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Schwarz和Altman教授正在开发一种新的方法，用于单分子表面化学的可视化和表征。这种新方法在探索分子和反应的大集合表面化学方面提供了比现有途径显著的进步；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的，可以随意选择特定和独特的反应途径，并量化该途径上势能最小值之间的能量势垒。在每一步中，负责驱动反应的相互作用都以前所未有的精度进行表征，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。以芳香族化合物的加氢、脱氢和碳-碳键形成为例，说明了该技术及其产生的新结果。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。“看到化学在起作用”的能力与控制和量化它的每一个细节相结合，有望促进向公众推广。更具体地说，新方法建立在扫描探针显微镜的最新进展之上，扫描探针显微镜不仅可以在表面上成像分子，而且可以1)绘制分子周围的整个表面电位，从而发现增强局部反应性的位点；2)翻译分子、原子和簇，同时测量位点之间的扩散屏障，从而检测这些屏障作为化学环境的功能；​4)待反应物在表面整齐排列后，通过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，以获得表面反应中所有基本步骤的完整定量图像。新方法的核心是测量将分子和/或原子推到一起以使它们发生反应所需的力；通过沿路径积分，可以恢复势极小值的能垒和深度。为了达到表面分子的必要稳定性，所有的研究都是在低温下使用自制的扫描隧道/原子力显微镜进行的。首先，苯、碘苯和氢沉积在铂的（111）表面。随后，通过施加电压脉冲产生单个H和I原子，并对产生的自由基和原子进行操作，以确定可能的操作路径和势能最小值之间的扩散屏障，同时绘制势能景观。最后，联苯是由两个苯基自由基或苯氢化连接产生的，要么是自发的，要么是通过施加偏置电压脉冲，一旦物种靠近在一起。因此，在单分子水平上获得了完整的能量信息。

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.