| AVS 64th International Symposium & Exhibition | |
| Applied Surface Science Division | Thursday Sessions |
| Session AS+BI+SA+SS-ThM |
| Session: | Spectroscopy of the Changing Surface |
| Presenter: | Ryo Taga, Tohoku University, Japan |
| Authors: | R. Taga, Tohoku University, Japan S. Ogawa, Tohoku University, Japan Y. Takakuwa, Tohoku University, Japan |
| Correspondent: | Click to Email |
Nitrogen contained in the air is oxidized and then harmful nitrogen oxide (NOX) is formed in the combustion chamber of engine. Accordingly, the exhaust gas which contains NOX is purified by catalysts. However, platinum group metals, whose prices are likely to rise by the depletion of resources in the future, are used as the catalysts, so the reduction of the amount used is an important matter for industrial and environmental fields. On the other hand, it has been already known that Ni has an effect to NOX reduction, but the its catalytic ability disappears when the Ni surface is oxidized. If O atoms on the Ni surface can be efficiently desorbed, Ni is expected as a catalyst for NO reduction. In the previous studies, some of researches have studied about reduction of oxidized Ni surfaces, but the relation between oxide reduction kinetics and behavior of O atoms has not yet been clarified. In this study, the oxidation and reduction kinetics on Ni(111) surfaces was investigated by real-time ultraviolet photoelectron spectroscopy (UPS). to investigate the amount of O atom adsorption and the changes of work function.
The experiments were performed using UPS apparatus with base pressure of ~3×10-8 Pa. The Ni(111) surface was firstly cleaned by the Ar+ ion bombardment, and the annealed at 600℃. O2 gas (1×10-5 Pa) was directly introduced to UPS apparatus at the sample temperature of 100℃. After the end of the introduction of O2 gas, the sample heated up to and H2 gas (1×10-5 Pa) was introduced in order to investigate the Ni oxide reduction process. The photoelectron spectra were measured repeatedly each 72 s during oxidation and reduction.
From the time evolution of O 2p photoelectron spectra, we obtained the O 2p uptake curve and the change in work function. When O2 gas was introduced, O 2p intensity increases linearly, so it turned out that the oxidation of Ni(111) surfaces was a zero order reaction. After introduction of H2 gas, O 2p intensity decreases gently for about 500 s and then decreased rapidly. On the other hand, the work function slightly increased and then rapidly decreased. The work function reaches the same value on the clean Ni(111) surface. Therefore, Ni oxide can be reduced completely using H2 gas.
These changes after introduction of H2 gas can be divided into two areas. In the first area, O atoms are drawing from subsurface because of slight increase of work function. In the second area, then, it is suggested that the reduction progresses and the clean Ni surface area enlarges as like to island growth. In the symposium, we will discuss the reduction process of the oxidized Ni surface by NO gas.