Deposition of a Titania Coating on Silica by Means of the Chemical

Langmuir 1999, 15, 5807-5813

5807

Deposition of a Titania Coating on Silica by Means of the Chemical Surface Coating† K. Schrijnemakers,* N. R. E. N. Impens, and E. F. Vansant University of Antwerpen (UIA), Department of Chemistry, Laboratory of Adsorption and Catalysis, Universiteitsplein 1, 2610 Wilrijk, Belgium Received September 14, 1998. In Final Form: April 12, 1999 A chemically bound TiO2 layer was created on silica by using a grafting method called “Chemical Surface Coating”. TiCl4 and H2O were used as reagents in successive cycles. The surface reactions and surface species attached to the silica were characterized by elemental analysis as well as X-ray diffraction, Fourier transform infrared, Raman, and UV-diffuse reflectance spectroscopy. The TiOx layer constitutes of nanoparticles of anatase homogeneously spread over the surface. Calcination of the TiO2-SiO2 materials leads to a particle size increase. An extensive pore size analysis was also undertaken to investigate the morphological changes as a function of the number of reaction cycles.

Introduction Titania is used in many fields of contemporary technology. As catalyst, catalyst support, and pigment in the paint industry it has proven its thoroughness. As a support, titania is an excellent choice for the preparation of catalysts for the selective catalytic reduction (SCR) of NOx with ammonia.1-4 However, the use of bulk titania has some serious drawbacks. It sinters easily at elevated temperatures and is difficult to obtain in high surface area form through extrusion. To overcome these problems many attempts have been undertaken to disperse TiO2 on high surface area solids. This was mostly achieved via grafting,5-16 whereby the surface silanol groups of silica react with a metal compound such as, for example, a titanium alkoxide, or via the solution sol-gel route.11,16-24 † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998. * Author to whom correspondence should be addressed. E-mail: [email protected].

(1) Baiker, A.; Dollenmeier, P.; Glinski, M. Appl. Catal. 1987, 35, 351. (2) Proceedings of EPA/EPRI Joint Symposium on Stationary Combustion NOx Control, Washington, DC 1991. (3) Ramis, G.; Yi, L.; Busca, G. Catal. Today 1996, 28, 373. (4) Willi, R.; Koppel, R. A.; Baiker, A. Ind. Eng. Chem. Res. 1997, 36, 3013. (5) Hanprasopwattana, A.; Srinivasan, S.; Sault, A. G.; Datye, A. K. Langmuir 1996, 12, 3173. (6) Gala`n-Fereres, M.; Alemany, L. J.; Mariscal, R.; Ban˜ares, M. A.; Anderson, J. A.; Fierro, J. L. G. Chem. Mater. 1995, 7, 1342. (7) Srinivasan, S.; Datye, A. K.; Smith, M. H.; Peden, C. H. F. J. Catal. 1994, 145, 565. (8) Fernandez, A.; Leyrer, J.; Gonza´lez-Elipe, A. R.; Munuera, G.; Kno¨zinger, H. J. Catal. 1988, 112, 189. (9) Castillo, R.; Koch, B.; Ruiz, P.; Delmon, B. J. Catal. 1996, 161, 524. (10) Klaas, J.; Schulz-Ekloff, G.; Jaeger, N. I. J. Phys. Chem. B 1997, 101, 1305. (11) Hutter, R.; Mallat, T.; Baiker, A. J. Catal. 1995, 153, 177. (12) Alemany, L. J.; Ban˜ares, M. A.; Pardo, E.; Martin, F.; Gala`nFereres, M.; Blasco, J. M. Appl. Catal. B Environ. 1997, 13, 289. (13) Lakomaa, E.-L.; Haukka, S.; Suntola, T. Appl. Surf. Sci. 1992, 60/61, 742. (14) Srinivasan, S.; Datye, A. K.; Hampden-Smith, M.; Wachs, I. E.; Deo, G.; Jehng, J. M.; Turek, A. M.; Peden, C. H. F. J. Catal. 1991, 131, 260. (15) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159. (16) Montes, M.; Getton, F. P.; Vong, M. S. W.; Sermon, P. A. J. Sol Gel Sci. Technol. 1997, 8, 131.

The so formed titania-silica materials exhibit special properties. They have stronger acidic properties than the composing single oxides caused by the formation of nonequivalent bridged heterometal-oxygen-silicon bonds (M-O-Si),14,25 and as such, they can be used as acidic catalysts.21,26 In addition, the “strong metal support interaction” (SMSI) effect ascribed to TiO216,27,28 and the observation that transition metals supported on titania-silica exhibit higher activities than on silica alone29-32 show the great potential of these materials as support. Not only are they used as a support, the titania-silica mixed oxides themselves have also been shown to be effective catalysts for the epoxidation of alkenes with organic hydroperoxides,11,24,25,33-35. On the other hand, TiO2 in the form of anatase is known to be an effective photocatalyst.12,36,37 (17) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (18) Stahkheev, A. Yu.; Shpiro, E. S.; Apijok, J. J. Phys. Chem. 1993, 97, 5668. (19) Klein, S.; Thorimbert, S.; Maier, W. F. J. Catal. 1996, 163, 476. (20) Liu, Z.; Davis, R. J. J. Phys. Chem. 1994, 98, 1253. (21) Liu, Z.; Tabora, J.; Davis, R. J. J. Catal. 1994, 149, 117. (22) Dutoit, D. C. M.; Schneider, M.; Baiker, A. J. Catal. 1995, 153, 165. (23) Imamura, S.; Ishida, S.; Tarumoto, H.; Saito, Y.; Ito, T. J. Chem. Soc., Faraday Trans. 1993, 89, 757. (24) Dusi, M.; Mallat, T.; Baiker, A. J. Catal. 1998, 173, 423. (25) Odenbrand, C. U. I.; Lundin, S. T.; Andersson, L. A. H. Appl. Catal. 1985, 18, 353. (26) Odenbrand, C. U. I.; Andersson, S. L. T.; Andersson, L. A. H.; Brandin, J. G. M.; Busca, G. J. Catal. 1990, 125, 541. (27) Apijok, J.; Shpiro, E. S.; Dmitriev, R. V.; Trachenko, O. P.; Sokolova, V. I.; Stakheev, A. Yu.; Minachev, Kh. M. Proc. All-Union Conference on Mechanism of Catalytic Reactions 5th, Moscow, 1991; 195. (28) Dropsch, H.; Baerns, M. Appl. Catal. A: Gen. 1997, 158, 163. (29) Quaranta, N. E.; Soria, J.; Corberan, V. C.; Fierro, J. L. G. J. Catal. 1997, 171, 1. (30) Reddy, B. M.; Reddy, E. P.; Ganesh, I. Res. Chem. Intermediates 1997, 23, 703. (31) Ueshima, M.; Sano, I.; Ikeda, M.; Yoshino, K.; Okamura, J. Res. Chem. Intermediates 1998, 24, 133. (32) Neumann, R.; LevinElad, M. J. Catal. 1997, 166, 206. (33) Handy, B. E.; Baiker, A.; Schraml-Marth, M.; Wokaun, A. J. Catal. 1992, 133, 1. (34) Bjorklund, R. B.; Odenbrand, C. U. I.; Brandin, J. G. M.; Andersson, L. A. H.; Liedberg, B. J. Catal. 1989, 119, 187. (35) Kanai, H.; Shono, M.; Imamura, S.; Kobayashi, H. J. Mol. Catal. A Chem. 1998, 130, 187. (36) Dias, C. R.; Portela, M. F.; Gala`n-Fereres, M.; Ban˜ares, M. A.; Granados, M. L.; Pena, M. A.; Fierro, J. L. G. Catal. Lett. 1997, 43, 117.

10.1021/la9812469 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/29/1999

5808 Langmuir, Vol. 15, No. 18, 1999

Figure 1. Presentation of the vacuum apparatus used to perform the CSC reactions.

A special application, called “Molecular Layering Technology,” was demonstrated by the research group of Malygin (St. Petersburg, Russia). Pigments and fillers were developed by jacketing industrial waste materials with a TiO2 coating.38 In most of the cited references, however, the mixed oxides are prepared in the liquid phase. An accurate control of temperature, pH, and concentration is necessary to obtain a homogeneous dispersion. In the present work, a grafting technique is presented to prepare high surface area catalyst supports, in which the silica is reacted in the gaseous phase, avoiding the use of organic solvents while also offering a higher degree of control. A thin coating is obtained using the Chemical Surface Coating (CSC) method.39 In this technique, a substrate carrying functional groups is reacted with different reagents in a cyclic way resulting in a polymeric coating precursor chemically bound to the support. Thermally treating this coating precursor yields a thin surface layer, which is covalently connected to the substrate. In the case of a TiO2 coating on silica, TiCl4 and H2O are used as the reactive vapors. First, reaction with TiCl4 results in the removal of the OH groups on the silica, creating titanium chloride groups on the surface. In a second step, these titanium species are hydrolyzed with water vapor in order to re-create OH groups necessary for the next TiCl4 treatment. For the first time a N2 adsorption/desorption study was undertaken to investigate the pore changes as a function of the number of CSC cycles. Experimental Section A well-defined silica gel (Kieselgel 60 from Merck) was used, having a specific surface area of 351 m2/g. Prior to modification, the silica gel was pretreated at 673 K for 17 h in air, resulting in a hydroxyl density on the surface of 3.2 OH/nm2.40 To prevent rehydration, the silica was allowed to cool in a N2-purged glovebox before it was transferred to the sample holder. The modification of the silica was carried out in a flow reactor (Figure 1). Two glass bulbs were filled respectively with TiCl4 obtained from Fluka (>99%) and distilled water and also attached to the vacuum line. The water was outgassed while the TiCl4 was further purified by several freeze-thaw cycles prior to use. In the first step, reaction between the silica bed and TiCl4 occurred at room temperature. A reaction time of 10 min seemed (37) Wittenberg, R.; Pradera, M. A.; Navio, J. A. Langmuir 1997, 13, 2373. (38) Malygin, A. A. Russ. J. Appl. Chem. 1996, 69, 1419. (39) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Elsevier: Amsterdam, 1995. (40) Gillis-D’Hamers, I. Synthesis of a boron nitride precursor on silica gel by means of the C. S. C method, Ph.D. thesis, University of Antwerpen (UIA), Belgium, 1993.

Schrijnemakers et al. sufficient to ensure complete reaction. After the reaction, the sample was evacuated till the pressure dial indicated vacuum. The reaction with water and evacuation afterwards took place at 373 K in order to remove physisorbed water. Chlorine was determined argentometrically taking extreme care to prevent hydrolysis of the samples before the measurement. Therefore, all transactions were carried out in a N2 glovebox. Titanium was determined by UV-vis spectrophotometry after leaching the samples with dilute sulfuric acid. Addition of hydrogen peroxide leads to the formation of a yellow complex having a maximal absorbance at 407 nm.41 A Unicam 8700 UVvis photometer was used to perform the measurements. IR spectra were recorded on a Nicolet 5DXB FTIR spectrophotometer equipped with a photoacoustic detector. The PA detector is a prototype of the MTEC-100 cell, constructed by J. F. McClelland. The photoacoustic cell was flushed with zeolitedried helium and the mirror velocity set to 0.16 cm/s. UV-diffuse reflectance spectra (UV DR) of the samples were recorded on a Unicam 8700 UV-vis photometer equipped with a diffuse reflectance accessory by Philips. All reflectance spectra are converted to Kubelka-Munk units using a standard white as the reference. N2 adsorption/desorption isotherms at 77 K were measured with a Quantachrome Autosorb 1 MP instrument. The BET model was used to calculate the specific surface area.42 Pore size distributions were calculated according to the Barrett-JoynerHalenda (BJH) model.43 XRD diffractograms of the samples were taken on a Philips PW1840 powder diffractometer using Ni-filtered Cu KR radiation (λ ) 1.542 Å). Raman spectra were recorded on a Bruker IFS-66v spectrometer equipped with a Bruker FRA 106 FT-Raman accessory using a Nd:YAG laser. The samples are represented as X CSC, where X stands for the number of reaction cycles.

Results and Discussion Reactions of TiCl4 and H2O with the surface can be expressed by the following idealized reactions.

(-OH)n + TiCl4 f (-O-)nTiCl4-n + nHCl

(1)

(-O-)nTiCl4-n + (4 - n)H2O f (-O-)nTi(OH)4-n + (4 - n)HCl (2) In the first step, TiCl4 reacts with the surface OH groups forming TiClx groups,13,44-50 where x can be 3 (monodentate reaction) or 2 (didentate reaction). The relative amount of these groups depends on the pretreatment temperature of the silica.44 The contribution of monodentate groups increases with increasing pretreatment temperature, while on the other hand, a lower amount of Ti is attached due to the lower silanol density. In the second step, these TiClx groups are hydrolyzed with water vapor in order to re-create OH groups, necessary for the next cycle of TiCl4 and H2O. According to these reactions a uniform film would be formed covering the whole substrate. (41) Vogel, A. I. A Textbook of Quantitative Inorganic Analysis; Longman: London, 1961. (42) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (43) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (44) Haukka, S.; Lakomaa, E.-L.; Root, A. J. Phys. Chem. 1993, 97, 5085. (45) Kinney, J. B.; Staley, R. H. J. Phys. Chem. 1983, 87, 3735. (46) Ellestad, O. H.; Blindheim, U. J. Mol. Catal. 1985, 33, 275. (47) Morrow, B. A.; McFarlan, A. J. Langmuir 1991, 7, 1695. (48) Morrow, B. A.; McFarlan, A. J. J. Non-Cryst. Solids 1990, 120, 61. (49) Murray, J.; Sharp, M. J.; Hockey, J. A. J. Catal. 1970, 18, 52. (50) Haukka, S.; Lakomaa, E.-L.; Jylha¨, O.; Vilhunen, J.; Hornytzkyj, S. Langmuir 1993, 9, 3497.

Deposition of Titania on Silica by CSC

Langmuir, Vol. 15, No. 18, 1999 5809

Figure 2. FTIR-PA spectra of silica pretreated at 673 K before (a) and after (b) reaction with TiCl4. The inset shows the difference spectrum (curve b minus curve a).

Unfortunately, the real mechanism is not that simple. Alternative reactions can take place leading to complication of the reaction studied. In a study of the atomic layer epitaxy (ALE) growth of TiO2 films with TiCl4 and H2O, Ritala et al.51 observed a surface roughening leading to the formation of small aggregates. They proposed the roughening to be due to formation of Ti(OH)2Cl2 either through ligand exchange (direct chlorination) (reaction 3)

2 -OH + TiCl4 f 2 - Cl + Ti(OH)2Cl2

(3)

or reaction with HCl liberated as a product of reactions 1 and 2. Reaction of HCl with the growing TiO2 leads to liberation of H2O as presented in reaction 4.52

Ti-OH + HCl h Ti(H2O)-Cl h Ti-Cl + H2O

(4)

For the CSC process a pretreatment temperature of 673 K was chosen in order to obtain a large degree of anchoring while still having a high modification level of the available surface OH groups. In Figure 2 the FTIR-PA spectra are shown of silica pretreated at 673 K before and after reaction with TiCl4 at room temperature. The band at 3746 cm-1 due to νOH of isolated silanol groups disappears while in the lower frequency domain bands appear at 995 and 924 cm-1 and at 780 and 733 cm-1. These bands are ascribed respectively to νSi-O and νTi-O of both mono- and didentate surface groups showing the chemical bonding between coating and substrate.45,48 The FTIR-PA spectrum of the hydrolyzed TiCl4modified silica is presented in Figure 3. One can clearly see the re-formation of silanol groups on the basis of the band present at 3744 cm-1, which is slightly shifted compared to the silanol band in the pure silica. The presence of Si-OH cannot be the result of unreacted (51) Ritala, M.; Leskela¨, M.; Johansson, L.-S.; Niinisto¨, L. Thin Solid Films 1993, 228, 32. (52) Parfitt, G. D.; Ramsbotham, J.; Rochester, C. H. Trans. Faraday Soc. 1971, 67, 3100.

Figure 3. FTIR-PA spectra of silica coated with an increasing amount of CSC cycles: (a) 0 CSC, (b) 1 CSC, (c) 2 CSC, (d) 3 CSC, and (e) 4 CSC.

silanols during TiCl4 reaction; as from Figure 2 it can be seen that almost all OH groups have reacted. However, direct chlorination during TiCl4 treatment gives rise to Si-Cl groups on the surface. On hydrolysis these groups form the observed Si-OH vibrations. Si-OH groups may also be formed by hydroxylation of siloxane bridges during the first water treatment as the water treatment took

5810 Langmuir, Vol. 15, No. 18, 1999

Schrijnemakers et al.

Table 1. Amount of Ti Expressed per Gram of Silica as a Function of the Number of CSC Cycles cycle no.

Ti (mmol/g)

0 1 2 3 4 5 6 7 8

0 1.4 2.7 3.9 5.0 6.1 7.2 7.4 6.8

place at 373 K while the pretreatment temperature was 673 K. A broad feature situated at 3420 cm-1 is probably caused by adsorption of undissociated water.53 In the spectral window between the Si-O vibrations of 1050 and 820 cm-1 and in the region below 700 cm-1 very broad features are observed which are ascribed to absorption of titaniumoxide species.6,11,19,20,22 The broad band at 930 cm-1 in the spectrum of 1 CSC (Figure 3b) is assigned to Si-O-Ti vibrations. The increasing absorption below 700 cm-1 with increasing number of reaction cycles is attributed to formation of Ti-O-Ti bonds. Anatase is known to absorb strongly in this region.20 The spectrum of 1 CSC exhibits a broad and barely visible band around 3670 cm-1. Whether this band is due to Ti-OH (surface OH groups of anatase absorb around this frequency44,53,54) or to intraglobular Si-OH (which is known to absorb around 3650 cm-1) cannot be unambiguously proven because of the low signal-to-noise ratio of the PA spectra. On the other hand, reaction of HCl with TiO2 surface (reaction 4) cannot be excluded as not all chlorine could be removed during the water treatment. An increasing amount of chlorine was found with each cycle. Table 1 represents the Ti concentration expressed per gram of silica against the number of reaction cycles. The uptake of Ti is linearly during the first 6 reaction cycles. Afterwards no further uptake of Ti was found. At that point a plateau is reached in the Ti concentration. In Figure 3 a decrease of OH groups can be observed from the second CSC cycle onward. At 4 cycles almost no hydroxyl groups are detected. We can therefore conclude that reaction occurs mostly through reaction with silanol groups. Islands of TiOx are formed on reaction of silica with TiCl4 and H2O. After each water treatment, Si-OH groups reappear at the surface but in decreasing amounts. After 6 cycles all Si-OH have reacted, indicating that the silica surface is completely covered. As a consequence, no additional Ti deposition is accomplished. This is evident in Table 1, which shows that the Ti loading remains constant after the 6th cycle. Structural Characterization. (a) Precursor Analysis. To obtain some information about both the structure of the attached layer and the local environment of the Ti atoms, XRD and UV DR measurements of the samples were made. None of the samples showed diffraction peaks due to a crystalline TiO2 phase. Either no crystalline phase is present or crystals are present but they are too small (