Facet-dependent NiO reduction revealed by surface-sensitive in situ scanning transmission electron microscopy

Abstract

In situ surface reduction has been widely adopted as an activation process on Ni-based catalysts, which can reduce NiO to highly active metallic Ni for catalysis. The detailed surface reduction mechanism and pathways, however, remain largely unclear. Here we present an in situ microscopic observation of the process of NiO surface reduction using surface-sensitive scanning transmission electron microscopy (STEM). The specially prepared NiO crystals with well-defined facet structure enable us to identify the facet-dependent reduction behavior: Reduced Ni nanoparticles preferentially exsolve on {111} facets of NiO surface initially, followed by particle growth and coalescence at higher temperature, and eventually migrate to {100} facets with higher binding energy to stabilize Ni nanoparticles. Remarkably, the formed Ni nanoparticles maintain the nearly epitaxial orientation relationship with the NiO matrix, with lattice mismatch accommodated by periodic edge dislocations at the Ni/NiO interface. Such facet-dependent reduction behavior is governed by the surface energies derived by density functional theory calculations. Besides shedding light on the fundamental mechanism of NiO surface reduction, our work also demonstrates a powerful approach combining surface-sensitive STEM with in situ capability to interrogate various physical and chemical processes on the surface.