Structure and Valency of a Cobalt-Phosphate Water Oxidation Catalyst Determined by in Situ X-ray Spectroscopy
Structure and Valency of a Cobalt-Phosphate Water Oxidation Catalyst Determined by in Situ X-ray Spectroscopy
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DOI：
10.1021/ja1023767
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发表时间：
2010-10-06
影响因子：
15
通讯作者：
Nocera, Daniel G.
中科院分区：
文献类型：
--
作者：
Kanan, Matthew W.;Yano, Junko;Nocera, Daniel G.
用原位x射线吸收光谱法研究了由含有磷酸盐和Co-Pi的水溶液电沉积生成的水氧化催化剂。得到了两种不同厚度Co-Pi膜在支持水氧化催化的外加电位和开路时的光谱。扩展的x射线吸收精细结构（EXAFS）光谱表明，Co- pi中存在双氧/羟基桥接的Co亚基，这些亚基被合并到高核簇中。在1.25 V vs NHE下沉积的相对厚的薄膜（类似于40-50 nmol Co离子/cm(2)）比在1.1 V下沉积的极薄薄膜（类似于3 nmol Co离子/cm(2)）的平均团簇核更大。x射线吸收近边结构（XANES）光谱和电化学数据支持Co- pi样品在1.25 V催化水氧化时的Co价大于3。当Co-Pi切换到开路时，由于残余水的氧化催化作用，Co-Pi持续还原，这可以从边缘能量的负移看出。减少的速度取决于平均簇大小。根据对两种不同厚度Co- pi的EXAFS数据拟合提取的结构参数，并与Co-氧化化合物的EXAFS光谱进行比较，提出了Co- pi的Co-氧/羟基簇由钴酸盐中发现的共享边的CoO6八面体组成的模型。而钴酸盐含有扩展平面的CoO6八面体，Co-Pi簇是分子尺寸的。
A water oxidation catalyst generated via electrodeposition from aqueous solutions containing phosphate and Co2+ (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 (similar to 40-50 nmol Co ions/cm(2)) deposited at 1.25 V vs NHE than in an extremely thin film (similar to 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 CoO6 octahedra, the structural motif found in cobaltates. Whereas cobaltates contain extended planes of CoO6 octahedra, the Co-Pi clusters are of molecular dimensions.