Carbon-decorated FePt nanoparticles

FePt magnetic nanoparticles are an important candidate material for many future magnetic applications. FePt exists as two main phases, that is, a disordered face-centered cubic (fcc) structure, which is generally prepared by chemical methods at low temperatures, and the high-temperature chemically ordered face-centered tetragonal (fct) structure. The fee FePt, with low coercivity but associated with superparamagnetic properties, may find applications as a magnetic fluid or as a nanoscale carrier for chemical or biochemical species in biomedical areas, while fct FePt is proposed for use in ultrahigh-density magnetic recording applications. However, for both of these applications an enhancement of the intrinsically weak magnetic properties, the avoidance of magnetic interferences from neighbor particles, and the improved stability of the small magnetic body remain key practical issues. We report a simple synthetic method for producing FePt nanoparticles that involves hydrothermal treatment of Fe and Pt precursors in glucose followed by calcination at 900 degrees C. This new method produces thermally stable spheroidal graphite nanoparticles (large and fullerene-like) that encapsulate or decorate FePt particles of ca. 5 nm with no severe macroscopic particle coalescence. Also, a low coercivity of the material is recorded; indicative of small magnetic interference from neighboring carbon-coated particles. Thus, this simple synthetic method involves the use of a more environmentally acceptable glucose/aqueous phase to offer a protective coating for FePt nanoparticles. It is also believed that such a synthetic protocol can be readily extended to the preparation of other graphite-coated magnetic iron alloys of controlled size, stoichiometry, and physical properties.

Heterogenisation of Os species on MCM-41 structure for epoxidation of trans-stilbene

Metal organic chemical vapour deposition technique (MOCVD) has been used to immobilise Os species onto the internal porous structure of MCM-41. Evidence suggests that volatile Os-3(CO)(12) cluster reacts with surface silanol groups of the MCM-41 via an oxidative addition reaction to yield a trinuclear HOs3(CO)(10)(OSi-) surface species. After heat treatment in air or at their very low surface coverage, these triangular sites break up to partially oxidised mononuclear surface species. In the presence of tert-butyl hydroperoxide (TBHP) as an oxidant, we demonstrate that the mononuclear species form extremely active species that catalyse the oxidation of trans-stilbene selectively to the corresponding epoxide. By carefully controlling the parameters of the MOCVD method (loading and calcination temperature), we report a new class of optimised MCM-41 porous heterogeneous catalysts carrying isolated but active Os sites for the selective oxidation of trans-stilbene in liquid phase. The reaction selectivity of the solid supported Os is apparently higher than the soluble homogeneous Os-3(CO)(12) cluster. It is envisaged that our solid supported catalysts not only facilitate separation from products but also offer an excellent utilisation of Os for catalysis. (C) 2003 Elsevier Science B.V. All rights reserved.

Surface science studies of strong metal-oxide interactions on model catalysts

The strong metal support interaction (SMSI) was first described in 1978 by Tauster [1-4]. The effect was observed as a severely negative effect on CO and H2 uptake on the catalyst after high temperature calcination under reducing conditions (heating above ~ 700 K) [1,2]. It also had a negative effect on the reaction rate for reactions, such as alkane hydrogenolysis [5,6]. It appeared that the effect occurred for catalysts comprised of reducible supports which were treated at elevated temperature in reducing conditions [2-4]. A classic support which has manifested this behaviour in many studies is TiO2. Over the years following the first discovery of SMSI it has been recognised that the effect is not always negative – for instance for the CO-H2 reaction for which it appears to have a positive effect [5,6]. Further it was noted that hydrogen reduction was not necessary to observe the effect of CO adsorption suppression, it also occurs by vacuum treatment [7], though it should be noted that vacuum treatment at elevated temperature is, in effect, a reducing environment.

Following the creation of active gold nanocatalysts from phosphine-stabilized molecular clusters

The phosphine-stabilised gold cluster [Au6(Ph2P-o-tolyl)6](NO3)2 is converted into an active nanocatalyst for the oxidation of benzyl alcohol through low-temperature peroxide-assisted removal of the phosphines, avoiding the high-temperature calcination process. The process was monitored using in-situ X-ray absorption spectroscopy, which revealed that after a certain period of the reaction with tertiary butyl hydrogen peroxide, the phosphine ligands are removed to form nanoparticles of gold which matches with the induction period seen in the catalytic reaction. Density functional theory calculations show that the energies required to remove the ligands from the [Au6Ln]2+ increase significantly with successive removal steps, suggesting that the process does not occur at once but sequentially. The calculations also reveal that ligand removal is accompanied by dramatic re-arrangements in the topology of the cluster core.