Hydrolysis forced

As increasing amounts of NaOH are added to the Cr(N03) solution, the hydrolyzed Cr forms dimeric, polymeric and three-dimensional species. Gelled, amorphous and colloidal Cr(OH) is eventually formed. E. Matijevic reported the preparation of a monodispersed Cr(0H)g sol by forced hydrolysis of Cr(III) salt at 90° C (7). Because of the differences in structural features, each olated species (n=l,2,3) should react differently with polymers and form gels of different properties. 

The structure derived from a Rietveld fit of a neutron diffraction pattern of a 6-line ferrihydrite which showed more and sharper lines (Fig. 2.9, lower) than an XRD pattern, was in agreement with the structure proposed by Drits et al. (1993) except that it was not necessary to assume the presence of hematite in order to produce a satisfactory fit (Jansen et al. 2002). The unit cell of the defect free phase had a = 0.29514(9) nm and c = 0.9414(9) nm and the average domain size derived from line broadening was 2.7(0.8) nm. Since forced hydrolysis of an Fe solution at elevated temperatures will ultimately lead to hematite, it is likely that incipient hematite formation may occur under certain synthesis conditions. Neither these studies nor Mbssbauer spectroscopy, which showed only a singular isomer shift at 4.2 K characteristic of Fe, supported the presence of " Fe (Cardile, 1988 Pankhurst Pollard, 1992). However, the presence, at the surface, of some Fe with lower (<6) coordination, perhaps as tetrahedra (Eggleton and Fitzpatrick, 1988) which may have become unsaturated on heating, has been suggested on the basis of XAFS results (Zhao et al. 1994). 

Somatoids are often twinned on the (322) plane to give star-shaped or x-shaped twins (Eig. 4.15 a). Incorporation of low levels of Si in the structure promotes twinning with 0.04 mol mol Si, akaganeite was almost 100% twinned (Cornell, 1992). These crystals have a visibly roughened surface. Increasing citrate concentration during forced hydrolysis at 100 °C and pH 1 reduced the length of the somatoids from... 

Hematites grown hy forced hydrolysis of acidic Fe " solutions at elevated temperatures also show a range of crystal morphologies. The type of anion, the acidity and the presence of additives appear to be main factors that influence the shape (Matijevic Scheiner, 1978 Kandori et al., 1991 Bailey et al., 1993). Additives appear to act by adsorbing on specific planes of the growing crystal. At close to 100 °C, rhombohedral crystals, 50-100 nm in size, sometimes showing intergrowths (stepped appearance)... 

Fig. 4.21 Hematites grown by forced hydrolysis of acid Fe " salt solutions at 98 °C a) from 0.02 M Fe(N03)3 b) from 0.2 M Fe(CI03)3 c) from 0.02 M FeCl3. (Schwertmann, Cornell, 2000, with permission).

All samples were produced by forced hydrolysis of Fe solutions except for where the sol-gel method was used. 

Akaganeite and schwertmannite form in acidic solutions by forced hydrolysis of FeCl3 or FeFs and Fe2(S04)3 solutions, respectively. For akaganeite a threshold concentration of cr or F ions must be present. 

Hematite forms by holding Fe " salt solutions at temperatures close to 100 °C ( forced hydrolysis ) (Matijevic Scheiner, 1978), from 2-line ferrihydrite in aqueous media at around pH 7, by high temperature solid-state transformation of var-... 

Hydrolysis is commonly induced by addition of a base", by heating (forced hydrolysis) or by dilution it can also be induced by solvent extraction or ion exchange (Segal, 1984). A1 in the system enhances hydrolysis (Shah Singh Kodama, 1994). The many investigations of this process have been reviewed by Sylva (1972), Flynn (1984), Schneider and Schwyn (1987), Cornell et al. (1989), Rose et al. (1997) and Schwertmann et al. (1999). 

Kanai, H., Navarrete, R.C., Macisko, C.W. Scriven, L.E. (1992) Rheol. Acta 31 333 Kandori, K. Ishikawa,T. (1991) Selective adsorption of water on amorphous ferric oxide hydroxide. Langmuir 7 2213-2218 Kandori, K. Aoki,Y. Yasukawa, A. Ishikawa, T. (1998) Effects of metal ions on the morphology and structure of hematite particles produced from forced hydrolysis reaction. 

Kandori, K Hori, I. Yasukawa, A. Ishikawa, T. (1995) Effects of surfactants on the precipitation and properties of colloidal particles from forced hydrolysis of EeCl3-HCl solution. J. Mat. Sci. 30 2145-2152 Kandori, K Kawashima,Y. Ishikawa, T. 

Matijevic, E. Partsch, R.E. (2000) Synthesis of monodispersed colloids by chemical reactions. In Sugimoto.T. (ed.) Fine Particles Synthesis, Characterization and Mechanism of Growth. Marcel Dekker, New York Matijevic, E. Sapieszko, R.S. (2000) Forced hydrolysis in homogeneous solutions. In Sugi-moto, T. (ed.) Fine Particles. Basel, Marcel Dekker, 2-34... 

Deprotonation of Hydrated Metal Ions (Forced Hydrolysis)... 

The observation that the pH of salt solutions, especially of those containing polyvalent metal ions, decreases on heating has lead to a different, yet very elegant, technique to generate well-defined metal (hydrous) oxides. Obviously, the increased acidity of such solutions must be due to the release of protons from the hydrated cations, which in turn change to hydroxide complexes. This process was termed by the senior author as forced hydrolysis (7,9-11). At appropriate temperatures and... 

This chapter summarizes the present state of the art of the forced hydrolysis approach by considering specific cations, particularly those of greatest practical and theoretical interest, using aqueous solutions of common salts. In addition to being economical in the manufacture of different products, the described procedure can also help in the development of a better understanding of different processes, such as corrosion of metals or formation of minerals, to mention a few. It should be emphasized that the focus of this chapter is on dispersions of narrow particle size distributions, normally designated as monodispersed systems. While a number of genera reviews have been published on monodispersed colloids (7,9-21), this chapter specifically addresses the problems related to metal (hydrous) oxides. 

The forced hydrolysis approach can be employed for the generation of monodispersed simple or internally composite metal (hydrous) oxides. In the latter case, the resulting particles are mostly internally inhomogeneous. 

In the formation of metal (hydrous) oxides, hydrolyzed metal ions are the primary constituent species. In the "forced hydrolysis" method, the latter are generated by deproionation of the coordinated water of the hydrated cation at elevated temperatures, according to ... 

Forced hydrolysis of zirconium sulfate solutions yielded reasonably uniform spheroidal particles in the presence of formamide (118). Smaller zirconia (—80 nm)... 

---