Thermal Desorption Curves Research Articles

IR Spectrometric Studies of CCL4 and Ar Gas Mixtures

IR spectrometric studies were performed to examine the cryo-deposition processes and properties of thin cryo-vacuum condensate films containing a mixture of carbon tetrachloride and argon, which were obtained by physical cryo-vacuum deposition at various concentrations. The measurements were carried out in a temperature range of 11–100 K, and the pressure of the gas phase was 10–8 Torr-10–4 Torr. The film thickness in all experiments performed was d = 2 μm. The vibrational spectra of CCl4 in an argon matrix were measured in a frequency range of 400–4200 cm−1 at mixture concentrations (CCl4 + Ar) of 10–90%, 5–95%, and 1–99% and after the argon matrix underwent sublimation. Thermal desorption curves for the yield of the matrix gas (Ar) were obtained in the temperature range of 40–90 K for the (CCl4 and Ar) system. It was found that the CCl4 molecules in the Ar matrix substance underwent cryocapture. The results obtained suggest that carbon tetrachloride can be used as a matrix in low-temperature studies on self-organization processes in thin films.

The effect of structural disorder on the hydrogen loading into the graphene/nickel interface

Hybrid materials composed by porous Ni foams coated by graphene appear appealing architectures to be applied in the field of hydrogen storage, where, due to the high surface-to-volume ratio of both components, are expected to be much more performant than their flat counterparts. In order to explore this possibility, in this study we have grown single layer graphene on nickel foams by chemical vapour deposition in ultra-high vacuum and have investigated the interaction with H atoms as a function of the temperature. By using high resolution C1s core level spectroscopy we found that nearly half of the graphene layer interacts with the support almost as strongly as with the flat ordered Ni substrate, whereas the other half is nearly free standing. Such dual electronic and structural coupling drives the hydrogenation of the graphene/foam interface. By using thermal programmed desorption combined with x-ray photoelectron spectroscopy we found that in the weakly interacting graphene regions, even at very low temperatures, H atoms easily intercalate below graphene, and enter in the bulk of the foam, from where they start to desorb around 180 K. This behaviour mimics what happens when dosing H atoms on the bare Ni foam. On the contrary, H intercalation below the strongly interacting graphene regions occurs only for temperatures around and above 200 K. The thermal desorption curves demonstrated that the presence of the graphene layer does not reduce the effectiveness of H loading in the bulk of the foam. On the other hand, it does not even increase significantly the stored amount with respect to the uncoated support, but contributes to stabilizing the stored hydrogen. Although the fundamental aspects of graphene/foam hydrogenation were here investigated in a regime far below the saturation of the bulk absorption, these measurements can be the starting point for further investigations aimed at establishing the ultimate storage capability of these hybrid nanostructured tanks.

Study of physisorbed carbon dioxide on zeolites modified by addition of oxides or acetate impregnation

Fundamental study of carbon dioxide (CO2) physisorption and chemisorption is investigated with the NaX zeolite modified by addition of magnesium oxide (mechanical mixtures) or by impregnation of magnesium acetate (impregnated zeolites). Defined by their textural properties, these samples are characterized using the CO2 adsorption and desorption. Complementary results are obtained with infrared experiments. This study enabled to conclude that the addition of small amounts of magnesium oxide improved the interactions of physisorbed CO2 with the materials without noticeable increase of the chemisorption. The CO2 desorption energies are increased when small magnesium amounts are added. As shown with the deconvolution of the thermal desorption curves, all the “high” temperatures physisorbed species (300–400K) are improved at the expense of the intermediate ones when the samples are modified.

Quantification and speciation of mercury in soils from the Tripuí Ecological Station, Minas Gerais, Brazil

Contents of total mercury, organic carbon, total sulfur, iron, aluminum and grain size and clay mineralogy were used along with Pearson's correlation and Hg thermal desorption technique to investigate the presence, distribution and binding behavior of Hg in soils from three depths from the Tripuí Ecological Station, located near Ouro Preto, Minas Gerais State, Brazil. The soils studied had predominantly medium and fine sand texture (0.59–0.062 mm), acid character and Hg contents ranging from 0.09 to 1.23 μg/g. The granulometric distribution revealed that Hg is associated with coarse sand (2–0.59 mm) and silt and clay (< 0.062 mm) and presents similar Hg concentrations in both fractions. Mercury distribution in soil profiles showed that Hg was homogeneously distributed throughout the depths at most sites. Hg thermal desorption curves show that mercury occurs not only as Hg 2+ predominantly bound to organic components in most of the samples, but also in the form of cinnabar in some. Pearson's correlation confirmed that mercury is associated with organic matter and sulfur and possibly with sulfur-bearing organic matter in most samples.

Adsorption dynamics of CO2 on copper-precovered ZnO(0001)–Zn: A molecular-beam scattering and thermal-desorption spectroscopy study

Initial, S(0), as well as coverage-dependent adsorption probability measurements, S(Theta), have been conducted at normal impact angle and as a function of the impact energy of CO(2), E(i), adsorption temperature, T(s), and copper precoverage, Theta(Cu) (at 300 K). S(0), which decreased from approximately 0.4 exponentially to approximately 0.05 with E(i) was independent of Theta(Cu). Astonishingly, S(0) for Cu on ZnO(0001)-Zn is smaller than for the clean support which indicates a chemical modification of the support by the Cu deposits. S(Theta) curves consist of two regimes, a Kisliuk-type and Langmuirian-type section. The first is consistent with capture zone models; the second may indicate direct adsorption of CO(2) on the Cu cluster. The thermal-desorption (TDS) curves for Cu on ZnO(0001)-Zn consist of two structures with binding energies of 26 and approximately 40 kJmol (nu=1 x 10(13) ls). The TDS results indicate that CO(2) populates predominantly the Cu deposits and the rim along the Cu nanoparticles. No indications for CO(2) dissociation could be obtained with Auger electron spectroscopy.

The effect of lattice defects on hydrogen thermal desorption of cathodically charged Co 3Ti

Using the thermal hydrogen desorption spectroscopy (THDS), the effects of lattice defects such as vacancies, dislocations and grain boundaries on the desorption behavior of hydrogen in the cathodically charged Co 3Ti were investigated. The low-temperature and high-temperature peaks were observed in the hydrogen thermal desorption curves, and as the trap sites of hydrogen, interstitial sites in L1 2 lattice and grain boundaries were suggested. The low-temperature peak was enhanced by deformation while the high-temperature peak was affected by grain size. As the holding time after hydrogen charging proceeded, the low-temperature peak decreased while the high-temperature peak increased. This change was faster and more significant in the deformed specimen than in the recrystallized specimen. It was suggested that during holding time hydrogen atoms migrate from the interstitial sites in L1 2 lattice to grain boundaries that are energetically deeper trap sites. The hydrogen atoms in the deformed specimen can migrate through dislocations or, with the help of excess vacancies, more quickly than those in the recrystallized specimen. The results were discussed in the association with the environmental embrittlement of L1 2 intermetallic compounds.

The size dependent adsorption properties of ferroelectric particles

We investigated the ability of a ferroelectric oxide surface to adsorb gaseous molecules with a dipole moment or an induced dipole moment. All the ferroelectric crystals studied readily adsorb carbon dioxide, thus our studies are centered on the adsorption studies of this molecule. We describe in this paper the present status of our studies: carbon dioxide–thermal desorption curves are obtained for powders of barium titanate, potassium niobate and lithium niobate with main particle sizes of 1000, 500, 200, 100 and 50μm in diameter. Two adsorption states are observed in these curves: a stronger state and a weaker state which appears only on the smallest particles measured (50μm). The percentage of surface area occupied by the CO2 molecules on the ferroelectric crystals is calculated after measuring the total surface area of the samples using the N2 single point BET method. Raman characterization as well as X-ray diffraction of the powders indicates that the dominant surface orientation of the crystallites changes when the particles become smaller than 100μm.

Evaluation of parameters from thermal desorption spectra – methods borrowed from the analysis of thermoluminescence

A number of methods for extracting the activation energy, pre-exponential factor and reaction order associated with the Polanyi–Wigner rate formulation have been reported in the literature. An entirely analogous equation had been utilized in the study of thermoluminescence (TL) and several methods have been developed for the extraction of the relevant parameters. Some of these methods are practically identical to those developed for the study of thermal desorption, whereas others have not been developed for thermal desorption but can be used with only very minor adjustment to the analysis of thermal desorption curves. In the present work, the utilization of the methods developed for the analysis of TL for the study of thermal desorption data is advocated.

Spectra and Photochemistry of Trifluoronitrosomethane Adsorbed on Alkali Halide Films

Trifluoronitrosomethane (CF3NO), which is readily photolyzed in the gas phase when exposed to light in the 600−700 nm portion of the spectrum, does not undergo significant photolysis when irradiated while adsorbed on sublimated films of alkali halides (NaCl, NaBr, KCl, KBr) at temperatures of 10−100 K. This quenching does not occur when the molecule is irradiated while adsorbed on argon films (thickness > 0.1 μm) or in multilayer coverage. Comparison of the quantum efficiencies for photolysis indicates that vibrational relaxation induced by surface phonons is primarily responsible for the alkali halide quenching, but lateral diffusion of the photofragments and/or excited-state molecules may also play a role in the kinetic scheme. The photochemistry was followed using infrared spectroscopy, providing the first vibrational characterization of adsorbed sub-monolayer CF3NO. A photodimer of CF3NO was also observed and vibrationally characterized on the argon films and on the overlayers. The NO stretching frequency for the dimer is significantly lower than had been previously reported for the N-nitritoamine photodimer {(CF3)2NONO} and may indicate the existence of a second photodimer species. Adsorption potentials were calculated for various sites on the four alkali halide films. The lowest energy orientation in each case had the C−N bond parallel to the surface, aligned in the [110] direction. The vibrational spectra and thermal desorption curves are consistent with the calculated energies.

Hydrogen bonding and surface interactions in protic solvents. Coadsorption of ammonia and hydrogen fluoride with water on silver(110)

Coadsorption of water with either ammonia or hydrogen fluoride on an Ag(110) substrate has been studied to examine the relationships between intermolecular hydrogen bonding near the surface and preferential adsorption at the surface. The experiments were conducted in ultrahigh vacuum (UHV) with facilities for thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS). Adsorption was performed at 110 K for ammonia and water and at 90 K for hydrogen fluoride and water. Ammonia and water are simply coadsorbed at 110 K with no evidence of specific hydrated complexes nor of ionization to NH4+. Coadsorbed water enhances the population of the chemisorbed state of ammonia, which we estimate to have a saturation coverage of 0.12 monolayer (ML) without water, through a mechanism of dielectric screening. At coverages approaching one monolayer and higher, water stabilizes the bulk of ammonia, increasing the average desorption temperature by as much as 21 K from 134 K. Stabilization energies estimated from these temperature shifts are of the order of 5 kJ mol–1, much less than typical hydrogen bond energies, ca. 25 kJ mol–1. In contrast, hydrogen fluoride and water form a well defined monohydrate HF · H2O, as evidenced by the coincident thermal desorption curves of both species. The monohydrate behaves as an adlayer azeotrope and thermal desorption measurements can be used to follow the sublimation temperature as a function of adlayer composition. From these data so-called bubble-point and dew-point curves can be developed by analogy with normal binary solutions.

Monte Carlo Study of Thermal Desorption Curves of Water from Zeolite Type A

We have studied temperature programmed desorption curves of water from zeolite Linde 4A using the Monte Carlo method taking into account the effects of readsorption and repulsive lateral interactions. It is shown that the peak maximum shifts to higher temperatures as readsorption increases and that lateral interactions only cannot reproduce experimental results.

Adsorption of hydrogen on a Ge covered Si(100) surface

In the present work we study hydrogen adsorption on Ge layers deposited by molecular beam epitaxy (at 600 K) on a Si(100) surface using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and work function measurements. The hydrogen TPD spectra exhibit a multipeak structure which gradually changes with increasing Ge coverage (up to 3 ML). Using Redhead's theoretical thermal desorption curves for the shape of the individual constituents of the H 2 TPD spectra, we have found five different states ( β 1− β 5). Accepting that hydrogen desorption occurs throug a transition state localized near a single atom, the behaviour of the deconvoluted peaks upon increasing Ge and hydrogen coverages, suggests the existence of long and short range effects of deposited Ge. The work function changes, observed upon hydrogen exposure and after mild annealing, give strong evidence of an initial uniform hydrogen adsorption on a Ge Si(100) surface and migration from Ge to Si related sites, during the annealing procedure.

Synthesis and characterization of high-silica zeolite RHO prepared in the presence of 18-crown-6 ether as organic template

Pure high-silica RHO-type zeolite samples with a framework Si Al molar ratio as high as 4.5 were prepared by using 18-crown-6 ether as an organic template from a starting aluminosilicate gel containing cesium cations. The solids were characterized by elemental analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD) thermal analysis, solid-state nuclear magnetic resonance (NMR) spectroscopy and n-hexane adsorption. The NH 3 stepwise thermal desorption (STD) curve of high-silica zeolite rho shows stronger acid sites than for a FAU-type material with similar Si Al molar ratio. High-temperature XRD analyses performed on the protonated hydrated form of high-silica zeolite rho showed no framework flexibility, presumably because of a higher covalent character of the framework.

Conductivity of [R 4PcFeL] n and [ R8PcFeL] n films on NO 2 doping

Films of soluble substituted polymerized iron phthalocyanine compounds [R 4PcFeL] n and [R 8PcFeL] n are elaborated and doped at various temperatures with NO 2 diluted in N 2 and their conductivity is recorded. Doping/dedoping cycles and thermal desorption curves provide information on NO 2 adsorption and on the stability of phthalocyanine compounds on exposure to NO 2 versus ligand species and number of alkoxy substituents.

Effects of oxygen loss from Bi-2201 phase doped by Pb and La on superconductivity

Effects of annealing of Bi 1.6Pb 0.4Sr 1.6La 0.4CuO x in vacuum on lattice parameters and superconductivity are studied. The amount of evolved oxygen (Δy) is obtained as 6.5×10 −3 by integration of thermal desorption curve of oxygen detected by a mass spectrometer. Upon annealing, the a-axis extends and the T C increases from 22 to 34 K. These results are briefly discussed in terms of redistribution of charge in the system.

Dissolution and diffusion of helium in graphite

Thermal desorption measurements were performed on three isotropic graphite samples which were exposed to helium gas at 100–700° C under 7–130 kPa for 1 h. Helium atoms which were desorbed from the sample during ramp heating at 10° C/min in vacuum were identified with a quadrupole mass spectrometer. Helium was dissolved into graphite for a few ppm in atomic fraction. The thermal desorption curve for ISO-880U appears to consist of two peaks (450 and 620° C) after helium gas exposure at above 500° C. The helium desorption can be ascribed to the diffusion-controlled process where the helium atoms may leave through bulk surface and open pores.

Some aspects of the adsorption properties of silanized silica gel at the solid/liquid interface part 2. Thermogravimetric nalysis

Thermal analysis was applied to the investigations of the adsorption process from the liquid phase on completely hydroxylated silica gel and silica gel with a bonded octyl phase. The results obtained from thermal desorption curves were compared with some parameters characterizing the adsorption process of n-butanol and benzene on different types of silicas. Good correlation between parameters obtained using different methods is observed.

Thermal desorption behavior of he implanted into graphite

Thermal desorption of He from various graphite samples irradiated with 20 keV He + ions has been studied. Thermal desorption behavior of He from Poco graphite was slightly different from that of Isograph-88 and above a dose of approximately 10 18 ions/cm 2 both isotropic graphites had almost identical desorption curves with peak temperatures of approximately 330°C. In the case of the graphitized paper Papyex, the amount of released He reached a maximum at a dose of 5 × 10 17 ions/cm 2 and the peak temperature on the thermal desorption curve became constant at approximately 200°C above a dose of 1 × 10 18 ions/cm 2 . The activation energy ( E d ) for He desorption from Poco graphite increased with the irradiation dose and was estimated to be 0.72 eV for a dose of 5 × 10 16 ions/cm 2 , 0.90 eV for 5 × 10 17 and 1 × 10 18 ions/cm 2 , respectively.

Estimation of thermal desorption efficiencies from activated carbon for determination of adsorbed organic solvents

A method for estimating thermal desorption curves and desorption efficiencies of organic solvents from activated carbon was developed for the determination of adsorbed solvent by thermal desorption method, and the estimated values were compared with the experimental values. The Freundlich or the Langmuir isotherm was assumed in obtaining the solution. Thermal desorption experiments were conducted by the following two methods: a) Desorption of solvents which were adsorbed in the activated carbon at the desorption temperature under the nitrogen gas flow containing vapor until the system reached its equilibrium state. b) Desorption of solvents which were adsorbed in the activated carbon previously at room temperature. The model was based on a) method, but the estimated desorption curves and desorption efficiencies agree well with the experimental ones not only for a) method but also for b) method.

活性炭に吸着した混合有機溶剤の加熱脱着率の簡易推算法

活性炭に吸着した混合有機溶剤の加熱脱着率の簡易推算法