Microstructural determination of a metastable phase during rhodium oxidation
In heterogeneous catalysis, oxidation of the catalyst is frequently used as a means of removing impurities from the active surface. However, bulk oxidation severely alters the microstructure of the metal due to atom repositioning and valence changes. Transformation of Rh to Rh2O3 causes the lattice structure to change from fcc to hexagonal with a resulting volume expansion of 90% due to density changes. This is the reported cause for fracturing of crystallites and variations in reactivity. In this study, microstructural changes during progressive oxidation of 5 nm metal Rh particles have been examined.A 2 wt% Rh/Silica catalyst was prepared using Rh(III)-2,4 pentanedionate as a precursor and nonporous silica spheres as a support. The catalyst was then reduced in flowing H2 at 473 K. High resolution electron microscopy (HREM) was performed on a JEM-4000EX having a point resolution of 0.17 nm. The catalyst was oxidized in research purity O2 (Matheson 99.99%) using an all glass volumetric chemisorption system.