Pre-exponential Factor For Desorption Research Articles

Mitigating Global Methane Emissions Using Metal-Organic Framework Adsorbents

Global emission of methane reached a record high in 2020. Furthermore, it is expected that methane emissions will continue to rise in the coming years despite the economic slowdown stemming from the coronavirus pandemic. Adsorbents can be used to reduce methane emissions. However, the question remains as to which adsorbents perform best for enhanced methane capture. In this work, it is demonstrated that metal-organic frameworks (MOFs) exhibited the best methane uptakes at 1 bar and 298 K from experiments as compared to tested carbonaceous materials, polymers, and zeolites. In addition, the adsorption entropy, an important thermodynamic property indicating adsorption capacity and kinetics, is determined on well-defined MOFs using a global predictive equation for porous materials. A correlation was used to describe the effect of translation and rotation of methane in the porous material for methane emission abatement. This information and the entropy of adsorption of methane on MOFs has not been reported before. The predicted results were compared to experimental data obtained from adsorption isotherms. Optimum isosteric heats were calculated by the Bhatia and Myers correlation. Finally, the pre-exponential factor of desorption is determined to aid in the design of materials for global methane emissions mitigation.

Real-time observation of cation exchange kinetics and dynamics at the muscovite-water interface

Ion exchange at charged solid–liquid interfaces is central to a broad range of chemical and transport phenomena. Real-time observations of adsorption/desorption at the molecular-scale elucidate exchange reaction pathways. Here we report temporal variation in the distribution of Rb+ species at the muscovite (001)–water interface during exchange with Na+. Time-resolved resonant anomalous X-ray reflectivity measurements at 25 °C reveal that Rb+ desorption occurs over several tens of seconds during which thermodynamically stable inner-sphere Rb+ slowly transforms to a less stable outer-sphere Rb+. In contrast, Rb+ adsorption is about twice as fast, proceeding from Rb+ in the bulk solution to the stable inner-sphere species. The Arrhenius plot of the adsorption/desorption rate constants measured from 9 to 55 °C shows that the pre-exponential factor for desorption is significantly smaller than that for adsorption, indicating that this reduced attempt frequency of cation detachment largely explains the slow cation exchange processes at the interface.

Low temperature adsorption and site-conversion process of CO on the Ni(111) surface

Low-temperature (25K) adsorption states and the site conversion of adsorbed CO between the ontop and the hollow sites on Ni(111) were studied by means of temperature programmed desorption and infrared reflection absorption spectroscopy. The activation energy and pre-exponential factor of desorption were estimated to be 1.2eV and 2.6×1013s−1, respectively, in the limit of zero coverage. At low coverage, CO molecules preferentially adsorbed at the hollow sites below 100K. With increasing temperature, the ontop sites were also occupied. Using a van't Hoff plot, the enthalpy and the entropy differences between the hollow and ontop CO were estimated to be 36meV and 0.043meVK−1, respectively, and the vibrational entropy difference was estimated to be 0.085meVK−1. The positive entropy difference was the result of the low-energy frustrated translational mode of the ontop CO, which was estimated to be 4.6±0.3meV. With the harmonic approximation, the upper limit of the activation energy of site hopping from ontop sites to hollow sites was estimated to be 61meV. In addition, it was suggested that the activation energy of hollow-to-hollow site hopping via a bridge site was less than 37meV.

Hydrogen surface coverage of as-prepared nanocrystalline porous silicon

Porous silicon (PoSi) surface coverage by hydrogen has been investigated by thermal desorption spectroscopy (TDS). The desorption rate of H 2 and SiH x ( x=1–4) molecules was monitored with a quadrupole mass spectrometer during linear heating of the samples in vacuum. The almost simultaneous desorption of several species containing hydrogen gives rise to complicated TD spectra. The full spectra can be understood as due to H-atom desorption from different bonding states at the PoSi surface, namely a SiH(a) surface monohydride, a SiH 2(a) dihydride and a SiH 3(a) trihydride. In order to obtain clean spectra for a kinetic analysis, as prepared samples were annealed at selected temperatures to isolate the contributions from the different bonding states. A detailed knowledge of the kinetic parameters for the desorption process has been obtained from these experiments. In this paper, the order of the reaction, activation energy and pre-exponential factor for desorption from each H bonding state will be presented.

Desorption kinetics of model polar stratospheric cloud films measured using Fourier Transform Infrared Spectroscopy and Temperature‐Programmed Desorption

AbstractThis study combines Fourier transform infrared (FTIR) spectroscopy and temperature‐programmed desorption to examine the evaporation kinetics of thin films of crystalline nitric acid hydrates, solid amorphous H2O/HNO3 mixtures, H2O–ice, ice coated with HCl, and solid HNO3. IR spectroscopy measured the thickness of each film as it evaporated, either at constant temperature or during a linear temperature ramp (temperature‐programmed infrared, TPIR). Simultaneously, a mass spectrometer measured the rate of evaporation directly by monitoring the evolution of the molecules into the gas phase (temperature‐programmed desorption, TPD). Both TPIR and TPD data provide a measurement of the desorption rate and yield the activation energy and preexponential factor for desorption. TPD measurements have the advantage of producing many data points but are subject to interference from experimental difficulties such as uneven heating from the edge of a sample and sample‐support as well as pumping‐speed limitations. TPIR experiments give clean but fewer data points. Evaporation occurred between 170 and 215 K for the various films. Ice evaporates with an activation energy of 12.9 ± 1 kcal/mol and a preexponential factor of 1 × 1032±1.5 molec/cm2 s, in good agreement with the literature. The beta form of nitric acid trihydrate, β–NAT, has an Edes of 15.6 ± 2 kcal/mol with log A = 34.3 ± 2.3; the alpha form of nitric acid trihydrate, α–NAT, is around 17.7 ± 3 kcal/mol with log A = 37.2 ± 4. For nitric acid dihydrate, NAD, Edes is 17.3 ± 2 kcal/mol with log A = 35.9 ± 2.6; for nitric acid monohydrate, NAM, Edes is 13 ± 3 kcal/mol with log A = 31.4 ± 3. The α–NAT converts to β–NAT during evaporation, and the amorphous solid H2O/HNO3 mixtures crystallize during evaporation. The barrier to evaporation for pure nitric acid is 14.6 ± 3 kcal/mol with log A = 34.4 ± 3. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 295–309, 2001

Determination of the desorption parameters of hydrogen on a stainless steel surface by analysis of time-pressure curves

As the course of the pressure in a vacuum system after an adsorption equilibrium impairment is strongly influenced by the sojourn times of the gas molecules on the surface the idea arose to determine the sojourn times, and consequently the Arrhenius' parameters of desorption, by analysis of the time-pressure curves. The gas release was caused by mechanical action of a tool to a stainless steel sample. Such action was attended by pressure pulse and after it the pressure fell to approximately the initial value. The experiments were carried out at various temperatures of the sample surface and the partial pressures of the released gases were registered. The obtained data for hydrogen were analysed according to a mathematical model describing the time-pressure curves as a multicomponent exponential decay and values of the sojourn times were found. The activation energy and pre-exponential factor for desorption of hydrogen on a stainless steel sample were determined from the Arrhenius' plot.

NO adsorption and thermal behavior on Pd surfaces. A detailed comparative study

The adsorption and thermal behavior of NO on ‘flat’ Pd(111) and ‘stepped’ Pd(112) surfaces has been investigated by temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and electron stimulated desorption ion angular distribution (ESDIAD) techniques. NO is shown to molecularly adsorb on both Pd(111) and Pd(112) in the temperature range 100–373 K. NO thermally desorbs predominantly molecularly from Pd(111) near 500 K with an activation energy and pre-exponential factor of desorption which strongly depend on the initial NO surface coverage. In contrast, NO decomposes substantially on Pd(112) upon heating, with relatively large amounts of N 2 and N 2O desorbing near 500 K, in addition to NO. The fractional amount of NO dissociation on Pd(112) during heating is observed to be a strong function of the initial NO surface coverage. HREELS results indicate that the thermal dissociation of NO on both Pd(111) and Pd(112) occurs upon annealing to 490 K, forming surface-bound O on both surfaces. Evidence for the formation of sub-surface O via NO thermal dissociation is found only on Pd(112), and is verified by dissociative O 2 adsorption experiments. Both surface-bound O and sub-surface O dissolve into the Pd bulk upon annealing of both surfaces to 550 K. HREELS and ESDIAD data consistently indicate that NO preferentially adsorbs on the (111) terrace sites of Pd(112) at low coverages, filling the (001) step sites only at high coverage. This result was verified for adsorption temperatures in the range 100–373 K. In addition, the thermal dissociation of NO on Pd(112) is most prevalent at low coverages, where only terrace sites are occupied by NO. Thus, by direct comparison to NO Pd(111) , this study shows that the presence of steps on the Pd(112) surface enhances the thermal dissociation of NO, but that adsorption at the step sites is not the criterion for this decomposition.

Time-resolved EELS studies of molecular surface residence times: the CO/Cu(111) desorption system

Time-resolved high resolution electron energy loss Spectroscopy was used to measure directly the coverage of carbon monoxide on Cu(111) following pulsed molecular beam dosing. The surface residence time of adsorbed CO was a strong function of both substrate temperature and adsorbate coverage. Analysis of the data shows a dramatic increase in the pre-exponential factor for desorption with increasing CO coverage in the absence of any compensation in activation energy. These results are discussed in light of the generalized quasi-chemical approximation for a triangular lattice. The data is also compared with results obtained by temperature programmed desorption.

Monte carlo simulation of temperature programmed desorption of Ag atoms from a Ru(001) surface

A Monte Carlo simulation method was applied to study the desorption process of submonolayer amounts of Ag atoms from Ru(001). The attractive lateral interactions between Ag atoms leading to island formation are also manifested in the desorption kinetics. First the equilibrium configuration of the Ag overlayer of hexagonal symmetry was simulated for different coverages. In the simplest case the desorption rate is evaluated as the sum of first order desorption rates of Ag atoms having 0, 1, 2, ...6 nearest Ag-Ag bonds, assuming the preexponential factor for desorption to be constant. The stimulated desorption curves exhibit quasi-zeroth order desorption kinetics for initial Ag coverages less than 0.35 ML. At higher coverages the desorption spectra are shifted towards higher temperatures and no unique slope can be identified. When the pre-exponential factor, however, is assumed to increase with Ag coverage according to a simple combinatory formula, the calculated desorption spectra exhibit the quasi-zeroth order behaviour also up to higher coverages in qualitative agreement with experimental results.

Effects of degree of mixing on quantitative interpretation of TPD spectra

The influence of the degree of mixing on the qualitative and quantitative interpretation of TPD spectra is investigated by simulation of physicochemical processes occurring within TPD cells. A methodology for accounting for readsorption effects is also developed and utilized in the simulations. It is shown that, as a result of imperfect mixing conditions, peak temperatures shift to lower values. Computed values of the heat of adsorption and the preexponential factor of desorption are always less than the true values, which can be obtained only under conditions of perfect mixing. Errors introduced by the presence of axial concentration gradients are large when desorption follows first-order kinetics and significantly smaller when second-order desorption kinetics are followed. An experimental procedure accounting for mixing effects is proposed.

The morphology of As terminated Si(111) from desorption kinetics

Isothermal desorption measurements of film/substrate systems yield rate parameters characteristic of the thermodynamical driving forces which determine the film morphology. We show from the activation energy for desorption for As on Si(111) that it is energetically favourable to form a uniform layer rather than clustering (for coverages up to ≈ 1 monolayer). This is supported by the fact that the desorption process follows a second-order kinetic law. The pre-exponential factor for desorption is consistent with a restricted surface mobility of the adatoms. These results bear on issues associated with the growth of GaAs on Si.

A thermal He scattering study of CO adsorption on Pt(111)

CO adsorption on Pt(111) is investigated by means of thermal He scattering. The relation between the relative height of the He specular peak and the coverage (θ = n CO n Pt ) is approximated well by an exponential at low coverages (θ < 0.17): I I 0 = exp(−18.5θ) for 63 meV He atoms incident at 40°. This relationship is used to monitor the CO coverage throughout this paper. The activation energy for CO desorption has been found to decrease linearly with increasing coverage up to θ ≈ 0.13, i.e. E d = E d0 − ϵθ, where E do = 133 kJ/mol and ϵ = 68.5 kj/mol. The preexponential factor for desorption is about 1.4 × 10 14 s −1 at T s = 400 K. This set of three parameters allows a fully consistent description of the CO adsorption, desorption and equilibrium measurements.

A thermal He scattering study of CO adsorption on Pt(111)

Abstract CO adsorption on Pt(111) is investigated by means of thermal He scattering. The relation between the relative height of the He specular peak and the coverage (θ = n CO n Pt ) is approximated well by an exponential at low coverages (θ I I 0 = exp(−18.5θ) for 63 meV He atoms incident at 40°. This relationship is used to monitor the CO coverage throughout this paper. The activation energy for CO desorption has been found to decrease linearly with increasing coverage up to θ ≈ 0.13, i.e. E d = E d 0 − ϵθ , where E do = 133 kJ/mol and ϵ = 68.5 kj/mol. The preexponential factor for desorption is about 1.4 × 10 14 s −1 at T s = 400 K. This set of three parameters allows a fully consistent description of the CO adsorption, desorption and equilibrium measurements.

Kinetics of hydrogen desorption from silica-supported rhodium

Temperature programmed desorption spectroscopy has been used to characterize the kinetics of H/sub 2/ desorption from a Rh/SiO/sub 2/ catalyst. Desorption is observed to occur from two states, designated ..beta../sub 1/ and ..beta../sub 2/. By simulation of the experimentally observed spectra, it is determined that the preexponential factor for desorption from both states is 3.8 x 10/sup -3/cm/sup 2//s. The activation energy for desorption from the ..beta../sub 1/ state is 14 kcal/mol and that for the ..beta../sub 2/ state is 21 kcal/mol. 4 figures, 1 table.

Adsorption of CO on Pd(100)

Adsorption of CO on a Pd(100) surface was studied in detail mainly by LEED, UPS, work function and thermal desorption measurements. Analysis of the ordered c(2√2×√2) R 45° structure occurring at Θ=0.5 revealed that in this phase each CO molecule is bridge bonded to 2 Pd atoms with Pd–C distances of 1.93±0.07 Å and a C–O bond length of 1.15±0.1 Å, the molecular axis being oriented normal to the surface. The mutual configuration of the adsorbed molecules is explained in terms of a short-range repulsive interaction model, which is supported by the observation that the isosteric heat of adsorption (Ead=38.5 kcal/mole) is constant up to a coverage of Θ?0.45. The photoelectron spectra exhibit two maxima at 7.9 (5σ+1π level) and 10.8 eV (4σ level) below the Fermi level which are in agreement with the observations with other Pd planes. This also holds for an electronic excitation associated with an energy of 13.5 eV as observed by electron energy loss spectroscopy. The variation of the sticking coefficient with coverage is described in terms of a second-order precursor state model with repulsive interactions. The pre-exponential factor for desorption (3×1016 sec−1) varies only little with coverage. The dipole moment of the adsorbate remains constant up to Θ?0.35 (0.17 D) where the overlayer starts to order, and exhibits an appreciable higher value near Θ=0.5. The maximum work function increase is Δφmax=0.93 V at Θ=0.5. The differential entropy of the adsorbed layer around 450 K was derived from the experimental adsorption isotherms. Up to Θ=0.35 the data fall well between the limits of two theoretical models for localised and delocalised adsorption of noninteracting particles. Deviations at higher coverages reflect again the onset of ordering. The energetic and work function data are in some variance to results reported earlier which is ascribed to the fact that these quantities may be sensitively influenced by spurious amounts of carbon impurities on the surface.

Adsorption of hydrogen on Pd(100)

The energetic, kinetic and structural properties of hydrogen chemisorbed on a Pd(100) surface were studied by means of thermal desorption, work function and LEED measurements. Under the applied conditions no interference with bulk dissolution occurs and dissociative adsorption gives rise to a continuous increase of the work function by up to 0.20 eV. The dipole moment of the adsorbate complex is constant up to θ ≈ 0.9 and then increases until saturation at θ ≈ 1.35 (at 170 K) is reached. The formation of a second adsorbed state at high coverages manifests itself also by a low-temperature shoulder in the thermal desorption spectra and in the variation of the isosteric heat of adsorption, E ad, with coverage: E ad remains practically constant ( 24.5 kcal mole ) up to θ ≈ 0.9 and then decreases. The sticking coefficient is initially rather high ( s 0 ≈ 0.5) and varies with coverage in a way which can be described by a precursor-state model. The preexponential factor for desorption is about 10 −2 cm 2 atom −1 s −1. Desorption follows second order kinetics only at very low coverages, at high θ it exhibits quasifirst order. This effect is attributed to the existence of lateral interactions between adsorbed hydrogen atoms which manifest themselves also in the appearance of a c(2 × 2) LEED pattern at low temperatures. The “extra” diffraction spots attain their maximum intensity at θ = 0.5, and a structural model is proposed whereafter in this phase the H atoms occupy next-nearest neighboring adsorption sites with local fourfold symmetry. Order-disorder transitions were followed by recording the intensity of the half-order spots as a function of temperature at various coverages. The resulting phase diagram exhibits a critical temperature T c = 260 K at θ = 0.5 and is slightly asymmetric with respect to this coverage. The data are analysed in terms of a lattice gas model and estimates for the pairwise interaction energies yield repulsion between nearest neighbors ( w 1 = 0.5 kcal mole ) and attraction between next-nearest neighbors ( w 2 = −0.3 kcal mole ). The additional operation of non-pair-wise interactions is made responsible for the asymmetric shape of the phase diagram. Whereas the adsorbed layer is obviously localized at T ⩽ 270 K, a detailed analysis of the adsorption entropy reveals that for T ⩾ 370 K a rather good description can be obtained with a model of delocalized two-dimensional translation.

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.