Chemical state analysis of size-controlled particles during redox reactions between NiO and metallic Ni

The chemical state change between NiO and Ni for size-controlled particles supported on SiO 2 was analyzed by means of in situ X-ray absorption fine structure technique. The reduction of NiO particles by H 2 gas occurred in two stages at approximately 380 °C and 650 °C, and the relative contribution of each stage varied with the particle size. Based on the particle size and the composition, it was interpreted that the NiO component, which was reduced at high temperatures, was stabilized by forming a silicate-like structure at the interface with SiO 2 . When metallic Ni particles were exposed to O 2 gas at room temperature, NiO was produced in an amount dependent on the surface area calculated from the particle size, suggesting that a 0.5–0.9 nm surface layer of the Ni particles was oxidized to NiO. As the temperature increased, all remaining metallic Ni in the particle core was oxidized to NiO. The higher oxidation completion temperature in larger particles suggests that the NiO layer expanded inward from the surface. The NiO formed on the surface of the metallic Ni particles was reduced by heating up to 300 °C in an H 2 gas atmosphere. This low reduction temperature indicates a high capacity of the surface NiO species for oxide ion dissociation, likely due to lattice mismatch with the internal metallic Ni. The redox processes between NiO and Ni generated an inhomogeneous distribution of chemical states within individual particles.