Low-Temperature Hydrogenation and Keto–Enol Tautomerization of Carbonyl Compounds: Effect of Distant Substituents on Reactivity

Understanding the mechanisms driving the low-barrier hydrogenation of aldehydes and ketones is crucial both for rational design of new molecular systems for reversible hydrogen storage and for optimizing heterogeneously catalyzed hydrogenation of carbonyl compounds in general. Recent theoretical predictions and experimental studies have proven that this process can proceed via two consecutive low-barrier steps in a temperature range, which is significantly lower than that required for direct hydrogen insertion into the highly stable C═O bond. This alternative reaction mechanism involves keto–enol tautomerization of the carbonyl group to the enol form, followed by hydrogenation of the newly formed C═C bond. In this study, we addressed both elementary processes for two carbonyl compounds, acetylpyridine and acetophenone, on a Pd(111) model catalyst employing a combination of molecular beam techniques, infrared reflection absorption spectroscopy, and scanning tunneling microscopy. The specific focus of this study was on exploring the way how the chemical and electronic structure of a distant substituting group, such as phenyl and pyridine rings, can affect both keto–enol tautomerization and low-barrier hydrogenation of the carbonyl group. The reactivity was investigated on the Pd surface containing different types of hydrogen atoms including hydrogen adsorbed on the surface and absorbed in the subsurface region of the catalyst. Specifically for acetylpyridine, both processes were found to strongly depend on the availability of subsurface hydrogen. While keto–enol tautomerization is merely enhanced with a growing concentration of subsurface hydrogen as compared to pristine Pd(111) or Pd covered with only surface-adsorbed hydrogen species, the hydrogenation process was detected only in the presence of subsurface hydrogen. In contrast, acetophenone was observed to readily undergo keto–enol tautomerization on all investigated surfaces irrespective of the availability of coadsorbed or absorbed hydrogen; however, hydrogenation was detected for this reactant on neither of these surfaces. With this, the chemical compositions of the distant substituting groups, phenyl vs pyridine ring, were found to strongly affect both keto–enol tautomerization and low-barrier hydrogenation of the acetyl group. This observation holds great potential for the rational design of molecular hydrogen carriers for the storage of green hydrogen that can be efficiently operated under low-temperature conditions.