Mechanistic Studies of Heterogeneously Catalyzed Reactions of Ammonia and Acetic Acid on Platinum Surfaces

The design and operation of a versatile microreactor capable of studying the rates of both steady-state and batch heterogeneous reactions on a wire, a foil or a single crystalline surface at pressures between 10-7 and 1000 Torr are described. The residence time distribution of the microreactor was characterized in order to evaluate the validity of using the continuous stirred tank reactor approximation to calculate reaction rates. Absolute reaction rates (i.e. the rate-per-unit catalyst surface area) have been measured for both the catalytic decomposition of NH3 and ND3 and the NH3 + D2 exchange reaction over a polycrystalline platinum wire. The pressure was varied between 5 x 10-7 and 0.5 Torr, and the temperature ranged from 400 to 1200 K. At relatively low pressures and/or high temperatures, the order of the decomposition reaction is unity with respect to ammonia, and the reaction rate is dictated by a competition between the surface reaction and the desorption of molecularly adsorbed ammonia. Under these conditions a primary isotope effect was observed for the decomposition of ND3. At relatively high pressures and/or low temperatures, the reaction rate is independent of ammonia pressure, and the recombinative desorption of nitrogen controls the rate of ammonia decomposition. The measured kinetics of the NH3 + D2 exchange reaction were employed together with adsorption-desorption parameters of NH3, N2 and H2 to develop a mechanistic model that describes the reaction rate over the entire (wide) range of conditions studied. Steady-state absolute reaction rates are reported also for the catalytic decomposition of NH3 on the Pt(110)-(1x2) single crystalline surface at pressures between 1 x 10-6 and 2.6 x 10-6 Torr and at temperatures between 400 and 1000 K. Qualitatively, the kinetics is similar to those observed for ammonia decomposition on the polycrystalline platinum surface. Thermal desorption measurements conducted during the steady-state decomposition reaction demonstrate directly that nitrogen adatoms are the predominant surface species, and that the recombinative desorption of nitrogen is the major elementary reaction that produces molecular nitrogen. The decomposition of CH313COOH at 7 x 10-4 Torr on a polycrystalline platinum wire at temperatures between 300 and 900 K was examined in the microreactor. The major reaction products on the initially clean surface are 13CO, CO, 13CO2, H2 and adsorbed carbon-12. The adsorbed carbon accumulates on the surface until the reactions that produce these products are poisoned by the graphitic overlayer that is formed. On the graphitized platinum surface, acetic acid dehydrates catalytically to ketene and water. The relative quantities of 13CO and 13CO2 that are formed depend both on the surface temperature and on the surface carbon coverage. The catalytic dehydration of acetic acid to ketene was investigated over a graphitized polycrystalline platinum surface at pressures between 8 x 10-7 and 7 x 10-4 Torr and temperatures between 500 and 800 K. Steady-state absolute reaction rates, thermal desorption measurements, and the reactivities of functionally related compounds suggest that the reaction proceeds via an irreversibly adsorbed intermediate, which is formed by dissociation of the oxygen-hydrogen bond of acetic acid. For temperatures below 540 K at pressures of 3.5 x 10-4 Torr and above, the rate of decomposition of the surface intermediate controls the overall rate of the reaction. At 675 K or above for the entire range of pressures studied, the rate of dehydration is determined by a competition between the rates of desorption and surface reaction of molecularly adsorbed acetic acid.</p

Thermodynamically Tuned Nanophase Materials for reversible Hydrogen storage

This program was devoted to significantly extending the limits of hydrogen storage technology for practical transportation applications. To meet the hydrogen capacity goals set forth by the DOE, solid-state materials consisting of light elements were developed. Many light element compounds are known that have high capacities. However, most of these materials are thermodynamically too stable, and they release and store hydrogen much too slowly for practical use. In this project we developed new light element chemical systems that have high hydrogen capacities while also having suitable thermodynamic properties. In addition, we developed methods for increasing the rates of hydrogen exchange in these new materials. The program has significantly advanced (1) the application of combined hydride systems for tuning thermodynamic properties and (2) the use of nanoengineering for improving hydrogen exchange. For example, we found that our strategy for thermodynamic tuning allows both entropy and enthalpy to be favorably adjusted. In addition, we demonstrated that using porous supports as scaffolds to confine hydride materials to nanoscale dimensions could improve rates of hydrogen exchange by &gt; 50x. Although a hydrogen storage material meeting the requirements for commercial development was not achieved, this program has provided foundation and direction for future efforts. More broadly, nanoconfinment using scaffolds has application in other energy storage technologies including batteries and supercapacitors. The overall goal of this program was to develop a safe and cost-effective nanostructured light-element hydride material that overcomes the thermodynamic and kinetic barriers to hydrogen reaction and diffusion in current materials and thereby achieve &gt; 6 weight percent hydrogen capacity at temperatures and equilibrium pressures consistent with DOE target values

Hydrogenation of Magnesium Nickel Boride for Reversible Hydrogen Storage

We report that a ternary magnesium nickel boride (MgNi_(2.5)B_2) mixed with LiH and MgH_2 can be hydrogenated reversibly forming LiBH_4 and Mg_2NiH_4 at temperatures below 300 °C. The ternary boride was prepared by sintering a mechanically milled mixture of MgB_2 and Ni precursors at 975 °C under inert atmosphere. Hydrogenation of the ternary, milled with LiH and MgH_2, was performed under 100 to 160 bar H_2 at temperatures up to 350 °C. Analysis using X-ray diffraction, Fourier transform infrared, and ^(11)B magic angle spinning NMR confirmed that the ternary boride was hydrogenated forming borohydride anions. The reaction was reversible with hydrogenation kinetics that improved over three cycles. This work suggests that there may be other ternary or higher order boride phases useful for reversible hydrogen storage

Neutron Vibrational Spectroscopy and First-Principles Calculations of the Ternary Hydrides Li\u3csub\u3e4\u3c/sub\u3eSi\u3csub\u3e2\u3c/sub\u3eH(D) and Li\u3csub\u3e4\u3c/sub\u3eGe\u3csub\u3e2\u3c/sub\u3eH(D): Electronic Structure and Lattice Dynamics

Using combined neutron spectroscopy and first-principles calculations, we investigated the electronic structure and vibrational dynamics of the recently discovered class of ternary hydrides Li4Tt2H (Tt=Si and Ge). In these compounds, all hydrogen atoms are located in a single type of Li6-defined octahedral site. The Tt atoms form long-range Tt-Tt chains sandwiched between each Li6-octahedra layer. The Li-H interactions are strongly ionic, with bond lengths comparable to those in LiH. Our density functional theory calculations indicate that Li atoms transfer their electrons to both H and Tt atoms. Tt atoms within the Tt-Tt chain are bonded covalently. The electronic density of states reveals that both hydrides exhibit metallic behavior. The observed vibrational spectra of these hydrides are in good overall agreement with the calculated phonon modes. There is evidence of dispersion induced splitting in the optical phonon peaks that can be ascribed to the coupling of H vibrations within the Li6-octahedra layers

Zeolite-Templated Carbon Materials for High-Pressure Hydrogen Storage

Zeolite-templated carbon (ZTC) materials were synthesized, characterized, and evaluated as potential hydrogen storage materials between 77 and 298 K up to 30 MPa. Successful synthesis of high template fidelity ZTCs was confirmed by X-ray diffraction and nitrogen adsorption at 77 K; BET surface areas up to ~3600 mT2 g^(–1) were achieved. Equilibrium hydrogen adsorption capacity in ZTCs is higher than all other materials studied, including superactivated carbon MSC-30. The ZTCs showed a maximum in Gibbs surface excess uptake of 28.6 mmol g–1 (5.5 wt %) at 77 K, with hydrogen uptake capacity at 300 K linearly proportional to BET surface area: 2.3 mmol g^(–1) (0.46 wt %) uptake per 1000 m^2 g^(–1) at 30 MPa. This is the same trend as for other carbonaceous materials, implying that the nature of high-pressure adsorption in ZTCs is not unique despite their narrow microporosity and significantly lower skeletal densities. Isoexcess enthalpies of adsorption are calculated between 77 and 298 K and found to be 6.5–6.6 kJ mol^(–1) in the Henry’s law limit

Neutron vibrational spectroscopy and first-principles calculations of the ternary hydrides Li4Si2H(D) and Li4Ge2H(D): Electronic structure and lattice dynamics

Using combined neutron spectroscopy and first-principles calculations, we investigated the electronic structure and vibrational dynamics of the recently discovered class of ternary hydrides Li4Tt2H (Tt=Si and Ge). In these compounds, all hydrogen atoms are located in a single type of Li6-defined octahedral site. The Tt atoms form long-range Tt-Tt chains sandwiched between each Li6-octahedra layer. The Li-H interactions are strongly ionic, with bond lengths comparable to those in LiH. Our density functional theory calculations indicate that Li atoms transfer their electrons to both H and Tt atoms. Tt atoms within the Tt-Tt chain are bonded covalently. The electronic density of states reveals that both hydrides exhibit metallic behavior. The observed vibrational spectra of these hydrides are in good overall agreement with the calculated phonon modes. There is evidence of dispersion induced splitting in the optical phonon peaks that can be ascribed to the coupling of H vibrations within the Li6-octahedra layers

Electrolyte-Assisted Hydrogen Storage Reactions

Use of electrolytes, in the form of LiBH_4/KBH_4 and LiI/KI/CsI eutectics, is shown to significantly improve (by more than a factor of 10) both the dehydrogenation and full rehydrogenation of the MgH_2/Sn destabilized hydride system and the hydrogenation of MgB_2 to Mg(BH_4)_2. The improvement revealed that interparticle transport of atoms heavier than hydrogen can be an important rate-limiting step during hydrogen cycling in hydrogen storage materials consisting of multiple phases in powder form. Electrolytes enable solubilizing heavy ions into a liquid environment and thereby facilitate the reaction over full surface areas of interacting particles. The examples presented suggest that use of electrolytes in the form of eutectics, ionic liquids, or solvents containing dissolved salts may be generally applicable for increasing reaction rates in complex and destabilized hydride materials

Electrolyte-Assisted Hydrogen Storage Reactions

Use of electrolytes, in the form of LiBH_4/KBH_4 and LiI/KI/CsI eutectics, is shown to significantly improve (by more than a factor of 10) both the dehydrogenation and full rehydrogenation of the MgH_2/Sn destabilized hydride system and the hydrogenation of MgB_2 to Mg(BH_4)_2. The improvement revealed that interparticle transport of atoms heavier than hydrogen can be an important rate-limiting step during hydrogen cycling in hydrogen storage materials consisting of multiple phases in powder form. Electrolytes enable solubilizing heavy ions into a liquid environment and thereby facilitate the reaction over full surface areas of interacting particles. The examples presented suggest that use of electrolytes in the form of eutectics, ionic liquids, or solvents containing dissolved salts may be generally applicable for increasing reaction rates in complex and destabilized hydride materials

Real-time observation of hydrogen absorption by LaNi₅ with quasi-dynamic neutron tomography

The uptake of hydrogen by lanthanum pentanickel (LaNi₅) to form lanthanum nickel hydride (LaNi₅H₆) is followed with three-dimensional imaging by neutron tomography. The hydrogen absorption process is slower than the time needed for acquiring a single radiograph, about 10 s, but fast relative to the time to acquire a fully-sampled tomographic data set, about 6000 s. A novel data acquisition scheme is used with angles based upon the Greek Golden ratio, a scheme which allows considerable flexibility in post-acquisition tomography reconstruction. Even with tomographic undersampling, the granular structure for the conversion of LaNi₅ particles to LaNi₅H₆ particles is observed and visually tracked in 3D. Over the course of five sequential hydrogen uptake runs with various initial hydrogen pressures, some grains are repeatedly observed.Keywords: Neutron tomography, Hydrogen storage, Dynamic tomograph