Hydrothermal method

Fig. 14. Equipment for the hydrothermal method, with hydrohalic acids as solvents.

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

Various methods are applied to the synthesis of titania particles including sol-gel method, hydrothermal method [2], citrate gel method, flame processing and spray pyrolysis [1]. To utilize titania as a photocatalyst, the formation of ultrafme anatase titania particles with large crystallite size and large surface area by various ways has been studied [4]. 

Another distinguishing feature of titania prepared by flame spray pyrolysis is the draar e of anatase crystallite size with the increase of flame temperature. Generally, the increase of preparation temperature increases the crystallite size in other processes such as sol-gel method, hydrothermal method [2, 3], flame processing and conventional spray pyrolysis. The decrease of crystallite size was directly related to the decrease of particle size. Fig. 5 shows SEM and TEM images of titania particles prepared by flame spray pyrolysis. 

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

Figure 8.1 FE-SEM images of K-OMS-2 [(a)-(c)], y-Mn02 [(d) and (e)], and Rb-OMS-2 [(f) and (g)] nanomaterials synthesized using the hydrothermal method. Reprinted with permission from [9-11] (2011) American Chemical Society and Wiley-VCH GmbH Co. KGaA.

Li, Y Su, X., Jian, J. and Wang, J. (2010) Ethanol sensing properties of tungsten oxide nanorods prepared by microwave hydrothermal method. Ceramics International, 36, 1917-1920. 

Cao, C., Cui, Z., Chen, C., Song, W. and Cai, W. (2010) Ceria hollow nanospheres produced by a template-free microwave-assisted hydrothermal method for heavymetal ion removal and catalysis. Journal of Physical Chemistry C, 114, 9865-9870. 

Xu, L., Ding, Y., Chen, C., Zhao, L., Rimkus, C., Joesten, R. and Suib, S.L. (2008) 3D flowerlike a-nickel hydroxide with enhanced electrochemical activity synthesized by microwave-assisted hydrothermal method. Chemistry of Materials, 20, 308-316. 

Moreira, M.L., Andres, J., Varela, J.A. and Longo, E. (2009) Synthesis of fine microsized BaZr03 powders based on a decaoctahedron shape by the microwave-assisted hydrothermal method. Crystal Growth and Design, 9, 833-839. 

In this paper, three zeolitic materials were used silicalite-2, pure silica ZSM-11 with MEL framework, synthesized according to Bibby et.al. [7] zeolite A synthesized via the Chamell hydrothermal method [8] SAPO STA-7 was synthesized according to Wright et. al. [9]... 

Fig. 2 Scheme of the transformations in the preparation of TiOz nanorods by hydrothermal method, the corresponding SEM images and framework models. Elaborated from data and pictures reported by Kolen ko et al.16°... 

Nanosized anatase (< 10 nm) and brookite ( 70 run) particles have been successfully synthesized via sonication and hydrothermal methods. Figure 5.1 shows the powder XRD patterns of as-synthesized anatase and brookite nanoparticles. The particle sizes were characterized by XRD and scanning electron microscopy (SEM) (Fig. 5.2). 

Rao, Jayalakshmi and coworkers have prepared NPs of SnO, SnS and ZnS in order to study their capacitance behavior [88-90]. The metal oxide and metal sulfide NPs were prepared using hydrothermal methods. After immobilization in PIGE electrodes, their electrochemical properties were examined. Capacitance values in the 4—15 Fg were reported for SnS. Comparable values were reported for ZnS. 

Tomita K, Petrykin V, Kobayashi M, Shiro M, Yoshimura M, Kakihana MA (2006) Water-soluble titanium complex for the selective synthesis of nanocrystaUine brookite, rutile, and anatase by a hydrothermal method. Angew Chem Int Edit 118 2438-2441... 

Increased homogeneity can be obtained with the hydrothermal method discussed in Section 3.5.1, which also operates at lower temperatures than conventional methods. 

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