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

International audienceThe 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

Innovative way to stabilize catalysts for biomass transformation methods

SSCI-VIDE+CDFA:ING+EGR:ATU:MBEInternational audienceNon

Innovative way to stabilize alumina for biomass transformation reactions

SSCI-VIDE+CDFA:ING+EGR:ATU:MBENational audienceNon

Innovative way to stabilize catalysts for biomass transformation methods

SSCI-VIDE+CDFA:ING+EGR:ATU:MBEInternational audienceNon

Locating the weak spots of alumina in water combiningmetadynamics simulations and shaped-controlled synthesis

International audienceγ-Al2O3 has remarkable properties as a support, but it is very sensitive to water, either as a liquid or even as steam. These structural changes can be retarded by the presence of polyols. The mechanism of action of these additives is believed to lie in their chemisorption on the surface that would make γ-Al2O3 water-resistant. However little is known on which exposed facet(s) and what site(s) need to be particularly targeted for protection. The optimization of the structure of inhibitors is therefore a challenging task since the exact mechanism of the decomposition of γ-Al2O3 in liquid water remains unknown. We combine here experiments and theory to provide the atomistic mechanism for the early-stage decomposition of γ-Al2O3 in liquid water. Beyond its impact on the stability of this important support, the present achievement constitutes an unprecedented milestone in the understanding of solid/liquid interface transformation in term of methodology.Alumina samples containing particles with different shapes were exposed to a hydrothermal treatment in presence of various concentrations of polyol (sorbitol and xylitol). We determined the minimum surface coverage of polyol at which the decomposition of alumina gets inhibited, a quantity referred to as inhibiting coverage. Since this inhibiting coverage strongly correlates with the fractional area of the (110) facet of the alumina sample and only with this area, we concluded that the decomposition is initiated at the (110) facet .Then, performing ab initio metadynamics simulations, we probed the reactivity of this Al2O3(110)/water interface and identified specific aluminum tetrahedral centers that are particularly reactive with water. We showed that interfacial water molecules are involved in the mechanism, both as reactants for the hydration of aluminum and as intermediates for the proton reshuffling required by the decomposition mechanism. We managed to determine the barriers (all lower than 80kJ.mol-1) for the 6 successive addition/elimination steps in a single simulation. Furthermore, the effect of xylitol was rationalized: its adsorption locally rendered the surface more hydrophobic and pushes water molecules away from the Al sites identified as water-sensitive

New Aluminosilicate Materials with Hierarchical Porosity Generated by Aerosol Process

This paper underlines the great advantage of using a spray-drying process (aerosol process) to synthesise new families of aluminosilicate materials with a hierarchical porosity from the microporous to the macroporous scales. In concrete terms, high aluminum content mesostructured solids, zeolite - mesostructured aluminosilicate composites and amorphous hierarchical aluminosilicate solids with micro, meso and macroporosity have been successfully synthesised. Notice that the latter family is characterised by non usual acidic properties in cumene cracking catalytic reaction. Such results open then new perspectives in the development of innovating acidic catalysts for hydrocracking reactions

Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions

Despite their wide use in the oil refining process, little is known about the distribution of aluminium and silicon atoms in amorphous aluminosilicates (ASAs). In this paper, we report the synthesis of both Al/SiO2 and Si/Al2O3 ASAs by grafting aluminium and silicon alkoxides on silica respectively alumina under various conditions. For both supports, we evidence the central roles of the precursor molecule size and reactivity in the grafting yield and the deposit structure. Unless hydrolysis of the alkoxy groups by water and/or thermal decomposition occurs, deposition is saturated at a monolayer of precursor molecules on the support oxide surface. Additional species can be deposited by repeating the grafting process provided that hydroxyl groups of the top layer are recovered after calcination. 27Al NMR indicates the presence of five-coordinated aluminium species on Al/SiO2 materials prepared by two successive grafting steps. Transmission electron microscopy (TEM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirm the controlled deposition of species by selecting the appropriate synthesis conditions. Homogeneous and regular deposition is favoured in anhydrous condition and at low temperature, while water addition during synthesis leads to inhomogeneous deposits. In case of Al/SiO2 grafting in aqueous conditions, alumina nanoparticles form. The accurate knowledge of the surface structure of these ASAs opens the way to a better understanding of the origin of their Brönsted acidity

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

Improving the catalytic performances of metal nanoparticles by combining shape control and encapsulation

SSCI-VIDE+ECI2D+NBT:LPIInternational audienc