Selective Reduction of Ni/Co Metal Nanoparticles from Spinel Oxides

<i>In-situ</i> techniques offer insights into reaction mechanisms and microstructure development that may be missed entirely when studying complex materials systems. Nickel/cobalt aluminate spinel is an interesting case study, where reducing atmospheres extract reducible cations from the MAl2O4 spinel starting phase. The spinel materials are chemically highly tailorable, and selectively reduced materials are useful as catalysts for fuel reforming, for example. The microstructures, however, are of critical importance. Here we present a study employing <i>in-situ</i> scanning electron microscopy as the primary tool for tracking the processes dynamically. In-situ neutron powder diffraction, Raman spectroscopy, and S/TEM complement the FEGSEM data. The talk will show how we track first the evolution of the faceting of the base oxide particles, then sintering of particles, and finally Ni/Co exsolution and resorption – as functions of temperature and oxygen partial pressure. Reduction of the metal nanoparticles from the oxide yields surface sockets and the time/temperature profiles determine the self-organization of the metal particles on the oxide surfaces, as well as the particle sintering. We find that rapid mass transport occurs on the surface, enabling the metal nanoparticles to diffuse along the surface and coalesce. Using catalysis measurements, we show that the oxide surfaces are in fact oxygen deficient, allowing rapid near-surface oxygen ion transport and that simple control over the reduction reactions yields optimized catalysis behavior. We consider possible mechanisms that enable rapid diffusion of particles on these unique oxide surfaces.