High-temperature Desorption Research Articles

Nano‐structured MgH2 catalyzed by TiC nanoparticles for hydrogen storage

AbstractBACKGROUND: Magnesium hydride is considered to be a promising hydrogen storage material because of its high gravimetric and volumetric storage capacities. However, its slow kinetics and high desorption temperature of > 300 °C limit practical applications. In this work, TiC nanoparticles were selected to modify the hydrogen storage properties of MgH2. Composite mixtures (MgH2 + TiC) were prepared using both cryogenic milling and high‐energy ball milling.RESULTS: The resulting morphology and crystallite structure of the composites were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The milled samples show good mixing of the hydride and carbide particles, with MgH2 particles around 0.09–1 µm and TiC particles 10–20 nm. The (MgH2 + TiC) composites consist of γ‐MgH2, β‐MgH2 and TiC. MgH2 nano‐crystallites of 25 nm were formed after cryomilling. Thermogravimetry reveals that the composites release ∼6.5 mass % hydrogen from 190–400 °C at a heating rate of 10 °C min−1 under He flow, with the onset and peak temperatures at 190 and 280 °C, respectively, for the (MgH2 + TiC) after 8 h cryomilling and 60 h ball milling.CONCLUSION: Results indicate that TiC is an effective catalyst for hydrogen desorption of MgH2. Copyright © 2010 Society of Chemical Industry

Method for measuring chemical diffusion coefficients in solids

Experiments were performed with temperature programmed desorption of hydrogen and deuterium adsorbates on small platinum spheres. Beyond the expected desorption peak of these adsorbates at around 300 K sample temperature an additional desorption peak at higher temperatures was observed. This additional peak is explained by the diffusion of hydrogen or deuterium atoms from the inside of the spheres to their surfaces with final desorption from these surfaces. The visibility of this second high temperature desorption peak is supported by a small diameter of the platinum spheres. Platinum spheres with diameters around 64 μm were used. The sample temperature at which the second peak was observed depends on the parameters: diameter of the platinum spheres, heating rate of the sample and chemical diffusion coefficient of hydrogen or deuterium in platinum. A theory, which assumes that the chemical diffusion coefficient can be described with an Arrhenius ansatz, was developed to simulate the occurrence of the second peak. The combination of these kinds of experiments with the theory gives a method to measure chemical diffusion coefficients. This method can be called temperature programmed diffusion. At 510 K sample temperature the diffusion coefficient 1.61×10−12 m2/s of hydrogen in platinum and the diffusion coefficient 1.40×10−12 m2/s of deuterium in platinum was measured.

High-temperature absolute hydrogen desorption kinetics of zirconium hydride under clean and oxidized surface conditions

High-temperature absolute hydrogen desorption kinetics of zirconium hydride specimens with H/Zr ratio from 1.7 to 0 with no hydrogen backpressure for oxidized and clean surface conditions were discussed. The temperature under study was in the range of 750–820 °C in order to create a hypothetical reactor accident condition manageable by the experimental setup. For the case of the clean surface state, hydrogen release was so rapid that most of hydrogen had escaped from zirconium hydride specimens over the course of only 10 min in the temperature range of ∼600–700 °C. For the case of the oxidized surface condition, the release rate was much slower and exhibited a strange and unexplainable linear fashion regardless of the temperature and H/Zr ratio. The zeroth-order absolute dehydriding rate was determined with an activation energy of 313 ± 17 kJ mol −1.

Dissociation of water on oxygen-covered Rh{111}

The adsorption of water and coadsorption with oxygen on Rh{111} under ultrahigh vacuum conditions was studied using synchrotron-based photoemission and photoabsorption spectroscopy. Water adsorbs intact on the clean surface at temperatures below 154 K. Irradiation with x-rays, however, induces fast dissociation and the formation of a mixed OH+H(2)O layer indicating that the partially dissociated layer is thermodynamically more stable. Coadsorption of water and oxygen at a coverage below 0.3 monolayers has a similar effect, leading to the formation of a hydrogen-bonded network of water and hydroxyl molecules at a ratio of 3:2. The partially dissociated layers are more stable than chemisorbed intact water with the maximum desorption temperatures up to 30 K higher. For higher oxygen coverage, up to 0.5 monolayers, water does not dissociate and an intact water species is observed above 160 K, which is characterized by an O 1s binding energy 0.6 eV higher than that of chemisorbed water and a high desorption temperature similar to the partially dissociated layer. The extra stabilization is most likely due to hydrogen bonds with atomic oxygen.

Temperature programmed desorption of coal gases – Chemical and carbon isotope composition

Fresh and stored coals from the United States (New Mexico, Colorado and Wyoming) and Czech Republic (North Bohemian and Upper Silesian Basins) were studied by the method of temperature programmed desorption. Desorbed gases were analyzed for their chemical and carbon isotope composition. Upon heating from room temperature with a constant rate of 40 °C/min, two desorption phases were observed: low temperature desorption of CH 4 and CO 2 with a maximum intensity between 50 and 80 °C and high temperature desorption of CO 2 only between 150 and 210 °C. The desorption of (residual) primary coalbed gas was compared with the desorption of re-adsorbed gases. The δ 13C values of the desorbed gases changed due to isotopic fractionation during coal degassing. Kinetic isotope effects were evaluated by comparing the gas desorption from fresh and stored coals from the same seams. Mean values of isotope enrichment during desorption are 2‰ and 1.9‰ for CO 2 and CH 4, respectively.

Chemisorption of water and carbon dioxide on nanostructured BaTiO3–SrTiO3(001) surfaces

The interaction of water and carbon dioxide with nanostructured epitaxial (Ba,Sr)TiO3(001) thin film and bulk single crystal SrTiO3(001) surfaces was studied using x-ray photoemission spectroscopy (XPS), thermal desorption spectroscopy (TDS), and density functional theory (DFT). On both surfaces, XPS and TDS indicate D2O and CO2 chemisorb at room temperature with broad thermal desorption peaks (423–723 K) and a peak desorption temperature near 573 K. A comparison of thermal desorption Redhead activation energies to adsorption energies calculated using DFT indicates that defect surface sites are important for the observed strong adsorbate-surface reactivity. Numerical calculations of the competetive adsorption/desorption equilibria for H2O and CO2 on SrTiO3(001) surfaces show that for typical atmospheric concentrations of 0.038% carbon dioxide and 0.247% water vapor the surfaces are covered to a large extent with both adsorbates. The high desorption temperature indicates that these adsorbates have the potential to impact measurements of the electronic structure of BaTiO3–SrTiO3(001) surfaces exposed to air, or prepared in high vacuum deposition systems, as well as the electrical properties of thin film ATiO3-based devices.

Surface Phenomena of Deuterized Ethanol Exposed Zircaloy-4 Surfaces

We report the results of the surface chemistry of deuterized ethanol exposed Zircaloy-4 (Zry-4) surfaces with various amount of <TEX>$C_2D_5$</TEX>OD exposures at 190 K. This system was examined with Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) techniques. In TPD study, <TEX>$D_2$</TEX> was evolved at two different desorption temperature regions accompanying with broad desorption background. The lower temperature feature at around 520 K showed first-order desorption kinetics. The high temperature desorption peak at around 650 K shifted to lower desorption temperature as the exposure of <TEX>$C_2D_5$</TEX>OD increased. The Zr(MNV) Auger peak shifted about 2.5 eV from 147 eV to lower electron energy followed by 300 L of <TEX>$C_2D_5$</TEX>OD dosing. This implies metallic zirconium was oxidized by deuterized ethanol adsorption. After stepwise annealing of the oxidized Zry-4 sample up to 843 K, the shifted Zr(MNV) peak was gradually shifted back to metallic zirconium peak position. After the sample was heated to 843 K, the oxygen content near the Zry-4 surface was recovered to clean surface level. The concentration of carbon, however, was not recovered by annealing the sample.

Metalorganic Vapor-Phase Epitaxial Growth of (211)B CdTe on (211) Si Using Ge Interfacial Layer

We have achieved metalorganic vapor-phase epitaxial growth of (211)B CdTe on Si without the requirement of a pregrowth high-temperature oxide desorption step. This was achieved by growing a thin Ge film on the starting (211) Si substrates. To get (211)B CdTe orientation, the Ge surface was exposed to As prior to the start of CdTe growth. A thin ZnTe interlayer between Ge and CdTe has been shown to improve the CdTe surface morphology.

Synthesis of nanocomposite hydrides for solid-state hydrogen storage by controlled mechanical milling techniques

The following composite hydride systems: NaBH 4–MgH 2, MgH 2–LiAlH 4, MgH 2–VH 0.81 and MgH 2–NaAlH 4, were synthesized in a wide range of compositions by controlled reactive/mechanical (ball) milling in a magneto-mill. In effect, composites having nanometric grain sizes of the constituent phases (nanocomposites) were produced. It is shown that the hydrogen desorption temperature of the composite constituent with the higher desorption temperature in the systems such as NaBH 4 + MgH 2, MgH 2 + VH 0.81 and MgH 2 + LiAlH 4 substantially decreases linearly with increasing volume fraction of the constituent having lower desorption temperature according to the well-known composite rule-of-mixtures (ROM). It is also shown that the ROM behavior can break down due to an ineffective milling of a composite. The composite system MgH 2 + NaAlH 4 does not obey the ROM behavior.

Desorption of H2O from Flat and Stepped Pt(111)

To investigate the effect of steps on H2O binding on a nominal Pt(111) surface, we used thermal desorption spectroscopy of water adsorbed on purposefully nanostructured surfaces: a rippled surface containing densely packed (100)-microfaceted and (111)-microfaceted steps was created using grazing incidence ion bombardment, and a surface with triangular mounds mainly consisting of (111)-microfaceted steps was fabricated through homoepitaxial growth. These morphologies are determined by scanning tunneling microscopy. We find two additional high-temperature H2O desorption peaks using the rippled surface, whereas only the peak with the highest desorption temperature is present on the (111)-microfaceted mound. Thus, water preferentially binds to steps and especially favors (111)-microfaceted ones. Furthermore, the large step concentration on our nanostructured surfaces precludes the coexistence of a condensed and a diluted phase in a monolayer of water and suppresses the formation of crystalline ice multilayers...

Study of Aluminoborane Compound AlB4H11 for Hydrogen Storage

Aluminoborane compounds AlB4H11, AlB5H12, and AlB6H13 were reported by Himpsl and Bond in 1981, but they have eluded the attention of the worldwide hydrogen storage research community for more than a quarter of a century. These aluminoborane compounds have very attractive properties for hydrogen storage: high hydrogen capacity (i.e., 13.5, 12.9, and 12.4 wt % H, respectively) and attractive hydrogen desorption temperature (i.e., AlB4H11 decomposes at ∼125 °C). We have synthesized AlB4H11 and studied its thermal desorption behavior using temperature-programmed desorption with mass spectrometry, gas volumetric (Sieverts) measurement, infrared (IR) spectroscopy, and solid state nuclear magnetic resonance (NMR). Rehydrogenation of hydrogen-desorbed products was performed and encouraging evidence of at least partial reversibility for hydrogenation at relatively mild conditions is observed. Our chemical analysis indicates that the formula for the compound is closer to AlB4H12 than AlB4H11.

Structure-dependent deuterium release from ion implanted beryllium: Comparison between Be(1 1 [formula omitted] 0) and Be(poly)

The temperature-driven release of deuterium implanted as keV ions into metallic beryllium is measured by temperature-programmed desorption (TPD). TPD spectra from single and polycrystalline Be implanted with 1keV ions are compared. The high-temperature desorption stage (T>700K) is attributed to the release of deuterium trapped at several types of energetically different ion-induced defects. A release peak around 850K is recorded in the single crystal, while in the polycrystal all deuterium desorbs below this temperature. An increase in the maximum release temperature is observed after implantation of the polycrystal with higher ion energies (2 and 3keV). We propose an interpretation of the experimental results based on two types of traps, with depth distributions adapted to the implantation energy. Preliminary TMAP7 calculations qualitatively reproduce the shifts in the maximum desorption temperature, observed in the polycrystal at different implantation energies. The difference between the single and the polycrystal is explained by a higher density of surviving defects in the single crystal. Diffusion of mobile defects to grain boundaries and subsequent annihilation is proposed as the dominant mechanism for differences in deuterium desorption from Be(112¯0) and Be(poly).

IR observation on O–D vibration in LiNbO 3 and LiTaO 3 single crystal irradiated by 3 keV [formula omitted

The behavior and existence states of deuterium irradiated into LiNbO 3 and LiTaO 3 single crystals were studied by in situ FT-IR observations on multiple O–D vibration peaks and by TDS on the desorption behavior of deuterium. Two O–D vibration peaks were observed in both LiNbO 3 and LiTaO 3, one at 2610–2615 cm −1 and the other at 2560–2570 cm −1. The former O–D vibration was attributed to –OD − affected by Nb defect in LiNbO 3 or Ta defect in LiTaO 3. For attribution of the latter O–D vibration, two candidates remained, –OD − affected by Li vacancy and –OD − of interstitial D +. By heating after the ion irradiation, the irradiated deuterium was mainly desorbed as hydrogen molecular forms (D 2 and HD). The –OD − affected by Nb defect in LiNbO 3 or Ta defect in LiTaO 3 had a higher desorption temperature than the other –OD −s. The two materials showed similar results in the existence states and the desorption behavior of irradiated D +, although some differences were observed due to more formation of Nb defect in LiNbO 3 than Ta defect in LiTaO 3 by displacement. It was indicated that the release temperature of hydrogen isotopes is heightened by the influence of M defect in ternary lithium oxides (Li x MO y ).

MTW zeolites for reducing cold-start emissions of automotive exhaust

Increasingly strict environmental legislations have led to the need for better control of vehicle cold-start emissions. In this research work, a series of one-dimensional channel molecular sieves with 12 oxygen ring apertures (12R) having MTW structure (MTW is the designation for ZSM-12 in the IZA nomenclature of zeolite structures), have been synthesized and characterized by different techniques such as: XRD, SEM, BET surface area, elemental analysis by atomic absorption spectroscopy, FTIR of adsorbed pyridine and temperature-programmed desorption (TPD). The synthesized samples were tested as hydrocarbons (HCs) trap adsorbents using toluene and ethylene as heavy (aromatic) and light (olefin) probe molecules present in the exhaust stream at engine cold-start. TPD tests were performed after adsorption under four different mixture conditions: binary (toluene–ethylene), ternary (toluene–ethylene–CO2), ternary (toluene–ethylene–H2O) and quaternary (toluene–ethylene–CO2–H2O). The results demonstrated that a silver exchanged MTW zeolite, Al-ZSM-12 (Ag) exhibited high and stable trapping capacities for both probe molecules in all mixtures investigated in this study, thus showing essentially no sensitivity to CO2 and/or H2O. Moreover, a high desorption temperature, particularly for toluene, was associated with the large amount of strong Lewis and Brönsted acid sites. Isomorphous substitution of Al by Fe in MTW structures did not lead to drastic changes in adsorption capacities and desorption temperatures of the two sorbates.

Low-energy electron-induced reactions in condensed HMDSO

Reactions induced by electrons at an incident energy of 15 eV in multilayer condensed films of hexamethyldisiloxane (HMDSO) serving as model for siloxane materials have been investigated by thermal desorption spectrometry (TDS). TDS of the non-exposed material as a function of coverage reveals the subsequent formation of two phases with different desorption temperature. The higher-temperature desorption peak, ascribed to an ordered phase, vanishes more rapidly under exposure to electrons than the low-temperature peak, ascribed to a disordered phase. Production of CH 4 is also more rapid during the initial depletion of the high-temperature desorption peak of HMDSO than at higher electron exposure. This gives evidence that two physically different phases with different reactivity under electron exposure must be present already at the deposition temperature. An estimate of the HMDSO depletion cross sections for the two phases is given and possible reasons for the different reactivity are discussed.

Temperature-programmed desoprtion of H2 from the surfaces of pd/support and Pd-Ag/support catalysts (support = Al2O3, SiO2)

Thermal desorption of H2 from the surface of Pd/support and Pd-Ag/support (support = Al2O3, SiO2) catalysts has been investigated. Two wide desorption peaks can be observed for the 5% Pd/support catalyst. The presence of these peaks in the thermogram indicates that several adsorption states exist, which is the result of occurrance of different adsorption centers of specific bond strengths for hydrogen. The addition of silver to the palladium catalysts causes a considerable decrease in the size of the high temperature desorption peak. It is also worth noting that the temperature of the maximum of the desorption rate remains practically constant for all bimetallic catalysts studied. This means that the activation energy of the hydrogen desorption process does not change after the introduction of silver to the palladium catalyst.

Ab Initio Alloying of Mg for Hydrogen Storage

AbstractA candidate hydrogen storing material should have high storage capacity and fast dehydrogenation kinetics. On this basis, magnesium hydride (MgH2) is an outstanding compound with 7.6 wt% storage capacity, despite its slow dehydriding kinetics and high desorption temperature. Therefore in this study, formation energies of alloyed bulk MgH2, adsorption energies on alloyed magnesium (Mg) and MgH2 surface structures were calculated by total energy pseudopotential methods. Also, the effect of substitutionally placed dopants to the dissociation of hydrogen molecule (H2) at the surface of Mg was investigated via Molecular Dynamics (MD). The results show that 31 out of 32 selected dopants decreased the formation energy of bulk MgH2, within a range of ˜37 kJ/mol-H2 where only Sr did not display any such effect. The most favorable elements in this respect are; P, K, Tl, Si, Sn, Ag and Pb. Moreover, surface adsorption energy values display that all elements are adsorbed substitutionally on the clean (0001) surface of Mg where adsorption on MgH2 (001) surface is possible only for alloying elements other than Zn, Au, In, Ag, Li, Tl, Cd, Na and K. Finally, results of MD simulations point out that the elements giving rise to the dissociation of hydrogen molecule came out to be Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Ru, Rh ve Hf.

Interaction of carbon dioxide with Cu overlayers on Pt(1 1 1)

Experimental and theoretical studies on the interaction of carbon dioxide with pseudomorphic and rough copper layers deposited on a platinum (1 1 1) single crystal are reported. Evidence for carbon dioxide dissociation and carbonate formation is presented and the relevance to methanol synthesis is discussed. Intermediate species on the surfaces were identified using in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) at pressures up to ∼550 mbar and electron energy loss spectroscopy (EELS) in ultra-high vacuum. Temperature programmed desorption (TPD) reveals a broad high temperature desorption state for CO 2 with peak maximum around 450 K. X-ray photoelectron spectroscopy (XPS) shows that approximately one third of the oxygen accumulated on the surface upon CO 2 exposure remains after TPD, indicative of carbonate formation via CO 2 dissociation supplying O ads and then facile CO 2 + O ads association, as well as subsequent decomposition at higher temperatures. Density functional theory studies of stepped Cu and Cu/Pt slabs reproduce vibrational frequencies of the carbonate, suggesting a nearly flat tridentate configuration at steps/defect sites.

Destabilizing LiBH4 with a Metal (M = Mg, Al, Ti, V, Cr, or Sc) or Metal Hydride (MH2 = MgH2, TiH2, or CaH2)

We experimentally investigate several hydrogen storage reactions based on thermodynamic destabilization of LiBH4. The destabilized mixtures include nine M(H2)−LiBH4 compositions, where M(H2) = Al, Mg, Ti, Sc, V, Cr, MgH2, CaH2, or TiH2, which were selected on the basis of favorable thermodynamics predicted by recent first-principles computational study (Siegel, D. J.; Wolverton, C.; Ozoliņš, V. Phys. Rev. B: Condens. Matter, Mater. Phys. 2007, 76, 134102). For all compositions, our measurements reveal significant kinetic barriers for hydrogen release, evidenced by high desorption temperatures (>300 °C) and exceedingly slow hydrogen release rates. Characterization of the desorbed reaction phases indicate that less than half of the mixtures examined (M(H2) = MgH2, Mg, Al, and CaH2) follow the thermodynamically expected reaction pathway, resulting in the formation of metal boride products (MgB2, AlB2, and CaB6, respectively). Hydrogen release/uptake data for these compositions indicate that the MgH2−(LiBH4)2 and Al−(LiBH4)2 reactions are reversible (10.2 and 6.7 wt %, respectively) at increased desorption pressures of 5 and 3 bar H2, respectively, while CaH2−(LiBH4)6 is irreversible under the conditions tested. For the remaining compositions, M(H2) = Sc, V, Cr, Ti, and TiH2, we surmise that substantial limitations in kinetics inhibit the expected formation of metal boride products. Finally, we discuss how the relative physical properties, in particular the melting point of the reactants and products, correlate with desorption route and reversibility.

Attenuated Total Reflection Fourier Transform Infrared Spectroscopic Investigation of CO Adsorption on Hydrated Ru/Al2O3

The adsorption of CO on hydrated 5 wt % Ru/Al2O3 produced νCO absorbance features at ∼2048, 1992, and 1924 cm-1 that are red-shifted by 50−116 cm-1 from those seen in the absence of water (2020−2040, 2080, and 2140 cm-1). This red-shift most likely arises from dipole−dipole interaction between coadsorbed CO and water molecules since (1) the exact frequency of the νCO absorbance feature depends upon the amount of coadsorbed water and (2) the presence of flowing liquid water further red-shifts the frequencies. These νCO absorbance features are uncorrelated, since the relative intensities of the νCO absorbances at 2049, 1992, and 1924 cm-1 depend on the amount of coadsorbed water and CO on the surface. Temperature programmed desorption done with TGA-MS indicated three different high-temperature CO2 desorption peaks. These CO2 peaks (T ≈ 350, 400, and 550 °C) are most likely the result of the oxidation of adsorbed CO reacting with surface adsorbed water (COads + H2Oads → H2 + CO2) and/or the disproportionatio...