Atomic-scale mechanisms of dehydration processes in Earth's near-surface environments: High-resolution solid-state NMR study of temperature-induced structural transitions in amorphous/crystalline oxide nanoparticles and zeolites

Abstract

Nano-scale earth materials are ubiquitous in diverse Earths environments. Nanoparticles can be produced through diverse geochemical processes, such as weathering of silicates and chemical reactions in acid mine drainage, and through biological processes involving microorganisms. In spite of the abundance in nanoparticles in the Earth, the effect of nanoparticles on the global and geological processes is not fully understood, yet. This dissertation is for a systematic exploration of temperature-induced changes in atomic structure of prototypical oxide nanoparticles and zeolites —a model system for natural earth materials— using high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopies with varying temperature and/or particle. The main objective of the dissertation includes exploration to how the oxide nanoparticles or zeolites affect the temperature-induced geochemical reactions such as dehydration, phase transition, and amorphization. The structural changes in diverse hydroxyl group and water molecules on the surface of amorphous silica nanoparticles upon dehydration were obtained using fast magic-angle spinning (MAS) and 2D HetCor (heteronuclear correlation) NMR spectroscopy. The atomic-scale dehydration model for amorphous silica nanoparticles was proposed in this dissertation. The results demonstrate that the particle size of nanoparticles plays an important role in controlling the hydrogen contents, and thus overall hydrogen bond strength of hydroxyl groups and atomic structure of silanols can control dehydroxylation of amorphous silica nanoparticles. Similar dehydration models can be applied to understand the nature of hydrogen bonding between water and natural amorphous silica including amorphous opal. The nature of temperature-induced phase transitions in alumina nanoparticles was explored using high-resolution solid-state 27Al 2D triple-quantum (3Q) MAS and 1D MAS NMR spectroscopies. The transition temperature from metastable disordered phase to stable ordered phase in alumina apparently increases as the particle size increases, indicating a larger energy penalty for phase transitions into α-Al2O3 in larger alumina nanoparticles. The mechanistic details of phase transitions among alumina polymorphs provide insights into the nature of the phase transition mechanisms for other oxide nanoparticles in the earths surface environment. The temperature-induced amorphization mechanism and Al-Si ordering behavior in dehydrated zeolites were investigated using 17O 2D 3QMAS NMR spectroscopy. The results demonstrated that both extent of topological disorder and chemical disorder in Na-zeolite A are affected by temperature induced amorphization. The observed structural changes in Na-zeolite A and its polymorphs with increasing temperature can provide an improved understanding of order-disorder transition mechanism of diverse aluminosilicates. The kinetics of oxygen isotope exchange between crystalline clinoenstatite (MgSiO3) and water under hydrothermal environments was explored using high-resolution solid-state 17O 2D 3QMAS NMR spectroscopies. The results suggest that crystallographically distinct oxygen site affect the isotope exchange reaction rate between magnesium silicate and aqueous fluid. The structural information and mechanistic details obtained from the current study provide insights into the structure of hydrous species and dehydration mechanisms in crystalline and amorphous silicates in diverse geological settings, highlighting usually unknown effects of particle size on the dehydration processes.