Metal nanoparticles formed by thermal transformation of M-MIL140C (M=In, Rh, Pd)

MIL-140C-10 containing various post-synthetically-added metals (Rh, Pd & In) was heated under CO₂/H₂ which is an atmosphere that can be used for catalytic hydrogenation reactions. A higher rate of weight loss increased the final ZrO₂ crystallite size. This size and, also, the extent of incorporation of lower-valence ions from the added metal into the ZrO₂ structure determined the oxygen vacancy concentration and hence the tetragonal-monoclinic phase ratio in the ZrO₂ support. Inclusion of In had smaller effects on the pyrolysis rate and oxidation state of the Zr than Rh and Pd perhaps, at least in part, because some of it was located on the surface rather than attached to the pyridine-containing linkers, and also because the metal species were more volatile and tended to evaporate during the heating. The morphology of the sample was greatly changed when the MOF structure was thermally treated. Under CO₂/H₂, up to 400 °C, some MOF structure was retained for Rh, but not for Pd. The particle size of ZrO₂ and the proportion of monoclinic as opposed to tetragonal ZrO₂ obtained as the MOF structure decomposed tended to increase as the final heating temperature increased but, like the particle size, was also dependent on the nature of the added metal. The surface of ZrO₂ after heating to 500 °C showed oxygen vacancies, the proportion of vacancies varying with the added metal. Thus the nature of the added metal has a powerful influence on composition and metal speciation of the MOF-derived products, which could have important implications on the catalytic activity, quite apart from the intrinsic catalytic activity of the added metal.