Synthesis and characterisation of ambient pressure dried composites of silica aerogel matrix and embedded nickel nanoparticles

Addressing the Effectiveness and Molecular Mechanism of the Catalytic CO2 Hydration in Aqueous Solutions by Nickel Nanoparticles

Hydration of carbon dioxide in water solution is the rate limiting step for the CO2 mineralization process, a process which is at the base of many carbon capture and utilization (CCU) technologies aiming to convert carbon dioxide to added-value products and mitigate climate change. Here, we present a combined experimental and computational study to clarify the effectiveness and molecular mechanism by which nickel nanoparticles, NiNPs, may enhance CO2 hydration in aqueous solutions. Contrary to previous literature, our kinetic experiments recording changes of pHs, conductivity, and dissolved carbon dioxide in solution reveal a minimal effect of the NiNPs in catalyzing CO2 hydration. Our atomistic simulations indicate that the Ni metal surface can coordinate only a limited number of water molecules, leaving uncoordinated metal sites for the binding of carbon dioxide or other cations in solution. This deactivates the catalyst and limits the continuous re-formation of a hydroxyl-decorated surface, which was a key chemical step in the previously suggested Ni-catalyzed hydration mechanism of carbon dioxide in aqueous solutions. At our experimental conditions, which expand the investigation of NiNP applicability toward a wider range of scenarios for CCU, NiNPs show a limited catalytic effect on the rate of CO2 hydration. Our study also highlights the importance of the solvation regime: while Ni surfaces may accelerate carbon dioxide hydration in water restricted environments, it may not be the case in fully hydrated conditions.

Morphology control of nickel nanoparticles prepared in situ within silica aerogels produced by novel ambient pressure drying

Silica aerogels are low density solids with high surface area and high porosity which are ideal supports for catalyst materials. The main challenge in aerogel production is the drying process, which must remove liquid from the pores of the wet gel while maintaining the solid network. In this work, the synthesis of silica aerogels and nickel-doped silica aerogels by a low energy budget process is demonstrated. Silica aerogels are produced by ambient drying using ammonium bicarbonate, rather than a conventional low surface tension solvent. Heating dissociates the ammonium bicarbonate, so generating CO₂ and NH₃ within the pores of the wet gel which prevents pore collapse during drying. Nickel-doped aerogels were produced by reducing nickel ions within pre-synthesised silica aerogels. The morphology of the resulting nickel particles—spheres, wires and chains—could be controlled through an appropriate choice of synthesis conditions. Materials were characterized using nitrogen adsorption/desorption isotherms, scanning electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis and X-ray diffraction. The surface area of undoped aerogel is found to increase with the concentration of ammonium bicarbonate salts from 360 to 530 m² g⁻¹, and that of nickel-doped silica aerogel varies from 240 to 310 m² g⁻¹ with nickel doping conditions.

Metallic nickel nanoparticles and their effect on the embryonic development of the sea urchin Paracentrotus lividus

The presence of nanoparticles in many industrial applications and daily products is making it nowadays crucial to assess their impact when exposed to the environment. Metallic nickel nanoparticles (Ni NPs) are of high industrial interest due to their ability to catalyze the reversible hydration of CO2 to carbonic acid at ambient conditions. We characterized metallic Ni NPs by XRD, HRTEM and EDS and determined the solubility of free nickel ions from 3 mg/L metallic Ni NPs in seawater by ICP-MS over 96 h, which was below 3%. Further, embryonic development of the sea urchin Paracentrotus lividus was investigated for 48 h in the presence of metallic Ni NPs (0.03 mg/L to 3 mg/L), but no lethal effects were observed. However, 3 mg/L metallic Ni NPs caused a size reduction similar to 1.2 mg/L NiCl2*6 H2O. The obtained results contribute to current studies on metallic Ni NPs and point to their consequences for the marine ecosystem.