In Situ Scanning Transmission Electron Microscopy of Ni Nanoparticle Redispersion via the Reduction of Hollow NiO

Abstract

Oxidation and reduction cycles are used in the regeneration of nanoparticle catalysts that have deactivated due to sintering or poisoning. Nickel oxidation and reduction cycles for the redispersion of nickel nanoparticles were studied via in situ high angle annular dark field environmental scanning transmission electron microscopy. Cycling the Ni/NiO system through successive redox cycles shows that the particles retain the same general size distributions even though Ostwald ripening and particle migration and coalescence is occurring. The regeneration of the smallest nanoparticle sizes, which disappear due to sintering processes, occur by the ejection of small (2–3 nm) nickel particles during the reduction of the hollow nickel oxide nanostructures. The nickel nanoparticles above ∼3.5 nm in size form hollow polycrystalline nickel oxide nanostructures upon oxidation. Upon reduction, the grains making up the shell of the hollow nickel oxide reduce separately at the grains surface and at the grain boundaries...