F. Le Normand

Modification by lanthanide (La, Ce) promotion of catalytic properties of palladium: Characterization of the catalysts

Pd/γ-Al2O3 catalysts modified by cerium or lanthanum oxide were prepared according to a standard procedure. The nature of the interaction between the metallic particles and the promoted support was addressed. The effects of changing the following parameters were studied: (i) precursor palladium salt (chloride or nitrate), (ii) content of lanthanide (from the unpromoted catalyst to the pure lanthanide oxide) and (iii) nature of the lanthanide (La or Ce). This paper focuses on the study of the transition metal, whereas the lanthanide redox properties have been reported in an earlier paper (J. Phys. Chem., 1988, 92, 2561). Both structural [transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS) at the Pd K edge] and spectroscopic investigations (XPS at the Pd 3d core level) emphasize the role of the precursor palladium salt. When derived from a palladium chloride precursor and after a standard treatment including calcination and reduction, the chloride ion is quantitatively trapped by the support and the metal–support interaction is strong. This is thought to be due to the preferential localization of the chlorine atoms in bridge positions between the metal and the lanthanide. When exposed to an air atmosphere, the palladium atoms at the interface are partially chlorided and cerium is in the +4 oxidation state. After a mild H2 treatment at 150°C, palladium is fully reduced, except when the cerium content is within 1–5% where the reduction is only partial. Cerium is converted into the trivalent oxidation state either as a chlorinated or an oxychlorinated form. It is thought that this metal–support interaction at the interface, which does not exist in the nitrate precursor catalysts, might explain the catalytic behaviour of such catalysts by the creation of new active sites or a promoting effect (J. Mol. Catal. A, in the press). The strength of this interaction depends on the cation of the support at the interface (Ce3+>La3+>Al3+) and is stronger for a cerium content between 1 and 5%.

Manifestation of coherent magnetic anisotropy in a carbon nanotube matrix with low ferromagnetic nanoparticle content

The influence of the magnetic medium can lead to peculiar interaction between ferromagnetic nanoparticles (NPs). Most research in this area involves analysis of the interplay between magnetic anisotropy and exchange coupling. Increasing the average interparticle distance leads to the dominant role of the random magnetic anisotropy. Here we study the interparticle interaction in a carbon nanotube (CNT) matrix with low ferromagnetic NP content. Samples were synthesized by floating catalyst chemical vapor deposition. We found that below some critical NP concentration, when NPs are intercalated only inside CNTs, and at low temperatures, the extended magnetic order, of up to 150 nm, presents in our samples. It is shown by analyzing the correlation functions of the magnetic anisotropy axes that the extended order is not simply due to random anisotropy but is associated with the coherent magnetic anisotropy, which is strengthened by the CNT alignment. With increasing temperature the extended magnetic order is lost. Above the critical NP concentration, when NPs start to be intercalated not only into inner CNT channels, but also outside CNTs, the coherent anisotropy weakens and the exchange coupling dominates in the whole temperature range. We can make a connection with the various correlation functions using the generalized expression for the law of the approach to saturation and show that these different correlation functions reflect the peculiarities in the interparticle interaction inside CNTs. Moreover, we can extract such important micromagnetic parameters like the exchange field, local fields of random and coherent anisotropies, as well as their temperature and NP concentration dependencies.

Impact of CNT medium on the interaction between ferromagnetic nanoparticles

We present results on low-temperature magnetization approaching the saturation law in aligned bundles of CNTs with ferromagnetic nanoparticles embedded inside inner channels of nanotubes for two directions of the magnetic field, parallel and perpendicular to the CNT axes. Elaborating experimental data, we were able to extract the explicit form of the correlation functions describing the orientation of the magnetic anisotropy axes in real space. In the parallel field the long-range coherence in the magnetic anisotropy axes is the characteristic feature. In the perpendicular direction the peculiar feature is the 2D exchange coupling. The nature of the exchange interaction and the role of the CNT medium in it is also discussed.