Cerium hydroxide samples prepared with different precipitating agents (NaOH, KOH, NH4-OH) have been studied by thermogravimetry, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). The untreated samples are a mixture of CeO2 and Ce(OH)4 with a small quantity of Ce(OH)3 present in the bulk of the solid. A part of the Ce3+ species remains stable even after calcination a t high temperatures (1073 K) under air. The residual alkali elements issuing from the precipitating agents induce variations on the final state of the solid.

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Vanadium-cerium oxide catalysts, with different V/Ce atomic ratios, were prepared using vanadyl oxalate (VOC 2 O 4 ) impregnated on ceria (CeO 2 ) as precursors. Subsequently, the freshly prepared solids were calcined under a flow of dried air at different temperatures from 400 to 800 °C. These solids have been characterized with different techniques: Raman spectroscopy, thermal analysis (TG-DSC), specific area measurements (BET), EPR, and solidstate 51 V MAS NMR. Different vanadium species in the vanadium-cerium oxide catalysts have been evidenced. Polymeric V-O-V chains, vanadium tetrahedral surface species, and V 2 O 5 phase are stabilized mainly when solids are calcined in the temperature range 400-500 °C. Their formation also depends on the vanadium content. For higher calcinations temperatures (g500°C), CeO 2 reacts with vanadium (V) species to form a CeVO 4 mixed phase. Further to that formation, a single electron is trapped in an oxygen vacancy in the CeVO 4 phase and can be considered as its probe.

Oxovanadium(IV) complexes of hydroxysalen derivatives have been prepared and tested as DNA reactive agents. The nuclease activity has been investigated under oxidative or reducing conditions, on the basis of the various oxidation states of vanadium: V(III), V(IV) and V(V). In the absence of an activating agent, none of the compounds tested was able to induce cleavage of DNA, whereas in the presence of mercaptopropionic acid (MPA) or Oxone the four complexes induced DNA modifications. Under both conditions, the para-hydroxy complex was found to be the most active compound. Reaction of these salen complexes with DNA occurs essentially at guanine residues and is more efficient in the presence of Oxone than under reducing conditions. The extent of Oxone-mediated DNA oxidation by the four vanadyl complexes was clearly superior to VOSO(4) and was observed without piperidine treatment. EPR studies provided information on the reactive metal-oxo species involved under each conditions and a mechanism of reaction with DNA is discussed.

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