Adsorption and keto-enol-tautomerisation of butanal on Pd(111)

Microscopic-level understanding of the interaction of hydrocarbons with transition metal surfaces is an important prerequisite for rational design of new materials with improved catalytic properties. In this report, we present a mechanistic study on the keto-enol tautomerisation of butanal on Pd(111), which was theoretically predicted to play a crucial role in low-barrier hydrogenation of carbonyl compounds. These processes were addressed by a combination of reflection-absorption infrared spectroscopy, molecular beam techniques and theoretical calculations at the density functional theory level. Spectroscopic information obtained on Pd(111) suggests that butanal forms three different aldehyde species, which we indicate as A1-A3 as well as their enol counterpart E1. The electronically strongest perturbed and strongest binding species A1 is most likely related to the η₂(C,O) adsorption configuration, in which both C and O atoms are involved in the bonding with the underlying metal. The species A2 weakly binds and is less electronically perturbed and can be associated with the η₁(O) adsorption configuration. The third type of aldehyde species A3, which is nearly unperturbed and is found only at low temperatures, results from the formation of the butanal multilayer. Importantly, the enol form of butanal was observed on the surface, which gives rise to a new characteristic band at 1104 cm^-1 related to the stretching vibration of the C-O single bond (ν(C-O)). With increasing temperature, the multi-layer related species A3 disappears from the surface above 136 K. The population of aldehyde species A1 and the enol species E1 noticeably increases with increasing temperature, while the band related to the aldehyde species A2 becomes strongly attenuated and finally completely disappears above 120 K. These observations suggest that species E1 and A1 are formed in an activated process and - in view of the strongly anti-correlated population of the species E1 and A2 - it can be concluded that enol species E1 is most likely formed from the weakly bound aldehyde species A2 (η₁(O)). Finally, we discuss the possible routes to enol stabilization via intermolecular bonding and provide the possible structure of the enol-containing stabilized complex, which is compatible with all spectroscopic observations. The obtained results provide important insights into the process of keto-enol tautomerisation of simple carbonyl compounds.