Investigations of adsorption sites on oxide surfaces using solid-state NMR and TPD-IGC

The number and chemical identity of reactive sites on surfaces of glass affects the processing, reliability, and lifetime of a number of important commercial products. Surface site densities, distributions, and structural identities are closely tied to the formation and processing of the glass surface, and exert a direct influence on strength and coating performance. The surface of a glass sample may vary markedly from the composition and chemistry of the bulk glass. We are taking a physicochemical approach to understanding adsorption sites on pristine multicomponent glass fibers surfaces, directly addressing the effect of processing on surface reactivity. This project aimed to understand the energy distributions of surface adsorption sites, the chemical/structural identity of those sites, and the relationship of these glasses to glass composition, thermal history, and in future work, surface coatings.;We have studied the bulk and surface structure as well as the surface reactivity of the glass fibers with solid-state nuclear magnetic resonance (NMR) spectroscopy, inverse gas chromatography (IGC), and computational chemistry methods. These methods, solid-state NMR and IGC, typically require high surface area materials; however, by using probe molecules for NMR experiments or packing a column at high density for IGC measurements, lower surface area materials, such as glass fibers, can be investigated. The glasses used within this study were chosen as representative specimens of fibers with potentially different reactive sites on their surfaces. The two glass compositions were centered around a nominal E-glass, which contains very little alkali cations and mainly alkaline earth cations, and wool glass, which contains an abundance of alkali cations. The concentration of boron was varied from 0 to 8 mole % in both fiber compositions. Fibers were drawn from each composition at a variety of temperatures and draw speeds to provide a range of glass samples with varying diameters and thermal histories.;The bulk structural features in both compositions of glass fibers were identified using high-resolution 29Si, 27Al, and 11B magic-angle spinning (MAS) NMR spectroscopic measurements. In multi-component glasses, the determination of silicon, aluminum, and boron distributions becomes difficult due to the competitive nature of the network-modifying oxides among the network-forming oxides. In pure silicates, 29Si MAS NMR can often resolve resonances arising from silicate tetrahedron having varying numbers of bridging oxygens. In aluminoborosilicate glasses, aluminum is present in four-, five-, and six- coordination with oxygen as neighbors. The speciation of the aluminum can be determined using 27Al MAS NMR. The fraction of tetrahedral boron species in the glass fibers were measured using 11B MAS NMR, which is typically used to study the short-range structure of borate containing glasses such as alkali borate, borosilicate, and aluminoborosilicate glasses.;While solid-state NMR is a powerful tool for elucidating bonding environments and coordination changes in the glass structure, it cannot quantitatively probe low to moderate surface area samples due to insufficient spins. Chemical probes either physisorbed or chemisorbed to the fiber's surface can increase the surface selectivity of NMR for analysis of samples with low surface areas and provide information about the local molecular structure of the reactive surface site. Common chemical probe molecules contain NMR active nuclei such as 19F or may be enriched with 13C. A silyating agent, (3,3,3-trifluoropropyl)dimethylchlorosilane (TFS), reacts with reactive surface hydroxyls, which can be quantified by utilizing the NMR active nucleus (19F) contained in the probe molecule. The observed 19F MAS NMR peak area is integrated and compared against a standard of known fluorine spins (concentration), allowing the number of reactive hydroxyl sites to be quantified.;IGC is a method used to study the surface properties of a material by examining the retention behavior of a probe molecule. The IGC method provides information on the surface chemistry of a wide range of samples based on high sensitivity, linearity, and stability of flame ionization detectors (FIDs) employed in IGC, but especially to probe and characterize the distribution of both weak and very strong adsorption sites on oxide surfaces. The drawback to IGC is the inability to identify the surface species reacting with the probe molecule. By adsorbing gas phase 13C labeled ethanol onto glass fiber surfaces and analyzing with 13C cross-polarization (CP) MAS NMR, the structural identity of the adsorption sites can be determined based upon chemical shifts of the carbon nuclei. The 13C chemical shifts are indicative of chemisorption and physisorption, and in the case of chemisorption, the next nearest neighbor atoms can be identified. The glass fiber surfaces exhibited three distant peaks in TPD-IGC plots and an extra peak in the 13C CP MAS NMR spectra and when adsorbing an alcohol to the surface.;A simpler model system assisted with the identification of two of the three/four peaks from the multicomponent glass fibers. The model system chosen was fumed silica, which contains only silicon sites. Alcohol was adsorbed to the surface of fumed silica and IGC and 13C CP MAS NMR experiments were performed. In each of the experiments on fumed silica, two peaks were present. One peak corresponded to a weakly adsorbed alcohol, since the desorption temperature was low and had a narrow linewidth in the NMR spectrum. The second peak corresponded to a chemisorbed alcohol, which was identified by the higher desorption temperature and the broad peak in the NMR spectrum. The desorption temperatures and the chemical shift values from the reactive sites on fumed silica relate to two of the peaks in the multicomponent experiments and are identified as a physisorbed alcohol and a chemisorbed alcohol bound to a silicon site. The third and fourth peaks (middle temperature range in TPD-IGC) present in the 13C CP MAS NMR spectra were identified through ab initio chemical shift calculations using model oxide clusters that underwent condensation reactions with ethanol. The third and fourth reactive sites were identified as a chemisorbed alcohol to a three-coordinated and four-coordinated boron sites, respectively.;This study investigated the bulk structural distributions, as well as investigating reactive sites via desorption plots using IGC methods, interactions with NMR sensitive probe molecules, ab initio chemical shift calculations, and density functional theory reaction pathways. Multicomponent fibers containing 8 mole % boron indicated the presence of four distant reactive sites: physisorbed alcohol, chemisorbed alcohol to tetrahedral silicon species, chemisorbed alcohol to three-coordinated boron species, and chemisorbed alcohol to a four-coordinated boron species.