Alumina and Silica Particles

The importance of particle size in packed column chromatography was discussed in Chapter 3. Only the surface of the stationary phase participates in the retention process, so chromatography (and SPE) are dynamic interface (adsorption) phenomena. As a result, other important parameters are the pore size and pore size distribution, since these determine the adsorbent surface area available to those analyte molecules that can penetrate inside the particle. This is important because the ratio of the outer particle surface to its inner one can be about 1 1000, i.e., the surface molecular interaction with analytes mainly 

Si —O silanol groups with physically adsorbed water 

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The frictional characteristics of abrasive alumina and silica particles were investigated and are shown in Fig. 16.28. The alumina slurry was very sensitive to the slurry chemistry. The highest frictional force of 9 kgf was observed in DI water, and the lowest frictional force of 4 kgf was measured when citric acid was added into the alumina slurry. The frictional forces of silica particles (6 kgf) were about the same with or without citric acid during CMP. This is consistent with the fact that citrate has little adsorption onto silica particles. Yoon et al. [69] reported that higher adhesion force between two surfaces caused higher friction force on them. 

The purpose of this study was to explore the interaction between slurry particles and wafer surfaces by the measurements of their zeta potentials. The zeta potentials of slurry particles such as fumed and colloidal silica, alumina, ceria and MnOj and substrates such as silicon, TEGS, W, and A1 have been measured by electrophoretic and electroosmosis method to evaluate the electrical properties of surfaces, respectively. The zeta potential of oxide and metal surfaces showed similar values to those of particles as a function of pH. The interaction energy between alumina and silica particles and TEOS, W and A1 substrate were calculated based on DLVO theory. No deposition of silica particles on TEOS and the heavy deposition of alumina particles on metal substrates were observed in the particle deposition test. Experimental results were well agreed with the theoretical calculation. 

The nature of the materials obtained by high temperature treatment of silica-alumina synthetic zeolites can be envisaged as follows. Such zeolites consist essentially of a mixture of 7-alumina and silica particles, the terminal silicons and aluminums sharing oxygens at the points of contact between alumina and silica particles. The exact coordination of the aluminum ions located at the interface is not known although it appears likely that they are in a three-coordinated state. This would correspond to the presence of the anhydride of HAIO2 at the interface sites. It further seems that the acid would be of the Lewis acid type rather than of the Brpnsted acid type, that is, it would have no proton associated with it. The data obtained on the thermal destruc-... 

The ability of anionic and cationic dispersants to modify the surface characteristics of ceramic powders in water is shown in Figure 8. Zeta potential curves obtained for alumina and silica particles in either the presence or absence of dispersant were taken as illustrative examples. As can be observed, the adsorption of the anionic species onto the alumina surface causes a shift of the zeta potential curve toward acidic values, leading to negatively charged particles over a broad pH range. The opposite behavior occurs in the case of the cationic dispersant, whose adsorption produces a positive net charge on the surface of silica particles within a wide pH interval. 

Figure 8 Examples of the effect of (a) anionic (0.06 wt% citric acid) and (b) cationic (0.33 wt% polyethyleneimine, PEI) dispersants on the zeta potential curve of alumina and silica particles, respectively. (The curves shown in graph b were kindly supplied by H. Wyss, ETH-Zurich.)...

Alumina and silica columns, formerly used for PAH separation, were later replaced with chemically bonded phases [685]. RP-HPLC based on the use of C18 columns is nowadays the most popular mode for PAH separation, and specially designed colnmns from different vendors are commercially available. Colnmns of 100-250cm length (3.0-4.6mm i.d.), packed with 3-5pm particles. 

A variety of industrial catalytic processes employ small metal-particle catalysts on porous inorganic supports. The particle sizes are increasingly in the nanometre size range which gives rise to nanocatalysts. As described in chapter 1, commonly used supports are ceramic oxides, like alumina and silica, or carbon. Metal (or metallic) catalysts in catalytic technologies contain a high dispersion of nanoscopic metal particles on ceramic oxide or carbon supports. This is to maximize the surface area with a minimum amount of metal for catalytic reactions. It is desirable to have all of the metal exposed to reactants. 

SMSI is also thought to affect methanation catalysts (normally transition metal or noble metals supported on alumina), which are used in the producton of substitute natural gas (SNG). In general, heating in H2 causes sintering on alumina and silica supports and heating in O2 or steam can cause dispersion and particle coalescence at 200 °C (Rukenstein and Lee 1984,1987, Nakayama et al 1984). The data have been based on ex situ EM studies. Here EM methods, especially under dynamic reaction conditions, can provide a wealth of new insights into metal-support interactions under reaction conditions. 

In situ ETEM permits direct probing of particle sintering mechanisms and the effect of gas environments on supported metal-particle catalysts under reaction conditions. Here we present some examples of metals supported on non-wetting or irreducible ceramic supports, such as alumina and silica. The experiments are important in understanding metal-support interactions on irreducibe ceramics. 

Specifically, catalysts are typically in the form of a ceramic support carrying small amounts of metals such as chromium, nickel, or platinum. Alumina and silica are commonly used in the construction of the ceramic support. Die catalysts lose their activity progressively via various deactivation mechanisms (Pavel and Elvin, 1994). Tliermal regeneration is often employed for regaining catalytic activity, if applicable, but some of the particles break during this process. Once the catalyst particles become too small to be useful, they constitute a waste disposal problem, since catalysts may contain heavy metals that are considered hazardous, or other harmful components. 

The commercial alumina and silica gel sorbents are mesoporous, i.e., with pores mostly larger than 20 A (see Fig. 1). Activated alumina is produced by thermal dehydration or activation of aluminum trihydroxide, A1 (OH)3 (Yang, 1997), and is crystalline. Commercially, silica is prepared by mixing a sodium silicate solution with a mineral acid such as sulfuric or hydrochloric acid. The reaction produces a concentrated dispersion of finely divided particles of hydrated Si02, known as silica hydrosol or silicic acid ... 

The frictional and adhesion forces between the abrasive particles and wafer surfaces were experimentally measured using alumina and silica slurries with and without citric acid. Although citric acid did not affect the zeta potential of the silica particles, it resulted in a more negative zeta potential of the alumina particles due to the adsorption of the negatively charged citrate ions onto the alumina surfaces. The highest particle adhesion force was measured in an alumina slurry without the addition of citric acid. However, the alumina slurry with the addition of citric acid had the lowest particle adhesion force due to the adsorption of citrate ions onto the alumina surfaces. Although citrate ions could easily adsorb onto alumina particles, the silica particles did not appear to benefit in terms of reduced frictional force when in citric acid solutions. 

Vahlas and coworkers recently tested a new delivery system based on sublimation in a fluidized bed, to improve mass and heat transport. A mixture of solid Al(acac)3 (minor component) and inert alumina or silica particles was fluidized with a combination of water vapor and oxygen at 150 °C, by which amorphons AI2O3 films were obtained on Ti6242 alloy wafers immersed in the bed. Pauleau and Dnlac compared the kinetics of vaporization of Al(acac)3, Al(tfac)3 and Al(hfac)3 by isothermal TGA. The satnration vapor pressure of Al(hfac)3 is 10-fold and 100-fold higher than those of Al(tfac)3 and Al(acac)3, respectively. 

The second step, the Interpretation in terms of -potentials and conductivities. requires theory. For non-dilute dispersions this implies consideration of hydrodynamic and electrostatic particle Interaction. James et al. l, working with poly(styrene) and poly(methyl methacrylate) latlces, alumina and silica sols confirmed that u obtained from ESA agreed with the (static) values, obtained mlcro-electrophoretlcally. if the theory by O Brien (see [4.3.45-481) was applied in the analysis. Marlow et al. already noted the same for dilute rutile dispersions their mobility (or Q curves as a function of pH agreed with those in flg. 3.63. 

Another major reason for studying mixed metal oxide membranes from double metal alkoxides is the potential for preparing zeolite>like membranes which can exhibit not only separation but also catalytic properties. It has been suggested that combinations of silica and alumina in a membrane could impart properties similar to those of natural and synthetic zeolites [Anderson and Chu, 1993]. Membranes with a pore diameter of 10 to 20 nm and consisting of combinations of titania, alumina and silica have been demonstrated by using a mixture of a meta>titanic acid sol, an alumina sol and silicic acid fine particles followed by calcining at a temperature of 500 to 900 C [Mitsubishi Heavy Ind., 1984d]. 

A method for determining the particle size distribution from a single X-ray diffraction profile when strain is present was applied to co-precipitated nickel oxide on alumina and silica. Appreciable strain occurred in the NiO, possibly due to the pressure developed in the small particles to balance the surface tension forces and the distortion produced by the deformation of the f.c.c. structure into a rhombohedral form. Apart from errors in the size distribution created by neglected lattice strain, the measurement of strain itself is important because its correlation with catalytic activity has been suggested. 

For the purification of fullerenes from the fullerene oxides the activated alumina and silica can be used. Fulllerene oxides are adsorbed strongly on such adsorbents from solution and the oxides are removed from fullerene samples. For the preparative separation of fullerenes at present activated carbons and graphite are used [11-14], For this purpose silica with the deposited carbon layer [16] can be used also. In this case it is very easily to regulate the pore diameter and specific surface area of adsorbents as well as particle diameter. Such adsorbents is very important for the decreasing of fullerenes loss. On preparative separation of fullerenes on LiChrosorb SI 60 with deposited carbon layer by modified method [15] on glass column first fractions contained quite pure Csq. 

Tombacz. E. et al., pH-dependent aggregation state of highly dispersed alumina, titania and silica particles in aqueous medium. Prog. Colloid Polym. Sci, 98, 160, 1995. Wieland. E.. The weathering of sparingly soluble minerals—a coordination chemical approach for describing their dissolution kinetics, Ph.D. Thesis, ETH, Zurich, 1988, cited after [865]. 

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