Mark E. Bowden

Spectroscopic Studies of the Phase Transition in Ammonia Borane: Raman spectroscopy of single crystal NH3BH3 as a function of temperature from 88 to 330 K

Raman spectra of single crystal ammonia borane, NH3BH3, were recorded as a function of temperature from 88 to 300 K using Raman microscopy and a variable temperature stage. The orthorhombic to orientationally disordered tetragonal phase transition at 225 K was clearly evident from the decrease in the number of vibrational modes. However, some of the modes in the orthorhombic phase appeared to merge 10-12 K below the phase transition perhaps suggesting the presence of an intermediate phase. Factor group analysis of vibrational spectra for both orthorhombic and tetragonal phase is provided. In addition, electronic structure calculations are used to assist in the interpretation and assignment of the normal modes.

Perovskite Sr‐Doped LaCrO3 as a New p‐Type Transparent Conducting Oxide

Epitaxial La1-x Srx CrO3 deposited on SrTiO3 (001) is shown to be a p-type transparent conducting oxide with competitive figures of merit and a cubic perovskite structure, facilitating integration into oxide electronics. Holes in the Cr 3d t2g bands play a critical role in enhancing p-type conductivity, while transparency to visible light is maintained because low-lying d-d transitions arising from hole doping are dipole forbidden.

Reductive Sequestration of Pertechnetate (99TcO4-) by Nano Zerovalent Iron (nZVI) Transformed by Abiotic Sulfide

Under anoxic conditions, soluble pertechnetate (⁹⁹TcO₄⁻) can be reduced to less soluble TcO₂·nH₂O, but the oxide is highly susceptible to reoxidation. Here we investigate an alternative strategy for remediation of Tc-contaminated groundwater whereby sequestration as Tc sulfide is favored by sulfidic conditions stimulated by nano zerovalent iron (nZVI). nZVI was pre-exposed to increasing concentrations of sulfide in simulated Hanford groundwater for 24 h to mimic the onset of aquifer biotic sulfate reduction. Solid-phase characterizations of the sulfidated nZVI confirmed the formation of nanocrystalline FeS phases, but higher S/Fe ratios (>0.112) did not result in the formation of significantly more FeS. The kinetics of Tc sequestration by these materials showed faster Tc removal rates with increasing S/Fe between 0 and 0.056, but decreasing Tc removal rates with S/Fe > 0.224. The more favorable Tc removal kinetics at low S/Fe could be due to a higher affinity of TcO₄⁻ for FeS than iron oxides, and electron microscopy confirmed that the majority of the Tc was associated with FeS phases. The inhibition of Tc removal at high S/Fe appears to have been caused by excess HS(-). X-ray absorption spectroscopy revealed that as S/Fe increased, the pathway for Tc(IV) formation shifted from TcO₂·nH2₂ to Tc sulfide phases. The most substantial change of Tc speciation occurred at low S/Fe, coinciding with the rapid increase in Tc removal rate. This agreement further confirms the importance of FeS in Tc sequestration.

The diammoniate of diborane: crystal structure and hydrogen release

[(NH(3))(2)BH(2)](+)[BH(4)](-) is formed from the room temperature decomposition of NH(4)(+)BH(4)(-), via a NH(3)BH(3) intermediate. Its crystal structure has been determined and contains disordered BH(4)(-) ions in 2 distinct sites. Hydrogen release is similar to that from NH(3)BH(3) but with faster kinetics.

High capacity hydrogen storage in a hybrid ammonia borane–lithium amide material

The direct (solventless) reaction of lithium amide with ammonia borane is described and is demonstrated to form a new hybrid material approximating to LiNH2BH3NH3. This material rapidly loses 3.2 mole equivalents of hydrogen, (11.9 wt%) on heating to 250 °C, with the most rapid hydrogen release occurring at temperatures as low as 60 °C. The product was an insoluble, amorphous polymer with the general formula –(NLi–BNH2)n– which did not rehydrogenate directly on exposure to hydrogen gas pressure. The hybrid material and its decomposition products were characterised by solid state 11B NMR. Selective deuterium labelling helped to elucidate a reaction sequence for the hydrogen release using mass spectrometry of the released gases.

Thermal stability of MnBi magnetic materials

MnBi has attracted much attention in recent years due to its potential as a rare-earth-free permanent magnet material. It is unique because its coercivity increases with increasing temperature, which makes it a good hard phase material for exchange coupling nanocomposite magnets. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. MnO formation is irreversible and harmful to magnet performance. In this paper, we report our efforts toward developing MnBi permanent magnets. To date, high purity MnBi (>90%) can be routinely produced in large quantities. The produced powder exhibits 74.6 emu g(-1) saturation magnetization at room temperature with 9 T applied field. After proper alignment, the maximum energy product (BH)max of the powder reached 11.9 MGOe, and that of the sintered bulk magnet reached 7.8 MGOe at room temperature. A comprehensive study of thermal stability shows that MnBi powder is stable up to 473 K in air.

Forsterite [Mg2SiO4)] Carbonation in Wet Supercritical CO2: An in Situ High-Pressure X-ray Diffraction Study

Mechanisms controlling mineral stabilities in contact with injected supercritical fluids containing water are relatively unknown. In this paper, we discuss carbonation reactions occurring with forsterite (Mg(2)SiO(4)) exposed to variably wet supercritical CO(2) (scCO(2)). Transformation reactions were tracked by in situ high-pressure X-ray diffraction in the presence of scCO(2) containing dissolved water. Under modest pressures (90 bar) and temperatures (50 °C), scCO(2) saturated with water converted >70 wt % forsterite to a hydrated magnesium carbonate, nesquehonite (MgCO(3) · 3H(2)O), and magnesite (MgCO(3)) after 72 h. However, comparable tests with scCO(2) at only partial water saturation showed a faster carbonation rate but significantly less nesquehonite formation and no evidence of the anhydrous form (MgCO(3)). The presence and properties of a thin water film, observed by in situ infrared (IR) spectroscopy and with isotopically labeled oxygen ((18)O), appears to be critical for this silicate mineral to carbonate in low water environments. The carbonation products formed demonstrated by temperature and water-content dependence highlights the importance of these kinds of studies to enable better predictions of the long-term fate of geologically stored CO(2).

Organic Matter Remineralization Predominates Phosphorus Cycling in the Mid-Bay Sediments in the Chesapeake Bay

Chesapeake Bay, the largest and most productive estuary in the U.S., suffers from varying degrees of water quality issues fueled by both point and nonpoint nutrient sources. Restoration of the Bay is complicated by the multitude of nutrient sources, their variable inputs, and complex interaction between imported and regenerated nutrients. These complexities not only restrict formulation of effective restoration plans but also open up debates on accountability issues with nutrient loading. A detailed understanding of sediment phosphorus (P) dynamics provides information useful in identifying the exchange of dissolved constituents across the sediment-water interface as well as helps to better constrain the mechanisms and processes controlling the coupling between sediments and the overlying waters. Here we used phosphate oxygen isotope ratios (δ(18)O(P)) in concert with sediment chemistry, X-ray diffraction, and Mössbauer spectroscopy on sediments retrieved from an organic rich, sulfidic site in the mesohaline portion of the mid-Bay to identify sources and pathway of sedimentary P cycling and to infer potential feedbacks on bottom water hypoxia and surface water eutrophication. Authigenic phosphate isotope data suggest that the regeneration of inorganic P from organic matter degradation (remineralization) is the predominant, if not sole, pathway for authigenic P precipitation in the mid-Bay sediments. This indicates that the excess inorganic P generated by remineralization should have overwhelmed any pore water and/or bottom water because only a fraction of this precipitates as authigenic P. This is the first research that identifies the predominance of remineralization pathway and recycling of P within the Chesapeake Bay. Therefore, these results have significant implications on the current understanding of sediment P cycling and P exchange across the sediment-water interface in the Bay, particularly in terms of the sources and pathways of P that sustain hypoxia and may potentially support phytoplankton growth in the surface water.

Kinetic and thermodynamic investigation of hydrogen release from ethane 1,2-di-amineborane

The thermodynamics and kinetics of hydrogen (H2) release from ethane 1,2-di-amineborane (EDAB, BH3NH2CH2CH2NH2BH3) were measured using Calvet and differential scanning calorimetry (DSC), pressure-composition isotherms, and volumetric gas-burette experiments. The results presented here indicate that EDAB releases ∼ 10 wt.% H2 at temperatures ranging from 100 °C to 200 °C in two moderately exothermic steps, approximately −10 ± 1 kJ mol−1 H2 and −3.8 ± 1 kJ mol−1 H2. Isothermal kinetic analysis shows that EDAB is more stable than ammonia borane (AB) at temperatures lower than 100 °C; however, the rates of hydrogen release are faster for EDAB than for AB at temperatures higher than 120 °C. In addition, no volatile impurities in the H2 released by EDAB were detected by mass spectrometry upon heating with 1 °C min−1 to 200 °C in a calorimeter.

Low viscosity alkanolguanidine and alkanolamidine liquids for CO2 capture

Global carbon dioxide (CO2) emission is expected to increase tremendously with the shift to coal-powered plants for energy generation. Capture and sequestration of CO2 are needed to mitigate environmental effects. Solvents currently used for this are the energy-intensive aqueous amines. Here we present 10 advanced solvents called alkanolguanidines and alkanolamidines that are potentially energy-efficient CO2-capture solvents. These solvents were synthesized in 1–3 steps from commercially available materials. One alkanolamidine derived from a 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) base core has a low vapor pressure and a high viscosity, resulting in low CO2 uptake capacity at standard temperature and pressure (STP). Three imidazoline base derived alkanolamidines were non-viscous but do not bind CO2 at STP, however, under mild pressure they effectively capture 7–10 wt%, making them suitable for high-pressure CO2 capture. Six novel alkanolguanidine molecules have low vapor pressure and low viscosity (<10 cP) which enable high CO2 uptake at STP. These compounds bind CO2 chemically via the alcohol moiety forming zwitterionic guanidinium and amidinium alkylcarbonate ionic liquids. These materials can be regenerated thermally by heating the alkylcarbonate to 75 °C. CO2 binding capacities of up to 12 wt% were achieved using several of these compounds at STP. Through this study we found that alkanolguanidines have low viscosity, are non-volatile, have high CO2 uptake at STP and are tolerant to water; thus we selected one compound for physical property testing.