The present study examined the effects of dry grinding, using ball-milling, on the structure of reference well-crystallized (KGa-1) and poorly crystallized (KGa-2) kaolinite powders from Georgia. Grinding produced a strong structural alteration, mainly along the c axis, resulting in disorder and total degradation of the crystal structure of the kaolinite and the formation of an amorphous product. The surface area increased with grinding time, mainly in KGa-2 (maximum value 50.27 m 2 /g), a result associated with particle-size reduction. These particles became more agglomerated with grinding, and the surface area decreased after 30 min, as confirmed by scanning electron microscopy and particle-size-distribution analysis. There was a limit to particle-size reduction with grinding time. When grinding time was increased, the original endothermic differential thermal analysis (DTA) effects of dehydroxylation in both samples shifted to lower temperatures, decreased in intensity, then disappeared completely after 120 min of grinding. The temperature of the characteristic first exothermic effect shifted slightly to lower temperatures with grinding, although the DTA effects did not increase with grinding time in either kaolinite sample, at least up to 325 min. The amorphous, mechanically activated kaolinite converted into low-crystalline mullite nuclei at a lower temperature than did the unground samples, as deduced by thermal and X-ray observations. This effect was especially important for the KGa-2 sample. Grinding did not seem to influence the formation of silicon-aluminum spinel from kaolinite. The present results may explain why ground kaolinite samples prepared via different routes-e.g., with differences in grinding-behave differently during high-temperature transformations, as reported in the related literature.

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The use of hydrogen as a potential future energy carrier is limited due to the problems of its storage. Hydrolysis of hydrogen storage materials such as sodium borohydride (NaBH 4 , SB) has been one of the most investigated approaches for hydrogen generation. SB is stable in dry air and combines lightweight with high hydrogen content (10.8 wt%). Although spontaneous, the SB hydrolysis (reaction 1) needs catalysts to occur at appreciable rates. Co has demonstrated to be a good choice because its compromise between activity and cost. However, its major drawback is related to stability: these materials deactivate upon cycling. Despite the great number of works reporting Co and Co‐B based catalysts, the nature of the active phase and deactivation mechanisms are still under intense discussion. We have recently reported the preparation of supported Co metallic catalysts as thin films by magnetron sputtering for sodium borohydride hydrolysis [1]. Magnetron sputtering is a very versatile technique used in this work to fabricate Co and CoB catalytic coatings under different deposition conditions and supported on different substrates (i.e. silicon and polymeric membranes). In this work we have been able to study by electron microscopy the catalytic coatings as grown on the wires of the polymeric membranes. The structural and compositional characterization by SEM and (S)TEM techniques has been performed before and after the catalytic tests (19 wt% SB in NaOH 4 wt%, 90 min reaction time). Fig. 1 and 2 show the SEM morphology of a Co thin film on the polymeric membrane before and after the catalytic test, respectively, showing the growth of a new layer onto the catalysts upon operation. Further nano‐analysis of the structure and compositional distribution have been performed by (S)TEM techniques coupled to EELS. They also reveal the formation of fiber/nanoflake‐like nanostructures onto the catalytic coatings (Fig. 3 and Fig. 4). Compositional analysis have pointed out the formation of Co‐borates and most likely cobalt oxide/hydroxide nanoflakes (i.e. CoO(OH)) which could be the origin of the leaching and deactivation mechanisms of the Co‐based catalysts for the investigated reaction.

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