Surface-Assisted Keto-Enol Tautomerism as a First Step in Heterogeneously Catalyzed Hydrogenation of Carbonyl Compounds

多相催化的羰基化合物加氢反应是许多技术相关过程的关键步骤。它依赖于高度稳定的C=O键的激活，而这一点很难实现。最近，理论上预测了一种替代的低势垒的羰基化合物氢化途径，它基于两步过程：(1)酮-烯醇互变异构化为烯醇形式，然后(2)H插入到新形成的烯醇的C=C键中。在这些研究中，这两个反应步骤的活化势垒都比直接将氢插入到原来稳定的羰基键上要低得多。这一替代机制为简单羰基氢化开辟了一种低势垒途径，依赖于酮-烯醇互变异构化作为反应的第一步。最近，我们首次在实验上证实了预测的反应路线。我们还证明了氢插入不是发生在理论上预测的烯醇单体中，而是发生在烯醇酮二聚体中，其中通常不太稳定的烯醇物种通过烯醇的-OH基团和相邻分子的羰基之间的氢键来稳定。在烯醇介导的低势垒加氢反应中，外来的含酮基吸附体对烯醇物种的外部稳定是至关重要的一步。这一观察打开了通过使用能够稳定和活化烯醇的稳定的外来修饰剂分子来控制催化表面的官能化来控制这一过程的前景。然而，对于潜在的表面过程的微观水平的了解仍然很少。通过拟议的研究，我们的目标是在原子水平上理解发生在金属表面的烯醇介导的多相加氢催化，无论是原始的还是带有改性剂物种的功能化表面，都能够实现烯醇的稳定和活化。这项研究的重点将集中在探索不同类别的酮基烯醇异构化、烯醇稳定和烯醇介导的加氢反应的机理；在反应条件下形成的表面物种的光谱和微观鉴定；监测它们的动态变化，并将这种结构信息与通过分子束技术在受控等温条件下获得的反应动力学相关联。我们将在定义明确的模型催化剂上应用独特的表面敏感技术组合-金属单晶和金属纳米颗粒负载在模型氧化物上，原始的和官能化的-探索烯醇介导的羰基化合物加氢的基本原理，并获得详细的结构-反应关系。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。

Heterogeneously catalysed hydrogenation of carbonyl compounds is a key step for many technically relevant processes. It relies on the activation of a highly stable C=O bond, which is difficult to achieve. Recently, an alternative low-barrier hydrogenation pathway of carbonyl compounds was predicted theoretically, which is based on a two-step process: (1) keto-enol tautomerisation of carbonyl species to the enol form followed by (2) H insertion into the newly formed C=C bond of enol. In these studies, both reaction steps exhibit significantly lower activation barriers as compared to the direct H insertion into the original stable carbonyl bond. This alternative mechanism opens up a prospect of a low-barrier pathway for hydrogenation of simple carbonyls relying on keto-enol tautomerisation as the first reaction step. Recently, we obtained the first experimental confirmation of the predicted reaction route. We also demonstrated that H insertion occurs not in an enol monomer as predicted theoretically, but in enol-ketone dimers, in which normally less stable enol species is stabilized via hydrogen bonding between the –OH group of the enol and the carbonyl group of a neighbouring molecule. External stabilization of enol species by foreign carbonyl-containing adsorbates was identified as a crucial step in enol-mediated low-barrier hydrogenation. This observation opens up a prospect of controlling this process via functionalization of the catalytic surface with stable foreign modifier molecules capable of enol stabilization and activation. 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 enol-mediated heterogeneous hydrogenation catalysis occurring at metal surfaces, both pristine and functionalized with modifier species, that are capable of enol stabilization and activation. The focus of this study will be on exploring the mechanisms of keto-enol tautomerisation, enol stabilization and enol-mediated hydrogenation for different classes of carbonyl compounds; spectroscopic and microscopic identification of surface species formed under the reaction conditions; monitoring their dynamic changes and correlating this structural information with the reaction kinetics obtained under controlled isothermal conditions by molecular beam techniques.We will apply a unique combination of surface-sensitive techniques on well-defined model catalysts – both metal single crystals and metallic nanoparticles supported on model oxides, pristine and functionalized – to explore the fundamentals of enol-mediated hydrogenation of carbonyl compounds and obtain detailed structure-reactivity relationships. The outcome of this research holds a great potential for developing new concepts towards rational-based design of surfaces with tailor-made catalytic properties.