Hexagonally arranged surface oxygen atoms, oxygen point defects, and multiple oxygen defects at oxygen-terminated CeO2(111) surfaces in different oxidation states were visualized by noncontact atomic force microscopy (NC-AFM). The multiple defects such as line defects and triangular defects were stabilized by a local reconstruction, where edge oxygen atoms surrounding the multiple defects were displaced and gave enhanced brightness due to a geometric reason. Successive NC-AFM measurements of the same area of a slightly reduced CeO2(111) surface revealed that hopping of surface oxygen atoms faced to metastable multiple defects was thermally activated even at room temperature. In contrast, no hopping was observed either at a point oxygen vacancy or a line defect that is stabilized by local reconstruction. It was also confirmed from atom-resolved NC-AFM observations that the surface oxygen defects were easily healed by exposure to O2 gas at room temperature.

The catalytic dehydration reaction processes of formic acid on a TiO2(110) surface at 350 K have been studied to visualize reaction intermediates and their dynamic behaviors by scanning tunneling microscopy. Three types of configurations of adsorbed formates on the surface were identified by their shapes and positions in STM images. Successive STM observations revealed transformations among the three configurations, i.e., bridge formate on a 5-fold coordinated Ti4+ row, bridge formate on an oxygen vacancy site with an oxygen atom of formate and on a 5-fold coordinated Ti4+ ion and with the other formate oxygen atom, and a monodentate formate on an oxygen vacancy site with an oxygen atom of formate. The decomposition of the monodentate formate to carbon monoxide and hydroxyl was also imaged, which is a rate-determining step in the catalytic dehydration of formic acid. Combined with first-principle DFT calculations, the overall reaction processes of the catalytic dehydration of formic acid on the surface have been elucidated. Oxygen vacancies on the surface that can be produced by dehydration of two hydroxyls in situ under the catalytic reaction conditions are essential for the reaction.

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