Thermal Desorption Mass Spectra Research Articles

An insight into the catalytic mechanism of perovskite ternary oxide for enhancing the hydrogen sorption kinetics of MgH2

Solid-state hydrogen storage has emerged as an efficient and reliable technique to commercialize hydrogen energy on a large scale. Magnesium hydride (MgH2), amongst other materials, shows excellent hydrogen storage capability. However, it suffers from setbacks like sluggish kinetics and high thermodynamic stability. Several studies have shown that doping with suitable materials is an effective method to improve the sorption kinetics. Current work is concerned with doping of MgH2 with perovskite-type ternary metal oxide NaNbO3 at 5,10 & 15 wt% doping concentration via ball milling and study of its sorption properties. Thermal desorption mass spectra (TDMS) with thermogravimetry (TG) and Differential scanning calorimetry (DSC) validate the optimum doping concentration to be 10 wt% NaNbO3 in MgH2. According to the isothermal hydrogenation plots, 10 wt% catalyzed sample was capable of absorbing 5.29 wt% hydrogen in just 4.2 min at 150ºC which by far outperforms the 2 h milled MgH2 sample which under the same set of conditions absorbs only 0.67 wt% H2. The catalyst starts affecting the absorption rate right from the room temperature whereas the milled sample has minimal absorption throughout the experiment. At room temperature, the average rate of absorption grows by factor 7 which is staggeringly high. The Kissinger analysis reveals activation energy for hydrogen release as 73.12 kJ/mol for 10 wt% doped NaNbO3-MgH2 system while 137.13 kJ/mol for as milled MgH2. The pressure-temperature isotherm (PCI) at four different temperatures gives a quantitative measure of the thermodynamic stability of the system. To fully comprehend the catalytic process, XRD, SEM, and XPS analysis were conducted after each stage of experiment. XPS suggested possible reduction of Nb valance state from + 5 to + 2 due to surface reduction reaction which further accelerated the sorption kinetics due to the electron transfer process.

PMMA – C60 composite: thermal decomposition experiments in mass spectrometer and quantum chemical modeling

Mass-spectrometric thermal decomposition experiments with submicron films of neat polymethylmethacrylate (PMMA), and PMMA-fullerene composite (PMMA-C60) after UV irradiation are discussed. The experiment registers thermal desorption mass spectra (TDMS), that is the monomer desorption rate versus time upon gradual heating the PMMA films in a given heating regime. The spectra provide information on the amount of the monomer desorbed at different decomposition stages upon heating the given amount of film material as well as on the spectral shape changes. It is shown that both amount of monomer and the TDMS spectral shape are sensitive to the presence of fullerene and UV irradiation. The experimental results are discussed in terms of quantum chemical models of binding. The DFT/B3LYP-D3/def2/J RIJCOSX level of theory was used. The MMA-C60 structures which can yield different amounts of monomer have been compared.

Thermal-Decomposition Spectra of Submicrometer Films of Polybutylmethacrylate–C60 Fullerene Composite: Effects of Filler Concentration and UV Irradiation

Thermal-desorption mass spectra of optically thin films of polybutylmethacrylate–C60 fullerene composite with various fullerene concentrations have been analyzed. The effects of UV radiation on the films under atmospheric conditions have been examined.

Structural and magnetic changes in hydrogenated TiFe1−xNix alloys

A series of crystalline B2 and amorphous TiFe1−xNix samples with different compositions and hydrogen loading ratios was characterized by XRD, VSM, TG, mass-spectrometry, and ¹H NMR in order to investigate possible correlations between the sample magnetization and hydrogen content. In crystalline samples, a reversible second-order phase transition from α-phase (B2 with soluble hydrogen) through a mixture of α- and ß-phase (B19 TiFe1−xNixHy) into a pure ß-phase was found. An increase of magnetization upon hydrogen loading was observed, depending on the concentration of iron in the samples. No unambiguous correlation between the hydrogen content and the saturation magnetization was found. The saturation magnetization of as-prepared amorphous samples was found to be significantly higher than in as-prepared crystalline ones. For both types of samples, magnetization remained high after dehydrogenation as well. Thermal desorption mass-spectra was found to strongly depend on composition, whereas structure, charging pressure, and H-content did not have tremendous effect. By ¹H NMR, enhanced relaxation due to strong magnetic moments was observed; however, by TEM the existence of Fe-clusters in hydrogenated samples could not be verified.

The thermal desorption mass spectra of polytetrafluoroethylene modified under the action of radiation at elevated temperatures

The products of the pyrolysis of polytetrafluoroethylene (PTFE) in a vacuum modified under the action of gamma irradiation in the molten state were studied mass spectrometrically at various temperatures. Radiation modification was found to cause the appearance of an additional stage of the thermal decomposition of PTFE at decreased temperatures compared with not irradiated PTFE. The chemical composition of the gas components of thermal decomposition substantially changes after the irradiation of PTFE, which increases the molecular weight of decomposition products. A comparison with the results of an additional study of the thermal decomposition of tetrafluoroethylene copolymer with hexafluoropropylene is evidence of the formation of side CF3 groups in PTFE under irradiation. Data on the composition of gaseous products of PTFE radiolysis under gamma irradiation were obtained.

Hydrogen desorption properties of the Ca–N–H system

After synthesizing calcium amide (Ca(NH 2) 2) through the reaction of calcium hydride (CaH 2) with ammonia (NH 3), the thermal decomposition properties have been examined. The results indicated that Ca(NH 2) 2 decomposed to calcium imide (CaNH) and NH 3 gas with increasing temperature, where the desorption of NH 3 started from about 60 °C and peaked around 300 °C in the thermal desorption mass spectra (TDMS). From the X-ray diffraction profiles, the product after heat treatment up to 350 °C was CaNH, but an unknown phase appeared in further heating up to 500 °C. Hydrogen (H 2) desorption properties of a mixture of Ca(NH 2) 2 and CaH 2 with 1:3 molar ratio were also examined. The H 2 gas of 3.5 wt.% in total was desorbed with two-peak structure in TDMS. The first peak corresponds to the reaction: Ca(NH 2) 2 + CaH 2 → 2CaNH + 2H 2, and the second one originates from the reaction: CaNH + CaH 2 → Ca 2NH + H 2. Simultaneously performed differential thermal analysis indicated that the former and the latter reactions were, respectively, exothermic and endothermic, so that the H 2 desorption corresponding to the first peak in TDMS disappeared after long-time-milling at room temperature.

New Metal—N—H System Composed of Mg(NH2)2 and LiH for Hydrogen Storage.

AbstractFor Abstract see ChemInform Abstract in Full Text.

New Metal−N−H System Composed of Mg(NH2)2 and LiH for Hydrogen Storage

We have investigated the hydrogen storage properties of a ball-milled mixture of 3Mg(NH2)2 and 8LiH after first synthesizing Mg(NH2)2 by ball milling MgH2 under an atmosphere of NH3 gas at room temperature. The thermal desorption mass spectra of the mixture without any catalysts indicated that a large amount of hydrogen (∼7 wt %) was desorbed from 140 °C, and the desorption peaked at ∼190 °C under a heating rate of 5 °C/min with almost no ammonia emission. Moreover, the reversibility of the hydrogen absorption/desorption reactions was confirmed to be complete. The above results indicate that this system is one of the promising metal−N−H systems for hydrogen storage.

C58on HOPG: Soft-landing adsorption and thermal desorption

Electron-impact induced dissociation/ionization of C60 molecules was used to produce an intense beam of C58+ ions. This was directed towards a HOPG (highly oriented pyrolytic graphite) substrate under nominally perpendicular impact conditions in order to generate deposits by soft-landing (kinetic energy < 0.1 eV atom−1). Deposited C58 molecules could subsequently be thermally desorbed intactly. Thermal desorption mass spectra of the deposits exhibit only C58. Surface deposited C58 can react with background gases to generate hydride derivatives C58Hn which are also desorbable. The apparent desorption energy of C58 and C58Hn molecules from the HOPG surface varies with increasing adsorbate coverage from 2 ± 0.1 to 2.2 ± 0.1 eV as determined by a Redhead analysis. These values are 0.7 ± 0.1 eV larger than found for C60 desorbed from the same substrate.

Catalytic effect of 3d transition metals on hydrogen storage properties in mechanically milled graphite

In this paper, we examined the catalytic effect of 3d transition metals on hydrogen storage properties in nanostructural graphite prepared by ball milling under hydrogen atmosphere. The Fe-doped nanostructured graphite shows the most marked hydrogen storage properties among the Fe-, Co-, Ni- and Cu-catalyzed graphite systems. The absorbed hydrogen concentration reaches up to ∼4 wt% by mechanically milling for 32 h (∼7 wt% for 80 h), and two peaks of hydrogen (mass number=2) around 730 and 1050 K were observed in the thermal desorption mass spectra (TDS). The starting temperature for hydrogen desorption was ∼600 K. On the other hand, the Co-doped graphite indicates that absorbed hydrogen concentrations reaches up to ∼2 wt% by mechanically milling for 32 h. The TDS spectrum showed only a broad peak around 1100 K, but the starting point for hydrogen desorption lowered down to ∼500 K. The Ni- and Cu-doped graphites did not show any significant improvement for hydrogen storage. These results suggest that the catalytic effect on hydrogen storage properties strongly depends on the affinity of graphite and doped metals.

Lithium nitride for reversible hydrogen storage

In this paper, we examined the basic properties in the 1:1 mixture of lithium amide LiNH 2 and lithium hydride LiH as a candidate of reversible hydrogen storage materials. The thermal desorption mass spectra of the ball milled mixture without any catalysts indicated that hydrogen H 2 is released in the temperature range from 180 to 400 °C while emitting a considerable amount of ammonia NH 3. On the other hand, the ball milled mixture containing a small amount of TiCl 3 as a catalyst showed the most superior hydrogen storage properties among the 1:1 mixtures with a small amount of catalysts, Ni, Fe, Co metals and TiCl 3 (1 mol.%). That is, the product desorbs a large amount of hydrogen (∼5.5 wt.%) in the temperature from 150 to 250 °C under the condition of a heating rate of 5 °C/min, but it does not desorb ammonia at all within our experimental accuracy. In addition, we confirmed that the product shows an excellent cycle retention with an effective hydrogen capacity of more than 5 wt.% and a high reaction rate until at least 3 cycles.

Thermal desorption from ordered chemisorbed phases studied by helium scattering: Oxygen on Ag(110)

We relate the helium specular beam intensity during adsorbate desorption (He desorption curve) to the instantaneous surface coverage. In this way the He desorption curve is shown to provide a picture of the desorption process which can be fruitfully compared to the one coming from thermal desorption mass spectra (TDS), obtained under strictly comparable experimental conditions. We tested the combination of thermal energy atom scattering (TEAS) and TDS in the case of the associative desorption from long range ordered O(2×1)–Ag(110) phase. Using the so-called overlap approach and assuming intense adsorbate–adsorbate (and vacancy–vacancy) attractions along Ag–O–Ag rows we obtain an instantaneous coverage which is in good agreement with TDS results. He desorption curves confirm the extreme sharpness of the desorption transition further indicating that the tails of the TDS peak bear small contributions from oxygen which did not belong to the O(2×1) phase. Opportunities and limitations inherent to the use of the He desorption curve (and its first derivative) as a marker of the temperature position and sharpness of the desorption transition are also addressed.

Interaction of D(H) atoms with physisorbed benzene and (1,4)-dimethyl-cyclohexane: Hydrogenation and H abstraction

Benzene and (1,4)-dimethyl-cyclohexane monolayers were physisorbed on graphite covered Pt(111) surfaces. Exposure of benzene monolayers at 125 K to D atoms (1700 K) initially hydrogenates sp2 hybridized C atoms with a cross section of ca. 8 Å2 producing C6H6D intermediates. Additional D atom reactions either transform this intermediate via a second hydrogenation reaction to cyclohexadiene-d2, C6H6D2, or restore benzene, C6H5D, via H abstraction. Once the aromaticity is broken, successive hydrogenation of the diene occurs rapidly generating the saturated cyclohexane-d6, C6H6D6. The C6H5D reaction product can undergo further H/D exchange reactions and, at any level of deuteration, the benzene species might get hydrogenated. Monolayers of the saturated hydrocarbon (1,4)-dimethyl-cyclohexane (DMCH) that are exposed to D atoms produce deuterated DMCH via successive abstraction/hydrogenation reactions. Thermal desorption mass spectra revealed that H atoms at the ring were exchanged with an apparent cross section of 1.7 Å2. Methyl groups H atoms were exchanged much more slowly than ring H atoms. It was also observed that D exposed molecules/radicals exhibit a tendency to desorb from the surface, which is ascribed to the exothermicity of the reactions which lead to these species.

Analysis of thermal desorption mass spectra obtained with exponential heating

Analysis of thermal desorption mass spectra obtained with exponential heating

Analysis of thermal desorption mass spectra obtained with exponential heating

An analysis of thermal desorption spectra obtained with non-linear healing of the sample is proposed. The paper is divided in two parts. Firstly, the calculation is performed for any heating function T( t) and, secondly, the results are applied to an exponential heating: T( t) = T a−( T a− T 0) exp( −α t). As in the linear case, in the first order of desorption, the temperature T p of the peak in the desorption rate is found to be independent of T 0 and of the initial coverage n 0, this second property being true for any continuous temperature change; while in the second order, T p depends on both T 0, and T 0.

Thermal desorption spectra of hydrogenated and water treated diamond powders

The thermal desorption mass spectra of hydrogenated and water treated diamond powders were measured in the range from room temperature to 1250°C. After preliminary outgassing up to 1150°C, samples were either hydrogenated under 1 Pa hydrogen at 500–1000°C or treated in water vapor at room temperature to 800°C. Hydrogenated diamond exhibited two desorption peaks due to H 2 at about 900 and 1050°C, while water treated one desorbed mainly H 2 and CO, and small amounts of H 2O and CO 2. Oxidation after hydrogenation and treatment with a mixture of hydrogen and oxygen showed some correlation between adsorption states of hydrogen and oxygen. The maximum amount of hydrogen desorbed (0.81 × 10 15 atom/ cm 2) was only one-third of the valu estimated as full surface coverage.

A method for assessing the coverage dependence of kinetic parameters: Application to carbon monoxide desorption from iridium (110)

A method for analyzing thermal desorption mass spectra has been developed for determining the coverage dependence of the pre-exponential factor and the desorption energy in the Arrhenius equation for a one-step desorption process. The method, which involves variable heating schedules, is applicable to spectra in which several features appear. However, if the desorption process involves multiple steps, or if substantial desorption from multiple sites occurs for any one coverage, this method cannot be used. The method is applied to CO desorption from the (110) surface of Ir. Three features can be resolved in the desorption spectrum. Both the preexponential factor and the desorption energy vary strongly with coverage, and a compensation effect occurs.

An analysis of thermal desorption mass spectra. II

A quantitative analysis is given concerning the combined effect of heating rate and pumping speed on thermal desorption mass spectra. It is shown that considerable error in the determination of the activation energy and the pre-exponential factor of desorption can be introduced by the combined effects of high heating rate and low pumping speed. It is found that for an accurate determination of the activation energy and the pre-exponential factor of first-order desorption, the reciprocal of the product of heating rate and pumping time constant should be large, i.e., greater than approximately 0.5.

An analysis of thermal desorption mass spectra. I.

Analytic expressions have been derived which allow a determination of the activation energy of desorption and the pre-exponential factor of the desorption rate coefficient using parameters obtained easily from thermal desorption mass spectra. In particular, the spectral peak widths and the temperature at which the maximum rate of desorption occurs may be used to describe both first- and second-order desorption kinetics. An explicit application of this method in the analysis of several important classes of desorption reactions is presented.