¹²⁹Xe NMR analysis of pore structures and adsorption phenomena in rare-earth element phosphates

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 REEPO