Atomically Dispersed Nickel in Cerium Oxide Aerogel Catalysts Characterized Using Scanning Transmission Electron Microscopy and Spectroscopy.

Catalytically active nickel-substituted cerium oxide (CeO2) aerogels, engineered to maximize yield in the water-gas shift reaction (WGS: H2O + CO ↔ H2 + CO2), are characterized using scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). STEM shows that the architected CeO2 catalysts with Ni substituted at 2.5, 5, and 10 atomic percentage (at.%) contain atomically distributed Ni within the ∼7 nm CeO2 nanocrystalline domains that comprise the covalently bonded oxide network. The aerogels prepared at the two higher Ni-to-Ce compositions also contain >100 nm nickel-oxide particles, which correlates with our prior report that these catalysts generate undesired CH4 as a byproduct during WGS reactions run at mild temperatures. The micrographic absence of large nickel-oxide particles in 2.5 at. % Ni-substituted CeO2 aerogel substantiates why this WGS catalyst is methane-free to the limit of detection. We find that high electron fluence modifies the cerium oxidation state and nickel distribution, resulting in a mix of Ce3+ and Ce4+ and ∼1 nm nickel aggregates. Density functional theory indicates that Ni cations are mobile and tend to aggregate in a CeO2 fluorite lattice, even in the absence of electron flux, particularly when located near oxygen vacancies. The nature of Ni within the CeO2 aerogel, as observed by STEM/EELS/EDS, validates the need to prevent Ni aggregation in WGS catalysts.