Adsorption of isophorone and trimethyl-cyclohexanone on Pd(111): A combination of infrared reflection absorption spectroscopy and density functional theory studies

Understanding the interaction of α,β-unsaturated carbonyl compounds with late transition metals is a key prerequisite for rational design of new catalysts with desired selectivity towards C = C or C = O bond hydrogenation. The interaction of the α,β-unsaturated ketone isophorone and the saturated ketone TMCH (3,3,5-trimethylcyclohexanone) with Pd(111) was investigated in this study as a prototypical system. Infrared reflection–absorption spectroscopy (IRAS) and density functional theory calculations including van der Waals interactions (DFT + vdW^surf) were combined to form detailed assignments of IR vibrational modes in the range from 3000 cm^{− 1} to 1000 cm^{− 1} in order to obtain information on the binding of isophorone and TMCH to Pd(111) as well as to study the effect of co-adsorbed hydrogen. IRAS measurements were performed with deuterium-labeled (d₅-) isophorone, in addition to unlabeled isophorone and unlabeled TMCH. Experimentally observed IR absorption features and calculated vibrational frequencies indicate that isophorone and TMCH molecules in multilayers have a mostly unperturbed structure with random orientation. At sub-monolayer coverages, strong perturbation and preferred orientations of the adsorbates were found. At low coverage, isophorone interacts strongly with Pd(111) and adsorbs in a flat-lying geometry with the C = C and C = O bonds parallel, and a CH₃ group perpendicular, to the surface. At intermediate sub-monolayer coverage, the C = C bond is strongly tilted, while the C = O bond remains flat-lying, which indicates a prominent perturbation of the conjugated π system. Pre-adsorbed hydrogen leads to significant changes in the adsorption geometry of isophorone, which suggests a weakening of its binding to Pd(111). At low coverage, the structure of the CH₃ groups seems to be mostly unperturbed on the hydrogen pre-covered surface. With increasing coverage, a conservation of the in-plane geometry of the conjugated π system was observed in the presence of hydrogen. In contrast to isophorone, TMCH adsorbs in a strongly tilted geometry independent of the surface coverage. At low coverage, an adsorbate with a strongly distorted C = O bond is formed. With increasing exposure, species with a less perturbed C = O group appear