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A study of niobia deposition on α-Al2O3(0001) and oxidized Al
Applied Surf;ice Science 55 (It1~J2) 135-1.12 Nl~rth-tIolland
~C)~Cl surface
science
A study of niobia deposition on ~r-A1203(0001) and oxidized AI S. R o b e r t s a n d R.J. G o r t c * lhTarttncnt t~l Chemical Engim'ermg. L'nil ,'r~itr o/Pemi~vll area. l'hih~h'll,lm~, l~4 I01O4. U~.4 Received 2 Augur,t l~vl: ~lcccptcd fi~r publication 24 Septemher lttgl
The deposition ill niobia tm oxidized AI fiim~:lntl on :m ,-Al:();(tltllil ) c~'st~+lw:ls ex:imined using Auger elect:on .,pcctrtl~copy ~md high-rc~olutitm TEM. V:~por deposition ~tl niobi;i rc~ultcd in amorphous, t~o-dimen~ion:d films ~hich ~ere stable upon healing up to ;it Iezl,,I t~(l() K in ".~tcuum. 7he prc~cnce ~f niohi;i had [1o mea~u¢cablc cflcct on lh¢ acidity of the samples. Temperature-progr;lmmed desorption of 2-propanol ;rod isopropyl~lmin¢occurred fr, lm shar.,- det.orptl(m lcuturc~ at 185 and 1411 K. respectively on all surfaces examined, indicating that no ;l£1dsitc~ v,ere present on any ol the szlmplcs.An alternate approach to deposition ol niobili ur,ing Nb(('2tl~O) ~ re~ulted in signific~mlcarbon cont~lminution. The i,llplicutio.lS of these results to the lorn~ilti~n of model. ~upportcd-oxid,~ ¢;,t;ll~t~ i~ discussed.
!. Introduction O x i d e - s u p p o r t e d , m e t a l oxides are used as catalysts in scvcral i m p o r t a n t industrial processes including partial oxidation of hydrocarbons, hydrodesulfurization, and a c i d - b a s e reactions, in many cases, the support composition and structure can be i m p o r t a n t for d e t e r m i n i n g the final catalytic properties. This is particularly truc for ~,cid catalysts where the acidity may be much stronger than that of e i t h e r of the pure oxides [1]. Spectroscopic studies suggest that the structure of the catalyst may be affected by the support [2,3] and q u a n t u m calculations indicate that the support may allow more dclocalization of c h a r g e on the s u p p o r t e d metal cations [4]. However, i, d e t a i l e d u n d e r s t a n d i n g of the interactions between two dissimilar oxide p h a s e s is lacking. W e have recently investigated support effects for metal catalysts by studying model catalysts. p r e p a r e d by d e p o s i t i n g metal films on singlecrystal oxides, using s t a n d a r d surface analysis t e c h n i q u c s [5,6]. O u r work showcd that the growth of the m e t a l film, the structure of the m e t a l
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particles, and the a d s o r p t i o n p r o p e r t i e s of the m e t a l can all be affected by the oxide substrate. In this paper, we have e x t e n d e d our studies to s u p p o r t e d oxide films. W e chose to examine niobia on a l u m i n a since this catalyst has been rep o r t e d to be highly acidic [7] and since niobia can form several interesting p h a s e s which a p p e a r to bc influenced by the support [8,9]. Two types of a l u m i n a s u p p o r t s were used: an ~-AI2Os(II(}01) ~inglc crystal and an oxidized AI film which was shown to be p r e d o m i n a n t l y t,-AI20 ~. While we were u n a b l e to observe strong acidity on any of the model catalysts we prepared, we did obserw-• some interesting p r o p e r t i e s for the growth and structure of niobia film~ on alumina. Some of these p r o p e r t i e s may hay,: implications for p r e p a r i n g novel catalyst supports and lor bonding of different p h a s e s to alumina. O u r results also d e m o n s t r a t e some of the p r o b l e m s associated with the p r e p a r a t i o n of model, s u p p o r t e d - o x i d e catalysts.
2. Experimental The e q u i p m e n t and p r o c e d u r e s used in this study have been described previously [5.6], Briefly.
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s. Roberts. R.Z Gorte / Niobia deposition on tr-Al:O~(O001) atul oxidked AI
the TPD and AES measurements were carried out in an ion-pumped, UHV chamber which had a base pressure of ~ 2 × 10- ~o Torr. This system was equipped with a cylindrical mirror analyzer (CMA) for AES, an ion gun for sample cleaning, a quadrupolc mass spectrometer housed inside a desorption cone for measuring TPD, a Nh evaporation source, and a calibrated film-thickness monitor for measuring niobia cvaporation rates. Calibration of the quartz-crystal, film thickness monitor has been discussed previously [5]. An oxidized, polycrystalline AI foil and an aAI20~(0001) single crystal were used as supports. The AI foil was oxidized in 2 × 10 -s Torr of either 02 or H , O at 773 K, and the oxidation was monitored using the A1 Auger peak shift from 68 eV for AI° to 51 eV for AI "~+. The a-A1203(0001) single crystals were purchased from Crystal Systems Inc., and were received without need for polishing or orientation. Both samples were mounted by wrapping the back and edges with a Ta foil, which could be heated or cooled by conduction. The sample temperature was measured using a chromel-alumel thermocouple attached to the back of the alumina supports using a UHV-compatible, ceramic adhesive. The TEM and T E D experiments were carried out on a Philips EM 400T operated at an accelerating voltage of 120 kV and a JEOL 4000EX operated at 400 kV. The a-Al20.~(0001) microscopy specimens were thinned by mechanical g-inding and polishing, dimpled to a thickness of ~ 30 /.tm, and finally ion milled to perforation with the sample held at liquid N 2 temperatures. Thin films of polycrystalline Al20 3 suitable for TEM were removed from the AI foils following oxidation by scribing the foils on one side to remove the oxide layer and floating the specimens on a dilute solution of mercuric chloride. After the Al foil dissolved, the thin films of AI203 left floating on top of the solution were picked up on 3 mm copper grids and washed with distilled water. Electron diffraction experiments showed the structure to be predominantly yAI203. Following preparation, the AI20~ specimens were loaded into the UHV chamber and cleaned using procedures similar to those used for the larger samples. After niobia deposition,
the samples were transferred in air to the microscopes for examination. Firally, adsorption experiments were also performed on y-AI203 from the Engelhard Corp., which had a BET surface area of 180 m2/g. Simultaneous T P D - T G A (thermogravimctric analysis) measurements of 2-propanol were made using a system which has been described in more detail elsewhere [10]. For the purposes of this present paper, it is important to notice only that adsorption was carried out by exposing the sample to ~ l0 Torr of adsorbatc for a few seconds and that desorption was carried out in ~ l0 -7 Torr, using ~ 20 mg of sample spread evenly over the sample pan of the microbalance. The heating rate was l0 K/rain.
3. Results 3.1. Vapor deposition a n d growth
Wc first examined the growth of the niobia films prepared by vapor dep6sition. Nb metal was vaporized in 5 × 10 -8 Torr 02 in front of the film thickness monitor to determine the flux, and the sample was moved in front of the metal source for a specified time. Since the film thickness monitor measures the mass of a film deposited on a quartz oscillator, we assumed that the Nb was completely oxidized to Nb205 in order to calculate the oxide fluxes. While the oxidation of Nb may not be complete under these conditions [11], the calculated flux would not differ significantly if oxidation werc only to NbO. The growth of the film was monitored by AES as a function of coverage on both of the alumina substrates, with the Nb(167 eV) and AI(51 eV) peak intensities as a function of coverage shown in fig. 1. For these experiments, the alumina substrates were held at 300 K. Results for coverages above 1.5 × 1015 N b / c m 2 on a-A1203(0001) are not shown because charging made it difficult to obtain reliable results, a problem which did not occur on the oxidized AI film. Apparently, the presence of approximately one monolayer of niobia on a-AI20~(0001) affects the electronic density of states at the surface, which then results
s. Roherts. R.,L Gorte / Niohla depo.iition rm t~-,4i 2 0 ~(tXX)l l and (ztidtzed AI olo
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Nixl015 Nb/cm21 Fig. I. Normalized AES intensities lot All51 eV) (e) and Nb(lh7 eVI (o) followingdeposition of niobia on T-AI,O~. along with low-coverage data for Nb(167 eV) (+) on aAI20~(0001). The solid line is the calculated curve for the AI(51 eV) peak assuming two-dimensional growth of niobia using parameters given in the text. The dashed line is a best fit to the data fl)r the Al(51 eV) peak followingdeposition of Pt. which is known to fl)rm three-dimenskmal particles at this temperature.
in charging upon exposure to the electron beam. The solid line drawn through the Al(51 eV) data was calculated assuming layer-by-layer growth of niobia. I x Ill j5 N b / c m 2 in a monolayer, a niobia density of 6.27 g / c m 3 (the density of bulk NbO), and a mean free path of 0.75 nm for the Al(51 eV) electrons [12]. While the above assumptions may not be exactly correct, the plot indicates that niobia probably grows in a two-dimensional manner on both aluminas. This conclusion is supported by the comparison with results for Pt on a-AI20.~(0001). It has been shown that Pt grows as three-dimensional particles for deposition at 3(111 K on a-AI2Od(0001) [13]. The dashed line in fig. I gives the data for the Al(51 eV) peak as a function of Pt coverage and demonstrates that the Auger intensity of the substrate decreases m u c h more slowly when three-dimensional growth occurs. In previous studies of Pt overlayers on ZnO and Z r O , [5,6]. i* had been shown that growth of the metal overlayer was two-dimensional, but that the two-dimensional films were unstable upon heating. However, the niobia films on both aluminas were stable to as high as ~ 900 K in vacuum. There were no observable changes in the Auger spectra during this pretreatment. We did observe that, after annealing, it became more difficult to remove all of the niobia on the a-AI2Od(0001) crystal by sputtering. This may indicate migration
of small amounts of Nb into alumina at high temperatures, but mixing may also have occurred during ion bombardment. The structure of niobia films was studied using electron microscopy. Films containing 5 x l(I j5 N b / c m 2 were prepared on the a-Al20~(O001) and T-AI20 3 samples in 5 × 10 " Torr 0 2 as discussed above for the AES measurements. The samples were then oxidized in 5 x (0 -7 Torr 0 2 for 30 min at 750 K before transferring them in air to the microscope. Typical results are given in fig. 2, which shows an HREM image of the niobia-covered, a-Al2Od(0001)crystal. This image was obtained by allowing the transmitted beam and the six equivalent (1120) reflections to pass through the objective aperture. The only features observed are the (1120) lattice fringes of aAl20:t0001) which have a spacing of ~ 0.238 nm. No islands of niobia were visible, even in a through-focus series of imag:s recorded with 0 to - 5 0 0 nm defoeus. The diffraction pattern also showed only the hexagonal array of reflections from ot-AI2Od(0001) and did not give any indication of scattering due to the presence of niobia. This suggests that the niobia overlayer may be amorphous. If three-dimensional niobia islands were formed, they should have been relatively easy to observe because of the differences in the electron scattering properties of Nb compared to Al. Post-microscopy Auger analysis of the specimen indicated that Nb remained on the sample in the vicinity of the microscopic examinations. The results for niobia on y-Al:O.~ were similar. Again, no new features due to niobia were observed, Diffraction patterns showed only a series of concentric rings due to diffraction from polycrystalline T-AI20 3. Small-aperature, selected-area diffraction patterns from single grains of y-AI203 were also measured but, again, no scattering from niobia was observed. As a final test that the niobia films formed by vapor deposition on alumina were amorphous, microscopy experiments were carried out with niobia deposited directly onto copper grids precoated with formvar. The deposition procedure and conditions were identical to that used on the alumina samples except that the film thickness corresponded to l x l0 t7 Nb/em-'. Following de-
s. Roberts, R.J. Gorte / Niobia deposition on t~-AI20¢(O001) and oxidized AI
position, the sample was heated briefly in air at 773 K to remove the formvar. The micrographs of this sample were featureless and the diffraction pattern exhibited only diffuse halos indicative of an amorphous material. This is in agreement with previous work by Liu and co-workers [11] who also observed the formation of amorphous niobia films with similar deposition procedures and
found that these films did not crystallize until heated above 1200 K in vacuum. 3 . 2 N b ( C eH~O).s / c~-AI .O.~(O001)
Conventional niobia catalysts are usually prepared by impregnation of alumina with a solution of Nb(C2HsO)s; therefore, we also attempted to
Fig. 2. (a) HREM image ~-AI:O3({}001)fi~llowingdeposition and oxidationof 5 × I()15 Nb/cm 2.
S. Roberts. R.J, Gorte / Niobia deposition on a-Al.,O.~lIR)Ol) and oxidized AI
1l~,,,93x30 100 200 300 400 500 600 T(I<) Fig. 3. TPD curve obtained following adsorption of Nb(C.~Hy,O)5 on a-Al~O:~((l(K|l). deposit a niobia film on a-AIzO3(0001) by decompeting Nb(C2HsO) 5 on the surface. Niobium ethoxide vapor was exposed to the clean aAIzO3(0001) crystal using a dosing tube directed at the crystal. When the alumina crystal was held at 300 K or higher, there was no adsorption and the surface remained clean. However, multilayer adsorption of the alkoxide did occur when the crystal was held at 100 K. Following adsorption, the temperature of the sample was ramped in an attempt to decompose the alkoxide and leave niobia on the surface. Several of the peaks observed in the mass spectrum of the desorbing products are shown in fig. 3. It was not possible, using the mass spectrometer, to identify all of the products; however, it is clear that a number of products corresponding to various complex processes are formed during the temperature ramp. The sharp features at ~ 130 K are probably due to desorbing alkoxide. Clearly, the m / e = 93 peak at this temperature indicates that niobium is leaving the surface with the species that are desorbing in this temperature region. The peak at m / e = 28 is partially due to desorbing alkoxide but, since m / e = 28 is a major feature for most hydrocarbons and CO, this feature in the T P D also monitors other products. Above 150 K, other products formed from secondary reactions dominate desorption. Following the T P D experiment, AES indicated that the a-A1203(0001) surface was highly contaminated with carbon and we were not able to see any niobium. Removing the carbon required extensive
Ar ion bombardent. This problem with contamination is consistent with other recent studies of organometallics on semiconductor surfaces [14]. Since the alkoxide did not adsorb on the aA1203(0001) crystal or contaminate it at 300 K. we tried to limit surface polymerization reactions of the alkoxide by varying the dosing procedure. Two methods were tried in order to maintain low surface concentrations. First, we dosed the crystal at various intermediate temperatures between 100 K and 300 K. Second, we adsorbed at low temperatures and then heated the crystal to 130 K to remove as much of the excess alkoxide as possible. In each case, however, we obtained either a carbon-contaminated surface or a clean surface. We were unable to deposit clean niobia starting from the alkoxide, and we conclude that deposition of niobia using an alkoxide was not practical for use on the model catalysts in UHV. 3.3. TPD o f simple bases
Our interest in the niobia films was to determine whether we could generate and characterize acid sites on the samples. For this reason, we measured T P D curves for 2-propanol and isopropylamine (1) on the clean a-Al203(0001) crystal, (2) on the a-Al203(0001) crystal with various coverages of niobia, (3) on alumina formed by oxidizing the AI foil in either 02 or H 2 0 , and (4) on alumina with various coverages of niobia. 2Propanol and isopropylamine were chosen for this study since they were found to be useful for examining acid sites in zeolites and other solid acids [15-17]. In figs. 4a and 4b, the T P D results for 2-propanol and isopropylamine are shown for desorption from the clean a-AI20~(0001)crystal following adsorption at 100 K. Only a single, sharp desorption feature was observed at 140 K for isopropylamiue and at 185 K for isopropanol. Increased exposures resulted in larger peaks, and it was not possible to saturate the surface. At very low exposures, the peaks shifted to slightly lower temperatures ( ~ 5 K) but remained very narrow. AES was used to prove that desorption wzs indeed from the oxide surface and not from the sample leads.
S. Rohert.~. R.J. Gorw / Ni,hia th'po.~ition on ~-Ale()~(tlOt)II and oxi~fi=ed AI
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1OO 300 T(K) 500 700 Fig. 4. TPD of (a) isopropylamine and (b) 2-propanol from clean a-AI20~(0(101) folh)wingadsorption at Ill0 K.
The narrow TPD curves are typical of a "'surface explosion", a phenomenon which has been observed by others for adsorbates which form strong, intermolecular hydrogen bonds [18]. When the first molecules dcsorb, they destabilize the remaining layer, which results in the very fast desorption process. The fact that the peaks were narrow at even the lowest exposures suggests that the molecules form islands during adsorption, which implies that interactions between the molecules and the surface are not the dominant factor in desorption. That the oxide surface only weakly influences desorption is further shown by the fact that the desorption curves for each of the substrates were identical. The TPD curves were unchanged by either the type of alumina used or by the presence of niobia. We attempted to introduce hydroxyls onto the surfaces of niobia and oxidized AI by oxidizing in H~O at 300 K and at 600 K but without effect on the TPD results. We also checked for activated adsorption by exposing the surface to the adsorbates at higher temperatures, but we were again unable to detect strong adsorption. On one oxidized AI sample which had Si impurities, we did detect high-temperature features ( > 600 K) for isopropylamine. However,
purc alumina and the niobia-alumina films prepared in this study did not form strong acid sites which could interact with 2-propanol or isopropylamtne. It is probable that the surfaces of the films prepared in this stuoy differ significantly from conventional, high-surface-area aluminas in ways that have yet to be determined. As a demonstration of this, we show in fig. 5 the T P D - T G A curves obtained following 2-propanol adsorption on a conventional 7-AI20 3 obtained from the Engelhard Corp. We found rzpld ?dsorption of 2-propanol at 300 K, and even after prolonged evacuation the coverage remained at ~ 2 × 10 ~4 molecules/cm -~. In TPD, most of this adsorbed 2-oropanol reacted to propene and water between 400 and 500 K, indicating that the alumina contained relatively strong Lewis acid sites. Various other aluminas, including a &-AI,O3 sample and model, o~-AI_,O3 spheres, were also investigated and gave very similar results. In each case, 2-proparol dcsorbed as propene and water from an initial c o v e r a g e close to 2 × 1(1 n4 molecules/cm 2, even though the surface areas of the aluminas differed dramatically. The pretreatme[d conditions used to prepare the aluminas
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3OO 400 500 600 700 T{K) Fig. 5. (a) TGA and (b) TPD curves I~.)r2-propam)l from a high-surface-area sample of "),-Al~O:vMost ~1 the 2-propamq (tu/e = 45) reacts; t~ desorb as propene (m/e = 41 ) and water (m/e = I 8) durungthe TPD experiment.
S. Roherts. R.J. Gorte / Niohia depo~it,m on ,-Al_,t) dO0011 and oxidized AI
had little effect other than to change the surface areas following very harsh pretreatments, Since we estimate that our limits for detection of 2-propanol on the model thin films is near 11)13 molecules/cm 2, it is clear that we should have been able to observe adsorption had the sites on the films been similar to those present on the high-surfacc-arca materials. While differences in the experimental conditions between fiat and porous samples can affect TPD peak temperatures [19,20], it does not seem likely that the extreme differences observed in this study can be explained by this.
4. Discussion There are several interesting conclusions to be reached from this study. First, the growth of niobia films on a'-A1203(00{}l) and oxidized AI is two-dimensional and the two-dimensional structure appears to be rclatively stable. This is in sharp contrast to what i:~ observed with metal films, where three-dimensional particles appear to be favored in most cases. The explanation for this is probably in part kinetic. Metal particles and atoms are far more mobile, allowing migration into the thermodynamically favored positions at lower temperatures. The fact that the niobia films in our study were amorphous to relatively high temperatures demonstrates that mobility is low. However, there also appears to be an attraction between niobia and alumina in our study. The apparent migration of some of the niobia into the a'-AI203(00l)l) crystal suggests this. and evidence for two-dimcnsional films in supportedoxide catalysts gives further indication for this attraction between some dissimilar oxides [I,2], This binding between oxides appears to have practical applications fi~r metal/ceramic interfaces. It is known that metals interact with certain oxidcs more strongly than with others. For example, there appears to be a much stronger attraction for most Group VIII metals with niobia than with alumina [2t]. Prcsumably, if one wanted to fi~rm a strong adhesion between alumina and a metal, the alumina could first bc covered with an interacting oxide before deposit-
ing the metal. This idea appears already to have been used to form Fe/alumina interfaces. It has been demonstrated that Fc films would adhere to alumina much better if the Fe initially del:osited were oxidized [22]. In this case, the iron oxide apparently binds strongly to both the alumina and the Fe, forming more stable films. A disappointing part of this study was that the model films did not exhibit the same types of adsorption sites that are present on convcntional catalysts, and we were not able to model the sites which are formed by ~he presence of niobia. The niobia resulls are not difficult to explain. On conventional catalysts, the supported niobium cations which lead to strong acidity exist in a tetrahedral environment which is ,~Imost certainly different from that of the Nb in the films prepared in this study [21. Also. the a-AI20~(0001) surface is unlikely to have many hydroxyl groups and. therefore, is different from high-surface-area aluminas and is probably nonacidic. However. it is not clear why the alumina films prepared by oxidizing AI behaved so differently from conventional alumina. The structure of the crystallites in the film was that of ~,-AI20 3. While it is possible that thc experimental conditions for the TPD measurements affected the results, it seems more likely that the procedures we used for preparing alumina films did not result in the same types of surface sites as those present on aluminas prepared by conventional means. It may be that oxidation at very low pressures leads to a different structure at the surface and. therefore, different types of sites. Clearly. more work needs to be done to understand alumina films if good model catalysts are to be prepared. Discovering the nature of these differences could lead to a better understanding of acidity in aluminas,
5. Summary Vapor deposition of niobia on alumina at room temperature results in two-dlmensional amorphous films which are m~changed by heating up to 9011 K in vacuum. Attempts to make these films acidic were unsuccessful and suggested that the nature of the alumina substrate is a very
142
S. Roberts. R..L Gorte / Niobia deposition on t,-Al eO flO001) and tztidized AI
i m p o r t a n t f a c t o r in t h e p r o d u c t i o n o f m o d e l , s u p ported-oxide catalysts.
Acknowledgements T h i s w o r k w a s s u p p o r t e d by t h e D O E , Basic E n e r g y S c i e n c e s , G r a n t No. D E - F G 0 3 - 8 5 - 1 3 3 5 0 , S u p p o r t for t h e e l e c t r o n m i c r o s c o p y e x p e r i m e n t s w a s p a r t i a l l y p r o v i d e d by t h e N S F , M R L P r o g r a m , G r a n t N o . D M R 88-19885. W e w o u l d also like to t h a n k Prof. D a v e L u z z i for his h e l p w i t h the electron microscopy experiments.
References [1] S.L. Soled, G.B. McVicker, L.L. Murrell, L.G. Sherman. N.C. Dispenziere, S.L. Hsu and D. Waldrnan. J. Catal. 111 119881 286. [2] J.A. llorsley, I.E. Wachs, J.M. Brown. G.II. Via and F.D. ttardcastle. J. Phys. Chem. 91 (It)',:7) 41114. 3] L. Salwlti. L.E. Makovsky, J.M. Stencel. F.R. Brown and D.M. Hercules, J. Phys. Chem. 85 1t9811 3700. 14] T. Bernholc, J.A. Ilorsley, L.L. Murrell, L.G Sherman and S. Soled, J. Phys. Chem. 91 (Iq87) 1526, [5] S. Roberts and R.J. Gorte, J. Chem. Phys. q3 (1991t) 5337.
(hi S. Roberts and R.J. Gone. J. Phys. Chem.. in press. [7] L.L. Murrell. D.C. Grenoble, C.J. Kim and N.C, Dispenziere, J. Catal. 107 (1087) 463. [g] J.G. Weissman, E,I. Ko and P. Wynblan. J, Catal. 1118 (1987) 383. [91 J.G. Weissman. E.I. Ko and P. Wynblan. J, Vac. Sci. Technol. A 5 (19871 1694. (Ill] T.G.K. Kotke, PhD Thesis. University of Pennsylvania ( 19891, [11] J,R, Ding. X. Zhou. J.N. Bat and B.X. Liu. J. Vae. Sci, Technol. A 8 (199t}) 3349. [12] P.W. Palmberg, G.E. Riach. R.E. Weber and N.C. MacDonald, Ilandhook of Auger Electron Spectroscopy (Physical Electronics Industries, Eden Prairie, MN. 19721. [13] E.I. Ahman and R.J. Gtlrle. J. Catal. l tO 119881 191. [14] A. Forster and tt. Luth. J. Vac. Sci. TechnoL B 7 11989) 7211. II5] T.G.K. Kc,fke. R.J. Gorte and W.E. Farneth, J. Catal. 114 (1988) 34. [16] T.J.G Kofke. R.J. Gortc and G.T. Kokotailu. J. Catal. 116 (1989) 252. [[7] T.J.G. Kolke, R.J. Ciorte and G.T. Kokotailo, Appl. Calal. 54 119891 177. [18] J.L. Falconer and R.J. Madix. Surf. Sci. 4fi (19741 473. [lt)] R.J. Gurte. J. Catal. 75 (19821 164. [211] R.A. Demmin and R,J, Gortc, J. Catal. 911(19841 32. [21] S.J. Tauster. S.C. Fung and R.L. Garten, J. Am. Chem. She. 1(1(]( 19781 170. [22] M.A. Smith and D.P. Polae, Mater. Sci. Eng,, in press.
~C)~Cl surface
science
A study of niobia deposition on ~r-A1203(0001) and oxidized AI S. R o b e r t s a n d R.J. G o r t c * lhTarttncnt t~l Chemical Engim'ermg. L'nil ,'r~itr o/Pemi~vll area. l'hih~h'll,lm~, l~4 I01O4. U~.4 Received 2 Augur,t l~vl: ~lcccptcd fi~r publication 24 Septemher lttgl
The deposition ill niobia tm oxidized AI fiim~:lntl on :m ,-Al:();(tltllil ) c~'st~+lw:ls ex:imined using Auger elect:on .,pcctrtl~copy ~md high-rc~olutitm TEM. V:~por deposition ~tl niobi;i rc~ultcd in amorphous, t~o-dimen~ion:d films ~hich ~ere stable upon healing up to ;it Iezl,,I t~(l() K in ".~tcuum. 7he prc~cnce ~f niohi;i had [1o mea~u¢cablc cflcct on lh¢ acidity of the samples. Temperature-progr;lmmed desorption of 2-propanol ;rod isopropyl~lmin¢occurred fr, lm shar.,- det.orptl(m lcuturc~ at 185 and 1411 K. respectively on all surfaces examined, indicating that no ;l£1dsitc~ v,ere present on any ol the szlmplcs.An alternate approach to deposition ol niobili ur,ing Nb(('2tl~O) ~ re~ulted in signific~mlcarbon cont~lminution. The i,llplicutio.lS of these results to the lorn~ilti~n of model. ~upportcd-oxid,~ ¢;,t;ll~t~ i~ discussed.
!. Introduction O x i d e - s u p p o r t e d , m e t a l oxides are used as catalysts in scvcral i m p o r t a n t industrial processes including partial oxidation of hydrocarbons, hydrodesulfurization, and a c i d - b a s e reactions, in many cases, the support composition and structure can be i m p o r t a n t for d e t e r m i n i n g the final catalytic properties. This is particularly truc for ~,cid catalysts where the acidity may be much stronger than that of e i t h e r of the pure oxides [1]. Spectroscopic studies suggest that the structure of the catalyst may be affected by the support [2,3] and q u a n t u m calculations indicate that the support may allow more dclocalization of c h a r g e on the s u p p o r t e d metal cations [4]. However, i, d e t a i l e d u n d e r s t a n d i n g of the interactions between two dissimilar oxide p h a s e s is lacking. W e have recently investigated support effects for metal catalysts by studying model catalysts. p r e p a r e d by d e p o s i t i n g metal films on singlecrystal oxides, using s t a n d a r d surface analysis t e c h n i q u c s [5,6]. O u r work showcd that the growth of the m e t a l film, the structure of the m e t a l
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particles, and the a d s o r p t i o n p r o p e r t i e s of the m e t a l can all be affected by the oxide substrate. In this paper, we have e x t e n d e d our studies to s u p p o r t e d oxide films. W e chose to examine niobia on a l u m i n a since this catalyst has been rep o r t e d to be highly acidic [7] and since niobia can form several interesting p h a s e s which a p p e a r to bc influenced by the support [8,9]. Two types of a l u m i n a s u p p o r t s were used: an ~-AI2Os(II(}01) ~inglc crystal and an oxidized AI film which was shown to be p r e d o m i n a n t l y t,-AI20 ~. While we were u n a b l e to observe strong acidity on any of the model catalysts we prepared, we did obserw-• some interesting p r o p e r t i e s for the growth and structure of niobia film~ on alumina. Some of these p r o p e r t i e s may hay,: implications for p r e p a r i n g novel catalyst supports and lor bonding of different p h a s e s to alumina. O u r results also d e m o n s t r a t e some of the p r o b l e m s associated with the p r e p a r a t i o n of model, s u p p o r t e d - o x i d e catalysts.
2. Experimental The e q u i p m e n t and p r o c e d u r e s used in this study have been described previously [5.6], Briefly.
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s. Roberts. R.Z Gorte / Niobia deposition on tr-Al:O~(O001) atul oxidked AI
the TPD and AES measurements were carried out in an ion-pumped, UHV chamber which had a base pressure of ~ 2 × 10- ~o Torr. This system was equipped with a cylindrical mirror analyzer (CMA) for AES, an ion gun for sample cleaning, a quadrupolc mass spectrometer housed inside a desorption cone for measuring TPD, a Nh evaporation source, and a calibrated film-thickness monitor for measuring niobia cvaporation rates. Calibration of the quartz-crystal, film thickness monitor has been discussed previously [5]. An oxidized, polycrystalline AI foil and an aAI20~(0001) single crystal were used as supports. The AI foil was oxidized in 2 × 10 -s Torr of either 02 or H , O at 773 K, and the oxidation was monitored using the A1 Auger peak shift from 68 eV for AI° to 51 eV for AI "~+. The a-A1203(0001) single crystals were purchased from Crystal Systems Inc., and were received without need for polishing or orientation. Both samples were mounted by wrapping the back and edges with a Ta foil, which could be heated or cooled by conduction. The sample temperature was measured using a chromel-alumel thermocouple attached to the back of the alumina supports using a UHV-compatible, ceramic adhesive. The TEM and T E D experiments were carried out on a Philips EM 400T operated at an accelerating voltage of 120 kV and a JEOL 4000EX operated at 400 kV. The a-Al20.~(0001) microscopy specimens were thinned by mechanical g-inding and polishing, dimpled to a thickness of ~ 30 /.tm, and finally ion milled to perforation with the sample held at liquid N 2 temperatures. Thin films of polycrystalline Al20 3 suitable for TEM were removed from the AI foils following oxidation by scribing the foils on one side to remove the oxide layer and floating the specimens on a dilute solution of mercuric chloride. After the Al foil dissolved, the thin films of AI203 left floating on top of the solution were picked up on 3 mm copper grids and washed with distilled water. Electron diffraction experiments showed the structure to be predominantly yAI203. Following preparation, the AI20~ specimens were loaded into the UHV chamber and cleaned using procedures similar to those used for the larger samples. After niobia deposition,
the samples were transferred in air to the microscopes for examination. Firally, adsorption experiments were also performed on y-AI203 from the Engelhard Corp., which had a BET surface area of 180 m2/g. Simultaneous T P D - T G A (thermogravimctric analysis) measurements of 2-propanol were made using a system which has been described in more detail elsewhere [10]. For the purposes of this present paper, it is important to notice only that adsorption was carried out by exposing the sample to ~ l0 Torr of adsorbatc for a few seconds and that desorption was carried out in ~ l0 -7 Torr, using ~ 20 mg of sample spread evenly over the sample pan of the microbalance. The heating rate was l0 K/rain.
3. Results 3.1. Vapor deposition a n d growth
Wc first examined the growth of the niobia films prepared by vapor dep6sition. Nb metal was vaporized in 5 × 10 -8 Torr 02 in front of the film thickness monitor to determine the flux, and the sample was moved in front of the metal source for a specified time. Since the film thickness monitor measures the mass of a film deposited on a quartz oscillator, we assumed that the Nb was completely oxidized to Nb205 in order to calculate the oxide fluxes. While the oxidation of Nb may not be complete under these conditions [11], the calculated flux would not differ significantly if oxidation werc only to NbO. The growth of the film was monitored by AES as a function of coverage on both of the alumina substrates, with the Nb(167 eV) and AI(51 eV) peak intensities as a function of coverage shown in fig. 1. For these experiments, the alumina substrates were held at 300 K. Results for coverages above 1.5 × 1015 N b / c m 2 on a-A1203(0001) are not shown because charging made it difficult to obtain reliable results, a problem which did not occur on the oxidized AI film. Apparently, the presence of approximately one monolayer of niobia on a-AI20~(0001) affects the electronic density of states at the surface, which then results
s. Roherts. R.,L Gorte / Niohla depo.iition rm t~-,4i 2 0 ~(tXX)l l and (ztidtzed AI olo
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Nixl015 Nb/cm21 Fig. I. Normalized AES intensities lot All51 eV) (e) and Nb(lh7 eVI (o) followingdeposition of niobia on T-AI,O~. along with low-coverage data for Nb(167 eV) (+) on aAI20~(0001). The solid line is the calculated curve for the AI(51 eV) peak assuming two-dimensional growth of niobia using parameters given in the text. The dashed line is a best fit to the data fl)r the Al(51 eV) peak followingdeposition of Pt. which is known to fl)rm three-dimenskmal particles at this temperature.
in charging upon exposure to the electron beam. The solid line drawn through the Al(51 eV) data was calculated assuming layer-by-layer growth of niobia. I x Ill j5 N b / c m 2 in a monolayer, a niobia density of 6.27 g / c m 3 (the density of bulk NbO), and a mean free path of 0.75 nm for the Al(51 eV) electrons [12]. While the above assumptions may not be exactly correct, the plot indicates that niobia probably grows in a two-dimensional manner on both aluminas. This conclusion is supported by the comparison with results for Pt on a-AI20.~(0001). It has been shown that Pt grows as three-dimensional particles for deposition at 3(111 K on a-AI2Od(0001) [13]. The dashed line in fig. I gives the data for the Al(51 eV) peak as a function of Pt coverage and demonstrates that the Auger intensity of the substrate decreases m u c h more slowly when three-dimensional growth occurs. In previous studies of Pt overlayers on ZnO and Z r O , [5,6]. i* had been shown that growth of the metal overlayer was two-dimensional, but that the two-dimensional films were unstable upon heating. However, the niobia films on both aluminas were stable to as high as ~ 900 K in vacuum. There were no observable changes in the Auger spectra during this pretreatment. We did observe that, after annealing, it became more difficult to remove all of the niobia on the a-AI2Od(0001) crystal by sputtering. This may indicate migration
of small amounts of Nb into alumina at high temperatures, but mixing may also have occurred during ion bombardment. The structure of niobia films was studied using electron microscopy. Films containing 5 x l(I j5 N b / c m 2 were prepared on the a-Al20~(O001) and T-AI20 3 samples in 5 × 10 " Torr 0 2 as discussed above for the AES measurements. The samples were then oxidized in 5 x (0 -7 Torr 0 2 for 30 min at 750 K before transferring them in air to the microscope. Typical results are given in fig. 2, which shows an HREM image of the niobia-covered, a-Al2Od(0001)crystal. This image was obtained by allowing the transmitted beam and the six equivalent (1120) reflections to pass through the objective aperture. The only features observed are the (1120) lattice fringes of aAl20:t0001) which have a spacing of ~ 0.238 nm. No islands of niobia were visible, even in a through-focus series of imag:s recorded with 0 to - 5 0 0 nm defoeus. The diffraction pattern also showed only the hexagonal array of reflections from ot-AI2Od(0001) and did not give any indication of scattering due to the presence of niobia. This suggests that the niobia overlayer may be amorphous. If three-dimensional niobia islands were formed, they should have been relatively easy to observe because of the differences in the electron scattering properties of Nb compared to Al. Post-microscopy Auger analysis of the specimen indicated that Nb remained on the sample in the vicinity of the microscopic examinations. The results for niobia on y-Al:O.~ were similar. Again, no new features due to niobia were observed, Diffraction patterns showed only a series of concentric rings due to diffraction from polycrystalline T-AI20 3. Small-aperature, selected-area diffraction patterns from single grains of y-AI203 were also measured but, again, no scattering from niobia was observed. As a final test that the niobia films formed by vapor deposition on alumina were amorphous, microscopy experiments were carried out with niobia deposited directly onto copper grids precoated with formvar. The deposition procedure and conditions were identical to that used on the alumina samples except that the film thickness corresponded to l x l0 t7 Nb/em-'. Following de-
s. Roberts, R.J. Gorte / Niobia deposition on t~-AI20¢(O001) and oxidized AI
position, the sample was heated briefly in air at 773 K to remove the formvar. The micrographs of this sample were featureless and the diffraction pattern exhibited only diffuse halos indicative of an amorphous material. This is in agreement with previous work by Liu and co-workers [11] who also observed the formation of amorphous niobia films with similar deposition procedures and
found that these films did not crystallize until heated above 1200 K in vacuum. 3 . 2 N b ( C eH~O).s / c~-AI .O.~(O001)
Conventional niobia catalysts are usually prepared by impregnation of alumina with a solution of Nb(C2HsO)s; therefore, we also attempted to
Fig. 2. (a) HREM image ~-AI:O3({}001)fi~llowingdeposition and oxidationof 5 × I()15 Nb/cm 2.
S. Roberts. R.J, Gorte / Niobia deposition on a-Al.,O.~lIR)Ol) and oxidized AI
1l~,,,93x30 100 200 300 400 500 600 T(I<) Fig. 3. TPD curve obtained following adsorption of Nb(C.~Hy,O)5 on a-Al~O:~((l(K|l). deposit a niobia film on a-AIzO3(0001) by decompeting Nb(C2HsO) 5 on the surface. Niobium ethoxide vapor was exposed to the clean aAIzO3(0001) crystal using a dosing tube directed at the crystal. When the alumina crystal was held at 300 K or higher, there was no adsorption and the surface remained clean. However, multilayer adsorption of the alkoxide did occur when the crystal was held at 100 K. Following adsorption, the temperature of the sample was ramped in an attempt to decompose the alkoxide and leave niobia on the surface. Several of the peaks observed in the mass spectrum of the desorbing products are shown in fig. 3. It was not possible, using the mass spectrometer, to identify all of the products; however, it is clear that a number of products corresponding to various complex processes are formed during the temperature ramp. The sharp features at ~ 130 K are probably due to desorbing alkoxide. Clearly, the m / e = 93 peak at this temperature indicates that niobium is leaving the surface with the species that are desorbing in this temperature region. The peak at m / e = 28 is partially due to desorbing alkoxide but, since m / e = 28 is a major feature for most hydrocarbons and CO, this feature in the T P D also monitors other products. Above 150 K, other products formed from secondary reactions dominate desorption. Following the T P D experiment, AES indicated that the a-A1203(0001) surface was highly contaminated with carbon and we were not able to see any niobium. Removing the carbon required extensive
Ar ion bombardent. This problem with contamination is consistent with other recent studies of organometallics on semiconductor surfaces [14]. Since the alkoxide did not adsorb on the aA1203(0001) crystal or contaminate it at 300 K. we tried to limit surface polymerization reactions of the alkoxide by varying the dosing procedure. Two methods were tried in order to maintain low surface concentrations. First, we dosed the crystal at various intermediate temperatures between 100 K and 300 K. Second, we adsorbed at low temperatures and then heated the crystal to 130 K to remove as much of the excess alkoxide as possible. In each case, however, we obtained either a carbon-contaminated surface or a clean surface. We were unable to deposit clean niobia starting from the alkoxide, and we conclude that deposition of niobia using an alkoxide was not practical for use on the model catalysts in UHV. 3.3. TPD o f simple bases
Our interest in the niobia films was to determine whether we could generate and characterize acid sites on the samples. For this reason, we measured T P D curves for 2-propanol and isopropylamine (1) on the clean a-Al203(0001) crystal, (2) on the a-Al203(0001) crystal with various coverages of niobia, (3) on alumina formed by oxidizing the AI foil in either 02 or H 2 0 , and (4) on alumina with various coverages of niobia. 2Propanol and isopropylamine were chosen for this study since they were found to be useful for examining acid sites in zeolites and other solid acids [15-17]. In figs. 4a and 4b, the T P D results for 2-propanol and isopropylamine are shown for desorption from the clean a-AI20~(0001)crystal following adsorption at 100 K. Only a single, sharp desorption feature was observed at 140 K for isopropylamiue and at 185 K for isopropanol. Increased exposures resulted in larger peaks, and it was not possible to saturate the surface. At very low exposures, the peaks shifted to slightly lower temperatures ( ~ 5 K) but remained very narrow. AES was used to prove that desorption wzs indeed from the oxide surface and not from the sample leads.
S. Rohert.~. R.J. Gorw / Ni,hia th'po.~ition on ~-Ale()~(tlOt)II and oxi~fi=ed AI
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1OO 300 T(K) 500 700 Fig. 4. TPD of (a) isopropylamine and (b) 2-propanol from clean a-AI20~(0(101) folh)wingadsorption at Ill0 K.
The narrow TPD curves are typical of a "'surface explosion", a phenomenon which has been observed by others for adsorbates which form strong, intermolecular hydrogen bonds [18]. When the first molecules dcsorb, they destabilize the remaining layer, which results in the very fast desorption process. The fact that the peaks were narrow at even the lowest exposures suggests that the molecules form islands during adsorption, which implies that interactions between the molecules and the surface are not the dominant factor in desorption. That the oxide surface only weakly influences desorption is further shown by the fact that the desorption curves for each of the substrates were identical. The TPD curves were unchanged by either the type of alumina used or by the presence of niobia. We attempted to introduce hydroxyls onto the surfaces of niobia and oxidized AI by oxidizing in H~O at 300 K and at 600 K but without effect on the TPD results. We also checked for activated adsorption by exposing the surface to the adsorbates at higher temperatures, but we were again unable to detect strong adsorption. On one oxidized AI sample which had Si impurities, we did detect high-temperature features ( > 600 K) for isopropylamine. However,
purc alumina and the niobia-alumina films prepared in this study did not form strong acid sites which could interact with 2-propanol or isopropylamtne. It is probable that the surfaces of the films prepared in this stuoy differ significantly from conventional, high-surface-area aluminas in ways that have yet to be determined. As a demonstration of this, we show in fig. 5 the T P D - T G A curves obtained following 2-propanol adsorption on a conventional 7-AI20 3 obtained from the Engelhard Corp. We found rzpld ?dsorption of 2-propanol at 300 K, and even after prolonged evacuation the coverage remained at ~ 2 × 10 ~4 molecules/cm -~. In TPD, most of this adsorbed 2-oropanol reacted to propene and water between 400 and 500 K, indicating that the alumina contained relatively strong Lewis acid sites. Various other aluminas, including a &-AI,O3 sample and model, o~-AI_,O3 spheres, were also investigated and gave very similar results. In each case, 2-proparol dcsorbed as propene and water from an initial c o v e r a g e close to 2 × 1(1 n4 molecules/cm 2, even though the surface areas of the aluminas differed dramatically. The pretreatme[d conditions used to prepare the aluminas
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3OO 400 500 600 700 T{K) Fig. 5. (a) TGA and (b) TPD curves I~.)r2-propam)l from a high-surface-area sample of "),-Al~O:vMost ~1 the 2-propamq (tu/e = 45) reacts; t~ desorb as propene (m/e = 41 ) and water (m/e = I 8) durungthe TPD experiment.
S. Roherts. R.J. Gorte / Niohia depo~it,m on ,-Al_,t) dO0011 and oxidized AI
had little effect other than to change the surface areas following very harsh pretreatments, Since we estimate that our limits for detection of 2-propanol on the model thin films is near 11)13 molecules/cm 2, it is clear that we should have been able to observe adsorption had the sites on the films been similar to those present on the high-surfacc-arca materials. While differences in the experimental conditions between fiat and porous samples can affect TPD peak temperatures [19,20], it does not seem likely that the extreme differences observed in this study can be explained by this.
4. Discussion There are several interesting conclusions to be reached from this study. First, the growth of niobia films on a'-A1203(00{}l) and oxidized AI is two-dimensional and the two-dimensional structure appears to be rclatively stable. This is in sharp contrast to what i:~ observed with metal films, where three-dimensional particles appear to be favored in most cases. The explanation for this is probably in part kinetic. Metal particles and atoms are far more mobile, allowing migration into the thermodynamically favored positions at lower temperatures. The fact that the niobia films in our study were amorphous to relatively high temperatures demonstrates that mobility is low. However, there also appears to be an attraction between niobia and alumina in our study. The apparent migration of some of the niobia into the a'-AI203(00l)l) crystal suggests this. and evidence for two-dimcnsional films in supportedoxide catalysts gives further indication for this attraction between some dissimilar oxides [I,2], This binding between oxides appears to have practical applications fi~r metal/ceramic interfaces. It is known that metals interact with certain oxidcs more strongly than with others. For example, there appears to be a much stronger attraction for most Group VIII metals with niobia than with alumina [2t]. Prcsumably, if one wanted to fi~rm a strong adhesion between alumina and a metal, the alumina could first bc covered with an interacting oxide before deposit-
ing the metal. This idea appears already to have been used to form Fe/alumina interfaces. It has been demonstrated that Fc films would adhere to alumina much better if the Fe initially del:osited were oxidized [22]. In this case, the iron oxide apparently binds strongly to both the alumina and the Fe, forming more stable films. A disappointing part of this study was that the model films did not exhibit the same types of adsorption sites that are present on convcntional catalysts, and we were not able to model the sites which are formed by ~he presence of niobia. The niobia resulls are not difficult to explain. On conventional catalysts, the supported niobium cations which lead to strong acidity exist in a tetrahedral environment which is ,~Imost certainly different from that of the Nb in the films prepared in this study [21. Also. the a-AI20~(0001) surface is unlikely to have many hydroxyl groups and. therefore, is different from high-surface-area aluminas and is probably nonacidic. However. it is not clear why the alumina films prepared by oxidizing AI behaved so differently from conventional alumina. The structure of the crystallites in the film was that of ~,-AI20 3. While it is possible that thc experimental conditions for the TPD measurements affected the results, it seems more likely that the procedures we used for preparing alumina films did not result in the same types of surface sites as those present on aluminas prepared by conventional means. It may be that oxidation at very low pressures leads to a different structure at the surface and. therefore, different types of sites. Clearly. more work needs to be done to understand alumina films if good model catalysts are to be prepared. Discovering the nature of these differences could lead to a better understanding of acidity in aluminas,
5. Summary Vapor deposition of niobia on alumina at room temperature results in two-dlmensional amorphous films which are m~changed by heating up to 9011 K in vacuum. Attempts to make these films acidic were unsuccessful and suggested that the nature of the alumina substrate is a very
142
S. Roberts. R..L Gorte / Niobia deposition on t,-Al eO flO001) and tztidized AI
i m p o r t a n t f a c t o r in t h e p r o d u c t i o n o f m o d e l , s u p ported-oxide catalysts.
Acknowledgements T h i s w o r k w a s s u p p o r t e d by t h e D O E , Basic E n e r g y S c i e n c e s , G r a n t No. D E - F G 0 3 - 8 5 - 1 3 3 5 0 , S u p p o r t for t h e e l e c t r o n m i c r o s c o p y e x p e r i m e n t s w a s p a r t i a l l y p r o v i d e d by t h e N S F , M R L P r o g r a m , G r a n t N o . D M R 88-19885. W e w o u l d also like to t h a n k Prof. D a v e L u z z i for his h e l p w i t h the electron microscopy experiments.
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