General discussion

Abstract

opened the discussion of Prof. Goringe's paper: In describing the nanoparticles of metals lodged, epitaxially perhaps, inside your carbon nanotubes, you raised the possibility that such novel preparations of metals could serve as interesting new catalysts. I have heard others talk in such terms, but I am a little puzzled by it all. For as long as the nanoparticle of metal remains inside (at the tip?) of a nanotube, access to it for a reactant species is going to be difficult. Likewise, egress of products formed at such a nanoparticle is also going to be difficult. I wonder whether you can enlighten us as to what you had in mind, or where I may have missed something?Prof. Goringe replied: You are quite correct in stating that gas access is likely to be restricted; the suggestion on catalytic action was made with reference to the possibility of specific, favourable facets being forced on particles by their being inside the nanotube.Dr. Tsang commented: In response to Prof. Sir John Meurig Thomas's question to Prof. Goringe on the diffusion problems associated with metal-encapsulated nanotube catalysts, I would like to make the following comment. We have recently been studying nanotube catalysis and have not encountered any diffusion problems in the gas-phase catalytic hydrogenation of CO to CH, over Pd-encapsulated nanotubes. However, the rates of liquid phase hydrogenation of nitroaromatics seemed to suffer from diffusion limitations especially in some batches of samples with a large proportion of long nanotubes (pm in length). This suggests that the mass transfer of reactants/products to and from the metal sites encapsulated deep inside the nanotubes are indeed quite slow processes. Research is underway to synthesise shorter nanotubes.Concerning in situ microscopy studies of encapsulated metal and metal oxide inside the carbon nanotubes, I have some questions for Prof. Goringe. He has observed some encapsulated metal particles, e.y., Ni which appeared to be in plug form. Do these metal plugs completely block the tubes? It has been suggested that the cross-section of a carbon nanotube can be either circular or polygonal in shape. What are the preferred crystallographic orientations of Ni crystallites with respect to these tubes? Prof. Goringe responded: We have studied too few particles in total to be able to make any definitive statement on preferred orientations. Likewise with the question of filling: some plugs appear to fill the whole cross-section, but with flat facets at their unconstrained ends (e.~]., in some cases with nickel particles), while others certainly do not (e.y., the iridium example in Fig. 4 of the paper).'