Structural and ion exchange study of nanostructured materials

Contini, Alessandro

Abstract

In this thesis, the structural and acidic properties of two zeolites LTL and MAZ are compared and reported. Commercial LTL and MAZ were ion exchanged with metal alkali cations, to obtain the corresponding ion exchanged forms, and characterised by IR spectroscopy, X-Ray diffraction, N2 adsorption-desorption and SEM-EDX. Attention was given to the interactions of cations with zeolites and to the cation migration mechanism to enter to the structure of the material. The acidic properties of produced materials were evaluated by pyridine adsorption to discriminate Brønsted and Lewis acid sites. X-Ray diffraction analysis and Rietveld method were employed to understand cation migration mechanism involved during the ion exchange process. This was complemented by several investigation techniques, such as N2 physisorption evaluating the surface area and pore size distribution. Modeling through the application of the Rietveld method was employed to investigate the migration of cations preferential route by producing simulated patterns as a predictive model. Results showed that cations of reduced size were able to reach inner cages at increasing ion exchange steps through the 12MR (main channel) of the zeolite LTL. For zeolite MAZ only the main channels were accessible. Respectively, up to 3 Cs+ for LTL and 2 Cs+ for MAZ, per unit cell were found. FTIR results, through Py adsorption, showed preferential poisoning of Brønsted acid sites located in the main cage of LTL after ion exchange with large cations and low BAS concentrations were found in comparison with the samples ion exchanged with smaller cations. The same trend was found for MAZ samples but with lower concentrations of BAS.

In this project zeolites such as LTL, NaX and NaY were also combined with graphite in a uniform mixture which was employed to produce electrodes. The sensing properties of these materials were investigated through potentiometry, cyclic voltammetry and impedance spectroscopy. Sensors were employed to observe the selectivity of the composite materials towards cations in solution by potentiometry. Results for NaX and NaA showed very low response, around 14 mV/decade and far from the target Nernstian response (59.2 mV/decade). In comparison, K-LTL gave a closer response around 44 mV/decade. Cyclic voltammetry was performed on potassium ferricyanide. Results suggested the presence of an irreversible system, which is not in agreement with the redox couple employed and the literature data. These aspects are justified by the insulating nature of the zeolites employed which affected the redox processes occurring at the electrodes. Electrochemical Impedance spectroscopy was also employed to observe the diffusivity aspects related to the cation migration of species from the solution onto the electrode in contact. Results showed high resistance between the substrate and the composite deposited on it, as consequence of a slow electron transfer between these two components.

Citation

Contini, A. (2024). Structural and ion exchange study of nanostructured materials. (Thesis). Keele University. Retrieved from https://keele-repository.worktribe.com/output/857282

• Thesis Type: Thesis
• Deposit Date: Jun 25, 2024
• Publicly Available Date: Aug 16, 2024
• Public URL: https://keele-repository.worktribe.com/output/857282
• Award Date: 2024-08