Nickel oxide reduction studied by environmental TEM

In situ reduction of an industrial NiO powder is performed under 1.3 mbar of H2 (2 mlN/min) in a differentially pumped FEI Titan 80-300 environmental transmission electron microscope (ETEM). Images, diffraction patterns and electron energy loss spectra (EELS) are acquired to monitor the structural and chemical evolution of the system during different temperature ramps (at 2, 4 and 7°C/min). High-resolution ETEM is also performed during similar experiments.
Ni nucleation on NiO is observed to be either epitaxial in thin areas or randomly oriented on thicker regions and when nucleation is more advanced. The growth of Ni crystallites and the movement of interfaces create pores within the NiO grains to accommodate the volume shrinkage associated with the reduction. EELS analysis illustrates that reduction proceeds quickly at temperatures below 400°C up to a reduced fraction of about 0.6, until the reaction is slowed down by water created upon reduction. Using the data obtained at different heating rates and the Kissinger method, an activation energy for the NiO reduction of 70 ± 20 kJ/mol could be obtained. Densification is then observed at temperatures higher than 550°C: pores created at lower temperatures disappear and Ni grains coarsen. This reorganization of Ni is detrimental to both the connectivity of the Ni catalyst and to the redox stability of the SOFC. A model for the structural evolution of NiO under H2 is proposed.