Initial Activation Energy Research Articles

Characterization of Pt Nanoparticles Encapsulated in Al2O3 and Their Catalytic Efficiency in Propene Hydrogenation

Pt nanoparticles supported in nanoporous Al2O3 catalyst are prepared by reduction of K2PtCl4 solution using H2 in the presence of Al2O3 and poly(acrylic acid) as capping material. After thorough washing with water to remove Pt nanoparticles located on the external surface of the Al2O3 and drying at 70 °C for 12 h, they were used in propene hydrogenation to evaluate catalytic activity as measured by the value of the activation energy in the temperature range between 30 and 90 °C. The Pt nanoparticles are characterized by using transmission electron microscopy (TEM). The particles in Pt/Al2O3 are found to be encapsulated and uniformly dispersed inside the Al2O3; however, the size and shapes are not clearly seen. After extraction of the Pt nanoparticles from the Al2O3 channels by using an ethanol-diluted HF solution, various shapes such as truncated octahedral, cubic, tetrahedral, and spherical with a size around 5 nm are observed. The encapsulated particles have various shapes but are smaller in size than those prepared in K2PtCl4 solution with polyacrylate in the absence of Al2O3. Using FT-IR studies, the capping material initially used in Pt/Al2O3 is not found in the Al2O3 channels. This might be due to the fact that the polymer (average MW 2100) is too large to be accommodated within the Al2O3 pores. The nanopores of Al2O3 have several roles in the synthesis of these nanoparticles. It allows for uniform dispersion and encapsulation of Pt nanoparticles. It controls the Pt sizes with narrow distribution that is determined by the pore dimension (5.8 nm). It protects against metal particle aggregation and produces various shapes even in the absence of the capping material. Using these Pt nanoparticles, the catalysis of hydrogenation of propene gas was studied. The initial rates, reaction order, rate constants, and activation energy for the hydrogenation are determined by use of mass spectrometric techniques. The activation energy is found to be 5.7 kcal/mol, which is about one-half that previously reported for catalysis by Pt metal deposited in SiO2 and TiO2 synthesized by using H2PtCl6 and Pt(allyl)2 by impregnation method.

1H NMR study of the kinetics of substituted aniline polymerization. I. Homopolymerization of 2‐methoxyaniline

AbstractWe studied the kinetics of the oxidative chemical homopolymerization of 2‐methoxyaniline (OMA) in aqueous acid solutions by monitoring OMA depletion with 1H NMR spectroscopy. We used the same semiempirical kinetic model used for aniline (ANI) homopolymerization to evaluate the experimental data. The reaction kinetics of OMA homopolymerization was similar to that of ANI, although we obtained longer induction and propagation times for OMA. This was attributed to steric hindrance of the bulky methoxy substituent during the coupling reaction. Furthermore, it was suggested that a lower OMA polymerization rate could also be related to a lower concentration of nonprotonated OMA molecules in the reaction solution due to a higher pKa value for OMA than for ANI. This may also explain the lower OMA end conversion (90%) compared with that of ANI (96%). The OMA end conversion was not influenced substantially by reaction conditions; it was lower than 90% only when high acid or low oxidant (oxidant‐deficient oxidant/OMA ratio) concentrations were applied. Because the oxidant took an active part in polymerization, it markedly influenced the polymerization rate, especially the initiation rate. The OMA initiation and propagation rates increased with increasing oxidant and initial monomer concentrations and with the reaction temperature, but there was no uniform trend in the correlation between the homopolymerization rate and acid concentration. The activation energies of the OMA initiation and propagation were 57 and 10 kJ/mol, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2471–2481, 2001

Catalytic Oxidative Polymerization of Vinyl Monomers Using Cobalt Phthalocyanine Complex and an Exploratory Investigation on the Polymerization of Vinyl Acetate

This is the first report on the oxidative polymerization of vinyl monomers using cobalt(II) phthalocyanine pyridine {CoIIPc(Py)} complex. The mechanism of polymerization is described and it has been supported from the kinetic data, UV−vis, ESR and 1H NMR studies. It is found that the rate of polymerizations (Rp) and number-average molecular weights (M̄n) of the vinyl polyperoxides are higher for CoIIPc(Py) catalyzed polymerization compared to the usual free radical, AIBN, initiated polymerization. This is supported by their activation energy of initiation (Ei) and total activation energy (Ea) of polymerization. The peroxide macroinitiator has been used for the polymerization of vinyl acetate. The polymerization obeys classical kinetics. The K2 values of the peroxide macroinitiator has been determined, which reveals that it can also be used at higher temperature for the polymerization compared to monomeric peroxide initiator, benzoyl peroxide. The “active” poly(vinyl acetate) has also been synthesized, which contains peroxy segments at the chain ends.

Mechanism of carbothermal reduction of iron, cobalt, nickel and copper oxides

A new scheme of thermal dissociation which is based on the dissociative evaporation of the reactant with simultaneous condensation of the low-volatile product has been invoked to interpret the kinetics of reduction of FeO, CoO, NiO and Cu 2O by carbon. A critical analysis of literature data and their comparison with theoretical calculations has shown that the main kinetic characteristics of carbothermal reduction, including the initial decomposition temperature and activation energy are in full agreement with the proposed mechanism of decomposition. Condensation of the low-volatile product (metal vapour) in the reaction zone and partial transport of condensation energy to the oxide account for the features which are typical of solid state reactions and manifest themselves in the appearance of periods of induction and acceleration in the course of the process. Carbon fulfils the role of buffer in this process. This is supported by an appearance of metals in the condensed phase and a higher than equilibrium partial pressure of oxygen in high-vacuum experiments with Knudsen cells.

Effect of lactic/glycolic acid side chains on the thermal degradation kinetics of chitosan derivatives

The thermal degradation properties of chitosan and lactic and/or glycolic acid grafted chitosan have been studied by differential scanning calorimetry (DSC) and thermogravimetric analyses (TG) in the range of 25–500°C. Both DSC and dynamic TG results show that the samples are thermal degraded easily after grafting the lactic and/or glycolic acid. From the isothermal TG experiments, the initial activation energy and the activation energy at different stages is obtained. The initial activation energy of all grafted samples is much lower than that of chitosan, especially for the sample GA/CS=2 and it varies with degree of conversion. The FT–IR spectra of thermally degraded residues give an indication of the chitosan polyscaccharide ring degradation after 30min in 280°C, while sample GA/CS=2 will degrade only after 15min.

The inhibited polymerization of lithium methacrylate and copolymerization with acrylonitrile

Results for the determination of initiation rate of the polymerization reaction of lithium methacrylate and its copolymerization with acrylonitrile in dimethylformamide at various temperatures are reported. The inhibition method involving the stable radical N, N-diphenyl- N′-picrylhydrazyl has been used. The initiation rate constant 2k df and the activation energy of initiation E 2k df have been determined. An influence of the initial monomer mixture concentration on the measured quantities has been revealed.

Kinetic model for methane oxidation by paddy soil as affected by temperature, moisture and N addition

The process of CH 4 oxidation by a paddy soil incubated under an initial CH 4 concentration of approximately 900 μl l −1 was simulated by a kinetic model, which assumed that the CH 4 oxidation activity of soil, a product of the specific activity of the individual and the number of methanotropic bacteria, was dependent on the CH 4 concentration during the incubation. The results showed that the model could very well simulate the process of CH 4 oxidation by the paddy soil in a closed system. The simulated results implied that CH 4 oxidation activity of the soil was induced at the beginning of the incubation, suggesting the growth of the methanotrophic bacterial community, then declined after reaching a peaked value. Additions of both NH 4Cl and KCl not only significantly inhibited the initial special activity of the individual, but suppressed population growth as well. There was no significant difference of NH 4Cl and KCl addition to inhibit the initial specific activity of the individual bacteria at the same added concentration, while NH 4 + was more effective to suppress population growth than K +. The optimum temperature for CH 4 oxidation by the paddy soil was about 35°C and the initial apparent activation energies varied from 59.7 to 159.1 kJ mol −1. The growth of methanotrophic bacteria almost ceased at temperatures above 40°C or below 12.5°C. The optimum water content for the soil to oxidize CH 4 was 280 g kg −1, at which point, the specific activity of the individual bacteria was the highest and the increase in the oxidation activity was the fastest. The response of the CH 4 oxidation activity to drying below the optimum was more sensitive than to wetting above the optimum. The prompt response of CH 4 oxidation activity to elevated CH 4 concentration and the decrease in sensitivity to wetting above the optimum moisture would be important for paddy soils to mitigate the emission of CH 4 produced endogenously.

Control of interfacial reactions during liquid phase processing of aluminum matrix composites reinforced with INCONEL 601 fibers

A comprehensive investigation is made of the parameters affecting the extent of interface reactions during squeeze casting of composites consisting of a matrix of either pure Al or alloy AS13 reinforced with fibers of INCONEL 601. The process parameters are the preform thickness and temperature, the fiber volume fraction, the temperature and mass of the liquid metal, and the temperature of the die. Adjustment of these process parameters made possible the full control of reactions. It is found that reactions proceed mainly in the solid state after decomposition of the oxide barrier layer covering the fibers. A simple kinetic model is developed that enlightens the role of this barrier layer. No trace of reaction could be detected in composites processed using preoxidized preforms. Alloying Al with Si also induces a drastic reduction of reactivity. The high ductility of the composites attests to the processing quality. An original procedure is proposed for measuring the activation energy for initiation of reactions by differential thermal analysis.

Photoageing of rigid PVC—III. Influence of exposure conditions on the thickness distribution of photoproducts

Unfilled, non photostabilized PVC bulk samples were exposed to accelerated ageing in various photochemical reactors equipped with fluorescent or filtered high pressure mercury lamps in which the UV intensity, determined by poly carbonate actinometry, varied between about 7 × 10 18 and 7 × 10 19 photons per cm h −1. IR (carbonyls) and UV-visible (conjugated polyenes) spectra were recorded on 20 μm thin slices cut with a microtome parallel to the irradiated surface. The carbonyl thickness profiles have a pseudo exponential shape with a maximum in the superficial layer, whereas the polyene profiles display a subcutaneous maximum. Despite the fact that a steady-state regime is rapidly established, the carbonyl profiles display no horizontal plateau in their superficial part, which can be interpreted (in mechanistic terms) by the fact that alkyl radicals play a noticeable role in terminations, even in superficial layers where the oxygen concentration is close to its equilibrium value. The thickness of the oxidized layer ( X co ) determined from the carbonyl profile, displays a tendency to increase with the temperature. It is not surprising in the case of photooxidation where the activation energy of initiation is practically zero, so that X co is essentially governed by the change of oxygen diffusivity with temperature. In contrast, the changes of X co with UV light intensity I seem to be within experimental error despite the very large variations of I. The kinetic and mechanistic consequences of this surprising result are discussed. The depth of the layer corresponding to the maximum polyene concentration seems to be a decreasing function of the UV light intensity and an increasing function of the temperature. A kinetic model is tentatively proposed to explain these results.

Determination of the initiation rate in the polymerization of zinc, cobalt and nickel acrylates

Results for the determination of the initiation rate of polymerization reaction of zinc, cobalt and nickel acrylates, initiated by 2,2′-azobisisobutyronitryle in dimethylformamide at different temperatures are reported. The inhibition method involving the stable radical N, N-diphenyl- N′-picryl-hydrazyl has been used. The initiation rate constants 2 k df and activation energy of initiation E 2 k d f have been determined.

Effects of DICY content and metal oxide on the curing behavior of a brominated epoxy resin

The curing characteristics of a brominated epoxy resin/dicyandiamide (DICY) system in the presence of different DICY contents and metal oxides are studied using isothermal differential scanning calorimetry (DSC). From the exotherms obtained, it is found that the reaction heats increase with increasing DICY content and curing temperature because of greater amounts of DICY reacted. The amine–epoxy-related reaction dominates the major curing behavior and the Tg especially at the high curing temperature, while the etherification is more significant at low temperature and conversion and plays an important role in determining the rate of liquid-to-solid transition during the cure. The addition of metal oxides, Fe2O3, and ZnO, results in changes in the initial transition rate, Tg, activation energy, reaction heat, reaction rate, and reaction order. Three fillers respond differently because of a difference in the surface-activated reaction. Regardless of the complex curing mechanisms involved in the specimens, a simple kinetic expression can describe the curing extent at 180°C with good accuracy. © 1996 John Wiley & Sons, Inc.

Molecular Weight in Polymerization of Methyl Methacrylate Initiated with Cyclohexanone

Abstract Molecular weight investigations of the poly(MMA) obtained in the polymerization initiated with cyclohexanone were carried out. The chain-transfer constants to the monomer, solvent, and initiator, using a new formal approach for chain-transfer constants calculations based on the Mayo relation, have been evaluated: C M = 1.1 × 10−3, C 1 = 2.2 × 10−4, C s = 1.8 × 10−4. The comparatively high C M value is interpreted in terms of the increased susceptibility of the MMA molecules toward the chain-transfer reaction due to the interactions between the monomer and initiator molecules. The infleuence of the temperature on the DP n of poly(MMA) is correlated with activation energy of initiation E i of MMA polymerization initiated with cyclohexanone.

Isotope-exchange reaction between hydrogen molecules and surface hydroxy groups on bare and modified ZrO2

The isotope-exchange reaction between molecular D2 and OH species on bare (b-) and modified with adsorbed formate (f-) and methoxy (m-) ZrO2 has been studied. On the b-ZrO2, the reaction rate was proportional to PD21/2, suggesting that the rate-determining step is the migration of D atoms. The initial activation energies (E‡) were 35 and 41 ± 4 kJ mol–1 for terminal (t-) and bridging (b-) OH groups, respectively, and increased as the reaction progressed, indicating the heterogeneity of the surface activation barrier for migration of D atoms. The kinetics of the exchange reaction on f- and m-ZrO2 was found to be different from that of b-ZrO2.

Correlation of thermal analysis data and hydrolysis resistance of polycarbonates

The initial activation energy at zero conversionE o of thermooxidative decomposition has been taken as a measure of thermal stability of polycarbonates (PC) andE o has been correlated with the relative hydrolysis rater h as a measure of their hydrolyse resistance. It is suggested that both decomposition processes are initiated by the same mechanism, the attack of hydrolytic agent onto ester C−O bonds. The following values ofE o have been found:187 (PC-M)>87 (PC-A)>43 (PC-C) kJ/mol, and they are correlated with values ofr h being 0.01 (PC-M)<1 (PC-A)<4.4 (PC-C). It has been found, using a computer modeling technique, that bothE o andr h depend on the minimized energy of conformations.

Synthesis and radical polymerization of α‐ethyl β‐hexafluoroisopropyl itaconate

AbstractThe synthesis of α‐ethyl β‐hexafluoroisopropyl itaconate (EFiPI) was performed. The polymerization of EFiPI initiated with dimethyl 2,2′‐azobisisobutyrate (MAIB) was investigated kinetically in benzene. The polymerization rate (Rp) was expressed by Rp = k[MAIB]0.5[EFiPI]2.0 at 50°C. The overall activation energy of the polymerization was calculated to be a low value of 57.38 kJ/mol. Rate constants of propagation (kp) and termination (kt) were estimated under different polymerization conditions, using the ESR‐determined concentrations of the propagating polymer radical. The increase in kp value (3.3‐6.0 L/mol s) and the decrease in kt value (0.94‐3.7 × 105 L/mol s) with monomer concentration are mainly responsible for the high dependence of Rp on the monomer concentration. Activation energies of initiation (Ei), propagation (Ep), and termination (Et) were also determined: Ei = 130.7 kJ/mol, Ep = 18.0 kJ/mol, Et = 57.4 kJ/mol. The large apparent activation energy of termination is a main origin of low overall activation energy of the polymerization. Thermal behavior of the resulting poly(EFiPI) was investigated by differential scanning calorimetry (DSC) and dynamic thermogravimetry (TG). Copolymerization of EFiPI (M1) with St (M2) was also examined at 50°C in benzene. The following copolymerization parameters were obtained according to the curve‐fitting method: r1 = 0.05, r2 = 0.46, Q = 0.46, e = +1.14. Using these results, the rate constants of cross propagations in the copolymerization were estimated. © 1995 John Wiley &amp; Sons, Inc.

Effect of copper oxides on the thermal oxidative degradation of the epoxy resin

AbstractThe effect of copper oxides on the thermal oxidative degradation of a brominated epoxy resin–dicyandiamide system was studied using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The addition of small amounts of Cu2O or CuO fillers to the epoxy resins affected the relative amounts of highly reacted cyclic species formed during thermal aging and induced catalytic degradation of the epoxy resins. The overall and initial activation energies of the degradation process were found to decrease, and the order in the degradation kinetics of the epoxy resin changed from a near zero order to negative domain (autocatalytic nature) in the presence of copper oxides. © 1994 John Wiley &amp; Sons, Inc.

Kinetic and ESR studies on radical polymerization. Radical polymerization of diisopropyl itaconate

The polymerization of diisopropyl itaconate (DIPI) initiated with dimethyl 2,2′-azobisisobutyrate(MAIB) was investigated kinetically in benzene. The polymerization rate ( R p) was expressed by R p = k[MAIB] 0.46[DIPI] 1.6 at 50°. The overall activation energy of the polymerization was calculated to be a low value of 60.2 kJ/mol. Rate constants of propagation ( k p) and termination ( k 1) were estimated different polymerization conditions, using the ESR-determined concentrations of the propagating polymer radical. The k p value (2.4–2.9 1/mol · sec) at 50° shows a little increase with monomer concentration. The k 1 value (1.8–4.9 × 10 4 1/mol · sec) was fairly dependent on the chain-length of the propagating polymer radical, which is mainly responsible for the high dependence of R p on the monomer concentration. Activation energies of initiation ( E i), propagation ( E p) and termination ( E t) were also determined; E i = 129.1 kJ/mol, E p = 28.5 kJ/mol, E t, = 66.1 kJ/mol. The large apparent activation energy of termination is a main origin of the low overall activation energy of the polymerization. Copolymerization of DIPI ( M 1) with St ( M 2) was also examined at 50° in benzene. The following copolymerization parameters were obtained according to the Kelen-Tüdős method; r 1 = 0.11, r 2 = 0.56, Q = 0.47, e = + 0.87. Using the above results, the rate constants of cross-propagations in the copolymerization were estimated.

NiO Reducibilities: Structural and Catalytic Properties of Their Pure and Potassium-Doped Reduced Forms

Abstract The study of the activation energies of reduction of stoichiometric and nonstoichiometric potassium-free and potassium-doped NiO was carried out by means of temperature-programmed reduction experiments, using two compared theoretical methods. Nonstoichiometric NiO shows the lower initial activation energy of reduction, whereas stoichiometric NiO shows a higher initial activation energy for starting the autocatalytic nucleation. Studies of the chemical preparation, BET surface areas, X-ray diffraction, X-ray photoelectron spectra, scanning electron microscopy, and measurements of catalytic activities of the nickel samples for the catalytic hydrogenation of 1,6-hexanedinitrile, in a continuous process at 1 atm pressure and 443 K, in the absence of ammonia, were also carried out. Surface areas dcrease when NiO reduction temperature increase. XRD, XPS, TPR, and SEM measurements detect NiO incipiently reduced between 463 and 498 K, a NiO reducibility inhibitor character for potassium, and 99.9% reduction above 573 K for all NiO forms. 100% selectivities with respect to 6-aminohexanenitrile may be obtained at 60% conversions for catalysts with potassium contents of 10.5 × l0 −4 g K 2 O/g Ni on reduced nonstoichiometric NiO. A mechanism for the continuous process is proposed.

XRD investigation of the crystallization process in Fe 40Ni 40B 20 metallic glass

X-ray diffraction analysis showed that two crystalline solid solutions simultaneously arise when Fe 40Ni 40B 20 glassy metallic ribbons, aged at room temperature for about 1 year, are heated in an inert atmosphere at 653 and at 663 K (first steps of the crystallization process). These two compounds are orthorhombic (Fe, Ni) 3B and fcc γ-(Fe, Ni). The first compound had larger unit cell parameters than those of both Ni 3B and Co 3B, while the second phase had a unit cell edge shorter than that of equiatomic γ-(Fe, Ni). These unit cell parameters indicate the formation of an orthorhombic solid solution richer in Fe than in Ni and, conversely, of a cubic solid solution richer in Ni than in Fe. The orthorhombic phase had the largest volume fraction and crystallites larger than those of the fcc phase (average sizes of 36 and 6 nm, respectively). The crystalline fraction of the metallic ribbons, partially crystallized after isothermal heating at the above temperatures, was determined as a function of the heating time. A modified Avrami kinetic analysis was carried out with computed values of the crystallized fraction. Dependence of the ‘local’ Avrami exponent on crystallized fraction is indicative of a crystal growth mechanism based on pre-existing nuclei (surface plus quenched-in nuclei). By applying the well known Arrhenius equation to the kinetic constants, as obtained by the ‘locally’ applied Avrami analysis, an initial activation energy of 250±50 kJ mol −1 was determined for the growth process.

Kinetic parameters of thermal decomposition of thiocyanatobismuthates(III): Part 3. Ammonium salts

Kinetic parameters of thermal decomposition of thiocyanatobismuthates(III) of formulae (NH 4) 3[Bi(SCN) 6], (NH 4) 2M[Bi(SCN) 6] (where M is Li, Na, K, Rb) and NH 4Ca[Bi(SCN) 6] · 5H 2O have been determined. The thermal stabilities (expressed by initial decomposition temperature and activation energy of the first step of decomposition) of thiocyanatobismuthates(III) containing alkali metal and ammonium group are compared with those of alkali metals in the outer sphere.