129Xe NMR analysis of pore structures and adsorption phenomena in rare-earth element phosphatesShow others and affiliations
2022 (English)In: Microporous and Mesoporous Materials, ISSN 1387-1811, E-ISSN 1873-3093, Vol. 344, article id 112209Article in journal (Refereed) Published
Abstract [en]
Rare-earth elements (REEs) are indispensable in various applications ranging from catalysis to batteries and they are commonly found from phosphate minerals. Xenon is an excellent exogenous NMR probe for materials because it is inert and its 129Xe chemical shift is very sensitive to its local physical or chemical environment. Here, we exploit, for the first time, 129Xe NMR for the characterization of porous structures and adsorption properties of REE phosphates (REEPO4). We study four different REEPO4 samples (REE = La, Lu, Sm and Yb), including both light (La and Sm) and heavy (Lu and Yb) as well as diamagnetic (La and Lu) and paramagnetic (Sm and Yb) REEs. 129Xe resonances are very sensitive to the porous structures and moisture content of the REEPO4 samples. In the samples treated at a lower temperature (80 °C), free water hinders the access of hydrophobic xenon into small mesopores, but the treatment at a higher temperature (200 °C) removes the free water and allows xenon to explore the mesopores. Based on a standard two-site exchange model analysis of the variable-temperature 129Xe chemical shifts, as well as its proposed, novel modification for paramagnetic materials, the average mesopore sizes were determined. The size was the largest (79 nm) for the La sample with mixed monazite (70%) and rhabdophane (30%) phases and the smallest (6 nm) for the Yb sample with pure xenotime phase. The mesopore sizes of the Lu and Yb samples (12 and 6 nm) differed by a factor of two regardless of their similar xenotime phase. The 129Xe NMR analysis revealed that the heats of adsorption of the samples are similar, varying between 8.7 and 10.1 kJ/mol. For diamagnetic samples, computational modelling confirmed the order of magnitude of the chemical shifts of Xe adsorbed on surfaces and therefore the validity of the two-site exchange model analysis. Overall, 129Xe NMR provides exceptionally versatile information about the pore structures and adsorption properties of REEPO4 materials, which may be very useful for developing the extraction processes and applications of REEs.
Place, publisher, year, edition, pages
Elsevier, 2022. Vol. 344, article id 112209
Keywords [en]
Rare-earth element phosphate, 129Xe NMR spectroscopy, DFT calculations
National Category
Metallurgy and Metallic Materials Theoretical Chemistry
Research subject
Chemistry of Interfaces
Identifiers
URN: urn:nbn:se:ltu:diva-93052DOI: 10.1016/j.micromeso.2022.112209ISI: 000872120400001Scopus ID: 2-s2.0-85138521061OAI: oai:DiVA.org:ltu-93052DiVA, id: diva2:1696131
Funder
EU, Horizon 2020, 772110Swedish Research Council Formas, 2018-00630
Note
Validerad;2022;Nivå 2;2022-11-10 (hanlid);
Funder: Academy of Finland (331008, 340099); Kvantum institute, University of Oulu
2022-09-152022-09-152022-11-16Bibliographically approved