A comparative study on the physicochemical and hydrogen adsorption properties of Ag-exchanged Demirci (Türkiye) clinoptilolites

The increase in global energy demand is primarily met by the use of fossil fuels, including coal, oil and natural gas. The depletion of these reserves has necessitated the development of other sustainable energy sources. One such potential source is hydrogen. Physical adsorption, a process of hydrogen storage, is defined as the reversible binding of hydrogen gas in the channels and voids of a porous material. In this study, the textural changes and hydrogen adsorption properties of two natural clinoptilolite-rich (>80 %) samples were investigated after cation exchange with AgNO₃ at different molarity values. The samples (A-raw and B-raw) with different (Na₂O + K₂O)/(CaO + MgO) ratios from the Demirci district (Manisa, Türkiye) were treated with 0.01, 0.02, 0.04 and 0.06 M AgNO₃ at 80 °C for 4 h. The mineralogical and physicochemical properties of Ag+-exchanged and raw clinoptilolites were compared using X-ray fluorescence, X-ray diffraction, and N₂ gas adsorption analysis at 77 K. There were differences in the XRD traces of raw samples and the Ag ⁺ -exchanged forms. Adsorption isotherms of N₂ and H₂ were measured at 77 K. There is a relationship between the (Na₂O + K₂O)/(CaO + MgO) ratio of clinoptilolite and its micropore surface area and micropore volume values. The values for the micropore surface area and the micropore volume of the clinoptilolite decreased with increasing AgNO₃ concentration. The hydrogen adsorption capacities of the modified forms of both clinoptilolite were found to decrease above certain thresholds. Treatment of natural clinoptilolite with Ag ⁺ resulted in a significant contribution to hydrogen adsorption. In this study, the hydrogen adsorption capacities of Ag⁺-exchanged clinoptilolites (1.391–3.239 mmol g⁻¹) were found to be higher than those of many differently modified clinoptilolites (1.181–2.752 mmol g⁻¹). Sample A-0.02 has the highest hydrogen adsorption capacity of all the samples in this study and can therefore be recommended for use in hydrogen storage applications.