Selective Carbon Deposition on γ-Alumina Acid Sites: toward the Design of Catalyst Supports with Improved Hydrothermal Stability in Aqueous Media.

γ-Alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides under hydrothermal (HT) conditions, resulting in strong modification of its intrinsic properties. Most of the strategies that have been proposed to prevent or at least minimize its transformation into oxy-hydroxides consist in covering the alumina surface with a hydrophobic carbon layer, making it less sensitive to modifications induced by water. However, such methods necessitate high carbon contents, which significantly modifies structural and chemical properties of alumina. Here, we propose a new method based on a series of adsorption/pyrolysis cycles using sorbitol molecules previously adsorbed on specific hydration sites of the (110) faces of γ-alumina crystals. These sites, which are responsible for the dissolution of γ-alumina crystals in water, are thus selectively protected by carbon clusters, with the rest of the surface being totally exposed and accessible to adsorbates. Under HT conditions (10 h in water at 200 °C), the formation of hydroxides is almost totally suppressed by covering less than 25% of the surface with only 7 wt % carbon, which is far below the amount necessary to achieve similar results with more conventional carbon deposition methods.

Thermal behavior of Pd@SiO2 nanostructures in various gas environments: a combined 3D and in situ TEM approach.

The thermal stability of core-shell Pd@SiO2 nanostructures was for the first time monitored by using in situ Environmental Transmission Electron Microscopy (E-TEM) at atmospheric pressure coupled with Electron Tomography (ET) on the same particles. The core Pd particles, with octahedral or icosahedral original shapes, were followed during thermal heating under gas at atmospheric pressure. In the first step, their morphology/faceting evolution was investigated in a reductive H2 environment up to 400 °C by electron tomography performed on the same particles before and after the in situ treatment. As a result, we observed the formation of small Pd particles inside the silica shell due to the thermally activated diffusion from the core particle. A strong dependence of the shape and faceting transformations on the initial structure of the particles was evidenced. The octahedral monocrystalline NPs were found to be less stable than the icosahedral ones; in the first case, the Pd diffusion from the core towards the silica external surface led to a progressive decrease of the particle size. The icosahedral polycrystalline NPs do not exhibit a morphology/faceting change, as in this case the atom diffusion within the particle is favored against diffusion towards the silica shell, due to a high amount of crystallographic defects in the particles. In the second part, the Pd@SiO2 NPs behavior at high temperatures (up to 1000 °C) was investigated under reductive or oxidative conditions; it was found to be strongly related to the thermal evolution of the silica shell: (1) under H2, the silica is densified and loses its porous structure leading to a final state with Pd core NPs encapsulated in the shell; (2) under air, the silica porosity is maintained and the increase of the temperature leads to an enhancement of the diffusion mechanism from the core towards the external surface of the silica; as a result, at 850 °C all the Pd atoms are expelled outside the silica shell.

Aerosol processing: a wind of innovation in the field of advanced heterogeneous catalysts.

Aerosol processing is long known and implemented industrially to obtain various types of divided materials and nanomaterials. The atomisation of a liquid solution or suspension produces a mist of aerosol droplets which can then be transformed via a diversity of processes including spray-drying, spray pyrolysis, flame spray pyrolysis, thermal decomposition, micronisation, gas atomisation, etc. The attractive technical features of these aerosol processes make them highly interesting for the continuous, large scale, and tailored production of heterogeneous catalysts. Indeed, during aerosol processing, each liquid droplet undergoes well-controlled physical and chemical transformations, allowing for example to dry and aggregate pre-existing solid particles or to synthesise new micro- or nanoparticles from mixtures of molecular or colloidal precursors. In the last two decades, more advanced reactive aerosol processes have emerged as innovative means to synthesise tailored-made nanomaterials with tunable surface properties, textures, compositions, etc. In particular, the "aerosol-assisted sol-gel" process (AASG) has demonstrated tremendous potential for the preparation of high-performance heterogeneous catalysts. The method is mainly based on the low-cost, scalable, and environmentally benign sol-gel chemistry process, often coupled with the evaporation-induced self-assembly (EISA) concept. It allows producing micronic or submicronic, inorganic or hybrid organic-inorganic particles bearing tuneable and calibrated porous structures at different scales. In addition, pre-formed nanoparticles can be easily incorporated or formed in a "one-pot" bottom-up approach within the porous inorganic or hybrid spheres produced by such spray drying method. Thus, multifunctional catalysts with tailored catalytic activities can be prepared in a relatively simple way. This account is an overview of aerosol processed heterogeneous catalysts which demonstrated interesting performance in various relevant chemical reactions like isomerisation, hydrogenation, olefin metathesis, pollutant total oxidation, selective oxidation, CO2 methanation, etc. A short survey of patents and industrial applications is also presented. Our objective is to demonstrate the tremendous possibilities offered by the coupling between bottom up synthesis routes and these aerosol processing technologies which will most probably represent a major route of innovation in the mushrooming field of catalyst preparation research.

Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations.

Despite the widespread use of amorphous aluminosilicates (ASA) in various industrial catalysts, the nature of the interface between silica and alumina and the atomic structure of the catalytically active sites are still subject to debate. Here, by the use of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) and density functional theory (DFT) calculations, we show that on silica and alumina surfaces, molecular aluminum and silicon precursors are, respectively, preferentially grafted on sites that enable the formation of Al(IV) and Si(IV) interfacial sites. We also link the genesis of Brønsted acidity to the surface coverage of aluminum and silicon on silica and alumina, respectively.

Aerosol route to functional nanostructured inorganic and hybrid porous materials.

The major advances in the field of the designed construction of hierarchically structured porous inorganic or hybrid materials wherein multiscale texturation is obtained via the combination of aerosol or spray processing with sol-gel chemistry, self-assembly and multiple templating are the topic of this review. The available materials span a very large set of structures and chemical compositions (silicates, aluminates, transition metal oxides, nanocomposites including metallic or chalcogenides nanoparticles, hybrid organic-inorganic, biohybrids). The resulting materials are manifested as powders or smart coatings via aerosol-directed writing combine the intrinsic physical and chemical properties of the inorganic or hybrid matrices with defined multiscale porous networks having a tunable pore size and connectivity, high surface area and accessibility. Indeed the combination of soft chemical routes and spray processing provides "a wind of change" in the field of "advanced materials". These strategies give birth to a promising family of innovative materials with many actual and future potential applications in various domains such as catalysis, sensing, photonic and microelectronic devices, nano-ionics and energy, functional coatings, biomaterials, multifunctional therapeutic carriers, and microfluidics, among others.

CO2 binding by dynamic combinatorial chemistry: an environmental selection.

We now report that a dynamic combinatorial selection approach can quantitatively provide, from trivial building blocks, an architecturally complex organic material, in which carbon dioxide is reversibly but covalently incorporated as a guest with a mass content of 20%. Solid-state analyses combined with covalent disconnection and quantization of the liberated components allowed identification of a three-component monomeric unit repeated within a range of assembled oligomeric adducts whose repartition and binding capacity can be finely tuned through the starting stoichiometries. The self-assembly of these architectures occurs through the simultaneous creation of more than 25 covalent bonds per molecular entity. It appears that the thermodynamic selection is directed by the packing efficiency of these adducts, explaining the spectacular building block discrimination between homologues differing by one carbon unit. This selectivity, combined with the reversible nature of the system, provided pure molecular building blocks after a simple chemical disconnection, promoting CO(2) as a green auxiliary to purify polyaldehyde or polyamine from mixtures of homologous structures. Moreover, the gas template could be expelled as a pure compound under thermodynamic control. This cooperative desorption process yielded back the initial libraries of high molecular diversity with a promising reduction of the energetic costs of capture and recycling.

Direct aerosol synthesis of large-pore amorphous mesostructured aluminosilicates with superior acid-catalytic properties.

An old dream comes true: A direct and environmentally benign synthetic strategy was developed for the aerosol-based mass production of large-pore mesostructured aluminosilicate powders (see TEM image). Although amorphous, some powders exhibit higher activity towards m-xylene isomerization and lower coke formation than a Y-zeolite based industrial reference catalyst.