Crystal plane dependent dispersion of cobalt metal on metastable aluminas

Metallic Co nanoparticles, widely used and studied as supported heterogeneous catalysts for Fischer-Tropsch synthesis (FTS), display catalytic properties that can vary significantly depending on their size and crystal structure. In this work, we used 59Co Internal Field NMR (59Co IF NMR) complemented by high-resolution transmission electron microscopy (HRTEM) to demonstrate the influence of strong metal-support interaction on two noticeably different metastable alumina phases - γ-Al2O3 and χ-Al2O3. According to 59Co IF NMR and HRTEM, the metallic particles supported on χ-Al2O3 were larger and displayed a significantly higher content of hcp Co phase, which are known to be more active and selective to C5+ in FTS. The 1H NMR chemical shifts of hydroxyl groups anchored to the (110) and (111) spinel crystal planes were calculated by DFT. It revealed that the hydroxyl coverage of γ-Al2O3 facilitates the dispersion of Co precursor over the surface of the support, ultimately leading to the formation of smaller metal Co nanoparticles on γ-Al2O3, than on χ-Al2O3

AN EXPERIMENTAL REVIEW OF THE REACTION PATHS FOLLOWED BY ALKALI-ACTIVATED SLAG CEMENTS

Solid-state NMR, XRD, mechanical strength measurements, quantification of bound water and isothermal calorimetry were combined to contrast the reaction paths followed by slag under the influence of three alkaline activators: NaOH, Na2CO3, and Na2Si1.7O4.4. NaOH-activation led to the concomitant formation of a very ordered C-A-S-H gel and of hydrotalcite, giving rise to early mechanical strength. For Na2Si1.7O4.4, a N-A-S-H gel formed first due to the high quantities of silicon in solution. This led to quick setting but no mechanical strength. Later, an amorphous C-A-S-H gel provided mechanical strength, while an aluminate phase precipitated. Finally, Na2CO3-activation led also to the formation of N-A-S-H favored by the initial consumption of calcium to form calcite. This did not bring any real structuration and mechanical strength to the paste. Only after a few days of hydration did the mechanical strength improve with the precipitation of amorphous C-A-S-H, an ill-defined hydrated aluminate phase, and gaylussite

THE ROLE OF SOLVENT QUALITY AND OF COMPETITIVE ADSORPTION ON THE EFFICIENCY OF SUPERPLASTICIZERS IN ALKALI-ACTIVATED SLAG PASTES

The loss of dispersing ability by polycarboxylates ether superplasticizers in alkali-activated slag cements has been widely reported. However, no clear-cut explanation of this phenomenon can be found to date. Therefore, the behaviour of poly(methacrylate-g-poly(ethylene glycol)) superplasticizers in NaOH or Na2CO3 -activated slag pastes was investigated. The observed loss of efficiency of the polymer was not due to a specific property of the slag particles, nor to structural degradation of the polymer in the alkaline solutions. Actually, the ionic strength of the activating solution decreased the solvent quality and changed the polymer conformation, leading to a deterioration of the steric repulsion brought by the side-chains. Moreover, in the Na2CO3 -activated systems, the adsorption behaviour of the polymers was also significantly altered. Here, this was not caused by a low calcium concentration or by a preferential adsorption of the superplasticizer on calcite crystallites. The most plausible explanation was a competitive adsorption with CO32- ions