Nature et réactivité des sites basiques de l'oxyde de magnésium (rôle des hydroxyles)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Revisiting γ-Alumina Surface Models through the Topotactic Transformation of Boehmite Surfaces

International audienceThe rational understanding of γ-alumina (γ-Al2O3) supported catalysts requires an ever more improved atomic scale determination of the support’s surface properties. By using density functional theory (DFT) calculations, we show how the structural and energetic surface properties of alumina crystallites intrinsically depend on its synthesis pathway. Considering the case study of the topotactic transformation of boehmite (γ-AlOOH) into γ-Al2O3 taking place during calcination, we propose a methodology to mimic this pathway by reconstructing relevant slabs of boehmite into γ-alumina slabs following 3 steps: dehydration, contraction/translation and Al migration into spinel or non-spinel sites. On the one hand, we confirm the reliability of some earlier , and surface structures determined by standard bulk cleavage approach. Moreover, we find new γ-alumina surfaces harboring Brønsted acid sites (BAS) and Lewis acid sites (LAS) with specific local structures. More strikingly, we find that the basal surface of alumina inherited from the (0 1 0) basal surface of boehmite, exhibits a larger number of isolated µ2-OH groups than the lateral surface. For the lateral (respectively ) orientation, four (respectively three) thermodynamically competing surfaces are identified, including models earlier proposed. These results are induced by finite size and morphology effects during the topotactic transformation of boehmite crystallites. Thanks to a thorough comparative analysis of morphology and nature of BAS and LAS as a function of thermal treatment and water pressure for each surface, we identify coherent chemical families of surfaces across the main crystallographic orientations. These features open the door to a better differentiation of the reactivity of the basal alumina surfaces from the lateral ones

The Origin of the Activity of Amine-Functionalized Metal-Organic Frameworks in the Catalytic Synthesis of Cyclic Carbonates from Epoxide and CO2

INGENIERIE+TLE:DF

Modeling Ammonia and Water Co-Adsorption in CuI-SSZ-13 Zeolite Using DFT Calculations

International audienceCu-SSZ-13 efficiently catalyzes the selective catalytic reduction (SCR) of NO by NH3 but the structure of the active site and, particularly, the redox state of the copper (+I or +II) is still debated. This paper focuses on the possible contribution of CuI species using quantum chemistry of adsorption and co-adsorption of NH3 and H2O on CuI species. The calculations show that CuI clearly migrates upon adsorption of NH3 and H2O. All the CuI complexes sit in the cage containing the 8 MR and interact with the zeolite framework through several H-bonds. In the experimental temperature and pressure domain of SCR conditions, calculated phase diagrams show that a coordination number of two is predicted for the co-adsorption of NH3 and H2O on CuI. Finally, the calculated phase diagrams of CuI-SSZ-13 are discussed together with those of CuII-SSZ-13 and recent experimental characterizations, providing a wider picture of the real catalyst in SCR conditions

PtOx Cly (OH)z (H2O)n Complexes under Oxidative and Reductive Conditions: Impact of the Level of Theory on Thermodynamic Stabilities

International audiencePlatinum-based catalysts with Cl- , OH- , O2- and H2O ligands, are involved in many industrial processes. Their final chemical properties are impacted by calcination and reduction applied during the preparation and activation steps. We investigate their stability under these reactive conditions with density functional theory (DFT). We benchmark various functionals (PBE-dDsC, optPBE, B3LYP, HSE06, PBE0, TPSS, RTPSS and SCAN) against ACFDT-RPA. PBE-dDsC is well adapted, although hybrid functionals are more accurate for redox reactions. Thermodynamic phase diagrams are determined by computing the chemical potential of the species as a function of temperature and partial pressures of H2O, HCl, O2 and H2. The stability and nature of the Pt species are highly sensitive to the activation conditions. Under O2, high temperatures favour PtO2 while under H2, platinum is easily reduced to Pt(0). Chlorine modifies the coordination sphere of platinum during calcination by stabilizing PtCl4 and shifts the reduction of platinum to higher temperatures under H2

Stability of Pt10Sn3 Clusters Supported on γ−Al2O3 in Oxidizing Environment: a DFT Comparison of Alloying and Size Effects

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Competition of Secondary versus Tertiary Carbenium Routes for the Type B Isomerization of Alkenes over Acid Zeolites Quantified by Ab Initio Molecular Dynamics Simulations

International audienceThe skeletal isomerization of alkenes catalyzed by zeolites involves secondary and tertiary carbenium ions for which respective reactivity cannot be easily assessed by standard theoretical approaches. Thanks to ab initio molecular dynamics, starting from 4-methyl-hex-1-ene (a monobranched C7 alkene), we identify and compare two mechanistic routes for skeletal isomerization: (i) a type B isomerization transforming a secondary carbenium into a tertiary carbenium (conventional route), and (ii) a two-step route involving an intramolecular 1,3 hydride-shift producing a tertiary carbenium, followed by a type B isomerization between two tertiary carbenium ions. We find that, in the case of the secondary cation, the relevant species from a kinetic point of view is the corresponding π-complex. The transition states found for type B isomerization reactions are edge-protonated cyclopropanes (edge-PCP) that exhibit similar stabilities and structures. The transition state for the 1,3-hydride shift is an edge-type PCP with one elongated C–C bond that is more stable than the one found for type B isomerization. From this analysis, we deduce relevant kinetic constants and quantify the respective contribution of both pathways to the global reaction rate. Although the secondary carbenium ions are poorly stable species, we show that they can hold a significant part of the reaction flux. Finally, we discuss, in detail, our kinetic and mechanistic insights with previous kinetic modeling data reported in the literature

Atomistic Models for Highly‐Dispersed PtSn/γ‐Al2O3 Catalysts: Ductility and Dilution Affect the Affinity for Hydrogen

International audienceSupported platinum-based sub-nanometric particles play a central role in many catalytic applications. In particular, platinum-tin active phases supported on γ-Al2O3 are largely employed for dehydrogenation of alkanes and catalytic reforming of naphta cuts, although geometric and electronic effects of the active phase in the presence of hydrogen still remains highly debated. Thanks to periodic density functional theory (DFT), we propose structural models of such systems containing thirteen metal atoms (PtxSn13-x with 0 ≤ x ≤ 13) deposited on the (100) γ-Al2O3 surface. We thus unravel the intricate effects of the composition (Pt/Sn ratio), of the support (γ-Al2O3) and the hydrogen coverage on the stability of platinum-tin sub-nanometric clusters, in the case where tin is reduced (Sn 0). A detailed investigation of the interaction of the supported Pt10Sn3 cluster with hydrogen by velocity-scaled molecular dynamics provides a mapping of the hydrogen coverage as a function of the operating conditions (T, P(H2)). Our study highlights significant differences between Pt13 and PtxSn13-x clusters in terms of ductility and dilution (also called ensemble) effects which may be at the origin of the different reactivities usually reported for Pt and PtSn supported catalysts

The French Conference on Catalysis-FCCat 1

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