Nanoscale Chemical Imaging of Three-Way Catalyst Pt/Ce2Zr2Ox particles by Ptychographic-XAFS
Nanoscale Chemical Imaging of Three-Way Catalyst Pt/Ce2Zr2Ox particles by Ptychographic-XAFS
复制标题
通过 Ptychography-XAFS 对三效催化剂 Pt/Ce2Zr2Ox 颗粒进行纳米级化学成像
DOI：
10.1017/s1431927618012497
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发表时间：
2018
影响因子：
2.8
通讯作者：
Yukio Takahashi
中科院分区：
文献类型：
--
作者：
Makoto Hirose;Nozomu Ishiguro;Kei Shimomura;Hirosuke Matsui;Mizuki Tada;Yukio Takahashi
X射线重叠关联成像是相干X射线衍射成像的扫描版本，用于以超出X射线透镜制造精度的技术限制的空间分辨率重建扩展对象的复杂透射函数[1]。X射线重叠关联成像最重要的应用之一是与X射线吸收光谱相结合，称为重叠关联-XAFS（X射线吸收精细结构）。这种方法可以从重建的空间分辨XAFS光谱中分辨出样品在纳米尺度上的化学状态。到目前为止，Ptychographic-XAFS应用于使用软X射线观察薄材料[2，3]。为了观察微米级的散装材料，必须使用硬X射线。在硬X射线区域实现重叠关联XAFS的一个挑战是弱XAFS信号的准确重建。为了克服这个问题，我们提出了先进的迭代相位恢复算法使用的Kramers-Kronig关系（KKR）之间的吸收和相移作为一个额外的约束。在原理验证实验中，成功重建了Mn-K边缘周围Mn 2 O3薄膜的XAFS光谱（6.539 keV）[4]。这里，将叠层XAFS应用于微米级三效催化剂材料。Ce基混合氧化物被称为高效储氧/释氧材料，用于三效汽车尾气净化系统中的促进剂。特别是Ce 2 Zr 2 Ox（CZx，x= 7-8）固溶体材料由于其优异的储氧性能而引起关注[5]。然而，CZx的异质性阻止了通过常规方法理解真实的反应行为。Ptychographic-XAFS被用来了解CZx颗粒的纳米级异质性。用H2或O2处理1重量%的Pt负载的CZx颗粒（Pt/CZx）以产生微米级的完全还原的Pt/CZ 7、完全氧化的Pt/CZ 8和部分氧化的Pt/CZ 7。6，然后将这些颗粒分散在500 nm厚的Si 3 N4膜上。Ptychographic-XAFS实验在BL 29 XUL SPring-8上进行。在5.707-5.817 keV之间使用了28个X射线能量，包括Ce-L3边缘。入射到样品上的X射线通过一对Kirkpatrick-Baez（KB）反射镜二维聚焦到500 nm的半峰全宽光斑尺寸。以400 nm的步长在9 × 9位置扫描样品。这种扫描条件导致可靠的图像重建，尽管重叠率相对较小，因为KB反射镜的聚焦光束具有较强的次峰。使用真空像素阵列检测器（EIGER 1 M）以4.0 s的曝光时间测量多个衍射图案。利用Pt/CZ_（7. 7）的X射线荧光光谱确定中间价态。6可以近似地由Pt/CZ 7和Pt/CZ 8的XAFS的线性组合表示[6]。重建图像的像素尺寸为13 nm。而Pt/CZ 7和Pt/CZ 8颗粒几乎完全还原或氧化，Pt/CZ 7. 6颗粒表现出复杂的氧化态分布，其来源于局部区域中储氧行为的差异，如图1所示[7]。
X-ray ptychography is a scanning version of coherent X-ray diffractive imaging for reconstructing a complex transmission function of an extended object with a spatial resolution beyond the technical limitation of the fabrication accuracy of the X-ray lens [1]. One of the most important applications of X-ray ptychography is a combination with X-ray absorption spectroscopy called ptychograhic-XAFS (X-ray Absorption Fine Structure). This approach can discriminate chemical state of the specimen at the nanoscale from the reconstructed spatially resolved XAFS spectrum. So far, ptychographic-XAFS was applied to observe thin materials using soft X-rays [2, 3]. To observe micrometer-sized bulk materials, it is necessary to use hard X-rays. A challenge towards realizing ptychographic-XAFS in the hard X-ray region was the accurate reconstruction of the weak XAFS signals. To overcome this problem, we proposed the advanced iterative phase retrieval algorithm using the Kramers–Kronig relation (KKR) between absorption and phase shift as an additional constraint. In a proof-of-principle experiment, XAFS spectra of a Mn2O3 film around Mn-K edge (6.539 keV) was successfully reconstructed [4]. Here, ptychographic-XAFS was applied to micrometer-sized three-way catalyst materials.Ce-based mixed oxides are known as efficient oxygen storage/release materials and used for promoter in three-way automobile exhaust cleaning systems. Especially, Ce2Zr2Ox (CZx, x= 7-8) solid-solution materials attract attention due to their excellent oxygen storage property [5]. However, the heterogeneity of the CZx prevented understanding of the real reaction behaviour by conventional methods. Ptychographic-XAFS is utilized to understand the nanoscale heterogeneity of the CZx particles. 1 wt% Pt-suppoted CZx particles (Pt/CZx) were treated with H2 or O2 to produce micron-sized fully reduced Pt/CZ7, fully oxidized Pt/CZ8, and partially oxidized Pt/CZ7. 6, then these particles were dispersed on 500-nm-thick Si3N4 membranes. Ptychographic-XAFS experiment was carried out at BL29XUL SPring-8. Twenty-eight X-ray energies were used between 5.707–5.817 keV, including Ce-L3 edge. Incident X-rays onto the samples were two-dimensionally focused to a spot size of 500 nm at full width at half maximum by a pair of Kirkpatrick–Baez (KB) mirrors. The samples were scanned in 9 by 9 positions with a step size of 400 nm. This scan condition results in the reliable image reconstruction despite of the relatively small overlap ratio since the focusing beam by KB mirrors has strong subpeaks. Multiple diffraction patterns were measured using an in-vacuum pixel-array detector (EIGER 1M) with an exposure time of 4.0 s. Intermediate valence can be decided by using the fact that XAFS of Pt/CZ7. 6 can be approximately expressed by a linear combination of the XAFS of Pt/CZ7 and Pt/CZ8 [6]. The pixel size of the reconstructed images was 13 nm. While Pt/CZ7 and Pt/CZ8 particles were almost fully reduced or oxidized, Pt/CZ7. 6 particles exhibited a complicated oxidation state distribution derived from the differences in the oxygen storage behaviors in the local domains as shown in Fig. 1 [7].