Takehiro Tamaoka1 2 Ryotaro Aso1 Hideto Yoshida1 Seiji Takeda1

1, Osaka University, Ibaraki, Osaka, Japan
2, Osaka University, Suita, Osaka, Japan

Investigating the surface dynamic structure of metals in gaseous environments is important for catalysts and gas sensing. Palladium is a well–known material which is used for hydrogen storage, hydrogen sensing and exhaust catalysis. Environmental TEM (ETEM) has been used for revealing the phase transition of palladium in hydrogen. Though the interaction between palladium surface and gases has been extensively studied, much less is known on the surface process at the atomic scale.

Here, we investigated the surface structure of a wedge–shaped palladium specimen in gases including hydrogen and oxygen by means of in-situ atomic resolution ETEM. As is well known, the surface of palladium is oxidized in air and at room temperature by several nanometers. After introducing hydrogen (100Pa) in ETEM, the native oxide layer (PdO) was reduced to metallic fcc palladium even at room temperature. During ETEM observation in hydrogen, the stable high index facets such as {311} and {511} appeared on the surface. After the exposure to hydrogen, we exhausted hydrogen and then introduced oxygen (100Pa) in ETEM. We confirmed that the surface oxide layer was reproduced. However, after prolonged exposure to hydrogen over 90 min., we have found that the surface was not oxidized. We could not detect the transition from palladium to palladium-hydrogen by electron energy loss spectroscopy (EELS) even after the prolonged exposure to hydrogen.

The results indicate that, although the phase transition did not occur, the prolonged exposure to hydrogen makes the oxidation–prevention state on the palladium surface. We will present atomic-scale in-situ movies on the surface dynamics in palladium in various processing.