Mass-selected FeCo clusters embedded in a carbon matrix as Benchmark Nanocatalysts

Abstract

Despite cobalt and iron−carbon interactions playing an important role in environmental chemistry activities, major questions are still unresolved on the prevailing mechanism, surface state, required structure, and the origin of apparent particle size dependence in carbon nanotube synthesis or other heterogeneous catalytic process such as Fischer−Tropsch (F−T) synthesis. In this paper, we focus our attention on well-defined bimetallic FeCo clusters embedded in a carbon matrix to study separately size, alloy, and interface effects on physical properties. First the size-dependent evolution of the intrinsic structural and magnetic properties was estimated by using density-functional ab initio calculations for relaxed B2 (CsCl-like) FeCo nanoclusters. Then, assemblies of mass-selected clusters preformed in the gas phase by using the low-energy cluster beam deposition (LECBD) technique and coated with a carbon film were investigated by use of various experimental techniques that include transmission electron microscopy (TEM) and superconducting quantum interference device (SQUID) magnetometry as well as synchrotron techniques such as extended Xray absorption fine structure (EXAFS) and X-ray magnetic circular dichroism (XMCD). All these results show that the carbon solubility is a size-dependent process and that annealing allows us to reach the chemically ordered B2 phase only for FeCo nanoparticles larger than 5 nm in diameter.