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Fundamentals of gas-surface interactions on metal oxides
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Applied Surface Science 72 (1993) 277-284 North-Holland
applied
surface s c i e n c e
Fundamentals of gas-surface interactions on metal oxides * V i c t o r E. H e n r i c h * * Department of Applied Physics, Yale University, New Haven, CT 06520, USA and P.A. Cox Inorganic Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QR, UK Received 2 February 1993; accepted for publication 7 April 1993
In an attempt to understand the fundamental ways in which molecules interact with metal-oxide surfaces, chemisorption has been studied on single-crystaloxide samples. Chemisorption behavior is determined both by the electronic configuration of surface cations and by the geometric structure of the surface. Stoichiometric, well ordered oxide surfaces are, for the most part, relatively unreactive (although striking exceptions exist). However, point defects, which consist predominantly of oxygen vacancies, significantly change the surface electronic structure on most oxides and are the active sites for many types of chemisorption. The interaction between 0 2 and transition-metal-oxide surfaces is determined primarily by cation electronic structure, while the dissociation of H20 seems to be promoted by particular structural features at defect sites and is less sensitive to electronic structure. For organic molecule chemisorption, a general feature of oxide surfaces is the rather facile breaking of C-H bonds, but not of C-C bonds; this is important in selective oxidation catalysis.
I. Introduction D u r i n g the past few years surface scientists have increasingly t u r n e d their a t t e n t i o n to m a t e rials that are of e n v i r o n m e n t a l i m p o r t a n c e . O n e aspect of that work has consisted of studies of g a s - s u r f a c e i n t e r a c t i o n s o n well c h a r a c t e r i z e d surfaces. T h e vast majority of that work has b e e n p e r f o r m e d o n m e t a l oxides, a n d this p a p e r att e m p t s to s u m m a r i z e some of the i m p o r t a n t p r i n ciples as well as to p r e s e n t specific results. D u e to space limitations, discussion will be c o n f i n e d to the results of m e a s u r e m e n t s o n single-crystal
* Part of the Proceedings of the Environmental Interfaces Topical Conference held as part of the 39th Annual Symposium of the American Vacuum Society. ** Contact author. Tel.: (203) 432-4399; Fax: (203) 432-4283.
samples. F o r a m o r e c o m p l e t e t r e a t m e n t of this topic, the r e a d e r is r e f e r r e d to ref. [1]. T h e types of g a s - s u r f a c e i n t e r a c t i o n that occur o n metal-oxide surfaces differ significantly from those o n metals. T h e relatively ionic n a t u r e of oxides leads to a p r e d o m i n a n c e of acid-base, or d o n o r - a c c e p t o r , interactions. Surface cations, which act as Lewis-acid sites, may interact with d o n o r molecules such as H 2 0 t h r o u g h a combin a t i o n of i o n - d i p o l e a t t r a c t i o n a n d orbital overlap. Similarly, oxide ions act as basic sites, interacting with acceptors such as H ÷. O n e of the most c o m m o n dissociative reactions o n metaloxide surfaces is the d e p r o t o n a t i o n of a n adsorbate to p r o d u c e surface hydroxyl groups, which are almost always p r e s e n t o n polycrystalline oxides [2]. O t h e r types of heterolytic dissociation (which result in a d s o r b e d ions) may be favored,
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even for molecules that would dissociate homolytically (that is, into neutral radicals) on metals. Another important feature of molecule-oxide interactions is the involvement of lattice oxygens. They are essentially basic towards acidic molecules such as CO 2 (forming surface CO 2- in that case). But in addition to their acid/base properties, many oxides are able to perform selective oxidation reactions. Oxygen may be added to an adsorbate in the form of a neutral O atom, accompanied by a corresponding reduction of the substrate. This is one type of oxidation/reduction reaction that can take place on metal-oxide surfaces. It might result in electrons that are free carriers, but more often it will lead to a localized decrease in the oxidation state of metal cations at the surface. In a catalytic cycle, re-oxidation takes place by filling of surface oxygen vacancies by dissociative adsorption of 0 2 . Oxidation/reduction reactions can also occur in which only electrons are transferred between the surface and the adsorbate. An example of such a reaction is the dissociative adsorption of CI2, which results in two adsorbed C l - ions and the attendant removal of two electrons from the substrate. An important concept in the reactivity of oxide surfaces is coordinative unsaturation. The coordination of metal and oxygen ions at surface sites is, if course, lower than in the bulk. Coordinatively unsaturated ions can act as acidic or basic adsorption sites, but their bonding to the lattice is also weaker than that of coordinatively saturated ions; thus they may be more easily removed, as in oxidation reactions. Different surfaces exhibit different degrees of coordinative unsaturation, but in general it is rather small for the most stable low-index faces, which explains both their stability and general lack of reactivity. Step and defect sites necessarily exhibit a greater degree of coordinative unsaturation. This increases their acidic and basic properties and may lead to greater reactivity. Such sites may also have different electronic properties; for example, oxygen may exist as O - and metal ions may exist in a lower oxidation state than on the perfect surface. Such sites may promote oxidation or reduction reactions more readily. Chemisorption measurements on well charac-
terized surfaces are almost always performed at very low pressures under essentially U H V conditions; that is true of all of the experimental results to be discussed in this paper. However, environmentally relevant reactions almost always occur at or near atmospheric pressure. One must be careful in extrapolating the results of low-pressure experiments to higher pressures. At high pressures the surface generally has a much higher steady-state coverage of adsorbed species than at low pressure. At high surface coverages the way in which a species is adsorbed may well be different than at low coverage, resulting in types of reaction that are not possible at low pressures and coverages. Another problem in interpretation can occur because stoichiometric low-index faces of oxides are often rather unreactive, and chemisorption, especially when dissociative, is promoted by surface defects and steps. Since small concentrations of surface defects are difficult or impossible to detect by most surface-science techniques, a surface that appears to be stoichiometric and well ordered and shows almost no chemisorption under surface-science conditions may be reactive at higher pressures even though the reaction takes place at a small fraction of surface sites.
2. General features of the adsorption of environmentally relevant molecules The interaction of 0 2 with metal oxides is important in many catalytic applications. Oxygen is a powerful electron acceptor and can be reduced via several routes: O 2 + e - - ~ 0 2 ; 0 2 + 2e-~O2-'2 , O2+2e-~20-; or O 2 + 4 e - ~ 2 0 2 - . There is ESR evidence in many polycrystalline studies for the existence of surface O~ and O - species. On single crystals the adsorbed species may sometimes be inferred from UPS, but frequently they are unknown. Stoichiometric surfaces of n-type semiconductors such as ZnO and SnO 2 are generally assumed to adsorb O 2 molecularly, although weakly; dissociative adsorption is promoted by defects such as oxygen vacancies on reduced surfaces of both transition-metal and non-transition-metal oxides.
V.E. Henrich, P.A. Cox / Fundamentals of gas-surface interactions on metal oxides
H 2 0 has a large dipole moment and lone-pair electrons, making it a good donor. Molecular adsorption occurs by acid-base interaction with surface metal ions, although this may be quite weak on defect-free low-index surfaces. Stronger adsorption occurs at steps and defects and is often dissociative: H 2 0 q- O 2----~ 2 O H - . This reaction is common on polycrystalline surfaces, so that they are frequently covered with hydroxyl groups [2]. After prolonged exposure of very basic oxides such as BaO and some high-Tc oxide superconductors to H 2 0 , hydroxide formation may occur and extend into the bulk of the solid. H 2 is a non-polar molecule having a very low polarizability and very weak donor or acceptor properties. Thus H E in general only weakly physisorbs. The H - H bond is very strong, and, although dissociative chemisorption takes place on many metals, it is only possible on the most reactive oxide surfaces. CO has a small dipole moment and is only a weak donor. Its bonding to surfaces depends upon the "back-donation" of electrons from surface cations into the antibonding ~r-orbital; that is, it acts simultaneously as a donor and an acceptor. Thus the strength of bonding on metal oxides depends upon the availability of electrons at the surface, so adsorption will be favored at cations having relatively low oxidation states. CO frequently seems to react with pre-adsorbed or lattice oxygen to give CO 2, which may further react to form carbonate. CO 2 can act either as a weak donor or an acceptor. The common reaction on metal oxides is an acid-base interaction to give surface carbonate, CO32-. On BaO and high-To oxide superconductors, this may proceed to yield bulk carbonate; indeed, oxides such as YBaECUaO7_y may be covered with a layer of BaCO 3. Although no experimental studies have been reported for the interaction of N 2 with metal oxides, any reaction would be expected to be extremely weak. A variety of types of adsorption has been observed for the interaction of organic molecules with metal-oxide surfaces. However, a general feature of oxide surfaces, which is important in selective oxidation catalysis, is the rather facile
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breaking of C - H bonds, but not of C - C , bonds. Nevertheless, many studies report a proportion of "deep oxidation" products (CO and CO z) and other species that can only be formed by breaking C - C bonds.
3. Chemisorption on non-transition-metal oxides
The chemisorption of molecules on non-transition-metal oxides, whose cation valence electrons are s- or p-orbitals, differs from that on transition-metal oxides, where the valence electrons are d-orbitals, in that non-transition-metal cations can usually exist in only one valence state. (The exception is Sn, which can exist as either Sn 2 ÷ or Sn4+.) The addition or removal of electrons from cations in non-transition-metal oxides requires so much energy that it does not usually occur during chemisorption. As for all types of metal oxide, the lattice O ions have very nearly an 0 2- 2p 6 closed-shell configuration, and, since the ionization energy for an O 2p electron is several eV, the transfer of charge from O z- ions to adsorbates is also unlikely. However, surface O ions may form molecular complexes with adsorbates, and in some cases the adsorbate may abstract lattice O ions from the surface. It is also possible for O - ions to exist at surface defects, and such ions are generally very active in chemisorption. The only stable surface of the rocksalt oxide MgO is (100). Nearly all work shows that the defect-free surface is quite inert. However, chemisorption of some molecules is promoted by the defects that are present on etched and facetted surfaces. Cleaved or annealed MgO(100) surfaces studied in U H V do not adsorb H z O at room temperature, but ion-bombarded surfaces exhibit features characteristic of O H - , presumably adsorbed at surface defect sites [2]. When cleaved MgO(100) surfaces are exposed to air, however, a high density of point defects is formed due to reaction with water vapor [4]. MgO(100) is quite inert towards most other molecules, although there has been one experimental report of CO 2 adsorbing at surface defect sites to yield CO ] - [3]. In addition, methanol, formic acid, acetic acid, formaldehyde and methyl formate
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have been found to interact dissociatively with MgO(100) [3,5-7]. Chemisorption on MgO doped with aliovalent cations is more complex than on stoichiometric MgO, since species such as O can then be stabilized [8]. Chemisorption on ZnO is important both in catalytic and gas-sensor applications, and ZnO is thus the most extensively studied of all the nontransition-metal oxides. The only clear conclusion that can be drawn from all of the work that has been performed is that the chemisorption behavior of Z n O is extremely complicated. Several stable polar and non-polar surfaces exist, as do acidic Zn and basic O sites. Virtually all types of chemisorption have been observed on ZnO. Also, many of the results obtained conflict with one another and with the expectations of simple models. Molecular 02 interacts only weakly with all ZnO surfaces. Below room temperature only physisorption occurs. Between 300 and 650 K, chemisorption involving charge transfer from the surface to the molecule results in adsorbed 0 2 , although the coverage on the surface is very small [9,10]; adsorption clearly occurs at defect sites. Steps on ZnO surfaces have also been found to be relatively more reactive for 0 2 adsorption, although the total surface coverage of adsorbed 0 2 is still no more than a few % of a monolayer [11]. Thus the O 2 - Z n O interaction is weak even for stepped surfaces. H 2 0 does not adsorb on annealed ZnO surfaces for temperatures above 130 K, so the interaction is clearly weak [12]. H 2 essentially does not interact with ZnO surfaces. At room temperature and above, both CO and CO 2 interact only weakly with ZnO, although changes in surface conductivity, which are a very sensitive measure of adsorption, have been observed [13]. There have been a great many studies of the interaction of organic molecules with ZnO surfaces; however, they are beyond the scope of this paper. SnO 2 differs from the other non-transitionmetal oxides in that the composition of the thermodynamically most stable (110) surface can easily by changed from stoichiometric, in which half of the surface cations are five-fold coordinated with O ions and the other half are six-fold coordinated, to reduced, in which all of the bridging O
ions have been removed. This occurs since Sn can exist in either the Sn 4+ or Sn 2+ valence states [14]. Surface reduction does not appreciably increase the surface conductivity because of the localized nature of the Sn 2+ ions on that surface. 0 2 interacts only very weakly with nearly perfect SnO 2, although it does chemisorb on reduced SnO 2 surfaces [15]. The interaction is believed to be dissociative, resulting in adsorbed 0 2- . The interaction with defects is stronger at elevated temperatures, showing that the healing of surface defects is a thermally activated process. H 2 0 acts as an electron donor on SnO 2, with the reaction being stronger on slightly reduced surfaces than on heavily reduced ones [16]. H 2 also interacts slightly with SnO 2 surfaces, probably dissociating at surface defects with the creation of H + ions, which bond to lattice 0 2- ions as O H - , and electrons [17]. The interaction of several organic molecules with SnO 2 surfaces has also been investigated, and in most cases adsorption is thought to involve deprotonation, with the involvement of lattice 0 2- ions. Surprisingly, virtually nothing is known about gas-surface interactions on well characterized surfaces of A1203 .
4. G a s - s u r f a c e interactions on transition-metal oxides
Transition-metal oxides exhibit a wider range of adsorption behavior than do non-transitionmetal oxides since the transition-metal cations can exist in more than one valence state. The energy necessary to change the valence state of the cations is generally fairly small, so electrons can be added to, or removed from, cation dorbitals during chemisorption. However, in transition-metal oxides whose d-electron orbitals are normally empty, there are no cation d-electrons available for transfer to adsorbates. Stoichiometric surfaces of those d o oxides are generally less active for chemisorption than are those of oxides whose d-orbitals are partially occupied on the perfect surface. When O-vacancy defects are present on the surfaces of transition-metal oxides, however, the requirement of maintaining local
V.E. Henrich, P.4. Cox / Fundamentals of gas-surface interactions on metal oxides
charge neutrality results in an increase in the population of the d-electron orbitals on cations adjacent to the defect. These reduced surface cations provide the active sites for much of the chemisorption and catalysis that takes place on d o transition-metal oxides. By far the most thoroughly studied d o transition-metal oxide is TiO 2, largely because of its potential use as a photocatalytic electrode in photoelectrolysis. (In most respects the perovskite oxide SrTiO 3 behaves similarly to TiO2, although there are differences in detail.) Thus there have been many studies of the interaction of H 2 0 with both stoichiometric and defective TiO 2 surfaces [1]. The experimental work has not resulted in unanimous agreement on the details of adsorption. Conclusions drawn from room-temperature adsorption measurements on nominally stoichiometric TiO 2 surfaces range from determining that the surface is completely inert to saying that H 2 0 adsorbs dissociatively. One reason for the different conclusions is that each nominally "stoichiometric, nearly perfect" surface has in reality a different density of step and point defects, regardless of how carefully it is prepared. We believe that a truly perfect TiO 2 (110) or (100) surface would be inert to H 2 0 at room temperature, and that the wide range of results reported is due to either extended or point defects on the surface. For defective TiO 2 surfaces, the weight of the evidence indicates that H 20 dissociates at room temperature, but that there is no interaction between H 2 0 and the electrons on reduced cations at surface defect sites [18,19]. This confirms that the interaction of H 2 0 with the surface is of the donor-acceptor type, with no appreciable transfer of charge from reduced surface cations to the adsorbed species. However, it is still not clear just what features of reduced TiO 2 surfaces are responsible for dissociating H 20. The lack of interaction between the adsorbate and Ti 3d electrons at defect sites suggests that the presence of occupied Ti3d orbitals is not in itself sufficient for dissociative adsorption of H20; this conclusion is supported by the results of H 2 0 adsorption experiments on UHV-cleaved Ti2Oa(1012) surfaces [20]. It appears that somehow it is the geometry more than the electronic
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structure of O-vacancy point defects that is necessary for dissociation of H 20. The role of steps on stoichiometric surfaces is not clear, but on TiO2(ll0) they do not appear to interact as strongly with H 2 0 as do point defects; thus the presence of reduced Ti cations, which are not present at step sites, probably also plays some role in the dissociation of H 20. The results of several studies of the interaction of 02 with TiO 2 surfaces suggest that it adsorbs only at point defects on TiO2, and not on either terraces or steps [1]. There is a strong interaction at room temperature between 02 and O-vacancy point defects on TiO 2 that results in depopulation of the band-gap defect surface states. UPS difference spectra indicate that the initial stage of 02 chemisorption on reduced TiO 2 is dissociative, and that the adsorbed species is O 2-, with charge transferred to the adsorbed O from the surface Ti 3+ ions [21]. What happens for larger 02 exposures is not clear, although the possibility of adsorbed 0 2 and 022- has been suggested. The adsorption of CO on TiO 2 surfaces occurs only in the presence of O-vacancy point defects [22]. It is proposed that when CO adsorbs at O-vacancy sites, the C end of the molecule bonds to an adjacent O ion. This surface complex acts as a weak electron acceptor. At higher temperatures, the CO 2 formed upon CO adsorption desorbs, creating more surface O vacancies. The adsorption of CO2 on TiO2(l10) was found to be independent of the density of surface O-vacancy defects, and the presence of adsorbed CO 2 does not interfere with reactions between 02 and surface defects. (This behavior is very different than that for CO 2 adsorption on ZnO.) It is therefore assumed that CO2 bonds to surface O ions. SO 2 is an extremely strong oxidizing agent for defects on TiO 2 surfaces, significantly stronger than 02 [23]. With the lower-valence oxide Ti203, SO 2 interacts corrosively, with no limit to the reaction for large exposures [24]. Of the organic molecules whose interaction with TiO 2 has been studied, most adsorb on reduced surfaces via deprotonation and the formation of surface hydroxyls. Transition-metal oxides in which the cations have partially filled d-orbitals in the ground state
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differ from d o oxides in that at least one of the d-electrons can often be removed relatively easily from the cation; i.e., d n oxides may be easily oxidized. The d n oxides are often much more reactive than are d o oxides, and they exhibit a wider range of interaction with adsorbed atoms and molecules. The behavior of stoichiometric surfaces of d n oxides is in many cases similar to that of defective surfaces of the corresponding d o oxide, since reduced cations are present at the defect sites. (An exception to this is the interaction of H 2 0 with titanium oxides discussed above.) The creation of O-vacancy point defects on d ~ oxides results in smaller relative changes in surface properties than for d o oxides, since even the stoichiometric surface contains d-electrons that are available to take part in surface chemical reactions. The oxides of vanadium exhibit a wide range of gas-surface interactions. Stoichiometric surfaces of the d o oxide V205 are quite inert, except that CO appears to reduce the stoichiometric surface with the formation of CO2 [25]. Reduced V205 reacts most strongly with 0 2 and SO/, with both molecules oxidizing the surface [25]. The corundum oxide V203 behaves in many respects like its isostructural cousin Ti203, including dissociatively adsorbing 0 2 on the stoichiometric surface and the dissociative adsorption of H 2 0 on reduced surfaces [26]. Large differences between the two oxides are found for the adsorption of SO 2, however [27]. Whereas stoichiometric Ti203 interacts violently with SO2, largely via its 3d electrons, V20 3, which has twice as many d-electrons per cation, hardly reacts at all. On the stoichiometric V203(1012) surface, the interaction saturates by 1 L, and the surface is inert thereafter. Only a few studies have been performed of the interaction of molecules with well characterized single-crystal Fe oxides. Nearly stoichiometric aFe203(0001) surfaces are relatively inert to 0 2. Reduced a-Fe203(0001) surfaces are more reactive than stoichiometric ones, although in both cases an observed increase in work function indicates a negative adsorbed species, possibly 0 2[28]. An extremely interesting interaction between 0 2 and an Fe oxide surface occurs for
Fe304(110). Both Fe 2+ and Fe 3+ cations should be present on that surface. By using resonant photoemission to differentiate between the Fe E+ and Fe 3÷ ions, the interaction of O 2 with the surface was shown clearly to be via the Fe 2÷ ions, with virtually no effect of 0 2 on the Fe 3÷ emission [29]. The adsorption of H 2 0 has been studied on both stoichiometric and reduced otFe203(0001) surfaces [28,30]. Stoichiometric surfaces are almost inert to H 2 0 . Reduced aFe203(0001) surfaces interact slightly with H E 0 , with the adsorbed species identified as O H - . H z is found to interact weakly with polished and annealed a-FeeOa(0001) surfaces [28]. At room temperature H 2 is believed to dissociate homolytically to produce adsorbed hydroxyl ions, but the results are not definitive. Only a weak interaction occurs between SO 2 and the a-FeEO3(0001) surface [28]; the interaction is even weaker than that for V203. The adsorbed species is thought to be a sulfate complex, although both SO 2- and SO 2 would also be consistent with the experimental results. While the NiO(100) surface behaves in a manner similar to many other transition-metal oxides with respect to O z adsorption (i.e., the stoichiometric surface is inert, with dissociative adsorption occurring at defects on reduced surfaces), its behavior toward H 2 0 is unique and extremely interesting [31]. The cleaved surface is inert to HEO at room temperature, but the reduced surface exhibits only very weak dissociative adsorption. However, pre-adsorption of O on the defective surface results in greatly enhanced dissociative adsorption of HE0. UPS difference spectra indicate that the adsorbed species is O H - . It thus appears that the presence of non-lattice O is necessary in order to break the O - H bond in H 2 0 and initiate adsorption. There are two extremely important gas-surface interactions for Cu-oxide high-Tc superconductors. They are technologically important because some superconductors react chemically with ambient gases to form non-superconducting compounds. This effect becomes crucial for thin-film superconductors, where a significant fraction of the film may become non-superconducting as a result of environmental degradation, and for
V.E. Henrich, P.A. Cox / Fundamentals of gas-surface interactions on metal oxides
polycrystalline materials, where chemical reactions at grain boundaries and the attendant changes in electrical properties there can interrupt the superconducting pathways. The two molecules that are most reactive with the Cuoxide compounds are H 2 0 and CO 2. When YBa2CUaOT_y powders having 0 < y < 1 are immersed in water, "the water fizzes like seltzer" [32]. The interaction occurs primarily with the Ba, which readily forms hydroxide. The BiESr2CaCu 2O8÷y compounds do not react readily with H 2 0 [33]. Equally destructive to the superconducting properties of the Cu-oxide compounds is CO 2. For YBaECU30 7 the reaction is thought to be [34]: 2 Y B a 2 C u 3 0 7 + 3 C O 2 --~ Y E B a C u O 5 + 5 C u O + 3 B a C O 3 + ~1- O 2. This reaction is strongly catalyzed by water vapor, with Ba(OH) 2 formed as an intermediate product. Thus the most destructive environment for high-Tc superconducting Cu oxides is a mixture of H 2 0 and CO2, with temperature also accelerating the reaction.
5. Conclusion The fundamental interactions between environmentally important molecules and the surfaces of metal oxides have been studied for both transition-metal and non-transition-metal oxides by using surface-sensitive techniques applied to single-crystal oxide samples. To a much larger extent than on metals, point defects dominate adsorption of most molecules on metal oxides; steps are relatively inactive as adsorption sites. The degree of coordinative unsaturation of surface ions is important in adsorption, as is the charge state of ions at point defect sites. The possibility of transferring lattice O to an adsorbate during chemisorption makes metal oxides useful catalysts for partial oxidation reactions.
Acknowledgements This work was partially supported by NSF, Division of Materials Research Grant DMR-
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9015448; and DoE, Office of Basic Energy Sciences Grant DE-FG02-87ER13773.
References [1] V.E. Henrich and P.A. Cox, The Surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1993). [2] For a thorough discussion of hydroxylated metal-oxide surfaces, see H. Kung, Transition Metal Oxides: Surface Chemistry and Catalysis (Elsevier, Amsterdam, 1989). [3] H. Onishi, C. Egawa, T. Aruga and Y. Iwasawa, Surf. Sci. 191 (1987) 479. [4] C. Duriez, C. Chapon, C.R. Henry and J.M. Rickard, Surf. Sci. 230 (1990) 123. [5] X.D. Peng and M.A. Barteau, Langmuir 7 (1991) 1426. [6] X.D. Peng and M.A. Barteau, Catal. Lett. 7 (1990) 395. [7] W.T. Petrie and J.M. Vohs, Surf. Sci. Lett. 259 (1991) L750. [8] E.A. Colbourn, J. Kendrick and W.C. Mackrodt, Surf. Sci. 126 (1983) 550. [9] W. G6pel, R.S. Bauer and G. Hansson, Surf. Sci. 99 (1980) 138. [10] P. Esser and W. G6pel, Surf. Sci. 97 (1980) 309. [11] W.H. Cheng and H.H. Kung, Surf. Sci. 122 (1982) 21. [12] G. Zwicker and K. Jacobi, Surf. Sci. 131 (1983) 179. [13] W. G6pel, Prog. Surf. Sci. 20 (1985) 9. [14] J.M. Themlin, R. Sporken, J. Darville, R. Caudano, J.M. Gilles and R.L. Johnson, Phys. Rev. B 42 (1990) 11914. [15] D.F. Cox, T.B. Fryberger and S. Semancik, Phys. Rev. B 38 (1988) 2072. [16] D.F. Cox, S. Semancik and P.D. Szuromi, J. Vac. Sci. Technol. A 4 (1986) 627. [17] D.F. Cox, T.B. Fryberger, J.W. Erickson and S. Semancik, J. Vac. Sci. Technol. A 5 (1987) 1170. [18] R.L. Kurtz, R. Stockbauer, T.E. Madey, E. Rom~in and J.L. de Segovia, Surf. Sci. 218 (1989) 178. [19] S. Eriksen, P.D. Naylor and R.G. Egdell, Spectrochim. Acta 43a (1987) 1535. [20] R.L. Kurtz and V.E. Henrich, Phys. Rev. B 26 (1982) 6682. [21] V.E. Henrich, G. Dresselhaus and H.J. Zeiger, J. Vac. Sci. Technol. 15 (1978) 534. [22] W. G6pel, U. Kirner, H.D. Wiemh6fer and G. Rocker, Solid State Ionics 28-30 (1988) 1423. [23] K.E. Smith, J.L. Mackay and V.E. Henrich, Phys. Rev. B 35 (1987) 5822. [24] K.E. Smith and V.E. Henrich, Phys. Rev. B 32 (1985) 5384. [25] Z. Zhang and V.E. Henrich, unpublished. [26] R.L. Kurtz and V.E. Henrich, Phys. Rev. B 28 (1983) 6699. [27] K.E. Smith and V.E. Henrich, Surf. Sci. 225 (1990) 47. [28] R.L. Kurtz and V.E. Henrich, Phys. Rev. B 36 (1987) 3413.
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[29] R.J. Lad and V.E. Henrich, J. Vac. Sci. Technol. A 7 (1989) 1893. [30] M. Hendewerk, M. Salmeron and G. Somorjai, Surf. Sci. 172 (1986) 544. [31] J.M. McKay and V.E. Henrich, Phys. Rev. B 32 (1985) 6764. [32] R.L. Barns and R.A. Laudise, Appl. Phys. Lett. 51 (1987) 1373.
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Applied Surface Science 72 (1993) 277-284 North-Holland
applied
surface s c i e n c e
Fundamentals of gas-surface interactions on metal oxides * V i c t o r E. H e n r i c h * * Department of Applied Physics, Yale University, New Haven, CT 06520, USA and P.A. Cox Inorganic Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QR, UK Received 2 February 1993; accepted for publication 7 April 1993
In an attempt to understand the fundamental ways in which molecules interact with metal-oxide surfaces, chemisorption has been studied on single-crystaloxide samples. Chemisorption behavior is determined both by the electronic configuration of surface cations and by the geometric structure of the surface. Stoichiometric, well ordered oxide surfaces are, for the most part, relatively unreactive (although striking exceptions exist). However, point defects, which consist predominantly of oxygen vacancies, significantly change the surface electronic structure on most oxides and are the active sites for many types of chemisorption. The interaction between 0 2 and transition-metal-oxide surfaces is determined primarily by cation electronic structure, while the dissociation of H20 seems to be promoted by particular structural features at defect sites and is less sensitive to electronic structure. For organic molecule chemisorption, a general feature of oxide surfaces is the rather facile breaking of C-H bonds, but not of C-C bonds; this is important in selective oxidation catalysis.
I. Introduction D u r i n g the past few years surface scientists have increasingly t u r n e d their a t t e n t i o n to m a t e rials that are of e n v i r o n m e n t a l i m p o r t a n c e . O n e aspect of that work has consisted of studies of g a s - s u r f a c e i n t e r a c t i o n s o n well c h a r a c t e r i z e d surfaces. T h e vast majority of that work has b e e n p e r f o r m e d o n m e t a l oxides, a n d this p a p e r att e m p t s to s u m m a r i z e some of the i m p o r t a n t p r i n ciples as well as to p r e s e n t specific results. D u e to space limitations, discussion will be c o n f i n e d to the results of m e a s u r e m e n t s o n single-crystal
* Part of the Proceedings of the Environmental Interfaces Topical Conference held as part of the 39th Annual Symposium of the American Vacuum Society. ** Contact author. Tel.: (203) 432-4399; Fax: (203) 432-4283.
samples. F o r a m o r e c o m p l e t e t r e a t m e n t of this topic, the r e a d e r is r e f e r r e d to ref. [1]. T h e types of g a s - s u r f a c e i n t e r a c t i o n that occur o n metal-oxide surfaces differ significantly from those o n metals. T h e relatively ionic n a t u r e of oxides leads to a p r e d o m i n a n c e of acid-base, or d o n o r - a c c e p t o r , interactions. Surface cations, which act as Lewis-acid sites, may interact with d o n o r molecules such as H 2 0 t h r o u g h a combin a t i o n of i o n - d i p o l e a t t r a c t i o n a n d orbital overlap. Similarly, oxide ions act as basic sites, interacting with acceptors such as H ÷. O n e of the most c o m m o n dissociative reactions o n metaloxide surfaces is the d e p r o t o n a t i o n of a n adsorbate to p r o d u c e surface hydroxyl groups, which are almost always p r e s e n t o n polycrystalline oxides [2]. O t h e r types of heterolytic dissociation (which result in a d s o r b e d ions) may be favored,
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
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even for molecules that would dissociate homolytically (that is, into neutral radicals) on metals. Another important feature of molecule-oxide interactions is the involvement of lattice oxygens. They are essentially basic towards acidic molecules such as CO 2 (forming surface CO 2- in that case). But in addition to their acid/base properties, many oxides are able to perform selective oxidation reactions. Oxygen may be added to an adsorbate in the form of a neutral O atom, accompanied by a corresponding reduction of the substrate. This is one type of oxidation/reduction reaction that can take place on metal-oxide surfaces. It might result in electrons that are free carriers, but more often it will lead to a localized decrease in the oxidation state of metal cations at the surface. In a catalytic cycle, re-oxidation takes place by filling of surface oxygen vacancies by dissociative adsorption of 0 2 . Oxidation/reduction reactions can also occur in which only electrons are transferred between the surface and the adsorbate. An example of such a reaction is the dissociative adsorption of CI2, which results in two adsorbed C l - ions and the attendant removal of two electrons from the substrate. An important concept in the reactivity of oxide surfaces is coordinative unsaturation. The coordination of metal and oxygen ions at surface sites is, if course, lower than in the bulk. Coordinatively unsaturated ions can act as acidic or basic adsorption sites, but their bonding to the lattice is also weaker than that of coordinatively saturated ions; thus they may be more easily removed, as in oxidation reactions. Different surfaces exhibit different degrees of coordinative unsaturation, but in general it is rather small for the most stable low-index faces, which explains both their stability and general lack of reactivity. Step and defect sites necessarily exhibit a greater degree of coordinative unsaturation. This increases their acidic and basic properties and may lead to greater reactivity. Such sites may also have different electronic properties; for example, oxygen may exist as O - and metal ions may exist in a lower oxidation state than on the perfect surface. Such sites may promote oxidation or reduction reactions more readily. Chemisorption measurements on well charac-
terized surfaces are almost always performed at very low pressures under essentially U H V conditions; that is true of all of the experimental results to be discussed in this paper. However, environmentally relevant reactions almost always occur at or near atmospheric pressure. One must be careful in extrapolating the results of low-pressure experiments to higher pressures. At high pressures the surface generally has a much higher steady-state coverage of adsorbed species than at low pressure. At high surface coverages the way in which a species is adsorbed may well be different than at low coverage, resulting in types of reaction that are not possible at low pressures and coverages. Another problem in interpretation can occur because stoichiometric low-index faces of oxides are often rather unreactive, and chemisorption, especially when dissociative, is promoted by surface defects and steps. Since small concentrations of surface defects are difficult or impossible to detect by most surface-science techniques, a surface that appears to be stoichiometric and well ordered and shows almost no chemisorption under surface-science conditions may be reactive at higher pressures even though the reaction takes place at a small fraction of surface sites.
2. General features of the adsorption of environmentally relevant molecules The interaction of 0 2 with metal oxides is important in many catalytic applications. Oxygen is a powerful electron acceptor and can be reduced via several routes: O 2 + e - - ~ 0 2 ; 0 2 + 2e-~O2-'2 , O2+2e-~20-; or O 2 + 4 e - ~ 2 0 2 - . There is ESR evidence in many polycrystalline studies for the existence of surface O~ and O - species. On single crystals the adsorbed species may sometimes be inferred from UPS, but frequently they are unknown. Stoichiometric surfaces of n-type semiconductors such as ZnO and SnO 2 are generally assumed to adsorb O 2 molecularly, although weakly; dissociative adsorption is promoted by defects such as oxygen vacancies on reduced surfaces of both transition-metal and non-transition-metal oxides.
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H 2 0 has a large dipole moment and lone-pair electrons, making it a good donor. Molecular adsorption occurs by acid-base interaction with surface metal ions, although this may be quite weak on defect-free low-index surfaces. Stronger adsorption occurs at steps and defects and is often dissociative: H 2 0 q- O 2----~ 2 O H - . This reaction is common on polycrystalline surfaces, so that they are frequently covered with hydroxyl groups [2]. After prolonged exposure of very basic oxides such as BaO and some high-Tc oxide superconductors to H 2 0 , hydroxide formation may occur and extend into the bulk of the solid. H 2 is a non-polar molecule having a very low polarizability and very weak donor or acceptor properties. Thus H E in general only weakly physisorbs. The H - H bond is very strong, and, although dissociative chemisorption takes place on many metals, it is only possible on the most reactive oxide surfaces. CO has a small dipole moment and is only a weak donor. Its bonding to surfaces depends upon the "back-donation" of electrons from surface cations into the antibonding ~r-orbital; that is, it acts simultaneously as a donor and an acceptor. Thus the strength of bonding on metal oxides depends upon the availability of electrons at the surface, so adsorption will be favored at cations having relatively low oxidation states. CO frequently seems to react with pre-adsorbed or lattice oxygen to give CO 2, which may further react to form carbonate. CO 2 can act either as a weak donor or an acceptor. The common reaction on metal oxides is an acid-base interaction to give surface carbonate, CO32-. On BaO and high-To oxide superconductors, this may proceed to yield bulk carbonate; indeed, oxides such as YBaECUaO7_y may be covered with a layer of BaCO 3. Although no experimental studies have been reported for the interaction of N 2 with metal oxides, any reaction would be expected to be extremely weak. A variety of types of adsorption has been observed for the interaction of organic molecules with metal-oxide surfaces. However, a general feature of oxide surfaces, which is important in selective oxidation catalysis, is the rather facile
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breaking of C - H bonds, but not of C - C , bonds. Nevertheless, many studies report a proportion of "deep oxidation" products (CO and CO z) and other species that can only be formed by breaking C - C bonds.
3. Chemisorption on non-transition-metal oxides
The chemisorption of molecules on non-transition-metal oxides, whose cation valence electrons are s- or p-orbitals, differs from that on transition-metal oxides, where the valence electrons are d-orbitals, in that non-transition-metal cations can usually exist in only one valence state. (The exception is Sn, which can exist as either Sn 2 ÷ or Sn4+.) The addition or removal of electrons from cations in non-transition-metal oxides requires so much energy that it does not usually occur during chemisorption. As for all types of metal oxide, the lattice O ions have very nearly an 0 2- 2p 6 closed-shell configuration, and, since the ionization energy for an O 2p electron is several eV, the transfer of charge from O z- ions to adsorbates is also unlikely. However, surface O ions may form molecular complexes with adsorbates, and in some cases the adsorbate may abstract lattice O ions from the surface. It is also possible for O - ions to exist at surface defects, and such ions are generally very active in chemisorption. The only stable surface of the rocksalt oxide MgO is (100). Nearly all work shows that the defect-free surface is quite inert. However, chemisorption of some molecules is promoted by the defects that are present on etched and facetted surfaces. Cleaved or annealed MgO(100) surfaces studied in U H V do not adsorb H z O at room temperature, but ion-bombarded surfaces exhibit features characteristic of O H - , presumably adsorbed at surface defect sites [2]. When cleaved MgO(100) surfaces are exposed to air, however, a high density of point defects is formed due to reaction with water vapor [4]. MgO(100) is quite inert towards most other molecules, although there has been one experimental report of CO 2 adsorbing at surface defect sites to yield CO ] - [3]. In addition, methanol, formic acid, acetic acid, formaldehyde and methyl formate
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have been found to interact dissociatively with MgO(100) [3,5-7]. Chemisorption on MgO doped with aliovalent cations is more complex than on stoichiometric MgO, since species such as O can then be stabilized [8]. Chemisorption on ZnO is important both in catalytic and gas-sensor applications, and ZnO is thus the most extensively studied of all the nontransition-metal oxides. The only clear conclusion that can be drawn from all of the work that has been performed is that the chemisorption behavior of Z n O is extremely complicated. Several stable polar and non-polar surfaces exist, as do acidic Zn and basic O sites. Virtually all types of chemisorption have been observed on ZnO. Also, many of the results obtained conflict with one another and with the expectations of simple models. Molecular 02 interacts only weakly with all ZnO surfaces. Below room temperature only physisorption occurs. Between 300 and 650 K, chemisorption involving charge transfer from the surface to the molecule results in adsorbed 0 2 , although the coverage on the surface is very small [9,10]; adsorption clearly occurs at defect sites. Steps on ZnO surfaces have also been found to be relatively more reactive for 0 2 adsorption, although the total surface coverage of adsorbed 0 2 is still no more than a few % of a monolayer [11]. Thus the O 2 - Z n O interaction is weak even for stepped surfaces. H 2 0 does not adsorb on annealed ZnO surfaces for temperatures above 130 K, so the interaction is clearly weak [12]. H 2 essentially does not interact with ZnO surfaces. At room temperature and above, both CO and CO 2 interact only weakly with ZnO, although changes in surface conductivity, which are a very sensitive measure of adsorption, have been observed [13]. There have been a great many studies of the interaction of organic molecules with ZnO surfaces; however, they are beyond the scope of this paper. SnO 2 differs from the other non-transitionmetal oxides in that the composition of the thermodynamically most stable (110) surface can easily by changed from stoichiometric, in which half of the surface cations are five-fold coordinated with O ions and the other half are six-fold coordinated, to reduced, in which all of the bridging O
ions have been removed. This occurs since Sn can exist in either the Sn 4+ or Sn 2+ valence states [14]. Surface reduction does not appreciably increase the surface conductivity because of the localized nature of the Sn 2+ ions on that surface. 0 2 interacts only very weakly with nearly perfect SnO 2, although it does chemisorb on reduced SnO 2 surfaces [15]. The interaction is believed to be dissociative, resulting in adsorbed 0 2- . The interaction with defects is stronger at elevated temperatures, showing that the healing of surface defects is a thermally activated process. H 2 0 acts as an electron donor on SnO 2, with the reaction being stronger on slightly reduced surfaces than on heavily reduced ones [16]. H 2 also interacts slightly with SnO 2 surfaces, probably dissociating at surface defects with the creation of H + ions, which bond to lattice 0 2- ions as O H - , and electrons [17]. The interaction of several organic molecules with SnO 2 surfaces has also been investigated, and in most cases adsorption is thought to involve deprotonation, with the involvement of lattice 0 2- ions. Surprisingly, virtually nothing is known about gas-surface interactions on well characterized surfaces of A1203 .
4. G a s - s u r f a c e interactions on transition-metal oxides
Transition-metal oxides exhibit a wider range of adsorption behavior than do non-transitionmetal oxides since the transition-metal cations can exist in more than one valence state. The energy necessary to change the valence state of the cations is generally fairly small, so electrons can be added to, or removed from, cation dorbitals during chemisorption. However, in transition-metal oxides whose d-electron orbitals are normally empty, there are no cation d-electrons available for transfer to adsorbates. Stoichiometric surfaces of those d o oxides are generally less active for chemisorption than are those of oxides whose d-orbitals are partially occupied on the perfect surface. When O-vacancy defects are present on the surfaces of transition-metal oxides, however, the requirement of maintaining local
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charge neutrality results in an increase in the population of the d-electron orbitals on cations adjacent to the defect. These reduced surface cations provide the active sites for much of the chemisorption and catalysis that takes place on d o transition-metal oxides. By far the most thoroughly studied d o transition-metal oxide is TiO 2, largely because of its potential use as a photocatalytic electrode in photoelectrolysis. (In most respects the perovskite oxide SrTiO 3 behaves similarly to TiO2, although there are differences in detail.) Thus there have been many studies of the interaction of H 2 0 with both stoichiometric and defective TiO 2 surfaces [1]. The experimental work has not resulted in unanimous agreement on the details of adsorption. Conclusions drawn from room-temperature adsorption measurements on nominally stoichiometric TiO 2 surfaces range from determining that the surface is completely inert to saying that H 2 0 adsorbs dissociatively. One reason for the different conclusions is that each nominally "stoichiometric, nearly perfect" surface has in reality a different density of step and point defects, regardless of how carefully it is prepared. We believe that a truly perfect TiO 2 (110) or (100) surface would be inert to H 2 0 at room temperature, and that the wide range of results reported is due to either extended or point defects on the surface. For defective TiO 2 surfaces, the weight of the evidence indicates that H 20 dissociates at room temperature, but that there is no interaction between H 2 0 and the electrons on reduced cations at surface defect sites [18,19]. This confirms that the interaction of H 2 0 with the surface is of the donor-acceptor type, with no appreciable transfer of charge from reduced surface cations to the adsorbed species. However, it is still not clear just what features of reduced TiO 2 surfaces are responsible for dissociating H 20. The lack of interaction between the adsorbate and Ti 3d electrons at defect sites suggests that the presence of occupied Ti3d orbitals is not in itself sufficient for dissociative adsorption of H20; this conclusion is supported by the results of H 2 0 adsorption experiments on UHV-cleaved Ti2Oa(1012) surfaces [20]. It appears that somehow it is the geometry more than the electronic
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structure of O-vacancy point defects that is necessary for dissociation of H 20. The role of steps on stoichiometric surfaces is not clear, but on TiO2(ll0) they do not appear to interact as strongly with H 2 0 as do point defects; thus the presence of reduced Ti cations, which are not present at step sites, probably also plays some role in the dissociation of H 20. The results of several studies of the interaction of 02 with TiO 2 surfaces suggest that it adsorbs only at point defects on TiO2, and not on either terraces or steps [1]. There is a strong interaction at room temperature between 02 and O-vacancy point defects on TiO 2 that results in depopulation of the band-gap defect surface states. UPS difference spectra indicate that the initial stage of 02 chemisorption on reduced TiO 2 is dissociative, and that the adsorbed species is O 2-, with charge transferred to the adsorbed O from the surface Ti 3+ ions [21]. What happens for larger 02 exposures is not clear, although the possibility of adsorbed 0 2 and 022- has been suggested. The adsorption of CO on TiO 2 surfaces occurs only in the presence of O-vacancy point defects [22]. It is proposed that when CO adsorbs at O-vacancy sites, the C end of the molecule bonds to an adjacent O ion. This surface complex acts as a weak electron acceptor. At higher temperatures, the CO 2 formed upon CO adsorption desorbs, creating more surface O vacancies. The adsorption of CO2 on TiO2(l10) was found to be independent of the density of surface O-vacancy defects, and the presence of adsorbed CO 2 does not interfere with reactions between 02 and surface defects. (This behavior is very different than that for CO 2 adsorption on ZnO.) It is therefore assumed that CO2 bonds to surface O ions. SO 2 is an extremely strong oxidizing agent for defects on TiO 2 surfaces, significantly stronger than 02 [23]. With the lower-valence oxide Ti203, SO 2 interacts corrosively, with no limit to the reaction for large exposures [24]. Of the organic molecules whose interaction with TiO 2 has been studied, most adsorb on reduced surfaces via deprotonation and the formation of surface hydroxyls. Transition-metal oxides in which the cations have partially filled d-orbitals in the ground state
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differ from d o oxides in that at least one of the d-electrons can often be removed relatively easily from the cation; i.e., d n oxides may be easily oxidized. The d n oxides are often much more reactive than are d o oxides, and they exhibit a wider range of interaction with adsorbed atoms and molecules. The behavior of stoichiometric surfaces of d n oxides is in many cases similar to that of defective surfaces of the corresponding d o oxide, since reduced cations are present at the defect sites. (An exception to this is the interaction of H 2 0 with titanium oxides discussed above.) The creation of O-vacancy point defects on d ~ oxides results in smaller relative changes in surface properties than for d o oxides, since even the stoichiometric surface contains d-electrons that are available to take part in surface chemical reactions. The oxides of vanadium exhibit a wide range of gas-surface interactions. Stoichiometric surfaces of the d o oxide V205 are quite inert, except that CO appears to reduce the stoichiometric surface with the formation of CO2 [25]. Reduced V205 reacts most strongly with 0 2 and SO/, with both molecules oxidizing the surface [25]. The corundum oxide V203 behaves in many respects like its isostructural cousin Ti203, including dissociatively adsorbing 0 2 on the stoichiometric surface and the dissociative adsorption of H 2 0 on reduced surfaces [26]. Large differences between the two oxides are found for the adsorption of SO 2, however [27]. Whereas stoichiometric Ti203 interacts violently with SO2, largely via its 3d electrons, V20 3, which has twice as many d-electrons per cation, hardly reacts at all. On the stoichiometric V203(1012) surface, the interaction saturates by 1 L, and the surface is inert thereafter. Only a few studies have been performed of the interaction of molecules with well characterized single-crystal Fe oxides. Nearly stoichiometric aFe203(0001) surfaces are relatively inert to 0 2. Reduced a-Fe203(0001) surfaces are more reactive than stoichiometric ones, although in both cases an observed increase in work function indicates a negative adsorbed species, possibly 0 2[28]. An extremely interesting interaction between 0 2 and an Fe oxide surface occurs for
Fe304(110). Both Fe 2+ and Fe 3+ cations should be present on that surface. By using resonant photoemission to differentiate between the Fe E+ and Fe 3÷ ions, the interaction of O 2 with the surface was shown clearly to be via the Fe 2÷ ions, with virtually no effect of 0 2 on the Fe 3÷ emission [29]. The adsorption of H 2 0 has been studied on both stoichiometric and reduced otFe203(0001) surfaces [28,30]. Stoichiometric surfaces are almost inert to H 2 0 . Reduced aFe203(0001) surfaces interact slightly with H E 0 , with the adsorbed species identified as O H - . H z is found to interact weakly with polished and annealed a-FeeOa(0001) surfaces [28]. At room temperature H 2 is believed to dissociate homolytically to produce adsorbed hydroxyl ions, but the results are not definitive. Only a weak interaction occurs between SO 2 and the a-FeEO3(0001) surface [28]; the interaction is even weaker than that for V203. The adsorbed species is thought to be a sulfate complex, although both SO 2- and SO 2 would also be consistent with the experimental results. While the NiO(100) surface behaves in a manner similar to many other transition-metal oxides with respect to O z adsorption (i.e., the stoichiometric surface is inert, with dissociative adsorption occurring at defects on reduced surfaces), its behavior toward H 2 0 is unique and extremely interesting [31]. The cleaved surface is inert to HEO at room temperature, but the reduced surface exhibits only very weak dissociative adsorption. However, pre-adsorption of O on the defective surface results in greatly enhanced dissociative adsorption of HE0. UPS difference spectra indicate that the adsorbed species is O H - . It thus appears that the presence of non-lattice O is necessary in order to break the O - H bond in H 2 0 and initiate adsorption. There are two extremely important gas-surface interactions for Cu-oxide high-Tc superconductors. They are technologically important because some superconductors react chemically with ambient gases to form non-superconducting compounds. This effect becomes crucial for thin-film superconductors, where a significant fraction of the film may become non-superconducting as a result of environmental degradation, and for
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polycrystalline materials, where chemical reactions at grain boundaries and the attendant changes in electrical properties there can interrupt the superconducting pathways. The two molecules that are most reactive with the Cuoxide compounds are H 2 0 and CO 2. When YBa2CUaOT_y powders having 0 < y < 1 are immersed in water, "the water fizzes like seltzer" [32]. The interaction occurs primarily with the Ba, which readily forms hydroxide. The BiESr2CaCu 2O8÷y compounds do not react readily with H 2 0 [33]. Equally destructive to the superconducting properties of the Cu-oxide compounds is CO 2. For YBaECU30 7 the reaction is thought to be [34]: 2 Y B a 2 C u 3 0 7 + 3 C O 2 --~ Y E B a C u O 5 + 5 C u O + 3 B a C O 3 + ~1- O 2. This reaction is strongly catalyzed by water vapor, with Ba(OH) 2 formed as an intermediate product. Thus the most destructive environment for high-Tc superconducting Cu oxides is a mixture of H 2 0 and CO2, with temperature also accelerating the reaction.
5. Conclusion The fundamental interactions between environmentally important molecules and the surfaces of metal oxides have been studied for both transition-metal and non-transition-metal oxides by using surface-sensitive techniques applied to single-crystal oxide samples. To a much larger extent than on metals, point defects dominate adsorption of most molecules on metal oxides; steps are relatively inactive as adsorption sites. The degree of coordinative unsaturation of surface ions is important in adsorption, as is the charge state of ions at point defect sites. The possibility of transferring lattice O to an adsorbate during chemisorption makes metal oxides useful catalysts for partial oxidation reactions.
Acknowledgements This work was partially supported by NSF, Division of Materials Research Grant DMR-
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9015448; and DoE, Office of Basic Energy Sciences Grant DE-FG02-87ER13773.
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