In situ environmental transmission electron microscopy to determine transformation pathways in supported Ni nanoparticles

We have applied in situ environmental transmission electron microscopy (ETEM) to follow the dynamic phase transformations that take place in SiO2 supported Ni nanoparticles during oxidation and reduction processes. The gas environments used for in situ ETEM studies were relevant to partial oxidation of methane (POM) reaction. In the presence of the CH4 + O2 gas mixture (in 2:1 ratio) at 400 °C, Ni transforms to NiO due to the high O-chemisorption energy. NiO void structures were formed during the oxidation because of the Kirkendall type process where diffusion of Ni cations along NiO grain boundaries is eight orders of magnitude greater than the diffusion of O anions. Reduction was performed under a CO + H2 mixture at 400 °C (in 1:2 ratio) and also in the presence of CH4 at 500 °C. Particle reduction processes also takes place via the diffusion of Ni cations along the NiO grain boundaries leaving NiO on the surface of the nanoparticle. NiO is the phase that is present on the surface of the nanoparticle during the intermediate stage of reduction.

► Ni nanoparticles transform to NiO void structures during the oxidation process.
► Grain boundaries in NiO acts as faster diffusion paths for Ni cations.
► NiO is the phase that is present on the surface of the nanoparticle during the intermediate stage of reduction.