Desorption Rate Coefficients Research Articles

Monte Carlo simulations of the effect of pressure on isothermal and temperature-programmed desorption kinetics

A Monte Carlo simulation technique is presented for describing the adsorption, surface diffusion, and desorption kinetics of molecules from metal surfaces. Lateral interactions between adsorbed molecules are taken into account using the bond-order-conservation-Morse-potential method. The rate of desorption observed in the presence of a gas-phase species is higher than that observed in a vacuum. The increase in the apparent rate coefficient for desorption with increasing pressure can be ascribed to the effects of repulsive lateral interactions on the activation energy for desorption. The simulated kinetics are in good agreement with the experimentally-observed kinetics for the isothermal desorption of CO from polycrystalline Pd and for the temperature-programmed desorption of CO from Ni(100).

Hydrogen behavior in graphite at elevated temperatures

We have examined deuterium retention in the graphite exposed to D 0 at elevated temperatures by means of reemission and thermal desorption techniques. The total retention increases with exposure times showing a t 1 2 dependence without any saturation behavior, which is interpreted as diffusion-limited formation and growth of a modified layer. On the other hand, desorption spectra obey second order desorption kinetics and their peak structure is hardly dependent on exposure time. The deuterium retention under D 0 exposure and succeeding thermal desorption are well interpreted by deuterium diffusion in the saturated layer and recombinational release in the saturated layer, respectively, and both the second order desorption rate and diffusion coefficient of deuterium in the saturated layer are determined

Emulsion polymerization: determinations of the average number of free radicals per particle and kinetic parameters by use of the particle size distribution

Abstract For emulsion polymerization of a ‘zero-one’ system, the method of moments is applied to the model proposed by Lichti et al. to describe the particle size distribution (PSD). Using the explicit expressions so obtained for the first four moments, the average number of free radicals par particle, n ss , and kinetic parameters of the system involving the rate coefficients for adsorption and desorption of free radicals, ϱ and κ, and the propagation rate coefficient, κp, can be obtained easily by use of PSD data without involving a complicated curve fitting procedure as was required in their work. Detailed calculations on the styrene system data of Lichti et al. show that both ϱ and κ are proportional to particle surface area. The result for κ is opposite to that proposed by those workers, in which κ was considered to be inversely proportional to the particle surface area. After further manipulation and approximation of the expressions so obtained, explicit expressions for the number-average volume, gn n , the standard deviation, σ, and the skewness, u′3, in terms of surfactant and initiator levels, temperature and reaction time are also obtained. The effects of the variations of these variables on the three characteristic parameters of the PSD can be determined.

Perturbations in the surface chemistry of hydrogen by nitrogen adatoms on platinum(110)-(1.times.2)

The kinetics of adsorption and desorption of deuterium have been studied on Pt(110)-(1x2) surfaces on which various fractional coverages of nitrogen adatoms are present. The nitrogen was adsorbed via the decomposition of ammonia at 400 K, at which temperature the initial probability of dissociative adsorption is approximately 4 x 10/sup -3/. Nitrogen selectively blocks the high-temperature ..beta../sub 2/-state of deuterium prior to poisoning the low-temperature ..beta../sub 1/-state. No evidence of a long range electronic perturbation of the surface by the nitrogen adatoms was found. The adsorption kinetics of deuterium on both clean and nitrogen-precovered Pt(110)-(1x2) surfaces were Langmuirian. In particular, adsorption into the ..beta../sub 2/-state was first order with respect to the fraction of vacant surface sites, and adsorption into the ..beta../sub 1/-state was second order in all cases. The modification of the desorption rate coefficients of deuterium on Py(110)-(1x2) by the nitrogen adatoms was measured quantitatively. Nitrogen modifies the preexponential factor and the activation energy of desorption of deuterium on Pt(110)-(1x2) by essentially rescaling the effective coverage of the deuterium. For the ..beta../sub 2/-state, the activation energy and the preexponential factor of the rate coefficient of deuterium desorption from both clean and nitrogen-precovered Pt(110)-(1x2) display a compensation effect. Themore » activation energy of desorption of nitrogen from Pt(110)-(1x2) was determined to be 24 kcal x mol/sup -1/ and relatively independent of coverage, whereas the preexponential factor decreases from approximately 5 x 10/sup -5/ cm/sup 2/ x s/sup -1/ at low coverages (theta/sub N/ less than or equal to 0.15) to approximately 5 x 10/sup -7/ cm/sup 2/ x s/sup -1/ at higher coverages (theta/sub N/ greater than or equal to 0.3).« less

Wave packet studies of gas–surface inelastic scattering and desorption rates

A previously formulated semiclassical wave packet method is used to investigate the importance of different surface phonon modes and the Debye surface temperature upon inelasticity in atomic gas–surface collisions. Desorption rates are calculated as a function of potential-well depth and the rate law for the process is examined. The incident beam is represented by a quantum mechanical wave packet whose momentum distribution is nearly square. This wave packet is coupled to a three-dimensional model lattice through a time-varying potential field obtained by solution of the classical motion equations for the lattice. Calculated final-state momentum and energy distributions are found to be strongly dependent upon the particular surface phonon mode into which the initial lattice energy is partitioned. In general, energy transfer occurs predominantly to and from those modes for which the lattice atom in the impact region have motion in the direction of the momentum vector of the incoming wave packet. The inelasticity of the collision is found to increase as the lattice force constants and the surface Debye temperature decrease. The peak spacings in the final-state momentum and energy distributions are found to correlate well with the surface phonon frequencies. Desorption is found to be well described by a first-order rate law for small potential-well depths. For larger well depths, the first-order decay plots begin to show an increasing amount of curvature. Desorption rate coefficients obtained from the slopes of the decay plots show an approximate exponential dependence upon the potential-well depth.

Kinetic and equilibrium studies at the solid–liquid interface. The adsorption of sodium hexadecyl sulphate to polystyrene latex

Binding isotherms and kinetic measurements associated with the adsorption of the surfactant sodium hexadecyl sulphate onto polystyrene latex particles at two latex concentrations are reported. The surfactant monomer concentrations were estimated in situ for all dispersions from e.m.f. measurements of a cell containing a surfactant ion-selective electrode. Both isotherms are of the classical B.E.T. type. At low surfactant concentration the initial portions of the isotherms (region I) can be fitted by the Langmuir equation and do not depend on the concentration of latex particles. At higher concentrations (region II) the isotherms diverge. Bound surfactant–counter-ion interactions could be responsible for the difference between the binding isotherms. The kinetic data were measured with the pressure jump relaxation technique and revealed a single relaxation process in region II only. The observed relaxation times decrease with increasing surfactant concentration. Application of linear phenomenological theory to the combined equilibrium and kinetic data shows clearly that the adsorption rate coefficient increases with increasing amount of bound surfactant and that the desorption rate is proportional to the amount bound in region II. The rate coefficients measured for the two dispersions agree reasonably well. These findings are consistent with the formation of hemimicelle type aggregates. Application of the Aniansson–Wall kinetic treatment associated with monomer–aggregate exchange to region II gives a linear plot for each solution, but the agreement between the desorption rate coefficients is worse than that obtained using the phenomenological approach.

Steady-state decomposition of ammonia on the ruthenium(001) surface

Steady-state specific reaction rates for the catalytic decomposition of ammonia at pressures of 1 x 10/sup -6/ and 2 x 10/sup -6/ Torr have been measured on the Ru(001) surface at temperatures between approximately 500 and 1250 K. For temperatures above 750 K, the reaction rate approaches first order in ammonia pressure, and the apparent activation energy is 5.0 +/- 0.3 kcal x mol/sup -1/. A kinetic isotope effect is observed at these high temperatures, and the activation energy for the decomposition of deuteriated ammonia is 6.6 +/- 0.3 kcal x mol/sup -1/. This apparent activation energy is associated with the difference in activation energies between that of the cleavage of a nitrogen-hydrogen (nitrogen-deuterium) bond in chemisorbed ammonia and that of the desorption of molecularly chemisorbed ammonia. At lower temperatures, the reaction rate is independent of ammonia pressure, and the apparent activation energy is 43 +/- 3 kcal x mol/sup -1/, which is associated with the recombinative desorption of nitrogen. From transient thermal desorption mass spectrometric measurements, the activation energy of the recombinative desorption of nitrogen on Ru(001) is 44.0 +/- 0.5 kcal x mol/sup -1/, and the preexponential factor of the desorption rate coefficient is (1.3 +/- 0.6) xmore » 10/sup -3/ cm/sup 2/ x s/sup -1/. On the basis of thermal desorption measurements during the steady-state decomposition of ammonia at 2 x 10/sup -6/ Torr, nitrogen adatoms are shown to be the dominant surface species under these experimental conditions. A mechanistic model that has been introduced previously describes accurately the pressure and temperature dependence of both the measured decomposition kinetics and the steady-state coverage of nitrogen adatoms.« less

Flume Tests on Hydrocarbon Reaeration Tracer Gases

Laboratory studies of the modified tracer gas technique were conducted in a tilting, recirculating flume. Simultaneous rates of hydrocarbon gas desorption and reaeration were measured for three flow conditions. The hydrocarbon gases used were propane (C3H8) and ethylene (C2H4). The ratio of the desorption rate coefficient for propane to the reaeration rate coefficient was found to be independent of flow conditions. The same was found to be true for the ratio of the ethylene desorption rate coefficient to the rearation rate coefficient. The results of the flume tests were compared with two sets of mixing tank studies of hydrocarbon desorption and reaeration. Results were comparable for all tests.

Inhibition by hydrogen of the heterogeneous decomposition of ammonia on platinum

Absolute reaction rates have been measured in a continuous flow microreactor for the inhibition by hydrogen of the heterogeneous decomposition of ammonia over a polycrystalline platinum wire at pressures between 10 T and 0.6 torr, with ammonia to hydrogen partial pressure ratios varying from 1:1 to 1:4 and temperatures between 400 and 1200 K. Inhibition of the decomposition is observed at relatively high total pressures (0.2-0.6 torr) for all temperatures studied. The reaction orders with respect to hydrogen and ammonia are -1.5 and 1.0, respectively, with an apparent activation energy of 38 kcal/mol. For relatively low total pressures (10 T-5 x 10 T torr), inhibition is only observed at temperatures below 650 K, where the activation energy varies from 33 to 55 kcal/mol and the reaction order with respect to hydrogen approaches -1.5 at low temperatures. The decomposition becomes uninhibited at high temperatures and the activation energy is 4.5 kcal/mol. The kinetics of this reaction as well as previous results for the NH3 + D2 exchange reaction are described quantitatively by a mechanistic model employing independently measured adsorption-desorption parameters of NH3, N2 and H2, where the rate coefficient for hydrogen desorption is a function of the fractional surface coverage ofmore » nitrogen adatoms. The hydrogenation of NH2(a) to produce molecularly adsorbed ammonia is predicted to be the dominant factor in the inhibition of the ammonia decomposition. 37 references, 5 figures, 1 table.« less

Kinetics of emulsion copolymerization. II. Effect of free radical desorption on the rate of emulsion copolymerization of styrene and methyl methacrylate

AbstractThe rate coefficient for radical desorption from the polymer particles is derived for an emulsion copolymerization system, assuming, for simplicity, that only monomer radicals can desorb from the particles. The effect of free radical desorption on the rate of emulsion copolymerization and the copolymer composition is theoretically analyzed, using the rate coefficient for radical desorption developed in this paper and a mathematical reaction model proposed earlier by the present authors for an emulsion copolymerization system where the average number of total radicals per particle does not exceed 0.5. The validity of the analysis is demonstrated experimentally using the seeded emulsion copolymerization of styrene (ST) and methyl methacrylate (MMA). Radical desorption from the particles does not affect the copolymer composition, but the desorption of MMA—monomer radicals plays an important role in determining the rate of emulsion copolymerization, while the desorption of ST—monomer radicals from the particles can be neglected from a kinetic point of view.

Chemisorption and reaction of cyclopropane on the (110) surface of iridium

Thermal desorption mass spectrometry (TDMS), UV photoelectron spectroscopy (UPS), low-energy electron diffraction (LEED), and contact potential difference measurements have been employed to study the interaction of cyclopropane with the reconstructed Ir (110)–(1×2) surface. At an adsorption temperature of 100 K cyclopropane dissociates on the surface with a reaction probability of 0.87±0.1. This overlayer saturates at a coverage of cyclopropane of (2.1±0.2)×1014 cm−2. Upon further exposure, cyclopropane adsorbs molecularly into two different adsorption sites up to a saturation coverage of 6.6×1014 cm−2. The parameters of the desorption rate coefficients based upon Arrhenius constructions for low coverages of the two desorption states are Ed = 6.4±1 kcal mol−1 and νd = 1.0×109±1 s−1, Ed = 8.3±1 kcal mol−1 and νd = 4.5×1010±2 s−1. The work function decreases by 0.5 eV during formation of the dissociated hydrocarbon residue at 100 K and decreases further (a total decrease of 0.7 eV) during the adsorption of molecular cyclopropane.

Rate coefficient for radical desorption in emulsion polymerization

Investigators have proposed the rate coefficient for radical desorption from polymer particles to explain the kinetic deviation of the emulsion polymerization of water-soluble monomers such as vinyl acetate and vinyl chloride from the classical Smith and Ewart theory.6 In this article, the rate coefficient for radical desorption is theoretically derived by a different approach, and its applicability to vinyl acetate and vinyl chloride emulsion polymerization is examined in detail using experimental data available in the literature. The theory developed here predicts the average number of radicals per polymer particle in the emulsion polymerization of vinyl acetate and vinyl chloride.

Kinetics of Potassium Desorption in Soil using Miscible Displacement

AbstractKinetics of K desorption were conducted on samples from the Ap, A2, B21t, and B22t horizons of two Dothan (Plinthic Paleudults) soils. Aluminum‐ and calcium‐ saturated samples were equilibrated with K for 96 hours and then continuously leached with 0.01M CaCl2 until K was not detected in the leachate. The rate of K desorption from all samples increased rapidly initially and levelled off with time. Desorption was nearly complete in approximately 3 to 4 hours for the Ap, A2, and B21t horizons, and in 8 to 9 hours for the B22t horizons. Approximately 95–98% of the adsorbed K was subsequently desorbed suggesting K adsorption‐desorption in the Dothan soils was reversible. A linear relationship between time½ and percent K desorption indicated that diffusion was the predominant mechanism of K desorption in these soils. Diffusion‐controlled exchange would be expected due to the vermiculitic clay minerals present in the soils.Potassium desorption conformed to first‐order kinetics. Apparent desorption rate coefficients (k′d) ranged from 0.3 to 1.3 hour−1. The magnitude of the k′d values decreased as clay content increased in the soils. This was ascribed to increased intraparticle transport and to increased diffusion in the more clayey samples. The k′d values were generally higher in the Al‐than in the Ca‐ saturated samples. The effect of flow velocity on rate of K desorption was investigated using velocities of 0.5, 1.0, and 1.5 ml min−1. The rate of K desorption increased only slightly with flow velocity.

The interaction of oxygen with the Rh(111) surface a

The adsorption of oxygen on Rh(111) at 100 K has been studied by TDS, AES, and LEED. Oxygen adsorbs in a disordered state at 100 K and orders irreversibly into an apparent (2 × 2) surface structure upon heating to T⩾ 150 K. The kinetics of this ordering process have been measured by monitoring the intensity of the oxygen (1, 1 2 ) LEED beam as a function of time with a Faraday cup collector. The kinetic data fit a model in which the rate of ordering of oxygen atoms is proportional to the square of the concentration of disordered species due to the nature of adparticle interactions in building up an island structure. The activation energy for ordering is 13.5 ± 0.5 kcal mole . At higher temperatures, the oxygen undergoes a two-step irreversible disordering ( T⩾ 280 K) and dissolution ( T⩾400K) process. Formation of the high temperature disordered state is impeded at high oxygen coverages. Analysis of the oxygen thermal desorption data, assuming second order desorption kinetics, yields values of 56 ± 2 kcal/ mole and 2.5 ± 10 −3 cm 2 s −1 for the activation energy of desorption and the pre-exponential factor of the desorption rate coefficient, respectively, in the limit of zero coverage. At non-zero coverages the desorption data are complicated by contributions from multiple states. A value for the initial sticking probability of 0.2 was determined from Auger data at 100 K applying a mobile precursor model of adsorption.

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.

Effects of surface heterogeneity in non-linear and non-equilibrium gas-adsorption chromatography

It is shown that one can investigate quantitatively the effects of surface heterogeneity in non-linear and non-equilibrium gas-solid chromatography on the basis of elution theories developed for homogeneous surfaces. One such theory, that of Zhitomirskii et al., was accepted for the purpose of this work and a new formulation for the kinetics of adsorption on heterogeneous surfaces was developed and applied. The effect of surface heterogeneity on elution curves is shown, and the possibility of determining quantitatively adsorption and desorption rate coefficients in adsorption systems with heterogeneous surfaces is demonstrated. A parameter that describes quantitatively the degree of surface heterogeneity has been determined.