Crucial Effect of Subsurface Hydrogen on Low-Barrier Hydrogenation and Keto–Enol Tautomerization of Carbonyl Compounds

Hydrogenation of a normally highly stable carbonyl group is an important step in many technological applications, including emerging molecular systems for reversible hydrogen storage. In this report, we present a mechanistic study of low-temperature hydrogenation of carbonyl compounds over Pd, proceeding via keto–enol tautomerization in the first step. The specific focus of this study is on the role of subsurface hydrogen absorbed in the nearest region below the Pd surface. Employing a combination of real space microscopic and operando spectroscopic surface sensitive techniques, as well as molecular beams, we show that subsurface H plays a crucial role in both keto–enol tautomerization and hydrogenation of the carbonyl compound acetylpyridine on the Pd(111) model catalyst. We demonstrate that a growing amount of subsurface H results in an enhanced abundance of enol species followed by hydrogenation already at cryogenic temperatures. In contrast, if only H adsorbed on the surface is present, no hydrogenation occurs, and substantially smaller amounts of enol species are formed. The population of subsurface H is also accompanied by a change in the mechanism of enol stabilization via hydrogen bonding: while in the presence of subsurface H specific enol-containing dimers are predominately formed, which strongly interact via the enol-acetyl or enol–enol groups, in the absence of subsurface H, a weaker interaction between the adsorbates occurs, which is realized mainly via the enol group of one molecule with a H atom belonging to the pyridine ring of the neighboring adsorbate. The observed strongly correlated behavior between the growing concentration of subsurface H, enhanced abundance of the enol form of acetylpyridine, and the onset of hydrogenation prove the crucial role of subsurface H species in the low-barrier hydrogenation pathway of carbonyl compounds. The obtained atomistic-level insights offer a prospect of controllable low-temperature hydrogenation of carbonyl compounds by tuning the abundance of subsurface H, which has not been available so far.