Kinetics of dissociative chemisorption of methane and ethane on Pt(110)-(1X2)

The initial probability of dissociative chemisorption Pr of methane and ethane on the highly corrugated, reconstructed Pt(110)‐(1×2) surface has been measured in a microreactor by counting the number of carbon atoms on the surface following the reaction of methane and ethane on the surface which was held at various constant temperatures between 450 and 900 K during the reaction. Methane dissociatively chemisorbs on the Pt(110)‐(1×2) surface with an apparent activation energy of 14.4 kcal/mol and an apparent preexponential factor of 0.6. Ethane chemisorbs dissociatively with an apparent activation energy of 2.8 kcal/mol and an apparent preexponential factor of 4.7×10^(−3). Kinetic isotope effects were observed for both reactions. The fact that P_r is a strong function of surface temperature implies that the dissociation reactions proceed via a trapping‐mediated mechanism. A model based on a trapping‐mediated mechanism is used to explain the observed kinetic behavior. Kinetic parameters for C–H bond dissociation of the thermally accommodated methane and ethane are extracted from the model

In situ surface coverage analysis of RuO₂-catalysed HCl oxidation reveals the entropic origin of compensation in heterogeneous catalysis

In heterogeneous catalysis, rates with Arrhenius-like temperature dependence are ubiquitous. Compensation phenomena, which arise from the linear correlation between the apparent activation energy and the logarithm of the apparent pre-exponential factor, are also common. Here, we study the origin of compensation and find a similar dependence on the rate-limiting surface coverage term for each Arrhenius parameter. This result is derived from an experimental determination of the surface coverage of oxygen and chlorine species using temporal analysis of products and prompt gamma activation analysis during HCl oxidation to Cl2 on a RuO2 catalyst. It is also substantiated by theory. We find that compensation phenomena appear when the effect on the apparent activation energy caused by changes in surface coverage is balanced out by the entropic configuration contributions of the surface. This result sets a new paradigm in understanding the interplay of compensation effects with the kinetics of heterogeneously catalysed processes

On the Apparent Activation Energy for Clustering in Dilute Al-Zn Alloys

The clustering phenomenon was observed when dilute Al-Zn alloys were annealed at temperatures higher than the solvus of the G.P.zones. In this report the apparent activation energy for clustering is estimated and compared with the experimental results. The estimated value of the apparent activation energy for clustering in several Al-Zn alloys comes to 0.51 eV, which is larger than the effective migration energy 0.43 eV of Zn atoms in Al-Zn alloys

Effect of amylose content on estimated kinetic parameters for a starch viscosity model.

The apparent viscosity profile of starches during gelatinization varies with different amylose content. This study focused on the influence of amylose content on the kinetic parameters of a starch viscosity model for corn starches. The five parameters were: gelatinization rate constant (kg), gelatinization activation energy (Eg), relative increase in apparent viscosity during gelatinization (Aα), relative decrease in apparent viscosity during shearing (B), and viscous activation energy (Ev). The parameters were estimated at different amylose content using both ordinary least squares nonlinear regression and the sequential method. The mixer viscometry approach was used to measure apparent viscosity. The first part of this paper presents parameter estimation results for waxy corn starch. The model was validated by using the parameters to predict viscosity for the same starch in a different measuring system, i.e., the RVA. The second part of this paper presents the estimated parameters for corn starch blends at different amylose content. The following parameters were significantly affected by amylose content: kg and Eg both decreased with amylose content by an power-law relationship. Activation energy of gelatinization ranged from 121 to 1169 kJ/mol. The other parameters Aα, B, and Ev were not significantly influenced by amylose content. In summary, the gelatinization parameters kg and Eg dramatically decreased as amylose increased from 3% to 35% (waxy corn starch blends)

Apparent activation energy during surface evolution by step formation and flow

During growth and etching by step flow as examples of anisotropic surface processing the apparent activation energy of the growth/etch rate depends on orientation, increasing gradually as a principal, terrace-rich surface is approached. This behaviour is traditionally explained as a change in the dominating process from step propagation to island/pit nucleation. We show that the orientation dependence of the activation energy is actually the result of a traditionally disregarded temperature dependence in the number of active step sites and is not attributable to an increasing role of step nucleation nor to a purely geometrical decrease in the number of step sites. This modifies the traditional picture of the apparent energy for a principal surface and explains how the energy can be higher than, equal to or even lower than that for vicinal orientations.Peer reviewe

Pseudogap Formation in the Symmetric Anderson Lattice Model

We present self-consistent calculations for the self-energy and magnetic susceptibility of the 2D and 3D symmetric Anderson lattice Hamiltonian, in the fluctuation exchange approximation. At high temperatures, strong f-electron scattering leads to broad quasiparticle spectral functions, a reduced quasiparticle band gap, and a metallic density of states. As the temperature is lowered, the spectral functions narrow and a pseudogap forms at the characteristic temperature $T_x$ at which the width of the quasiparticle spectral function at the gap edge is comparable to the renormalized activation energy. For $T << T_x$, the pseudogap is approximately equal to the hybridization gap in the bare band structure. The opening of the pseudogap is clearly apparent in both the spin susceptibility and the compressibility.Comment: RevTeX - 14 pages and 7 figures (available on request), NRL-JA-6690-94-002

Interim analysis of long time creep behavior of columbium C-103 alloy

Analysis of 16 long time creep tests on columbium C-103 alloy (Cb-10Hf-1Ti-0.7Zr) indicates that the calculated stresses to give 1 percent creep strain in 100,000 hours at 1,255 K (1800 F) are 7.93 and 8.96 MPa (1,150 and 1,300 psi) for fine grained and course grained materials, respectively. The apparent activation energy and stress dependence for creep of this alloy are approximately 315 KJ/gmol (75,300 cal/gmol) and 2.51, respectively, based on Dorn-Sherby types of relations. However, the 90 percent confidence limits on these values are wide because of the limited data currently available

Kinetics of the thermal degradation of Erica arborea by DSC: Hybrid kinetic method

The scope of this work was the determination of kinetic parameters of the thermal oxidative degradation of a Mediterranean scrub using a hybrid method developed at the laboratory. DSC and TGA were used in this study under air sweeping to record oxidative reactions. Two dominating and overlapped exothermic peaks were recorded in DSC and individualized using an experimental and numerical separation. This first stage allowed obtaining the enthalpy variation of each exothermic phenomenon. In a second time, a model free method was applied on each isolated curve to determine the apparent activation energies. A reactional kinetic scheme was proposed for the global exotherm composed of two independent and consecutive reactions. In fine mean values of enthalpy variation and apparent activation energy previously determined were injected in a model fitting method to obtain the reaction order and the preexponential factor of each oxidative reaction. We plan to use these data in a sub-model to be integrated in a wildland fire spread model