Desorption Peak Research Articles

Rapid hydrogenation at 30 °C of magnesium (Mg) and iron (Fe) nanocomposite obtained through a decomposition of Mg 2FeH 6 precursor

A ternary Mg 2FeH 6 hydride was synthesized applying the processing route described in detail in [4]. Subsequently, a nanocomposite of 2Mg + Fe was produced through a thermal decomposition of Mg 2FeH 6 precursor. Microstructural studies by SEM, STEM, EDS, EELS and XRD show a microstructure consisting of nanocrystalline nearly spherical Fe particles with an average diameter of 20–30 nm uniformly distributed in the nanocrystalline Mg matrix exhibiting the grain size on the order of 5–10 nm. The 2Mg + Fe nanocomposite was subsequently hydrogenated at 30 °C under 4 MPa of hydrogen pressure. After 10 h about 1.1 mass% of hydrogen has been absorbed but the pertinent desorption curve shows that the hydrogenation process is not saturated yet. Temperature programmed desorption (TPD) of hydrogenated 2 Mg + Fe sample shows an asymmetrical desorption peak with a tail extending from ∼130 to 200 °C, then sharp increase in the peak height and eventually a sudden drop around a maximum at ∼250 °C. This asymmetrical shape strongly suggests that a large population of nanocrystalline grains being in an intimate contact with the nanocrystalline particles of Fe which, most likely, act as a catalyst, is able to desorb hydrogen at very low temperatures from ∼130–250 °C under helium flow used in a TPD test.

Consequences of deuterium retention and release from Be-containing mixed materials for ITER Tritium Inventory Control

In order to assess the tritium removal procedure currently suggested for ITER (wall baking at 513K (240°C) for the main chamber, at 623K (350°C) for the divertor), deuterium (D) retention and release behaviour of beryllium (Be)-containing materials are investigated. In pure Be, D is predominantly released around 470K with a relatively sharp desorption peak. Mixing of tungsten (W) or carbon (C) into Be changes the D desorption behaviour causing less efficiency D removal by the baking procedure. Especially, high C concentrations in Be affect the D release behaviour significantly and prevents removal of the retained D by baking at 623K. As a consequence, the baking operation in ITER would work for tritium removal from the first wall and Be-rich deposited layers formed at low temperature areas, while it does not work for C-rich codeposited layers and/or plasma-facing surfaces heated above 623K during a discharge.

Effect of Fe and C doping on the thermal release of helium from aluminum

The effect of Fe and C doping on the thermal release of helium from Al implanted with 10 keV, 4.0 × 10 21 ion/m 2 He at room temperature (RT) has been investigated by thermal helium desorption spectrometry (THDS) and transmission electron microscope (TEM). The results show that Fe and C doping have significant impact on the release of helium from Al and the extent depends on the doping fluence. Proper fluence of Fe and C doping would lead to the retardation of the release of helium from Al, while excessive fluence of Fe and C doping would result in more desorption peaks and the release of helium in lower temperature ranges. Fe and C doping have different influence on the release of helium from Al, and the difference is related with the secondary phases forming in the samples.

Adsorption, Desorption, and Reaction of Methyl Radicals on Surface Terminations of α-Fe2O3

The adsorption, desorption, and reaction of gas phase methyl radicals were studied on the (0001)-oriented α-Fe2O3 surface in ultrahigh vacuum. Two different surface terminations were compared: An Fe3O4 (111) layer and the so-called “biphase” surface thought to be a mixture of FeO and Fe2O3 terminations. Gas phase methyl radicals were prepared by pyrolysis of azomethane. On Fe3O4 (111) methyl radical adsorption forms surface methoxide species as determined by the C(1s) XPS binding energy. Temperature programmed reaction spectroscopy produced direct desorption of methyl radicals at all coverages and the formation of ethane at high coverages in two desorption peaks at 331 and 439 K. The activation energies for desorption were 84 and 133 kJ/mol in the two regimes. The two surface terminations exhibit saturation coverages that differ by ca., 30×: 1.5 × 1014 and 5.2 × 1012 per cm2 for the Fe3O4(111) and “biphase” terminations, respectively. These results are interpreted in terms of bonding models and difference...

The composites of magnesium hydride and iron-titanium intermetallic

Hydride-intermetallic composites MgH 2 + X wt.% FeTi ( X = 10, 30, 50) were synthesized by Controlled Mechanical Milling (CMM) in a magneto-mill. Their thermal behavior was investigated by Differential Scanning Calorimetry (DSC) and Temperature Programmed Desorption (TPD). It is found that the DSC hydrogen desorption peak temperature as well as the activation energy of hydrogen desorption of the MgH 2 constituent in composites decreases linearly with increasing volume fraction of FeTi with a coefficient of fit R 2 = 0.98. A doping of the MgH 2 + FeTi composites with 5 wt.% of nanometric-size nickel (n-Ni) produced by Vale Inco Ltd. further reduces the DSC hydrogen desorption peak temperature of MgH 2 to the temperature range below 300 °C for the MgH 2 + 10 and 30 wt.% FeTi composites. The most effective reduction of the DSC hydrogen desorption peak temperature of the MgH 2 constituent by as much as 60 °C due to the catalytic effect of n-Ni is observed for the MgH 2 + 10 wt.% FeTi + 5 wt.% n-Ni composite. At this composition the composite also has hydrogen capacity slightly higher than 5 wt.%.

Carbon−Chlorine Bond Scission in Li-Doped Single-Walled Carbon Nanotubes: Reaction of CH3Cl and Lithium

The doping of single-walled carbon nanotubes (SWNTs) under ultrahigh vacuum by Li atoms has been explored experimentally and theoretically. The chemical effect of Li in breaking the C−Cl bond in chloromethane has been observed. Temperature programmed desorption (TPD) experiments show that at low coverage CH3Cl is physisorbed to the undoped SWNT sample, exhibiting a desorption process near 178 K. The CH3Cl desorption peak shifts to about 240 K for lithiated SWNTs, indicating an increase in binding energy of about 0.16 eV. More importantly, the integrated intensity of the CH3Cl desorption peak is dramatically reduced in the lithiated SWNT case, and CH3, C2H6, or related species are not observed in significant quantities in the TPD experiments up to a temperature of 500 K. This strongly indicates that CH3Cl reacts on lithiated SWNTs to produce an irreversibly bound species. Products LiCl and Li2Cl2 are observed to desorb near 700 K. Density functional theory calculations present possible reaction mechanisms ...

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.

Structural and thermal gas desorption properties of metal aluminum amides

Metal aluminum amides M[Al(NH 2) 4] x ( M = K, Mg, and Ca; x = 1 and 2) were synthesized along with previously reported LiAl(NH 2) 4 and NaAl(NH 2) 4 by ball milling technique under liquid NH 3. The profiles of synchrotron radiation X-ray diffraction suggest that KAl(NH 2) 4, Mg[Al(NH 2) 4] 2 and Ca[Al(NH 2) 4] 2 have been indexed with single phases, which have never been reported so far. Combination of both FT-IR and 27Al MAS/3QMAS nuclear magnetic resonance suggest that they all have an anion complex unit [Al(NH 2) 4] − as a basic component, indicating successful synthesis of the metal aluminum amides. Thermogravimetry-differential thermal analysis coupled with mass spectroscopy showed the release of NH 3 below the temperatures of 140 °C during the thermal decomposition and a NH 3 desorption peak temperature ( T des) decreased with the increasing atomic number. Additionally, a relationship between 27Al isotropic chemical shift and T des was discussed. The present study gives an useful information that the thermal stability of the anion complex [Al(NH 2) 4] − can be controlled by a cation M.

Oxygen desorption from polycrystalline palladium: Thermal desorption of O2 from a chemisorbed layer of Oads in the course of the decomposition of PdO surface oxide and in the release of oxygen from the bulk of palladium

The desorption of oxygen from polycrystalline palladium (Pd(poly)) was studied using temperature-programmed desorption (TPD) at 500–1300 K and the amounts of oxygen absorbed by palladium (n) from 0.05 to 50 monolayers. It was found that the desorption of O2 from Pd(poly), which occurred from a chemisorbed oxygen layer (Oads), in the release of oxygen from a near-surface metal layer in the course of the decomposition of PdO surface oxide, and in the release of oxygen from the bulk of palladium (Oabs), was governed by repulsive interactions between Oads atoms and the formation and decomposition of Oads-Pd*-Oabs structures (Pd* is a surface palladium atom). At θ ≤ 0.5, the repulsive interactions between Oads atoms (ɛaa = 10 kJ/mol) resulted in the desorption of O2 from Pd(poly) at 650–950 K. At 0.5 ≤ n ≤ 1.0, the release of inserted oxygen from a near-surface palladium layer occurred during TPD in the course of the migration of Oabs atoms to the surface and the formation-decomposition of Oads-Pd*-Oabs structures. As a result, the desorption of O2 occurred in accordance with a first-order reaction with a thermal desorption (TD) peak at Tmax ∼ 700 K. At 1.0 ≤ n ≤ 2.0, the decomposition of PdO surface oxide occurred at a constant surface cover-age with oxygen during TPD in the course of the formation-decomposition of Oads-Pd*-Oabs structures. Because of this, the desorption of O2 occurred in accordance with a zero-order reaction at low temperatures with a TD peak at Tmax ∼ 675 K. At 1.0 ≤ n ≤ 50, oxygen atoms diffused from deep palladium layers in the course of TPD and arrived at the surface at high temperatures. As a result, O2 was desorbed with a high-temperature TD peak at T > 750 K.

Water Adsorption on a Co(0001) Surface

We have investigated the adsorption of water on a Co(0001) surface by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At 130 K, the interaction between adsorbed water (H2O(a)) and Co(0001) is quite weak, and water adsorption forms an intact, nonwetting, and three-dimensional layer on Co(0001). The monolayer and multilayer of H2O(a) could be distinguished by XPS, and they desorb molecularly upon heating but only giving a single water desorption peak. An ordered p(2 × 2) LEED pattern was observed at low exposures of water. At room temperature, water adsorbs dissociatively on Co(0001), forming chemisorbed atomic oxygen (O(a)) and atomic hydrogen (H(a)). Upon heating, H(a) recombines into H2 desorbing from the surface, whereas O(a) remains on the surface. The interaction of water with Co(0001) is greatly influenced by the presence and nature of the oxygen species on Co(0001). Water adsorption on Co(0001) precovered with 0.45 ML O(a) at 130 K forms a mixture layer of OH(a) and H(a) via the reaction of 2H2O(a) + O(a) → 3OH(a) + H(a). However, on oxide-like Co(0001) surface, the water decomposition is completely passivated even at room temperature. These results broaden our fundamental understanding of water interaction with metal surfaces and provide insights into the water-involved catalytic reactions catalyzed by cobalt.

Density Functional Investigations of Methanol Dehydrogenation on Pd−Zn Surface Alloy

Methanol dehydrogenation on Pd(111) and various Pd-Zn surface alloy films supported on Pd(111) have been investigated using density functional method in combination with periodic slab models. Calculations show that compared to Pd(111) the interaction between CH(3)O and the films is enhanced, whereas that for CH(2)O and CHO is weakened. Zn in top layer facilitates the CH(3)O stability. At variance, the subsurface Zn reduces the interaction of CH(2)O and CHO with the substrate significantly. Addition of Zn promotes the O-H breaking of CH(3)OH and the dehydrogenation of CHO but hinders the dehydrogenation of CH(3)O and CH(2)O. Comparison shows that the third-layer Zn atoms have essentially no effect on the reactions. Our calculations demonstrate that the experimentally observed 360 K desorption peak cannot be originated from CH(2)O adsorbed at flat Pd-Zn alloy surfaces, and it is very likely that CH(2)O combines preferentially with some species before decomposing into CHO during methanol steam reforming if CH(2)O is an intermediate. Finally, we show that the newly proposed relationship between the energy of the initial states and transition states exhibits better correlation than the classical BEP relation.

Deuterium retention of boron–titanium and reduction of deuterium retention after helium ion irradiations

Deuterium ion irradiations with an ion with energy of 1.7 keV were conducted for boron–titanium (B–Ti) film prepared by electron beam evaporation and hot pressed titanium-boride, TiB 2. The amount of retained deuterium was measured for these materials using a technique of thermal desorption spectroscopy. The amount of deuterium retained in TiB 2 was comparable with that in B–Ti. Desorption peaks of deuterium in B–Ti were 470 K and 620 K, corresponding to a desorption in the low temperature regime observed in boron (B) and a desorption in titanium (Ti), respectively. The desorption peaks in TiB 2 were 620 K and 750 K, which correspond to the desorption in Ti and that in the high temperature regime in B, respectively. The desorption temperature in B–Ti was approximately 100 K lower than that in TiB 2. This difference is discussed based upon chemical bindings and amorphous/crystal structures of B–Ti and TiB 2. Irradiation of helium ion with energy of 5 keV was conducted for B–Ti after the deuterium ion irradiation. The amount of retained deuterium decreased and the desorption temperature shifted to the lower temperature regime, as the helium ion fluence increased. The shift to the low temperature regime is due to the enhancement of amorphous structure of B in B–Ti.

Copper underpotential deposition on Ru quasi-single-crystal films

Ru quasi-single-crystal electrodes, prepared by resistive heating of Ru deposited on Pt single-crystal surfaces in a nitrogen atmosphere, have been used to study copper underpotential deposition (Cu-UPD), Bulk and surface Cu stripping experiments were performed on the low-index Ru/Pt(1 0 0), Ru/Pt(1 1 0) and Ru/Pt(1 1 1) quasi-crystalline film electrodes and also stepped Ru/Pt(S)[ n(1 0 0) × (1 1 1)] and Ru/Pt(S)[4(1 1 1) × (1 0 0)] electrodes vicinal to the (1 0 0) and (1 1 1) planes respectively. Upon performing Cu-UPD experiments in the potential range from 0 to 500 mV vs. Cu/Cu 2+ on the Ru/Pt(1 0 0) quasi-single-crystal film electrode, a single peak for Cu-UPD was observed at a potential of about 200 mV vs. Cu/Cu 2+. Its charge density was close to that for copper on the clean Pt(1 0 0) single-crystal substrate consistent with a coverage of one monolayer (1.3 × 10 15 atoms cm −2). In contrast, on Ru/Pt(1 1 0) and Ru/Pt(1 1 1), no significant amounts of Cu could be deposited – possibly due to competitive adsorption by adsorbed oxygen-species. For Cu-UPD on stepped Ru/Pt(S)[ n(1 0 0) × (1 1 1)] film electrodes vicinal to the (1 0 0) plane, a peak was also found in the same potential range as observed for Ru/Pt(1 0 0). From the linear variation of the charge densities of this Cu-UPD peak with the step density, we conclude that Cu-UPD is related to the adsorption of Cu on (1 0 0) terrace sites. Hence Cu-UPD is a structural probe of (1 0 0) terrace sites on quasi-crystalline Ru films supported on platinum. In order to reduce the amount of oxygen-species blocking the Ru/Pt(1 1 1) and vicinal surfaces, diffusion controlled progressive over-potential deposition (OPD) of Cu (in the sub-monolayer regime but at very negative potentials) from very dilute Cu 2+(aq) containing solutions was performed. For the Ru/Pt(1 1 1) film electrode, a desorption peak in the UPD range around 300 mV vs. Cu/Cu 2+ for Cu desorption from the (1 1 1) sites was then observed, while for experiments on the Ru/Pt(3 1 1), Ru/Pt(5 1 1) and Ru/Pt(5 3 3) electrodes, we could also observe a peak at around 200 mV, which we ascribe to the desorption of Cu from the (1 0 0) oriented sites. From these experimental results, we conclude that after preparation of the Ru-film electrodes, the terrace and step sites of the underlying Pt single-crystal substrate are preserved on the Ru film. These results for Cu-UPD on the Ru-film electrodes also demonstrate that free Pt domains do not exist on the Ru-film electrodes.

Adsorption kinetics and dynamics of CO on silica supported Au nanoclusters—Utilizing physical vapor deposition and electron beam lithography

The adsorption kinetics and dynamics of CO on silica supported Au clusters have been studied by thermal desorption spectroscopy (TDS) and molecular beam scattering. Physical vapor deposition (PVD) and electron beam lithography (EBL) have been used to fabricate the samples. According to our prior study (Kadossov et al., in press [49]), a maximum in the initial adsorption probability, S 0, occurs for 3 nm Au clusters. Therefore, in the present study, we have focused on this cluster size for PVD samples. In addition, 12 nm EBL Au clusters have been studied. This is presently one of the smallest achievable cluster sizes using EBL for macroscopic samples. Auger electron spectroscopy and scanning electron microscopy have been used to further characterize the samples. TDS revealed well-known features for PVD samples; i.e., two peaks assigned to CO adsorption on different defect sites were present. Interestingly, TDS data of EBL samples were dominated solely by one CO desorption peak. The initial adsorption probability of CO, S 0, decreased as a function of impact energy, E i, and adsorption temperature, T s, for both samples, which is consistent with non-activated molecular adsorption. The coverage dependence of the adsorption probability and S 0 are discussed in the framework of the capture zone model.

Effect of Cr and Zr Dopes on Hydrogen Behaviour in Rapidly Solidified Aluminium Foils

Hydrogen (H) behaviour in materials was investigated in rapidly solidified (RS) foils of pure aluminium (Al), Al-0.4 Cr and Al-0.25 Zr alloys (at %) by means of thermal desorption spectroscopy (TDS). In addition, Al-0.25; 0.3 Zr alloys were examined with respect to microstructure and its instability during the thermal process using SEM and microhardness measurements. The effect of dopes and heating rate on H desorption was summarized. The lowest energy desorption is attributed with significant thermal desorption peak which temperature was found is correlated with sample composition.

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO ( α 1 peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites ( α 2 peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H 2 in the same temperature regime, but again no reaction between the two species is observed.

Electro-thermal simulation and characterization of preconcentration membranes

Preconcentrators have been much studied last years to enhance sensitivity of gas sensors. We use electro-thermal simulations to design thermal isolated membranes that supports thin film heaters as components of gas preconcentrators, these heaters provide a good thermal homogeneity ensuring a narrow desorption peak and improving in this way the sensors response. We have validated our simulations with experimental results.

Reaction pathways for hydrogen desorption from magnesium hydride/hydroxide composites: bulk and interface effects

This manuscript investigates the thermal desorption behaviour of MgH(2)/Mg(OH)(2) composites by means of thermal desorption spectroscopy. Besides the H(2)O and H(2) desorption events due to Mg(OH)(2) dehydration and MgH(2) decomposition reactions, respectively, two additional H(2) desorption peaks arise at lower temperatures. These peaks are related to solid-state reactions between magnesium hydride and magnesium hydroxide through different channels. The low temperature H(2) peak ( approximately 150 degrees C) is related to reaction between a H atom diffusing from MgH(2) and a surface OH group, whereas the intermediate temperature H(2) peak ( approximately 350 degrees C) is due to an interface reaction between the hydride and the hydroxide. The present work supports the theory that the onset of the H(2) desorption coming from MgH(2) decomposition is controlled by an incubation process, consisting in the formation of catalytically active vacancies at the MgO/Mg(OH)(2) surface by dehydration. Possible ways to improve the H(2) desorption kinetics from MgH(2) are discussed in the light of the results obtained.

Significant Reduction in Adsorption Energy of CO on Platinum Clusters on Graphite

We have studied desorption of CO on a Pt-deposited highly oriented pyrolytic graphite (HOPG) surface by temperature programmed desorption of CO (CO-TPD), scanning tunneling microscope (STM), and He atom scattering (HAS). A desorption peak of CO at a significantly lower temperature of ∼270 K with a heating rate of 0.5 K/s is observed for Pt clusters on a flat terrace of HOPG. STM results indicate that the height of Pt clusters on the terraces of the HOPG surface is monatomic. It is concluded that the significant reduction in CO adsorption energy is due to a modified electronic structure as a result of the interface interaction between Pt clusters and the HOPG surface.

Studies on metal hydride electrodes containing no binder additives

Electrochemical properties of hydrogen storage alloys (AB 5 type: LaMm-Ni 4.1Al 0.3Mn 0.4Co 0.45) were studied in 6 M KOHaq using Limited Volume Electrode (LVE) method. Working electrodes were prepared by pressing alloy powder (without binding and conducting additives) into a metal net wire serving as a support and as a current collector. Cyclic voltammetry curves reveal well defined hydrogen sorption and desorption peaks which are separated from other faradic processes, such as surface oxidation. Voltammograms of LVE resemble the curves obtained by various authors for single particle metal alloy electrodes. Hydrogen diffusion coefficient calculated at room temperature for LV electrodes and for 100% state of charge reaches a constant value of ca. 3.3 × 10 −9 and 2.1 × 10 −10 cm 2 s −1, for chronoamperometric and chronopotentiometric measurements, respectively. A comparison of the electrodes with average alloy particle sizes of ca. 50 and 4 μm allows us to conclude that at room temperature hydrogen storage capability of AB 5 alloy studied is independent on the alloy particle size. On the other hand, reduction of the particle size increases alloy capacity at temperatures below −10 °C and reduces time of electrochemical activation of the electrode.