Hafnium species

The adsorption of hafnium species on glass was found to increase with the solution pH and hafnium concentration. The effects on the adsorption of the solution preparation and age were studied and the equilibration time for the adsorption process was determined. The surface area of the glass sample was determined by the B.E.T. method using water vapor. The results are discussed in terms of the hydrolyzed hafnium(IV) species. At equilibrium, nearly monolayer coverage was obtained at pH > 4.5. Under these conditions hafnium is in the solution in its entirety in the form of neutral, soluble Hf(OHspecies. In the close packed adsorption layer the cross-sectional area of this species is 24 A which is nearly the same as for water on silica surfaces. 

At least some of these difficulties have been overcome in the work to be reported in this study, which deals with the adsorption of hafnium hydrolyzed species on powdered glass as a function of the acidity of the medium. The adsorption of hafnium species from aqueous solution has apparently never been investigated, yet this ion lends itself conveniently to studies of the problems discussed above. The chemistry of the hafnium ion in water is fairly well understood (23) and a suitable isotope, 181Hf, is available for adsorption studies. What makes hafnium a particularly interesting system is the fact that it forms the entire series of hydrolyzed species Hf(OH)n(4 r )+ where n 4. At intermediate acidities (pH > 4) the solutions of low concentrations contain only the neutral, soluble species Hf(OH)4. It should be emphasized that there is a pH and a concentration range over which this species is present without simultaneous formation of hafnium hydroxide. Thus, it is possible to elucidate the effect of the ionic charge upon the adsorption of hydrolyzed species in systems void of colloidal hydroxides. The glass powder was used in... 

Precipitation of Hafnium Hydroxide. In order to interpret the adsorption data it was necessary to determine the conditions which lead to the precipitation of hafnium hydroxide. It is not usually advisable to depend on the solubility product because the information on this quantity is often unreliable for hydroxides of polyvalent metal ions. In addition, "radiocolloids may apparently form much below saturation conditions in radioactive isotope solutions. In the specific case of hafnium hydroxide only two measurements of the solubility seem to have been reported. According to Larson and Gammill (16) K8 = [Hf(OH)22+] [OH ]2 — 4 X 10"26 assuming the existence of only one hydrolyzed species Hf(OH)22+. The second reported value is Kso = [Hf4+] [OH-]4 = 3.7 X 10 55 (15). If one uses the solubility data by Larson and Gammill (Ref. 16, Tables I and III) and takes into consideration all monomeric hafnium species (23) a KBO value of 4 X 10 58 is calculated. 

Tyree (37) found evidence of highly polymerized hafnium species, but he studied solutions of high salt concentrations at elevated temperatures. 

It is also easily understood why in the case of hafnium the surface coverage increases as the pH becomes higher. Under these conditions the adsorbate consists only of neutral hafnium species. Since there is no lateral repulsion between them, the uncharged Hf(OH)4 can adsorb until a close packed monolayer is formed. The experiments indicate no evidence of multilayer adsorption. Once saturation is reached the adsorbed amount remains constant regardless of the equilibration time or the concentration of the hafnium salt in solution. 

At lower pH values the areas per molecule at maximum coverage are larger. For example, at pH —3 the calculated area per hafnium species is 42 A.2. This is to be expected if one considers the hydrogen ion competition in the adsorption process. 

Figure 2. Reciprocal of the degree of aggregation of solute hafnium species vs.

From the pH data alone, it can be concluded that solutions of hafnium salts for [Hf]tot — lO -lO M, pH 1, and at 25°C require a year or more to reach a steady state. Thus, the solutions are not at equilibrium. Nonetheless, it is possible to calculate much about the nature of the hafnium species from the data at any particular time, in accordance with Sillen s model (13). In order to make such calculations, it is necessary to calculate Z from pH data for each solution. Values of Z define the actual stoichiometry of the hafnium species, [Hf(OH)z] (4 z)iV+. For experimental solutions at 25°C after two days, the Z vs. pH plot (with [Hf]ToT as the parameter) is shown in Figure 3. Clearly, the hydrolysis products are polynuclear. 

Pinnavaia and Fay 432) have found that the zirconium and hafnium species containing the same halogen are isomorphous, but the chloro species are not isomorphous with the bromo species. The monohalogen substitution products M(acac)3X, except for the iodide show no significant dissociation as observed by molecular weight and conductivity experiments. In the case of the iodide in tetrahydrofuran, the ionic species is assumed to be Zr(acac)a+. In the chloro and bromo compounds it appears that the species in solution is the 7-coordinate molecule. The infrared spectra show that all the carboxyl groups are coordinated. The dihalo species in solution are assumed to be 6-coordinate since conductance data in nitrobenzene indicate only 2-5% dissociation. 

Equilibrium constants for the formation of nitrate complexes at hydrogen ion concentrations of 2 and 4 M and metal ion concentrations of 5 X 10 M or less, were determined using ion exchange techniques (353, 465) (Table XVIII). Activity coefficient data for aqueous zirconium and hafnium species are scarce, although there is one report (319) of activity coefficients for metal nitrate solutions as determined by the isopiestic method. 

New mixed borides of Hf and Mo (HfgMo B) and Zr and Mo (Zr9Mo4B) have been prepared and characterized as fc-borides by 2f-ray methods. The hafnium species will dissolve up to 14 atom% A1 at 1400 °C. 

Incorporation of various cations into mesoporous silica often leads to the creation of Bronsted acid sites. For example, adsorption of ammonia on Ti/ MCM-41 (430,443), [A1]MCM-41 (444), and [AlJSBA-15 (182), as well as on titanium, zirconium, and hafnium species supported on mesoporous silica (445), and on Ti02-Si02 (110) leads to the appearance of bands ofpro-tonated ammonia. However, the adsorption is reversible (430) or partly reversible (182,443,445) at ambient temperature, indicating a weaker acidity of these materials in comparison to zeolites. 

The stepwise bond dissociation enthalpies Dj (Ti-Me) and D2(Ti-Me) in TiMe2Cp2 established from thermochemical data and extended Huckel calculations are compared to the respective halides and hydrides.34 The isolated C-H stretches of M(CH2D)2Cp2 (M = Ti, Zr, Hf) are lower than any other studied methyl complexes and the C-H bonds in the hafnium species are the longest and weakest yet characterised by this method.35... 

Table 10.9 Thermodynamic data for hafnium species at 25 °C determined in the present review together with data available in the literature.

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