Catalysis beyond frontier molecular orbitals: Selectivity in partial hydrogenation of multi-unsaturated hydrocarbons on metal catalysts

The mechanistic understanding and control over transformations of multi-unsaturated hydrocarbons on transition metal surfaces remains one of the major challenges of hydrogenation catalysis. To reveal the microscopic origins of hydrogenation chemoselectivity, we performed a comprehensive theoretical investigation on the reactivity of two a,b-unsaturated carbonyls—isophorone and acrolein—on seven (111) metal surfaces: Pd, Pt, Rh, Ir, Cu, Ag, and Au. In doing so, we uncover a general mechanism that goes beyond the celebrated frontier molecular orbital theory, rationalizing the C--C bond activation in isophorone and acrolein as a result of significant surface-induced broadening of high-energy inner molecular orbitals. By extending our calculations to hydrogen-precovered surface and higher adsorbate surface coverage, we further confirm the validity of the “inner orbital broadening mechanism” under realistic catalytic conditions. The proposed mechanism is fully supported by our experimental reaction studies for isophorone and acrolein over Pd nanoparticles terminated with (111) facets. Although the position of the frontier molecular orbitals in these molecules, which are commonly considered to be responsible for chemical interactions, suggests preferential hydrogenation of the C--O double bond, experiments show that hydrogenation occurs at the C--C bond on Pd catalysts. The extent of broadening of inner molecular orbitals might be used as a guiding principle to predict the chemoselectivity for a wide class of catalytic reactions at metal surfaces

Selectivity in hydrogenation of alpha, beta-unsaturated carbonyl compounds on model palladium catalysts

Die Umsetzung von alpha,beta-ungesättigten Aldehyden und Ketonen mit Wasserstoff über Pd-Modellkatalysatoren wurde anhand von Molekularstrahlmethoden, kombiniert mit Infrarot-Reflexions-Absorptions-Spektroskopie (IRAS), Quadrupol-Massenspektrometrie (QMS) und Röntgen-Nahkanten-Absorptions-Spektroskopie (NEXAFS), unter wohldefinierten Ultrahochvakuumbedingungen untersucht. Das Ziel dieser Arbeit war es, ein atomistisches Verständnis der strukturellen Faktoren zu gewinnen, die die Aktivität und Selektivität eines Pd(111)-Einkristalls und Eisenoxid-geträgerter Pd-Nanopartikel für die Hydrierung der C=C- und C=O-Bindungen bestimmen. Exemplarisch für diese Art von Kohlenwasserstoffen wurden das Aldehyd Acrolein und das Keton Isophoron gewählt. Die NEXAFS- und IRAS-Studien zeigten, dass Isophoron bei niedrigen Bedeckungen auf Pd(111) in einer flachliegenden Geometrie adsorbiert wird. Die Neigungswinkel der C=C- und C=O-Bindungen in Bezug auf die Pd(111)-Ebene nehmen mit zunehmender Oberflächenbedeckung zu. Auf reinem Pd(111) ist die Neigung der C=C-Bindung stärker ausgeprägt, was auf eine Verzerrung des konjugierten pi-Systems hindeutet. Bei Anwesenheit von Wasserstoff wird eine schwächere Bindung von Isophoron an Pd beobachtet. Die selektive partielle Hydrierung über einer Pd(111)-Oberfläche und geträgerten Pd-Nanopartikeln unterschiedlicher Größen wurde unter Verwendung von Acrolein untersucht. Molekularstrahlmethoden wurden mit IRAS- und QMS-Messungen kombiniert, um gleichzeitig die Bildung von Adsorbaten auf der Oberfläche und die der Produkte in der Gasphase verfolgen zu können. Über einem Pd(111)-Kristall wird Propenol mit nahezu 100% Selektivität durch Hydrierung der C=O-Gruppe gebildet, während über Pd-Partikeln Propanal durch selektive Hydrierung der C=C-Gruppe erzeugt wird. IRAS-Untersuchungen zeigten, dass die Propenol-Bildung eine Modifikation der Pd(111)-Oberfläche mit einer dichten Oxopropyl-Monolage voraussetzt.The conversion of alpha,beta-unsaturated aldehydes and ketones with hydrogen over model Pd catalysts was investigated using molecular beam techniques combined with infrared reflection-absorption spectroscopy (IRAS), quadrupole mass spectrometry (QMS), and near-edge X-ray absorption fine structure (NEXAFS) studies under well-defined ultra-high vacuum conditions. The aim of this work was to gain atomistic-level understanding of structural factors governing the selectivity and activity of a Pd(111) single crystal and iron-oxide-supported Pd nanoparticles for C=C and C=O bond hydrogenation. The ketone isophorone and the aldehyde acrolein were chosen as prototypical alpha,beta-unsaturated carbonyl compounds. NEXAFS and IRAS studies showed that isophorone is adsorbed on Pd(111) in a flat-lying geometry at low coverages. With increasing coverage, both C=C and C=O bonds tilt with respect to the surface plane. The tilting is more pronounced for the C=C bond on pristine Pd(111), indicating a strong distortion of the conjugated pi system upon interaction with Pd. Co-adsorbed hydrogen leads to a conservation of the in-plane geometry of the conjugated pi system, pointing to a much weaker interaction of isophorone with Pd in the presence of hydrogen. The selective partial hydrogenation over a Pd(111) surface and supported Pd nanoparticles with different particle sizes was investigated using acrolein. Molecular beam techniques were combined with IRAS and QMS measurements in order to simultaneously monitor the evolution of surface species and the formation of the final gas-phase products. Over a Pd(111) single crystal, acrolein is hydrogenated at the C=O bond to form propenol with nearly 100% selectivity, while over supported Pd particles, selective conversion of the C=C bond to propanal occurs. IRAS investigations showed that a distinct modification of the Pd(111) surface with a dense overlayer of an oxopropyl species is required for propenol formation

Trends In The Binding Strength Of Surface Species On Nanoparticles: How Does The Adsorption Energy Scale With The Particle Size?

The binding energy of oxygen on Pd nanoparticles was measured by a direct calorimetric method as a function of the particle size. The reduced dimensionality of the Pd substrate results in two counteracting trends: an increase of the Pd-O binding strength due to a change of the local configuration of the adsorption site and a strong decrease of the Pd-O bonding due to the reduction of the cluster size. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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

Toward Low-Temperature Dehydrogenation Catalysis: Isophorone Adsorbed on Pd(111)

Adsorbate geometry and reaction dynamics play essential roles in catalytic processes at surfaces. Here we present a theoretical and experimental study for a model functional organic/metal interface: isophorone (C9H14O) adsorbed on the Pd(111) surface. Density functional theory calculations with the Perdew-Burke-Ernzerhoff (PBE) functional including van der Waals (vdW) interactions, in combination with infrared spectroscopy and temperature-programmed desorption (TPD) experiments reveal the reaction pathway between the weakly chemisorbed reactant (C9H14O) and the strongly chemisorbed product (C9H10O), which occurs by the cleavage of four C-H bonds below 250 K. Analysis of the TPD spectrum is consistent with the relatively small magnitude of the activation barrier derived from PBE+vdW calculations, demonstrating the feasibility of low-temperature dehydrogenation

Near Field Guided Chemical Nanopatterning

This article demonstrates the possibility of creating well-defined and functional surface chemical nanopatterns using the optical near field of metal nanostructures and a photosensitive organic layer. The intensity distribution of the near field controlled the site and the extent of the photochemical reaction at the surface. The resulting pattern was used to guide the controlled assembly of colloids with a complementary surface functionality onto the substrate. Gold colloids of 20 nm diameter were covalently bound to the activated nanosites and proved the functionality of the suboptical wavelength structures and enabled direct visualization by means of electron microscopy. Our results prove, for the first time, the possibility of using optical near field to perform chemical reactions and assembly at the nanoscale

Toward Low-Temperature Dehydrogenation Catalysis: Isophorone Adsorbed on Pd(111)

Adsorbate geometry and reaction dynamics play essential roles in catalytic processes at surfaces. Here we present a theoretical and experimental study for a model functional organic/metal interface: isophorone (C₉H₁₄O) adsorbed on the Pd(111) surface. Density functional theory calculations with the Perdew–Burke–Ernzerhoff (PBE) functional including van der Waals (vdW) interactions, in combination with infrared spectroscopy and temperature-programmed desorption (TPD) experiments, reveal the reaction pathway between the weakly chemisorbed reactant (C₉H₁₄O) and the strongly chemisorbed product (C₉H₁₀O), which occurs by the cleavage of four C–H bonds below 250 K. Analysis of the TPD spectrum is consistent with the relatively small magnitude of the activation barrier derived from PBE+vdW calculations, demonstrating the feasibility of low-temperature dehydrogenation