Higher Pre-exponential Factor Research Articles

Parametric study of membrane reactors for hydrogen production via high-temperature water gas shift reaction

This study presents numerical studies of hydrogen production performance via water gas shift reaction in membrane reactor. The pre-exponential factor in describing the hydrogen permeation flux is used as the main parameter to account for the membrane permeance variation. The operating pressure, temperature and H2O/CO molar ratio are chosen in the 1–20 atm, 400–600 °C and 1–3 ranges, respectively. Based on the numerical simulation results three distinct CO conversion regimes exist based on the pre-exponential factor value. For low pre-exponential factors corresponding to low membrane permeance, the CO conversion approaches to that obtained from a conventional reactor without hydrogen removal. For high pre-exponential factor, high CO conversion and H2 recovery with constant values can be obtained. For intermediate pre-exponential factor range both CO conversion and H2 recovery vary linearly with the pre-exponential factor. In the high membrane permeation case CO conversion and H2 recovery approach limiting values as the operating pressure increases. Increasing the H2O/CO molar ratio results in an increase in CO conversion but decrease in H2 recovery due to hydrogen permeation driving force reduction. As the feed rate increases in the reaction side both the CO conversion and hydrogen recovery decrease because of decreased reactant residence time. The sweep gas flow rate has a significant effect on hydrogen recovery. Low sweep gas flow rate results in low CO conversion H2 recovery while limiting CO conversion and hydrogen recovery can be reached for the high membrane permeance and high sweep gas flow rate cases.

The energy required to ignite micropyretic synthesis. Part I: stable Ni + Al reaction

In the previous study, the influences of micropyretic parameters on the ignition energy for lower exothermic heat and lower activation energy of stable Ni–Al reaction have been studied. In this study, we studied the effect of ignition energy on the unstable Ti + 2B micropyretic reaction, which has higher exothermic heat and higher activation energy. Three-dimensional maps are generated to illustrate the influences of micropyretic synthesis parameters on the required ignition energy. In addition, the comparisons in the ignition energy for the micropyretic reactions with Ni + Al and Ti + 2B are studied. The numerical calculation indicates that the changes in the ignition energy caused by the thermal conductivity for the micropyretic reaction with Ti + 2B is smaller as compared with for the micropyretic reaction with Ni + Al. However, the required ignition power is found to significantly change with the thermal conductivity for the NiAl micropyretic reaction when a higher pre-exponential factor is taken in the calculation. In addition, the difference in the required ignition energy caused by the thermal conductivity is noted to be larger for the Ni–Al micropyretic reaction with a lower pre-exponential factor.

Atomic diffusion of boron and other constituents in amorphous Si–B–C–N

The self-diffusion of boron is studied between 1550 and 1700°C in polymer-derived amorphous solids of composition Si3BC4.3N2. Ion implanted stable 10B isotopes are used as tracers and secondary ion mass spectrometry for depth profiling. The experimentally determined diffusivities obey an Arrhenius behavior with a high activation enthalpy of ΔH=(7.3±1.3)eV and a high pre-exponential factor of D0=1.6m2/s. The results are discussed in relation to the diffusivities of the other constituents (Si, C, N) and possible diffusion mechanism are suggested.

Picosecond and Steady-State Emission of [Ru(phen)2dppz]2+ in Glycerol: Anomalous Temperature Dependence

The excited-state deactivation of the “light-switch” compound [Ru(phen)2dppz]2+, where phen = 1,10-phenanthroline and dppz = dipyrido[3,2-a:2‘,3‘-c]phenazine, has been investigated in glycerol using single-photon counting at picosecond time resolution. Relaxation back to the ground state occurs in about 8 ns at 20 °C, which is much faster than previously reported in monohydric alcohols, though still slow compared to that in water. Multivariate kinetic analysis reveals three distinct excited species involved in the relaxation process in glycerol. Using a matrix exponential approach for the kinetic data analysis, including global fitting of the relaxation data collected at many wavelengths, individual emission spectra for all three excited species could be resolved. The resolved emission profile for the most short-lived species was found to resemble the steady-state emission spectrum of [Ru(phen)3]2+ in glycerol whereas the emission profile of the intermediate species resembled that of [Ru(phen)2dppz]2+ in ethanol. The spectrum of the third species is considerably red-shifted compared to those of the other two. The longest lifetime as well as the emission quantum yield show pronounced nonmonotonic variations with temperature in apparent conflict with the Arrhenius equation. This anomalous temperature dependence can be accounted for by a model based on the equilibrium between two excited species, corresponding to the two resolved emission spectra retrieved at 20 °C. Thermodynamic data indicates that transfer to the fast-relaxing, red-shifted species is accompanied by a substantial lowering in enthalpy. The thermodynamic data, as well as an abnormally high preexponential factor for the back reaction from the third to the second excited species, could be explained in terms of the formation of two hydrogen bonds, one to each of the aza nitrogens of the dppz moiety.

Kinetics of the low-temperature pyrolysis of chromated copper arsenate-treated wood

In this paper, a kinetic evaluation of non-isothermal thermogravimetric (TG) data is carried out by the method of least squares for both untreated and chromated copper arsenate (CCA)-treated wood. The type of wood used is Pinus sylvestris and the slow-heating decomposition in an inert (nitrogen) atmosphere (low-temperature pyrolysis) is studied. In the case of untreated wood, best fit parameters are determined for the three independent parallel reactions scheme, proposed by several other researchers. For CCA treated wood a comparative study is performed, in which five different kinetic models are considered. These are based on the assumption of: (1) a single reaction; (2) independent parallel reactions; (3) competitive reactions; (4) a chain of successive reactions and (5) a combination of two independent parallel reactions and one subsequent reaction. The choice between which of these models offer the best fit, is based partly on chemical considerations and partly on the quality of the fit. Altogether, the DTG curve of CCA treated wood is well described by assuming two independent parallel reactions at low temperature, which may be catalyzed by the metals, followed by the successive decomposition of the residue. A comparison is made with the kinetic parameters derived for untreated wood, resulting in the following observations and conclusions drawn therefrom: hemicellulose decomposition is catalyzed by the metals (lower activation energy, pre-exponential factor and peak temperature and higher contribution to volatiles production); cellulose decomposition starts once the major part of chromium(III) arsenate is decomposed and volatilised (higher activation energy) and may then even be catalyzed by the metals (higher pre-exponential factor) and lignin decomposes very slowly at higher temperatures together with intermediate products from hemicellulose and cellulose decomposition (low pre-exponential factor and activation energy). During this slow charring process the metals act no more as catalysts. This hypothesis is confirmed by a TG analysis, performed on a chromium(III) arsenate precipitate, which is the major arsenic compound present in CCA treated wood, and by a study concerning the microdistribution of copper, chromium and arsenic in the pyrolysis residue of CCA treated wood.

Microscopic Molecular Diffusion Enhanced by Adsorbate Interactions

The diffusion of carbon monoxide molecules on the (110) surface of copper was investigated in the temperature range between 42 and 53 kelvin. The activation energy for thermal motion was determined directly by imaging individual molecular displacements with a scanning tunneling microscope. An attractive interaction between carbon monoxide molecules gave rise to the formation of dimers and longer chains. Carbon monoxide chains diffused substantially faster than isolated molecules although the chains moved by a sequence of single-molecule jumps. A higher preexponential factor in the Arrhenius law was found to be responsible for the observed efficiency of chain hopping.

Diffusion of Hydrogen in Amorphous Silicon in the Low Concentration Regime

We report the first measurements of hydrogen diffusion kinetics in a-Si in the regime of low H concentration (<2×l019 cm−3). The results differ substantially from the diffusion behavior typically observed in hydrogenated a-Si:H at H concentrations >1020 cm−3. The activation energy and pre-exponential factor for low-concentration H diffusion are found to be 2.70 ± 0.02 eV and 2.2 × 104 cm2s−1, respectively, and are shown to be independent of both annealing time and H concentration. It is difficult to reconcile the combination of high activation energy and large pre-exponential factor with a simple deep-trap-limited diffusion model. Consequently, an alternative mechanism for H diffusion involving the migration of dangling bonds coupled with a H bond-switching step is proposed.

Single crystal polarized Raman spectra of the solid electrolyte Cu 2HgI 4; attempt frequencies for ion motion in Ag 2HgI 4

The polarized Raman scattering from small single crystals of Cu 2HgI 4 provided assignments for the more prominent Raman features to specific irreducible representations. The E symmetry assignment, mass dependence, and pressure dependence of the 36 cm −1 band in Cu 2HgI 4 and 24 cm −1 band in Ag 2HgI 4 indicate that these features approximate the attempt frequency for ion hopping. The unusually high pre-exponential factor in the Arrhenius expression for ion hopping is discussed in light of the observed attempt frequency; we conclude that despite the high activation energy the conduction mechanism is similar to other heavy-metal solid electrolytes.

Classical theory of desorption rate velocity distribution of desorbed atoms; possibility of a compensation effect

A three-dimensional theory of atomic desorption is formulated using the thermal equilibrium hypothesis. The velocity distribution of desorbed atoms obtained is Maxwellian at the temperature of the crystal, whatever the interaction potential. As expected the desorption rate is equal to the product of a Boltzmann factor, exp(- U/ kT), by a so-called preexponential factor. The case of an adatom linked by harmonic forces to four surface atoms of a unit cell is considered in particular. The result obtained, as well as those given by three-dimensional linear theory, show a compensation effect which seems to be too low if one compares calculated and experimental data. However, the calculated preexponential factor is mainly dependent on: (a) the adatom vibrational frequency; (b) the amount of potential energy which can be concentrated in the bond, characterized by its mean thermal value. It increases with respect to this quantity. This result suggest that high preexponential factor values are due to a long range collective interaction between the adatom and surface atoms, corresponding to the formation of a large “quasi molecule”.

États d'adsorption à faible densité du potassium sur le cuivre

The properties of metallic potassium adsorbed on a (100) copper surface have been studied using a beam technique. Two different states of adsorption were observed, one being characterized by a very low concentration (0.001 monolayer) and a high mobility, and the other, by a high concentration and a much lower mobility. The high mobility phase (surface gaz) desorbs as ions following a first order activitation process with an unusually high preexponential factor. The condensed phase desorbs simultaneously as ions and atoms with complicated kinetics. The surface density determines which phase exists, and has a critical value which is weakly temperature dependent. The phase transition cannot be studied completely by thermodesorption alone.

Study of the transition state in the isomerization of trans-N2F2 to cis-N2F2 by extended H�ckel theory

The quantitative agreement with experimental barrier height and the extra stabilization energy of cis-N2F2 over trans-N2F2 may not be considered to be very good, but this is only to be expected as small differences between large quantities are being calculated. It appears from our present calculations that the use of Cusachs relation for evaluating the off-diagonal matrix elements in this system is open to question since the experimentally observed planarity of N2F2 is not correctly predicted by the use of this relation. We therefore prefer the use of the Wolfsberg-Helmholz relation in this system. A value of K= 1.75 appears to be preferable. The results of all the E.H.T. calculations made here for difluorodiazine clearly show that a twisted transition state is more probable than a linear one (LT or LC) in the isomerization of trans-N2F2 to cis-N2F2. This conclusion also receives further support from a consideration of the high preexponential factor in the rate equation [1] and the multiplicity of the transition state.

Thermal Decomposition of Azomethane. II. The Unimolecular Reaction on the Quantum Harmonic Model

The decomposition of azomethane is a complex process, but previous work has shown that the residual, fully NO-inhibited reaction has all the features of a unimolecular reaction. This residual reaction is now treated by the quantum harmonic version of the Marcus—Rice theory of unimolecular reactions. Extrapolation to infinite pressure gives log10k∞=17.32 — 55 500/2.303 RT. In accordance with the high pre-exponential factor (and hence high entropy of activation) a very loose activated complex is postulated involving free rotations of the two methyls and the central nitrogen. Two models of this complex are considered: one having a planar methyl and the other a tetrahedral methyl. From the pre-exponential factor there are deduced six more active rotations in the complex than in the molecule. Exact vibrational—rotational energy sums are evaluated on a computer, as is the rate constant integral itself. Within the relatively large experimental error, and subject to the uncertainty in collision diameter and collisional deactivation efficiency, good pressure fit between theory and experiment in the rate constant pressure fall-off is obtained for a theoretical model having nine or more active rotations in the complex and three or more in the molecule. It turns out that the vibrational frequency pattern of the complex is so much less important than the number of active rotations that both models for the complex, tetrahedral or planar, lead to identical fall-off behavior. Theory also predicts an activation energy fall-off with pressure that is, on the whole, in accord with the rather limited experimental data.

The Mechanism of the Depolymerization of Polytetrafluoroethylene With Pyrolytic and Radiolytic Initiation.

The rate of decomposition of polytetrafluoroethylene, exposed to gamma radiation in a flowing helium atmosphere, was studied in the range of temperatures 330 to 450 °C, dose rates 0.1 to 7.6 MR/h, and sample thicknesses 0.0006 to 0.01 cm. Rate measurements were made with a thermal conductivity cell in the exit stream, which recorded essentially the difference between the thermal and radiation-induced rate, in total molecules of volatiles per second. At temperatures below about 400 °C, the volatile products were a complex mixture. Above 400 °C, tetrafluoroethylene was the major product, and the radiation-induced rate had an activation energy* of 70 to 84 kJ/mol. The radiation-induced rate is proportional to square root of dose rate, after appropriate allowance for the thermal initiation is made. Analysis of the results suggests that the rate of the thermal initiation has a very high preexponential factor, 1028 to 1030 s-1. The activation energy for termination is large, E4 > 155 kJ/mol. Diffusion and reverse reaction appear to be minor factors above 400 °C, but a surface reaction may be important. The thermal decomposition mechanism seems clearly to consist of random initiation, termination by bimolecular recombination and a short kinetic chain (13 at 480 °C).

Mechanism of the Gas Phase, Thermal Decomposition of Ozone

The pyrolysis of ozone has been reinvestigated experimentally, and it is now shown that most of the known data are quantitatively explained on the basis of the simple mechanism, M+O3[open phi] lim 21M+O2+OO+O3→ lim 32 O2,where M may be O2, O3, CO2, N2, He, etc. There is no evidence for a direct bimolecular reaction of ozone to produce O2 nor is there any evidence for important surface effects or energy chains. On the other hand, the results at very fast rates of decomposition indicate an acceleration which can be accounted for in terms of temperature gradients in the system. This region is usually very close to the thermal explosion limit. The values found for the rate constants are (for M equal to O3), k1=4.61±0.25×1012 exp(−24 000/RT) liter/mole−sec,k2=6.00±0.33×107exp(+600/RT) liter2/mole2−sec,k3=2.96±0.21×1010exp(−6000/RT) liter/mole−sec. The relative efficiencies of O2, N2, CO2, and He in activating O3 (compared to O3 itself) are, respectively, 0.44, 0.41, 1.06, and 0.34. As expected for energy transfer processes, k1 has an abnormally high pre-exponential factor, and k2 has correspondingly a negative energy of activation. It is further inferred that Reaction 3 which produces two O2 molecules with 99 kcal of excess energy between them does not produce more than one excited electronic state of O2 and that these hot O2 molecules are not very efficient in exciting O3 to decomposition. Calculations of the entropy of activation of Reaction 1 can be made and are shown to be in good agreement with this conclusion. From these findings it appears that the high quantum yields found in the photolysis of O3 at short wavelengths, if real, probably are attributable to metastable O atoms [1D] produced in the primary process which can generate chains in O3. It can be shown that the homogeneous production of O atoms from O2 at high temperatures proceeds through two different paths having different activation energies, the lower energy path involving O3 as an intermediate. Finally, possible mechanisms are considered for the chemical sensitization of the thermal explosion of O3 and its catalytic decomposition by H donors.