Reactivity of shape-controlled crystals and metadynamics simulations locate the weak spots of alumina in water.

The kinetic stability of any material in water relies on the presence of surface weak spots responsible for chemical weathering by hydrolysis. Being able to identify the atomistic nature of these sites and the first steps of transformation is therefore critical to master the decomposition processes. This is the challenge that we tackle here: combining experimental and modeling studies we investigate the stability of alumina in water. Exploring the reactivity of shape-controlled crystals, we identify experimentally a specific facet as the location of the weak spots. Using biased ab initio molecular dynamics, we recognize this weak spot as a surface exposed tetra-coordinated Al atom and further provide a detailed mechanism of the first steps of hydrolysis. This understanding is of great importance to heterogeneous catalysis where alumina is a major support. Furthermore, it paves the way to atomistic understanding of interfacial reactions, at the crossroad of a variety of fields of research.

Fast 'Operando' electron nanotomography.

Electron tomography in transmission electron microscopy provides valuable three-dimensional structural, morphological and chemical information of condensed matter at nanoscale. Current image acquisitions require at least tens of minutes, which prohibits the analysis of nano-objects evolving rapidly such as under dynamic environmental conditions. Reducing the acquisition duration to tens of seconds or less permits to follow in 3D the same object during its evolution under varying temperatures and pressures. We report Operando Electron nanotomography using image series acquired in less than 230 seconds instead of typically 15 min in the best cases so far. The in situ calcination of silica zeolites encaging silver nanoparticles, a catalytic nanosystem of potential interest for, e.g., nuclear waste treatments or selective heterogeneous catalysis, was successfully studied. Kinetic environmental Operando 3D electron microscopy becomes possible, as well as real time observation of beam sensitive samples (polymers, biological objects) without prior preparation, which reduces their contrast and reactivity.

A combined experimental and theoretical evaluation of the structure of hydrated microporous aluminophosphate AlPO4-18.

Water adsorption in the microporous aluminophosphate AlPO4-18 is studied by a combination of solid-state NMR, X-ray diffraction, and density functional theory calculations. The change of the framework structure upon hydration is moderate, and NMR gives local information on the environment of Al and P atoms. The structural distribution of water molecules in the channels has been explored by a combination of first-principle molecular dynamics simulations and of static geometry optimizations. Two starting points have been considered for the calculations. If the structure of the dehydrated aluminophosphate is used, the simulation result is not satisfactory with an incomplete hydration and no agreement with NMR results. Starting from a partial refinement of the aluminophosphate framework for the hydrated system, a structure with six tetrahedral and six octahedral Al atoms in the unit cell is obtained, involving twelve water molecules coordinated to Al atoms and twelve others in the channels, and in good agreement with experimental data.

EXAFS determination of the size of Co clusters on silica.

The metallic Co catalyst for the Fischer-Tropsch reaction is prepared by reduction of Co salts impregnating microporous silica. The average size of the metallic Co clusters is determined from the average number of neighbours deduced from Co K-edge EXAFS of catalyst samples. A model EXAFS signal constructed from the scattering paths of Co metal fcc lattice with lengths up to 5 A is calibrated on a reference spectrum of Co metal foil. Catalyst spectra are interpreted with the same model expanded with variable neighbour fractions of the four nearest shells. Cluster size is obtained from comparison with the neighbour-fractions of consecutive fcc magic-number clusters.

Study by 31P NMR spin echo mapping of vanadium phosphorus oxide catalysts.

Vanadium phosphorus oxides (VPO) containing vanadium ions in the +3 and +4 oxidation states, namely VPO4, (VO)2P2O7, VOHPO4.0.5H2O and VO(H2PO4)2 have been characterized using 31P solid state NMR spectroscopy. Because of couplings between the unpaired electrons of V4+ or V3+ ions and 31P nuclei, 31P NMR lines are drastically broadened and shifted by more than 4500 ppm in the case of the V3+ phases. It is therefore necessary to use the Spin Echo Mapping technique to obtain the complete NMR information. The technique has proved to be particularly interesting as it permits us to distinguish between various V4+ phases. The observed frequency shift is proportional to the atomic magnetic susceptibility of the material and, therefore, changes with temperature. For V4+ phases, we have observed that the inverse shift was proportional to temperature in the working temperature range and that good estimations of the Weiss temperatures could be obtained. Moreover, at low temperature, the 31P NMR spin echo mapping of the pyrophosphate (VO)2P2O7 showed four distinct peaks that could be assigned to various unpaired electron spin densities on the phosphorus atoms. This has been correlated with previous structural determinations, particularly the oxidation state of vanadium ions in these solids. We have also observed that the inverse of the line width increased linearly with temperature in the working range. Special attention was given to VPO4 where both the evolution with temperature of the observed shift and the line width were very different from those observed on V4+ phases.