Negative Apparent Activation Energy Research Articles

Low temperature catalytic NO oxidation over microporous materials

It is shown that NO oxidation is catalyzed by chabazite zeolites in the proton (H+), sodium (Na+) and siliceous forms, by microporous carbons and by the metal-organic framework (MOF) material Basolite A100® at temperatures between 298K and 423K. Reaction orders at low conversion for NO and O2 are 2 and 1, as observed for the gas-phase reaction. At higher conversion and low temperatures, the rate decreases because of the presence of various NxOy species within the pores of the materials. Catalytic rates decrease with increasing temperature and yield negative apparent activation energies (−24.9 to −37.5kJmol−1). The catalytic properties of the samples are attributed to their ability to stabilize a [N2O4]‡ transition state within the micropores through van der Waals forces. Na-SSZ-13 samples exhibit faster catalytic rates than siliceous chabazite due to the additional presence of electrostatic forces stabilizing the transition state. An enhancement of catalytic rates on H-SSZ-13 is also observed but is the result of more complex interactions due to the formation of NO+ and NO3- in the zeolite pores that can also stabilize the [N2O4]‡.

Catalysis by Confinement: Enthalpic Stabilization of NO Oxidation Transition States by Micropororous and Mesoporous Siliceous Materials

Mesoporous silica and purely siliceous zeolites with voids of molecular dimensions (MFI, CHA, BEA) catalyze NO oxidation by O2 at near ambient temperatures (263–473 K) with reaction orders in NO and O2 identical to those for homogeneous routes and with negative apparent activation energies. These findings reflect the stabilization of termolecular transition states by physisorption on surfaces or by confinement within voids in processes mediated by van der Waals forces and without the involvement of specific binding sites. Such interactions lead to the enthalpic stabilization of transition states relative to the gaseous reactants; such enthalpic benefits compensate concomitant entropy losses upon confinement because of the preeminent role of enthalpy in Gibbs free energies at low temperatures. These data and their mechanistic interpretation provide clear evidence for the mediation of molecular transformations by confinement without specific chemical binding at active sites.

Kinetic Expression for the Carbonation Reaction of K2CO3/ZrO2 Sorbent for CO2 Capture

For the kinetic study of K2CO3/ZrO2 sorbent in the carbonation reaction to capture CO2 from the flue gas, reaction experiments were carried out at temperatures between 328 and 343 K for CO2 gas compositions not exceeding 18% at 1 bar, and a phenomenological kinetic model was proposed to fit the carbonation conversion data obtained. Time-dependent carbonation conversions of the sorbent appeared as sigmoid curves. Sigmoid characteristics of the conversion curve were more pronounced for the sorption reaction at lower temperature and lower gas phase concentration of CO2. Such conversion behavior of fresh-dried K2CO3/ZrO2 sorbent could be closely described with the reaction rate equation in the form: r = kf(X)yCO2n . The reaction rate constant k as a temperature dependent term could be represented by Arrhenius’ equation with the negative apparent activation energy of −17.43 kJ/mol and the pre-exponential factor of 3.83 × 10–3 1/min. The term f(X) was a function introduced to reflect the carbonation rate change...

Effect of A and B-site cations on surface exchange coefficient for ABO3 perovskite materials

A novel approach, called isothermal isotope exchange (IIE), was applied to varying A- and B-site lanthanum manganites, ferrites, and cobaltites in the perovskite crystal structure in order to extract accurate surface exchange coefficients (k*). Pure electronic conductors revealed temperature dependent isotope exchange, while for mixed ionic and electronic conductors (MIEC) the extent of exchange was independent of temperature. MIEC materials have higher k* values than pure electronic conductors in the temperature range from 500-850 °C, demonstrating the importance of both electronic species and oxygen vacancies being present for surface exchange. Strontium doped perovskites exhibited opposite temperature dependencies to parent materials. Some perovskites exhibited an apparent negative activation energy for k*, the behavior of which is explained by a precursor-mediated mechanism for dissociative adsorption. The results have significant implications for the improvement of the oxygen reduction reaction for fuel cells, metal-air batteries, and numerous other energy technologies.

Characterization of Low-k Dielectric SiCOH Films Deposited with Decamethylcyclopentasiloxane and Cyclohexane

Ultra low-k dielectric SiCOH films were deposited with decamethylcyclopentasiloxane (DMCPSO, C10H30O5Si5) and cyclohexane (C6H12) precursors by plasma-enhanced chemical vapor deposition at the deposition temperature between 25 and 200 degrees C and their chemical composition and deposition kinetics were investigated in this work. Low dielectric constants of 1.9-2.4 were obtained due to intrinsic nanoscale pores originating from the relatively large ring structure of DMCPSO and to the relatively large fraction of carbon contents in cyclohexane. Three different deposition regions were identified in the temperature range. Deposition rates increased with temperature below 40 degrees C and decreased as temperature increased to 75 degrees C with apparent activation energies of 56 kJ/mol x K at < 40 degrees C, -26 kJ/mol x K at 40-100 degrees C, respectively. In the temperature region of 40-100 degrees C hydrocarbon deposition and decomposition process compete each other and decomposition becomes dominant, which results in apparent negative activation energy. Deposition rates remain relatively unaffected with further increases of temperature above 100 degrees C. FTIR analysis and deposition kinetic analysis showed that hydrocarbon deposition is the major factor determining chemical composition and deposition rate. The hydrocarbon deposition dominates especially at lower temperatures below 40 degrees C and Si-O fraction increases above 40 degrees C. We believe that dielectric constants of low-k films can be controlled by manipulating the fraction of deposited hydrocarbon through temperature control.

Kinetics of nitrogen absorption in molten aisi 316 stainless steel during immersion nitrogen blowing

Nitrogen absorption in molten metal for stainless steel AISI316 has been investigated by immersion nitrogen blowing through an immersed alumina nozzle with an internal diameter of 3 mm. Based on these experimental data, some kinetic parameters of nitrogen absorption, such as reaction order, rate constant and apparent activation energy of nitrogen absorption reaction, have been obtained. Effect of stirring by immersion nitrogen blowing through an immersed alumina nozzle on nitrogen absorption reaction has been observed. Results show the following: (1) Nitrogen absorption reaction is the −1.5th order reaction. The rate constant k N is of the order of 10−5 wt%2.5·min−1. Nitrogen absorption reaction for AISI 316 has negative apparent activation energy of −92.40 kJ·mol−1. This indicates that the nitrogen absorption reaction has a complex and multistep reaction mechanism. (2) The rate of nitrogen absorption reaction in molten stainless steel is mixture control by the adsorption of monatomic nitrogen on the surface of molten stainless steel and mass transfer in molten metal. (3) A rate equation of nitrogen absorption reaction has been derived based on a mixed control mechanism by both the -1st order nitrogen absorption reaction and mass transfer in molten metal.

Co3O4–SiO2 Nanocomposite: A Very Active Catalyst for CO Oxidation with Unusual Catalytic Behavior

A high surface area Co(3)O(4)-SiO(2) nanocomposite catalyst has been prepared by use of activated carbon as template. The Co(3)O(4)-SiO(2) composite, the surface of which is rich in silica and Co(II) species compared with normal Co(3)O(4), exhibited very high activity for CO oxidation even at a temperature as low as -76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H(2)O content ~3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co(3)O(4)-SiO(2) catalyst led to the conclusion that adsorption of H(2)O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions.

Determination of Surface Exchange Coefficients of LSM, LSCF, YSZ, GDC Constituent Materials in Composite SOFC Cathodes

A novel approach to processing and modeling isothermal isotope exchange (IIE) data was developed to extract kinetic rate coefficients for the oxygen reduction reaction (ORR) on cathode materials used for solid oxide fuel cells (SOFC). IIE is capable of testing powders with particle sizes on the nano-scale, where the effective sample thickness (particle size) was shown to be below the characteristic thickness (Lc) for ionically conducting materials. This allows for accurate kinetic measurements in surface exchange controlled regimes, in contrast to secondary ion mass spectrometry depth profiling and electrical conductivity relaxation techniques where sample thicknesses are typically in the diffusion limited or mixed regimes. Surface exchange coefficients (k*) were extracted and Lc values calculated from cathode (La0.6Sr0.4)(Co0.2Fe0.8)O3–δ (LSCF) and (La0.8Sr0.2)MnO3 (LSM), and electrolyte (Ce0.9Gd0.1)O2–δ (GDC) and (Zr0.8Y0.2)O2 (YSZ) materials. Additionally, diffusion coefficients (D*) were extracted for LSM. In a surface exchange controlled regime LSCF exhibits a low activation energy for k*, while for LSM k* was observed to increase with decreasing temperature consistent with a precursor-mediated mechanism in which there is a negative apparent activation energy for the dissociative chemisorption of O2. GDC was shown to exhibit a low activation energy for k*, lower than YSZ, which is attributed to higher concentration of electrons in GDC than YSZ.

Surface Exchange Coefficients of Composite Cathode Materials Using In Situ Isothermal Isotope Exchange

Isothermal isotope exchange (IIE) was used to evaluate the surface kinetics of the oxygen reduction reaction on composite cathode materials. With IIE, powder materials can be tested to ensure surface exchange behavior is isolated. Macroscopic diffusion coefficients and surface exchange coefficients were extracted from composite cathode powders (LSCF/GDC), (LSM/YSZ), and (LSM/GDC) using IIE, and characteristic thicknesses were calculated from the ratio of . Mixed ionic and electronic conductor LSCF/GDC was shown to have the highest values between 500 and and values above . Adding an ionically conducting phase to LSM significantly improved the magnitude of , which below were highest for LSM/YSZ of the three composites. In addition, LSM/YSZ and LSM/GDC exhibited an apparent negative activation energy, which can be explained by a precursor-mediated mechanism for dissociative adsorption. Sample thicknesses (particle size) were shown to be well below characteristic thicknesses , ensuring that samples were in a surface exchange controlled regime and validating the accuracy of the extracted values.

Compatibility of ZrN and HfN with molten LiCl–KCl–NaCl–UCl 3

The reaction kinetics of ZrN and HfN immersed in a quaternary salt of composition of 28.5% LiCl–36.3% KCl–29.4% NaCl–5.8% UCl 3 (in weight percent) were assessed. Coupons of ZrN and HfN were exposed to the quaternary salt at 525–900 °C for 4–485 h. The reaction kinetics of the salt-refractory interactions were assessed through physical and microstructural characterization including scanning electron microscopy, X-ray diffraction and mass spectrometry. The results indicated that ZrN and HfN lose weight under all conditions investigated. While multiple mechanisms were evident, it is proposed that dissolution and oxidation were the dominant reactions that influence the weight loss. For the overall reaction, negative apparent activation energy values of −46 and −28 kJ/mol were observed in ZrN and HfN, respectively. These seemingly anomalous activation energies were associated with the simultaneous occurrence of electrochemical dissolution and surface oxide formation.

An experimental study of magnesite dissolution rates at neutral to alkaline conditions and 150 and 200 °C as a function of pH, total dissolved carbonate concentration, and chemical affinity

Steady-state magnesite dissolution rates were measured in mixed-flow reactors at 150 and 200 °C and 4.6 < pH < 8.4, as a function of ionic strength (0.001 M ⩽ I ⩽ 1 M), total dissolved carbonate concentration (10 −4 M < ΣCO 2 < 0.1 M), and distance from equilibrium. Rates were found to increase with increasing ionic strength, but decrease with increasing temperature from 150 to 200 °C, pH, and aqueous CO 3 2− activity. Measured rates were interpreted using the surface complexation model developed by Pokrovsky et al. (1999a) in conjunction with transition state theory ( Eyring, 1935). Within this formalism, magnesite dissolution rates are found to be consistent with r d = k Mg { > MgOH 2 + } 4 1 - exp - 4 A RT , where r d represents the BET surface area normalized dissolution rate, { > MgOH 2 + } stands for the concentration of hydrated magnesium centers on the magnesite surface, k Mg designates a rate constant, A refers to the chemical affinity of the overall reaction, R denotes the gas constant, and T symbolizes absolute temperature. Within this model decreasing rates at far-from-equilibrium conditions (1) at constant pH with increasing temperature and (2) at constant temperature with increasing pH and ΣCO 2 stem from a corresponding decrease in { > MgOH 2 + } . This decrease in { > MgOH 2 + } results from the increasing stability of the > MgCO 3 - and >MgOH° surface species with increasing temperature, pH and CO 3 2− activity. The decrease in constant pH dissolution rates yields negative apparent activation energies. This behavior makes magnesite resistant to re-dissolution if formed as part of mineral carbon sequestration efforts in deep geologic formations.

In Situ High‐Temperature Diffraction Study of the Thermal Dissociation of Ti3AlC2 in Vacuum

In this paper, the effect of high‐temperature vacuum annealing on the dynamic phase stability and phase transition of Ti3AlC2 at up to 1550°C was studied using in situ neutron diffraction. The decomposition of Ti3AlC2 into TiCx at an elevated temperature was observed to occur through the continuous sublimation of Al and Ti species in a dynamic environment of high vacuum. The rate of decomposition decreased with increasing temperature, resulting in negative apparent activation energy (i.e., −71.9 kJ/mol). Depth profiling of vacuum‐annealed Ti3AlC2 by grazing‐synchrotron radiation diffraction has revealed a graded composition of TiCx at the near‐surface.

Influence of Particle Size on Reaction Selectivity in Cyclohexene Hydrogenation and Dehydrogenation over Silica-Supported Monodisperse Pt Particles

The role of particle size during the hydrogenation/dehydrogenation of cyclohexene (10 Torr C6H10, 200–600 Torr H2, and 273–650 K) was studied over a series of monodisperse Pt/SBA-15 catalysts. The conversion of cyclohexene in the presence of excess H2 (H2: C6H10 ratio = 20:60) is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature (<423 K), an intermediate temperature range in which both hydrogenation and dehydrogenation occur; and a high temperature regime in which the dehydrogenation of cyclohexene dominates (>573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size. For the hydrogenation of cyclohexene, a non-Arrhenius temperature dependence (apparent negative activation energy) was observed. Hydrogenation is structure insensitive at low temperatures, and apparently structure sensitive in the non-Arrhenius regime; the origin of the particle-size dependent reactivity with temperature is attributed to a change in the coverage of reactive hydrogen. Small particles were more active for dehydrogenation and had lower apparent activation energies than large particles. The selectivity can be controlled by changing the particle size, which is attributed to the structure sensitivity of both reactions in the temperature regime where hydrogenation and dehydrogenation are catalyzed simultaneously.

Role of oxygen impurities in etching of silicon by atomic hydrogen

In a pure-hydrogen glow discharge plasma, the etch rate of silicon increases with increasing temperature up to about ≥1100 Å/s at 60–80 °C and, upon a further increase of the temperature, etch rate strongly decreases, showing Arrhenius-like dependence with negative apparent activation energy of −1.5 kcal/mol. When the Si sample is at the floating potential, oxygen impurities of ≥10 at. ppm strongly decrease the etch rate. At more than 70 ppm of oxygen, the etching stops. Oxygen adsorbed on the Si surface can be removed by ion bombardment when negative potential is applied to the Si sample and the Si is then etched chemically by H atoms. The etching by atomic hydrogen is isotropic in an oxygen-free system. A controllable addition of a few ppm of oxygen in combination with negative bias of the Si sample results in highly anisotropic etching with thin oxide acting as side-wall passivation.

Formation of multi-walled carbon nanotubes by Ni-catalyzed decomposition of methane at 600–750 °C

Ni-catalyzed decomposition of methane at high temperatures was examined by using a thermogravimetric apparatus. The catalyst (10 wt.% Ni supported on spherical alumina) gave quite a high carbon nanotube (CNT) yield at the temperatures below 680 °C. At > 700 °C, however, carbon formation rate decreased with increasing the reaction temperature. Temperature-programmed reaction also showed the maximum CNT growth rate at ~ 690 °C. This result ruled out the possibility that the apparent negative activation energy is caused by sintering of Ni particles. Detailed examination on the kinetic expression led us to a conclusion that the dissolution of carbon atoms formed by dissociation of methane into bulk of the nickel particles is the rate-determining step at high temperatures, while methane adsorption is the rate-determining step at lower temperatures. This idea also explains the fact that the carbon yield drastically decreased at high temperatures. The CNTs formed at these temperatures had thinner walls than those formed at lower temperatures. The latter fact also supports the idea that the solubility of carbon in the nickel particles decreases at high temperatures.

Mechanisms of Methane Decomposition over Ni Catalysts at High Temperatures

Decomposition of methane over nickel catalyst supported on spherical alumina was investigated using a thermogravimetric apparatus. The reaction products were hydrogen and multi-walled carbon nanotubes. Initial rate of carbon formation increased with reaction temperature up to 680°C. However, the initial rate decreased at higher reaction temperatures, implying that the reaction had an apparent negative activation energy, although thermodynamic considerations suggest that higher temperatures should favor the decomposition of methane. The reaction order with respect to methane was ca. 1.4, irrespective of the reaction temperature, whereas the reaction order with respect to hydrogen changed from −1/2 to zero by increasing the reaction temperature from <700°C to >720°C. The kinetic expression based on the Langmuir-Hinshelwood mechanism suggested that the rate- determining step changed from the adsorption of methane, which is disturbed by surface hydrogen atoms at below 700°C, to the dissolution of carbon species into the bulk of nickel particles at above 720°C. The apparent negative activation energy is interpreted by the decrease of solubility of carbon species into the bulk of nickel particles.

Characterization of fluorine-modified organosilicate glass

In this study, fluorine-modified organosilicate glass (F-OSG) films were deposited by using a plasma-enhanced chemical vapor deposition technique on 3MS∕O2∕SiF4 mixtures to change deposition temperatures. The films were characterized by using reflectometer data, Fourier transformation infrared spectroscopy, and x-ray photoelectron spectroscopy. The authors found that film deposition rates and fluorine contents in the F-OSG films decreased while the deposition temperature increased; moreover, negative apparent activation energies for film deposition were also observed, suggesting a deposition process dominated by surface adsorption/desorption reactions. In addition, the authors also investigated the effects of Si–C and Si–F bonding on the dielectric breakdown and leakage mechanism of the F-OSG films. They found that high and two-step breakdown voltage of the F-OSG films relative to that of the OSG films can be highlighted as a consequence of the structural change accompanied by the incorporation of fluorin...

Initiated Chemical Vapor Deposition (iCVD) of Poly(alkyl acrylates): An Experimental Study

In a two-part investigation, an experimental study and a kinetic model analysis of the initiated chemical vapor deposition (iCVD) of alkyl acrylate polymers are described. In this first part, an experimental study was performed to look at the effect of process parameters on iCVD polymerization. A homologous series of alkyl acrylates, from ethyl up to hexyl acrylate, were iCVD polymerized. The resulting polymers matched well spectroscopically with those from liquid-phase polymerization, demonstrating that stoichiometric polymers with no observable cross-linking can be achieved in a chemical vapor deposition environment. Deposition rate and molecular weight increased by a factor of over 300 and 60, respectively, when monomer saturated vapor pressure, Psat, was reduced from 42.6 to 0.584 Torr at equal gas pressures, PM. Over three times increase in deposition rate was observed for ethyl acrylate when substrate temperature was reduced from 29 to 17 °C. These trends are attributed to an increase in PM/Psat or, equivalently, monomer surface concentration in Henry's law limit at low PM/Psat. Evidence for adsorption-limited iCVD kinetics came from an apparent negative activation energy of −79.4 kJ/mol obtained experimentally that agreed well with a mathematically derived activation energy of −81.8 kJ/mol equal to twice the heat of desorption in the negative sense. Adsorption measurements found Henry's law limit to be valid and, when fitted to a BET equation, allowed the heat of desorption to be calculated. On the basis of this experimental study, process guidelines were made to define the appropriate parameter space for future iCVD polymerization, with PM/Psat in the range of 0.4−0.7 recommended as an optimal iCVD window.

Kinetic study on the combined effect of high pressure and temperature on the physico-chemical properties of egg white proteins

A kinetic study was conducted on the effect of pressure (525–600 MPa) at different temperature levels (10, 25, and 40 °C) on selected physico-chemical properties of egg white solutions (turbidity, solubility, residual denaturation enthalpy, surface hydrophobicity, and sulfhydryl content). The pressure induced decrease in residual denaturation enthalpy, solubility and buried SH content, and increase in turbidity and surface hydrophobicity could be described by a fractional conversion model, while the loss of SH groups due to SH oxidation could be described by a second order kinetic model. At 25 °C, pressure-induced denaturation was favoured by increased pressure, as evidenced by the negative activation volumes for the Eyring equation. Thus, higher rate constants were obtained for higher pressures. Furthermore, pressure-induced denaturation was enhanced by pressure treatment at lower temperatures. For all properties, except total SH content, a negative apparent activation energy for the Arrhenius equation was obtained, resulting in lower rate constants at higher temperatures. This is opposite to what is observed for heat treatment at ambient pressure, indicating an antagonistic effect of temperature and pressure in the temperature–pressure range studied. At higher temperature (60 °C), pressure-induced egg white protein denaturation more closely resembles heat-induced denaturation.

Kinetics of Liposome Adhesion on a Mercury Electrode

The adhesion of liposomes on a mercury electrode leads to capacitive signals due to the formation of islands of lecithin monolayers. Integration of the current-time transients gives charge-time transients that can be fitted by the empirical equation Q(t) = Q(0) + Q(1)(1 - exp(-t/tau(1))) + Q(2)(1 - exp(-t/tau(2))), where the first term on the right side is caused by the docking of the liposome on the mercury surface, the second term is caused by the opening of the liposome, and the third term is caused by the spreading of the lecithin island on the mercury surface. The temperature dependence of the two time constants tau(1) and tau(2) and the temperature dependence of the overall adhesion rate allow determination of the activation energies of the opening, the spreading, and the overall adhesion process both for gel-phase 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and for liquid-crystalline-phase DMPC liposomes. In all cases, the spreading is the rate-determining process. Negative apparent activation energies for the spreading and overall adhesion process of liquid-crystalline-phase DMPC liposomes can be explained by taking into account the weak adsorption equilibria of the intact liposomes and the opened but not yet spread liposomes. A formal kinetic analysis of the reaction scheme supports the empirical equation used for fitting the charge-time transients. The developed kinetic model of liposome adhesion on mercury is similar to kinetic models published earlier to describe the fusion of liposomes. The new approach can be used to probe the stability of liposome membranes.