In Situ Observation of a Surface Chemical Reaction by Fast X-Ray Photoelectron Spectroscopy
In Situ Observation of a Surface Chemical Reaction by Fast X-Ray Photoelectron Spectroscopy
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通过快速 X 射线光电子能谱原位观察表面化学反应
DOI：
10.1021/ja991858v
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发表时间：
1999
影响因子：
15
通讯作者：
R. M. Lambert
中科院分区：
文献类型：
--
作者：
A. Lee;K. Wilson;R. Middleton;A. Baraldi;A. Goldoni;G. Paolucci;R. M. Lambert
尽管它们具有巨大的社会经济意义，但新的多相催化剂的发现和优化仍然效率低下。这反映了通常采用的经验方法，而且在大多数情况下，缺乏对催化剂行为的微观了解。通过研究定义明确的单晶表面上的反应，结合对相应的实际分散催化剂的测量，有可能从根本上洞察反应途径并发展预测能力。然而，为了进一步合理地设计催化活性材料，需要新的原位分析技术。特别是，获得关于反应吸附层随温度变化的时间分辨信息将是有价值的。在这里，我们展示了首次使用时间分辨的快速X射线光电子能谱(XPS)作为化学特异性的定量探针来研究乙炔在催化活性Pd(111)表面上的三聚反应。我们测量了三聚反应的阈值温度和活化势垒，阐明了反应途径的细节，并确定了吸附层的构型变化。对于CO/Rh-(110)体系，最近证明了快速XPS作为分子脱附过程的探针的潜力。3这项技术的成功取决于当前第三代同步加速器的高光子通量和高分辨率。这些属性允许快速获取时间分辨的XP光谱，同时提高样品温度。在这里，我们将该技术扩展到一个表面催化的反应：乙炔在Pd(111)单晶表面上的三聚反应。该反应可视为典型的金属催化的炔烃偶联反应。这些过程之所以令人感兴趣，是因为它们构成了一个相对复杂的反应的多功能网络，根据条件的不同，可以产生各种产品，包括直链和环状碳氢化合物，甚至杂环。此外，它们可以在从超高真空到1bar的广泛压力范围内使用单晶样品和分散催化剂进行操作。因此，它们为催化科学的许多核心概念提供了一个有价值的试验场(见参考文献4和参考文献)。4个实验是在Elettra(的里雅斯特)同步辐射源的SuperESCA光束线上进行的，使用的是按标准工艺制备的、在超高真空(系统压力∼为1×10-10Torr)下保持的Pd单晶衬底。引用的曝光量在朗缪尔(1朗缪尔)1×10-6托S-1中给出。在光子能量为400 eV，能量分辨率为∼100 meV的条件下，获得了碳1S-XP谱。在乙炔覆盖的样品上应用线性加热斜率(∼0.4Ks-1)获得程序升温反应数据，乙炔三聚反应在低温(<；180K)下发生。因此，样品在100K下进行吸附，以避免立即生成苯。图1显示了C1S XP光谱序列作为乙炔覆盖率的函数。这些数据是在最初清洁的Pd(111)样品连续曝光期间获得的，时间间隔为∼850 S，每个光谱记录在∼10 S中。所有减去的低温C1S谱都可以很容易地用以283.88 eV为中心的单峰进行拟合，该单峰由与高斯卷积的Doniach-Sunjic函数建模。导出的峰值参数如表1所示，所得的C1S积分强度显示在图1的插图中。这是…
Despite their immense socioeconomic importance the discovery and optimization of new heterogeneous catalysts remains inefficient. This reflects the empirical methodologies often adopted and, in most cases, the absence of a microscopic understanding of catalyst behavior. By studying reactions on well-defined single-crystal surfaces, in conjunction with measurements on the corresponding practical dispersed catalysts, it is possible to obtain fundamental insight into reaction pathways and to develop predictive capabilities. 1, 2 However, to further the rational design of catalytically active materials, new in situ analytical techniques are required. In particular, it would be valuable to obtain time-resolved information about the temperature-dependent evolution of the reacting adsorbed layer. Here, we demonstrate the first use of timeresolved fast X-ray photoelectron spectroscopy (XPS) as a chemically specific, quantitative probe in a study of the trimerization of ethyne to benzene over a catalytically active Pd (111) surface. We have measured the threshold temperature and activation barrier for trimerization, elucidated details of the reaction pathway, and identified configurational changes in the adsorbed layer. The potential of fast XPS as a probe in studies of molecular desorption processes was recently demonstrated for the CO/Rh-(110) system. 3 The success of this technique hinges on the high photon flux and high resolution available at current third-generation synchrotrons. These attributes permit rapid acquisition of time-resolved XP spectra while ramping the sample temperature. Here, we extend the technique to follow a surface-catalyzed reaction: the trimerization of ethyne to benzene on a Pd (111) singlecrystal surface. This reaction may be regarded as the prototypical metal-catalyzed alkyne coupling reaction. These processes are of interest because they constitute a versatile network of relatively complex reactions which, depending on the conditions, can yield a variety of products including linear and cyclic hydrocarbons and even heterocycles. In addition, they can be operated over a wide range of pressure ranging from ultra high vacuum to 1 bar using both single-crystal samples and dispersed catalysts. They therefore provide a valuable testing ground for many concepts that are central to catalytic science (see ref 4 and refs therein). 4 Experiments were performed at the SuperESCA beamline of the ELETTRA (Trieste) synchrotron radiation source using on a Pd (111) single-crystal substrate prepared by standard procedures and maintained under ultra-high vacuum (system pressure∼ 1× 10-10 Torr). Quoted exposures are given in langmuirs (1 langmuir) 1× 10-6 Torr s-1). Carbon 1s XP spectra were acquired with a photon energy of 400 eV and energy resolution of∼ 100 meV. Temperature-programmed reaction data were acquired by application of a linear heating ramp (∼ 0.4 Ks-1) to the ethyne-covered sample.Ethyne trimerization occurs at low temperatures (< 180 K). Therefore, adsorption was carried out with the sample held at 100 K so as to avoid immediate benzene formation. Figure 1 shows a sequence of C 1s XP spectra as a function of ethyne coverage. These data were obtained during continuous exposure of the initially clean Pd (111) sample over a time interval of∼ 850 s, each spectrum being recorded in∼ 10 s. All of the backgroundsubtracted, low temperature C 1s spectra can be readily fitted using a single peak centered at 283.88 eV modeled by a Doniach-Sunjic function convoluted with a Gaussian. Derived peak parameters are given in Table 1 and the resulting C 1s integrated intensities are shown in the inset to Figure 1. This …