Michel Guelton

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.

Surface structure and reactivity of VTiO catalysts prepared by solid-state reaction 2. Nature of the active phase formed during o-xylene oxidation

Abstract The modifications occurring to V2O5 crystallites supported on TiO2 (anatase or rutile) in contact with an o-xylene/air flow at about 600 K are characterized by determining the change in the valence state of vanadium through chemical analyses, the variation in the vanadium coordination environment through FT-IR and solid-state 51V NMR spectroscopies, and the modification in the nature and distribution of the vanadium-oxide phase by means of XRD and XPS analyses. Results are analyzed with reference to the time-on-stream modifications in the catalytic behavior in o-xylene oxidation to phthalic anhydride of these catalysts and to the catalytic behavior and physicochemical characteristics of V3O7 supported on TiO2 and of an unsupported partially reduced vanadium-oxide phase. On anatase samples, the TiO2 surface is covered homogeneously by a VIVVV partially hydrated mono- or bilayer. Overlaying this phase a hydrated V3O7-like phase (VV: VIV ratio of about 2 : 1) is also present as amorphous multilayer patches. In rutile samples, the former phase is no longer present, but instead islands of partially oxidized V2O4 are found together with the same partially reduced V-oxide phase as in the anatase samples. Catalytic results indicate that both phases have roughly similar catalytic behaviors and that the role of the TiO2 support both in the anatase and rutile forms is to increase the number of surface sites, but not to modify their nature which depends only on the modifications occurring during the catalytic tests.

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.

Effect of potassium on the surface potential of titania

The surface potential, χ, of several commercial titania powders has been measured by the vibrating condenser method in the temperature range 100–450 °C in air. The K impurity detected by XPS on some of titanias lowers the χ value of TiO2 considerably. A series of K-doped TiO2 anatase samples containing from 1.1 to 11 K atoms nm–2 was prepared and examined by surface potential and XPS techniques. The surface potential of anatase decreases linearly with increasing potassium content up to ca. 2.4 K atoms nm–2, and then remains constant. The surface potential of K-doped samples varies to a smaller extent with temperature compared with undoped anatase. On the basis of the surface potential and XPS data obtained, it is suggested that the mode of K deposition on the surface of anatase changes at a coverage of about 2 K atoms nm–2.

Effect of the sequence of potassium introduction to V2O5/TiO2 catalysts on their physicochemical properties and catalytic performance in oxidative dehydrogenation of propane

Abstract Two series of K-promoted V 2 O 5 /TiO 2 catalysts were prepared by: (a) deposition of vanadia on K-doped TiO 2 support (TiK V preparations), and (b) deposition of K on vanadia-titania catalysts (TiV K preparation). They were characterized by 51 V NMR, XPS, surface potential (work function) techniques and isopropanol decomposition, a probe reaction for the acid-base properties, and tested in oxidative dehydrogenation of propane (ODH). It has been found that the sequence of the K introduction in the preparation step is preserved in the calcined preparations, with more potassium being present on the surface of TiV K than TiK V catalysts. The vanadium species on TiK V samples include V 2 O 5 and polymeric [VO x ] γ species. The TiV K sequence leads to the formation of potassium vanadates (KV 3 O 8 and possibly KVO 3 ), and to the decrease in the amount of V 2 O 5 . The TiV K catalysts are more active and selective in the ODH of propane, and more active in isopropanol dehydrogenation to the acetone (thus more basic) than the TiK V samples.

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.

51V MAS NMR characterization of V–Ce–O catalysts.

Abstract Vanadium–cerium oxide samples with different V/Ce atomic ratios were prepared using the impregnation method, and then calcined under air at 773 K, and characterized by XRD and 51V MAS NMR. Ceria is the only phase evidenced in low vanadium content samples while a CeVO4 phase and small amounts of V2O5 crystals appeared with the CeO2 phase for higher V contents. 51V MAS NMR study showed the presence of different V sites in solids: distorted sites in polymeric vanadates, highly tetrahedral sites in CeVO4 and octahedral sites in V2O5. The dependence of isotropic chemical shift line for both polymeric V sites and CeVO4 with temperature is evidenced and revealed the interaction of nuclear spin of V atoms with unpaired electrons associated to cerium species.

Effect of potassium addition to the TiO2 support on the structure of V2O5/TiO2 and its catalytic properties in the oxidative dehydrogenation of propane

Vanadium oxide has been deposited by a grafting technique onto TiO2 anatase, both pure and doped with potassium [(1.2 and 2.5) atoms nm–2]. The V content varied between 0.1 and 20 atoms nm–2[0.01–2 V2O5 monolayers (ML)]. The prepared samples were characterized by X-ray photoelectron spectroscopy (XPS), 51V magic-angle spinning (MAS) NMR and a surface potential (SP) technique and tested as catalysts in the oxidative dehydrogenation (ODH) of propane and propan-2-ol decomposition, a probe reaction for acid–base properties. From the XPS and SP data it has been inferred that VOx are located beside the K centres on the bare surface of TiO2 with the lower K content sample, whereas they cover the K-doped fraction of the surface for the sample with higher K content. Monomeric and polymeric VOx species and V2O5 were detected by 51V NMR on pure and K-doped catalysts. For the K-doped samples the polymeric species were observed only at high V content and new tetrahedral VOx species and traces of KVO3 appeared. It has been found that the presence of K on the TiO2 surface leads to (a) a decrease in the reducibility of the vanadia phase at low V content; (b) a decrease in the surface potential (electronic work function); (c) a decrease in acidity and increase in basicity and (d) a decrease in the total activity for ODH of propane. The pattern of the activity and selectivity changes with the total V content depends on the amount of K on the support surface: with K 1 ML) are required to obtain the same catalytic performance. Polymeric [VOx] species seem to be more active and selective in the ODH of propane than monomeric species or bulk V2O5.

Oxidative dehydrogenation of isobutyric acid on H4PVMo11O40, NaxH4-xPVMo11O40 and CUyH4-2yPVMo11O40 heteropolyacid catalysts supported on silica

Abstract The oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) was studied over the heteropolyacid H4VMo11O40 and the acidic salts NaxH4-xPVMo11O40 and CuyH4-2yPVMo11O4. supported on silica, as catalysts. The presence of copper as counter-cations in the heteropoly compounds increases the activity and the selectivity to MAA. This increase is probably due to the simultaneous presence in the solids of Bronsted acidity and oxygen vacancies with an adequate ratio. These latter sites can more easily be obtained in the presence of Cu2+ and V4+ than with Na+ counter-ions in the catalysts.

13C NMR investigation of the isomerization of 1-butene on a synthetic near-faujasite germanium zeolite

Abstract 13 C Nuclear magnetic resonance has been used to study the isomerization of 1-butene to the 2-butene isomers, the reaction being conducted at 573 K and below monomolecular coverage. The NMR data enable the reaction kinetics to be determined and the absence of polymerization to be proved. They also enable the equilibrium constants (at 573 K) between the various isomers to be measured. By comparison of the latter with gas-phase data, the relative adsorption constants can be calculated for both the cis -2- and trans -2-butene isomers. cis -2-Butene is found to be more stabilized upon adsorption than trans -2-butene. Although the adsorbed butene species observed by NMR are identified as π-complexes with active sites on the surface (probably Na + ions), it is proposed that the isomerization reaction occurs at 573 K via a cyclic transition state involving the butene molecule and the simultaneous presence of an acidic and a basic Bronsted site on the surface. That explains the differences in activity and selectivity (toward the formation of cis -2-butene) between the NaGeX and the NaHGeX zeolites. The equilibrium constants between the various isomers in the adsorbed state differ from the corresponding gas-phase values because of differences in the adsorption constants of the various isomers.