Apparent Activation Energies Of Desorption Research Articles

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.

Strain dependence of dissociative adsorption of H[formula omitted]O on epitaxially strained, out-of-plane polarized, BaTiO[formula omitted](001) thin films

The strain dependence of the apparent activation energy of desorption of H2O on epitaxial, TiO2-terminated, out-of-plane polarized, BaTiO3(100) thin films has been investigated using X-ray diffraction, X-ray photoelectron spectroscopy and temperature-programmed desorption. The apparent activation energy of desorption increases significantly with the in-plane strain, suggesting that the strain-induced ferroelectric distortion determines the strength of the chemical bond between the dissociatively adsorbed water and the surface.

Facile Synthesis of Carbon Supported Nano-Ni Particles with Superior Catalytic Effect on Hydrogen Storage Kinetics of MgH2

Metal nanocatalysis is an effective method to enhance the hydrogen storage properties of magnesium hydride (MgH2), and the catalytic effect can be further improved by a matrix material supported nanometal. In this work, carbon supported nano-Ni (Ni@C) was synthesized by calcination of dimethylglyoxime dinickel chelate, and then it was doped into MgH2 to improve the de/rehydrogenation kinetics. This shows that the homogeneously distributed Ni with refined particle size in carbon base leads to superior catalytic effects on hydrogen absorption/desorption of MgH2-5 wt % Ni@C. The MgH2-5 wt % Ni@C starts to desorb hydrogen at 187 °C, which is 113 °C lower than that of as-milled MgH2. Moreover, it takes only 500 s to thoroughly desorb hydrogen at 300 °C, which is 3000 s faster than as-milled MgH2 under the same dehydrogenation conditions. According to the Kissinger’s method, the apparent activation energy for desorption of the MgH2-5 wt % Ni@C is 66.5 ± 1.8 kJ mol–1, which is about 79.9 kJ mol–1 lower than that...

Enhanced adsorption of CO2 at steps of ultrathin ZnO: the importance of Zn-O geometry and coordination.

The interaction between CO2 and ultrathin ZnO supported on Au(111) has been studied using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. We find that CO2 binds weakly on the planar ZnO bilayer and trilayer surfaces, desorbing at T = 130 K. CO2 binds more strongly at the steps formed between ZnO bilayers and trilayers, desorbing at T = 285-320 K depending upon the CO2 exposure. The adsorption energies determined from DFT calculations for CO2 on the ZnO planar surfaces and at the steps are ∼5.8 and 19.0 kcal mol-1, respectively, agreeing with the apparent activation energies of desorption (Ed) estimated based on the TPD peaks at the limit of low CO2 exposures (7.7 and 19.5 kcal mol-1, respectively). The DFT calculations further identify that the most stable adsorption configuration of CO2 at the steps of ultrathin ZnO is facilitated by the geometry and coordination of the Zn cations and O anions near the step region. Specifically, the enhanced adsorption takes place via bonding of both the C and O atoms of the CO2 molecule to the tri-fold coordinated O anions at the trilayer edge and to the neighboring Zn cations on the bilayer terrace, respectively, leading to CO2 bending and formation of a carbonate-like species.

D2O Interaction with Planar ZnO(0001) Bilayer Supported on Au(111): Structures, Energetics and Influence of Hydroxyls

We investigate the interaction between D2O and the planar ZnO(0001) bilayer grown on Au(111) with temperature programmed desorption (TPD), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. We show that D2O molecules adsorbed on this planar surface form two ordered overlayers, a (3 × 3) and a (√3 × √3)R30°, not seen before on any of the bulk ZnO single crystal surfaces. The apparent activation energies of desorption (Ed) estimated from TPD peaks are 15.2 and 16.7–17.3 kcal/mol for (3 × 3) and (√3 × √3)R30°, respectively, which agree well with the adsorption energy values calculated from DFT (14.9–15.6 kcal/mol and 16.8–16.9 kcal/mol, respectively). The DFT calculations reveal that the formation of the overlayers takes place at different packing densities and is mediated by extensive hydrogen bonding among the molecules. The hydroxyl groups, which accumulate very slowly on the ZnO(0001) bilayer surface under the standard ultrahi...

Catalytic activity of titania polymorphs towards desorption reaction of MgH2

Hydrogen desorption properties of MgH2 ball milled with two different TiO2 polymorphs, rutile and anatase, were studied. Structural characterization has been done by X-ray diffraction (XRD) analysis, particle size analysis (PSD) and scanning electron microscopy (SEM). The mechanism of desorption and the apparent activation energy for desorption were investigated by applying isoconversional kinetic analysis of DTA spectra, while desorbed species were followed by TPD coupled with mass spectroscopy. It has been demonstrate that the addition of rutile TiO2 significantly decrease the apparent activation energy for desorption (Edesa), while addition of anatase titania has negligible influence on Edesa. Such behaviour could be explained in terms of the same crystal structure of rutile TiO2 and MgH2. Further, the mechanism of desorption changes from Avrami–Erofeev n = 3 for pure MgH2 to Avrami–Erofeev n = 4 for composite materials.

Apparent Activation Energies Associated with Protein Dynamics on Hydrophobic and Hydrophilic Surfaces

With the use of single-molecule total internal reflection fluorescence microscopy (TIRFM), the dynamics of bovine serum albumin (BSA) and human fibrinogen (Fg) at low concentrations were observed at the solid-aqueous interface as a function of temperature on hydrophobic trimethylsilane (TMS) and hydrophilic fused silica (FS) surfaces. Multiple dynamic modes and populations were observed and characterized by their surface residence times and squared-displacement distributions (surface diffusion). Characteristic desorption and diffusion rates for each population/mode were generally found to increase with temperature, and apparent activation energies were determined from Arrhenius analyses. The apparent activation energies of desorption and diffusion were typically higher on FS than on TMS surfaces, suggesting that protein desorption and mobility were hindered on hydrophilic surfaces due to favorable protein-surface and solvent-surface interactions. The diffusion of BSA on TMS appeared to be activationless for several populations, whereas diffusion on FS always exhibited an apparent activation energy. All activation energies were small in absolute terms (generally only a few kBT), suggesting that most adsorbed protein molecules are weakly bound and move and desorb readily under ambient conditions.

The effects of graphite on the reversible hydrogen storage of nanostructured lithium amide and lithium hydride (LiNH 2 + 1.2LiH) system

The addition of 5 wt.% of graphite was incorporated into the (LiNH 2 + 1.2LiH) hydride system in order to study its effect on the prevention of LiH from hydrolysis/oxidation which leads to the escape of NH 3. The composite hydride system was processed by ball milling for 25 h. Thermal behavior in DSC up to 500 °C and isothermal desorption in a Sieverts-type apparatus were carried out. XRD was used to obtain information about phase changes. It is found that after ball milling graphite becomes amorphous. DSC analysis shows that for the mixture ((LiNH 2 + 1.2LiH) + 5 wt.% graphite) graphite can prevent or at least substantially reduce the oxidation/hydrolysis of LiH since no melting peak of retained LiNH 2 is observed. Both the DSC and Sieverts-type tests show that the addition of graphite increases the apparent activation energy of desorption from the ∼57–58 to ∼85–90 kJ/mol range. On the other hand, the graphite additive increases measurably the desorbed/absorbed capacity of hydrogen at 275, 300 and 325 °C. The ((LiNH 2 + 1.2LiH) + 5 wt.% graphite) system is fully reversible desorbing/absorbing ∼5 wt.% H 2 at 325 °C in the following reaction: (LiNH 2 + LiH ↔ Li 2NH + H 2). Step-wise pressure–composition–temperature (PCT) tests show that the enthalpy and entropy change of this reversible reaction is −62.4 and −61.0 kJ/mol H 2 and 117.8 and 115.8 J/mol K for undoped and 5 wt.% G doped (LiNH 2 + 1.2LiH) system, respectively. It shows that within an experimental error there is no measurable effect of graphite additive on the thermodynamic properties. The Van’t Hoff analysis of the obtained thermodynamic data shows that the equilibrium temperature at atmospheric pressure of hydrogen (1 bar H 2) is 256.8 and 253.9 °C for the undoped and 5 wt.% G doped (LiNH 2 + 1.2 LiH) system ball milled for 25 h, respectively. Such high equilibrium temperatures render it rather obvious that both of these hydride systems cannot be employed for hydrogen desorption/absorption below 100 °C as required by the DOE targets for the automotive hydrogen storage materials.

Adsorptive removal of tert-butylmercaptan and tetrahydrothiophene using microporous molecular sieve ETS-10

The adsorptive removal of tert-butylmercaptan (TBM) and tetrahydrothiophene (THT) in methane gas using microporous molecular sieve ETS-10 was investigated at an ambient temperature and atmospheric pressure. Na,K-ETS-10 and Cu-exchanged Na,K-ETS-10 (Cu-ETS-10) were characterized by X-ray diffractometer (XRD) for the crystalline phase, X-ray fluorescence (XRF) analyzer for the chemical composition, and nitrogen adsorption-isotherm measurement for the BET surface area and porosity, respectively. Posterior to its treatment at 723K in He atmosphere for 2h, the Cu(II) in Cu-ETS-10 could be partially autoreduced to Cu(I), which was confirmed by X-ray photoelectron spectroscopy (XPS). The preferential adsorption of THT over TBM on Na,K-ETS-10 and the concurrent adsorption of TBM and THT on Cu-ETS-10 were achieved, which could be explained by the uptake curve in the binary component adsorption, the temperature-programmed desorption, and the apparent activation energy for desorption. The breakthrough sulfur adsorption capacity for both TBM and THT was attained to 2.50mmolS/g on Cu-ETS-10-0.1(4). This markedly high breakthrough sulfur adsorption capacity on Cu-ETS-10 is unprecedented in removing organic sulfur compounds from fuel gas by adsorption on zeolites.

TPD and ITPD study of materials used as chemoresistive gas sensors

Temperature-programmed desorption (TPD) and a differential form of it, called intermittent temperature-programmed desorption (ITPD), turned out to be powerful characterising techniques for chemoresistive materials applied to gas sensing. We investigated samples of SnO 2, TiO 2 and solid solutions of them (Ti x Sn 1 − x O 2). TPD and ITPD experiments were carried out in vacuum, with samples previously treated in pure O 2 (100 Torr, 500 °C, 30 min). Amounts of desorbed O 2 corresponded for all Ti-containing samples to less than 10% of a compact monolayer of ions O 2−. Corresponding values of the apparent activation energy of desorption ( E app) were calculated directly from the Arrhenius plots for each partial TPD and ranged from about 100 to 330 kJ mol − 1 (1.16 to 3.82 eV).

Study of desorption kinetics of polycyclic aromatic hydrocarbons (PAHs) from solid matrices using internally cooled coated fiber

The kinetics of desorption of hydrophobic organic compounds (HOCs) from soil and sediment particles is important both from environmental and analytical chemistry points of view. Reliable techniques are required for prediction of desorption behaviour of HOCs from contaminated soils and sediments. In this study internally cooled coated fiber device, in which a PDMS hollow fiber extraction phase is cooled with liquid CO 2, was used as an exhaustive extraction sorbent phase for extraction of desorbed organic compounds (e.g. polycyclic aromatic hydrocarbons, PAHs) from both laboratory-spiked and naturally contaminated solid sample into the gaseous headspace in a batch system. The extraction time profiles were obtained at two different elevated temperatures (above 100 °C) for spiked sand and silica gel matrices. The slow desorption rate constants at each temperature were determined from desorption plots and the apparent activation energies of desorption were obtained from Arrhenius equations. The apparent activation energies of desorption of naphthalene, acenaphthylene and acenaphthene, from spiked silica gel, were approximately 60 kJ mol −1, and were higher, 70 and 100 kJ mol −1 for fluoranthene and anthracene, respectively. The fast and slow desorption rates and apparent activation energies of desorption for PAHs were obtained by spiking a naturally contaminated sediment sample with deuterated PAHs (PAHs-d 10). The activation energies of native PAHs were higher than those of spiked deuterated PAHs, suggesting that the native compounds were more affected by retarded pore diffusion or slow mass transfer through glassy sorbent organic matter (SOM). The proposed technique in the present study is fully automated, and can extract the contaminants from the solid matrix fast and exhaustively, which makes it more time efficient and versatile compared to the commonly used technique for desorption studies, i.e. vial desorption.

Study of oxygen desorption from substituted perovskites: New possibilities through fast intermittent temperature-programmed desorption

A differential desorption technique, named intermittent temperature-programmed desorption (ITPD), has been used to get new insights into the properties of La 1 − x Sr x Co 0.8Fe 0.2O 3 − δ perovskites. The apparent activation energy of desorption ( E app) and the frequency factor ( ν) were calculated from ITPD data, for the two distinct oxygen species desorbing from perovskite at temperatures lower than 400 °C. The low values (about 10 7–10 8 s − 1 ) obtained for ν app indicated that re-adsorption occurred during the TPD process and therefore that E app is equal to the Heat of adsorption ( E = − Δ H). The first oxygen species, related to lower desorption temperatures, exhibits a distribution of E from 105 to 125 kJ mol − 1 . The second one, related to higher desorption temperatures, corresponds to E = 146 ± 4 kJ mol − 1 and 139 ± 5 kJ mol − 1 , for x = 0.2 and x = 0.3, respectively. The relative amounts of these species contributing to the desorption peak are dependent upon Sr content in the perovskite.

Study of Oxygen Desorption from SnO2: New Possibilities through Fast Intermittent Temperature-Programmed Desorption

A differential desorption technique called intermittent temperature-programmed desorption (ITPD) was used to gain new information on the superficial oxygen species of SnO2, a solid used extensively for the preparation of sensors and redox catalysts. The use of an original device involving an induction heating system enabled the time necessary for ITPD determination to be decreased by a factor of 20 or so, compared to that with a classical apparatus. TPD and ITPD studies carried out in vacuum showed the existence of at least two distinct states desorbing at temperatures higher than 700 K. The corresponding amount of adsorbed oxygen was less than 10% of that necessary for the formation of a compact monolayer of oxide ions. These two states exhibited apparent activation energies of desorption, Eapp, equal to 285 ± 10 kJ/mol and 334 ± 8 kJ/mol, respectively. The corresponding frequency factors, v, were far higher than 1013 s−1, indicating that re-adsorption was unlikely and that Eapp was equal to the activati...

Hydrogen desorption properties of MgH 2 nanocomposites with nano-oxides and Inco micrometric- and nanometric-Ni

The effects of doping with 5 wt.% of nano-oxides (n-Al 2O 3 and n-Y 2O 3), specialty Inco micrometric- and nanometric-Ni (m- and n-Ni) and their equal weight ratio mixtures on the hydrogen desorption properties of commercial MgH 2 were studied. The powder mixtures were mechanically milled for 20 and 100 h. Doping with n-Al 2O 3 does not have a beneficial effect and doping with n-Y 2O 3 has only a limited effect on the temperatures and hydrogen desorption kinetics while nanocomposites doped with specialty Ni show reduced hydrogen desorption temperatures, very fast desorption kinetics under 0.1 MPa H 2 pressure and substantially reduced apparent activation energies of desorption. Prolonged milling for 100 h is detrimental for the hydrogen storage properties of the MgH 2 doped with the specialty Inco m- and n-Ni.

Desorption kinetics studies on PAH-contaminated soil under varying temperatures

The purpose of this study was to investigate the effect of temperature on the release of polycyclic aromatic hydrocarbons (PAHs) from aged contaminated soil. The release of fluorene, phenanthrene, anthracene, fluoranthene and pyrene at 7, 15, 18 and 23 °C was studied using a column leaching method with a hydraulic retention time of 0.5 h. As the temperature declined from 23 to 7 °C the concentrations decreased by a factor of 11–12 for all the studied compounds except for anthracene, which only decreased by a factor 7. Rate constants at maximum release rate at the four studied temperatures were assessed. From temperature dependence studies, apparent activation energies of desorption, E des ∗ , were calculated. E des ∗ -values appeared to be in the range of 105–137 kJ mol −1 for the studied PAHs and increased with the LeBas molar volume of the compounds. The increase of E des ∗ with increased molecular size indicates stronger sorption with increased hydrophobicity of the compounds.

Anomalous Phenomena in Oxygen Desorption from Tungsten and Their Mathematical Modeling

A model is developed that describes unusual phenomena in oxygen desorption from tungsten surfaces: (1) in temperature-programmed desorption (TPD) in the transfer from the linear regime of sample heating to the isothermal regime, a maximum of the desorption rate is observed; (2) in the stepped partial desorption of oxygen, the beginning of each subsequent TPD peak shifted toward higher temperatures; (3) the apparent activation energy of desorption increased in the latter case by a factor of ∼3 (from 280 to 920 kJ/mol). Monte Carlo modeling that takes into account surface migration of adsorbed oxygen atoms and their lateral interactions gave semiquantitative description of experimental data.

Adsorption of oxygen on the paramagnetic defect centers in aluminophosphate molecular sieves

Temperature-programmed desorption (TPD) and electron spin resonance (ESR) techniques have been employed to investigate the oxygen adsorbed on the paramagnetic defect centers in dehydroxylated AlPO4-20, AlPO4-11, AlPO4-5, and VPI-5 molecular sieves. Two different adsorption sites are observed for the small-pore AlPO4-20, where oxygen is entrapped within the intracrystalline voids generated during the calcination step, as well as within the β-cages. However, the unidimensional molecular sieves AlPO4-11, AlPO4-5, and VPI-5 have only one type of adsorption site within their inner pores. The apparent activation energies of desorption (12.0-21.7 kcal mol-1) determined from the oxygen TPD results demonstrate that, as the pore size of the AlPO4 molecular sieve is larger, the interaction between the adsorbed oxygen and the molecular sieve framework becomes weaker.

Dynamics of OH desorption from single crystal Pt(111) and polycrystalline Pt foil surfaces

The laser-induced fluorescence technique in conjunction with an UHV apparatus was used to probe the desorption dynamics of OH radicals formed in the association reaction of chemisorbed O and H atoms on single crystal Pt(111) and polycrystalline Pt foil surfaces. The apparent activation energies of desorption and the rotational energy distributions of OH radicals were measured in these experiments. The apparent OH desorption energies for both surfaces were observed to increase by about 10 kcal/mol as the O/H reactant gas mixture ratio is decreased by a factor of 100, with the values for the Pt(111) single crystal at 5–10 kcal/mol below those for the Pt foil. Computer kinetic modeling of the temperature dependence of OH desorption rate as a function of O/H ratio showed similar trends as those observed in the experiments. Boltzmann-like OH rotational energy distributions with relatively high rotational to surface temperature ratios, Tr/Ts=0.85, were measured over the temperature range 1227–1479 K for the Pt(111) single crystal and over 1283–1475 K for the polycrystalline Pt foil. For both surfaces the OH spin-orbit temperatures were essentially the same as the rotational temperatures and no preference for any lambda-doublet state was observed. The Tr/Ts ratio does not seem to depend on the O/H reactant gas mixture ratio for each surface, suggesting that the rotational energy accommodation is insensitive to the apparent Pt-OH bond strength.

Some kinetic aspects of the oxidation of graphite and platinum graphimet

The catalytic effect of platinum on the oxidation of graphite has been examined for 1 wt% Pt-graphimet with the use of temperature programmed desorption technique. Analyses of results show that the addition of platinum particles enhances the overall rate of carbon oxides evolution by increasing the active surface area, with little or no change in the apparent activation energy of desorption. This is attributed to the scission of carbon-carbon bonds by platinum. Results of the mathematical modelling based on controlled atmosphere electron microscopy studies by Baker and his co-workers seem to suggest the existence of a viscous oxide layer and surface diffusion on the metal surface and a possibility that both electron and oxygen transfer mechanisms are operating in the carbon-oxygen reactions catalysed by platinum group metals.

Thermal desorption of n-butylamine from a silica-alumina surface

Temperature-programmed desorption (TPD) of n-butylamine ( n-ba) has been used as a tool to investigate the nature of the active sites on the surface of a silica-alumina gel. On dry samples, at low temperature and short times of adsorption, the TPD spectra show two peaks: the first peak corresponds to n-ba weakly held that desorbs around 150°C whereas the second one is composed of propylene and 1-butene formed during the TPD at temperatures about 400°C. As temperature or time of adsorption is increased, a slow conversion of the first into the second peak is observed. At increasing adsorption temperatures, an intermediate peak develops around 300°C. When water is preadsorbed, the height and shape of the first peak change slightly, the change is more pronounced in the last peak and is very important in the intermediate one. Conclusions about the nature of the active sites on the surface are drawn from these experimental facts. Finally, apparent activation energies of desorption for the first two peaks are calculated.