Chemical Imaging of Elementary Steps in Hydrogenation Reactions of Surfaces

在化学系化学测量和成像项目的支持下，耶鲁大学的施瓦茨教授和Altman教授正在开发一种用于单分子表面化学的可视化和表征的新方法。这种新的方法提供了显着的进步，现有的途径，探索表面化学的分子和反应的大集合;相反，反应的每一步都是由扫描探针显微镜的尖端单独诱导，特定的和独特的反应途径是随意选择的，并在该途径上的潜在最小值之间的能量障碍进行量化。在每一步中，负责驱动反应的相互作用都以前所未有的精确度进行了表征，这有可能揭示附近表面缺陷或其他分子的影响，从而为表面化学和催化的研究开辟了一条全新的途径。该技术和新的结果，它可以产生说明使用氢化，脱氢，和碳-碳键的芳香族化合物的形成为例。选择这些反应是因为它们对世界化学和石化工业具有非凡的技术重要性。这种“观察化学反应”的能力，结合对化学反应的每一个细节的控制和量化，有望促进对公众的宣传。更具体地说，这种新方法建立在扫描探针显微镜的最新进展之上，这不仅使表面分子成像成为可能，而且还可以1）绘制分子周围的整个表面电位，从而发现局部反应增强的位点; 2）平移分子、原子和簇，同时测量位点之间的扩散势垒，从而检测这些势垒作为化学环境的函数; 3）使用由尖端提供的能量分裂分子（解离）;以及4）在反应物已经被适当地布置在表面上之后，通过由尖端诱导的电压脉冲形成分子和分子键。这项研究首次将这些元素结合起来，实现了表面反应中所有基本步骤的完整定量描述。新方法的核心是测量将分子和/或原子推到一起使它们能够反应所需的力;通过沿路径沿着积分，可以恢复能量势垒和势能最小值的深度。为了实现表面分子的必要稳定性，所有研究都在低温下使用自制的扫描隧道/原子力显微镜进行。首先，苯、碘苯和氢沉积在铂的（111）表面上。随后，通过施加电压脉冲产生单个H和I原子，并且操纵所产生的自由基和原子以确定可能的操纵路径和势能极小值之间的扩散势垒，同时绘制势能景观。最后，联苯通过连接两个苄基自由基或苯通过氢化苄基而产生，自发地或通过一旦物种靠近在一起就施加偏压脉冲。结果，在单分子水平上获得完整的能量信息。

With support from the Chemical Measurement and Imaging Program in the Division of Chemistry, Professors Schwarz and Altman at Yale University are developing a new approach for the visualization and characterization of single molecule surface chemistry. This new approach offers dramatic advances over existing pathways to explore surface chemistry over large ensembles of molecules and reactions; instead each step of a reaction is individually induced by the tip of a scanning probe microscope, the specific and unique reaction pathway is chosen at will, and energy barriers between potential minima on that pathway are quantified. During each step, the interactions responsible to drive the reaction are characterized with unprecedented precision, which has the potential to reveal the influence of nearby surface defects or other molecules, thereby opening a whole new avenue to the study of surface chemistry and catalysis. The technique and the novel results it can generate are being illustrated using hydrogenation, dehydrogenation, and carbon-carbon bond formation in aromatic compounds as examples. These reactions are chosen because of their extraordinary technological importance for the world's chemical and petrochemical industry. The ability to "see chemistry in action" combined with controlling and quantifying every detail of it is expected to facilitate outreach to the general public.More specifically, the new approach builds on recent advances in scanning probe microscopy, which have made it possible to not only image molecules on surfaces, but also to 1) map the entire surface potential around the molecule, thereby uncovering sites of enhanced local reactivity; 2) translate molecules, atoms, and clusters while measuring the diffusion barrier between sites, thereby detecting these barriers as a function of the chemical environment; 3) split molecules (dissociation) using energy provided by the tip; and 4) form molecules and molecular bonds through voltage pulses induced by the tip after the reactants have been arranged properly on the surface. This research, for the first time, combines these elements to achieve a complete quantitative picture of all of the elementary steps involved in surface reactions. The new approach is centered on measuring the force needed to push molecules and/or atoms together so that they can react; by integrating along the path, the energy barriers and depths of the potential minima can be recovered. To achieve the necessary stability of the molecules on the surface, all investigations take place at low temperatures using a home-built combined scanning tunneling/atomic force microscope. First, benzene, iodobenzene, and hydrogen are deposited on the (111) surface of platinum. Subsequently, individual H and I atoms are produced by applying voltage pulses and the resulting radicals and atoms are manipulated to determine possible manipulation paths and diffusion barriers between potential minima while the potential energy landscapes are being mapped. Finally, biphenyl are produced by linking two benzyl radicals or benzene by hydrogenating benzyl, either spontaneously or through the application of bias voltage pulses once the species are brought close together. As a result, complete energetic information is obtained on a single-molecule level.