Hydrogen adsorption and phase transitions in fullerite

Hydrogen desorption and adsorption properties of the fullerene materials C60, C70, and fullerite (a mixture of C60 and C70) were measured volumetrically using a Sievert's apparatus. Over several cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite samples increased from an initial value of 0.4 wt % for the first cycle to a capacity of 4.4 wt % for the fourth cycle. Correspondingly, the surface area of this sample increased from 0.9 to 11 m^2/g, and there were changes in its x-ray powder diffraction pattern. In comparison, two other fullerite samples, prepared by a different procedure showed no such behavior. Pure C60 and pure C70 were also cycled and exhibited small and constant capacities of 0.7 and 0.33 wt %, respectively, as a function of number of cycles. The enhanced storage capacity of fullerite material is tentatively attributed to the presence of C60 oxide

Pore size distribution and supercritical hydrogen adsorption in activated carbon fibers

Pore size distributions (PSD) and supercritical H_2 isotherms have been measured for two activated carbon fiber (ACF) samples. The surface area and the PSD both depend on the degree of activation to which the ACF has been exposed. The low-surface-area ACF has a narrow PSD centered at 0.5 nm, while the high-surface-area ACF has a broad distribution of pore widths between 0.5 and 2 nm. The H_2 adsorption enthalpy in the zero-coverage limit depends on the relative abundance of the smallest pores relative to the larger pores. Measurements of the H_2 isosteric adsorption enthalpy indicate the presence of energy heterogeneity in both ACF samples. Additional measurements on a microporous, coconut-derived activated carbon are presented for reference

Kinetics and mechanism of formic acid decomposition on Ru(001)

The steady-state rate of decomposition of formic acid on Ru(001) has been measured as a function of surface temperature, parametric in the pressure of formic acid. The products of the decomposition reaction are C0_2, H_2, CO, and H_2)0, i.e., both dehydrogenation and dehydration occur on Ru (001). A similar product distribution has been observed on Ni(110), Ni(100), Ru(100), Fe(100), and Ni(111) surfaces; whereas only dehydrogenation to C0_2 and H_2 occurs on the Cu(100), Cu(110), and Pt(111) surfaces. Only reversible adsorption and desorption of formic acid is observed on the less reactive Ag(110) surface at low temperatures, whereas the more reactive Mo(100) surface is oxidized by formic acid at low temperatures with the products of this reaction being H_2, CO, and H_(2)O (Ref. 10). We report here the confirmation of earlier observations of the occurrence of both dehydrogenation and dehydration of formic acid on Ru(001), and more importantly, we provide a detailed mechanistic description of the steady-state decomposition reaction on this surface in terms of elementary steps

Stabilizing the surface morphology of Si1–x–yGexCy/Si heterostructures grown by molecular beam epitaxy through the use of a silicon-carbide source

Si1–x–yGexCy/Si superlattices were grown by solid-source molecular beam epitaxy using silicon carbide as a source of C. Samples consisting of alternating layers of nominally 25 nm Si1–x–yGexCy and 35 nm Si for 10 periods were characterized by high-resolution x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry to determine strain, thickness, and composition. C resonance backscattering and secondary ion mass spectrometries were used to measure the total C concentration in the Si1–x–yGexCy layers, allowing for an accurate determination of the substitutional C fraction to be made as a function of growth rate for fixed Ge and substitutional C compositions. For C concentrations close to 1%, high-quality layers were obtained without the use of Sb-surfactant mediation. These samples were found to be structurally perfect to a level consistent with cross-sectional TEM (< 10^7 defects/cm^2) and showed considerably improved homogeneity as compared with similar structures grown using graphite as the source for C. For higher Ge and C concentrations, Sb-surfactant mediation was found to be required to stabilize the surface morphology. The maximum value of substitutional C concentration, above which excessive generation of stacking fault defects caused polycrystalline and/or amorphous growth, was found to be approximately 2.4% in samples containing between 25 and 30% Ge. The fraction of substitutional C was found to decrease from roughly 60% by a factor of 0.86 as the Si1–x–yGexCy growth rate increased from 0.1 to 1.0 nm/s

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