Magic Angle Spinning NMR Study on Inversion Behavior and Vacancy Disorder in Alumina-Rich Spinel

Abstract

Solid state magic angle spinning nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) were used to investigate the inverse behavior and vacancy disorder in alumina-rich spinel, MgAlO (0 ≤ x ≤ 0.86). Simulation and integration of NMR spectra have been developed for probing the population of Al located in different coordinated environments. Rietveld profile refinements were performed for powder XRD spectra by combining with NMR analysis. With changes in the composition, inversion disorder and cationic vacancies coexisting in tetrahedral and octahedral coordinations fluctuate in amount in the crystal lattice. The coordination polyhedra in the crystal structure can adjust the volume to variations of composition, anion parameter, and inverse parameter. This opens a window to the design and functionalization of spinel materials.

Mg‐Ion Inversion in MgO@MgO−Al2O3 Oxides: The Origin of Basic Sites

“…The solid-state 27 Al NMR spectra of the x MgOÀAl 2 O 3 samples ( Figure 1c)r eveal the presence of three species, corresponding to the tetra-(Al IV ), penta-(Al V ), and octa-( Al VI )c oordinated Al species. The variousA lc oordinations detected by 27 Al NMR spectroscopy can be described as follows;o cta-coordinate Al sites are from the inherent alumina framework, penta-coordinate Al speciesa re derived from the amorphousp hase, and tetra-coordinate Al sites are relevant to the basicity and resultants of inversioni nM gAl 2 O 4 .T he relative amountso ft etra-coordinate Al and penta-coordinate Al speciesa re reduced as in-creasingM g/Al ratio in the x MgOÀAl 2 O 3 composite, which means higha mount of octahedral Mg species might be present in low Mg/Al composite. Moreover, at al ow Mg/Alr ange, the occupation of Mg species in octahedral sites of Al might be easier in an amorphous Al structure, which resulting in the inversion of Mg species.…”

“…Structural property analysis:I nversion in spinels and solid state 27 Al and 25 Mg NMR We describe herein ao ne-pot synthesis of x MgOÀAl 2 O 3 through the self-assembly of the magnesium and aluminum precursors in the presence of at riblock copolymer,w hich functions as as tructure-directing agent (Figure1a). We designed the synthesis methods uch that it would enableh igh metal oxide loading in mesoporous alumina with ah igh degree of dispersion.…”

“…Owing to their distinctive structural features and cation distributions, spinels (AB 2 O 4 ) are of great interest for various applications such as gas adsorption, energy storage/conversion, sensor, and catalysis. In the spinel structure, the A and B cations are distributed in different ratios at tetrahedral and octahedral interstices, respectively, and it is known that inversion phenomenon occurs by cation exchange of the trivalent (Al 3+ ) and divalent (Mg 2+ ) cations, but the degree of inversion is not well known . Furthermore, as the Mg/Al molar ratio changes, the inversion is more difficult to identify.…”

“…In the spinel structure, the Aa nd Bc ations are distributed in different ratios at tetrahedral and octahedral interstices,r espectively,a nd it is knownt hat inversion phenomenon occurs by cation exchange of the trivalent( Al 3 + ) and divalent (Mg 2 + )c ations, [26] but the degree of inversion is not well known. [27] Furthermore, as the Mg/Al molar ratio changes,t he inversion is more difficult to identify.A lthough much is knowna bouti nversion phenomenon, its correlation with specific applicationsh as been limited. There is thus a need to correlate the structural features with the physicochemical properties to potentially enhance their performance.…”

“…Furthermore, we investigated induced structural changes that resulted from the modification of cation distribution by different Mg/Al ratios in the spinel structure. Moreover, we identified inversion of the x MgOÀAl 2 O 3 composite by using solid-state 25 Mg and 27 Al NMR spectroscopy to examinet he detailed heterogeneouse nvironments of the metal species. We obtainedt he degree of inversion, whichr efers to the degree of exchanging of cation sublattices when Mg 2 + replaces Al 3 + in the octahedral sites, by quantifying the coordination number of Mg in the corresponding x MgOÀAl 2 O 3 composites.…”