Camille La Fontaine

A Time-Resolved In Situ Quick-XAS Investigation of Thermal Activation of Fischer–Tropsch Silica-Supported Cobalt Catalysts†

The Fischer–Tropsch (FT) process transforms coal-, natural gasor biomass-derived syngas (a CO/H2 mixture) into liquid hydrocarbons, which can be used as valuable petroleum substitutes because of their high cetane number and low content of sulfur and aromatics. Supported cobalt catalysts are suitable for low temperature FT process. Their catalytic performance is strongly affected by cobalt dispersion: higher metal dispersion supposes higher proportion of reduced metal in the catalyst and lower average nanoparticle size. This can be achieved by optimizing catalyst texture, adding organic compounds, or promoters during catalyst preparation, decomposing cobalt nitrate in a glow discharge or by controlling the catalyst thermal activation. 11] In particular, de Jong et al. prepared smaller Co3O4 and Co 0 particles on SBA-15 silicas by activating the catalyst in a NO-containing atmosphere instead of air prior to the reduction. In the present paper, in situ quick X-ray absorption spectroscopy (QXAS) has been used as a unique tool to accurately monitor the transformations of dispersed phases in supported catalysts under different atmospheres (air, helium and 5 % NO/He), both from the structural, quantitative, and kinetic standpoints. The time-resolved QXAS spectra were continuously collected at the Co K edge in the transmission mode during the catalyst activation. The experimental setup at SAMBA beamline (SOLEIL synchrotron) also allowed simultaneous in situ recording Raman spectra. The details of preparation, activation and characterisation of CoACHTUNGTRENNUNG(10 wt %)/SiO2 catalysts are given in Experimental Section. Figure S1 (see the Supporting Information) displays timeresolved in situ X-ray absorption near-edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) Fourier transform moduli obtained during the activation of SiO2-supported hexahydrated cobalt(II) nitrate [Co ACHTUNGTRENNUNG(H2O)6] ACHTUNGTRENNUNG(NO3)2 in air or helium. The shift of the edge position towards higher energies and changes in the white line intensity and shape above 160 8C are consistent with the transformation of the cobalt salt into Co3O4, characterised by a typical triangular XANES white line. 13] Actually, the presence of two successive series of isobestic points in the XANES spectra, below and above 140 8C, respectively, shows that decomposition of hydrated cobalt(II) nitrate takes place in two distinct steps, each involving two phases: first, dehydration to anhydrous Co ACHTUNGTRENNUNG(NO3)2 (with progressive replacement of aqua ligands by NO3 ions in Co ions coordination sphere), followed by decomposition of CoACHTUNGTRENNUNG(NO3)2 into cobalt oxide ([Eq. (1) and (2)]).

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

Applications in materials science of combining Raman and X-rays at the macro- and micrometric scale

Simultaneous combination of Raman with X-ray Absorption Spectroscopy (XAS) has become of great interest in Materials Science. Four applications are reviewed in this article that reflect the large range of possibilities offered by such coupling at the macro and micrometer scales. Special emphasis was laid on the macrometer scale on time-resolved studies of physico-chemical transformations, either for providing more complete analysis of structural changes or for looking at the sample integrity. The capability of Raman and XAS to provide complementary chemical and structural information was also pointed out for discriminating and characterizing chemical phases in heterogeneous materials at the micrometer scale.