Imai Hideto, Izumi Koichi, Matsumoto Masashi, Kubo Yoshimi, Kato Kazuo, Imai Yasuhiko
Nano Electronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan.
J Am Chem Soc. 2009 May 6;131(17):6293-300. doi: 10.1021/ja810036h.
The electrochemical oxidation behaviors of the surfaces of platinum nanoparticles, one of the key phenomena in fuel cell developments, were investigated in situ and in real time, via time-resolved hard X-ray diffraction and energy dispersive X-ray absorption spectroscopy. Combining two complementary structural analyses, dynamical and inhomogenous structural changes occurring at the surfaces of nanoparticles were monitored on an atomic level with a time resolution of less than 1 s. After oxidation at 1.4 V vs RHE (reversible hydrogen electrode) in a 0.5 M H(2)SO(4) solution, longer Pt-O bonds (2.2-2.3 A that can be assigned to OHH and/or OH species) were first formed on the surface through the partial oxidation of water molecules. Next, these species turned to shorter Pt-O bonds (2.0 A, adsorbed atomic oxygen), and atomic oxygen was incorporated into the inner part of the nanoparticles, forming an initial monolayer oxide that had alpha-PtO(2)-like local structures with expanded Pt-Pt bonds (3.1 A). Finally, quasi-three-dimensional oxides with longer Pt-(O)-Pt bonds (3.5 A, precursor for beta-PtO(2)) grew on the surface, at almost 100 s after oxidation. Despite the very complex oxidation mechanism on the atomic level, XANES analysis indicated that the charge transfer from platinum to the adsorbed oxygen species was almost constant and rather small, that is, about 0.5 electrons per oxygen, up to two monolayers of oxygen. This means that ionic polarization hardly develops at this stage of the surface platinum's "oxide" growth.
作为燃料电池发展中的关键现象之一，铂纳米颗粒表面的电化学氧化行为通过时间分辨硬X射线衍射和能量色散X射线吸收光谱进行了原位实时研究。结合两种互补的结构分析方法，以小于1秒的时间分辨率在原子水平上监测了纳米颗粒表面发生的动态和不均匀结构变化。在0.5 M H₂SO₄溶液中相对于可逆氢电极（RHE）在1.4 V下氧化后，首先通过水分子的部分氧化在表面形成较长的Pt - O键（2.2 - 2.3 Å，可归属于OHH和/或OH物种）。接下来，这些物种转变为较短的Pt - O键（2.0 Å，吸附的原子氧），并且原子氧并入纳米颗粒的内部，形成具有扩展的Pt - Pt键（3.1 Å）的类似α - PtO₂局部结构的初始单层氧化物。最后，在氧化后约100秒时，具有较长Pt - (O) - Pt键（3.5 Å，β - PtO₂的前体）的准三维氧化物在表面生长。尽管在原子水平上氧化机制非常复杂，但XANES分析表明，从铂到吸附氧物种的电荷转移几乎恒定且相当小，即每个氧约0.5个电子，直至两层氧。这意味着在表面铂“氧化物”生长的这个阶段几乎不会发生离子极化。
