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

Structural, thermodynamic, electronic and UV-visible optical properties of N-doped Bulk Anatase TiO2: a DFT study

International @ EAU+MHAInternational audienceNon

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

Size-Dependent Structural, Energetic, and Spectroscopic Properties of MoS 3 Polymorphs

International audienceIn spite of the interest of a-MoS3 based nanomaterials, the amorphous nature of the MoS3 phase makes it challenging to control and understand its chemical reactivity. In particular, the type of the structural building unit such as Mo3 triangular vs Mo chain is still debated, while the ambivalent interpretation of the nature of sulfur species (S2–, S22–) and Mo–Mo bonds leads to ambiguous interpretations of spectroscopic data and reactivity. By density functional theory (DFT), we simulate the energetic, structural, and spectroscopic features of relevant zero-dimensional 0D-, 1D-, and 2D-MoS3 triangular, chain-like polymorphs, including unprecedented ones (ring, wave, and helix) and revisit the interpretation of EXAFS, IR-RAMAN, and XPS experimental data. We analyze how MokS3k clusters of a few k atoms may grow up to infinite (periodic replica) MoS3 polymorphs. The evolution of the growth energy and the computed IR spectra within the density functional perturbation theory level suggest the coexistence of various polymorphs in the MoS3 phase as a function of sizes. Molecular dynamics simulations reveal how the small triangular MokS3k oligomers may transform into condensed MoS3 patches resembling embryos of the 2D 1T′-MoS2 phase. Finally, we discuss some plausible transformation pathways from one polymorph to another

Genesis of MoS2 from Model-Mo-Oxide Precursors Supported on γ-Alumina

International audienceThe molecular-scale understanding of the genesis of the MoS2 phase from Mo-oxide precursors supported on alumina is highly challenging with a strong impact on the activation process of heterogeneous industrial catalysts. By means of density functional theory (DFT), we quantify the activation free energies of the elementary steps involved in the sulfo-reduction mechanisms of Mo-trioxide oligomers and the stability of the corresponding Mo-oxysulfide intermediates supported on the γ-alumina (1 0 0) surface. The Gibbs free energy profiles highlight the characteristic chemical reactivity of various oxygen sites involved in the O/S exchange mechanism and reveal that interfacial oxygen atoms (Mo-O-Al) are the most challenging sites to be exchanged with S. We quantitatively compare the two main paths proposed experimentally: the one involving Mo-oxysulfide and Mo-trisulfide intermediates and the second one involving only Mo-oxysulfide. While O/S exchange requires moderate activation energies, the rate-determining steps correspond to S- and O-removal on small MonO3n-xSx or MonS3n (n ≤ 3) oligomeric intermediates. To overcome these high energy steps, the small Mo-trisulfide (MonS3n) oligomers are proposed to be fast diffusing surface species and to promote the growth process towards the targeted MoS2 phase. A reconstruction from a chain to a triangular Mo3S9 conformer also facilitates this phase transformation

Hydrogenolysis and β–elimination mechanisms for C S bond scission of dibenzothiophene on CoMoS edge sites

International audienceUnraveling the mechanisms of hydrodesulfurization (HDS) of dibenzothiophene (DBT) and the corresponding active sites represents a scientific challenge to improve the intrinsic performances of Co-promoted MoS2 (CoMoS) catalysts. By using density functional theory calculations, we compare two historical mechanisms for the Csingle bondS bond scission of DBT (direct desulfurization): direct hydrogenolysis of DBT and β–elimination of α,β–dihydro-diobenzothiophene (α,β–DHDBT) on four relevant sites of the two CoMoS M- and S-edges. On the Co promoted M-edge, the α,β–DHDBT is formed through dihydrogenation which is kinetically competing with hydrogenolysis (both exhibiting activation free energies, ΔG‡, smaller than +1.24 eV). On the S-edge, both dihydrogenation and hydrogenolysis exhibit higher ΔG‡ (>+1.78 eV). Interestingly, on the S-edge, the β–elimination (E2 type) on the α,β–DHDBT is found to be kinetically competing (ΔG‡ = +1.14 eV). The elimination of Hβ atom involves a S2 dimer close to the S-vacancy site where DHDBT is adsorbed. Since this leaving Hβ atom is distinct from the one added at dihydrogenation step, this may explain why direct desulfurization of 4,6-alkyl substituted DBT compounds is hampered according to the elimination mechanism. We finally discuss the possible synergy between the two edges of CoMoS for HDS of DBT

2D MoO3-xSx/MoS2 van der Waals Assembly: a Tunable Heterojunction with Attractive Properties for Photocatalysis

International audienceTwo-dimensional (2D) van der Waals (vdW) heterostructures currently have attracted much attention in widespread research fields where semiconductor materials are key. With the aim of gaining insights into photocatalytic materials, we use density functional theory (DFT) calculations within the HSE06 functional to analyze the evolution of optoelectronic properties and high-frequency dielectric constant profiles of various 2D MoO3–xSx/MoS2 heterostructures modified by chemical and physical approaches. Although the MoO3/MoS2 heterostructure is a type III heterojunction associated with a metallic character, we found that exchanging the terminal oxo atoms of the MoO3–xSx single layer (SL) with sulfur enables shifting its CB position above the VB position of the MoS2 SL. This trend gives rise to a type II heterojunction where the band gap and charge transfer within the two layers are driven continuously by the S concentration in the MoO3–xSx SL. This fine-tuning leads to a versatile type II heterostructure proposed to provide a direct Z-scheme system valuable for photocatalytic water splitting