Surface Chemistry of Titania (Anatase) and Titania-supported Catalysts Konstantin 1. Hadjiivanov and Dimitar G. Klissurski Institute of General and Inorganic Chemistry Bulgarian Academy of Sciences 1113 Sofia Bulgaria 1 Introduction Titania is the principal white pigment in the world and has wide applications in various branches of industry such as the production of plastics enamels artificial fibres electronic materials and rubber.' In the field of catalysis the interest in TiO is mainly due to its application as catalyst or support. In many catalytic reactions titania demonstrates neither a high activity nor a high selectivity,2 but in some cases it is among the most efficient of catalysts. For instance TiO is a classical photocatalyst for water decomposition and for other photocatalytic It shows very good prop- erties in the Claus process (SO + 2H,S +3s + 2H,0).6 Sulfate- modified titania is one of the few superacids that are at present the only known heterogeneous catalysts for Friedel-Crafts acylations and it is active in a number of other organic synthetic reaction^.^ However titania is mainly used as a support. This is of course due to some extent to its good mechanical properties its inertness and its low price. In contrast to silica which is one of the most widely used supports and whose normal role is to ensure that the supported phase has a large surface area titania interacts in many cases with the supported active phase. This behaviour determines the unique catalytic properties of the latter. Thus vanadia-titania catalysts are the best ones for a number of selective oxidation reactions. In par- ticular they have found industrial application in the oxidation of 0-xyleneX and in the DeNO process6 (NO + NH * N + H,O). Other titania-supported oxides such as Fe,O MOO WO MnO and CuO also exhibit a high activity and selectivity in the latter reac- tion. Noble metals are of special interest. Very often their activity with respect to CO hydrogenation strongly increases when they are sup- ported on titania.9 This is associated with the so-called strong metal-support interaction (SMSI) which will be considered below. Despite the fact that such catalysts have not yet found industrial applications they have been the subject of numerous investigations and the effective use of titania as a promoter is a result of these studies.'() TiO is also used as an additive for improving the prop- erties of some supports such as alumina. The use of titania in the field of catalysts is schematically presented in Figure 1. The properties of the titania surface are decisive for its catalytic application. On the one hand the catalytic activity and selectivity Konstantin Hadjiivanov was born in Sofia in 1958.He graduated in 1981 from the University of Sofia and started work as a Researcher at the Institute of General and Inorganic Chemistry of the Bulgarian Academy of Sciences where he received his PhD degree in chem- istry in 1990. During his scien- tific career he has served as a Visiting Scientist at the labora- tories of Professors A. Davydov (Novosibirsk Russia) G. Busca (Genova Italy) and H Knozinger (Munich Germany). He also joined the laboratories of Professors M. Che (Paris France) and J.-C. Lavalley (Caen France) as a Postdoctoral Fellow. He is cur- rently continuing his research on IR spectroscopic character- ization of catalyst surfaces. 61 of TiO itself depend directly on the kind and concentration of the different active sites and on the other the surface affects the forma- tion of definite structures of the active phase of titania-supported catalysts. Other important characteristics of TiO such as its adsorption ability and its pigmentation also depend on the surface properties. The breaking of the crystal lattice at the surface leads to the appear- ance of ions with a lower coordination number than that in the bulk. They are named coordinatively unsaturated surface (c.u.~) ions and have a tendency for additional coordination. The C.U.S. cations (in this case Tin+) possess an uncompensated positive charge and coor- dinate molecules with a free electron pair i.e. they are Lewis acids while the C.U.S. oxygen anions are Lewis bases and adsorb acidic molecules. Even after evacuation at high temperatures at which most admixtures are desorbed residual hydroxy groups exist on the oxide surfaces. They may have an acidic or basic character. The surface properties also depend on the deviations from stoichiometry e.g. the existence of various forms of excess oxygen possessing different oxidation activities and vice i'ersa formation of a reduced oxide which is responsible for the reduction properties of the samples. Various admixtures may also affect the adsorption sites. The adsorption sites are usually not directly observable (e.g the C.U.S.ions) or the data on them are scarce (surface OH groups). For that reason their interaction with so-called probe molecules is investigated. The properties of the adsorption sites are determined by the characteristics (stability and spectral features) of adsorbed probe molecules. Titania is usually encountered as three different modifications anatase rutile and brookite.' The latter modification has no practi- cal importance owing to its low stability. Anatase is thermodynam- ically stable up to 800 "C and an anatase-rutile transition occurs above this temperature. The rutile obtained however does not change into anatase after cooling owing to the high activation energy of the back transition. Both anatase and rutile crystallize in a tetragonal lattice. The coordination number of titanium is 6 and that of oxygen 3. The two modifications differ in number of common edges of the TiO octahedra 4for anatase and 2 for rutile. It is anatase that is more widely used for catalytic purposes. That is why we shall concentrate on this crystallographic modification. Prof. Dimitar Klissurski obtained his PhD (1968) and DSci (DHabil) (1981) degrees from the Bulgarian Academy of Sciences. He is author and coauthor of over 200 research papers 15 reviews,5 books and 45 patents. His research interests are mainly in the areas of cutalysis and material science. He is currently a member of the Internutionul Council oj the Congress on Catalysis und the Council of the international Mechanoc hemicul Association (MA) and President of the Bulgarian Mec hano- chemical Society. During the period 197G1992 he wm a member of the Editorial Board of the journal Matenals Chemistry and Physics and is currently on the Editorial Bourd of International Journal of Mechanochemistry. e g in alumina support \ nfl e g for CO hydrogenation /I Pr TI0 e g ft,O decomposition e g noble metals on TiO HeterogeneousCatalysis e g Claus reaction Figure 1 Application of TiO in the field of catalysis. 2 Preparation of Titania Different methods for synthesis of titanium dioxide result in prod-ucts with different structures (anatase or rutile) crystallinity and contaminants. As a consequence the surface properties of TiO strongly depend on the preparation technique. There are two main methods for obtaining titania for industrial purposes.'.' The first is the so-called 'sulfate method'. The titanium source (usually ilmenite FeTiO,) is dissolved in sulfuric acid the resulting solution is purified to remove iron and then hydrolysed. The precipitate titania in hydrous form is calcined in order to eliminate water The product obtained has the structure of anatase since the sulfate ions stabilize this modification. TiO prepared via the 'sulfate route' always contains sulfate ions which affect its surface acidity (see Section 4. I .). The other widespread technique for manufacturing titania is the vapour-phase oxidation of TiCl, TiCl + 0 -TiO + 2C1 (1) The products are characterized by a narrow particle size distrib- ution the average diameter being 20 nm. The main contaminant is C1 and sometimes Si and Al. The surface of this type of titania is deeply dehydroxylated. However the phase purity is not high. For instance the typical commercial fumed TiO (anatase) made by Degussa contains 25% of rutile. On a laboratory scale titania is usually prepared from TiCI or titanium alcoh01ates.I~ TiCl is hydrolysed by water at 5-10 "C and after that ammonia is added to obtain hydrous titania. The product formed is pure anatase whereas rutile is produced when the reaction is held above 80 "C. The degree of surface hydroxylation may be controlled by the temperature of the subsequent calcination. The samples prepared in this way usually contain chloride ions but their concentration may be minimized by washing of the final (cal- cined) product. To obtain chlorine-free titania one usually hydroly- ses titanium alcoholates [the preferred compounds are Ti(OC,H,) and Ti(OC,H9) since the lighter alcoholates react very violently]. The products are of high purity the carbon contaminants being eliminated by calcination of the products. Titanium dioxide is also produced by oxidation of metallic tita- nium by oxygen or water as well as by hydrolysis of titanium com- pound in the gas phase. As already noted heating of anatase at 800 "C results in the formation of rutile. Some contaminants and additives decrease the transition temperature which is often a reason for deactivation of anatase-supported catalysts. 3 Surface Chemistry of Anatase The name of anatase originates from the Greek avay7au~(upward tension upwards direction) and is due to the long form of its crys- tals. The cleavage planes mainly exposed on the anatase surface are 100 and 011101 (= means isostructural). However the faces 100=010 110 111 112 and 113 are also observed although to a smaller extent. I2 The same faces are characteristic of the crystallites CHEMICAL SOCIETY REVIEWS 1996 of disperse samples but the high concentration of crystal edges steps and comers should also be taken into account. The best way of studying the titania surface is to investigate the properties of the separate faces using single crystals and look for an anology with disperse samples. Since appropriate anatase single crystals are not available,' information on the surface chemistry of anatase has to be based on studies of disperse samples. Most results have been obtained by IR spectroscopy. Along with the properties reported in the literature which are common many differences have also been observed. The latter are due to (i) sample morphology (mainly quantitative differences) and (ii)the strong influence of impurities (which results in a qualitative change in the properties). 3.1 Surface Hydroxy Groups Hydrated titania obtained by hydrolysis of titanium salts has the crystal structure of anatase or rutile.14 It is assumed that its surface is completely hydroxylated. Drying even at room temperature leads to irreversible dehydroxylation. The degree of dehydroxylation is a function of the heating temperature and titania slowly loses its water when calcined up to 600 "C. Samples prepared by 'dry' methods (eg.fumed anatase) are characterized by a strongly dehydroxylated surface. However even after evacuation at 700 "C the so-called residual hydroxy groups are present on the anatase surface.''-20 They are observed in the 3800-3600 cm-I region of IR spectra. Their surface localization has been proved by isotopic exchange with heavy water' and by interaction with coadsorbed mole- cule~.~~,~~-~~)At least 12 kinds of OH groups have been reported by different authors. It has been established that some of them are due to impurities e.8. the band at 3740 cm-I appears as a result of the presence of ~i1icon.I~~~~) Most authors report the existence of two kinds of residual hydroxy groups on anatase which produce IR absorption bands at 3715 and 3675 cm-1.14-16 These two bands are readily observable after evacuation at about 300-400 "C. An increase in the evacuation temperature (combined with the use of modern evacuation techniques) results in a complex spectruml8.l9 containing a set of low-intensity bands in the 380&3600 cm-I region. The problem of the precise interpretation of the different hydroxy groups of anatase has not been solved. The more complex spectrum of highly dehydroxylated samples is probably due to different spectral behaviours of hydroxy groups which are identical from a crystallographic viewpoint but different with respect to their more distant surroundings (e.g. neighbouring sites which may or may not be occupied by OH groups). Water adsorption on strongly dehydroxylated anatase takes place in two ways part of the water molecules are dissociated forming hydroxy groups again while the major part of the water is adsorbed molec~larly~~[in the IR spectra this is demonstrated by the 6(H20) band at 1600 cm-I]. Repetition of the hydroxylation and dehydroxylation cycles leads to a decreasing concentration of the sites for dissociative adsorption probably owing to surface reconstruction.I There are many differences between the results obtained for the concentration of residual surface hydroxy groups on anatase. The methods which exclude OH groups from adsorbed water show that the concentration of residual hydroxyls is about 0.5 OH nrn-,.,' Thus irrespective of the fact that this concentration depends on the morphology of the samples and their pretreatment it may be stated that only a small part of the anatase surface is covered by hydroxy groups. 3.2 Surface Acidity In order to estimate surface acidity adsorption of weak (CO benzene) and strong (ammonia pyridine) bases is usually studied. l1 A typical probe molecule for fine determination of Lewis acidity is CO. Carbon monoxide is coordinated by a o-bond to metal cations which have no d electrons e.g. Ti4+. The stronger the bond the higher the IR stretching frequency of adsorbed CO. Testing oxi- dized anatase (containing no Ti3+ ions) with CO reveals the exis- tence of two kinds of c.u.s. Ti4+ ions differing in ele~trophility.'~-l~ The stronger site5 (vco at 2208 cm-I) are usually denoted by a SURFACE CHEMISTRY OF TITANIA-K I HADJIIVANOV AND D G KLISSURSKI t I 1 2250 2200 2150 2100 v/cm" Figure 2 FTIR spectra of CO adsorbed on anatase at 100 K (a+) Increasing amounts of CO and (f)an equilibnum pressure of 2 Torr CO (K Hadjiivanov and J C Lavalley unpublished results) while p is used for the weaker ones (vco at 2190 cm-') The increase in the equilibnum pressure results in saturation of the a sites with CO at about 2 Tom whereas the p sites are not fully occu- pied even at pressures above 100 Torr Both adsorption forms are weak and are easily destroyed by evacuation As a rule the concentration of a sites is lower and some samples with a regular crystal shape may even have no a sites l8 When CO is weakly adsorbed the number of monitored sites could be increased by lowenng the temperature of adsorption Carbon monoxide adsorption on anatase at CQ 100 K (see Figure 2) also leads to detection of both (a and p) types of sites and C-0 stretching modes are observed at 2206 and 2175 cm-l 22 The band at 2175 cm-l is very intense which implies that in addition to the sites detected at room temperature (p'),some sites which are inert at room temperature (p")are involved in the adsorption An addi- tional band at 2165 cm-I is also observable and corresponds to CO adsorbed on c u s Ti4+ sites possessing a very low electrophility (y sites) The increase in CO coverage (at both ambient and low tempera- tures) leads to a shift in maxima of the v(C0) bands to lower fre- quencies owing to two overlapping effects static and dynamic shifts The static shift is produced by interaction of the admolecules through the support while the dynamic interaction is usually of a dipole-dipole type and occurs through-space 22 The dynamic inter- action is observed when the admolecules are parallel are localized in the same neighbourhood and on the same plane and vibrate with the same intrinsic frequency The latter condition allows elimina- tion (and calculation) of the dynamic shift by the use of T0-l ,CO isotopic mixtures A dynamic shift is observed with the bands at 2175 and 2165 crn-I,,* I e the p and the ysites are situated on defi- inite anatase faces whereas with the carbonyls formed on the asites (2208 cm-1 band) at least one of the requirements for dynamic shift is not satisfied The question about the detection of Ti3+ ions using CO adsorp-tion is still open It has been reported that Ti3+-C0 carbonyls on partially reduced anatase give absorption bands at 21 15 cm- I According to other studies,18 Ti3+ ions cannot be observed by testing with CO because they are oxidized by the latter Indeed Ti3+ ions on anatase possess a very strong reduction ability and are oxi- dized even by water to Ti4+ lo Studies on the adsorption of strong bases lead to simultaneous and selective detection of both Lewis and Brgnsted acidities When ammonia is coordinated to c u s cations usually the S,(NH,) band at about 1200 cm-* is analysed This band is sensitive to the strength of the bond formed the stronger the bond the higher the frequency Ammonia protonation I e formation of NH; shifts the symmetnc N-H deformation modes of 1680 cm I However the band typical of ammonium ions is S,(NH;) at about 1450 cm I Ammonia adsorption on anatase leads to the appearance of two bands in the 1500-1000 cm region about 1220 (weak) and at 11 80 (strong) cm l5 l7 lX 2o The latter band is shifted to 1145 cm I at maximum coverage These results have been considered for a long time as evidence for the presence of two kinds (namely a and p) of titanium cations differing in electrophility I5 l7 This is in agreement with both the qualitative detection of a and p titanium cations by CO adsorption and the lower concentration of the a sites However some experimental results contradict this opinion (I) the amount of adsorbed ammonia significantly exceeds the amount of CO adsorbed at room temperature,lX and (21) using another strong base pyridine it is possible to detect one type of Lewis acid sites only l7 Recently it has been established that the band at 1220 cm is produced by dissociated ammonia since a new type of OH groups (3658 cm I) appear at the same time IxThe original OH groups of anatase do not protonate ammonia but those at 3658 cm-I display a weak Brgnsted acidity and form NH; groups at equilibnum ammonia presssures Thus the band at 1180 cm characterizes ammonia coordinated to Lewis acid sites and this mode is not sen- sitive enough to distinguish between a and p sites The data obtained by adsorption and coadsorption of NH and CO show the existence of several types of c u s titanium ions (Table 1) It is seen that in order to perform an efficient surface analysis of titania sup- ported catalysts (I e to establish the location of the active phase and the eventual existence of a bare titania surface) combined testing by CO and NH should be employed Ammonia and pyridine adsorption show that the original OH groups of anatase exhibit no Brgnsted acidity However titania hydroxy groups protonate tnmethylamine which is a stronger base l5 The acidity of hydroxy groups is determined more precisely and quantitatively by the hydrogen bond method Weak bases (e g benzene) form a hydrogen bond with the protons of the hydroxy groups during adsorption The higher the mobility (acidity) of this proton the stronger the bond formed and the more pronounced the weakening of the O-H bond respectively This causes a shift of the OH stretching modes to lower frequencies Benzene adsorption on anatase20 leads to an average shift of the OH stretching modes by about -120 cm I which corresponds to a weak acidity of the surface hydroxy groups It is interesting that preadsorption of ammonia on Lewis acid sites decreases the absolute value of this shift and it then amounts to -60 cm-l I e NH transmits electrons through the substrate and causes a twofold decrease of anatase surface hydroxyl acidity At present no sufficiently good probe molecule for determina- tion of surface basicity is known Most often CO is used for this purpose Adsorption of CO on anatase proceeds (I) on Lewis acid Table 1 Charactenstic IR frequencies of probe molecules (CO NH,) adsorbed on different sites of the anatase surface Characteristic frequencies (v/cm I) CO adsorbed CO adsorbed Sites at 293 K at l00K NH adsorbed at 293 K a 0; -2208 0 -2208 Omdxb-2206 Om -2206 P' 0 -2192 0,,,-2185 0 -2192 0 -1180 F -I Omax -2175 Om -1145 Y -0 -2165 Omax -2164 Sites for dissociative 7 7 1220 adsorption -OH groups -2155 a The value at zero coverage calculated by interpolation The value at saturation A . t ,rr.r I.,300 3600 3400 r 3200 3000 2800 1400 1200 1000 v/cm-' Figure 3 FTIR spectra of methanol adsorbed on (a) rutile and (b) anatase evacuated at room temperature (reproduced with permission from ref. 23). sites where linearly bonded CO is formed,17 and (ii)on basic sites i.e. 02-ions and OH groups. Carbonates are formed with the participation of the oxygen anions while the hydroxy groups form hydrogencarbonates.ls,l These compounds are decomposed during evacuation at room temperature which indicates a weak surface basicity of anatase. One can compare ammonia and water adsorption. In general it is nondissociative in both cases. However these molecules dissociate on particular centres whose concentration is low. This is evidence for the presence of a certain proportion of strong acid-basic pairs on the anatase surface. The centres of dissociative adsorption are more precisely monitored by alcohols. On most oxides alcohol adsorption proceeds with breaking of the 0-H bond. Dissociation of different alcohols also takes place on rutile while on the major- ity of the anatase sites these compounds are adsorbed coordina- ti~ely.~~The IR spectra of methanol adsorbed on both titania modifications are shown in Figure 3. The existence of coordina- tively bound methanol on anatase is shown by the S(C0H) band at 1365 cm-1 (arrowed) whereas this band was not observed with methanol adsorbed on rutile since the 0-H bond is broken during the dissociation. This essential difference in surface chemistry of the two TiO modifications is probably decisive for their different adsorption and catalytic properties. Thus both anatase and rutile are n type semi- conductors with almost the same bandgap. In this respect they meet CHEMICAL SOCIETY REVIEWS 1996 tence of face 001 only. However 001 cannot be the only plane exposed on the surface. Another model,21 focused on the four-co- ordinated Ti4+ sites from the 111 plane alone has the same dis- advantage. Busca et all7 have proposed a model considering the structure of most faces characteristic of the anatase crystallites (011 010 001 and 110). According to this model the four-coordinated titanium cations from face 110 are the strong Q sites the five-co- ordinated cations from the other faces represent the weaker p sites and the hydroxy groups are localized on crystal lattice defects. Although much more advanced this model cannot explain the variety of acid sites either. Evidently the difference in properties of the titanium cations cannot be due to differences in their coordination number alone. Hadjiivanov el al.'8 have proposed a model explaining the high heterogeneity of titanium cations in anatase taking also into consideration the effect of the second coordination sphere. According to this model titanium cations from isolated acidic-basic pairs (c.u.s. Ti4+-02-) are more electro- philic than the titanium ions from acidic-basic rows (c.u.s. -. --Ti4+a2-Ti4+-02-. . . ). In the latter case the electrophility decreases owing to the formation of stronger bonds with two and not with one C.U.S. oxygen ion. Figure 4 presents a scheme for the anatase surface. It is obvious that regardIess of their situation all titanium cations may be divided into the following groups I. Four-coordinated Ti4+ (faces 110 111 and 113 as well as the edges of the 110 face). These ions are referred to Q sites. Owing to the different directions of the oxygen vacancies CO molecules adsorbed on (Y sites are not parallel which is the reason why no dynamic shift is observed with the Q carbonyls. 11. Five-coordinated Ti4+ participating in acidic-basic pairs (on the most characteristic faces such as 101=011 and 100=010). These ions correspond to the p sites. At room temperature only half of the sites are occupied owing to an induced heterogeneity of the surface. 111. Five-coordinated Ti4+ participating in acidic-basic rows (faces 001 and I12 and edges 101 X 011). They are the y sites. IV. Ti3+ ions. Stoichiometric considerations show that the C.U.S. titanium cations situated on the edges of the 001 plane are of a Ti'+ type.'* Since Ti3+ ions are not characteristic of oxidized anatase they are assumed to be the places of localization of the residual hydroxy groups. Thus the titanium cations are stabilized in the fourth valency. The most appropriate centres for dissociative adsorption are situ- ated on the 110 face for two reasons (i) the 110 face contains the most acidic four-coordinated Ti4+ ions; (ii) the cationic vacancies on this plane are bridged i.e. two C.U.S. 02-ions act together as a Lewis base. This point of view is in agreement with the low concentration of the centres for dissociative adsorption but is not supported by direct experimental evidence. It is important to establish the reason for the predominant mole- the requirements for photocatalytic decomposition of However the anatase photoactivity is considerably higher. Obviously the local structure of the catalytic site is also important. It is established that the photocatalytic decomposition of water (as well as the interaction of water with alkanes) on titania involves mainly molecularly adsorbed H,0.3 Thus the lack of sites for dis- sociative adsorption on anatase seems to be an important reason for its photocatalytic properties. 3.3 Models of the Anatase Surface Different models of the anatase surface have been proposed on the basis of its adsorption properties. Primet et af.l5 have considered the 001 face supposing it to be the main face exposed on the surface. These authors are of the opinion that both types of surface hydroxy groups are localized on C.U.S.titanium cations the band at 3715 cm-' being characteristic of isolated hydroxy groups and that at 3675 cm-I of OH groups situated on neighbouring titanium cations. Dehydroxylation leads to surface reconstruction which explains the nondissociative water adsorption. The C.U.S. titanium cations thus obtained represent the /3 sites whereas the dehydroxy- lated titanium cations from unreconstructed centres are a sites. This pioneer model of the anatase surface takes into account the exis- ~ater.~.~cular adsorption of alcohols and water on anatase and for their dis- sociative adsorption on rutile. The cleavage rutile planes contain five-coordinated Ti4+ and two-coordinated 02-ions. Schemes of the 1 10and 100mile faces are presented in Figure 5. It is seen that in contrast with the case of anatase the coordinative vacancies of the Ti4+ and 0,-ions are either not parallel or are situated in differ- ent layers. The same applies to the other faces of rutile. Thus a two- centre adsorption of water (alcohol) molecules is possible on anatase whereas on rutile dissociation is necessary to ensure binding of adsorbed molecules to two surface sites simultaneously. The above model explains well the high heterogeneity of the anatase surface and the relative concentration of the different sites. It is also confirmed by the dependence of the concentration of the different sites on the sample morphology.'* The main problems remaining are (i) a detailed interpretation of the different OH groups; (ii)explanation of the heterogeneity of p sites (now it seems to us that this heterogeneity is induced i.e. an adsorbed CO mole- cule changes the acidity of the cationic sites in the vicinity) and (iii) exact interpretation of the sites for dissociative adsorption. Elaboration of a precise model of the anatase surface explaining all properties is a question for the future. SURFACE CHEMISTRY OF TITANIA-K. I. HADJIIVANOV AND D. G. KLISSURSKI 001 100 110 Figure 4 Scheme of an anatase crystal with different exposed faces. e C.U.S. Ti4+ ions; o saturated Ti4+ ions from the subsurface layers; C.U.S. 02-ions; @ saturated 02-ions from the surface layers 0 saturated 02-ions from the subsurface layers. 110 100 0.2nm Figure 5 Schemes of the ‘unit cells’ of the 110 and 100mile planes. Ti4+ cations 0 02-anions. The ions without vacancies (plane 1 lo) although situated on the surface are coordinatively saturated. 4 Surface Chemistry of Anion-modified Anatase A new type of catalyst may be developed by synthesis of new sub- stances with a new bulk structure or by modification of the surface of known substrates. Anatase is a typical example of the second possibility. It strongly adsorbs various anions and cations which causes a pronounced change in its surface properties. We shall con- sider anatase modified by sulfates phosphates and peroxides. In the two former cases the anions are adsorbed on Lewis acid sites and via ion exchange with surface hydroxy groups while the OH groups alone participate in the formation of peroxide compounds. 4.1 Sulfate-modified Anatase Sulfur is often present in the commercial titanias especially when they are prepared via the so-called ‘sulfate method.’I4 It is also accumulated on titania-based catalysts during the Claus reaction. However titania and zirconia modified by sulfuric acid or sulfates have been the subject of numerous investigations since 1983 when Hino and Arata7 reported their superacidity and their unusual cat- alytic properties (see Introduction). Titania may be sulfated by treat- ing with H,SO impregnation with (NH,),SO or adsorption of sulfur-containing compounds followed by oxidation. Part of the C.U.S.oxygen ions from the TiO surface are replaced by sulfate ions on sulfated titania (ST).24-25These sulfates are stable up to m.600 “C and are characterized by a strong band (typical for them) at 1380 cm-’ which corresponds to S=O stretching modes. In principle ST dehydroxylation is more difficult than that of pure anatase.I9 In cases of strongly dehydroxylated ST samples the sulfate ions attract electron density from a neighbouring titanium cation via an induc- tive effect. As a result the electrophility of the titanium ions increases. This enhanced Lewis acidity is detected by testing with CO; when adsorbed on sulfated a sites CO produces an absorption band at 2215 cm-l while on sulfated p sites absorption is observed at 2200 cm-’.24 Busca et af.17have also reported formation of dicar- bonyls. Ammonia adsorption on dehydroxylated samples shows mainly the presence of Lewis However weakly dehy- droxylated ST exhibits Brgnsted acidity (a strong IR band at 1445 cm-after ammonia adsorption resistant to evacuation) which is assumed to be due to monomeric SO,H groups24 or to polymeric sulfates.2s At present it is not clear whether the enhanced Lewis or the generated Brgnsted acidity are responsible for the unique cata- lystic properties of ST. The first supposition is supported by the absence of superacidity of sulfated silica (which in principle pos- sesses no Lewis acidity). Nevertheless the Lewis acidity of ST is not very high. The low temperature of the catalytic reactions indi- cates that most probably ST catalysts are bifunctional 1.e. both aprotonic and protonic acid sites participate in the catalytic process.24 4.2 Phosphate-modified Anatase Phosphorus is often present as a pollutant in commercial titania samples. It is also used as a promoter in Ti0,-supported catalysts.’ Phosphated anatase may be obtained by adsorption of phosphate ions from acidic solutions or by impregnation. By analogy with ST one may expect a change in surface chemistry of anatase after phosphatation. However in contrast to sulfates the phosphate anions block the Lewis acid sites of anatase.26 Thus in contrast with the pure oxide phosphated anatase does not adsorb C026and has a hydrophobic surface.*’ A hypothesis was proposed26 according to which the role of phosphorus as a promoter in vanadia-titania cat- alysts consists in blocking of the free C.U.S. titanium cations which usually catalyse parasitic reactions (e.g.production of CO and CO during o-xylene o~idation,,~ formation of N,O in the DeNO process,2xetc.). 4.3 Peroxide-modified Anatase Hydrogen peroxide also belongs to the class of anionic modifiers. As a rule univalent anions are not sorbed strongly on anatase but hydrogen peroxide interacts with the surface hydroxy groups according to the reaction:29 Ti-OH + H,O -Ti-OOH + H,O. (2) These surface hydroperoxo groups begin to decompose at about 80 "C evolving oxygen. Hence peroxide-modified anatase is not of interest for catalysis at high temperatures. Surface peroxide species are of importance in low-temperature catalytic and especially in photocatalytic reactions (in the latter case H,O is often a reaction intermediate). Peroxide-modified titania could find important applications in the synthesis of some molecularly deposited cata- lysts owing to its increased adsorption capacity relative to pure titania with respect to ions tending to form peroxides.3o 5 Vanadia-Titania Catalysts Among the titania-supported oxide catalysts V,O,/TiO (anatase) is the most important. It shows better catalytic properties in o-xylene oxidation than does vanadium oxide itself as well as vana- dium oxide deposited on other oxides and in particular on r~tile.~' V,O,/T'iO catalysts are also the most commonly used for the DeNOA process. Vanadia-titania catalysts are usually prepared by impregnation the most widely used vanadium source being vana- dium oxalate. In recent years much attention has focused on the so called 'molecularly deposited' catalysts. They are prepared via ion-exchange or by grafting i.e. by a reaction of vanadium compounds with the anatase surface hydroxy groups. When vanadia is deposited on anatase its surface acidity decreases. As a rule its distribution on anatase is not uniform. Probe molecules (CO NH,) detect bare titanium ions even on catalysts containing a higher amount of V,O than that necessary for mono- layer formation.72 This disadvantage is avoided in the so-called molecularly deposited catalysts where the distribution is more homogeneous. Many theories have been proposed to explain the unique effect of anatase as a support. Vejux and C~urtine~~ have shown the crystal- lographic similarity between faces 001 100 and 010 of anatase and face 010 of vanadium pentoxide. They are of the opinion that during deposition epitaxial growth of vanadium oxide crystals with exposure of the 010 face proceeds. This face contains V=O groups which are responsible for the good catalytic properties. In the case of active molecularly deposited catalysts however no separate V,O phase is formed and it may be assumed that the two-dimen- sional compounds possess a structure similar to that of face 0 10. A series of investigations have shown that depending on the vanadia coverage various surface compounds are formed. Isolated monooxovanadyl groups and two-dimensional vanadium oxide clusters correspond to low coverages whereas a separate V,O phase and polyvanadates are observed at higher coverages.,() The nature of the surface vanadium-oxo species also depends on the calcination temperat~re,,~ as shown in Figure 6. At high tempera- tures an anatase-rutile transition (favoured by vanadium additives) and dissolution of VO in the isostructural rutile crystals with formation of a VkTi -,02(r) phase are among the reasons for cata- lyst deacti~ation.~.~~ In all cases vanadidanatase catalysts contain species with a double V=O bond. This bond is well monitored in the IR spectra by the 2v(V=O) overtone at 2060-2054 cm-I. Part of the surface vanadia species are also characterized by the presence of V-OH groups (bands in the 3680-3660 cm-I region). These hydroxy groups cause the BrGnsted acidity of the catalysts [a ad5(NH+,) band CHEMICAL SOCIETY REVIEWS 1996 110-200 "C 350"c 450-575 "c 650"c 750 "C Figure 6 Model of evolution of V,O,RiO with calcination temperature (reproduced with permission from ref. 27b). at 1460 cm-I after ammonia adsorption]. According to Bond et al.34 it is the joint effect of the V=O and V-OH groups that determines the good catalytic properties of vanadidanatase catalysts in selec- tive oxidation reactions. The Lewis acidity of the V,O,/TiO cata-lysts is weak no VSf-CO carbonyls are formed after CO adsorption. However the stronger base ammonia is coordinated to Vs+ sites [S,(NH,) at 1240 cm-'] causing rearrangement in the VO surface complexes and disappearance of the V=O bond.30 It should be noted that all of the above considerations relate to oxidized cat- alysts which however are reduced and reoxidized during the reac- tions. Many researchers have pointed out the lack of activity of titania in the oxidation of o-xylene but Wachs et al.,' have demonstrated that anatase leads to the full oxidation of some of the reaction inter- mediates. Thus the bare titania surface of vanadidanatase catalysts lessens their selectivity. This point of view could also be extended to other reactions and catalysts. For instance during the DeNO process NH,NO,-like species are formed on the anatase surface.28 These species decompose to the undesired reaction product N,O. Hence one of the reasons for N,O formation over titania-supported oxide catalysts could be the incomplete covering of the titania by the active phase. These examples show that the design of the Ti0,- supported oxide catalysts is of great importance for their catalytic performance. 6 Titania-supported Metal Catalysts One of the main reasons for the numerous studies of titania-sup- ported metal catalysts is the so-called strong metal-support interac- tion (SMSI). However even in the absence of a SMSI effect these catalysts are of interest. Thus platinized titania exhibits a much higher activity in a series of photocatalytic reactions than pure anatase.s Combined Pt-RuO,/TiO catalysts are the most promising ones for photocatalytic water cleavage. 6.1 Preparation of Titania-supported Metal Catalysts In most cases metal salts are precursors for preparation of supported metals. After deposition the metal ions are reduced. Both the pre- reduction treatment and the reduction conditions are found to affect the structure of the metal formed and especially the average parti- cle size. In a number of catalytic reactions (the so-called structure- sensitive reactions) the catalytic properties strongly depend on the metal particle size.35 In addition the catalytic performance of bimetallic catalysts is affected by the homogeneity of distribution SURFACE CHEMISTRY OF TITANIA-K I HADJIIVANOV AND D G KLISSURSKI M '+ containing Titnnia ph ike ion exchange I M '' containing phase i low tenipernture reduction b reduction with hydrogen I C next ion exchange I d reduction with hydrogen I e Figure 7 Principal scheme of the multi ion-exchange process of the two metals in the separate particles That is why the design of supported metal and bimetal catalysts is crucial The techniques used most often for preparation of supported metals are impregnation and ion exchange 35 Here we shall discuss two methods of deposition of noble metals which are specific for titania TiO is an effective photocatalyst for deposition of noble metals from solutions of their salts ScThis process is used for the synthesis of mono- and bi-metallic catalysts supported on titanium dioxide 36 The amounts of deposited metals are controlled by the concentration of their ions in the solutions The main restriction to the method is that it can be applied to photosensitive systems only Recently Hadjiivanov et a1 37a proposed the multiple ion- exchange method the model system used being Pt-TiO Platinum-containing ions are adsorbed initially on anatase CO adsorption reveals the occupation of Ti4+ Lewis acid sites during the process Testing with CO again after the reduction of Ptn+shows that part of the initially occupied adsorption sites are liberated This phenome- non allows subsequent ion exchange efc (see Figure 7) The increase in platinum content is smaller than 100% owing to phys- ical blocking of part of the exchange sites by metal particles The catalysts obtained by repeated ion exchange are characterized by a high dispersion and a narrow metal particle size distribution It has been established that the process may also be applied to the system Ag-TiO 37h At present the question about whether other supports may be used is still open 6.2 A Strong Metal-Support Interaction (SMSI) The consideration of the surface and catalytic properties of titania- supported noble metals is associated with SMSI which is one of the most interesting and most studied effects in catalysis The term SMSI was introduced by Tauster et a1 38 to denote the effect respon- sible for the drasoc decrease in CO and H chemisorption on titania-supported metals after increasing the reduction temperature from 200 to 500 "C The following peculianties of SMSI have to be noted (1) the effect occurs only when the hydrogen adsorption on rpL1Titnnid high temper,iture redtiction I TiOv Titdnin reduction nhove 800 "ci I Titdnia I Figure 8 Titania supported metal catalysts reduced at different tempera tures (a) Before reduction (b) a metal particle on titania formed after low temperature reduction (c) partial coverage of the metal particle with TiOAmoieties after high temperature reduction and (d) full encapsulation of the metal particle into the support after reduction at a very high tern perature (see text) the supported metals is dissociative (zi) it is not due to change in the metal particle sizes Catalytic systems have been denoted as LTR (low temperature of reduction) and HTR (high temperature of reduction) depending on whether this temperature is below or above 300 "C The present enormous interest in SMSI could not have amen from the low chemisorption capacity of the HTR catalysts only However they have also demonstrated some unusual catalytic properties The TOF (turnover frequency z e the activity per surface metal atom) of LTR and HTR catalysts towards structure- insensitive reactions is almost the same but structure-sensitive reactions proceed at a much lower rate on HTR catalysts I e in the presence of SMSI Many hypotheses have been proposed to explain SMSI Initially electronic effects were considered to be the cause but now the fol- lowing explanation is accepted titania from HTR catalysts is partly reduced and a suboxide phase migrates onto the metal parti- cle (see Figure 8) Thus the part of the metal surface partially covered by TIO-~ is blocked As a result the chemisorption capacity of the metal strongly decreases and structure-sensitive reactions which need a larger ensemble of metal atoms are suppressed The existence of admixtures e g Na+ facilitates the migration of TiOx moieties and SMSI occurs at lower temperatures of reduction 75) At a higher reduction temperature partial or complete encapsulation of the metal particles in titania may occur Other effects that are reported to occur on reduction above 800 "C are the formation of intermetallic compounds and alloys The large interest in SMSI is strengthened by one more phenom- enon With some metals such as platinum supported on titania the TOF of hydrogenation of CO as well as of compounds containing a carbonyl group increases by up to two orders of magnitude after HTR It is supposed that the suboxide phase being a strong reducer favours the dissociation of CO extracting oxygen while the metal serves as catalyst for the hydrogenation of the carbon obtained However there is some evidence that although associated with CHEMICAL SOCIETY REVIEWS 1996 rn TI^+-co a Ru3'(CO) A RuO-CO I I I I 7 Figure 9 FTIR spectra of CO (1 Torr) adsorbed on a LTR Ru/TiO catalyst at 100 K (solid line) and after heating to 373 K in a CO atmosphere fol- lowed by cooling again to 100K (dotted line) (K Hadjiivanov and J C Lavalley unpublished results) SMSI the increase in the CO hydrogenation rate is not directly pro- voked by SMSI because (I) it has been established that water (which is produced in the CO + H reaction) oxidizes the suboxide and SMSI disappears (zz) in some cases even LTR Me/TiO cata- lysts have a higher TOF than does the corresponding metal deposited say on silica It is evident that the suboxide phase is not unchangeable during CO hydrogenation On the contrary reduced titanium ions can probably be oxidized directly not only by water but also by CO It seems that in this case HTR is needed to ensure the initial formation of the suboxide phase on the metal particles During the reaction the titanium cations formed are altervalent z e they are reduced by hydrogen but oxidized by water and CO thus facilitating CO dis-sociation This mechanism also explains the varying effect of HTR on different metals For instance CO is difficult to dissociate on Pt and that is why Pt/TiO catalysts strongly enhance their TOF after HTR The IR spectrum of CO adsorbed on a LTR RuDiO catalyst is shown in Figure 9 At 100 K all of the ruthenium is in the Ruo state but heating in a CO atmosphere up to 373 K is accompanied by formation of Run+ z e the activation of CO in this case is not hindered This explains the small effect of the reduction tempera- ture and the type of support on the TOF of supported ruthenium dunng hydrogenation of the CO bond 39 7 Conclusions 7.1 Heterogeneity The anatase surface is highly heterogeneous Three kinds of Lewis acid sites (differently coordinated Ti4+ ions) and at least two kinds of hydroxy groups are present on the surface This high hetero- geneity is due to the exposure of different planes on the real crys- tallites 7.2 Local Arrangement The local arrangement of the anionic and cationic vacancies on the anatase surface determines the lack of centres for dissociative adsorption of water and alcohols This particular anatase property is relevant to (I) the low hydroxyl coverage of anatase and (zz) its (photo)catalytic behaviour 7.3 Anions Adsorbed anions strongly affect the surface properties of anatase Surface sulfates increase the Lewis acidity whereas phosphates block the Lewis acid sites Both anions induce a Bronsted acidity 7.4 Design The design of titania-supported oxide catalysts is very important since a bare titania surface often leads to parasitic reactions and decreases the selectivity of the catalysts 7.5 SMSI Titania-supported metals reduced above 300 "C are charactenzed by a strong metal-support interaction (SMSI) This effect strongly decreases the chemisorption capacity of the supported metals and is due to their coverage by Ti-suboxide moieties 7.6 Turnover Frequency The TOF of some metals in CO hydrogenation is enhanced when supported on titania This effect is also due to covering of the metal particles with a Ti-containing phase bur the catalysts are not in the SMSI state during the reaction Acknowledgments This work was supported by the Bulgarian National Research Foundation (Project X-486) 8 References 1 J Whitehead Titanium Compounds Inorganic in Kirk Othmer Encyclopaedia of Chemical Technology 3rd edn executive ed M Grayson Wiley New York 1983 vol 23 p 131 2 G I Golodets Heterogeneous Catalytic Reactions Involving Molecular Oxygen (Studies on Surface Science and Catalysis vol 15) Elsevier Amsterdam 1982 3 M Anpo Res Chem Intermed ,1989,11,67 4 A Mills R H Davies and D Worsley Chem Soc Rev 1993,22,417 5 Energy Resources through Photochemistry and Catalysis ed M Gratzel Academic Press Harcourt Brace Jovanovich New York 1983 6 S Matsuda and A Kato Appl Catal 1983,8 149 7 K Arata and M Hino in Proc VII Sov Yaponsku Semin Katal ,ed A A Davydov Irkutsk 1983,Nauka Novosibirsk 1983,p 7 (Chem Abs 1985 102,61889t) 8 V Nikolov D Klissurski and A Anastasov Card Rev Sci Eng ,199 1 33,315 9 M A Vannice J Catal ,1982,74 199 10 G L Haller and D E Resasco Adv Catal ,1989,36 173 11 A A Davydov Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides ed C H Rochester Wiley Chichester 1990 12 I Kostov Mineralogy 3rd edn ,Nauka i Izkustvo Sofia 1973 13 Handbuch der Praparativen Anorganischen Chemie ed G Brauer Ferdinand Enke Verlag Stuttgart 1975 14 R Pletnev A Ivakin D Kleshchev T Denisova and V Bumistrov Hydrated Oxides of Group IV and V Elements Nauka Moscow 1986 (Chem Abs 1987,106,60379~) 15 M Pnmet P Pichat and M V Mathieu J Phys Chem ,1971,75,1216 1221 16 D J C Yates J Phys Chem ,1967,65,746 17 G Busca H Saussey 0 Saur J -C Lavalley and V Lorenzelly Appl Catd 1985,14,245 18 (a)K Hadjiivanov A Davydov and D Klissurski Kinet Katal ,1988 29 161 (6)K Hadjiivanov 0 Saur J Lamotte and J -C Lavalley 2 Phys Chem (Munich) 1994,187,281 19 C Morterra J Chem Soc Faraday Trans I 1988,84,1617 20 K Hadjiivanov D Khssurski G Busca and V Lorenzelli J Chem Soc Faraday Trans 1991,87 175 21 G Munuera F Moreno and J A Prieto Z Phys Chem (Munich) 1972 78 113 22 A A Tsyganenko L A Denisenko S M Zverev and V N Filimonov J Catal 1985,94 10 23 G Ramis G Busca and V Lorenzelli J Chem Soc Faraday Trans I 1987,83 1591 24 K Hadjiivanov and A Davydov Kinet Katal 1988,29,460 25 M Waqif 3 Bachelier 0 Saur and J -C Lavalley J Mol Catal ,1992 72 127 26 K Hadjiivanov D Klissurski and A Davydov J Catal 1989,116,498 27 (a)I E Wachs R Y Saleh S S Chan and C C Chersich Appl Catal 1985 15 339 (b) R Y Saleh I E Wachs S S Chan and C C Chersich J Catal ,1986,98 102 28 K Hadjiivanov V Bushev M Kantcheva and D Khssurski Langmuir 1994,10,464 SURFACE CHEMISTRY OF TITANIA-K I HADJIIVANOV AND D G KLISSURSKI 69 29 D Klissurski K Hadjiivanov M Kantcheva and L Gyurova J Chem 36 J M Henman J Disdier P Pichat A Gonzalez-Elipe G Munuera and Soc ,Faraday Trans 1990,86,385 C Leclercq J Catal 1991 132,490 30 M Kantcheva K Hadjiivanov and D Klissurski J Catal 1992 134 37 (a)K Hadjiivanov J Saint-Just,M Che J M Tatibouet,J Lamotte and 299 J -C Lavalley J Chem Soc ,Faraday Trans 1994 90 2277 (h) K 3 1 J Haber Oxygen in Catalysis Decker New York 1991 Hadjiivanov E Vassileva M Kantcheva and D Klissurski Marer 32 H Miyata Y Nakagawa T Ono and Y Kubokawa Chem Lett 1983 Chem Phys ,1991,28,367 1141 38 S J Tauster S C Funk and R L Garten,] Am Chem Soc 1978,100 33 A Vejux and P Courtine J Solid State Chem 1978,23,93 170 34 G C Bond J P Zunta S Flamerz P J Gellings H Bosch J G van 39 T Komaya A T Bell Z Weng-Sieh R Gronsky F Engelke T S King Ommen and B J Kip Appl Catal 1986,22,361 and M Pruski J Caral 1995,152 350 35 M Che and C 0 Bennett Adv Card 1989,36,55