Formation and Real-Space Distribution of Acetophenone Dimers on H-containing Pt(111)

Abstract

We present a mechanistic study on the formation and real-space distribution of acetophenone dimers on the H-containing Pt(111) surface. Their geometric configuration and the specific chemical nature are addressed by a combination of scanning tunneling microscopy (STM) and infrared reflected absorption spectroscopy (IRAS). The formation of different types of acetophenone dimers is reported here, which critically depends on the surface coverage of H species. When the surface coverage of H is close to a monolayer (ML), acetophenone is adsorbed mainly as monomer species, while only a small amount of adsorbates are agglomerated into dimers. In these dimers, the majority of species are related to acetyl/phenyl dimers, in which a carbonyl group of one molecule interacts with a H atom involved in the phenyl ring of the neighboring molecule. Depending on the position of this ring, H atom, ortho-, meta-, and para-dimers can be formed with comparable probabilities. On the surface precovered with a lower concentration of H (close to 0.5 ML) only, a new class of dimers prevail, which interacts via two acetyl groups forming ketone–enol dimers or partly hydrogenated dimers. Specifically, in the ketone–enol dimers, one molecule undergoes keto-enol tautomerization and a newly formed OH group becomes involved in H bonding with the neighboring carbonyl entity. In this way, a normally unstable enol species can be stabilized on the surface. The interaction between the carbonyl group and the phenyl ring becomes negligible on this type of surface. Additionally, a large fraction of the ketone–enol dimers becomes partly hydrogenated on this surface, producing a direct reaction intermediate toward hydrogenation of the carbonyl group in acetophenone. We discuss the chemical nature as well as the structure of all formed surface species observed on H-containing Pt.

Mechanisms Acting in Ligand-Directed Heterogeneous Catalysis: Electronic, Geometric, and Enol-Induced Effects

“…To further explore the possible effects of strong lateral interactions between the coadsorbed ligands and the reaction intermediates, the reactivity of acrolein hydrogenation was investigated on the surface precovered with acetophenone, which was shown in our previous studies to be capable of undergoing strong intermolecular interactions. ,− Figures and show the kinetic data of propenol formation and the related IR spectra obtained under operational conditions, correspondingly. In these experiments, the surface was covered with different amounts of acetophenone, which can be tentatively divided into low (exposure: 0.12–0.30 L) and high (exposure: 0.78–1.10 L) AP coverages.…”

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

“…Recently, we provided several examples of this kind of stabilization of the enol form of a carbonyl compound via formation of oligomer complexes, in which the enol species were stabilized by hydrogen bonding in oligomers. 9–16…”

“…Specifically, in our recent studies on acetophenone adsorption and hydrogenation over a Pt(111) model catalyst, we showed that a stable enol form of acetophenone can be formed. 9–11,13,15 However, it requires the formation of surface ketone–enol dimers or ketone–enol–enol trimers, in which the newly formed OH group of enol is stabilized by establishing a hydrogen bond with the O atom of a carbonyl group of neighboring ketone species. 9–11,13,15 The formation of dimer or trimer acetophenone species was found to be an activated process and the ketone–enol–enol trimers were shown to be stable on this surface even at relatively high (up to 350 K) temperatures.…”

Formation of 2D Self-Assembled Layers of Allyl Cyanide on Pd(111)

“…Such specific sites can be created by functionalization of the metal surface with a 2D layer of organic ligand adsorbates, exhibiting strong lateral interactions with the reactants. − By virtue of these reactant–ligand lateral interactions, the surface reaction can be directed toward the desired route, and the unwanted reactions become inhibited. The approach based on the lateral interactions between a reactant and a coadsorbed 2D ligand layer is currently being developed for numerous multipathway hydrogenation reactions, ,− such as selective hydrogenation of acetophenone, − trans-stilbene and phenylacetylene hydrogenation over Pd and Rh nanoparticles functionalized with N-heterocyclic carbens, , selective hydrogenation 1-epoxy-3-butene on Pd nanoparticles precovered with n -alkanethiol, hydrogenation of acetophenone to phenylethanol over Pt nanoparticles functionalized with proline, and chemoselective hydrogenation of the CO bond in α,β-unsaturated aldehydes over Pt 3 Co nanocrystals precovered with an oleyamine array …”

“…They comprise the geometric and electronic effects altering the formation of the desired reaction intermediate, the dynamic changes in the chemical structure of the ligands under the reaction conditions as well as the details of mutual intermolecular interactions between ligand and reactant species . The decisive effects of intermolecular interactions between different types of surface species on their reactivity were shown also for other reactive systems including hydrogenation of carbonyl and ester compounds. ,,− …”