상압 광전자 분광기를 이용한 Pt3Ni(111)의 산화연구

Abstract

연료 전지 연구에서 Pt3Ni( 111 ) 합금은 cathode 의 산소 환원 반응의 차세대 촉매 모델 시스템으로 잘 알려져 있다. 지금까지 많은 연구의 결과는 이 합금의 뛰어난 촉매작용의 원인으로 Pt3Ni( 111 ) 합금표면에 형성된 Ptskin 구조에 기인한다고 파악하고 있다. 그러나 실제 촉매환경인 높은 압력과 온도 하에서는 Pt-skin 구조가 불안정해지기 때문에 상용화가 진행되지 않고 있다. 본 연구는 Pt3Ni( 111 ) 합금 표면이 실제 상압-상온 조건에서 산소분자와 어떻게 반응하는지 관찰하기 위해서, 상압 광전자 분광기를 사용하여 표면특성을 연구하였다. 시료온도가 room temperature 이며 산소 압력이 100mTorr 까지 증가 될 경우, 표면에는 오직 화학적 흡착 산소 원자들이존재하고 Pt-skin 구조는 남아 있는 것으로 파악되었다. 그러나, 온도를200℃까지 증가 시킬 경우, Pt-skin 표면 위에 흡착되어 있던 산소 원자들은표면 아래 층에 있는 Ni 원자와 반응이 시작되는 것을 파악하였으며, 그 결과Ni 원자는 표면으로 노출되어 산화물을 형성하는 것을 확인하였다. 표면의산화물은 NiO 와 Ni2O3의 형태로 존재하는 것으로 파악되었으며, 특히 NiO 는표면 안쪽에서 성장하는 것으로 확인 되었다.|In the community of fuel cell electrochemistry, Pt3Ni( 111 ) alloy surface has been well known for the best catalytic surface of oxygen reduction reaction in cathode. Many studies on the surface identified the modified surface structure, i.e. the formation of Ptskin surface, and its altered electronic structure as the origin of the enhanced chemical reactivity. However, the practical applications of the alloy surface to the real catalytic system have not been fully realized due to unstable surface structures under elevated temperature and pressure conditions. In order to observe how the surface of Pt3Ni( 111 ) alloy reacts to oxygen molecules under realistic conditions, the ambient pressure x-ray photoemission spectroscopy is utilized, which can provide the surface chemical information under elevated pressure condition. As oxygen gas pressure is increased up to 100mTorr at room temperature, only chemisorbed oxygen species are present and Pt-skin structure remains on surface. However, as the temperature is increased up to 200oC under the elevated oxygen pressure conditions, the adsorbed oxygen atoms on the Pt-skin surface start to interact with the subsurface Ni atoms, resulting in pulling the Ni atoms to surface and forming the surface oxide, i.e. a surface segregation. In this presentation, we will provide the details of surface segregation profile of Pt3Ni ( 111 ) alloy surface as a function of temperature and oxygen pressure. The experiments are carried out at KEK-Photon Factory in Tsukuba and Advanced Light Source in Berkeley.; In the community of fuel cell electrochemistry, Pt3Ni( 111 ) alloy surface has been well known for the best catalytic surface of oxygen reduction reaction in cathode. Many studies on the surface identified the modified surface structure, i.e. the formation of Ptskin surface, and its altered electronic structure as the origin of the enhanced chemical reactivity. However, the practical applications of the alloy surface to the real catalytic system have not been fully realized due to unstable surface structures under elevated temperature and pressure conditions. In order to observe how the surface of Pt3Ni( 111 ) alloy reacts to oxygen molecules under realistic conditions, the ambient pressure x-ray photoemission spectroscopy is utilized, which can provide the surface chemical information under elevated pressure condition. As oxygen gas pressure is increased up to 100mTorr at room temperature, only chemisorbed oxygen species are present and Pt-skin structure remains on surface. However, as the temperature is increased up to 200oC under the elevated oxygen pressure conditions, the adsorbed oxygen atoms on the Pt-skin surface start to interact with the subsurface Ni atoms, resulting in pulling the Ni atoms to surface and forming the surface oxide, i.e. a surface segregation. In this presentation, we will provide the details of surface segregation profile of Pt3Ni ( 111 ) alloy surface as a function of temperature and oxygen pressure. The experiments are carried out at KEK-Photon Factory in Tsukuba and Advanced Light Source in Berkeley.