Antoine Aboukaïs

Highly resolved electron paramagnetic resonance spectrum of copper(II) ion pairs in CuCe oxide

A copper–cerium oxide sample prepared with an atomic ratio, Cu : Ce, of 0.01 has been studied by electron paramagnetic resonance (EPR) spectroscopy. A well resolved spectrum of copper(II) ion pairs has been evidenced and the hyperfine structure of the perpendicular components clearly appears. The high resolution can be explained by the presence of two nearly equivalent Cu2+ ions separated by an oxygen ion. A correlation in EPR parameters has been found between the signal corresponding to a single Cu2+ ion, a precursor of the dimer, and the copper(II) ion pair spectrum.

Evidence for different copper species in precursors of copper–cerium oxide catalysts for hydrogenation reactions. An X-ray diffraction, EPR and X-ray photoelectron spectroscopy study

Conditions for preparing copper–cerium mixed oxides by coprecipitation have been examined. X-Ray diffraction (XRD), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the oxides calcined at different temperatures up to 1073 K. In all cases, a solid solution is formed by the insertion of Cu2+ ions in substitutional positions in the ceria lattice; this occurs at the coprecipitation step, and is maintained during calcination. Four Cu2+ signals were found by EPR and attributed to monomers, dimers, clusters and small particles of CuO. Taking into account the eventual presence of additional crystallized CuO and the reduction in copper at the surface detected by XPS, a model for these oxides is proposed.

Transformation of tetragonal zirconia phase to monoclinic phase in the presence of Fe3+ ions as probes: an EPR study

EPR was mainly used to study the morphological, textural and structural behavior of zirconium hydroxide [ZrO(OH)2] with respect to calcination under air at different temperatures. For calcination temperatures less than 700°C, the tetragonal and monoclinic phases of the solid were present. In this range of temperatures an EPR signal with gxx=1.9755, gyy=1.9720 and gzz=1.9562 was observed and attributed to Zr3+ ions located in octahedral sites with strong tetragonal distortion. The dehydration of OH- groups from solids could be responsible for the Zr4+ reduction into Zr3+ ions. A second signal, centered at g=2.0018, was also observed and assigned to trapped single electrons located in oxygen vacancies of ZrO2. A third signal with gxx=2.0040, gyy=2.0082 and gzz=2.0334 was attributed to adsorbed O2- species. Finally, a fourth signal obtained at low magnetic field with different g values was attributed to Fe3+ ions located in sites with a purely rhombic field. For high calcination temperatures (>700°C), the tetragonal phase was completely transformed into monoclinic phase. In this phase, the trapped single electrons and the adsorbed O2- species disappeared whereas the number of Zr3+ ions increases when compared to that obtained at lower calcination temperatures. This increase could be related to the reduction of Zr4+ by the trapped single electrons and the formation of the monoclinic phase which stabilizes the Zr3+ ions. In this latter phase, the Fe3+ ions are located in sites which have the same environmental symmetry than in tetragonal phase but with specific EPR parameter values.

DNA modification by oxovanadium(IV) complexes of salen derivatives

AbstractOxovanadium(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: VIII, VIV and VV. 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 VOSO4 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.

Study of the Ru/Ce system in the oxidation of carbon black and volatile organic compounds

The relationship between the state of Ru on CeO2 and catalytic activity in the oxidation of carbon black (CB) and some volatile organic compounds (VOCs) was investigated for Ru/CeO2 catalysts prepared by wet impregnation. It was demonstrated that the addition of ruthenium to ceria significantly improved the reactivity of the latter. The temperature programmed reduction (TPR) experiments of Ru/CeO2 showed that the oxygen species of RuO2 was reduced at low temperatures. In addition, Electronic Paramagnetic Resonance (EPR) studies of outgassed samples at different temperatures showed an anisotropic signal indicating that Ru(IV) was reduced to intermediate valence states like Ru(III) before its total reduction to metallic Ru. It was concluded that Ru-O-Ce bonds in the well-dispersed Ru species are highly fragile and its mobile oxygen is the active species in the catalytic oxidation process.

X-ray photoelectron spectroscopic study of a Cu-Al-O catalyst under H2 or CO atmospheres

The surface changes of a Cu–Al–O catalyst under H2 or CO atmospheres have been followed by X-ray photoelectron spectroscopy (XPS). In both cases, the progressive reduction of copper(II) species in the octahedral and tetrahedral sites to lower oxidation states for temperatures above 250 °C is observed. When the catalyst is reduced by H2, the main Cu 2p3/2 peak is resolved into its components (Cu2+, Cu+ in tetrahedral and octahedral sites, Cu0) and the proportion of the different copper species has been calculated. The concentration of Cu+ species in the octahedral environment is maximum after reducing the catalyst at 300 °C, which corresponds to better catalytic activity in the hydrogenation of dienes. In case of Cu–Al–O treated by CO where the catalyst surface contained copper species with oxidation states lower than +2, the adsorption of carbon monoxide in a molecular state seems more probable than dissociation which leads to carbidic species.

Total oxidation of propene and toluene on copper/yttrium doped zirconia

The catalytic oxidation of propene and toluene has been investigated on pure ZrO2, pure Y2O3, and ZrO2 doped with 1, 5, and 10 mol % Y2O3 in the presence or absence of copper (0.5, 1, and 5 wt%). A synergetic effect has been detected since ZrO2 and Y2O3 exhibit significantly lower activities than the mixed oxides. The higher surface areas, related to structural change from mononoclinic (ZrO2) to tetragonal (ZrO2–;;Y2O3), partly explained the higher activity of ZrO2–;;Y2O3. However, it has been shown that the number of anionic vacancies, created by the substitution of Zr4+ by Y3+, in yttria-stabilized zirconia solids depends on the yttrium contents. Their effect on propene and toluene oxidation activity is significant. The anionic vacancies should induce better activity of the ZrO2—5 mol % Y2O3 catalyst with or without copper, which presents the higher number of Zr3+ species. This support should favor the formation of CuO particles, which should be the most active catalytic sites in the studied reaction.

Formation of oligonuclear complexes between copper(II) and 1-hydroxyethane-1,1-diphosphonic acid

Abstract The complex formation between Cu(II) and 1-hydroxyethane-1,1-diphosphonic acid was studied by means of pH-potentiometry and spectroscopy (UV–Vis and EPR). Speciation revealed the formation of a trinuclear complex [Cu 3 A 3 ] 6− besides the mononuclear 1:1 and 1:2 species. In the complex [Cu 3 A 3 ] 6− , which is not formed with other diphosphonic derivatives, the ligands adopt chelating and bridging modes via the four O atoms of the two phosphonates, the alcoholic-OH groups remaining in the protonated form.

E.S.R. investigations of γ-irradiated AgNa–Y zeolites

Five different types of paramagnetic species related to silver nuclei were detected for γ-ray irradiated type Y zeolite in which Na+ ions were exchanged by Ag+ ions to different extents. One first species was identified as silver atoms almost freely diffusing in water contained in the zeolite cages. The second one corresponded to silver atoms interacting with lattice oxygens yet easily displaced to aluminium sites by slight warming. The unpaired spin delocalization to lattice nuclei was then rather significant at 36.3 and 21.2 %, respectiveley. The third species was assigned to Ag2+ ions of d9 character whose e.s.r. parameters reversibly depended on dehydration extent, confirming the dynamic properties of ions within zeolitic material. The fourth species was attributed to AgH+ arising from the action of liberated H atoms on Ag+. Finally, V- and F-type centres were observed, but could not be clearly identified. However, their parameters and their presence, compared to the case of non exchanged Na–Y zeolite sample, show that the electronic properties of framework nuclei are greatly perturbed by silver ions. For hydrated samples OH· radicals were also detected.

Potential of Supported Copper and Potassium Oxide Catalysts in the Combustion of Carbonaceous Particles

Different oxide carriers (TiO2 and ZrO2) as supports for low amounts of Cu2+ and K+ species (2 wt % as equivalent oxide) were tested in the catalytic oxidation of carbon black. The K-Cu/oxide catalysts were shown to have a lower soot combustion temperature than K/oxide, Cu/oxide, and pure oxide carriers. The K-Cu/ZrO2 catalyst was found to be the most active; it exhibited activity in a loose contact nearly similar to that obtained in a tight contact mode. Physicochemical characterization by EPR, XPS, and TPR revealed the interaction of K+ species with Cu2+ species and the ZrO2 carrier in K-Cu/ZrO2 as well as a strongly distorted Cu2+ species on the ZrO2 surface. The potassium ions ensure promoting effects towards the contact between the carbon black and the catalyst surface. Although potassium ions were found to lower the reducibility of the cupric oxide species, the oxidation rate of carbon black increased in the presence of K/oxide and K-Cu/oxide.