Valence control

For n-type GaN crystals doped with Si or 0, from TABLE 1, we find a decrease in EM. It follows that valence control using donor dopants, Si and 0, is likely to prove extremely useful in fabricating high-conductivity n-type doped GaN crystals with stable ionic charge distributions. 

Photocatalytic Valence Control of Actinide Metal Ions I ... 

Neptunium Valence Control Photochemistry and Heterogeneous Catalysis 461... 

Choice of Semiconductor Material for Actinide Valence Control. 467... 

Choice of Hole Scavenger for Actinide Valence Control. 470... 

Keywords Actinides Colloidal semiconductor Nuclear fuel processing Photocatalysis Valence control... 

It is therefore the purpose of this chapter to review applications of photocatalysis in actinide ion valence control and nuclear fuel and actinide materials processing. 

In summary, potential improvements could be made to the PUREX process in the following areas (1) separation of Np from U and Pu prior to the U/Pu split and (2) in the requirement to use a large excess of U(IV) reductant to reduce Pu(IV) to Pu(III). The majority of published work on the applications of photo catalysis in actinide redox chemistry has concentrated on solving the first of these difficulties through Np valence control. A smaller volume of literature exists on the applications of photocatalysis in valence state control of U and the radioactive d block metal, technetium. This section will review both of these aspects. 

Homogeneous photochemical and heterogeneous catalytic processes are attractive because they do not increase the volume of secondary waste. Heterogeneous photocatalysis offers the further advantage that, post valence control, any additives such as HAN can be photocatalytically degraded to readily disposable gases such as N2 and C02. This will be explored further in the next section. 

Control of the particle valence/conduction band oxidation/reduction potential is not only achieved through a judicious choice of particle component material band edge redox thermodynamics of a single material are also affected by solution pH, semiconductor doping level and particle size. The relevant properties of the actinide metal are its range of available valence states and, for aqueous systems, the pH dependence of the thermodynamics of inter-valence conversion. Consequently, any study of semiconductor-particle-induced valence control has to be conducted in close consultation with the thermodynamic potential-pH speciation diagrams of both the targeted actinide metal ion system and the semiconductor material. 

Similar arguments can be used to discount ZrC>2, HfC>2, Si, SiC, Ge, ZnO, PbC>2 and CdS. However, inspection of the Sn-H20 potential-pH diagram indicates that SnC>2 may find wide utility as a photo catalyst in actinide valence control. 

The electron donor/hole scavenger to be used in actinide valence control must have two properties. The first is that it is capable of undergoing an irreversible oxidation, either by valence holes formed by the photo-excitation of the semiconductor photocatalyst or by means of the hydroxide radicals generated by the oxidation of water by the same valence band holes. Suitable materials include organic acids, alcohols, aldehydes, amino acids, and hydrazine and its oxidation products, such as hydroxylamine and dihydrox-ylamine. 

The quantum efficiency, 4> for the generation of useful charge carrier pairs (i.e. those that can potentially be used in a valence control process) per photon absorbed, was found to be 0.27. This is high compared to values reported for similar processes in the literature and is a direct consequence of the charge transfer mechanism in operation. Under the conditions employed in the experiment of Fig. 5, both the particle and the Ce4+ cation are... 

Factors found to be contributory to efficient actinide valence control include ... 

For valency-controlled perovskite-type mixed oxides, such as Lai j Sr cCoOs, the ease with which they form oxide vacancies increases the catalytic activity for hydrocarbon oxidation. ... 

Each transuranic element has many valencies and their behaviour in aqueous solution is very complicated because of disproportionation reactions. As stated above, the ion-exchange and solvent-extraction behaviours of transuranics are dependent on their valency state. Therefore valency control is very important in their analysis (Katz et al., 1986). Additional references discussing this problem include Karkisch (1989), Diamond et al. (1954), Stevenson and Nervik (1961), Abuzwidaet al. (1987), Bemabee et al. (1980), Budnitz (1973), Chu (1971), Fukai et al. (1976), Hampson and Tennaut (1973), Hindman (1986), Holm et al. (1979), Irlweck and Veselsky (1975), Jiang et al. (1986), Johns (1975), Scott and Teynolds (1975), Sekine et al. (1987). 

---