Study on State-sensitive Observation of Gold and Platinum Nano-particles and Promoter Sites by XAFS combined with X-ray Fluorescence Spectrometry

本工作是我们的一系列应用于催化剂和环境材料的状态敏感XAFS（X射线吸收精细结构）研究的延伸。本文测量了抑制SnK能级的芯-空穴寿命展宽的Pt-Sn催化剂的SnK α_1选择XAFS谱。结果表明，Pt颗粒（平均粒径约为1 nm）高度分散在SiO_2表面，Sn在Pt颗粒上存在吸附位，Sn的电子态也得到了证实。并合成了系列Au/TiO_2催化剂。平均Au颗粒尺寸被均匀地控制在2.9和8.7 nm之间。基于状态灵敏Au Lα_1-选择XAFS，在Au纳米粒子中发现了中性1，δ^-和δ^+的存在，这一时期的第一个主要的，显著的进展是由于SPring-8光束线上X射线束稳定性的提高，提高了状态灵敏XAFS的能量分辨率。光束位置稳定性表现出类似的效果，以减少样品位置处的光束尺寸。因此，Au纳米颗粒中每个位点的状态敏感结构分析变得更加熟悉，技术要求更低。第二，与我们在可见光下激发的钒催化剂的研究（优先领域研究“纳米动力学”）一样，建立了在反应条件下的状态敏感XAFS测量技术。结果表明，在反应条件下，我们成功地直接探测到了Au（δ^+）位对O_2分子的电子贡献。该方法可用于CO低温氧化反应中关键中间体的检测。

This work was performed as the extension of series of our state-sensitive XAFS(X-ray absorption fine structure) studies applied to catalysts and environmental materials. State-sensitive Sn Kα_1-selecting XAFS spectra were measured for Pt-Sn catalysts suppressing the core-hole lifetime broadening of Sn K level. The presence of Sn sites adsorbed on Pt particles (mean particle size approximately equal 1 nm) highly dispersed over SiO_2 surface and the electronic state of Sn were demonstrated. Also, series of Au/TiO_2 catalysts were synthesized. The mean Au particle size was controlled homogeneous to between 2.9 and 8.7 nm. Based on state-sensitive Au Lα_1-selecting XAFS, the presence of neutra1, δ^-, and δ^+ sites were demonstrated in Au nano-particles.First major, remarkable progress during this period was that the improvement of energy resolution of state-sensitive XAFS due to the advancement of X-ray beam stability at SPring-8 beamline. The beam position stability exhibited similar effect to reduce the beam size at the sample position. As the result, the state-sensitive structural analyses of each site in Au nano-particles became more familiar, technically less-demanding. Second, in common with our study of vanadium catalysts excited under visible light (Priority Area Research "Nano-Dynamics"), measurement technique of state-sensitive XAFS under on-reaction conditions was established. As the result, we were successful to directly detect electron donation from Au(δ^+)sites to O_2 molecule under on-reaction condition. The measurement is regarded as to detect the key intermediate in the low-temperature CO oxidation.