Tuning catalytic reactivity on metal surfaces: Insights from DFT

International audienceDensity Functional Theory (DFT) calculations show on two examples how catalytic reactivity qualitatively depends on the environment of the active sites. The example of butadiene selective hydrogenation to butenes on Pt or Pd first illustrates how alloying this catalytically active metal with a non- or less reactive elements (Sn or Au, respectively) allows weaker catalyst–molecule interactions and favors the desorption of butenes. It also shows that, from this weakening of the interactions, new reaction pathways are opened and non-selective pathways through a metallacycle intermediate are disadvantaged. Both effects concur on PtSn for an improved selectivity in butenes, while on the PdAu alloy, the gain in selectivity is explained mainly by the easier butene desorption. The dehydrogenation of ethanol on Rh and Pt surfaces shows how co-adsorbates, as water solvent molecules, can interfere with the catalytic elementary steps, assisting OH bond breaking and disadvantaging Csingle bondH dissociation. The balance between these two effects is metal dependent. Ethanol interacts by a strong H-bond with the H2O pre-covered surface, in a more stabilizing way than for the simple adsorption on the bare metal. The surface H2O (or surface-OH) complex can hence be seen as the true active site for the reaction. It is hence important to include the influence of the environment, from the surface part or from the reactant part, for an accurate modeling of heterogeneous catalytic reactions

Mechanistic investigation of the cis/trans isomerization of 2-butene on Pt(1 1 1): DFT study of the influence of the hydrogen coverage ,,

International audienceThe cis/trans isomerization of 2-butene on Pt(1 1 1) in the presence of hydrogen has been studied by means of calculations based on density functional theory (DFT). Two hydrogen precoverages have been considered, 0.11 and 1.00 ML. The di-σ bonding geometries are the most stable and the trans isomer is preferred at the low hydrogen precoverages, whereas the π bonding geometry of the cis isomer is the preferred structure at H saturation coverage instead. The set of hydrogenation/dehydrogenation reactions that leads to the conversion from one isomer to the other, which involves an adsorbed alkyl intermediate, has been studied for both coverages, and the transition states for those steps have been identified. The differences between the activation energies of the reactions involving the two isomers are small, but a different behavior can nevertheless be observed depending on the hydrogen coverage considered. At low hydrogen precoverages, conversion from the cis to the trans isomer was estimated to be preferred, whereas on the hydrogen saturated surface the reverse appears to be true. Surprisingly, an Eley–Rideal mechanism was identified as the lowest energy pathway in the latter case. The results from this work are consistent with previously published experimental data from temperature programmed desorption and infrared absorption spectroscopy studies

CH3-ReO3 on gamma-Al2O3: understanding its structure, initiation, and reactivity in olefin metathesis

Me-ReO3 on gamma-alumina: understanding the structure, the initiation and thereactivity of a highly active olefin metathesis catalyst Heterolytic splitting of the C-H bond of the methyl group of CH3ReO3 on AlsO reactive sites of alumina as a way to generate the active site of CH3ReO3 supported on gamma-Al203

Nature and Structure of Aluminum Surface Sites Grafted on Silica from a Combination of High-Field Aluminum-27 Solid-State NMR Spectroscopy and First-Principles Calculations

International audienceThe determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While Al-27 solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadnipolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found

Quantitative Investigation of MgO Bronsted Basicity: DFT, IR, and Calorimetry Study of Methanol Adsorption

66-3 FIELD Section Title:Surface Chemistry and Colloids Laboratoire Reactivite de Surface, UPMC, University Paris 06, Paris, Fr. FIELD URL: written in EnglishThe adsorption geometries, energies, and vibrational frequencies of methanol on MgO defective surfaces have been calcd. by periodic DFT simulations. The results are very comparable with those obtained with water and are also in very good accordance with microcalorimetry and IR expts. At low coverage, the dissocn. is obsd. on all defects involving ions in low coordinations. Over and above the coordination no. of surface ions, the adsorption energy is strongly governed by the surface topol.: dissocn. on confined sites gives rise to methoxy groups highly stabilized by bridging two or even three cations. The occurrence of such very strong sites on MgO powder is confirmed by microcalorimetry. The dissocn. ability depends on the methanol coverage because it modifies the surface relaxation and the network of H bonds, resulting, for a given defect, in similar adsorption energies for mol. and dissocd. species at high coverage. This explains why there are more strong sites (quantified by microcalorimetry) than dissocg. sites (quantified by IR). [on SciFinder(R)