Neoformation of DLH During Impregnation of α-Alumin

The adsorption of Co(II) or Ni(II) ammine complexes from aqueous solutions onto α-alumina at neutral pH and ambient temperature was investigated. The formation of coprecipitates including Al (III) ions extracted from the support was demonstrated by EXAFS for contact times and Ni or Co loadings higher than 0.5 h and about 2.0 wt %, respectively. The EXAFS technique makes it possible to distinguish the Ni or Co hydroxides and basic nitrates from coprecipitates with a double layer hydroxide (DLH) structure. Not only is EXAFS shown to be sensitive to the presence of aluminum in the coprecipitates, but in most cases, the M(lI)/Al(III) ratio (M= Ni or Co) in the supported coprecipitates can be estimated. Thus, alumina should not be considered systematically as inert even at pH values close to its isoelectric point. It is suggested that a dissolution-precipitation mechanism is involved and that the rate of alumina dissolution is promoted by adsorbed Ni(II) or Co(II) ions. Site-binding models have a considerable value for the early stages of impregnation, whereas a geochemical approach involving surface rehydration and coprecipitation have probably a greater validity for the later stages

Interplay between silicate and hydroxide ions during geopolymerization

International audienceTwo sets of activating solutions with increasing sodium hydroxide contents were prepared either with or without silicates. Their buffer capacities, i.e. their ability to resist changes in pH, were determined and compared using the Hammett acidity function, a measure of acidity appropriate for concentrated solutions. This is the first time the Hammett acidity function of sodium silicate solutions has been measured. The effects of the buffer capacity and of the initial Hammett acidity function on the reactivity of metakaolin-based pastes were assessed using isothermal conduction micro-calorimetry. The reactivity of metakaolin in sodium hydroxide solutions is shown to be directly related to the initial Hammett acidity function, whereas for sodium silicate mixtures, the buffer capacity is a more pertinent parameter. The mechanism deduced for the role of hydroxide ions during geopolymerization also highlights the role of silicate species as a hydroxide reservoir that nurtures the dissolution process

High-resolution 29Si solid-state NMR study of silicon functionality distribution on the surface of silicas.

29Si solid-state NMR allows us to distinguish silicon atoms involved in siloxane bridges (Q4), single silanol (Q3) and geminal silanol groups (Q2) on the surface of silica. The relative proportions of each species can be obtained by Magic Angle Spinning (MAS) associated with 1H-->29Si crosspolarization (CP/MAS). This preliminary study deals with the silicon functionality of MCM-41, a purely siliceous, amorphous and mesoporous silica. From the 29Si spectra the relative proportions are estimated to be 3.7% Q2 27.3% Q3 and 69.0% Q4, while the ratio Q2/Q2 + Q3) is 0.12. These results are found to be in perfect agreement with a binomial probability distribution of sites.Journal Articleinfo:eu-repo/semantics/publishe

High-Resolution Solid-State Nmr: a Versatile Tool for the Study of AI-O-P Clusters

International audienc

Impact of polyacrylamide adsorption on flow through porous siliceous materials: State of the art, discussion and industrial concern

International audienceHypothesis: Adsorption of high molecular mass polymers impacts flow in porous media. In the industrially crucial case of acrylamide-based polymers in porous silicates, the very occurrence of adsorption is still debated. Thus, the present work aimed at establishing a clear correlation between adsorption of acrylamide-based polymers and injectivity loss in porous silica. Experiments: A review of the literature revealed apparent discrepancies regarding the affinity of acrylamide-based polymers for siliceous materials having ostensibly the same chemical composition. Through a deeper analysis of the reported literature and new experimental measurements on well-defined polymers and surfaces, we investigated the relation between the silica surface properties and the acrylamide-based polymer adsorption. Our observations were confronted with water injection experiments in porous media of different surface compositions previously put in contact with polymers. Findings: The polymer affinity towards the silica surface depended on the density of hydroxyl groups at the surface of the oxide, its thermal treatment, storage condition and purity. This demonstrated that the impact of adsorption on acrylamide-based polymer flow within porous silicates heavily depends on the silicate surface composition and must be carefully evaluated. In view of the continually expanding use of acrylamide-based polymers, notably in enhanced oil recovery, such considerations provide interesting insights into the effect of adsorption on their flow into porous materials