Article pubs.acs.org/JPCC
Synthesis and Characterization of High-Surface-Area Silica−Titania Monoliths Andriy Budnyk, Alessandro Damin,* Silvia Bordiga, and Adriano Zecchina Department of Chemistry, NIS Centre of Excellence, University of Turin, Via G. Quarello 11, I-10135 Torino, Italy S Supporting Information *
ABSTRACT: Hybrid SiO2−TiO2 mesoporous monoliths were synthesized via sol−gel method. The water for hydrolysis of titanium precursor has been obtained in situ via esterification reaction between acid and alcohol without use of additional agents. The titania content in the samples has been varied from 2 to 40 M% molar content with respect to silica. The nitrogen adsorption isotherms showed that monoliths containing 2 M% of Ti retain a large surface area and micropores with size peaking in the 1 to 2 nm interval. For larger Ti content, a bimodal average micropores distribution is observed. The surface area decreases for titania content larger than 10 M%. The Raman spectra taken with excitation lines in the 244−785 nm interval and the UV−vis measurements demonstrate that in the 2 M% sample Ti is atomically dispersed and fully incorporated in the SiO2 framework and that the local structure is similar to that of titanium silicalite. This conclusion is further confirmed by XAS measurements. Also, the reactivity toward hydrogen peroxide is similar. For larger Ti concentrations, the presence of segregated anatase particles grown in restricted space is observed, as demonstrated by XRD, TEM, Raman, and UV−vis measurements. The surface properties of the silica matrix and of the embedded anatase nanoparticles have been fully characterized by FTIR spectra of CO probe adsorbed at 100 K. Unlike the samples with 5−20 M% Ti, the 40 M% Ti system is fully amorphous, and only small traces of crystalline TiO2 are present.
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INTRODUCTION Because of their wide applications in photocatalysis and as photovoltaic cells components, the catalytic properties of TiO2 phases anatase and rutile have been intensively investigated for a few decades.1 An important requirement for improvement of TiO2 catalytic activity lays in finding of an optimal combination of developed surface area and defined surface chemistry. Along this line, crystalline/amorphous hybrid oxide materials are good candidates, and among them, silica/titania nanocomposites must be cited because they are offering a good combination of the well-known adsorption properties of the porous (amorphous or crystalline) silica matrix and the high catalytic and photocatalytic activity of anchored Ti centers or entrapped crystalline titania nanoparticles.2 In these composites, the titania oxide particles are usually well-dispersed within pores of the silica matrix that allow the direct contact of reactants with titanium ions located on the particles. In extreme cases, only isolated titanium centers can be present, which are substituting Si in the silica matrix. When these centers are localized on the © 2012 American Chemical Society
walls of the pores, they confer unique catalytic properties to the system.3,4 In the popular catalyst titanium silicalite (TS-1), Ti is substituting Si in the tetrahedral positions of crystalline MFI framework. These tetrahedral centers can expand their coordination sphere under the action of adsorbates (like hydrogen peroxide), a fact that explains the outstanding shape selective catalytic activity of TS-1 in epoxidation reactions. It was found in studies by Notari5 and then by Millini et al.6 that the upper limit of Ti accommodated in TS-1 framework is 2.5 M% of Si content, and further increase in Ti concentration leads to the formation of extraframework Ti in form of TiO2. However the micropore size of TS-1 limits the accessibility of molecules. For this reason, some families of mesostructured silicas like MCM (Mobil Composition of Matter) or SBA Received: February 8, 2012 Revised: April 24, 2012 Published: April 24, 2012 10064
dx.doi.org/10.1021/jp3012798 | J. Phys. Chem. C 2012, 116, 10064−10072
The Journal of Physical Chemistry C
Article
The scope of this work is to investigate the surface properties of these systems with special attention to the Lewis acid character, coordination state, and reactivity of Ti centers both as isolated site and on anatase nanoparticles grown in the channels. These properties are of essential importance for many practical applications.18 In this regard, CO has been normally used to probe the coordination state of titanium cations and distribution of hydroxyl groups.19 The synthesis adopted in this Article is based on controlled hydrolysis of TTIP. Water for the hydrolysis is slowly released in situ by esterification of acetic acid with ethanol. After drying, the obtained xerogels in the form of monolith were calcined in air at 600 °C. This temperature is believed to be the optimal one to remove any organic residuals and to avoid excessive sintering of the glass. The texture and porosity have been determined by accurate N2 isotherms. The surface and bulk characterization at the atomic level has been performed by means of several physical methods. In particular, the coordination state and reactivity of Ti centers has been studied by Raman spectroscopy of the samples both in the pure form and in presence of hydrogen peroxide, by IR spectroscopy of adsorbed CO at low temperature and by UV− vis and XANES spectroscopies of the pure form or in presence of adsorbed H2O2, H2O, or both.
(Santa Barbara Amorphous) have been investigated as support for dispersed Ti centers. The most studied member of the former family has been MCM-41, which offers a large surface area of uniform mesoporous channels in hexagonal array. Zhang et al.7 obtained Ti-MCM-41 under acidic conditions with Ti incorporation at the 2 M% level. The poor hydrothermal stability of MCM family materials stimulated interest to SBA-15 (2D hexagonal) and SBA-12 (3D cubic/ hexagonal), the representatives of latter family. Incorporation of Ti into the framework of molecular sieves during the synthesis has always been a challenge, stimulated by tempered results obtained from postsynthesis grafting reactions.8 By accelerating the hydrolysis rate of the silicone precursor with fluoride, Zhang et al.9 found the optimal parameters for direct synthesis of Ti-SBA-15 and Ti/Si ≤ 1 M% to ensure high quality in terms of the absence of extraframework TiO2 content. Chen et al.10 extended that value to ∼3 M% by using titanium oxychloride to generate acidity of the solution close to the isoelectric point of silanol groups. The representative for Ti incorporation into SBA-12 3D hexagonal framework by direct hydrothermal synthesis is the study done by Kumar et al.,11 where the value of ∼3 M% was reached. Hereby, the research activities in the synthesis of large-pore titanium-containing catalysts such as mesoporous molecular sieves have opened new routes for catalytic oxidation of bulky molecules.12 Concerning the sol−gel processes that are utilized for the synthesis of metal-oxo-polymer networks, it is useful to recall that they are based on hydrolysis−condensation of metal alkoxides or metal salts.13 In general, the formation of oxobridges takes place at moderately low temperatures in aqueous or aquo-organic media. However, in the case of Ti precursors, hydrolysis with water is very fast and lead to uncontrolled formation of amorphous TiO2 precipitates. To have a better control over reaction kinetics and hence on Ti dispersion, a nonaqueous sol−gel approach has emerged, which is based on the controlled generation of water by means of an in situ esterification reaction.14 This method not only allows an accurate control of hydrolysis but also leads to the synthesis of hybrid mesoporous materials in the form of glassy monoliths. Zeleňaḱ et al.15 have already synthesized a set of Ti−Si mesoporous samples with Ti/Si mass ratio ranging from 5 to 30 wt %. For the samples treated at 550 °C, the specific surface area was found to span from 250 (30 wt % Ti) to 550 m2/g (5 wt % Ti). The formation of mixed phases has been checked by IR and UV−vis spectroscopies. The first method gave information on the Si−O−Si and Ti−O−Si framework vibrations, whereas the second showed that Ti species incorporated in the silica framework and TiO2 particles are both present in the samples. A similar study has been performed by G. Cernuto et al.,16 who prepared a set of Ti− Si mesoporous samples with Ti/Si molar ratio ranging in the 20−50% range and specific surface area in the 450−670 m2/g interval. As in the case of the work of V. Zeleňaḱ , the work of G. Cernuto did not include the characterization of surface properties. Concerning titania content, it is important to mention that by analyzing the ratio of six- to four-fold sites of Ti present in SiO2−TiO2 glasses produced by sol−gel. Dirken et al.17 revealed its increase with higher Ti content. Samples of 8 M% and of 12 M% of TiO2 have both sites and indicate the presence of Ti−O−Si bonds, whereas the sample with 41 M% of TiO2 shows phase separation.
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EXPERIMENTAL METHODS Synthesis. The silica−titania gels were produced by adding tetraethyl orthosilicate [Si(OC2H5), TEOS] to anhydrous ethanol [C2H5OH, AE] under stirring in fume hood at room temperature (RT). Then, titanium(IV)-isopropoxide Ti(O3C3H7), followed by acetic acid [C2H4O2, AA], was added, and the sealed beaker was left for 1 h stirring. The molar ratio TEOS/EA/AA was 1:8:4. The amount of TTIP was chosen to have Ti/Si molar ratio of 2, 5, 10, 20, and 40 M%. Additionally, the sample without titania and that containing only pure titania were prepared as references. All chemicals were supplied by Sigma-Aldrich and used without further purification. The mixture has been poured into a plastic tube then sealed with a scotch tape and directly transferred into an oven preheated at 70 °C. The samples have been kept at this temperature for 2/3 days to obtain gelation. After gel formation, the temperature was raised at once to 80 °C and left for 1 week for aging. During the subsequent drying stage (by pin-holing the plastic tube to allow the evaporation of solvents), the temperature was gradually increased over 1 week up to 120 °C. The dried gel was then calcined at 600 °C for 1 h in a programmable furnace after 15 h of steady increase in temperature from RT to 600 °C. After, all samples were in the form of monoliths with the shape of the reaction tube. Finally, all samples were further calcined at 550 °C for 3 h. Whereas the low concentrated samples (
