Quantum chemical investigation of support-metal interactions and their

Hydrogen Production via Steam Reforming of Ethanol on Phyllosilicate-Derived Ni/SiO2: Enhanced Metal–Support Interaction and Catalytic Stability. Ch...
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J . Phys. Chem. 1986, 90, 4322-4330

failed. Reaction of the residual silanols of a 3PDS silica with alkylsilylating agents appear to produce a significantly more complex or disperse excimer formation mechanism. There are important qualitative trends in the decay profiles shown in Figures 4 and 5. If one examines the first 120 ns of the decay profile, 3PDH/HMDS silica exhibits the same gradual grow-in as unmodified 3PDS silica and the calculated rise time in Table 111 supports this observation. Trimethylsilylation does not seem to effect the motion or flexibility of the 3PDS chains. Figure 5 indicates that subsequent octadecylation of the 3PDS silica has a dramatic effect on the 3PDS molecules. While the 3PDS molecules are still capable of forming excimers, a small but substantial number of 3PDS molecules form excimer in a much shorter time than in the unmodified 3PDS silica. Octadecylation may result in a much more ordered and hence restricted phase mass. If the 3PDS/ODS phase is contacted with methanol the decay profile of this material closely resembles the decay profile of the excimer from unmodified 3PDS in contact with water. This is further evidence for bonded phase aggregation in contact with hostile solvents. Conclusions

The results indicate that the texture of monomeric, hydrocarbon-bonded phases is controlled mainly by specific solvent-

hydrocarbon interactions and the n-alkyl surface coverage. Solvent-induced conformation changes of the bonded phase predominate at low surface coverages, where the alkyl chains are relatively unrestricted and free to adopt a number of different configurations. Solvent-hydrocarbon interactions strongly influence the flexibility and extension of the alkyl chains from the silica surface at low surface coverages; however, the magnitude of these textural changes become attenuated as the n-alkyl surface concentration approaches saturation. At near-saturation surface coverages, the bonded phase is pictured as being composed of highly organized n-alkyl clusters, where the octadecyl chains are initially fully extended from the surface in a dense and efficiently packed microstructure. Solvents compatible with n-alkanes tend to swell the microstructure and promote full extension of bound n-octadecyl chains, while incompatible solvents tend to promote collapse of the chains upon each other and toward the underlying surface. Thus, local n-alkyl density effects as well as specific solvent-hydrocarbon interactions influence the ultimate conformation of monomeric n-octadecyl bonded phases.

Acknowledgment. This work was supported, in part, by a grant from the National Science Foundation, No. CHD-8500658. Registry No. Methanol, 67-56-1 ; tetrahydrofuran, 109-99-9; hexane, 110-54-3; acetonitrile, 75-05-8.

Quantum Chemical Investfgatlon of Support-Metal Interactions and Their Influence on (M = Ti, Si)‘ Chemisorption. 2. Strong Metal-Support Interaction in H-NI-MO, Helmut Haberlandt* and Friedrich Ritschl Central Institute of Physical Chemistry, Academy of Sciences of the GDR, DDR-1199 Berlin, German Democratic Republic (Received: November 21, 1985)

Semiempirical CND0/2 cluster calculations are performed on several surface clusters of oxidized and partly reduced support surfaces and on their interaction with a nickel atom. An electron transfer directed to the metal is found for a reduced support surface that originates from occupied surface states in the band gap of clean surface clusters acting as electron donors. The direction of electron transfer is reversed for completely oxidized surfaces interacting with nickel. The binding energy of a single hydrogen atom to the nickel atom is lowered due to support influence by two effects: (i) There is a discontinuous decrease in going from oxidized to reduced support clusters which is connected with the Occurrence of doubly occupied interface states in the gap of nickel-support systems. This indicates a strong interaction between the nickel and the supporting oxide. (ii) The H binding energy decreases continuously due to the contributions of “support orbitals” to the highest occupied or lowest unoccupied molecular orbital which is expected to interact with hydrogen. There is, however, no direct and unique connection between electron transfer and diminution of H binding energy. The latter is as a rule closely related to a weakening of the nickel-support bond. The local bond strength of the Ni-H bond is hardly influenced except for one model where it is drastically decreased. The results are qualitatively equal for Ti02 and Si02as support. The consequences for the SMSI effect are discussed.

1. Introduction The metal-support and metal-additive effects in catalysis are widely discussed in the literature24 due to their importance with respect to the activity and selectivity of catalysts. BondS distinguished between weak (WMSI), medium (MMSI), and strong (SMSI) metalsupport interactions according to the strength of the effects observed. The WMSI occurs predominantly with transition metals supported on “nonreducible” oxides like SiOz, Alz03, and MgO. The SMSI is found mainly with transition (1) Part 1: Haberlandt, H.; Ritschl, F. J . Phys. Chem. 1983, 87, 3244. ( 2 ) Metal-Support and Metal-Additive Effects in Catalysis, Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1982. (3) Proceedings of the 8th International Congress-on Catalysis, Berlin ( W e s f ) 1984; Verlag Chemie: Weinheim, DECHEMA 1984. (4) Bond, G. C.; Burch, R. In Specialist Periodical Reports: Catalysis, Vol. 6, Royal Society of Chemistry; Burlington House: London, 1983; p 27. (5) Bond, G. C., ref 2, p 1.

0022-3654/86/2090-4322$01.50/0

metals supported on reducible (transition-metal) oxides like T i 0 2 and Nb20S. The so-called SMSI effect is usually defined as the strong diminution, indeed suppression, of H2 and CO chemisorption on transition metals supported on the oxides after a high-temperature reduction (at about 773 K) in hydrogen. This effect was observed for the first time by Tauster, Fung, and Garten6 in the system Pt-TiOZ. Recently, the SMSI effect has been observed also with transition metals supported on “nonreducible” oxides7**after reduction in a hydrogen atmosphere at temperatures near 1100 K. The origin of this effect is still unclear and remains a subject of current interest. A number of explanations have been proposed to describe the adsorption and (6) Tauster, S. J.; Fung, S . C.; Garten, R. L. J . Am. Chem. SOC.1978.100, 170.

(7) Ren-Yuan, T.; Rong-An, W.; Li-Wu, L. Appl. Catal. 1984, IO, 163. (8) Praliaud, H.; Martin, G . A. J . Catal. 1981, 72, 394.

0 1986 American Chemical Society

Strong Metal-Support Interactions

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catalytic behavior of titania-supported metals. Among them are the foll~wing:~ (i) localized10and delocalized electron transfer”-” between the metal and the support, (ii) changes in the metal structure such as formation of thin ~ a f t s , ’ O *(iii) ~ ~ migration of (reduced) titanium oxide onto the (decoration model), (iv) creation of special active sites at the interface between the metal and the support,18 (v) hydrogen spillover to sites on the t i t a ~ ~ i a . ~Several . ’ ~ of these mechanisms may act also simultaneously. Santos et al.I5 state that “the acronym ‘SMSI‘ is an umbrella under which a number of different phenomena may be covered”. There is an increasing number of papers considering heterogenous metal catalysts as well as specially prepared metalsupport systems by modern surfacespectroscopic methods like XPS, UPS, AES, EXAFS, X-ray absorption spectroscopy, and transmission electron microscopy. The findings of these studies are of importance both for the proposal of well-founded structural models (cf. section 3) and for a comparison with the theoretical results obtained with these models (section 4). Concerning SMSI systems, two basic questions remain from hitherto experimental and theoretical work (a) Is the SMSI effect of electronic or geometric (decoration model) nature or both? (b) If there occurs an electron transfer between the support and the metal, does it play a significant part in the SMSI effect? Many experimental investigations have been devoted to the metalsupport interaction in the system Ni-Ti02 (e.g., ref 18-35). Already about two decades ago SzabB and Solymosi20assumed a strong electronic interaction between nickel and the Ti02 support and an electron transfer to the metal. The activity and selectivity of this system with respect to reactions like hydrogenation of C 0 2 , have CO, or carbon and hydrogenolysis of ,-hexanel*~1b~23~5b,26,zg,33 been treated as well as its properties in C O and H2 chemisorption18J9,21,22,24,25aq27-3i,33-35 and its structural and electronic properties.18-20,21a,22-24,2628,30,32,34,35 The most relevant results of these papers will be included in the discussion of the theoretical findings in section 4.

(9) Vannice, M. A.; Sudhakar, Ch. J . Phys. Chem. 1984,88, 2429. (10) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121. (1 1) Meriaudeau, P.; Ellestad, 0.H.; Dufaux, M.; Naccache, C. J . Catal. 1982, 75, 243. (12) Herrmann, J. M.; Pichat, P. J . Card 1982, 78, 425. (13) Chen, B. H.; White, J. M. J . Phys. Chem. 1982, 86, 3534. (14) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. J. Catal. 1979.56, 390. 1979, 59, 293. (15) Santos, J.; Phillips, J.; DumesiE, J. A. J . Catal. 1983, 81, 147. (16) Resasco, D. E.; Haller, G. L. J . Card 1983, 82, 279. (17) Sadeghi, H. R.; Henrich, V. E. Appl. Surf. Sci. 1984, 19, 330. (18) Burch, R.; Flambard, A. R. J . Catal. 1982, 78, 389. (19) Jiang, X. Z.; Hayden, T. F.; DumcsiE, J. A. J. Card 1983, 83, 168. (20) (a) Szab6, Z.; Solymosi, F. Proceedings of the 2nd International Congress on Catalysis Paris, 1960; Technip.: Paris, 1961; p 1627. (b) Solymosi, F. Catal. Rev. 1967, I , 233. (21) (a) Vannice, M. A.; Garten, R. L. J . Catal. 1979, 56, 236. (b) Vannice, M. A,; Garten, R. L. ibid. 1980, 66, 242. (22) Smith, J. S.;Thrower, P. A.; Vannice, M. A. J . Catal. 1981,68,270. (23) Kao, C. C.; Tsai, S. C.; Chung, Y. W. J . Catal. 1982, 73, 136. (24) Simoens, A. J.; Baker, R. T. K.; Dwyer, D. J.; Lund, C. R. F.; Madon, R. J. J . Catal. 1984, 86, 359. (25) (a) Weatherbee. G. D.: Bartholomew. C. H. J . Caral. 1984.87, 55. (b)‘Bartholomew, C. H.; Vance, C. K. J . Card 1985, 91, 78. (26) Chung, Y. W.; Xiong, G.; Kao, C. C. J . Catal. 1984, 85, 237. (27) Takatani, S.; Chung, Y. W. Appl. Surf.Sci. 1984, 19, 341. (28) Takatani, S.; Chung, Y. W. J . Catal. 1984, 90, 75. (29) Vance, C. K.; Bartholomew, C. H. Appl. Catal. 1983, 7 , 169. (30) Herrmann, J. M. J . Catal. 1984, 89, 404. (31) Raupp, G. B.; DumesiE, J. A. J . Phys. Chem. 1984, 88, 660. (32) Baker, R. T. K.; Chludzinski, J. J.; DumesiC, J. A. J . Catal. 1985, 93, 312. (33) Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J . C m l . 1980.65, 335. (34) Mustard, D. G.; Bartholomew, C. H. J . Card 1981, 67, 186. (35) zur Loye, H. C.; Stacy, A. M. J . Am. Chem. SOC.1985, 107, 4507.

Up to now there is only one theoretical paper36considering the strong metalsupport interaction in the system Pt-Ti02. In brief, it was stated that the SMSI effect is due to a localized electron transfer via a Ti-Pt bond from the surface site of the reduced Ti0, surface to the metal. N o gas-phase species interacting with the metal have been investigated. Since our first paper in this series’ on the WMSI in the system H-.Ni-Si02 several theoretical papers have been published on the WMSI and on metal-additive effects. A recent paper3’ using the SCF-local spin density-scattered wave method deals with the C O chemisorption on Ni supported on a A1203cluster. The calculated magnetic moment, especially of the interfacial nickel atoms, is diminished by the support influence. The Ni-CO bond strength decreases, but the electron transfer is not discussed.37 K ~ n considers z ~ ~ the binding of an H atom on NiO and on metallic nickel atoms supported on silicon dioxide in the framework of an embedded cluster approach and the a b initio SCF-UHF method. Joyner et al.39discussed the short-range character of the metalsupport interaction in the system Ni(l11) supported on graphite. Using a Green function technique they pointed out that there is a remarkable effect of the support on the local density of states only in the case of a nickel(l11) monolayer. The relative number of nickel atoms affected by the support is larger for smaller clusters and for clusters forming a large interface with the support (e.g. wetting).39 In this connection explanation (ii) of the SMSI effect should be remembered. After completion of the present work a paper has been published on the strong metalsupport interaction in Pt-TiO, and Ni-TiOPa The energy level diagrams of metal clusters and Ti02 clusters in oxidized and reduced forms have been treated separately. The electron transfer to the metal is attributed to the presence of occupied surface states as a result of reduction pretreatment. In the present paper, dealing with theoretical models, we discuss the charge-transfer process between the support and the metal and the influence of the support upon binding of atomic hydrogen on the metal. The main questions are as follows: (1) What is the direction of the electron transfer between the support and the metal? Does the electron transfer depend on the degree of reduction and/or on the chemical composition of the support? (2) Does the electron transfer affect the binding energy of a hydrogen atom onto the metal? If not, what other properties of the metal-support system do? In section 2 the computational methods are outlined. The model structures are established on the basis of investigations of the structures of the catalysts and on data for the corresponding bulk solids in section 3. Models of completely oxidized and of partly reduced TiOz and SiO, surfaces are considered. The results are given and discussed with special emphasis to the comparison of SMSI and WMSI systems in section 4. Some conclusions are drawn in section 5. 2. Computational Methods The calculations were performed as in part 1 of this series by the semiempirical CNDO/2 method using the original parametrization for the first- and second-row elements.41 Metal-metal bonds are expected to be of crucial importance in SMSI systems both at the metalsupport interface (vide infra) and in the metallic clusters. Therefore the calculations were run in the CNDO version of B a e t ~ o l dfor ~ ~the transition metals rather than according to Clack’s version43as in part 1. Within the framework of Baetzold’s (36) Horsley, J. A. J . Am. Chem. SOC.1979, 101, 2870. (37) Raatz, F.; Salahub, D. R. Surf. Sci. 1985, 156, 982. (38) Kunz, A. B. Phil. Mag. B 1985, 51, 209. (39) Joyner, R. W.; Pendry, J. B.; Saldin, D. K.; Tennison, St. R. Surf. Sci. 1984, 138, 84. (40) Viswanathan, B. In Advances in Catalysis Science & Technology, Proceedings of the 7th National Symposium on Catalysis, Feb 6-8, 1985; Baroda: India; 63. (41) Pople, J. A.; Beveridge, D. L. Approximate Molecular Orbital Theory; McGraw-Hill: New York, 1970. All calculations were performed with K(Wo1fsberg-Helmoltz parameter) = 1 for all off-diagonal core matrix elements. (42) Baetzold, R. C. J . Chem. Phys. 1971, 55, 4335.

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CNDO version, the parameters of Blyholder" for nickel were used. Titanium parameters following Blyholder's procedure were determined recently by one of the present authors45and were found to be compatible with the nickel parameters. These titanium parameters work also quite well in Ti0, cluster calculation^^^ yielding realistic valence band properties. A satisfying description of valence band properties has been obtained also for SiO;' even when using very small clusters.' The computational procedures ( U H F open shell, R H F closed shell) are the same as in part 1. The binding energy AEHof the hydrogen atom H on the metal-support cluster X is calculated from the total energies E ( H ) , E(X), and E(HX): AEH = (E(X)

+ E ( H ) ) - E(HX)

(1)

Bond lengths and angles are (with one exception) not varied due to the limitations of the semiempirical method but are taken from well-known related inorganic compounds. Concerning the reliability of CNDO/2 charge-transfer results we refer to the corresponding paragraph in part 1. It should be pointed out in this connection, however, that the compatibility of nickel and titanium parameters which is crucial for this question has been checked for the system Ni3Ti.48 Thus, qualitative conclusions may be drawn from charge distributions in systems exhibiting chemical bonds between the nickel atom and surface atoms of the support. Nonempirical quantum chemical methods are required in order to address the weak physisorptive interaction between metallic species and surface sites on an ideal oxide surface, e.g. the siloxane bridge on S O 2 ,which is dominated by dispersion forces.49 Such interactions are excluded from the present investigation. All calculations were performed using the cluster approach. The outer oxygen atoms of the SiO, clusters with tetrahedrally coordinated silicon were saturated with hydrogen atoms as described in part l . The corresponding oxygen atoms of Si02and Ti02 clusters with octahedrally cooidinated Si and Ti were saturated with pseudo hydrogen atoms H exhibiting a nuclear charge different from one in order to provide for charge neutrality of the respectiv? oxide cluster and to avoid unrealistic surface state^.^^.^^ The 0-H distance is 97 pm as is the 0-H distance for S i 0 2 clusters-with tetrahedrally coordinated silicon,' and the (Si,Ti)-0-H angle is 180'. Charge neutrality of the oxide clusters is a prerequisite for obtaining qualitatively correct charge distributions in the nickel-support clusters.

3. Model Structures The basic idea of modeling of the metal-support system is the same as in part 1. The process of preparation and reduction of a heterogeneous catalyst is not addressed. Rather, a reduced heterogeneous catalyst is modeled by permitting a metal cluster (in the present case a metal atom: single-nickel-atom model) to interact with clusters representing special surface sites on the support surface. This theoretical modeling procedure is motivated by chemisorption and spectroscopic results for "experimental model systems" in the literature (e.g. for Ni-TiO,, ref 23, 26, 27, 31, 32). Explicitly only the electronic effect of the support is investigated. The geometric effect connected with the redistribution of metal particles on the support surface, e.g. flattening, sintering, or change of dispersion due to the influence of the support, cannot be considered rigorously without a tremendous effort. In part the ~~~~~

~~~

~

~

~

~

~~

(43) Clack, D. W.; Hush, N. S.; Yandle, J. R. J . Chem. Phys. 1972, 57, 3503. In this version the parameters are calibrated for metal monoxide and fluoride molecules. (44) Blyholder, G. Surf. Sci. 1974, 42, 249. (45) Haberlandt, H. Phys. Status Solidi B, submitted for publication. (46) Haberlandt, H.; Miessner, H., manuscript in preparation. (47) Haberlandt, H.; Ritschl, F. Phys. Status Solidi B 1980, ZOO, 503. (48) Haberlandt, H., unpublished results. (49) Sauer, J.; Haberlandt, H.; Schirmer, W. In Structure and Reacttiuity of Modified Zeolites, Jacobs, P . A,, et al., Eds.; Elsevier: Amsterdam, 1984; p 313. (SO) Fleisher, M. B.; Golender, L. 0.;Shimanskaya, M . V. React. Kine?. Catal. Lett. 1984, 24. 25.

Haberlandt and Ritschl geometric effect is involved, however, in the special geometry of the model and cannot be separated from the electronic effect. Thus, properly chosen models are of crucial importance in order to obtain reliable results. This theoretical approach has been used frequently in investigations of solid-solid interface^.^' Before presenting the individual models in detail we will summarize the main results of papers which have dealt with the structural properties of such metal-support systems and in particular with the interfacial structure. Most of the work on Ni-TiO, is concerned with the determination of size distributions or mean or with the question particle sizes of nickel whether there is a TiO, migration onto the nickel.'9,24~26~27,3',32 When the sample was reduced at about 700 to 800 K, mean particle diameters between 4.624and 10 nm22depending on catalyst preparation have been observed by transition electron microscopy or X-ray diffraction. About 10% of the particles have a diameter of about 1 nm.22,24,34 A change of surface morphology, e.g. a flattening of crystallites, is proposed by several authors. 18,22,23.26,32-34 Utilizing Auger electron spectroscopy and sometimes sputter profiling and other techniques shows that the TiO, migration In our opinion, these hypothesis is widely favored.'9,24,26*27,31,32 findings confirm the idea of an intimate contact between metal and supporting oxide through a large interface. With respect to Ni-SO,, the situation is different. There are almost no papers dealing with the structure of this catalyst in the SMSI state. Particle diameters as small as 0.5 nmSZare found in Ni-SiO, in the "normally" reduced state, the mean diameters being in the range between 3.9* and 10 nm.22 In the SMSI state the mean diameter is enlarged to 12 nm accompanied with a diminution of saturation magnetizations8 The latter is assumed to be caused by the occurrence of an Ni-Si alloy. Indeed the SMSI effect is also observed in the nickel intermetallic compound and in Pt3Ti.54 Turning to the interface structure shows that the results are not unequivocal. From EXAFS spectra the existence of Ni-O-Si subunits in "normally" reduced Ni-Si02 is c o n c l ~ d e whereas d~~~~~ in Co-Ti0, no indication of Co-0-Ti bonds has been found.57 The Ni-O bond length was determined to be 20556and 200 pm,55 respectively, depending on the preparation. Unfortunately, there are no EXAFS studies on Ni-TiO, either in the "normal" or in the SMSI state. Short et al.,58however, using EXAFS found no evidence of Pt-Ti or Pt-0 bonds in Pt-TiO, in the SMSI state. In a very recent studyS9fully reduced and highly dispersed RhAl,O, has been investigated. The Rh particle sizes ranged between 0 . 6 and 1.2 nm. Rh-0 distances of 270 pm have been found and attributed to the interaction of interfacial Rh atoms with 2 to 3 oxygen ions of the support. These results suggest that it is useful to investigate theoretical models of nickel in contact with special surface sites of the support also allowing for the occurrence of intermetallic bonds. Furthermore, it is well-known that reduction is of essential importance in preparing SMSI catalysts. This suggests the choice of models exhibiting different oxidation states of the metal cation (Si"+, Ti"+) which forms the supporting oxide. Three types of models are considered in the present study as suggested from experimental findings. These are models of (A) the completely oxidized support surface (further designated as M4+models since the corresponding Si and Ti cations have formal (51) Herman, F. J . Phys., Colloq. C5, Suppl. No. 4, 1985, 45, C5-375. (52) Coenen, J. W. E. In Preparation of Catalysts II, Delmon, B., et al. Eds.; Elsevier: Amsterdam, 1979; p 89. (53) Nuzzo, R. G . ;Dubois, L. H. Appl. Surf.Sci. 1984, 19, 407. (54) Bardi, U.; Somorjai, G . A.; Ross, P. N. J . Catal. 1984, 85, 272. (55) Ovsjannikova, J. A.; Kraisman, V. L.; Starzev, A. N.; Ermakov, Ju. I . Kine?. Katal. 1984, 25, 446. (56) Tohji, K.; Udagawa, Y . ;Tanabe, S.; Ueno, A. J . Phys. Chem. 1984, 106, 612. (57) Tohji, K.; Udagawa, Y . ;Tanabe, S.; Ida, T.; Ueno, A. J . Am. Chem. SOC.1984, 106, 5172. (58) Short, D. R.; Mansour, A. N.; Cook, J. W.; Sayers, D. E.; Katzer, J. R. J . Catal. 1983, 82, 299. (59) van Zon, J. B. A. D.; Koningsberger, D. C.; van't Blik, H. F. J.; Sayers, D. E. J . Chem. Phys. 1985, 82, 5742.

The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4325

Strong Metal-Support Interactions

Q I

I

I

bl

Figure 2. Nickel interacting with T i 0 surface clusters (rock salt structure): (a) 0-terminated (1 11) surface, (b) Ti-terminated surface.

Q I

dJ

Q

Figure 1. Models of nickel supported on an oxide surface: (a) Aa (surface cation M = Si4+), (b) Ab (Si4+),(c) Ac (Si4+),(d) Ad (Si4+), Cd (Si3+), pyramidal forms, (e) Ae (Si4+), Af (Ti4+), Ce (Si3+), Cf (Ti3+). Small circles represent oxygen atoms. Saturating H and H atoms are not drawn. For further explanation see text.

charges of +4), (C) the partly reduced support surface (M3+ models, the formal charge is +3 due to an oxygen vacancy at the interface), and (D) titanium monoxide (Mz+ models). The designations of all models and the respective surface cations are summarized in Table I (models of type B, cf. part 1, are not treated here). The models Aa, Ab, and Ac of isolated, geminal, and vicinal Si-0 groups interacting with a nickel atom are shown in Figure

la-. Therein silicon is tetrahedrally coordinated as usual in bulk Si02with the exception of stishovite60exhibiting octahedral silicon coordination. There exist, however, indications for the presence of higher coordinated silicon also on silica surfaces6' and in the bulk of low pressure silicates.60 Model Ae (Figure le) of a fivefold coordinated silicon is isomorphous to model Af (Figure le) representing a perfect Ti02(l 10) rutile surface.62 The latter model was chosen also by H o r ~ l e yin~his ~ investigation of the system Pt-TiO,. It has been criticized as being too but this criticism has been refuted64by qualitative arguments. We found by calculating one of the lafger clusters proposed by H e n r i ~ h ~ ~ that the small cluster Ti(OH)5" indeed yields a correct picture of the situation at the T i 0 2 surface (vide infra). Model Ad (Figure Id) is generated by splitting off an OHgroup from an isolated silanol (Si-OH) group or an 0-anion from model Aa. It is artificial and chosen as an oxidized counterpart to model Cd (Figure Id). The latter model may occur as a result of the reaction of the nickel compound with an isolated Si-H group on a partly reduced silica surface forming a Ni-Si bond. Such Si-H groups have been found by IR spectroscopf5 when reducing silica in a hydrogen atmosphere near 1073 K if traces of metals able to dissociate H2 are present. There is no doubt that we are concerned with such a situation in heterogeneous nickel-silica catalysts reduced at higher temperatures (see, eg., ref 8). It should be noted here that there are no reduced isomorphous counterparts to models Aa, Ab, and Ac. The occurrence of model Cd is also conceivable as a result of a reduction of model Aa but model Cd reveals no oxygen ion as a nickel nearest neighbor atom. Models Ad and Cd are the only ones that account for surface reconstruction at least in part. In both clusters a silicon dangling bond represents the surface. In the former model it is empty but in the latter model it is filled with a single electron. Thus, reconstruction is expected to be of crucial importance leading to a planar structure for model Ad (sp2 hybridization of Si) but to a pyramidal structure for model Cd (sp3 hybridization of Si). We performed a rough optimization of the structure taking three values of the (Ni-)Si-O angle, namely 109.47' (further designated as pyramidal, py), 90' (planar, pl), and an intermediate value of 99.735' (in). The bond lengths were kept constant the same as that of Si-0, being 155 pm in idealized P-cristobalite.66 Gibbs, V. Am. Mineral. 1982, 67, 421. Low, M. J. D.J . Phys. Chem. 1981, 85, 3543. Woning, J.; van Santen, R. A. Chem. Phys. Lett. 1983, 101, 541. Henrich, V. E. J . Cotol. 1984, 88, 519. Horsley, J. A. J . Carol. 1984, 88, 549. Fripiat, J. J. In Soluble Silicates, Falcone, J. S., Ed.; American Chemical Society: Washington, 1982; ACS Symp. Ser. No. 194, Chapter 11, p 179. (60) (61) (62) (63) (64) (65)

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Haberlandt and Ritschl

TABLE 1: Orbital Energies of Highest Occupied (toss) and Lowest Unoccupied (eEA) Surface States and Net Charges of Surface Cations M"+ (9M)and of the Nickel Atom (qNi)Interacting with the Respective Surface Site clean support clusters Ni-support clusters model Aa Ab Ac Adpl Ae Af CdPY Ce Cf Da Db

M"+ Si4+ Si4+ Si4+ Si4+ Si4+ Ti4+

Si3+ Si'+ Ti3+ Ti2+ Ti2+

multiplicity doublet triplet triplet singlet singlet singlet doublet doublet doublet singlet triplet

toss,

e"

-7.69 -5.09 -3.42 -4.39b -3.98

fEA,' eV 3.21 2.68 (3.20) 2.84 (3.58) 2.80b 2.29 3.10 (3.97)b3C 1.58 3.26 4.26 (4.54)b5c 2.32 2.1 4b

q M , lel 1.53 1.49 1.42 1.39 1.64 1.14 0.96 1.23 0.58 0.56 0.12

multiplicity doublet triplet triplet triplet triplet triplet doublet doublet doublet singlet triplet

qNir

lei

0.44 0.32 0.32 0.12 0.16 0.17 0.04 -0.08 -0.22 -0.31 -0.48

'The energy of a second unoccupied surface state is given in parentheses. bDoubly degenerate. 'Ti 3d, e symmetry.

Models Ce and Cf (Figure le) are the partly reduced counterparts to models Ae and Af. The same geometry is chosen in the former and latter clusters but the formal charge of Si and Ti is reduced to +3 as proposed by H o r ~ l e ydue ~ ~to partial reduction of the surface in hydrogen a t elevated temperatures. Models Ae and Cf may also be regarded as M+3-0 vacancy complexes on a (reduced) (1 10) rutile-type surface. The charge neutrality of all models is provided by a proper value of the nuclear charge of saturating H atoms as outlined in section 2. The Si-0 distance in models Ae and Ce is 178 pm60 as in stishovite; the Ti-0 distance in models Af and Cf is 196 pm.67 Models Da and Db represent oxygen and titanium terminated (1 11) surfaces of titanium monoxide interacting with nickel. They are depicted in Figure 2. The Ti-0 bond length is 209.5 pm as in bulk Ti0.69 Since information on the actual geometry at the metalsupport interface is scarce (vide supra) the corresponding bond distances are taken from well-known related inorganic structures. In all models investigated the Ni-Ti distance is 254.53 pm as in Ni3Ti.'& The Ni-0 distance of 209 pm] is similar to the experimental finding^^^,^^ given above. The Ni-Si distance is estimated from ionic and covalent radii to be 220 pm.70b Our theoretical results of clean surface clusters and their comparison with experimental results (cf. section 4.1) give a further motivation for using the models presented above. For type A models the influence of the support on the Ni-H distance is expected to be small (cf. part 1) and of minor importance for the qualitative conclusions to be drawn here. As a distance variation shows, this holds true also for type C models except for model Cdpy. Hence, in the former models the Ni-H distance is kept equal to 160 pm which is nearly identical with the equilibrium distance of the N i H molecule in the doublet state (159 pm) obtained within the CNDO-UHF frame using Blyholder parameters. In the latter model the optimized Ni-H distance of 203 pm has been used. In all models the Ni-H bond is perpendicular to the surface of the support cluster. The use of the single-nickel-atom model throughout this paper is a strong limitation compared with the actual situation. Nevertheless it is our opinion that an understanding of this lower-limit model is a prerequisite for investigations of extended models. The well-founded models selected here allow for preliminary conclusions which will be checked in future work on large clusters. (66) Wyckoff, R. W. G. Crystal Structures, Vol. I; Interscience: New York, 1951. (67) The Ti(OH)6n-.clusters have been calculated in Ohsymmetry. In bulk TiOl the corresponding subunits are slightly elongated exhibiting four T i 4 distances equal 194.4 pm and two distances equal 198.8 pm.68 The length of 196 pm is an average value. (68) Vos, K. J. Phys. C 1977, 10, 3917. (69) Mattheiss, L. F. Phys. Rev. B 1972, 5, 290. 1972, 5, 306. (70) (a) Fischer, T. E.; Kelemen, S. R.; Wang, K. P.; Johnson, K. H. Phys. Reo. B 1979, 20, 3124. (b) For models (Ae) and (Ce) the C N D 0 / 2 equilibrium distances are about 250 and 240 pm, respectively. Both the electron transfer and the position and character of interface states (cf. section 4) do not change appreciably. Due to the unreliability of CNDO/2 equilibrium distances and the lack of experimental information we prefer to use the estimated distance.

4. Results and Discussion 4.1. Clean Surface Clusters. The results of the clean surface sites of models Aa, Ab, and Ac were presented and discussed in part 1. Model Cdpy of an unreconstructed Si-terminated (1 1 1) SiOz surface reveals an occupied surface state in the band gap which has been observed also experimentally by XPS.7' This surface state corresponds to the paramagnetic E,' center observed in crushed S i 0 2 powder under UHV condition^.^^ The corresponding molecular orbital (MO) level is emptied when turning to model Ad by removing the unpaired electron. The explanation of the empty surface state observed by electron energy loss spectroscopy (ELS)73is still controversial, but this state is certainly caused by a surface defect (e.g. Si--Si interaction or an Si=O g r o ~ p ) . ' ~Indeed, , ~ ~ there exists no occupied dangling-bond surface state when "oxidizing" model Cd to become model Aa (see part 1 for a discussion). Thus, our theoretical results for models Aa and Cd are in agreement with the existing experimental findings and theoretical interpretation^.^^,'^ Considering the total energies, the first of models Cdpy, Cdin, and Cdpl is the most stable one as expected (cf. section 3). In the case of models Ad, the planar structure is the most stable. It reveals a doubly degenerate empty surface state. One M O is dominated by the Si3sand the other by the Si3porbitals. The high stability of model Adpl corresponding to a surface reconstruction resembles the situation expected at the Si-terminated S O 2 (111) surface.73 A reconstruction should occur similar to that observed at the S i ( l l 1 ) (2 X 1) surface. A splitted surface band was observed in the latter system which originates from the half-filled surface band at the unreconstructed surface (cf. model Cd). The lower part of the splitted surface band is occupied and the upper part is empty. Models Ce and Cf as well as model Cd reveal a half-filled M ( M = Si, Ti) dangling-bond state in the gap (Table I), the existence of which is crucial for the metal-support interaction as shown in section 4.3. For the oxidized counterparts Ae and Af, the corresponding M O levels are emptied as for model Ad. Several other empty surface states at higher energy are found in the level schemes of models Af and Cf due to unoccupied Ti3dorbitals. The overall agreement of the "bulky" stishovite and rutile clusters with the valence band properties of corresponding bulk materials is satisfying (for T i 0 2 see ref 46). The surface properties of our oxidized and reduced Ti02clusters outlined above are in line with the experimental and other theoretical results: UPS measurements gave no evidence of occupied surface states in the band gap on the clean oxidized T i 0 2 surface.23,75 An occupied surface state was found, however, 0.3 eV below the conduction band edge in the gap of reduced T i 0 2 using XPS, X-ray induced AES, and This state has been created (71) Stephenson, D.; Binkowski, H. J . Non-Cryst. Solids 1976, 22, 399. The observed peak at 0.75 eV above the bulk valence band edge was assumed to be due to an E' center at or near the surface by Bedford and K u n ~ . ' ~ (72) Hochstrasser, G.; Antonini, J. F. Surf. Sci. 1972, 32, 644. (73) Ciraci, S.;Ellialtio@u, S. Phys. Rev. B, 1982, 25, 4019. A detailed discussion of experimental results is given. (74) Bedford, K. L.; Kunz, A. B. Solid State Commun. 1981, 38, 411. ( 7 5 ) Henrich, V. E.; Kurtz, R. L. Phys. Rev. B 1981, 23, 6280.

The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4321

Strong Metal-Support Interactions 9Ni

r'e'J

I

I

0.51

/\

e \

*i

Figure 3. Dependence of electron transfer between metal and support on the degree of reduction of the clean support surface (see text). The designation of models is explained in section 3. Stars and crosses refer to octahedrally and tetrahedrally coordinated surface cations, respectively. Type A models are framed with a dash-dotted line and type C and D models with a dashed line.

by both Ar+ bombardment and thermal treatment (1 3 10 K, low oxygen pressure), the latter being similar to reducing conditions in the preparation of the catalysts. It is assumed to be due to a 2Ti3+-0 vacancy complex.76 Ti3+-related defects have been observed by ESR in reduced transition-metal-Ti02 systems (see, e.g., ref 33,77). These experimental findings have been confirmed by calculations of the electronic structure of perfect T i 0 2 surface~.~~ W e are not aware of any experimental or theoretical work concerning the surface of stishovite. However, the results should be similar to those for rutile. With respect to T i 0 we can state that our Ti404"bulky" cluster reveals a "valence band structure" which is in qualitative accordance with band structure calculation^.^^ The corresponding type D surface models are also expected to represent partly reduced T i 0 2 possibly existing on the support surface. In models Da and Db the number of atoms representing the surface (three) is too large compared with those representing the "bulk" (four). Thus, we only note that in both models Ti3d4sorbitals dominate the highest occupied and lowest unoccupied MOs. 4.2. Factors Governing Electron Transfer between Metal and Support. Which properties of the support surface or of the individual surface site influence electron transfer at the interface? Six possible factors will be. considered here: (a) degree of reduction of the support surface, (b) chemical composition of the support, (c) coordination number of the cation Mn+forming the supporting oxide, (d) number of interface bonds, (e) chemical nature of the nearest neighbor atom of supported nickel (02or M"'), (0 electron affinity of the respective surface site. The degree of reduction of the support is measured here from the net charge qMof a surface cation at the clean support surface calculated within the C N D 0 / 2 method. It must not be confused with the formal charge of the respective cation. The lower the (positive) net charge qM the higher is the degree of reduction. The electron transfer in these models may be expressed as the net charge qN,of the nickel atom on the support calculated the same way. The relationship between both quantities is depicted in Figure 3 (see also Table I). Roughly speaking, electron transfer toward nickel increases with increasing degree of reduction of the support. This holds for TiO, models as well as for SiO, models. The net charge on the nickel is negative in M3+and M2+ models with the (76) GBpel, W.; Anderson, J. A,; Frankel, D.; Jaehnig, M.; Phillips, K.; SchBfer, J. A.; Rocker, G. Surf. Sci. 1984, 139, 333. (77) Huizinga, T.; Prins, R. J . Phys. Chem. 1981, 85, 2156. (78) Munnix, S.; Schmeits, M. Phys. Rev. B 1983, 28, 7342.

exception of model Cdpy. In M4+models the nickel net charge is always positive, indicating an electron transfer to the support for a completely oxidized support surface (Figure 3). This trend can be even better visualized when one compares the respective M4+ model with its isomorphous reduced M3+ counterpart. In M3+ models nickel is always more negative than in the corresponding M4+ models (Table I). The different direction of electron transfer in the type A models compared with those of type C and D is fully understandable from the electronic structure of clean surface clusters which is in accord with the experimental results (section 4.1). All clean surface clusters of type C and type D models exhibit a partly filled surface state (OSS) in the band gap (Table I) which is acting as a donor state being responsible for electron transfer to the metal. A metal-support u bond is established in the type C models. This situation resembles that described by Solymosi20but in our case the donor state is of intrinsic nature whereas it is caused by extrinsic impurities in ref 20. For type A models, no occupied surface states are present but there are empty ones acting as electron acceptors. Therefore the direction of electron transfer is reversed. Comparing the results on SiO, with those on TiO,, we can state that nickel is more negative on reduced TiOz surfaces than on reduced S i 0 2 surfaces. It is, however, slightly more positive on completely oxidized Ti02 than on completely oxidized Si02 (model Af vs. model Ae). This effect of the chemical composition is not as marked as that of the degree of support reduction. The Ti3d orbitals are of minor importance here. The number of interfacial bonds is in most cases constant, namely one, with the exception of models Ab, Ac, Da, and Db. The dependence of electron transfer on this quantity should be checked further in subsequent studies. Models exhibiting K bonding between the metal and the support will be of particular interest. Concerning the dependence of electron transfer on the electron affinity cEA of the respective surface site (Table I), no straightforward correlation is possible. We attribute this behavior to the existence of several more empty surface states (Table I) of higher energy than the lowest unoccupied MO (LUMO) which are able to interact with the nickel atom. These states are either due to unoccupied Ti3d orbitals in TiO, models (Table I, not reported for models Da and Db) or to the existence of two surface oxygen dangling bonds (models Ab and Ac). The former indicates the (small) difference in behavior of TiO, models compared to the SiO, models as outlined above. The latter gives evidence of the effect of the number of interfacial bonds. Thus, the influence of the electron affinity pointed out in part 1, although present, is superimposed by other properties of the surface site (factors b and d mentioned a t the beginning of this paragraph). The influence of coordination number of the surface cation seems to be of minor importance with respect to electron transfer. The chemical nature of the nearest-neighbor atom of nickel leads to small changes in the amount of electron transfer but does not change the whole picture given above. In the case of 02-as the nearest neighbor, the nickel atom is more positive than in the case of Mn+ (model Aa vs. Adpl and model Da vs. Db) as expected from the corresponding electronegativities. In the former case, the contribution of "support orbitals" to the frontier orbital (Figure 6 ) of the respective M4+ model interacting with nickel is significantly higher. The consequences will be discussed in section 4.3. These theoretical results are in agreement with the experimental findings of several XPS4,16,23,7'83(see ref 40 for a review), X-ray absorption edge,83belectric c o n d ~ c t i v i t y and , ~ ~ ferromagnetic resonance83c investigations of different metal-support systems (79) Sexton, B. A.; Hughes, A. E.; Foger, K. J . Catal. 1982, 77, 85. (80) Chien, S. H.; Shelirnov, B. N.; Resasco, D. E.; Lee, E. H.; Haller, G . L. J . Catal. 1982, 77, 301. (81) Shpiro, E. S.; Djusenbina, B. B.; Antoshin G. B.; Tkachenko, 0. P.; Minachev, H. M. Kinet. Katal. 1984, 25, 1505. (82) Fung, S. C. J . Catal. 1982, 76, 225. (83) (a) Metal Microstructures in Zeolites, Jacobs, P. A,, et al., Eds.; Elsevier: Amsterdam, 1982. (b) Gallezot, P.; Bergeret, G. Ibid. p 167. (c) Sauvion, G. N.; Guilleux, M. F.; Tempere, J. F.; Delafosse, D. Ibid. p 229.

4328 The Journal of Physical Chemistry, Vol. 90, No. 18, 1986

Haberlandt and Ritschl

TABLE 11: Binding Energies AEH of an H Atom on Clusters Exhibiting a Single Ni-Support Bond, and Wiberg Bond Indices (WB) for Ni-H and Ni-M (M = Si, Ti) Bonds

model Aa Adpl CdPY Ae

Ce Af Cf

multiplicity triplet doublet triplet doublet

triplet doublet triplet

AEH,eV 1.77 2.70 0.30‘ 2.54 1.38 2.73 1.76

WBNi-H

WBNi-M

WB,;-M“

0.8880 0.9005 0.0572 0.8922 0.8606 0.8381 0.8387

O.330Ob 0.1743 0.9348 0.2356 0.3979 0.0742 0.3063

0.8354b 0.4353 0.9435 0.4454 0.8956 0.2562 0.9494

“Hydrogen-freecluster. * Wiberg bond index of the Ni-0 bond. CAtthe Ni-H equilibrium distance of including Ni-Ti0,.23~30*82 They are also in line with Horsley’s prediction. The general opinion is that the electron transfer is directed toward the metal in the case of SMSI systems or corresponding model systems whereas it is directed toward the support in “normally” reduced heterogeneous catalysts. Identifying M3+ and M2+ models with SMSI systems and M4+ models with “normally” reduced catalysts, our results are in accord with this opinion. There are, however, a number of papers reporting that there is no evidence of electron t r a n ~ f e r . ~It~should ~ ~ . ~be~ recalled that XPS chemical shifts may be affected not only by a charge transfer but also by relaxation effects which depend on the metal particle size (see, e.g., ref 7 9 ) . A very thorough preparation technique is needed in order to render such effects unlikely. Sexton et assuming that a flattening of metal clusters on the surface occurs (wetting), estimated a lower bound to that part of the chemical shift which originates from the charge transfer in their SMSI systems. This confirms the conclusion of an electron transfer to the metal for such s y ~ t e m s . ~ ~ ~ ~ ~ On the other hand, there is some doubt that electron transfer even if it really exists may have a remarkable effect on chemi* ~ ~ the electron transfer should be sorption or ~ a t a l y s i s . ~Indeed, very localized at the interface (see, e.g., ref 39 and 79) and could be of importance only if there is intimate contact between the metal and the support, e.g., due to a migration of reduced support particles or due to an ultrahigh metal dispersion. Currently, the main body of papers confirms the idea of migration of reduced support particles (cf. section 3). The models presented here are also in accord with this idea since they can be regarded not only as models of nickel supported on oxides but also as models of (partly reduced) oxide covering a nickel particle. With respect to low-temperature-reduced metal catalysts, the question arises whether the chemical shift is due to incompletely reduced metal clusters, at least in part. For nickel in physisorptive interaction with an oxygen bridge on the support surface very little electron transfer to the metal has been predicted49which is in line with conclusions from conductivity measurements on Ni-TiO, after low-temperature reduction.30 Finally, let us comment on the interaction between nickel and a larger cluster as proposed by H e n r i ~ hexhibiting ~~ two Ti3+ cations on the support surface. The C N D O calculations of a Ti209Hs4-cluster (singlet) interacting with a nickel atom yields a net charge on nickel of -0.391el. The respective clean surface cluster reveals a partly filled surface state in the gap. The nickel net charge on the oxidized counterpart is +O. 171el. This result is in accord with the direction of electron transfer obtained with the smaller clusters as discussed above. Thus, after having given a preliminary answer to the first question raised in the Introduction we turn to the secund question. 4.3. Support Influence on Binding Energy of Atomic Hydrogen on the Single-Nickel-AtomModel. The dependence of H binding energy AEH on electron transfer between support and metal is shown in Figure 4. We restrict the presentation to models exhibiting a single metal-support bond. It is obvious from Figure 4 that the correlation is not uniform for all models but that the models fall in two categories, with one set identical with type A models and the other set identical with type C models. In both

pm.

203

IC) I

/

I

I

“t

I

//*f

/ + d PY

Oi

Figure 4. Dependence of the binding energy of atomic hydrogen (AE,)

on the electron transfer between metal and support (see text). X designates the corresponding value of the NiH molecule within the CNDO framework used. Other symbols have the same meaning as in Figure 3.

categories the binding energy is proportional to the electron transfer to nickel. This change of AEHis further designated as change I. However, comparing H binding energies of models of the oxidized support surface with those of the reduced surface, a remarkable decrease of this quantity is observed. This decrease (change 11) is accompanied by a distinct transfer of electrons to nickel (cf. section 4.2). From the schematic representation of M O levels (Figure 5) the difference between type A and type C models (change 11) is understandable. All type C models reveal a doubly occupied nickel-support u-bonding MOS6in the gap which can be interpreted as an occupied interfacial state due to a M3+-Ni complex. It is caused by the interaction of the nickel atom with the half-filled surface state (OSS) in the band gap of clean M3+surface clusters (Table I). Models Ce and Cf, when interacting with hydrogen, show a redistribution of electrons in the metal-support system. ?this results in a weakening of the nickelsupport bond, the latter being governed in hydrogen-free clusters by the occupied interfacial state. An analysis of the Wiberg bond indices8’ (Table 11) in clusters without and with hydrogen on top the nickel reflects this weakening, whereas the local nickel-hydrogen bond strength is hardly changed comparing the M4+ and M3+ models. Thus, the diminution of formal hydrogen binding energy in these models (change 11) calculated within the supermolecule approach ac(86) This term is used to designate the corresponding two occupied a and

(84) Huizinga, T.; Prins, R., ref 2, p 11. ( 8 5 ) Ponec, V.,ref 2, p 63.

p MOs that are nearly identical and equal in energy. (87) Wiberg, K. B. Tetrahedron 1968, 24, 1083.

The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4329

Strong Metal-Support Interactions

L -

L -

L -

o]

I

A0

P

m

m

=

p Adpl

“

P Cd py

P O :

a AD

B Ce

B

B . (

u At

Cf

Figure 5. Schematic drawing of MO levels of nickelsupport models exhibiting a single interfacial bond. HOMO and LUMO levels are designated by H and L, respectively. a and j3 refer to spin-up and

spin-down orbitals and VB and CB to “valence band” and “conduction band” edges. MO levels below H are mainly due to nonbonding nickel states not reactive to hydrogen. Stars designate Ni dzz orbitals. cording eq 1 can be interpreted as originating from the decrease of the metal-support bond strength rather than from a decrease of the Ni-H bond strength. This effect is similar to the so-called demetallizationa8as observed for strongly localized chemisorption leading to a weakening of metallic bonding in the adsorption. For model Cpy the situation is different in that the C N D O optimization yields a considerably increased Ni-H bond length (cf. section 3). The H binding energy amounts to only 0.3 eV (Table 11). This drastic decrease of A& is due to a weakening of the local Ni-H bond strength while the corresponding Nisupport bond strength is practically the same (see Wiberg bond indices, Table 11). The above-mentioned doubly occupied interfacial state is not present in type A models, with the exception of model Aa (vide infra). In these cases the highest occupied M O (HOMO) is singly occupied and also metal-support bonding in character. The interaction with the hydrogen atom is dominated by the corresponding LUMO (Figure 5 ) . A consequence of this only singly occupied interface state is a weaker nickelsupport bond than in models of type C. This is indicated by a significantly lower Wiberg bond index (Table 11) in clusters free of hydrogen. We note here that a stronger Rhsupport bond for SMSI systems compared with “normally” reduced catalysts has been concluded recently also from S I M S investigations.*’ Interacting with hydrogen the nickelsupport bond in type A models is further diminished but not by as large an amount as in type C models. Consequently, the corresponding loss of energy is not as large either. In contradiction to models Adpl, Ae, and Af, model Aa reveals, as type C models do, a doubly occupied interfacial state being metalsupport bonding in character which is dominated by “support orbitals”. Hence, the nickel-support bond is strong (Table 11). Model Aa is the only model of those presented in Figure 1 exhibiting an oxygen atom as a nickel nearest-neighbor atom. The occurrence of a doubly occupied interface state shows that the latter is not a peculiarity of reduced surface models but may be present also at oxidized surfaces. There is, however, no reduced counterpart to model Aa exhibiting an oxygen atom neighboring the nickel. Change I, outlined above, occurs in both types of models. An increasing electron transfer to the support is accompanied by increasing contributions of ‘support orbitals” to the nickel-hydrogen bonding orbitals. These contributions were assumed to be related to the decrease of the H binding energy (cf. part 1). The H atom predominantly interacts with the frontier orbitals of clusters free of hydrogen. Hence, change I can nicely be illustrated by drawing the H binding energy vs. the contribution of “support orbitals” to the respective frontier orbital (Figure 6 ) . Again, type A models and type C models are clearly distinguished (88) Sachtler, W. M. H. Surf. Sci. 1970, 22, 468.

I

/ / /

/ + dPY

I 0

M

Contribution of support orbLta1s the frontier orbital

[%I

to

Figure 6. Dependence of H atom binding energy on the contribution of “support orbitals” to the LUMO for type A models and to the HOMO

for type C models. For model Aa also the contribution to the doubly occupied HOMO is used (see text).

due to change I1 as discussed above. However, they do not differ with respect to the amount of “support orbitals” contributing to the HOMO or LUMO. Change I is equivalent to the formation of oxidic nickel as in NiO which is well-known not to show any hydrogen adsorption (see,e.g., ref 31). This finding is in line with the theoretical results of K u n who ~ ~concluded ~ the presence of Ni,,, hybrid orbitals in nickel on S i 0 2 to be responsible for the reactivity of the system against hydrogen. These orbitals have not been found in Ni0.38 The metal-support effects in WMSI systems may originate in the first place from change I. Both changes of AEH observed here are of different character. Change I is continuous. Change 11, however, is discontinuous as is the change in chemisorption properties of the SMSI systems when switching from “normally” reduced to the high-temperature reduced state. Yet, the suppression of hydrogen adsorption is not necessarily caused by the diminution of binding energy of atomic hydrogen. There is evidence in the literature that the hydrogen adsorption energies on SMSI catalysts can even be i n c r e a ~ e d . ~ ’ , ~ ~ Another conceivable reason for the suppression of H2 chemisorption may be the occurrence of an activation barrier for H2 dissociation as assumed by DumesiE et al?’ for Ni-TiO,. Dubois and Nuzzog0 stated that no H2 dissociation occurs on the alloy Nisi, which also reveals a SMSI effect.53 While the present investigation does not discriminate between these two possibilities, it shows that a change of H binding energy is not caused in a straightforward manner by electron transfer. The occurrence of a doubly occupied interfacial state at the hydrogen-free nickel-(reduced) support systems provides the supported metal with a more inert character than in the case of a half-filled state which is more reactive. It is to be expected that this different behavior will also be of crucial importance in H, dissociation. Finally, the SiO, and TiO, models will be compared. As with electron transfer, the results concerning chemisorption of atomic hydrogen are qualitatively identical. This is caused by the use of structural isomorphous models and should not be confused with the Occurrence of the respective chemisorption properties in the real catalysts under the same conditions of preparation and reduction. Rather, identical chemisorption properties occur under different conditions of preparation and reduction (cf. sections 1 and 3). Thus, the SiO, models Cd and Ce represent a catalyst reduced at considerably higher temperature than the TiO, model Cf. 5. Conclusions In sum, let us offer five qualitative statements: (i) In accordance

with experimental data there is a distinct difference in the electronic structure of completely oxidized and partly reduced surface (89) Vannice, A.; Chou, P.J . Chem. SOC.,Chem. Commun. 1984, 1590. (90)Dubois, L. H.;Nuzzo, R. G . J . Am. Chem. SOC.1983, 105, 365.

4330

J . Phys. Chem. 1986, 90. 4330-4333

clusters calculated with the CNDO method. The former surface clusters reveal no occupied surface states in the band gap, whereas the latter ones do. (ii) An electron transfer to the metal is calculated for the interaction of nickel with surface sites on reduced support surfaces. It is caused by the existence of the occupied surface states on the clean surface exhibiting electron-donor capability. The direction of electron transfer is reversed for nickel interacting with completely oxidized surfaces. The amount of electron transfer to nickel increases with increasing degree of support reduction. (iii) The binding energy of atomic hydrogen is diminished by the support influence, but this diminution cannot be simply attributed to electron transfer between the support and the metal. Two explanations have been identified: First, turning from oxidized to reduced surface models we found a discontinuous decrease of H binding energy due to the existence of a doubly occupied interfacial state. The latter originates from an M3+-Ni complex at the interface. The corresponding M O is strongly nickelsupport u bonding and diminishes the reactivity of nickel against hydrogen. The lowering of H binding energy is closely related to the weakening either of the nickel-support bond (models Ce and Cf, or of the Ni-H bond (model Cdpy). Second, a continuous decrease of H binding energy is connected with the mixing of “support orbitals” to the frontier orbitals of metalsupport systems which are expected to interact with hydrogen. In this case as well a weakening of the nickelsupport bond is found whereas the local bond strength of the Ni-H bond is only slightly influenced. Thus, there exists a distinct electronic effect of the support and, in particular, of the degree of support reduction on the chemisorption of hydrogen atoms.

(iv) The conclusions hold for both TiO, and S i 0 2 as supports. It should be kept in mind, however, that the reduction occurs at considerably higher temperatures in the latter system. The Ti,, orbitals do not seem to be of crucial importance for the results obtained. (v) The SMSI effect defined as a strong diminution of H2 chemisorption after high temperature reduction of the catalyst is not necessarily connected with a decrease of hydrogen adsorption energy. There are results in the literature suggesting that the latter is even increased. However, the two causes for lowering H atom binding energy may also be of crucial importance in the process of H2 bond breaking enlarging the dissociation barrier. The picture outlined meets the assumption of Short et aLS8that the SMSI effect is not caused in a straightforward manner by electron transfer “but is due to more subtle and specific electronic changes”. Returning to the five explanations of the SMSI effect quoted in the Introduction, the present results are in line with the creation of special active sites at the metal-support interface as proposed by Burch and Flambard.’* The intimate contact between metal and support could be ensured by both the formation of thin rafts and the migration of (partly reduced) oxide particles onto the metal. The five points i-v emphasized above will be further examined in future work with larger model clusters. Particular attention will be given to their role in H2 dissociation.

Acknowledgment. The authors are grateful to Professor D. R. Salahub, MontrBal, for sending a preprint of the recent paper prior to publication, to Professor L. Ziilicke, Berlin, for critical reading of the manuscript and to Mr. K. Menning, Berlin, for useful hints at some relevant references.

Different Strong Metal-Support Interaction Effects on Rh/TIOp and Pt/TiO, Catalysts D. E. Resasco,* R. J. Fenoglio, M. P. Suarez, and J. 0. Cechini INTEMA (Institute of Materials Science and Technology), Universidad Nacional de Mar del Plata-CONICET, Mar del Plata, Argentina (Received: December 9, 1985; In Final Form: March 18, 1986)

Differences in the so-called strong metal-support interaction (SMSI) occurring in Pt/Ti02 and Rh/Ti02 catalysts following reduction at 770 K have been analyzed. The variation of the benzene hydrogenation reaction with temperature as well as the thermal programmed desorption of benzene suggests that the SMSI causes an increase in the heat of adsorption of benzene for Pt/TiOz catalysts. By contrast, no change in heats of adsorption is evident for Rh/TiO, catalysts. During the hightemperature reduction (HTR), a thermodynamic driving force operates, favoring an increase in the metal-reduced support interfacial area. As a response to this driving force, and due to their higher mobility, Pt particles can spread over the partially reduced titania while, at the same time, titanium suboxide species migrate onto the metal. Bulk interdiffusion and formation of new phases may even occur as well. These drastic changes in morphology would cause the observed changes in heats of adsorption on Pt/Ti02 catalysts. In the case of Rh/TiO, catalysts, the much lower mobility of rhodium particles would not allow any structure modification of the metal surface. Thus, as previously proposed (Resasco, D. E.; Haller, G. L. J . Catal. 1983,82, 279), only the surface migration of titanium suboxide species may occur for these catalysts during the HTR.

Introduction Following Tauster et al.,’ the term “strong metal-support interaction” (SMSI) has been generally used to describe the phenomena occurring on all Ti02-supported group VI11 (groups 8-10)19 metals during reduction at high temperatures. Even though SMSI may be caused by similar mechanisms in every group VI11 (groups 8-10) metal, its effect will not necessarily be identical for each one. As recently proposed,24 the SMSI can be ascribed to a surface migration of TiO, species (1 C x < 2) onto the metal particles. If the catalytic effect of SMSI were merely due to a physical blockage of metal sites by the Ti02 (1) Tauster, S . C.; Fung, S.C.; Garten, R. L. J . Am. Chem. SOC.1978,

moieties, one might expect similar SMSI effects among the group VI11 (groups 8-10) metals. However, there are some evidences indicating that, in fact, the modification of the metal catalysts by SMSI may differ from one metal to another. For instance, electron microscopy studies have shown that in the SMSI state the Pt particles adopt a pillbox-like s t r u c t ~ r ewhile , ~ Rh and Ir particles retain their original spherical shape.6 We are interested in studying the differences observed in titania-supported platinum and rhodium catalysts after a hightemperature reduction. In this work we have used benzene as a probe molecule to seek differences in the two catalysts. In particular, we have studied the benzene hydrogenation reaction and the thermal desorption of benzene after both low-temperature

100, 170.

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(2) Santos, J.; Phillips, J.; Dumesic, J. A. J . Card. 1983, 81, 147. (3) Resasco, D. E.; Haller, G . L. J. Caral. 1984, 82, 279. (4) Jiang, X.; Hayden, T.; Dumesic, J. A. J . Card. 1983, 83, 168.

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(5) Baker, R , Prestridge, E , Garten, R L J . Catal 1979, 56, 390 (6) Meriaudeau, P ; Ellestand, 0 , Dufaux, M , Naccache, C J Caral 1982, 75, 234

0 1986 American Chemical Society