Surface sol-gel process

The key effect of oxide supports on the catalytic activities of metal particles is exerted through the interface between oxides and metal particles. The key objective of this study is to develop synthesis methodologies for tailoring this interface. Here, an SSG approach was introduced to modify the surface of mesoporous silica materials with ultrathin films of titanium oxide so that the uniform deposition of gold precursors on ordered mesoporous silica materials by DP could be achieved without the constraint of the low lEP of silica. The surface sol-gel process was originally developed by Kunitake and coworkers.This novel technology enables molecular-scale control of film thickness over a large 2-D substrate area and can be viewed as a solution-based... 

Figure 5.5 shows the variation of the pore size distribution as a function of cycles of surface-modification-based N2 adsorption isotherms. The pore size decreases with the modification cycle number. The reduction of the mesopore size for each cycle should be about twice the single-layer thickness. Accordingly, the effective singlelayer thickness is about 6 to 7 A based on the above BET measurements. This value is close to those estimated from the frequency changes of a quartz crystal balance for ultrathin fihns prepared by the surface sol-gel process on 2-D substrates." " ... 

Another consequence of this surface sol-gel process on SBA-15 is the reduction of the surface silanol groups induced by the condensation reaction with titaniumbutoxide. The reaction, after hydrolysis, converts surface silanol functionality, (SiO)3SiOH, into Ti-OH functionality. Given the similar electronegativity of silicon (1.8) and titanium (1.7), the expected consequence of this modification in the solid state Si NMR is a decrease in the band and a corresponding increase in the Q band that is, the OTi(OH)3 and OSi(OH)3 ligands will cause equivalent NMR shift perturbations on the substituted silicon site. 

I. Ichinose, H. Senzu, and T. Kunitake, Stepwise adsorption of metal alkoxides on hydrolyzed surfaces A surface sol-gel process, Chem. Lett. (10), 831-832 (1996). 

Figure 6.3. Schematic illustrations of sequence of the surface sol-gel process. A QCM...

Precursors useful in the surface sol-gel process are not restricted to alkoxides. The requirements as precursors are chemisorption on surface hydroxyl groups and regeneration of the hydroxyl groups after hydrolysis. For example, TiO(acac)2 repeatedly adsorbs, when acid and alkali are added to the adsorption and hydrolysis media, respectively. Except for the case of Nb(0"Bu)5, all the compounds listed in Table 6.1 show linear frequency shifts. Adsorption conditions such as concentration, temperature, and immersion time are dependent on the solubility, reactivity, and the ease of hydrolysis of alkoxides. These conditions are varied as the structure of alkoxide units is changed. For example, Ti(01Pr)4, which exists as a monomer in solution [17], requires conditions different from Ti(0"Bu)4, which tends to form oligomer species. 

Ultrathin oxide gel films prepared by the surface sol-gel process include many nano-sized voids, and the film density is generally low. Their compositions and... 

Figure 6.14. Two different approaches of molecular imprinting by the surface sol-gel process the pre-mixing approach (a), and the layer-by-layer approach (b).

Ti(0 Bu)4 (100 mM) and carbobenzyloxy-L-alanine (Cbz-L-Ala, 25 mM) were mixed in toluene/ethanol and stirred at room temperature for more than 12 h. After addition of water and aging for several hours, the stock solution was diluted with toluene, and subjected to the surface sol-gel process. Uniform adsorption was observed up to 15 cycles with frequency shifts of 140-160 Hz per cycle. The template molecule, Cbz-L-Ala, was removed from the gel film by dipping in 1 wt % aqueous ammonia, as confirmed from the disappearance of characteristic peaks of the titanium-carboxylate complex and carbamate in reflection FT-IR spectra. 

The efficiency of molecular recognition may be improved by creating additional hydrogen bonding sites in the Ti02-gel. As an example, a composite gel film was prepared from trimethoxysilane with a urea unit (TMS-PU) and Ti(0 BU)4. TMS-PU and Cbz-(3-Ala were mixed in organic solvent in the presence of water. After aging and addition of Ti(0 Bu)4, the stock solution was subjected to the surface sol gel process, and the template molecule was removed... 

The metal oxides prepared by conventional baking or by the CVD method are, in general, chemically stable, crystalline materials, and show excellent mechanical, electrical, optical, and physical properties. Flexible porous gel films obtained by the surface sol-gel process are totally different. In this chapter, we described a new preparative method for ultrathin metal oxide films by stepwise adsorption of various metal alkoxides. We named this method the surface sol-gel process. Structural characterization of the gel films thus obtained, the electrical property, and formation of nano-composites with organic compounds, were also explained. The soft porous gel contains many active hydroxyl groups at the surface and interior of the film. This facilitates adsorption of organic compounds, and consequent preparation of ultrathin metal oxide/polymer nano-composite films and organization of functional small molecules. In the nano-composites, proper selection of polymer components leads to the design of new materials with unique electrical, optical, and chemi-... 

Ichinose, I., Senzu, H. and Kunitake, T. (1997). A surface sol-gel process of Ti02 and other metal oxide films with molecular precision. Chem. Mater. 9(6), 1296-1298. 

Mesoporous silica SBA-15 was uniformly covered with one-layer titanium oxide by a hydrol3dic surface sol-gel process by Dai and co-workers [151] and ultra-small gold nanoparticles (0.8-1 nm) were deposited on the resulting material via a DP technique. This catalyst has also shown high catal3dic activity for CO oxidation. 

A carbobenzyloxy-L-alanine-titanium n-butoxide complex was adsorbed repeatedly to the hydroxylated surface of the electrode of the QCM by a surface sol-gel process in order to prepare ultrathin layers with molecular recognition sites [8],... 

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