Sylvain Cristol

Parallel between infrared characterisation and ab initio calculations of CO adsorption on sulphided Mo catalysts

Abstract Carbon monoxide adsorption on sulphided Mo catalysts has been investigated by means of IR spectroscopy and DFT ab initio calculations. IR experiments show that CO adsorption on the sulphide phase of Mo/Al2O3 catalysts gives rise to various ν(CO) bands, the intensities of which are strongly modified when post-treatment of the catalyst with H2 or H2S is performed before CO adsorption, therefore, revealing strong modifications in the nature and the number of sites present on the sulphide phase. Ab initio periodic DFT calculations allow to define two types of edges for MoS2, which sulphur coverage and structure depend on the H2/H2S ratio in the surrounding atmosphere. Adsorption energies and stretching wavenumber of CO adsorbed on the various sites of these surfaces were computed, providing the possibility to compare for the first time results from theoretical calculations and spectroscopic measurements on these systems. A novel attribution of the main IR features of CO adsorbed on MoS2 is proposed.

Indirect Detection via Spin-1/2 Nuclei in Solid State NMR Spectroscopy: Application to the Observation of Proximities between Protons and Quadrupolar Nuclei

We present a comprehensive comparison of through-space heteronuclear correlation techniques for solid state NMR, combining indirect detection and single-channel recoupling method. These techniques, named D-HMQC and D-HSQC, do not suffer from dipolar truncation and can be employed to correlate quadrupolar nuclei with spin-1/2 nuclei. The heteronuclear dipolar couplings are restored under magic-angle spinning by applying supercycled symmetry-based pulse sequences (SR412) or simultaneous frequency and amplitude modulation (SFAM). The average Hamiltonian theory (AHT) of these recoupling methods is developed. These results are applied to analyze the performances of D-HMQC and D-HSQC sequences. It is shown that, whatever the magnitude of spin interations, D-HMQC experiment offers larger efficiency and higher robustness than D-HSQC. Furthermore, the spectral resolution in both dimensions of proton detected two-dimensional D-HMQC and D-HSQC spectra can be enhanced by applying recently introduced symmetry-based homonuclear dipolar decoupling schemes that cause a z-rotation of the spins. This is demonstrated by 1H-13C and 1H-23Na correlation experiments on l-histidine and NaH2PO4, respectively. The two-dimensional heteronuclear 1H-23Na correlation spectrum yields the assignment of 23Na resonances of NaH2PO4. This assignment is corroborated by first-principles calculations.

Direct conversion of methanol into 1,1-dimethoxymethane: remarkably high productivity over an FeMo catalyst placed under unusual conditions

We report here the highest productivity ever observed in the direct conversion of methanol into 1,1-dimethoxymethane (ca. 4.6 kgDMM h−1 kgcat−1 at 553 K), this result being obtained over an FeMo catalyst. This catalyst is industrially used to selectively convert methanol into formaldehyde but has never before been applied to the present reaction. Placing this FeMo catalyst under unusual reaction conditions, i.e., using a feed rich in methanol, completely changed its behaviour in terms of selectivity: the massive production of 1,1-dimethoxymethane, instead of formaldehyde, was observed.

A new experimental cell for in situ and operando X‐ray absorption measurements in heterogeneous catalysis

A new X-ray absorption cell dedicated to in situ and operando experiments in heterogeneous catalysis has been built and tested. The cell consists of several boron nitride and stainless steel plates linked together using graphite seals. It allows the measurement of XANES and EXAFS spectra of heterogeneous catalysts within a wide range of photon energies in transmission mode under the flow of various oxidative and reductive gas mixtures at elevated temperatures. The cell is compact and easy to build. Catalysts are loaded into the cell as powders. The use of boron nitride and a small beam pathlength in the cell result in a low absorption of the X-ray beam at lower energies. The cell was tested by in situ characterizing cobalt species during oxidative and reductive pre-treatments of a silica-supported Fischer-Tropsch catalyst. An operando study of methanol conversion over alumina-supported molybdenum catalysts was also carried out.

17O solid-state NMR and first-principles calculations of sodium trimetaphosphate (Na3P3O9), tripolyphosphate (Na5P3O10), and pyrophosphate (Na4P2O7).

The assignment of high-field (18.8 T) (17)O MAS and 3QMAS spectra has been completed by use of first-principles calculations for three crystalline sodium phosphates, Na 3P 3O 9, Na 5P 3O 10, and Na 4P 2O 7. In Na 3P 3O 9, the calculated parameters, quadrupolar constant ( C Q), quadrupolar asymmetry (eta Q), and the isotropic chemical shift (delta cs) correspond to those deduced experimentally, and the calculation is mandatory to achieve a complete assignment. For the sodium tripolyphosphate Na 5P 3O 10, the situation is more complex because of the free rotation of the end-chain phosphate groups. The assignment obtained with ab initio calculations can however be confirmed by the (17)O{ (31)P} MAS-J-HMQC spectrum. Na 4P 2O 7 (17)O MAS and 3QMAS spectra show a complex pattern in agreement with the computed NMR parameters, which indicate that all of the oxygens exhibit very similar values. These results are related to structural data to better understand the influence of the oxygen environment on the NMR parameters. The findings are used to interpret those results observed on a binary sodium phosphate glass.

First-principles calculations of NMR parameters for phosphate materials.

In this short review, we discuss the ability to reproduce NMR parameters in the case of phosphates materials through electronic structure calculation within density functional theory linear response. Indeed, the gauge‐including projector‐augmented wave is today largely used by the solid‐state NMR community as a tool for structural determination and it has been applied to a large variety of materials. We emphasise on the crucial points that should be taken into account to perform such calculations. In particular, we discuss the influence of the electronic structure and of the geometry on the calculation of NMR parameters. To illustrate the review, we present experimental and theoretical comparison of 31P, 1H and 23Na NMR data on a series of sodium phosphate systems. Copyright

New insights into oxygen environments generated during phosphate glass alteration: a combined 17O MAS and MQMAS NMR and first principles calculations study

In the present study, we used a combination of (17)O NMR methods at a high magnetic field with first-principles calculations in order to characterize the oxygen sites in a series of hydroxylated sodium phosphate compounds, namely the hydrogen pyrophosphate Na(2)H(2)P(2)O(7) and the hydrogen orthophosphates NaH(2)PO(4), NaH(2)PO(4) x H(2)O and NaH(2)PO(4) x 2 H(2)O. The chemical shifts and quadrupolar parameters of these compounds were interpreted in terms of local and semi-local environment, i.e., the chemical composition of the immediate surroundings and the nature of the bonds, e.g. hydrogen bonding. The magnitude of the quadrupolar interaction and its asymmetry were revealed to be a precise indicator of the local structure in sodium hydrogen phosphates. Our (17)O NMR experimental and computing approach allowed for identification and quantification of the different crystalline phases involved in the weathering mechanism of a sodium phosphate glass, even in small amount.

Theoretical study of benzothiophene hydrodesulfurization on MoS2

Abstract Benzothiophene (BT) and methylbenzothiophene (MBT) desulfurization on the catalytically active MoS 2 edge has been studied using density functional theory. The calculation of the stability of the (100) surface as a function of sulfur coverage indicates two potential active sites. The adsorption energies of BT, MBT and hydrogen molecules on these sites were calculated. The calculation of the binding energies of the various hydrogenated intermediates allow us to the build of energy profiles of BT and MBT desulfurization. It appears that the most endothermic step is the site regeneration (creation of the vacancy).

Structure of clean and hydrated alpha-Al2O3 (1(1)over-bar02) surfaces: implication on surface charge

Periodic DFT calculations coupled to a first-principle thermodynamic approach have allowed us to establish a surface phase diagram for the different terminations of the α-Al(2)O(3) (1102) surface in various temperature and water pressure conditions. Theoretical results are compared with previous experimental data from the literature. Under a wide range of temperature and water pressure (including ambient conditions) the most stable surface (denoted C2_1H(2)O in this work) is terminated with singly coordinated hydroxyls on four-fold coordinated aluminium (Al(4C)-μ(1)-OH) while most existing surface models are only considering six-fold coordinated surface Al atoms as in the bulk structure of alumina. The presence of more acidic Al(4C)-μ(1)-OH sites helps explain the low Point of Zero Charge (PZC) (between 5 and 6) determined from the onset of Mo oxoanions adsorption on (1102) single crystal wafers. It is also postulated that another termination (corresponding to the hydration of the non-polar, stoichiometric surface, stable in dehydrated conditions) may be observed in aqueous solution depending on the surface preparation conditions.

Synergy between XANES Spectroscopy and DFT to Elucidate the Amorphous Structure of Heterogeneous Catalysts: TiO2‐Supported Molybdenum Oxide Catalysts

The properties of heterogeneous catalysts are directly correlated to the molecular structure of the active sites which often consists of nanometer-scale particles of transition metals (in a metallic, oxide or sulfide form) dispersed on an oxide support. The complexity of physico-chemical phenomena occurring during the catalyst synthesis/activation stages often leads to the formation of unknown supported phases featuring new ill-defined sites. Their identification requires an indepth characterization at the molecular scale of the catalyst with the use of various spectroscopic tools. Despite the valuable information so-obtained, these techniques sometimes are not able to provide the overall structure of the active species responsible for the catalytic activity. The use of theoretical tools to establish direct correlation between spectroscopic fingerprints and structural/electronic properties of catalysts is a powerful and quite new approach for unraveling the structure of catalysts. Thanks to its chemical and electronic (orbital) selectivity, X-ray absorption near-edge structure (XANES) spectroscopy is the technique of choice for molecular-scale characterization of catalysts. Furthermore, in the XANES spectra, the long mean free path of the photoelectron, induced by its small kinetic energy (Ec< 50–100 eV), provides a high contribution of multiple scattering events of the photoelectron allowing to probe the three-dimensional structure (3D) around the absorbing atom. XANES spectroscopy is however highly dependent on electronic parameters that make its fine interpretation difficult. This often leads to a restricted use of XANES spectra for the determination of the oxidation state of an absorber atom or/and basic consideration on its local symmetry. It is however possible to obtain a finer interpretation of spectra by calculation of XANES transitions. Indeed, the multiple scattering (MS) theory is well adapted to reproduce the XANES transitions observed at K edges of elements heavier than Li and at L2,3 edges of elements heavier than Cd. We propose herein to use MS simulations based on structural models predicted by DFT calculations for interpreting the MoK edge fingerprints observed for TiO2-supported molybdenum oxide catalysts which are widely used for olefin metathesis, selective oxidation reactions, and hydrotreatment. The structure of the oxomolybdate species formed during the catalyst activation is still unknown even though it has been extensively characterized by various spectroscopies. A three-step methodology was followed to unravel the catalyst structure: first simulations of XANES spectra of Mo reference compounds (ammonium heptamolybdate (hereafter noted AHM), (NH4)3[Al(OH)6Mo6O18] (noted AlMo6), a-(NH4)4[Mo8O26] (noted Mo8O26)) were performed to obtain direct correlation between spectroscopic fingerprints and structural properties of known structures. Then, these fingerprints are identified in the spectrum of the supported catalyst to generate catalyst structures for DFT optimization. Finally, the optimized geometry is used for modeling XANES spectra using MS theory. Figure 1A presents MoK edge experimental spectra of the activated catalyst (350 8C in oxygen), labeled hereafter 7.5 MoTi, together with four reference compounds in which