HYDROGEN ADSORPTION ON Co2+-, Ni2+- EXCHANGED ZEOLITES ZSM-5 AND US-Y

---

Thesis Type: Postgraduate

Institution Of The Thesis: Middle East Technical University, Graduate School of Natural and Applied Sciences, Graduate School of Natural and Applied Sciences, Turkey

Approval Date: 2022

Thesis Language: English

Student: NURKAN SAROHAN

Supervisor: Bahar İpek Torun

Open Archive Collection: AVESIS Open Access Collection

Abstract:

The increase in greenhouse gases motivates the search for renewable energy sources and carriers. Hydrogen is an attractive option used as an energy carrier due to its high gravimetric energy density (140 kJ/kg). Although hydrogen energy has been used in fuel cell electric vehicles (FCEVs) at an increasing rate over the years, its storage is still the main concern. Zeolites, carbon-based materials, and metal organic frameworks (MOFs) are porous materials that store hydrogen through physical adsorption. In this thesis, Ni2+ or Co2+- exchanged micro-and mesoporous zeolites ZSM-5 and US-Y are prepared for the hydrogen adsorption tests. According to the experiments performed at 298 K and up to 10 bar, Ni2+-,Co2+- zeolites show gravimetric storage capacity in the range of 0.14–0.05 wt.%. The experimentally calculated initial isosteric heat of H2 adsorption values are between -23 and -40 kJ/mol for Ni2+-zeolites and -15 and -19 kJ/mol for Co2+-zeolites. Ni2+-, Co2+-zeolites show reversible storage at 298 K and up to 50 bar due to the optimum heat of adsorption values. Among the results obtained at 298 K and up to 50 bar experiments, the highest volumetric storage capacity is reached with Meso-Na+,Ni2+-ZSM-5 by 20 g H2/L, showing a great potential to be used as H2 storage material at 298 K. On the other hand, calculated high maximum storage capacity values on US-Y at 77 K (~2.3 wt.%) via the Sips model show that US-Y can be an appropriate adsorbent for higher pressure or low temperature adsorption studies. The site information of Co2+ and Ni2+ cations on the samples are investigated using diffuse reflectance (DR) UV–Vis spectra and synchrotron powder X-ray Diffraction (XRD). The results show that hydrogen adsorption mostly occurs on β and γ sites in Co2+-ZSM-5. According to synchrotron powder XRD analysis, the highest cation occupation is detected near Site III (in the supercage) of US-Y that are accessible to H2 molecule. The theoretical site dependent heat of adsorption values (-Eads) are calculated using both periodic and cluster density functional theory (DFT) for Ni2+-,Co2+-ZSM-5. β-site and γ-site occupation on Co2+-ZSM-5 as well as an additional α-site occupation on Ni2+-ZSM-5 are also predicted using DFT. Overall, it is found that the heat of adsorption values calculated from experiments and density functional theory (DFT) are within the same range.