C. F. Aissi

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.

Active form of 12-vanadomolybdophosphoric acids in n-butane selective oxidation

The catalytic behaviour of H5PV2Mo10O40 heteropoly acid in the oxidation of n-butane to maleic anhydride is analysed in relation to that of its partially salified Cs salt and their FTIR spectra, UV–VIS diffuse reflectance spectra, thermogravimetry and reducibility tests as well as solid state 51V NMR and EPR characterizations are also compared. At temperatures around 573 K the acid sample dehydrates with partial reduction and the formation of lacunar oxo-anions but with preservation of the Keggin unit structure. The creation of inter-Keggin V—O—V bonds during dehydration is suggested. The presence of the alkali metal in the secondary structure inhibits these effects. The superior catalytic performance of the acid sample in butane transformation is discussed in terms of the role of the presence of V5+ sites associated with an inter-Keggin interaction.

Reduction effect on cerium oxide catalysts doped with copper(II) ions. An electron paramagnetic resonance study

The reduction effect on CuCe oxide catalysts with an atomic ratio Cu : Ce = 0.01 has been studied mainly by EPR techniques. The surface species of copper(II) ion pairs and their precursors (Cu2+ ion monomers) were easily reducible by H2, C2H4 and CO. A bronze-like complex (HxCuCeO2) has been obtained before the beginning of the effective reduction of Cu2+ by heating the solid in the presence of hydrogen or ethene. Ce3+ ions have been evidenced near the solid surface during the reduction. By adsorption of CO molecules, CO–2 radicals have been formed on the catalyst surface.

Spectroscopic characterization of CuTh oxide catalysts in decomposition of isopropyl alcohol

Copper–thorium oxide catalysts prepared with different atomic ratios Cu : Th by coprecipitation of hydroxides and calcination in dry air at 1073 K, have been characterized by electron paramagnetic resonance (EPR) and UV–VIS spectroscopies before and after being tested as catalysts for the decomposition of isopropyl alcohol. The catalysts are selective for dehydrogenation to form acetone only. The simultaneous presence of Cu2+, Cu+ and/or Cu0 in the thoria with a ratio of (Cu0+ Cu+)/Cu2+≈ 2 is required for activity toward isopropyl alcohol dehydrogenation.

Redox behaviour of copper(II) ion pairs as surface species on CuTh oxide catalysts. Electron paramagnetic resonance and X-ray photoelectron spectroscopy study

EPR and XPS have been used to study the redox reaction on CuTh oxide catalysts with different Cu/Th atomic ratios. Apart from the substitutional (S) sites occupied by Cu2+ ions both in the bulk and on the surface of thoria, other Cu2+ ions, occupying different sites on the surface were susceptible to H2 and/or O2 treatment at different temperatures. In particular, copper(II) ion pairs which can be obtained at high copper contents (Cu/Th > 0.01) are formed from one copper(II) ion, reducible by hydrogen, and another, corresponding to substitutional surface sites (Ss), which remains stable towards hydrogen. The migration of some Cu2+ ions, from sites very susceptible to the cyclic redox treatment, is responsible for the growth of CuO crystallites on the surface of CuTh oxide catalysts.

Electron spin resonance study of the structural properties of copper–chromium oxide catalysts

Copper–chromium catalysts with spinel structure and containing different amounts of copper are shown to have residual ferrimagnetic behaviour at room temperature. The small super-exchange interactions, sensitively characterized by octahedral Cr3+ e.s.r. linewidth and magnetic susceptibility determination, are drastically modified by redox treatment and variation of the Cu/Cr ratio. Comparison with ESCA results leads these modifications to be assigned to octahedral Cu+ concentration changes, which are quantitatively determined. Existence of such copper sites, even in the presence of isoprene, which is hydrogenated to mono-olefin, suggests that octahedral Cu+ ions play an important role in the selective hydrogenation catalysis.