Browsing by Author "Bedford, NM"

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    Nanoporous zirconium phosphonate materials with enhanced chemical and thermal stability for sorbent applications
    (American Chemical Society, 2020-04-01) Veliscek-Carolan, J; Rawal, A; Oldfield, DT; Thorogood, GJ; Bedford, NM
    Nanoporous zirconium phosphonate (ZrP) materials are considered to be promising candidates for practical applications such as catalysis and separation, in particular because of their excellent stability, resulting from the strength of the P–O–Zr bond. However, the functionality of ZrP materials is dependent on the availability of free phosphonate groups uncoordinated by zirconium, the presence of which can decrease the stability. The mechanisms by which nanoporous ZrP materials degrade and lose functionality during thermal and chemical treatment are not well understood. Herein, we address this knowledge gap using nanoporous zirconium aminotris(methylenephosphonic acid) (Zr-ATMP) sorbent materials. Thermal treatment up to 150 °C caused collapse of the nanoporous structure of some Zr-ATMP materials without a significant effect on the chemical structure. On the other hand, contact with 5 M nitric acid changed the chemical structure of the Zr-ATMP materials by catalyzing the formation of P–O–Zr bonds and elemental leaching. Enhancement of the thermal and chemical stability of the Zr-ATMP materials was achieved by decreasing the pH of the synthesis and, interestingly, changing the counterion of the hydroxide used to control the pH also impacted the structure and stability of the resulting materials. The most stable Zr-ATMP material was produced at pH 3 using LiOH, but this material demonstrated lower selectivity than other Zr-ATMP materials, which decreases its practicality for separation applications. The Zr-ATMP material synthesized at pH 3 with NaOH showed an optimal balance between the stability and sorption performance. The enhanced chemical and thermal stability of this material drastically improves its applicability for use in harsh environments, such as in the treatment of radioactive wastes. © 2020 American Chemical Society
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    Radiolytic stability of metal (IV) phosphonate sorbents designed for minor actinide-lanthanide separations
    (CEA, 2024-09-01) Cataldo, T; Veliscek-Carolan, J; Bedford, NM; Le Caër, S
    Nuclear power is an intrinsically clean source of energy. However, improvements in nuclear waste treatment are required. The minor actinide (MA) elements in nuclear waste are problematic due to their radiotoxicity and long half-lives. In principle, minor actinides (MAs) in nuclear waste could be recycled. However, the chemical similarity of MAs and the lanthanide fission products also found in nuclear waste means that separating and recycling MAs is extremely challenging. Hence, there is a need for materials that can selectively separate MAs from lanthanides in nuclear waste, whilst also possessing the necessary acid and radiation resistance required to function in nuclear waste conditions. Metal (IV) phosphonates, such as titanium or zirconium phosphonates, are a type of material with promising potential for MA-lanthanide separation applications. Metal phosphonates are a coordination polymer: a material in which inorganic metal cations are structurally joined together by organic ligands via coordinate bonds. The hybrid inorganic-organic nature of metal phosphonates allows for a variety of chemical and physical properties. In the context of MA-lanthanide separations, the phosphonate component allows for the intramolecular incorporation of organic ligands that provide selectivity and efficiency for MA sorption. Furthermore, the strong M(IV)–O–P bonding of the inorganic component provides stability and resistance to acid and radiation damage. Post-synthesis, metal phosphonates are collected as a porous, solid powder; hence, they can be employed as a solid-phase phase sorbent in MA-lanthanide separations. Previous studies on zirconium (IV) phosphonate materials have demonstrated promising sorption capacity, selectivity for MA over lanthanides, and excellent stability1,2,3. Therefore, further study and optimization of these materials presents a potential pathway for solving the challenges of MA separation and recycling. In this study, a zirconium phosphonate sorbent that intramolecularly incorporates the MA-selective 2,6-bis(1,2,3-triazol-4-yl)pyridine (PTP) ligand was synthesised. The sorbent (ZrPTP) was irradiated with high energy electron radiation to doses of 2 MGy to study its radiation stability. Since ZrPs are highly amorphous, synchrotron light sources were employed to accurately assess the average local structure before and after irradiation using x-ray absorption spectroscopy (XAS) and atomic pair distribution function (PDF). Gas chromatography, solid-state NMR and infrared spectroscopy were also used to support the characterisation. Lastly, the MA-selectivity of ZrPTP before and after irradiation was compared using americium and europium. It was found that ZrPTP possessed excellent radiation stability for doses up to 2 MGy. Characterisation of ZrPTP exhibited only small amounts of radiation damage to its Zr-O bonds, aliphatic C-H bonds, and its N bonds in the triazole groups. Furthermore, ZrPTP demonstrated maintained selectivity for americium over europium even after a 2 MGy dose. Overall, the results extensively demonstrate the viability of metal phosphonate sorbents for nuclear waste treatment applications in terms of their radiation stability. © The Authors
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    Reconstructing Cu nanoparticle supported on vertical graphene surfaces via electrochemical treatment to tune the selectivity of CO2 reduction toward valuable products
    (ACS Publications, 2022-04-07) Ma, ZP; Tsounis, C; Toe, CY; Kumar, PV; Subhash, B; Xi, SB; Yang, HY; Zhou, SJ; Lin, ZH; Wu, KH; Wong, RJ; Thomsen, L; Bedford, NM; Ng, YH; Han, ZJ; Amal, R
    Reconstructing a catalyst with tunable properties is essential for achieving selective electrochemical CO2 reduction reaction (CO2RR). Here, a reduction–oxidation–reduction (ROR) electrochemical treatment is devised to advisedly reconstruct copper nanoparticles on vertical graphene. Undercoordinated sites and oxygen vacancies constructed on the Cu active sites during the ROR treatment enhance the CO2RR activity. Moreover, by varying the oxidation potential while maintaining the reduction potential during the ROR treatment, CO2RR selectivity can be tuned between *COOH- and *OCHO-derived products. Specifically, rich grain boundaries are formed on the ROR catalyst with a high oxidation potential (+1.2 VRHE), favoring the *COOH/*OCCO adsorption and leading C–C coupling to *COOH-derived products, while the catalyst undergoing ROR at a low oxidation potential (+0.8 VRHE) lacks grain boundaries, resulting in highly selective formate (*OCHO-derived) production. Our findings are evidenced by combined in situ and ex situ characterizations and theoretical calculations. © 2022 American Chemical Society