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International audience; A γ-alumina digital twin is created by means of a hierarchical pore network model. For this digital twin, nitrogen porosimetry is simulated, and the parameters of the pore network model are modified by means of a particle swarm optimization (PSO) algorithm until both branches of the digital isotherm match those of the experimental isotherm. After parameter identification, the resulting pore network model provides a digital isotherm that is in good agreement with the experimental results. The same set of parameters also allows to predict the experimental nitrogen scanning curves. The BJH pore size distribution obtained from the digital isotherm corresponds well to the experimental one. Therefore, the generated digital twin can statistically represent the topology of the industrial alumina sample. Diffusion simulations on the final structure allowed one to estimate the tortuosity factor of the structure. The predicted value differs by less than 20% from the tortuosity factor measured by PFG-NMR.

International audience; Obtaining an atomic-scale description of the chemical interactions of phosphates with an oxide support, such as γ-Al2O3, is essential to get a rational understanding of the role of phosphate additives for a great number of heterogeneous catalysts, as well as to improve the use of this element. Combining cutting-edge Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) techniques with Density Functional Theory (DFT) calculations, we provide an accurate molecular description of phosphate speciation on γ-Al2O3 surfaces for various P surface coverages after drying at 120 °C. Thanks to 31P double- and triple-quantum filtered NMR experiments as well as to 27Al–31P dipolar- and scalar-based correlation spectra, we demonstrate the presence of polyphosphates and of Al–O–P connectivities at the exposed facets of γ-Al2O3. DFT-based thermodynamics shows that phosphates (mono- or di-) are preferentially covalently bonded on the (1 1 0) γ-Al2O3 facet with high-dentation modes. These high-dentation modes are favored by entropy gain due to water desorption. We used the gauge-including projector-augmented wave (GIPAW) DFT method for 31P NMR chemical shifts calculations and propose a systematic identification of the various types of phosphates covalently or noncovalently bonded to the alumina surface. The calculations confirm the existence of polyphosphates as observed experimentally. Since the surface condensation into polyphosphates is endergonic, the presence of polyphosphates on the surface is likely to result from their direct adsorption in impregnation solution. The observed increasing concentration of polyphosphates with the coverage could be related to a less likely hydrolysis due to the reduced availability of sites to stabilize the fragmented oligomers. This understanding opens the way to a better control over the speciation of phosphate species that are known to be key in the preparation of supported catalysts over alumina.

International audience; An innovative approach using solid/liquid interface characterization (zetametry measurements, ATR-IR), coupled with ex situ FTIR has been used to build a phosphate complexation model at the alumina surface via the PHREEQC software. According to the phosphorus coverage impregnated on the alumina surface, two mechanisms of adsorption have been identified: a specific surface complex adsorbed on the predominant alumina facet and a surface precipitation process. At low phosphorus coverage (P coverage

International audience; γ-alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides under hydrothermal 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 by 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 (110) faces of γ-alumina crystals. Those sites, which are responsible for the dissolution f γ-alumina crystals in water, are thus selectively protected by carbon clusters, the rest of the surface being totally exposed and accessible to adsorbates. Under hydrothermal conditions (10 hours 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 get similar results with more conventional carbon deposition methods.  INTRODUCTION Molecules derived from existing biomass treatment processes such as fermentation and hydrolysis (sugars, alcohols, polyols, carboxylic acids, etc.) are platform molecules for bio-based products manufacture. They consist of oxygen-rich, highly functionalized and generally water soluble molecules and reactions to convert them into valuable bio-products in aque-ous media may require relatively high temperatures and sometimes high pressures. These particular conditions, which combine water and temperature, are called hydrothermal conditions (HT) and they represent a serious challenge in the field of today's heterogeneous catalysis. 1,2 Indeed, conventional catalysts used in the petroleum refining industry generally consist of an active metal phase deposited onto an inorganic support and they may not be appropriate for applications involving water as solvent under such HT conditions. For exam-γ-alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides by a dissolution/re-precipitation process when placed in a water-rich medium at moderate temperature, resulting in strong modification of its intrinsic properties. 3,4,5-8 The stability of alumina materials in water has been a matter of great interest during the last years and many strategies have been proposed to prevent or at least minimize their transformation into hydroxides. The approach generally consists in modifying the alumina surface in order to make it less sensitive to the modifications produced by water under hydrother-mal conditions. The benefits of organic additives (such as carbon 9,10), inorganic additives (such as silicon 11,12 or phosphorus 13) and metallic additives 14 on the HT stability of alumina have been reported. However, the main parameters that govern the dissolution of alumina remain poorly known, which makes difficult the development of rational methods to improve the stability of this material. Ravenelle and coworkers first noticed that alumina transformation did not occur when polyols were present in the aqueous medium during a hydrothermal process. 15,16 The effect is supposed to result from the chemisorp-tion of the organic molecules on alumina, which avoids direct contact between water molecules and the surface and significantly improves water resistance. 17 This result can be compared with the work of Pham et al. who increased alumina stability in water by covering its surface with an oxygenated carbonaceous layer resulting from sucrose pyrolysis at 400°C. 9 They showed that 10 wt. % of carbonaceous deposit on the alumina surface was enough to prevent the transformation of the solid into boehmite. In another study, the same group used methane as a carbon source to produce graphitic carbon on alumina surface by gas phase carbonization. In this case, a higher amount of carbon was required to effectively protect alumina (35 wt. %), leading to a complete modification of its original properties (textural properties and surface chemistry). 10 Despite a growing number of articles dedicated to the improvement of alumina based materials in hydrothermal conditions, rationalization of predominant parameters that control hydrothermal stability remain unclear. In a previous work, we proposed a mechanism of the first steps of alumina dissolution based on experimental and theoretical approaches. 18 This mechanism involves some specific surface sites located on basal (110) planes of alumina platelets which are particularly reactive towards liquid water. We also showed

International audience; Transitional aluminas are a very important class of materials in catalysis. The detailed understanding of their surface chemistry has attracted much attention with input from spectroscopic and theoretical studies. In this context, NMR is a key technique, which, thanks to recent methodological developments, provides insights into local structures and relative arrangements. In the present paper, we first assess two 1 H− 27 Al heteronuclear correlation NMR methods for studying the surface of γ-Al 2 O 3 dehydroxylated at 500°C. The commonly used cross-polarization (CP) transfer method is compared to the dipolar heteronuclear multiple quantum coherence (D-HMQC) experiment, revealing that the latter is significantly more robust and efficient with less signal distortion that hampers the recording of meaningful signals. Furthermore, a new MQ-D-HMQC pulse sequence is proposed to confirm the presence of sites, with a large quadrupolar coupling constant at the surface of alumina. The robustness of D-HMQC is further probed by two additional alumina-based catalysts, namely, aluminum hydrides supported on γ-Al 2 O 3 , which are active in olefin hydrogenation and polymerization, and ZrCp 2 Me 2 grafted onto γ-Al 2 O 3 , a single-site olefin polymerization catalyst. In both cases, this provides information on the configuration of surface aluminum species in the vicinity of catalytically active sites and on the residual hydroxyl networks. The present study combines advanced NMR methods and complex catalytic materials for which a critical, yet fruitful, spectroscopic approach is required for deeper understanding.

The reactions of Ga(i-Bu)3 (i-Bu = CH2CH(CH3)2) with the dehydrated and partially dehydroxylated surfaces of alumina (Al2O3–500) and silica (SiO2–700) were studied by IR, high field solid-state NMR and EXAFS spectroscopies, as well as elemental analysis. Grafting onto Al2O3–500 occurs selectively by protonolysis at individual surface hydroxyl groups, resulting in the formation of mononuclear [(≡AlO)Ga(i-Bu)2L] (L = surface oxygen) sites as the major surface organometallic entities. Conversely, grafting on silica affords dinuclear species [(≡SiO)2Ga2(i-Bu)3] by a combination of protonolysis and isobutyl group transfer to Si. Further evidence for the difference in nuclearity was obtained by analysis of the WT-EXAFS. The mononuclear alumina-supported Ga sites show much higher activity in propane dehydrogenation than their dinuclear silica-supported counterparts. The propane dehydrogenation reaction may require the presence of Al–O–Ga bonds to promote heterolytic C–H bond activation. Comparisons with benchmar...

The present work highlights for the first time that a thermal treatment of γ-Al2O3 in water at 80 °C for 24 h prior to Ag deposition (Ag/Al2O3-OH) leads to a drastic enhancement of H2-C3H6-SCR performance at low temperature. This enhancement is attributed to the higher NO x coverage on Ag/Al2O3-OH compared with that of Ag/Al2O3. The higher NO x adsorption capacity of the sample prepared from the γ-Al2O3 thermally-treated in water is proposed to result from the preferential interaction of Ag with newly-created Al2O3 anchoring sites (Al(OH)3 bayerite domains), which were not available on the untreated pristine support. Graphical Abstract

The nature of γ-alumina (γ-Al2O3) surface sites leaves many open questions today, and solid-state NMR spectroscopy has been proposed and used as a tool for assessing their structure. Here, we calculated 27Al NMR parameters from first principles in periodic boundary conditions for a large number of Al sites with different coordination, potentially present on the alumina surface. The nature and accordingly the NMR parameters of these sites change with the level of hydroxylation and thereby the pretreatment temperature of γ-Al2O3. While the Al chemical shift is little affected by hydroxylation, the magnitude of the quadrupolar interaction at the Al nucleus is strongly correlated to hydroxylation, with high to very high quadrupolar coupling constant (CQ) values (20–34 MHz) on the weakly hydrated major (110) termination, which contains highly reactive Lewis acidic “defect sites”, and gradual lowering to bulklike CQ values of around 5 MHz on highly hydrated surfaces. In addition, we studied the effects of local...

International audience

A combined use of DFT periodic calculations and spectroscopic studies (IR and solid-state NMR) shows that a gamma-alumina treated at 500 degrees C under high vacuum contains surface defects, which are very reactive toward H2 or CH4. The reaction of H2 on defect sites occurs at low temperature (ca. 25 degrees C) on two types of Al atoms of low coordination numbers, AlIII or AlIV, to give AlIV-H and AlV-H, respectively. The amount of defects as titrated by H2 at 25 and 150 degrees C is 0.043 and 0.069 site/nm2, respectively, in comparison with 4 OH/nm2). In contrast, CH4 reacts selectively at 100-150 degrees C on the most reactive AlIII sites to form the corresponding AlIV-CH3 (0.030 site/nm2). The difference of reactivity of H2 and CH4 is fully consistent with calculations (reaction and activation energy, DeltaE and DeltaE++).

The stresses in thin gold film deposited on a γ Al 2 O 3 substrate is investigated as a function of the deposit thickness by the bending beam method. The film morphology and structure are studied by electron microscopy and are related to the variations of the stresses during the growth. The influence of the substrate temperature and the deposition rate on the thin film stresses is considered and related to the modification of the various stages of the growth.