Relationships between crystalline structure and dielectric properties in Sr2Sm(1-x)NdxTi2Nb3O15 ceramics

In this study, tungsten-bronze type materials of Sr2Sm(1-x)NdxTi2Nb3O15 composition (x=0; 0,25; 0,5; 0,75 and 1) were elaborated by classical solid-state reaction. The structural characterization demonstrate that these compounds present tetragonal symmetry, using two space groups P4bm (N°100) and P4/mbm (N°127) respectively. The lattice parameters are a=b≈12,2Ǻ; c≈3,8Ǻ; V≈579,03Ǻ and Z=2. In this compounds, Ti and Nb cations show obvious off-center displacements along the c axis in both the Ti/Nb(1)O6 and the Ti/Nb(2)O6 octahedra. Besides, the unequal Ti/Nb(2)–O bonds length in the equatorial plane of Ti/Nb(2)O6 octahedra indicates the displacement of the Ti/Nb(2) cations in the ab plane, and no displacement of the Ti/Nb(1) cations exists in the ab plane. The results show that this material has two types of octahedral, first octahedron with little deformation around the Ti1/Nb1 and the second octahedron are more distorted around the Ti2/Nb2. The measurements of permittivity and dielectric losses of the ceramic samples performed between 25°C and 700°C (100Hz to 1MHz), high dielectric constants (εr=127 ~ 194) and low dielectric losses (tan(δ) around 10-4 at 1MHz) were found. The maximum value of the dielectric constant is obtained for x=0 (εr=194). The Curie temperature Tc decreases from 332 to 246°C as a function of the substitution of the samarium by neodymium. Detailed microstructural analysis by scanning electron microscope (SEM) and (EDS) for this compounds are also investigated

Unsupported Nimo Catalysts - Influence of the Sulfiding Temperature and Evolution of the Unsupported Nimos Phase During Reaction

A series of unsupported NiMo catalysts, containing 30% Ni atoms, were prepared by the homogeneous sulfide precipitation method (HSP) and sulfided at different temperatures in the range 673-1073 K. XPS, XRD, microelectrophoresis and BET surface area measurements were used to characterize the samples. The catalytic activity of the various catalysts for the hydrodesulfurization of thiophene and the hydrogenation of cyclohexene in a mixture containing both compounds was measured in mild conditions (573 K, 3 MPa). Both the evolution of the ''NiMoS phase'' during catalytic reaction and the influence of the sulfidation temperature on the properties of the unsupported NiMo catalysts were investigated. The sample sulfided at 673 K gave all the characteristic signals of the ''NiMoS phase''. As the sulfidation temperature increases, larger amounts of nickel segregate from ''NiMoS phase'' to form nickel sulfide. The ''NiMoS phase'' was found unstable under reaction conditions. The maximum catalytic activity for both HDS and HYD reactions was obtained when both ''NiMoS phase'' and bulk Ni3S2 were present in the catalyst. It thus appears as a precursor of a very active mixture of phases rather than of the stable catalyst itself. These results are discussed in relation to some models attempting to explain the synergy in the NiMo catalysts. They are compatible with the remote control mechanism

Relationships between crystalline structure and dielectric properties in Sr2Sm(1-x)NdxTi2Nb3O15 ceramics

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Synergy Between the Comos Phase and Supported Or Unsupported Cobalt Sulfide - Existence of a Remote-control Effect

Unsupported Co9S8, MoS2, and CoMoS sulfides (prepared by the HSP method) as well as supported MoS2/Al2O3 and CoSx/C (prepared by impregnation) were used to prepare a series of binary mechanical mixtures of various compositions. A variety of physicochemical techniques (temperature programmed decomposition and sulfidation, both followed by subsequent TPO, XRD, TPR, microelectrophoresis, XPS and AEM) were used to study the samples, verify the formation of the ''CoMoS phase'' in the mixed CoMo sulfide and investigate the existence of interactions between the sulfides forming the mechanical mixtures. The catalytic activity of the supported and unsupported HSP sulfides as well as that of the mechanical mixtures was tested in the hydrodesulfurization of thiophene (HDS) and the hydrogenation of cyclohexene (HYD) at 513 K and 3 MPa. The mechanical mixtures exhibit a synergy in HDS and HYD. This shows that cobalt sulfide (supported or unsupported HSP) can promote the activity not only of molybdenum sulfide, but also of the unsupported HSP ''CoMoS phase''. In addition, XRD and AEM (performed before and after catalytic tests) were used to study the evolution of the unsupported ''CoMoS phase'' during the catalytic reaction, ft was found that this phase is unstable; cobalt segregates from the CoMoS association to form Co9S8, even after a relatively short period in the reactor. These results provide more evidence that the remote control mechanism plays a major role in the synergetic behavior of the CoMo hydrotreating catalysts