Molecular Aspects of Catalytic Reactivity. Application of EPR Spectroscopy to Stuies of the Mechanism of Heterogeneous Catalytic Reactions

The basic objective of mechanistic studies of real catalytic processes is to dissect the course of the reaction into individual steps; ascertain their sequence; and determine the stoichiometry, structure, and electronic states of active sites and intermediates. The electron paramagnetic resonance (EPR) technique is at present widely used to explore many of these principal aspects of heterogeneous catalysis and surface chemistry. The extreme sensitivity compared to the usual spectroscopic methods is perhaps its most acknowledged advantage and makes EPR best suited to investigate and characterize low-abundance active sites and intermediates appearing during catalytic reaction. Additional information can be drawn from the theoretical analysis of the experimental spin Hamiltonian parameters within the ligand field and from angular overlap or Newman's superposition models as well as by more sophisticated quantum chemical calculations. The purpose of this paper is to show how catalysis benefits from EPR spectroscopy and to identify the issues and areas explored by this method. A comprehensive literature review is not attempted in this article; instead, attention is directed toward application of EPR for elucidation of the molecular reaction mechanism that can provide a scientific background for understanding many fundamental aspects of catalytic activity. The major events of mechanistic studies which involve the identification of active sites, activation of reagents, and determination of the reaction pathways are illustrated by selected examples and discussed. An approach that is complementary to mechanistic catalytic test studies is also presented. It consists of spectroscopic investigations of a set of partial reactions, driven by external creation of the supposed active sites and intermediates, with the aim of reproducing and verifying the feasibility of the postulated catalytic cycle. Moreover, to assure some consistency of the subject, basic characteristics of EPR spectroscopy related to surface studies and chemical theories of reactivity are concisely reviewed.     

A computer system for teaching the design of a power engineering plant

The scope and methodological assumptions of computer-aided teaching of the design of power engineering plant are presented. A plan of the course is given; a system of programs for computer-aided teaching is described and the authors practical experience in the use of computers for teaching is considered.     

Electrode processes of bismuth in weakly acidic,neutral and alkaline solutions

The electrode process Bi(III)+ 3e = Bi(Hg) has been investigated in weakly acidic, neutral and alkaline solutions by means of amalgam polarography and cyclic voltammetry and chronopotentiometry with the use of hanging amalgam drop electrode. It has been found that in weakly acidic and neutral solutions, oxidation of bismuth amalgam goes through aquocomplexes of bismuth. In the cathodic step on cyclic voltammetry curves four peaks of reduction are observed, with magnitude and shape strongly dependent on pH of the solution. These peaks have been ascribed to the reduction of Bi³⁺, Bi(OH)⁺₂, Bi(OH)⁺² and Bi(OH)₃.In alkaline solutions, oxidation of bismuth amalgam leads to Bi(OH)₃. In cathodic scan on cyclic voltammetry curves only one peak is observed, which corresponds to Bi(OH)₃ reduction. Formal potentials of electrode process and solubility product of Bi(OH)₃ have been calculated and discussed. The step-wise nature of the overall electrode reaction has been ascertained and discussed. At higher concentrations of OH^? ions, Bi(OH)₃ dissolves.     

Plastic zone in a thick layer with a disc-shaped crack

Summary The width of the annulus of plastic deformations for a layer of ideally elasto-plastic material with a very thin plastic zone has been discussed. The problem reduces to a Fredholm integral equation of the second kind, with the upper limit to be determined by a trial-and-error method from the condition of finiteness of the normal stress. The expression for the width of plastic zone has been found. The distribution of stresses is given in diagrams and its behavior discussed.

---

Zusammenfassung Es wurde die Weite eines Ringes einer plastischen Veränderung in einer Schicht von idealem, elastisch-plastischem Material mit einer sehr dünnen plastischen Zone, untersucht. Das Problem verringerte sich zu einer Fredholm Integralgleichung zweiter Ordnung. Die obere Grenze wurde durch eine Versuchs- und Fehlermethode auf den Gebrauchszustand des normalen Druckes bestimmt. Die Formel für die Weite der plastischen Zone wurde gefunden. Die Verteilung der Drucke ist in einem Diagram gezeigt und ihr Verhalten wurde besprochen.

---

Résumé Dans l'ouvrage on a discuté la largeur d'un anneau des déformations plastiques dans une couche d'un matériau élastoplastique avec une zone plastique très mince. Le problème est réduit à une equation intégrale de Fredholm de deuxième ordre, avee la limite supérieure déterminée par la méthode des essais successifs, appliquée sur la base d'une condition, que les contraintes normales ont des valeurs finies. Les formules pour la largeur de la zône plastique ont ététabliés et la répartition des contraintes est discutèe et prèsentée sur les graphiques.

     

Studies on the addition reaction of diethylamine to multiacrylate catalyzed by lithium diethylamide

Summary The nucleophilic addition reaction of diethylamine to vinyl groups of multiacrylate - oligo[(2-acryloyloxy-5-methyl-1,3-phenylene)methylene] and to p-cresyl acrylate as model compound were investigated. The reaction was carried out in benzene with excess amine in the presence of lithium diethylamide as catalyst within the temperature range from 21 to 50°C. Based on the obtained results, it has been shown that the addition reaction rate of diethylamine to multiacrylate and p-cresyl acrylate is proportional to [CH₂=CH-] and to [Et₂NLi] with the ratio [Et₂NH]_o/[Et₂NLi]_o= 5. Second-order addition reaction rate constants and activation energies for the systems under investigation have been determined. The chemical composition of the resultant addition products has been confirmed by spectroscopic methods: IR, ¹H-NMR, ¹³C-NMR and elemental analysis. Received: 29 August 1999/Revised version: 20 April 2000/Accepted: 20 April 2000     

Peritektoidische Reaktion in einem Stahl mit rd. 1% C und 18% Cr

Betrachtungen zur peritektoidischen Reaktion γ + M₂₃C₆ → M₇C₃ in einem Stahl mit rd. 1% C und 18% Cr im Zusammenhang mit früheren Schrifttumsangaben. Elektrolytische Isolation der Phasen nach unterschiedlichen Austenitisierungsbedingungen des Stahles und chemische sowie Röntgenanalysen der gebildeten Phasen sowie vergleichende metallographische Beurteilung der entstandenen Gefüge. Erörterung der Versuchsergebnisse im Hinblick auf Ablauf, Morphologie und Bildungsmechanismus der peritektoidischen Reaktion unter Berücksichtigung der Diffusionsvorgänge und vorangehender Schrifttumsangaben.     

Keimbildung und Wachstum von Vanadincarbiden auf 0,8% C enthaltendem Austenit

Untersuchung des Mechanismus der Bildung und des Wachstums von Vanadindiffusionsschichten beim Diffusionsvanadieren mittels Gasverfahren auf 0,8% C enthaltendem Austenit. Bestimmung des Verlaufs der Bildung von Schichten durch Analysieren ihres Zustandes während des Vanadierens.     

Wetting of Cu by Bi–Ag based alloys with Sn and Zn additions

Contact angles on copper substrate of Bi–Ag–Sn and Bi–Ag–Zn ternary alloys containing 3, 6, and 9 at.% of Sn and Zn, respectively, were studied with the sessile drop method. Wetting tests were carried out at 573 and 603 K with or without the use of a flux. Without the flux, the examined alloys do not wet copper, i.e., the observed contact angles are higher than 90°. However, in the presence of the flux wetting of copper is observed. In the case of alloys with Sn, the contact angles decrease with increasing content of Sn, while in the case of alloys with Zn no such tendency is observed. Solidified solder–substrate couples were cross-sectioned and examined with scanning electron microscopy coupled with electron dispersive X-ray analysis.