Abstract
The behavior of nickel crystallites supported on titania (rutile) upon heating in H 2 and O 2 atmospheres has been investigated by using transmission electron microscopy. When Ni TiO 2 specimens were heated in additionally purified ultrahigh-purity hydrogen (APH) at 500 and 700 °C, alternate changes in the shapes of the crystallites were observed. This shape alternation is found to be associated with the extension and contraction of the crystallites. During heating of Ni TiO 2 specimens in hydrogen (APH) at 700 °C, cavities were formed in the substrate probably because of the migration of TiO 2−x away from the substrate underneath and near the particle. The specimens were also heated alternately in oxygen and hydrogen (APH) atmospheres at 700 °C. During heating in an oxygen atmosphere, the particles extended considerably over the substrate surface and/or diffused into the substrate. On subsequent heating in a hydrogen atmosphere, large crystallites were formed on the substrate surface. On further heating in hydrogen, the crystallites extended again but to a much smaller extent than in an oxygen atmosphere. These results suggest that in the case of Ni TiO 2 the interactions between the particle and substrate are overall stronger in an O 2 atmosphere than in a H 2 atmosphere.
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