~(19)F-NMR study Al-oxyhydroxide surfaces and reactions

摘要

~(19)F-NMR spectroscopy is a potentially useful technique for characterizing the surface structure of diamagnetic materials. ~(19)F is a very sensitive, 100% naturally abundant NMR-active nucleus, which allows spectra to be obtained at relatively low concentrations, such as on mineral surfaces of moderate specific surface areas and modest loadings. Dissolved fluoride substitutes for both terminal water and bridging hydroxyl groups on Al-oxyhydroxide surfaces and could serve as a probe of reactive sites. A main problem is uncertainty in NMR peak assignments, and we show here that ~(19)F{~(27)Al} double-resonance techniques could help distinguish fluoride signals arising from terminal vs. bridging configurations. Application to formation of Al-oxyhydroxide gels from the "GaAl_(12)" polyoxocation in the presence of fluoride suggests that most terminal fluorides are converted to bridging sites by water-eliminating condensation reactions during precipitation.