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

羰基化合物的多相催化加氢是许多技术相关工艺的关键步骤。它依赖于激活一个高度稳定的C=O键，这是很难实现的。最近，从理论上预测了羰基化合物的另一种低势垒加氢途径，该途径基于两步过程：(1)羰基物种的酮-烯醇互变异构成烯醇形式，然后(2)在烯醇新形成的C=C键中插入H。在这些研究中，与H直接插入原始稳定羰基键相比，这两个反应步骤都表现出明显较低的激活障碍。这种替代机制为简单羰基的氢化开辟了低屏障途径，依赖于酮烯醇互变异构作为第一步反应。最近，我们首次通过实验证实了预测的反应路线。我们还证明，H插入并不像理论上预测的那样发生在烯醇单体中，而是发生在烯醇-酮二聚体中，在这种二聚体中，通常不太稳定的烯醇通过烯醇的-OH基团和邻近分子的羰基之间的氢键来稳定。外来含羰基吸附剂对烯醇的外部稳定作用是烯醇介导的低势垒加氢的关键步骤。这一观察结果开辟了控制这一过程的前景，通过稳定的外源修饰剂分子的催化表面功能化能够稳定和激活烯醇。然而，对潜在表面过程的微观层面的理解在很大程度上仍然缺失。通过提出的研究，我们的目标是在原子水平上理解烯醇介导的非均相氢化催化发生在金属表面，包括原始的和功能化的改性物质，能够稳定和激活烯醇。本研究的重点将是探索不同类型羰基化合物的酮-烯醇互变异构、烯醇稳定和烯醇介导的加氢机制；在反应条件下形成的表面物质的光谱和显微鉴定；监测它们的动态变化，并将这些结构信息与分子束技术在受控等温条件下获得的反应动力学相关联。我们将在定义良好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支持模型氧化物，原始和功能化-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-反应性关系。这项研究的结果对于开发基于理性设计的具有定制催化性能的表面的新概念具有很大的潜力。

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.