P. Panissod

The influence of sulfide structures on the hydrodesulfurization activity of carbon-supported catalysts

Abstract All transition metals in the first, second, third rows and some in the fourth row, supported on carbon at low concentration, have been tested in their sulfided form to evaluate their activity toward the hydrodesulfurization (HDS) reaction at four different temperatures. The first row shows two maxima on V Cr and Co whatever the temperature with two orders of magnitude between the lowest and the highest activities (one order between Ni and Co). The second and third rows present a single maximum on Rh and Ir, respectively, with three orders of magnitude between the lowest and the highest activities. Electronic considerations have been proposed (S. Harris and R.R. Chianelli, J. Catal. , 86 , 400, 1984) to account for these different activities. A new complementary approach is developed by introducing the structural properties of the Sulfides.

Quasi-2D XY Magnetic Properties and Slow Relaxation in a Body Centered Metal Organic Network of [Co4] Clusters

Octahedral Co(2+) centers have been connected by mu(3)-OH and mu(2)-OH(2) units forming [Co(4)] clusters which are linked by pyrazine forming a two-dimensional network. The two-dimensional layers are bridged by oxybisbenzoate (OBA) ligands giving rise to a three-dimensional structure. The [Co(4)] clusters bond with the pyrazine and the OBA results in a body-centered arrangement of the clusters, which has been observed for the first time. Magnetic studies reveal a noncollinear frustrated spin structure of the bitriangular cluster, resulting in a net magnetic moment of 1.4 microB per cluster. For T > 32 K, the correlation length of the cluster moments shows a stretched-exponential temperature dependence typical of a Berezinskii-Kosterlitz-Thouless model, which points to a quasi-2D XY behavior. At lower temperature and down to 14 K, the compound behaves as a soft ferromagnet and a slow relaxation is observed, with an energy barrier of ca. 500 K. Then, on further cooling, a hysteretic behavior takes place with a coercive field that reaches 5 T at 4 K. The slow relaxation is assigned to the creation/annihilation of vortex-antivortex pairs, which are the elementary excitations of a 2D XY spin system.

CoMo sulfide catalysts studies by metal solid NMR: The question of the existence of the chemical synergy

For the first time the 59Co NMR has been used for the study of the cobalt sulfides in the Co-based HDS catalysts. Four different cobalt sites were observed on these catalysts, two of them corresponding to the regular octahedral and tetrahedral Co9S8 sites, the two other sites, one distorted tetrahedral and one octahedral, were only found in highly dispersed catalysts. A good correlation has been found between the HDS activity on CoSiO2 and CoC catalysts and the amount of these new tetrahedral Co which are found very active. A parallel study of the HDS activity on mixed CoMoSiO2 has shown the classical so-called promoting effect of Co on Mo, but this effect can be simply described in terms of additivity of the Mo activity and of the new very active tetrahedral Co which are also observed to be dominant at the maximum of the volcano curve.

Spin-Frustrated Complex, [FeIIFeIII(trans-1,4-cyclohexanedicarboxylate)1.5]∞: Interplay between Single-Chain Magnetic Behavior and Magnetic Ordering

A three-dimensional mixed-valent iron(II,III) trans-1,4-cyclohexanedicarboxylate, 1,4-chdc, coordination polymer, [Fe(II)Fe(III)(mu(4)-O)(1,4-chdc)(1.5)](infinity), 1, has been synthesized hydrothermally by mixing iron powder and 1,4-chdcH(2) and investigated by X-ray diffraction, dc and ac magnetic susceptibility, and iron-57 Mossbauer spectroscopy over a wide range of temperatures. Single-crystal X-ray diffraction studies of 1 at 90(2), 293(2), and 473(2) K reveal a tetrahedral [Fe(II)(2)(mu(4)-O)Fe(III)(2)(mu(4)-O)](6+) mixed-spin-chain structure with no change in the P1 space group but with subtle changes in the Fe-O and Fe...Fe distances with increasing temperature. These changes are associated with the electron delocalization observed by Mossbauer spectroscopy above 225 K. Magnetic studies reveal three different magnetic regimes in 1 between 2 and 320 K. Above 36 K 1 is a one-dimensional ferrimagnetic-like complex with frustration arising from competing exchange interactions between the iron(II) and iron(III) ions in the chains. Between 36 and 25 K the interchain interactions are non-negligible and 1 undergoes three-dimensional ordering at 32.16 K but with some residual fluctuations. Below 25 K the residual fluctuations slow and eventually freeze below 15 K; the small net moment of 0.22 mu(B) per mole of 1 observed below 15 K may be attributed to a non-collinear or canted spin structure of the spins of the four iron ions in the [Fe(II)(2)(mu(4)-O)Fe(III)(2)(mu(4)-O)](6+) chains. Below 32 K the Mossbauer spectra of 1 exhibit sharp sextets for both the iron(III) and iron(II) ions and are consistent with either a static long-range or a short-range magnetic ground state or a slow relaxation between two canted magnetic states that are indistinguishable at the observed spectral resolution. The 85 and 155 K spectra reveal no electron delocalization and correspond solely to fixed valence iron(II) and iron(III). Between 225 and 310 K the spectra reveal the onset of electron delocalization such that, at 295 to 310 K, 25, 25, and 50% of the iron in 1 is present as iron(II), iron(III), and iron(II/III) ions, respectively. The absence of any spectral line broadening associated with this electron delocalization and the coexistence of four doublets between 225 and 310 K indicate that the delocalization occurs through electron tunneling via vibronic coupling. The sudden increase in the tunneling rate beginning above about 260 K and the presence of a cusp in the magnetic susceptibility centered at about 275 K strongly suggest the existence of a charge order/disorder transition whose nature and order are discussed.

Satellite structure of 59Co NMR spectra in some Co alloys

Co alloys with Cr, Fe, Ru and Cu in the low concentration range have been studied by 59Co NMR. Regularly spaced satellite lines are observed that develop with increasing impurity concentration. The satellite intensities as a function of the impurity concentration have been compared with the binomial distribution law of particular Co environments. Deviations from the exact binomial law in some cases are briefly discussed against the properties of their respective phase diagrams. From this comparison it is concluded that satellite frequencies are primarily determined by the number of impurity atoms in the first neighbour shell: each additional impurity atom in the shell introduces a constant (element-dependent) modification to the hyperfine field value on a Co site. This assignment of satellite frequency shifts to given configurations in the first coordination shell provides references for structural investigations in related binary metallic systems such as metallic multilayers. Preliminary results for CoPt alloys are also presented which do not follow the systematics of the previous alloys but rather suggest an extended oscillating perturbation around Pt impurities.

Nematic-like Organization of Magnetic Mesogen- Hybridized Nanoparticles

A fluid nematic-like phase is induced in monodisperse iron oxide nanoparticles with a diameter of 3.3 nm. This supramolecular arrangement is governed by the covalent functionalization of the nanoparticle surface with cyanobiphenyl-based ligands as mesogenic promoters. The design and synthesis of these hybrid materials and the study of their mesogenic properties are reported. In addition, the modifications of the magnetic properties of the hybridized nanoparticles are investigated as a function of the different grafted ligands. Owing to the rather large interparticular distances (about 7 nm), the dipolar interaction between nanoparticles is shown to play only a minor role. Conversely, the surface magnetic anisotropy of the particles is significantly affected by the surface derivatization.

NMR analysis of buried metallic interfaces

Owing to the sensitivity of the hyperfine field to the topological and chemical environment of the probe nuclei, NMR spectra can be considered as detailed histograms of the short range order ruling the structure of the material under investigation. Complementary to diffraction techniques this gives a local insight into the structure in the direct space. We present here recent developments of the method which have been used for structural investigations of cobalt layers imbedded in Co/X multilayers and particularly of buried interfaces. Indeed the use of spectrum modelling allows a detailed, quantitative, description of the way intermixing takes place at the interfaces. Co/Cu multilayers, a case of weakly miscible elements, have been specially investigated. Such a case is the most likely to yield sharp interfaces. Actually, the interface structure is shown to depend drastically on the preparation conditions and techniques, resulting in very different interfaces from abrupt to largely mixed. Solid solution forming elements (Co/Ru or Co/Cr) have also been studied for which interdiffusion at the interfaces is more expected. Indeed thick diffusion profiles, across several atomic planes, are observed which are much less dependent on the preparation technique than in the former case. The case of compound forming elements (Co/Fe) shows the largest interface admixture with a structure that seems also reminiscent of the bulk alloy phase diagram.

Size dependent dipolar interactions in iron oxide nanoparticle monolayer and multilayer Langmuir–Blodgett films

The dipolar interactions in monolayer and multilayer assemblies of iron oxide nanoparticles have been investigated as a function of the nanoparticle size. The magnetic properties of iron oxide nanocrystals of various sizes have been measured for particles as powders and assembled in mono- and multilayers by the Langmuir–Blodgett technique, and compared to the behavior of non-interacting nanoparticles. It is shown that increasing dipolar interactions lead to higher blocking temperatures and to deviation from the Neel–Brown law. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples. The effect of interactions increases strongly with the nanoparticle size in agreement with simulations, leading to an unusual behaviour for the larger particles assembled in monolayer, which could be a signature of a superferromagnetic state.

Inhomogeneous structure and magnetic properties of granularCo10Cu90alloys

P. Panissod, M. Malinowska, E. Jedryka, M. Wojcik, S. Nadolski, M. Knobel, and J. E. Schmidt Institut de Physique et Chimie des Materiaux de Strasbourg, 67 037 Strasbourg, France Institute of Physics, Polish Academy of Sciences, 02 688 Warszawa, Poland Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas 13083-970, Campinas, SP, Brazil Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, RS, Brazil ~Received 2 November 1999; revised manuscript received 12 May 2000; published 11 December 2000!

Structural defects in Sr2FeMoO6 double perovskite: Experimental versus theoretical approach

The lower than expected magnetization of imperfect Sr2FeMoO6 (SFMO) double perovskites is usually attributed to the presence of Fe at antisite positions that would be antiferromagnetically coupled to their regular neighbors. However, ab initio calculations suggest strongly that such defective Fe sites would be ferromagnetically coupled and, consequently, the magnetization reduction would originate from other kinds of defects. The magnetic, hyperfine, and structural properties of SFMO perovskites prepared by solid-state reaction under a variety of conditions are reported and correlated with ab initio calculations of the magnetic moments and hyperfine fields of Mo and Fe ions in different local environments (antisites, antisite neighbors, and neighbors of an oxygen vacancy). When plotted against the order parameter the experimental magnetization is found to decrease at a rate of about −7.6μB per Mo–Fe antisite pair as in other previous experiments, where the theoretical calculation predicts −6.56μB per anti...