Alumina, isoelectric point

The surface of uncalcined aluminas consists of amphoteric hydroxyl groups that are even less acidic than those in silica gel. Electrophoretic measurements by Stigter et al. (90) show that the isoelectric point for alumina is attained at a pH of 9, much higher-than the value of 2 obtained in the case of silica gel. Thus, hydroxylated aluminas are relatively non-acidic, unless they are promoted with acid-producing impurities. 

Adsorption of ETES and VTES on Alumina. In the absence of acid or base catalysts it was not possible to obtain ETES films on a-alumina by retraction and the VTES films were unstable toward the organic test liquids as well as water. This failure of both the ETES and VTES to form strongly held films can be attributed to the weakly basic character of alumina surfaces. The isoelectric point of alumina is usually at a pH of 7.5 to 9.0 (11) which is a rough measure of the base strength of the —A1—OH surface groups. Silica, on the other hand, has an isoelectric point at a pH of 2, signifying a moderately strong acid character for the... 

The acidic surfaces of quartz and tungsten oxide are noted in this table, as well as, the basic surfaces of alumina and magnesium oxide. It should also be noted that the method of powder fabrication is important in establishing the structure of the surface and, therefore, the isoelectric point of the powder surfiice. The lEP for a simple oxide is inversely proportional to the ratio of the valence, z, to the radius, Rcaom > of the metal cation making up the oxide. The regression equation for lEP data from Parks [53] is as follows ... 

Abrasive particles are a key component in CMP slurry. The most commonly used abrasive particles include silica, alumina, ceria, zirconia, titania, and diamond. Table 21.1 listed a set of information on each type of abrasive particles such as density, microhardness, and isoelectric points (lEP). It is important to point out that the specific values for these properties depend highly on the preparation techniques and the specific states of the samples. The values listed in the table represent an average of the most commonly reported data. For example, the isoelectric point for silica is a function of the number of hydroxyl groups, type and level of adsorbed species, metal impurity in the solid matrix, and the treatment history of the materials [1]. There are three major types of silica according to their preparation methods fumed, colloidal, and precipitated. The common sources for obtaining these abrasive particles are listed in Table 21.2. As examples, some of the more specific information on... 

Thus alumina membranes surface modified with silanes and sulfone [Shimizu et al. 1987] and with trimethyl chlorosilane TMS [Shimizu et al. 1989] and glass membranes adsorbed with surfactants [Busscher et al. 1987] have been studied this way. The results show that surface treatments alter the zeta potentials. Shimizu et al. [1989] have also demonstrated that under normal operating conditions the zeta potentials of alumina membranes do not change over time even for a period of two to three years. The isoelectric point for alumina particles thus determined is close to 4.00 as determined by direct measurement of membranes. 

For the nonionic surfactants, the maximum stabilization under either acidic (pH2) or basis (pH 10) conditions was by a factor of 3 albeit with different surfactants. At pH2, Brij 35 was the most effective, and was in the middle of the pack at pHlO. Thus, from the point of view of using only one surfactant at all values of the pH, Brij 35 is the best choice. As will be seen below, this remains true for the other (polish rate, surface roughness) criterion as well. The other nonionic surfactants employed were selected to span the entire HLB range in order to serve as a basis for comparison (Brij 35 has one of the highest HLB). Only two ionic surfactants were studied and both provided a maximum stabilization by a factor of 5 at appropriate pH anionic SDS at pH 2 and cationic CTAB at pHlO. Note that the isoelectric point of the alumina is about 9. These ionic surfactants were also examined in this study. 

The zeta potential of slurry particles was measured as shown in Figure 1. Fumed silica showed a higher isoelectric point (lEP) at which the net charge and electrophoretic mobility is zero, than that of colloidal silica as shown in Figure 1 (a). Also zeta potentials of colloidal silica were around 20mV lower than that of fumed silica. Figure 1 (b) shows the zeta potentials of alumina, ceria and Mn02 particles. Due to their lower hardness than alumina, ceria has been... 

Class 2 modifiers are simple inorganic acids and bases that can alter the chemistry of the support surface. The presence of acid significantly affects the quantity of chloroplatinic acid adsorbed on alumina with the effect of solution pH on the extent of adsorption (Fig. 13.10). As discussed in Chapter 9 and illustrated fiirther in the discussion on ion exchange presented later, the isoelectric point (lEP) for alumina occurs at a pH near 8 and no adsorption occurs at higher pH values. The amount of nickel nitrate or nickel chloride adsorbed on 7 alumina is very low below pH 4, with the amount adsorbed increasing abruptly at a pH near 5.2. 2> 3... 

The effects of pH and ionic strength on the performance of an a-alumina microfiltration membrane from U.S. Filter was evaluated by Nazzal and Wies-ner [46]. Concerning pH effect on flux, results obtained in this work perceptibly differ from the previous one. Here the membrane operated at a significantly higher permeation rate at a pH well below the isoelectric point of the membrane. This variance can be explained considering the isoelectric point of the membrane was found at pH = 8.3 in this case while it was at pH = 3.5 in the... 

The term isoelectric point used in [1258] is not related to electrophoresis. The authors studied the colors of the solution and of the sediment in a system containing alumina and a pH indicator (e.g., fuchsine) adjusted to different pH values by NaOH or H2SO4. Reference [1258] was quoted in [1] and then by a few other authors as an authoritative source of the IEP of alumina ([1258] was pubhshed in German, but was translated into German by someone other than one of the authors). 

Hackley, VA. et al.. Analysis of the isoelectric point in moderately concentrated alumina suspensions using electroacoustic and streaming potential methods, J. Dispersion Sci. Technol., 23. 601. 2002. 

Franks, G.V. and Meagher, L., The isoelectric points of sapphire crystals and alpha-alumina powder, Colloids Surf. A, 214, 99, 2003. 

Rezwan, K., Meier, L.P, and Gauckler, L.J., A prediction method for the isoelectric point of binary protein mixtures of bovine serum albumin and lysozyme adsorbed on colloidal titania and alumina particles, Langmuir, 21, 3493, 2005. 

Jiratova, K., Isoelectric point of modihed alumina, Appl. Catal., 1, 165, 1981. 

It is well known that alumina, titania [10,11,12] and magnesium oxide [13,14] dissolve in acidic aqueous solutions and even at pH values close to the isoelectric point [15,16], In this study, it will be shown that these support surfaces were modified with promoters to increase the inertness thereof to acidic/aqueous environments, and not to stabilise the support against sintering and loss in surface area at high temperatures [17,18], This paper will deal with the modification of alumina and titania supports for cobalt based slurry phase Fischer-Tropsch catalysts to ensure the successful operation of slurry phase bubble column reactors on commercial scale,... 

Small white crystals were observed on the surfece of the particles. A linescan elemental analysis was performed on a small crystal and was found to be pure boehmite (AIO(OH)). The alumina used in this study has been calcined at 750 °C and XRD analysis has shown that there was no boehmite present in the fi esh support. It was reported earlier that alumina supports could (partially) dissolve in aqueous solutions [24], even at pH values close to the isoelectric point [15]. Partial dissolution of the alumina used during the preparation of catalysts A was experimentally confirmed during the aforesaid model dissolution test (refer Fig. 4). 

At pH 7.2 the alumina is positively charged (the isoelectric point of alumina is... 

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