J . Chern. Soc., Furuduy Trans. I, 1988, 84(1), 37-40 Effect of the Surface Structure of Metal Oxides on their Adsorption Properties K. Hadjiivanov* and D. Klissurski Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1040, Bulgaria A. Davydov Institute of Catalysis, Siberian Division of the USSR Academy of Sciences, Novosibirsk 630090, U.S.S.R. The effect of the first and second coordination spheres on the electron acceptor properties of coordinatively unsaturated metal ions at oxide surfaces has been studied theoretically. Metal ions with the same coordination number can differ strongly in their electrophilic properties depending on the coordination state of the ligands. It is shown that cations at crystal edges can have stronger, equal or weaker electrophilic properties than the corresponding cations situated on the planes forming the edges.The properties of coordinatively unsaturated oxygen ions and surface hydroxyl groups as well as the localization of the strongest Lewis-acid and Lewis-base sites are discussed. It has been e~tablishedl-~ that the Lewis acid sites on a metal oxide surface are coordinatively unsaturated (c.u.s.) metal ions. The variety of these sites for one oxide is due to the variety of surroundings of the metal ions, which are usually classified on the basis of their coordination number with respect to the lattice oxygen. The abundant experimental data3-* on oxide systems show that the number of C.U.S. cations on metal- oxide surfaces is usually much larger than the number of Lewis-acid sites.In addition, there are several types of surface compounds bonded to the metal ions (surface hydroxyl groups, non-stoichiometric oxygen e t ~ . ) . ' - ~ ? 9-13 In these cases the variety of metal-ion properties cannot be attributed to the difference in their coordination number only. The purpose of the present paper is to estimate the electrophilic properties of metal cations with the same coordination number but with different coordination states of the ligands and to summarize the resulting consequences. With most transition-metal oxides the metal-oxygen bond is, to a high degree, covalent. l4 This means that the second coordination sphere, i.e. the coordination state of ligands, may substantially affect the electron-acceptor properties of the central metal ion.Proceeding from these factors, we shall denote the ion state as MR+(al- a,-. . .-a,), where the number of bracketed figures gives the coordination number, the figure itself indicating the vacancies of each ligand, e.g. Mg2+(1-l-l-l-O) denotes a pentacoordinated magnesium ion where four of the oxygen ligands have one vacancy each, and one is coordinatively saturated. Consider a surface complex consisting of an acid-base pair, i.e. a C.U.S. metal ion and a C.U.S. oxygen ion having one vacancy each [fig. l ( a ) ] . Fig. l ( b ) presents the coordination of the oxygen ion from a Lewis acid and the resulting redistribution of electron density. Obviously, the coordination leads to a decrease in electron density around the central metal ion and a corresponding increase in the electron-acceptor properties of this ion, the coordination number remaining the same.A similar coordination from carbon dioxide15 or sulphur trioxide16 is accompanied by a 3738 Effect of Surface Structure on Adsorption Properties Fig. 1. (a) A complex of a C.U.S. metal ion and a C.U.S. oxygen ion with one vacancy each; (b) the same complex after coordination of the oxygen ion with a Lewis acid and the corresponding change m electron density. 0, Metal ion; 0, oxygen ion; 8, Lewis acid. Fig. 2. Scheme of some planes and edges on the titanium dioxide surface. (a) Planes and edges containing the titanium ion in the Ti4+(l-1-0-0-0) state (see text): 1, rutile (101) plane; 2, anatase (001) plane; 3, anatase (101) x (011) edge. (b) Planes containing the.titanium ion in the Ti4+(l-0-0-0-0) state: 1, anatase (100) plane; 2, anatase (101) and (011) planes.(c) Rutile (1 10) plane containing the titanium ion in the Ti*+(O-O-O-O-O) state. a, Pentacoordinated Ti4+, 0, twofold-coordinated 02-, @, tricoordinated 02-. pronounced shift (by ca. 15-30 cm-l) to higher frequencies of the carbon-oxygen stretching modes in the M-CO surface complex. If a C.U.S. metal ion, which is a strong Lewis acid, participates in the coordination, an analogous effect would be observed. The states shown in fig. 1 correspond to metal ions with the same coordination number but situated on different planes (edges, corners). A typical example exhibiting several types of states is titanium dioxide. Fig. 2 shows the structures of some planes and edges characteristic of the surface of anatase and rutile, the states of the titanium ion being Ti4+( I--1-0-0-0), Ti4+( 1-0-0-0-0) and Ti4+(O-0-0-0-0) [fig.2 (a), (b) and (c), respectively]. According to the scheme in fig. 1, transition from the titanium state in fig. 2(a) to the state in fig. 2(b) and, subsequently, to that in fig. 2(c) can occur, the electron-acceptor properties of the titanium ion increasing in the same direction. Thus, the pentacoordinated metal ions on an oxide surface can be ordered in the sequence of increasing electron-acceptor (acidic) properties as follows : (1) Mn+(l--1-1-1-0) or Mn+(l-1-1-0-0), a metal ion in an acid-base network, (2) M"+(l-1-0-0-0), a metal ion from an acid-base row, (3) M"+(l--0-0-0-0), a metal ion participating in an acid-base pair, (4) M"+(O-0-0-0-0), a metal ion with no C.U.S.oxygen ions in its first coordination sphere. One or several of these states can exist on the surface of an oxide crystal. For instance, the pentacoordinated magnesium ions in the (001) plane of magnesium oxide are in theK. Hadjiivanov, D. Klissurski and A . Davydov 39 Mg2+( 1-1-1-1-0) state [excepting Mg2+(2-1-1-1-0) ions situated close to the crystal edges], whereas all four states can be found on the titanium dioxide surface. All ions having a definite coordination number can be ordered in such a way according to their electrophilic properties. The C.U.S. oxygen ions on the surface, which represent Lewis bases, can also be systematized in this way, e.g. the basic properties of the twofold-coordinated oxygen ions of titanium dioxide will increase from fig.2(a) to As a result of the effect of the second coordination sphere (1) hydroxyl groups bonded to metal ion(s) with the same coordination number can differ in properties and spectral registration depending on the state of the metal ion(s) and (2) the strongest acid and base sites of the surface can be situated on different planes (edges, corners). Let us consider two planes of an oxide crystal, one of them consisting of acid-base fig. 2(c). pairs : M"+( 1--0-0-0-0) - 02-( 1-0-0) the other containing a complex of a metal ion with two vacancies and two oxygen ions with one vacancy: O2-(2-O-0)-Mn+( 1-1-0-0)-02-(2-0-0). The stronger Lewis base will be characteristic of the first crystal plane, while the stronger Lewis acid will correspond to the second plane.(3) Metal ions at edges can have the same coordination number but weaker electrophilic properties than the corresponding ions on the planes forming the edge. This can be seen in fig. 2. Titanium ions on the (101) and (01 1) anatase planes have stronger electrophilic properties than those situated on the (101) x (01 1) edges. In view of the fact that the Lewis acid sites in the case of anatase amount to ca. 8 YO of the C.U.S. titanium ions,3 the cations from acid-base rows have to be considered to be inactive since they possess the weakest electrophilic properties. Hence, with decreasing particle size (increase of cation concentration at the edges) the number of acid sites per unit surface will decrease, in agreement with the experimental data presented in ref.(5) and (17). In the literature the properties of disperse oxides are often described by one crystal plane alone, i.e. that predominantly exposed on the surface.** '-137 On the basis of general consideration one may expect less exposed planes to have a higher activity for adsorption due to their higher surface tension.lg To elucidate the surface properties one has to discuss the structure of all forms exposed on the oxide surface to determine the state of ions and to order them according to the decrease in acidic (basic) properties. In high-dispersity systems, where ions situated on crystal edges and corners constitute several per cent of the C.U.S. ions,1'20 the structures of these forms should also be discussed irrespective of the fact that some of them may be inactive towards adsorption.We thank Professor A. Andreev for helpful discussion. References 1 A. Zecchina, S. Coluccia and C. Morterra, Appl. Spectrosc. Rev., 1985, 21, 259. 2 A. V. Kiselev and V. I. Lygin, Infrared Spectra of Surface Compounds (Nauka, Moscow, 1972). 3 A. A. Davydov, IR Spectroscopy Applied to Surface Chemistry of Oxides (Nauka, Novosibirsk, 4 G. Munuera, F. Moreno and J. A. Prieto, Z . Phys. Chem., 1972, 78, 113. 5 K. Hadjiivanov, A. Davydov and D. Klissurski, Kinet. Katal., in press. 6 K. Tanabe, Muter. Chem. Phys., 1985, 3-4, 347. 7 E. Garrone and F. S. Stone, Proc. VZZZth. Int. Congr. Catal., Berlin, 1984 (Verlag-Chemie, Weinheim, 8 R. J. Cvetanovid and Y. Amenomiya, Adu. Catal., 1967, 17, 103. 9 M. Primet, P. Pichat and M-V. Mathieu, J. Phys. Chem., 1971, 75, 1216. 10 M. Primet, P. Pichat and M-V. Mathieu, J. Phys. Chem., 1971, 75, 1221. 11 A. Zecchina, S. Coluccia, E. Guglielminotti and G. Ghiotti, J . Phys. Chem., 1971, 75, 2774. 1984). 1984), vol. 3, p. 441.40 Efect of Surface Structure on Adsorption Properties 12 A. Zecchina, S. Coluccia, L. Cerruti and E. Borello, J. Phys. Chem., 1971, 75, 2783. 13 A. Zecchina, S. Coluccia, E. Guglielminotti and G. Ghiotti, J. Phys. Chem., 1971, 75, 2790. 14 I. Kostov, Mineralogy (Nauka i Izkustvo, Sofia, 1973). 15 J. C. Lavalley, J. Saussey and T. Rais, Proc. VIth Soviet-French Seminar on Catalysis, Moscow, 1983, 16 K. Hadjiivanov and A. Davydov, Kinet. Kafal., in press. 17 P. Vergnon, J. M. Hermann and S . J. Teicher, Russ. J. Phys. Chem., 1978, 52, 3021. 18 V. Lorenzelli and G. Busca, Muter. Chem. Phys., 1985, 34, 261. 19 B. F. Ormont, Structure oflnorganic Compounds (Gosudarstveno Izdatelstvo NT literaturoi, Moscow- 20 K. Hadjiivanov, D. Klissurski and A. Davydov, Ann. Univ. Sofa Fac. Chim., 1985, 79, in press. p. 97. Leningrad, 1950). Paper 611738; Received 27th August, 1986