Toward Understanding the Reduction Process of Nickel Oxide and Ceria-Based Nickel Catalyst via Positron Annihilation Lifetime Spectroscopy

Abstract

This study presents in situ monitoring using positron annihilation lifetime spectroscopy (PALS) of the reduction process, at its very early stage, occurring on nickel oxide and ceria-based nickel catalyst designed for the steam reforming of ethanol. The PALS results show the formation of water on the surface of materials containing Ni species just after exposure to hydrogen at only 120 °C. For the first time, it has been presented that on the surface of both materials, water droplets exist in the form of plugs or bridges that can accommodate ortho-positronium bubbles of about 0.5 nm in diameter. The chemical composition and structure, and what follows, the availability of highly reactive oxygen species, affect the size of water droplets. The droplets are larger on the surface of NiO than on the catalyst (consisting of NiO dispersed on CeO2 and a solid solution of Ce1-XNiXO2), which is characterized by the greater availability of reactive oxygen species.