Direct Observation of Iron Nanoparticles within Mesoporous MCM-41 Silica Molecular Sieve

M.S.MORENO, M.WEYLAND, S.G.MARCHETTI AND P.A.MIDGLEY

Antwerp, Belgica

Congreso; 13th European Microscopy Congress; 2004

The improvement and development of nanocatalysts requires direct information on the size, distribution, location and detailed morphology of the nanoparticles constituting the active sites, especially in those catalysts with particles so small that nearly all the atoms are exposed to the reactant species. One of those materials is the mesoporous molecular sieve system with one-dimensional channels designated as MCM-41. There have been attempts to introduce Fe nanoparticles inside the pores but there has been no direct evidence given that the nanoparticles were incorporated into the pore structure. We studied the MCM-41 / nanoparticle system before and after reduction of the precursor. The supports were prepared according to [1] and loaded with Fe concentration of 5 and 10 wt%. The decomposition procedure of the impregnated salt produces haematite particles [2]. To detect the iron nanoparticles we used High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (STEM-HAADF) combined with EDS (Energy Dispersive X-ray Spectroscopy). This combination allowed us to correlate unambiguously features in the images with chemical information. Our observations revealed that a small fraction of the haematite nanoparticles are located at the surface but by far the majority of the particles are anchored inside the pores, as can be seen in Figure 1. It is also quite clear that the nanoparticles are not filling the pores homogeneously but they are located only in certain channels. The distribution of occupied channels seems to be random. Our results indicate also that the iron loading has no influence on this distribution, being similar for the sample loaded with 5 and 10 Fe wt%.Figure 2 shows results for the reduced sample. We can observe iron particles elongated with a variable longitudinal length but with a lateral dimension approximately the same as the pore size. We consider this fact is strongly indicating that the observed features in the images correspond to nanoparticles anchored inside the channels, that is they are not expeled to the surface after the reducing treatment as happens in iron/zeolite systems [3]. We suggest that the elongated morphology observed for the nanoparticles arises because of a tendency for the particles to sinter but are constrained by the walls of the tube-like channels. This observation acts as indirect proof that the elongated nanoparticles of the precursor oxide are anchored initially inside the channels. It is also likely that the sintered iron particles grow to fill the diameter of the channel, blocking the route for any catalytic reaction.