Ligand-directed heterogeneous catalysis for controlling chemoselectivity of multi-pathway surface reactions: towards mechanistic understanding via surface science approach.

多路径表面反应的选择性依赖于竞争反应活化势垒的细微差异，这是很难控制的。解决这一问题的最有希望的策略之一是在反应物和催化活性中心之间引入特定的选择性相互作用，将化学转化引导到所需的路线上。这种相互作用可以通过催化剂与配体的官能化来实现，通过空间约束或电子效应来促进所需的途径。近年来，一系列高选择性配体功能化粉末催化剂相继问世，开辟了配体导向多相催化的新领域。然而，微观水平上对潜在的表面过程的了解仍然很少。通过拟议的研究，我们的目标是在原子水平上理解有机配体组装的金属表面上发生的配体导向的多相催化作用。这项研究的重点将集中在原子水平上表征配体组装的几何和化学结构，以及探索发生在这些复杂界面上的多路径反应的分子反应机理和动力学。我们将在定义明确的具有配体功能的模型催化剂上应用独特的表面敏感技术组合-包括金属单晶和模型氧化物负载的金属纳米颗粒-在原子水平上探索配体导向催化的起源。我们的实验方法包括结合扫描隧道显微镜对配位层的结构进行表征；红外光谱允许对表面物种进行化学鉴定和操作监测，以及分子束技术允许高度可控的动力学研究。我们将系统地改变配位体组件的化学和几何结构来调节表面限制和电子效应，并找到这些配位层的结构和化学性质与吸附和反应过程之间的精确关联，例如反应物和反应中间体在配位层上的吸附几何结构，反应机理，竞争反应路径和反应动力学的活性和选择性。我们将研究两种类型的反应：多不饱和羰基化合物的部分选择性加氢反应和炔的部分选择加氢反应。这些最先进的实验技术的结合将首次用于模型配体导向催化的基础水平研究，并将允许获得对基本表面过程的新的原子水平的洞察。这项研究的结果具有巨大的潜力，可以开发基于理性的、具有定制催化性能的表面设计的新概念。

The selectivity of multi-pathway surface reactions depends on subtle differences in the activation barriers of competing reactions, which is difficult to control. One of the most promising strategies to overcome this problem is to introduce a specific selective interaction between the reactant and the catalytically active site, directing the chemical transformations towards the desired route. This interaction can be imposed via functionalization of a catalyst with ligands, promoting the desired pathway via steric constrain or electronic affects. Recently, a number of highly selective ligand-functionalized powdered catalysts was developed, opening up a new field of ligand-directed heterogeneous catalysis. The microscopic-level understanding of the underlying surface processes, however, is still largely missing.With the proposed research we are aiming at an atomistic-level understanding of ligand-directed heterogeneous catalysis occurring at the metal surfaces functionalized with organic ligand assemblies. The focus of this study will be at the atomistic-level characterization of the geometric and chemical structure of the ligand assemblies as well as at the exploring the molecular reaction mechanisms and kinetics of multi-pathway reactions occurring at these complex interfaces.We will apply a unique combination of surface-sensitive techniques on well-defined model catalysts functionalized with ligands – both metal single crystals and metallic nanoparticles supported on model oxides– to explore the origins of ligand-directed catalysis at the atomistic level. Our experimental approach includes a combination of scanning tunneling microscopy for structural characterization of the ligand layers; infrared spectroscopy enabling the chemical identification and in operando monitoring of the surface species and molecular beam techniques allowing for highly controlled kinetic studies.We will systematically change the chemical and the geometric structure of the ligand assemblies to tune the surface confinement and electronic effects and find exact correlations between these structural and chemical properties of the ligand-layer and the adsorption and reactive processes, such as the adsorption geometries of the reactants and the reaction intermediates over a ligand-layer, the reaction mechanisms, the activity and selectivity towards competing reaction pathways and reaction kinetics. Two types of reactions will be investigated: partial selective hydrogenation of multi-unsaturated carbonyl compounds and alkynes.The combination of these state-of-the-art experimental techniques will be employed for the first time in the fundamental-level studies on the model ligand-directed catalysis and will allow to obtain new atomistic-level insights into the underlying elementary surface processes. The outcome of this research holds a great potential for developing new concepts towards rational-based design of surfaces with tailor-made catalytic properties.