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

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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.