Preparation and hydrogen storage properties of nanostructured Mg2Cu alloy

We successfully synthesized Mg₂Cu alloys from the metal nanoparticles, which are produced from hydrogen plasma-metal reaction method, in two ways. One is under 0.1 MPa argon at 673 K and the other is under 4.0 MPa hydrogen at 673 K. The structure, morphology and reaction mechanism were studied. The hydrogen absorption and the pressure-composition isotherm properties of the obtained Mg₂Cu alloy under hydrogen were studied. The van’t Hoff equation and the formation enthalpy and entropy of the resulting hydride (MgH₂+MgCu₂) were obtained from the equilibrium plateau pressures of the desorption isotherms. Nanostructured Mg₂Cu shows excellent hydrogen storage properties because nanostructured materials have more surface area and more defects, which means more nucleation sites with hydrogen, and smaller particles, which means shorter diffusion distance for hydrogen in the alloys particles.     

Reaction Pathways Determined by Mechanical Milling Process for Dehydrogenation/Hydrogenation of the LiNH2/MgH2 System

The dehydrogenation/hydrogenation processes of the LiNH₂/MgH₂ (1:1) system were systematically investigated with respect to balller milling and the subsequent heating process. The reaction pathways for hydrogen desorption/absorption of the LiNH₂/MgH₂ (1:1) system were found to depend strongly on the milling duration due to the presence of two competing reactions in different stages (i.e., the reaction between Mg(NH₂)₂ and MgH₂ and that between Mg(NH₂)₂ and LiH), caused by a metathesis reaction between LiNH₂ and MgH₂, which exhibits more the nature of solid–solid reactions. The study provides us with a new approach for the design of novel hydrogen storage systems and the improvement of hydrogen‐storage performance of the amide/hydride systems.     

The Adsorption,Thermal Desorption and Photochemistry of Methyl Iodide on an Ag‐Covered TiO2(110) Surface

The thermal reactions and photochemistry of monolayer methyl iodide (CH₃I) on a silver covered TiO₂(110) surface have been studied using combinative techniques of temperature programmed desorption (TPD) and x‐ray photoelectron spectroscopy (XPS). About ? 60% of CH₃I at monolayer coverage on Ag/TiO₂(110) dissociates between 130 and 200 K yield adsorbed CH₃ and I, with the rest desorbing molecularly at a peak temperature of 200 K in a TPD study. Photochemistry of CH₃I on Ag/TiO₂(110) is wavelength dependent. Irradiation of monolayer CH₃I by 404 nm photon causes C‐I bond dissociation and CH₃ desorption. Upon 290 nm, UV irradiation, the depletion of CH₃I_(a) is dominated by photodesorption of molecular CH₃I.     

Na-coated hexagonal B36 as superior hydrogen storage materials

Based on van der Waals corrected density functional theory, we show that Na atoms acting as decoration metals are not inclined to form clusters due to a large binding energy of 3.31 eV, indicating a promising good reversible hydrogen storage. Both the polarization mechanism and the orbital hybridizations contribute to the adsorption of hydrogen molecules (storage capacity of 4.4 wt%) with optimal adsorption energy of 0.25 eV/H₂. Additionally, the dimerization of these isolated B₃₆ does not remarkably affect the number of adsorbed H₂ per Na atom. Our results may serve as a guide in the design of new hydrogen storage materials based on low-dimension boron clusters.     

Effect of titanium based complex catalyst and carbon nanotubes on hydrogen storage performance of magnesium

Mg (MgH₂)-based composites, using carbon nanotubes (CNTs) and pre-synthesized titanium based complex (TCat) as the catalysts, were prepared by high energy ball milling technique. The use of both catalysts demonstrated markedly improved the hydrogen storage performance, e.g. a significant increase of hydrogen release rate and decrease of desorption temperature. The synthesized composites can absorb almost 6 wt% of hydrogen within 3 min at 200 °C and desorb 6 wt% hydrogen in 10 min at 310 °C. The influence of CNTs and TCat on desorption temperature was also investigated by using temperature programmed desorption (TPD). The TPD results reveal that the peak desorption temperature and the onset temperature can be lowered by 109 °C and 155 °C, respectively, compared to the non-catalyzed MgH₂. The reaction enthalpy and entropy of hydrogen desorption for the synthesized MgH₂-based composites are calculated by the van’t Hoff analysis to be 73.1 kJ/mol H₂ and 130.2 J/mol H₂ K, respectively.     

The influence of preadsorbed hydrogen and CO on adsorption of ethylene on the Pd(111) surface

The coadsorption of C₂H₄ with H₂ and CO on Pd(111) has been investigated at 300 and 330 K At 300 K two forms of adsorbed ethylene coexist on the surface in the presence of ethylene gas: a molecular form desorbing as C₂H₄ at 330 K and a dissociatively adsorbed form (giving only hydrogen in desorption spectra) which is stable both in vacuum and in hydrogen at 10^?8 Torr. The molecular form seems to be a precursor state for hydrogenation and for dissociative adsorption. Both processes are controlled by the amount of coadsorbed hydrogen which in turn is controlled by CO coverage.     

Thermal-, photo- and electron-induced reactivity of hydrogen species on rutile TiO2(110) surface: Role of oxygen vacancy

The formation and reactivity of various types of hydrogen species on rutile TiO₂(110), including surface hydroxyl group, surface hydride species and bulk hydrogen species sensitively depend on the oxygen vacancy concentration and structure.     

甲烷在清洁 Pd(111) 及氧改性的 Pd(111) 表面解离的密度泛函理论研究

 采用广义梯度近似 (GGA) 的密度泛函理论 (DFT) 并结合平板模型, 研究了 CH4 在清洁 Pd(111) 及 O 改性的 Pd(111) 表面发生 C朒 键断裂的反应历程. 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能. 结果表明, CH4 采用一个 H 原子指向表面的构型在 Pd(111) 表面的顶位吸附, CH3 的最稳定的吸附位置为顶位, OH, O 和 H 的最稳定吸附位置均为面心立方. CH4 在清洁 Pd(111) 表面裂解的活化能为 0.97 eV, 低于它在 O 原子改性 (O 没有参与反应) 的 Pd(111) 表面的活化能 1.42 eV, 说明表面氧原子抑制了 CH4 中 C朒 键的断裂. 当亚表面 O 原子和表面 O 原子 (O 参与反应) 共同存在时, C朒 键断裂的活化能为 0.72 eV, 低于只有表层氧存在时的活化能 (1.43 eV), 说明亚表面的 O 原子对 CH4 分子的活化具有促进作用. CH4 在 O 原子改性的 Pd(111) 表面裂解生成 CH3 和 H, 以及生成 CH3 和 OH 的反应活化能分别为 1.42 和 1.43 eV, 说明 CH4 在 O 原子改性的 Pd(111) 表面发生这两种反应的难易程度相当.     

Pd(111), Pd(100)及Pd(110)表面H2和O2直接合成H2O2的密度泛函理论研究

采用周期性密度泛函理论研究了H₂和O₂在Pd(111),Pd(100)及Pd(110)表面上直接合成H₂O₂的反应机理,对反应的主要基元步骤进行了计算和分析.结果表明,Pd(111)表面对H₂O₂直接合成的催化选择性最好,表面原子密度较低的Pd(100)表面和Pd(110)表面上含有O-O键的表面物种解离严重,不利于H₂O₂的生成.H₂O₂的选择性与含有O-O键表面物种的O-O键能和表面物种的结合能有关.含有O-O键的表面物种在表面的结合能越大,越容易发生解离,不利于形成H₂O₂.     

Molecular rearrangement induced by hydrogen co-adsorption: C2H4 on Pd(110)

The symmetry of low-coverage (0.1 ML) ethylene adsorbed on Pd(110) and the co-adsorption effect of hydrogen have been investigated by high-resolution electron energy-loss spectroscopy. Ethylene adsorbs with π-bond character intact on both clean and H-covered Pd(110) with characteristic C–C stretching vibrational energies at 178 and 186 meV, respectively. The symmetry of the adsorbed ethylene, however, drastically changes upon the co-adsorption of hydrogen: the C–C axis which is tilted to the clean Pd(110) surface (C₁ symmetry) is rearranged such that it becomes parallel to the H-covered Pd(110) surface (C₂ symmetry).     

The mechanism of methanol formation and other reactions following formaldehyde adsorption on Ni(110)

The adsorption/desorption and reactive behavior of formaldehyde was studied on clean single-crystal Ni(110) at adsorption temperatures down to 200 °K. For low exposures of the surface to formaldehyde, hydrogen and CO binding states were populated due to decomposition of the molecule upon adsorption. Higher exposures gave rise to a decomposition-limited hydrogen peak exhibiting an activation energy of 20 kcal/gmol and an apparent frequency factor of 10¹⁴ sec^?1. At initial coverages of H₂CO exceeding about 0.5, monolayer methanol was observed to form. The formation of methanol involved a hydrogen atom transfer between two adsorbed H₂CO molecules and did not occur totally via surface hydrogen. Self-oxidation to form CO₂ was also observed. The surface exhibited reaction heterogeneity, and the surface reactivity was observed to depend on the temperature of adsorption of reactants, suggesting strong adsorbate-induced surface “reconstruction.”     

SO2 Surface Chemistry on Metal Substrates

The surface chemistry, induced by thermal and non-thermal methods, of SO₂ on metal substrates is reviewed. The substrate temperature during dosing is important; regardless of metal, adsorption is dissociative at 300 K and molecular at 100 K. On Ni, Pd, and Pt, molecular adsorption occurs through the S and one O atom, and the molecular plane is perpendicular to the surface. However, on Ag and Cu, adsorption occurs only through the S with the molecular plane perpendicular to the surface. The differences can be attributed to the structure of the metal's molecular orbitals and their interactions with the SO₂ orbitals. Upon heating, SO₂ dissociates on all transition metal surfaces with the exception of Ag, Au, and Cu, where only molecular desorption occurs. On Pt, Fe, and Pd, additional reactions are observed between SO₂ and its dissociation products. The nonthermal reactions induced by photons and electrons for monolayer coverages of SO₂ on Ag (111) are dominated by molecular desorption. Desorption cross sections for 313 nm photons and 50eV electrons were 2.8 × 10^?20 cm² and ?1 × 10^?16 cm², respectively. Nonthermal excitation mechanisms and quenching processes as well as interesting characteristics of SO₂ under irradiation are also reviewed.     

Interaction of methanol with ruthenium

The interaction of methanol with a clean (110) ruthenium surface has been studied using temperatures programmed desorption methods. Methanol dissociates upon adsorption at 300 K and yields H₂(g) and chemisorbed CO as the dominant products. Randomization of evolved hydrogen was shown to occur during methanol adsorption and also upon subsequent thermal desorption using isotopically labeled methanol, CH₃OD. In addition to hydrogen and CO, small amounts of H₂CO, CH₃OH, CO₂, and H₂O, are also observed upon thermal desorption. In contrast with a previous study of formaldehyde on Ru(110), no detectable CH₄ product is found upon methanol desorption.     

Diffusion of Formaldehyde on Rutile TiO2(110) Assisted by Surface Hydroxyl Groups

As the photo-dissociation product of methanol on the TiO₂(110) surface,the diffusion and desorption processes of formaldehyde (HCHO) were investigated by using scanning tunneling microscope (STM) and density functional theory (DFT).The molecular-level images revealed the HCHO molecules could diffuse and desorb on the surface at 80 K under UV laser irradiation.The diffusion was found to be mediated by hydrogen adatoms nearby,which were produced from photodissociation of methanol.Diffusion of HCHO was significantly decreased when there was only one H adatom near the HCHO molecule.Furthermore,single HCHO molecule adsorbed on the bare TiO₂(110) surface was quite stable,little photo-desorption was observed during laser irradiation.The mechanism of hydroxyl groups assisted diffusion of formaldehyde was also investigated using theoretical calculations.     

Identification of defect sites on MgO(100) thin films by decoration with Pd atoms and studying CO adsorption properties.

CO adsorption on Pd atoms deposited on MgO(100) thin films has been studied by means of thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. CO desorbs from the adsorbed Pd atoms at a temperature of about 250 K, which corresponds to a binding energy, E(b), of about 0.7 +/- 0.1 eV. FTIR spectra suggest that at saturation two different sites for CO adsorption exist on a single Pd atom. The vibrational frequency of the most stable, singly adsorbed CO molecule is 2055 cm(-)(1). Density functional cluster model calculations have been used to model possible defect sites at the MgO surface where the Pd atoms are likely to be adsorbed. CO/Pd complexes located at regular or low-coordinated O anions of the surface exhibit considerably stronger binding energies, E(b) = 2-2.5 eV, and larger vibrational shifts than were observed in the experiment. CO/Pd complexes located at oxygen vacancies (F or F(+) centers) are characterized by much smaller binding energies, E(b) = 0.5 +/- 0.2 or 0.7 +/- 0.2 eV, which are in agreement with the experimental value. CO/Pd complexes located at the paramagnetic F(+) centers show vibrational frequencies in closest agreement with the experimental data. These comparisons therefore suggest that the Pd atoms are mainly adsorbed at oxygen vacancies.     

水对甲醇在Rutile-TiO2(110)-(1 ×1)表面光催化解离的影响简

采用程序升温脱附方法研究了甲醇分子吸附在真空退火后的二氧化钛（110）表面的光催化过程,对比分析了单独吸附甲醇分子以及甲醇分子与水分子共吸附情况下的光催化解离过程. 结果表明,在二氧化钛（110）表面吸附的甲醇分子对共吸附水分子的光催化解离过程并没有直接的帮助作用. 共吸附状态下的水分子也同样没有影响到甲醇的光致解离过程,但是水分子的存在抑制了甲醇光解产物甲醛的光致脱附过程,同时促进了甲酸甲酯的形成.     

Identification of Hydroxyl on Ni(111)

Hydroxyl (OH) is identified and characterized on the Ni(111) surface by high‐resolution electron energy loss spectroscopy. We find clear evidence of stretching, bending, and translational modes that differ significantly from modes observed for H₂O and O on Ni(111). Hydroxyl may be produced from water by two different methods. Annealing of water co‐adsorbed with atomic oxygen at 85 K to above 170 K leads to the formation of OH with simultaneous desorption of excess water. Pure water layers treated in the same fashion show no dissociation. However, the exposure of pure water to 20 eV electrons at temperatures below 120 K produces OH in the presence of adsorbed H₂O. In combination with temperature‐programmed desorption studies, we show that the OH groups recombine between 180 and 240 K to form O and immediately desorbing H₂O. The lack of influence of co‐adsorbed H₂O at 85 K on the O? H stretching mode indicates that OH does not participate in a hydrogen‐bonding network.     

Decomposition of Nitrogen Oxides on Palladium and Angle-Resolved Measurements of Desorbing Products

Recent angle-resolved measurements of desorbing products were reviewed for decomposition of nitrogen oxides on noble metals. Two pathways for the removal of adsorbed nitrogen atoms, i.e., N(a) + NO(a) N₂O(a) N₂(g) + O(a) and 2N(a) N₂(g), were examined typically on Pd(110). The former takes place in the presence of gaseous CO and shows two-directional N₂ desorption collimated far from the surface normal in the normally directed plane along the [001] direction. The latter does not contribute in CO + NO reaction on Pd(110). The model proposed for the inclined desorption was also explained.     

Effect of ZrC Nanopowders on Enhancing the Hydro/Dehydrogenation Kinetics of MgH2 Powders

Hydrogen has been receiving great attention as an energy carrier for potential green energy applications. Hydrogen storage is one of the most crucial factors controlling the hydrogen economy and its future applications. Amongst the several options of hydrogen storage, light metal hydrides, particularly nanocrystalline magnesium hydride (MgH₂), possess attractive properties, making them desired hydrogen storage materials. The present study aimed to improve the hydrogen storage properties of MgH₂ upon doping with different concentrations of zirconium carbide (ZrC) nanopowders. Both MgH₂ and ZrC were prepared using reactive ball milling and high-energy ball milling techniques, respectively. The as-prepared MgH₂ powder was doped with ZrC (2, 5, and 7 wt%) and then high-energy-ball-milled for 25 h. During the ball milling process, ZrC powders acted as micro-milling media to reduce the MgH₂ particle size to a minimal value that could not be obtained without ZrC. The as-milled nanocomposite MgH₂/ZrC powders consisted of fine particles (~0.25 μm) with a nanosized grain structure of less than 7 nm. Besides, the ZrC agent led to the lowering of the decomposition temperature of MgH₂ to 287 °C and the reduction in its apparent activation energy of desorption to 69 kJ/mol. Moreover, the hydrogenation/dehydrogenation kinetics of the nanocomposite MgH₂/ZrC system revealed a significant improvement, as indicated by the low temperature and short time required to achieve successful uptake and release processes. This system possessed a high capability to tackle a long continuous cycle lifetime (1400 h) at low temperatures (225 °C) without showing serious degradation in its storage capacity.     

High-capacity hydrogen storage of magnesium-decorated boron fullerene

By theoretical analysis, we have explored the feasibility of functionalizing boron fullerene (B₈₀) by adsorbing Mg atoms for the application as hydrogen storage nanomaterials. Our results show that due to the charge transfer from Mg to B atoms Mg atoms reside above the pentagonal faces of the B₈₀ cage. The electric field induced around the positive charged Mg atoms polarizes H₂ molecules, and the resulting binding is strong enough to adsorb H₂ without dissociation. Further calculations indicated that the 12Mg-decorated-B₈₀ has a high hydrogen storage capacity storing up to 96 H₂ molecules with an ideal binding energy of 0.20 eV/H₂ according to the approximation of GGA and 0.5 eV/H₂ according to LDA, corresponding to a hydrogen uptake of 14.2%. This suggested a possible method of engineering new structure for high-capacity hydrogen storage materials with the reversible adsorption and desorption of hydrogen molecules.