In Situ TEM Investigation of Structural Changes in Ni Nanoparticle Catalysts under Gas Atmospheres: Implications for Catalyst Degradation

Ni catalysts with strong hydrogenation and dehydrogenation activities have been used to reform hydrocarbons in natural gas. In this study, microstructural changes, which relate to catalytic degradation for reforming hydrocarbons, in Ni nanocatalysts were observed in situ in oxygen, hydrogen, and methane atmospheres at elevated temperatures using an environmental transmission electron microscope. During oxidation and reduction under oxygen and hydrogen atmospheres, respectively, volumetric changes and mass transfer occurred in the Ni nanoparticle as well as in a larger Ni catalyst particle. On the other hand, the face-centered cubic (fcc) crystal structure of the Ni nanocatalysts transformed to a hexagonal close-packed (hcp) structure as the particles were heated from 250 to 350 °C in methane atmosphere at pressures of 30-40 Pa. The entire Ni nanocatalyst particle had the hcp structure at 350 °C. The spacing of close-packed planes was more than 5% wider in the hcp Ni crystals than it was in fcc Ni. We concluded that carbon and nickel solid solutions formed in the Ni particles as methane thermally decomposed to elemental carbon, which caused the transformation of the Ni crystal structure. Graphite layers appeared, surrounding the Ni particles, after the Ni transformation from fcc to hcp.