Titania surface grafting

Pietron J J, Rolison D R (2004) Improving the efficiency of titania aerogel-based photovoltaic electrodes by electrochemically grafting isopropyl moieties on the titania surface. J Non-Cryst Solids 350 107-112 Tursiloadi S, Imai H, Hirashima H (2004) Preparation and characterization of mesoporous titania-alumina ceramic by modified sol-gel method. J Non-Cryst Solids 350 271-276... 

To overcome both problems, inert oxides like silica, have been used as support to obtain high surface area dispersed titania, by grafting to the SiO support (refs. 3,4) through the impregnation from n-hexane solutions of alcoxides, that by hydrolysis and calcination would produce the final coated... 

Though Method 2 had been reported12 to give adherent titania films only on sulfonated surfaces, we compared PMR-15 with and without CISO3H activation in a 22-h deposition. In both cases, adherent anatase films with significant (004) orientation formed, 660-690 nm on the sulfonated surface and 520-630 nm thick on the untreated substrate (Table 1). In a similar study on PS,15 films formed with or without grafted sulfonic acid groups on the surface, but were only adherent on the sulfonated surface. 

The Fe(acac)3-x species are unstable on this surface so that the characteristic spectrum of iron acetylacetonate is not observed but instead a Fe-OH species is observed indicating that the iron has been grafted to the surface of the zirconia. The tris-Fe +(acac)3 does not decompose completely on other surfaces, such as silica and titania, but rather it may loose 1-2 ligands upon contacting the surface. Thus, the intrinsic chemistry of the surface can have a significant effect upon the reactivity of the Fe(acac)3 towards that surface. Upon heating to 300°C in air all evidence of the (acac) ligands had disappeared. 

Heferogeneous olefin epoxidation over solid titania-silica catalysts has been the subject of numerous publications in the open literature. The general picture that emerges is that isolated titanium (IV) species on a silica surface or in a zeolife mafrix are responsible for the high epoxidation activity [2]. This picture is supported by model catalyst work on titanium silasesquioxane complexes [3,4] that form active homogeneous epoxidation catalysts [5] and by various successful atfempfs fo prepare well-defined, site-isolated titanium complexes by grafting molecular precursors on mesoporous silica [6-9]. These site-isolated titanium complexes have been shown to possess catalytic activity in olefin epoxidation. 

Grafting has been used for the preparation of vanadia on titania catalysts [24]. Vanadyl triisobutoxide was used as precursor. The vanadia species were well dispersed. It was observed by electron microscopy that silica was present as an amorphous phase. Moreover, the amount of titania present in the catalysts influences the specific surface area and the pore volume. Lower titania concentrations give rise to both higher specific surface areas and pore volumes. The most stable catalyst contained equimolar titania-silica or pure titania. 

The aim within the frame of this book is not to survey the plethora of pubH-cations devoted to surface photografting. Typical work pubhshed in recent years is compiled in Table 11.6, which demonstrates that the enhancement of hydro-philicity and wettabiUty of hydrophobic polymers and the improvement of adhesion of polymers to various substrates are still major research topics (see also [99]). Moreover, the grafting of ultrafine inorganic particles, such as nanosized silica and titania, with vinyl monomers is an attractive subject. Relevant earHer work on surface photografting has been reviewed by Yagci and Schnabel [84]. 

AG values are nonlinear functions of the titania content because of several effects caused by textural changes of the materials and the formation of new phases (amorphous titania, anatase, and rutile) with particles of different sizes formed in pores and at the outer surface of the silica gel particles. Water becomes more strongly bound with increasing titania content (Figure 2.73). However, the AG(C J curves for all the SGT samples are above the curve for silica gel. The secondary porosity of silica gel decreases due to grafting of titania, and the peak intensity at / =40 nm of textural pores decreases (Figure 2.74), but a peak at / = 30 nm appears because of partial filling of these broad pores by titania nanoparticles. 

If we assume Scheme 2, grafted colloidal titania particles, similar to those obtained from simple hydrolysis of Ti-(0Pr ) with water, should be obtained and therefore their structure should not be very different from that recently proposed by Leaustic et al (ref.11). In fact, the XANES spectrum of our sample Is very similar to the spectrum recorded by these authors for such colloidal particles. However, there are big differences in the EXAFS region that can be explained by the smaller size of the titania particles obtained in our system. Moreover, after heating at 373K, these authors obtain crystalline anatase, as previously did Kozlowski et al.(ref. 12) and Reichmann et al (ref. 3) during the preparation of similar systems, while the crystalline structure of anatase could not be detected by XRD in our samples, even after calcination at 873K thus implying that the layered open structure, remains stabilized on the surface of the silica. 

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