Science Thrust 2: Interfacial Structure and Reactivity
Lead: Kevin Rosso
Goal: The research proposed for Science Thrust 2 (ST2) is focused on understanding chemical transformations that lead to the precipitation of solids such as gibbsite, boehmite, and aluminate salt hydrates, as well as the dynamics of processes occurring at interfaces with low-water, highly alkaline solutions.
Task 2.1: Identify the nucleation and precipitation processes driving interface formation under water-limited conditions.
ST2 is steadily unraveling mechanisms governing nucleation and growth during precipitation and crystallization of aluminum (oxy)hydroxide and alkaline aluminate salts shown in the phase diagram in Figure 1. Recent work has focused on identifying and understanding the roles of metastable intermediates that emerge during crystallization of aluminum-bearing phases in acidic, circumneutral, and alkaline conditions. Under acidic to circumneutral conditions, relict oligomeric aluminum hydroxide clusters, where the aluminum coordination is six-coordinate, were recently identified as inclusion defects in gibbsite nanoplatelets through X-ray scattering analysis, with the molecular structure of the cluster constrained by supporting magic angle spinning nuclear magnetic resonance spectroscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy. The retention of liquid-state motifs of aluminum oligomer motifs was observed in alkaline conditions during the crystallization of sodium aluminate hydrates under alkaline conditions. That is, the sodium mu-oxo aluminate dimer was observed for the first time with magic angle spinning nuclear magnetic resonance spectroscopy.
Ongoing research is building on these observations to probe for similar intermediates during aluminum hydroxide crystallization mechanisms in alkaline conditions. An aspect of this exploration is examining the effect of varying alkaline cations in the MOH solutions from which the aluminum hydroxide precipitates, where M+ = Li+, Na+, K+, Rb+ , and Cs+, to understand how nucleation may depend on water availability controlled in part by the hydration dynamics of the alkali cation. These studies will build upon recent work tracking the sensitivity of transformations between potassium aluminate salts and aluminum hydroxide in the form of gibbsite or dawsonite to the partial pressure of carbon dioxide, as well as on studies investigating the variation of the crystalline phase between gibbsite and the potassium mu-oxo aluminate dimer related to variations in system composition. These studies are intrinsically linked to those in IDREAM Science Thrust 1 (ST1), where the effects of cation identity on solution organization are also under investigation.
Part of understanding aluminum coordination change mechanisms at interfaces includes examining dissolution of aluminum oxyhydroxides. ST2 is exploiting high-resolution atomic force microscopy and high-precision synthetic gibbsite nanoplatelets (Cross-Cutting Theme 3 [XC3]: Synthesis and Materials Characterization) to probe dissolution mechanisms in situ at the atomic scale in highly alkaline solutions, complemented by computational molecular simulations (Cross-Cutting Theme 2 [XC2]: New Computational Tools and Theory) (Figure 2). In situ atomic force microscopy (AFM) on the basal (001) surface shows single-layer etch pit nucleation and growth defined by near simultaneous retreat of (100) and (110) step edges in aqueous NaOH solutions, suggesting similar aluminate detachment mechanisms at these two crystallographically distinct step edges. In our first generation of molecular simulations, we mapped the free-energy landscape of aluminate detachment from (110) step edges and found that the overall reaction combines kink formation and kink propagation. Two individual reactions were found to be rate-limiting for kink formation, that is, the displacement of Al from a step site to a ledge adatom site and its detachment from ledge/terrace adatom sites into the solution. As a result, a pool of mobile and labile adsorbed species, or adatoms, exists before the release of Al into solution. Because of the quasi-hexagonal symmetry of gibbsite, kink site propagation can occur in multiple directions at similar rates. Ongoing molecular simulation work will examine both (100) and (110) step edges to both test this hypothesis and also attempt quantitative agreement in predicted etch pit opening rates compared to those observed by AFM.
In addition to these activities, work is also underway to expand upon recent studies that discovered Cr(III) cluster formation during sorption on boehmite and a consequential decrease in boehmite dissolution kinetics. In addition to the presence of trivalent cations, changes in the solubility of aluminum hydroxides and other salts due to dissolved nitrogen oxyanions, such as nitrite and nitrate, are under investigation to better understand how ion–ion interactions in mixed electrolytes influence solubilities. Such work begins to approach the complexity of high-level nuclear waste in real-world scenarios, where processing is not only limited by the immense compositional variation in the stored waste but also by the presence of radiation fields.
Task 2.2: Quantify the impact of ionizing radiation in driving nucleation, growth, and dissolution.
Studies of the effects of ionizing radiation in conjunction with Cross-Cutting Theme 1 (XC1) on nucleation, growth, and dissolution have begun to unveil the molecular-level basis for how radiation-driven changes at interfaces alter phase stability and reactivity. X-ray, energetic photon, and electron irradiation can ionize and electronically excite target atoms and molecules. These excitations undergo complicated relaxation and energy-transfer processes that ultimately determine the manifold system responses to the deposited excess energy. For example, in weakly bound gas- and solution-phase samples, intermolecular Coulomb decay (ICD) and electron-transfer-mediated decay (ETMD) can occur with neighboring atoms or molecules, leading to efficient transfer of the excess energy to the surroundings. In ionic solids such as metal oxides, intra- and interatomic Auger decay produces localized final states that lead to lattice damage and typically the removal of cations from the substrate. The relative importance of Auger-stimulated damage (ASD) versus ICD and ETMD at microsolvated nanoparticle interfaces is not well known and is an important aspect of ST2 research.
In our studies of electron bombardment of microsolvated boehmite, essentially no lattice damage resulting from the ionization and electronic excitation could be detected. Instead, efficient energy transfer and soft ionization of interfacial water molecules was observed. This is likely a general phenomenon at gas– oxyhydroxide nanoparticle interfaces where the density of states of the ionized chemisorbed species significantly overlaps with the core hole states of the solid. To examine this process in more detail, ST2 and XC1 studied the production and release of H2 and O2 during and after electron (100–1000 eV) irradiation of boehmite nanoplatelets. H2 and O2 produced during irradiation likely correspond to electron-induced dissociation of the hydroxyls primarily in the terminal surface layers of the boehmite followed by combinative desorption and abstraction reactions. Post-irradiation temperature-programmed desorption shows a temperature-dependent release of H2 but not O2 that likely involves diffusion and recombination of hydrogen atoms initially trapped between the boehmite layers followed by recombination at edge and surface sites. H2 production upon thermal annealing irradiated material shows no dependence on flux, suggesting the precursor hydrogen atom species trapped within the bulk are long lived.
In contrast, ST2 studies using gamma irradiation indicate that trapped electrons and related O-centers are major radiolytic products. Gibbsite powders and their thermally dehydrated forms were rehydrated and irradiated with gamma rays to examine the effects of the collapse of the crystalline gibbsite structure as it is converted to alumina on the transport of precursors of molecular hydrogen to the surface. The amount of hydrogen gas production from irradiated gibbsite and its transition phases with adsorbed water increased substantially with increasing temperature of dehydration of the samples. H2 production for samples without water was considerably lower than the hydrated samples, signifying the importance of surface water and the transport of precursors to the surface. Electron paramagnetic resonance spectroscopy showed that, in this case, the major radiolytic products are trapped electrons and related O-centers.
Given the importance of water at interfaces to radiolysis, significant investments in the characterization of hydrated interfaces with vibrational sum-frequency-generation spectroscopy have provided interface-specific information on how structural hydroxyl groups and adsorbed water are modified by ionizing radiation (Figure 2). Gamma irradiation causes substantial scission of surface hydroxyl groups both on gibbsite and boehmite basal surfaces. Intriguingly, rehydration of gamma-irradiated boehmite and gibbsite interfaces was observed to recover the hydroxyl density on gibbsite but not boehmite surfaces. Additional characterization of the gibbsite and boehmite nanoparticles using electron paramagnetic spectroscopy indicates that radiolysis products such as oxygen radicals and/or F-centers in gibbsite and boehmite persist over many months. This work characterizing the interface in radiation fields is foundational to studies underway to quantify the effects of radiation on dissolution and precipitation processes via a novel RadAFM equipped for in situ irradiation of solid–liquid interfaces by X-rays. Given the complexity of dissolution and precipitation processes in the absence or presence of radiation, a key component of the strategy is to guide the analysis of experimental observations with XC2 MD simulations, as described in the cross-cutting themes.