 /aAl2 O3 (0 0 0 1) sample. During the ®rst two runs, the sample was heated to only 750 K, while in subsequent runs it was heated to 900 K. Data is presented for runs 1, 2, 3, 7, and 10 of the series. In the ®rst run, NO desorbed at 465 K and N2 desorbed in a broad feature between 400 and 750 K with a peak maximum at 590 K. This result is nearly identical to that obtained from the Rh/aAl2 O3 (0 0 0 1) sample. The areas of both the NO and N2 peaks decreased with each successive run in the series. A peak area analysis indicates that the total amount of NO adsorbed decreased by roughly 75% from run 1 to 10. This can again be attributed to a combination of site blocking by adsorbed O atoms and agglomeration of the Rh particles upon heating the sample to 750 and 900 K. For runs 7±10 a second N2 desorption feature is observed at 475 K. It is possible that this peak was present in the earlier runs in the series but could not be resolved from the large N2 peak at 590 K. The conversion of NO to N2 was 85% for run 1, but dropped to 58% for run 10. This decrease in

400

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Temperature (K) Fig. 7. A series of N15 O and N15 2 TPD spectra obtained from a N15 O-dosed Rh/CeO2 /a-Al2 O3 (0 0 0 1) sample. Only spectra for runs 1±3, 7, and 10 are displayed.

the fraction of adsorbed NO which dissociates is consistent with the presence of O atoms on the surface in the later runs. The XPS results indicated that heating the CeO2 /a-Al2 O3 (0 0 0 1) support to 900 K may result in some reduction of the ceria. Thus, in the later runs shown in Fig. 7 the CeO2 may have been partially reduced. Note that in these runs the Rh was also partially covered with adsorbed oxygen. In order to investigate the reaction of NO on a clean Rh/CeO2 /a-Al2 O3 (0 0 0 1) sample that had been previously heated above 900 K, a sample was prepared in the following manner. A CeO2 ®lm was grown on a a-Al2 O3 (0 0 0 1) substrate and then ¯ashed to 1100 K. Rh was then deposited on the pre-annealed CeO2 /a-Al2 O3 (0 0 0 1) support. The results of a NO TPD experiment performed using this sample are presented in Fig. 8, and show that NO desorbed in a narrow peak centered at 465 K,

R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

NO TPD experiments were also performed on a highly reduced Rh/CeO2 x /a-Al2 O3 (0 0 0 1) sample. This sample was reduced by annealing in 1  10 8 Torr of C2 H4 at 650 K for 100 s. We have previously shown that this treatment results in substantial reduction of the CeO2 ®lm [17]. A NO TPD series obtained from this highly reduced Rh/ CeO2 x /a-Al2 O3 (0 0 0 1) sample is presented in Fig. 9. In the ®rst run, NO desorption was not detected and N2 desorbed in a very broad peak spanning from 385 to 780 K. Although this temperature range is similar to that observed for N2 desorption from NO-dosed, sputtered CeO2 x /a-Al2 O3 (0 0 0 1) samples without Rh, the peak intensity is substantially larger. This result suggests that in the case of the reduced Rh/CeO2 x /a-Al2 O3 (0 0 0 1) sample, the N2 desorption feature is due primarily to reaction on the Rh rather than on the ceria. In

NO

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NO

Temperature (K)

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Fig. 8. TPD spectra obtained from an N15 O-dosed freshlyprepared Rh/CeO2 /a-Al2 O3 (0 0 0 1) sample that had been annealed at 1100 K after ceria ®lm growth but prior to Rh deposition.

2

3 4

while N2 desorbed in a broad feature between 425 and 800 K. The N2 feature can be resolved into at least two distinct peaks centered at 500 and 600 K. Additional desorption states may also be present between 625 and 800 K, but cannot be resolved. In this run the conversion of NO to N2 was 80%. The spectra in Fig. 8 closely resemble those for the freshly prepared sample in Fig. 7 (run 1) and those reported in the literature for the reaction of NO on single crystal Rh surfaces [26±29]. This result is consistent with the NO TPD results for CeO2 /aAl2 O3 (0 0 0 1) samples without Rh and suggests that the a-Al2 O3 (0 0 0 1)-supported CeO2 ®lms do not become signi®cantly reduced upon heating to 1100 K. These results also suggest that in the case of the XPS experiments, the observed reduction of the a-Al2 O3 (0 0 0 1)-supported CeO2 ®lm upon heating was most likely due to the presence of adsorbed carbon.

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300

400

500

600

700

800

900

1000

Temperature (K) Fig. 9. A Series of N15 O and N15 2 TPD spectra obtained from N15 O-dosed Rh/CeO2 /a-Al2 O3 (0 0 0 1). The Rh/CeO2 /a-Al2 O3 (0 0 0 1) sample was reduced prior to the TPD series by annealing in C2 H4 .

18

R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

the second run in the series, in addition to a similar broad N2 desorption feature, a small NO desorption peak centered at 440 K is observed. This NO peak increases in intensity in runs 3 and 4. In the latter runs the N2 desorption feature also becomes more distinct and appears to be composed of two overlapping peaks centered at 580 and 680 K. In run 4, roughly 80% of the adsorbed NO dissociated which is almost the same as that on the oxidized Rh/CeO2 /a-Al2 O3 (0 0 0 1) sample. Although the interpretation of this result is complicated by the fact that reactions may be taking place on both the CeO2 and the Rh, the high fraction of NO that dissociates in the latter runs suggests that there is not a build up of adsorbed oxygen on the Rh particles. This indicates that on highly reduced Rh/CeO2 x /a-Al2 O3 (0 0 0 1) NO dissociates on the supported Rh particles forming adsorbed N and O atoms and the O atoms then migrate from the Rh to the ceria to reoxidize the support. 3.3.3. Rh/CeO2 /YSZ(1 0 0) Results of a series of NO TPD experiments obtained using a freshly prepared Rh/CeO2 / YSZ(1 0 0) sample are presented in Fig. 10. In the ®rst three runs in the series the sample was heated to 750 K, while in all subsequent runs it was heated to 900 K. The results for the ®rst four runs are nearly identical: NO desorbed in a single peak centered at 450 K and N2 desorbed in a broad feature centered at 500 K which appears to be composed of at least two overlapping peaks. A much smaller N2 peak is also observed between 340 and 420 K. By comparison to the TPD results obtained from CeO2 /YSZ(1 0 0) this peak can be attributed to reaction of NO on reduced sites on the surface of the ceria. The larger, higher temperature N2 peaks are most likely due to reaction of N atoms on the Rh. The intensity of the NO and N2 peaks remained almost constant in the ®rst four runs. Since adsorbed oxygen blocks NO adsorption and dissociation, this result demonstrates that oxygen atoms produced via dissociation of NO do not remain on the Rh but migrate to the ceria. It is interesting that the N2 desorption temperature from Rh on CeO2 /YSZ(1 0 0) is 100 K lower than that from

NO 1 2 3 4 5 6

N2 1 2 3

4 5 6

300

400

500

600

700

800

900

1000

Temperature (K) Fig. 10. A series of N15 O and N15 2 TPD spectra obtained from N15 O-dosed Rh/CeO2 /YSZ(1 0 0) sample.

Rh/a-Al2 O3 (0 0 0 1) and Rh/CeO2 /a-Al2 O3 (0 0 0 1). This result may also be due to migration of oxygen from the Rh to the ceria. For Rh/CeO2 /YSZ(1 0 0), the N2 desorption temperature is indicative of N atoms desorbing from Rh. In contrast for Rh/aAl2 O3 (0 0 0 1) and Rh/CeO2 /a-Al2 O3 (0 0 0 1), oxygen atoms produced by NO dissociation remain on the Rh and thus the N2 desorption temperature for these samples is indicative of the recombinative desorption of nitrogen from a Rh surface partially covered with oxygen. The results for runs 5 and 6 show that heating the Rh/CeO2 /YSZ(1 0 0) sample to 900 K produced dramatic changes in the NO TPD results. NO desorption was not observed and N2 desorbed over a very broad temperature range extending from 400 K to above 800 K. The area of the N2 peaks also decreased signi®cantly relative to the earlier runs. This decrease is also consistent with agglomeration of the Rh particles upon heating above 750 K. The shape and position of the N2

R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

peaks are similar to those obtained from the reduced Rh/CeO2 /a-Al2 O3 (0 0 0 1) sample. 4. Discussion The TPD and XPS results demonstrate that the underlying oxide support in¯uences the structure, composition, and reactivity of supported ceria thin ®lms. On both CeO2 (1 1 1) and freshly-grown, fully-oxidized, ceria ®lms supported on a-Al2 O3 (0 0 0 1), NO does not adsorb at temperatures above 300 K. On CeO2 (1 1 1) and ceria ®lms on aAl2 O3 (0 0 0 1) that had been partially reduced by sputtering with Ar‡ , however, adsorption and dissociation of NO takes place. During NO TPD with these samples N2 desorbed in two broad peaks centered at 475 and 625 K. This result demonstrates that exposed Ce3‡ cations are active sites for the adsorption and dissociation of NO on ceria. The similarity in the results obtained from CeO2 (1 1 1) and CeO2 /a-Al2 O3 (0 0 0 1) indicates that the a-Al2 O3 (0 0 0 1) support does not in¯uence the chemical properties of the ceria thin ®lm. In contrast to the samples that had been reduced by sputtering, during TPD with NO-dosed CeO2 x ®lms grown in a low partial pressure of oxygen (i.e. 10 9 Torr), N2 desorption occurred between 300 and 400 K. As noted above, the surface species that give rise to the various N2 desorption peaks for the partially reduced samples can be identi®ed by comparison to the SXPS study of Overbury et al. [19]. In that study the production of N2 at temperatures below 400 K was assigned to the decomposition of adsorbed NO species, while the N2 produced at higher temperatures was assigned to decomposition of surface and bulk nitride species. Since nitride species are only formed on highly reduced surfaces, this result indicates that the sputtered CeO2 x surfaces were more highly reduced than those produced by oxidizing in 10 9 Torr of O2 . Unlike CeO2 ®lms on a-Al2 O3 (0 0 0 1), under the conditions used in this study, it was not possible to grow fully oxidized CeO2 ®lms on YSZ(1 0 0). Ceria ®lms on YSZ(1 0 0) that were grown in 10 7 Torr of O2 exhibited NO TPD results similar to those produced from ceria ®lms grown in 10 9

19

Torr of O2 on a-Al2 O3 (0 0 0 1). NO TPD spectra obtained from freshly prepared CeO2 /YSZ(1 0 0) samples contained N2 desorption peaks between 300 and 400 K which are indicative of the presence of Ce3‡ cations on the surface. This result is consistent with the XPS results which also showed that ceria ®lms on YSZ(1 0 0) that had been exposed to atmospheric conditions before analysis were not fully oxidized. In addition to being less oxidized than those on a-Al2 O3 (0 0 0 1), the YSZ(1 0 0)-supported ceria ®lms, were found to be less thermally stable and undergo signi®cant reduction when heated above 750 K. NO TPD spectra obtained from CeO2 / YSZ(1 0 0) samples that had been previously heated to 900 K exhibited two NO desorption features centered at 475 and 625 K which are indicative of the decomposition of cerium nitride species. This result is consistent with that obtained in our previous studies of the reaction of CO and C2 H4 on Rh supported on CeO2 /YSZ(1 0 0) [16, 17]. In those studies it was also observed that ceria ®lms on YSZ(1 0 0) underwent reduction upon heating to 900 K. The trends in the TPD results provide insight into the oxygen transport properties of the ceria ®lms. As noted earlier in the ®rst NO TPD experiment with a partially reduced ceria ®lm on aAl2 O3 (0 0 0 1) (grown in 10 9 Torr O2 ), adsorbed NO reacts to form N2 between 300 and 400 K. As shown in Fig. 1(c), after one TPD run this sample no longer adsorbs NO at 300 K. This indicates that the surface became completely oxidized during the ®rst TPD run. Note, however, that the conditions used to grow this ceria ®lm should result in a uniform extent of reduction throughout the ®lm. This suggests that migration of oxygen from the surface of the ®lm into the bulk is slow, even upon heating to 900 K. This is in sharp contrast to that obtained for the CeO2 /YSZ(1 0 0) sample. The TPD results obtained from a freshly grown ceria ®lm on YSZ(1 0 0) contained N2 peaks at 360 and 400 K which are indicative of a slightly reduced surface. The data from subsequent NO TPD runs were identical as long as the sample was not further reduced by heating to above 750 K. Thus, for CeO2 /YSZ(1 1 1), a single NO TPD run did not result in an oxidized CeO2 surface as was

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R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

the case for reduced CeO2 /a-Al2 O3 (0 0 0 1). Apparently the exchange of oxygen between the surface and the bulk is facile for the ceria ®lms on YSZ(1 0 0). It is possible that the di€erence in reoxidation behavior for the YSZ(1 0 0) and the a-Al2 O3 (0 0 0 1)-supported ceria ®lms is due to structural modi®cations caused by interactions at the CeO2 ±YSZ(1 0 0) interface as discussed below, but other factors such as di€erent ®lm morphologies on the two supports may play a role. The NO TPD results obtained from the samples with Rh particles supported on a CeO2 ®lm were similar to those obtained from Rh/a-Al2 O3 (0 0 0 1) as long as the ceria ®lms were nearly fully oxidized. A lower N2 desorption temperature, was observed for Rh supported on partially reduced ceria ®lms on both YSZ(1 0 0) and a-Al2 O3 (0 0 0 1) substrates. For these samples NO adsorbs dissociatively and the resulting oxygen atoms migrate from the Rh to the ceria at a relatively low temperature. This result provides further evidence for the facile migration of oxygen from Rh to a partially reduced ceria support. The results of this study provide new insight into how interactions at a CeO2 ±YSZ(1 0 0) interface in¯uences the oxygen storage properties of ceria. For CeO2 ®lms supported on YSZ(1 0 0) there is rapid exchange between surface and bulk oxygen. In contrast for CeO2 ®lms supported on aAl2 O3 (0 0 0 1) this exchange appears to be much less facile. Thus one possible explanation for the higher oxygen storage capacity of mixed ceria± zirconias compared to pure ceria is that both bulk and surface oxygen are accessible in ceria±zirconia, while only surface oxygen is accessible in pure ceria. Recent detailed characterization of the structure of polycrystalline ceria and mixed ceria± zirconias provide some clues into the mechanism by which the YSZ support in¯uences the oxygen transport properties in the CeO2 thin ®lm. Pulsed neutron scattering studies of nano-scale ceria powders [30] suggest that high oxygen ion di€usivity originates from the presence of interstitial oxygen defects coexisting with oxygen vacancies. In pure ceria, however, interstitial oxygen ions recombine with vacancies around 1000 K, resulting in a reduction in di€usivity [30]. On the other hand, pulsed neutron scattering studies of mixed

ceria±zirconia nano-particles [31] have shown that interactions between ceria and zirconia maintain the ceria in a slightly reduced state, and interstitial oxygen ions remain, and do not recombine with vacancies, up to high temperatures. These results are consistent with those obtained in the present study which show that interactions at the CeO2 ± YSZ(1 0 0) interface also result in a partially reduced ceria ®lm, and oxygen di€usion remains high even after high temperature treatments. 5. Conclusions The adsorption and reaction of NO on ceria was found to be in¯uenced by the oxidation state of the surface cerium cations. On fully oxidized ceria surfaces NO does not adsorb at 300 K. On slightly reduced surfaces that have exposed Ce3‡ cations NO adsorbs molecularly at 300 K. Upon heating to 400 K these molecularly adsorbed species dissociate resulting in oxidation of the surface and the production of gaseous N2 . On more highly reduced ceria surfaces, NO dissociates and forms surface nitride species that are stable at temperatures up to 400 K. At higher temperatures the nitrides decompose producing gaseous N2 . The reaction of NO on Rh supported on the various ceria thin ®lm samples was similar to that observed for reaction on Rh/a-Al2 O3 (0 0 0 1) and Rh single crystals as long as the surface of the ceria ®lm was fully oxidized. For Rh supported on partially reduced CeO2 , adsorbed oxygen atoms, formed via dissociation of NO, migrated from the Rh to the ceria resulting in oxidation of the surface of the ceria ®lm. The thermal stability and extent of reduction of the ceria thin ®lms was found to depend on the identity of the underlying support. Ceria ®lms grown on a-Al2 O3 (0 0 0 1) in 10 7 Torr of O2 were nearly fully oxidized and appeared to be thermally stable to temperatures in excess of 1000 K. In contrast, ceria ®lms grown on YSZ(1 0 0) in 10 7 Torr of O2 were not fully oxidized suggesting that interactions at the CeO2 ±YSZ interface helps maintain the ceria in a slightly reduced state. The YSZ(1 0 0)-supported CeO2 ®lms were also less thermally stable than those on a-Al2 O3 (0 0 0 1) and

R.M. Ferrizz et al. / Surface Science 476 (2001) 9±21

became signi®cantly reduced upon heating to temperatures in excess of 750 K. Acknowledgements This work was supported by the Department of Energy, Oce of Basic Energy Sciences, (grant no. DE-FG02-96ER14682). Some facilities used in this work were also partially funded by the National Science Foundation through the MRSEC program (grant no. DMR-963298). Special thanks to Adrian Sherrill and Dr. Mark Barteau of the University of Delaware for instruction and use of their XPS analysis system. References [1] K.C. Taylor, Catal. Rev. 35 (1993) 457. [2] A. Trovarelli, Catal. Rev. 38 (1996) 439±520. [3] H.W. Jen, G.W. Graham, W. Chun, R.W. McCabe, J.P. Cuif, S.E. Deutsch, O. Touret, Catal. Today 50 (1999) 309. [4] R.W. McCabe, J.M. Kisenyi, Chem. Industry 15 (1995) 605. [5] J.G. Nunan, H.C. Robota, M.J. Cohn, S.A. Bradley, J. Catal. 133 (1992) 309. [6] M. Shelef, G.W. Graham, Catal. Rev. 36 (1994) 433. [7] H.S. Gandhi, M. Shelef, Stud. Surf. Sci. Catal. 30 (1987) 199. [8] E.S. Putna, T. Bunluesin, X.L. Fax, R.J. Gorte, J.M. Vohs, R.E. Lakis, T. Egami, Catal. Today 50 (1999) 343±352. [9] M. Haneda, K. Miki, N. Katusa, A. Ueno, S. Matsuura, M. Sato, Nihon Kagaku Kaishi 8 (1990) 820. [10] T. Ohatu, Rare Earths 17 (1990) 37. [11] T. Murota, T. Hasegawa, S. Aozasa, H. Matsui, M. Motoyama, J. Alloys Comp. 193 (1993) 298.

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