Engineering metal-functionalized halloysite nanoclay for fundamental insights into H2-surface interactions: Effects of Mg-Ni functionalization, structural-thermodynamic characterization, and molecular interaction analysis

This study presents the synthesis and comprehensive characterization of halloysite nanoclay (HNT)-based materials modified with magnesium (HNT-Mg), nickel (HNT-Ni), and both metals (HNT-MgNi) as model systems for investigating fundamental H 2 -surface interactions on metal-modified aluminosilicates. The materials were systematically characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric/differential scanning calorimetry (TGA/DSC), Brunauer–Emmett–Teller (BET) surface area analysis, and HAADF-STEM/EDX. Hydrogen adsorption experiments were conducted at 35, 55, and 75 °C under atmospheric pressure and evaluated using Langmuir and Freundlich models. These conditions are substantially less demanding than those generally considered for practical H 2 storage, and the measured uptakes therefore remained below 0.1 mmol g −1 . The thermodynamic analysis revealed positive apparent enthalpy changes of +45.51, +44.74, and +34.50 kJ mol −1 for HNT-Ni, HNT-Mg, and HNT-MgNi, respectively, indicating an apparent endothermic character under the adopted thermodynamic treatment. The corresponding entropy changes were +112.33, +92.73, and +56.79 J mol −1 K −1 , respectively. HNT-Ni exhibited the highest H 2 uptake, reaching 0.078 mmol g −1 at 75 °C, while its ΔG° decreased from 10.93 to 6.45 kJ mol −1 between 35 and 75 °C. Positive ΔG° values were obtained for all materials over the investigated temperature range, indicating that adsorption was not thermodynamically favorable at the adopted reference state. Hirshfeld surface and RDF analyses revealed differences in the spatial organization of H 2 around the modified surfaces, including distinct H 2 -Ni and H 2 -Mg correlations. These results provide qualitative information on intermolecular proximity and local ordering but do not directly establish adsorption energies, charge transfer, chemical bonding, H 2 activation, or cooperative effects. Overall, the findings show that H 2 adsorption depends on the combined effects of metal identity, surface chemistry, structural organization, and accessible adsorption environments. Given the low uptake values (≤0.078 mmol g −1 ; <0.02 wt%), the materials are more appropriately considered as model systems for understanding H 2 -surface interactions than as practical hydrogen-storage sorbents.