Department of Chemistry, 6-335, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Am Chem Soc. 2010 Oct 6;132(39):13692-701. doi: 10.1021/ja1023767.
A water oxidation catalyst generated via electrodeposition from aqueous solutions containing phosphate and Co(2+) (Co-Pi) has been studied by in situ X-ray absorption spectroscopy. Spectra were obtained for Co-Pi films of two different thicknesses at an applied potential supporting water oxidation catalysis and at open circuit. Extended X-ray absorption fine structure (EXAFS) spectra indicate the presence of bis-oxo/hydroxo-bridged Co subunits incorporated into higher nuclearity clusters in Co-Pi. The average cluster nuclearity is greater in a relatively thick film (∼40-50 nmol Co ions/cm(2)) deposited at 1.25 V vs NHE than in an extremely thin film (∼3 nmol Co ions/cm(2)) deposited at 1.1 V. X-ray absorption near edge structure (XANES) spectra and electrochemical data support a Co valency greater than 3 for both Co-Pi samples when catalyzing water oxidation at 1.25 V. Upon switching to open circuit, Co-Pi undergoes a continuous reduction due to residual water oxidation catalysis, as indicated by the negative shift of the edge energy. The rate of reduction depends on the average cluster size. On the basis of structural parameters extracted from fits to the EXAFS data of Co-Pi with two different thicknesses and comparisons with EXAFS spectra of Co oxide compounds, a model is proposed wherein the Co oxo/hydroxo clusters of Co-Pi are composed of edge-sharing CoO(6) octahedra, the structural motif found in cobaltates. Whereas cobaltates contain extended planes of CoO(6) octahedra, the Co-Pi clusters are of molecular dimensions.
一种通过在含有磷酸盐和 Co(2+)（Co-Pi）的水溶液中电沉积生成的水氧化催化剂，已通过原位 X 射线吸收光谱进行了研究。在支持水氧化催化作用的外加电位和开路电位下，获得了两种不同厚度的 Co-Pi 薄膜的扩展 X 射线吸收精细结构（EXAFS）光谱表明，双-氧/羟桥联 Co 亚基存在于更高核的簇中，这些簇被整合到 Co-Pi 中。在相对于较厚的薄膜（约 40-50 nmol Co 离子/cm(2))（在 1.25 V 相对于 NHE 沉积）中，平均簇核性大于在非常薄的薄膜（约 3 nmol Co 离子/cm(2))（在 1.1 V 沉积）。X 射线吸收近边结构（XANES）光谱和电化学数据表明，当在 1.25 V 催化水氧化时，两种 Co-Pi 样品的 Co 价态均大于 3。当切换到开路时，由于残留的水氧化催化作用，Co-Pi 会连续还原，这表明边缘能量发生负移。还原的速率取决于平均簇大小。基于从具有两种不同厚度的 Co-Pi 的 EXAFS 数据拟合中提取的结构参数以及与 Co 氧化物化合物的 EXAFS 光谱的比较，提出了一种模型，其中 Co-Pi 的 Co 氧/羟簇由共边的 CoO(6)八面体组成，这是在钴酸盐中发现的结构基元。虽然钴酸盐含有扩展的 CoO(6)八面体平面，但 Co-Pi 簇具有分子尺寸。
