Energetics of C–C Bond Scission in Ethane Hydrogenolysis: A Theoretical Study of Possible Intermediates and Reaction Pathways

C–C bond scission steps, which are often considered as rate-determining in ethane hydrogenolysis, are studied by the Unity Bond Index–Quadratic Exponential UBI–QEP method. The binding energies of atomic carbon with Group VIII and IB metal surfaces Ni(111), Pd(111), Pt(111), Rh(111), Ru(001), Ir(111), Fe(110), Cu(111), and Au(111) are estimated using experimental data on the adsorption of various species on these surfaces. These estimates are corrected using data from density functional theory (DFT) on the adsorption heats of the CH _x species. Metal surfaces are arranged in the following series according to the binding strength of a carbon atom: Cu(111) < Au(111) < Pd(111) < Ru(001) Pt(111) < Ni(111) Rh(111) < Ir(111) < Fe(110). The values of chemisorption heats range from 121 kcal/mol for Au(111) to 193 kcal/mol for Fe(110). The activity of these surfaces toward C–C bond scission increases in the same series. The results of this work suggest that the most probable C–C bond scission precursors are ethyl, ethylidyne, adsorbed acetylene, CH₂CH, CH₂C, and CHC. Theoretical data obtained by different methods are compared and found to agree well with each other. An overview of experimental data on ethane hydrogenolysis mechanisms is given.     

BOILING IN CHANNEL WITH POROUS MATRIX

Experimental data on heat transfer and pressure drop in the case of forced-convection boiling in a channel with a porous matrix are presented. Correlations for the experimental data are given. Example calculation of thermohydrautic characteristics of a heat exchanger with a highly conductive porous matrix has been done using the obtained experimental data. Results of these calculations are presented.     

Choice of Efficient Linear Scale and Generalized Description of Internal Heat Transfer Coefficient in Porous Structures

The expedience of using the ratio of inertial β and viscous α hydraulic coefficients of a fluid flow in porous structures as the characteristic linear scale, when generalizing the experimental data on internal heat transfer in porous media, is shown. It is demonstrated that the correlation Nu = A · Pe, with both criteria based on β/α ratio, most efficiently describes the experimental data for a wide set of ordered and disordered porous structures, including sintered spheres, network materials, sintered felt and cellular foams of high porosity. The coefficient A depends on porosity and is equal to 0.004 for spheres, networks and felts, and 0.0004 for foams. For any specific case the values of α and β coefficients can be readily obtained from testing materials under consideration, control samples, or full-scale articles.     

Adsorption and Reactions of N2O on Transition Metal Surfaces

Data on the interaction of nitrous oxide with polycrystalline and low-index single crystal surfaces of transition metals (Cu, Ag, Pt, Pd, Ni, W, Ir, Rh, and Ru) are reviewed. The kinetics of N₂O adsorption, desorption, and dissociation N₂O on these surfaces, as well as the energetics and mechanisms of these processes, are considered. New calculated data on the energetics of nitrous oxide transformations on (111) single crystalline transition metal surfaces are reported.     

Experimental data on the mechanism of subcooled water boiling: high-speed video shooting

The article provides statistical data on the characteristics of subcooled liquid boiling and behavior of vapor and air bubbles resulted from high-speed shooting. The authors have described a number of new phenomena going along with water boiling.     

The UBI-QEP method: Basic formalism and applications to chemisorption phenomena on transition metal surfaces. Chemisorption energetics

The paper reviews the state of the art in the unity bond index-quadratic exponential potential (UBI-QEP) method. Assumptions made in the framework of the method, as well as their validity and generality, are discussed. The method is based only on well-defined observable energetic and structural parameters. UBI-QEP formulas for calculating reaction energetics (the binding energies of atomic and molecular adsorbates, the reaction enthalpies, and the intrinsic activation barriers) at different surface coverages are discussed. The UBI-QEP formalism is best suited for calculations of the adsorption of atoms and diatomic molecules but also allows one to consider polyatomic molecules in the quasi-diatomic approximation. A new formalism is discussed for determining the binding energies of various polyatomic molecules without resorting to hypothetical (and largely ambiguous) bond energy partitioning schemes. Instead, this new formalism considers the total bond energy of gas-phase species, which is an observable value. This formalism is the recent advance in the method. Various examples of calculating the energetics of atomic and molecular adsorption are presented. In most cases, the agreement of calculated and experimental values is good. The UBI-QEP method makes it possible to consider uniformly various processes on metal surfaces: adsorption, dissociation, diffusion, recombination, disproportionation, and desorption. Examples of complicated UBI-QEP calculations of molecular adsorption are presented.     

Mechanism of oxidative carbonylation of phenylacetylene and methylacetylene. Generation and experimental discrimination of hypotheses

Formal and informal methods for advancing hypotheses on mechanisms were used in a study of the oxidative carbonylation of phenylacetylene to methyl phenylpropiolate catalyzed by the PdCl₂−CuCl−CuCl₂ system. The hypotheses remaining after discrimination and consistent with all experimental data include the steps of formation of the Cu^I alkynyl complex, transfer of the phenylethynyl group from Cu^I to Pd^II, insertion of carbon monoxide into a Pd−C or Pd−OMe bond of the Pd^II σ-alkynyl complex. Comparison of the formal and informal methods for advancing hypotheses confirmed a higher effeciency of the first method. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 882–889, May, 1999.     

About lack of fragmentation for hot droplets at low subcooling of coolant

It was confirmed in experiments that during contact between cool and hot liquids, the lower sub-cooling of the cool liquid below the saturation temperature changes the characteristics of a vapor layer covering the fragments of hot liquid. This factor also decreases the probability of spontaneous direct contact between two kinds of liquid, explosive incipience of the cool liquid, and pressure pulse generation (the latter triggers fine fragmentation of hot coolant and vapor explosion). The mechanism that describes this trend in vapor layer behavior has been described.     

Prediction of Comparative Catalytic Activity in the Series of Single Crystalline Surfaces in a Water-Gas Shift Reaction

A method is proposed for predicting comparative catalytic activity. This method includes the following stages: (1) formulation of a general reaction mechanism that is applicable to the whole series of catalysts, (2) calculation of the Arrhenius activation energies and the preexponential factors, and (3) kinetic simulations for the preset conditions. The method is illustrated by the model water-gas shift reaction (WGSR) CO + H₂O = CO₂ + H₂ and a series of catalytic single crystalline surfaces Cu(111), Ag(111), Au(111), Ni(111), Pd(111), Pt(111), and Fe(110). The mechanism is formulated using the computer program MECHEM. The activation energies are calculated using the UBI-QEP method. The reaction kinetics is simulated for a plug-flow reactor. The following series of the catalytic activity is obtained: Cu(111) > Ni(111) > Fe(111) > Pt(111), Pd(111) > Ag(111) > Au(111).__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 543–549.Original Russian Text Copyright © 2005 by Zeigarnik, Callaghan, Datta, Fishtik, Shustorovich.