Super-Resolution Reactivity Mapping of Nanostructured Catalyst Particles
Super-Resolution Reactivity Mapping of Nanostructured Catalyst Particles
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DOI：
10.1002/anie.200904944
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
Hofkens, Johan
中科院分区：
文献类型：
--
作者：
Roeffaers, Maarten B. J.;De Cremer, Gert;Hofkens, Johan
近一个世纪以来，多相催化剂一直是无数工业化学过程的核心，但人们对其在分子水平上的操作通常比均相催化剂或酶的操作了解得少得多。主要原因是，尽管固体催化剂颗粒的宏观尺寸很大，但它们的活性似乎受到纳米尺度上的组成不均质性和结构特征的控制。因此，理解多相催化的进展要求纳米尺度的组成和结构数据[3-5]与局部催化活性数据相联系，这些数据记录在相同的小空间域中，并在原位反应条件下进行。光学显微镜是固体催化材料原位研究工具箱的最新补充。[5-7]它结合了高时间分辨率和灵敏度，在区分反应产物和试剂方面具有相当大的特异性。然而，基于透镜的显微镜受到光衍射的影响，这将光学分辨率限制在像平面上的250 nm。[8，9]这种分辨率太有限了，无法在固体催化剂上解析纳米尺寸的区域。通过将高斯分布函数与观察到的荧光点的强度(点扩散函数，PSF)进行拟合，可以实现对单个发射体的纳米级精确定位。这种方法已被用来绘制高度稀薄条件下介孔或粘土材料中的扩散路径。然而，对于浓度更高的体系，无法区分同时位于衍射限制区域内的几个分子。在时间上分离不同荧光标记的发射，例如通过选择性光活化，解决了静态系统的成像问题，[8，9，13-18]，但在观察工作催化剂的动力学时不是这样。在这里，我们使用小的含氟反应物的单一催化转化，它们随机地发生在催化剂的密集堆积的活性中心上，重建了催化剂颗粒的衍射无限反应性图。由于连续的催化反应在时间上不重叠，即使它们之间的距离只有10 nm(或更小，取决于信噪比)，人们也可以精确地确定在不同时刻显示转折的反应部位的位置，并以超分辨率重建催化活性区域的图像。尽管荧光底物在单分子酶学中得到了广泛的应用，但到目前为止，只有少数研究报道了利用荧光显微镜对固体化学催化剂进行单周转计数。因此，对微孔材料的类似实验关键取决于确定一种小试剂，该试剂可转化为单分子水平上可检测的产品。令人惊讶的是，糠醇就是这样一种试剂，而且似乎经过酸催化反应(见支持信息)，[26，27]包埋在孔中的产物具有足够的荧光，可以用配备单一激发源(532 nm半导体激光器)和灵敏CCD相机(实验细节见支持信息)的标准显微镜单独观察。我们将这种基于荧光底物催化转化的新的高分辨率重建方法称为纳斯卡显微镜，或随机催化反应的纳米精度。图1a和b分别显示了纳斯卡显微镜的概念和由酸性沸石晶体形成的单个产物分子的2D荧光强度图像。…中的荧光
For almost a century, heterogeneous catalysts have been at the heart of countless industrial chemical processes, but their operation at the molecular level is generally much less understood than that of homogeneous catalysts or enzymes. The principal reason is that despite the macroscopic dimensions of solid catalyst particles, their activity seems to be governed by compositional heterogeneities and structural features at the nanoscale.[1, 2] Progress in understanding heterogeneous catalysis thus requires that the nanoscale compositional and structural data [3–5] be linked with local catalytic activity data, recorded in the same small spatial domains and under in situ reaction conditions. Light microscopy is a recent addition to the toolbox for in situ study of solid catalytic materials.[5–7] It combines high temporal resolution and sensitivity with considerable specificity in distinguishing reaction products from reagents. However, lens-based microscopes are subjected to light diffraction which limits the optical resolution to 250 nm in the image plane.[8, 9] This resolution is far too limited to resolve the nanosized domains on solid catalysts. Nanometer-accurate localization of single emitters can be achieved by fitting a Gaussian distribution function to the intensity of the observed fluorescence spot (point-spread function, PSF). This method has been used to map out diffusion pathways in mesoporous or clay materials under highly dilute conditions.[10–12] However, for more concentrated systems, several molecules simultaneously located within a diffraction-limited area cannot be distinguished. Separating the emission of the different fluorescent labels in time, for example by selective photoactivation, solves the problem for imaging of static systems,[8, 9, 13–18] but not when looking at the dynamics of a working catalyst. Herein, we used single catalytic conversions of small fluorogenic reactants, which occurred stochastically on the densely packed active sites of the catalyst, to reconstruct diffraction-unlimited reactivity maps of catalyst particles. As successive catalytic reactions do not overlap in time, one can precisely determine the location of reaction sites that show turnovers at different moments in time, even if the distance between them is only 10 nm (or less, depending on the signal-to-noise ratio), and reconstruct images of catalytically active zones with super-resolution. Although fluorogenic substrates are widely used in singlemolecule enzymology,[19–23] so far only a few studies have reported single-turnover counting using fluorescence microscopy on solid chemocatalysts.[12, 24, 25] Such studies typically use large polycyclic substrates, which cannot enter the micropores of many heterogeneous catalysts. Hence, similar experiments on microporous materials critically depend on identifying a small reagent that is converted to a product detectable at the single-molecule level. Surprisingly, furfuryl alcohol is such a reagent, and it appears that after acid-catalyzed reaction (see the Supporting Information),[26, 27] the pore-entrapped products are sufficiently fluorescent to be individually observed using a standard microscope equipped with a single excitation source (532 nm diode laser) and sensitive CCD camera (for experimental details, see the Supporting Information). We refer to this novel high-resolution reconstruction method based on catalytic conversion of fluorogenic substrates as NASCA microscopy, or nanometer accuracy by stochastic catalytic reactions microscopy. Figure 1a and b show the concept of NASCA microscopy and a 2D fluorescence intensity image of individual product molecules formed by an acid zeolite crystal, respectively. The fluorescence …