Catalysts isoelectric point

Catalyst Structure Particle Size BET Surface Area Isoelectric Point pH... 

To an extent the surface charges are determined by the pH of the solution, and by the isoelectric point of the oxide, i.e. the pH at which the oxide surface is neutral. The surface is negative at pH values below the isoelectric point and positive above it. Obviously, the charged state of the surface enables one to bind catalyst precursors of opposite charge to the ionic sites of the support. 

One solution-based approach that works for gold catalysts, in that it produces highly active catalysts, is the deposition-precipitation (DP) method [8]. The DP method entails adjusting the pH, temperature, and gold concentration of an HAUCI4 solution to form a gold hydroxide species which is then deposited onto the support material [8]. This catalyst precursor is washed, dried, and annealed to form small (<5nm) catalyst particles [9]. The DP method has a number of limitations for example, DP cannot produce Au particles with diameters less than 5 nm on support materials with low-isoelectric points (lEPs) like SiOz and WO3 [5,10,11]. 

The surface structure and characteristics (density and acidity) of the hydroxyl groups presented in Fig. 13.21 (using CrystalMaker 2.1.1 software) give very useful information to understand the reactivity of the surface of the particles, particularly when adsorption of another complex is desired to synthesize a bimetallic catalyst, or to control the interaction with an oxide carrier (the deposition step). The isoelectric point calculated with the model (5.9) is in fair agreement with the experimental value (4.3). 

Secondly, the quaternised monomer may be replaced with a weakly basic monomer such as MEMA, which exists in its neutral, non-protonated form in alkaline media. Thus the desired zwitterionic block copolymer is prepared in its anionic/neutral form so that no isoelectric point is encountered during the copolymer synthesis. Afterwards, the solution pH can be adjusted to the isoelectric point by the addition of acid to protonate the weakly basic MEMA residues and precipitate the copolymer, which might be a useful alternative approach to column chromatography for the efficient removal of the ATRP catalyst. 

Hariita et al. (14) prepared spherical particles of molybdenum sulfide and cobalt sulfide with a narrow size distribution by reaction of dilute ammonium orthomolybdate or cobalt(II) acetate with sulfide ions liberated from thioacetamide as a reservoir of S2- ions in weakly acidic media. The compositions of these metal sulfides were estimated to be Mo S 0 = 1.0 1.7 3.0 and Co S 0 = 1,0 4.5 6.4 by chemical analysis. Figure 3.1.4 shows an SEM of a thus prepared uniform molybdenum sulfide particles sample. These sulfide particles were of no distinct crystal structure as shown by x-ray diffractometry. The isoelectric points of the Mo sulfide and Co sulfide particles in terms of pH were 1.9 and 3.1, respectively. Both of them are useful as hydrodesulfurization catalysts. 

Fig. 5. Arrhenius plots and optical yields of hydrogenations of MAA with MRNis. Catalyst RNi (digested at 20 2 "C and kept at 75-78 C for 45 min). Modifying conditions isoelectric point, O C. Reaction conditions MAA (neat), atmospheric pressure.

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... 

Figure 2.24 Variation of the emission maximum wavelength with the isoelectric point or pcz of the oxide support in supported vanadia catalysts. Reprinted from ref [111], with permission from Springer.

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). 

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