THE CHEMICAL REACTIVITY OF SOLIDS
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haracterizing the reaccompletely covered in a tintv of the solid state single meeting. A preview of the Symposium of the is, essentially, a problem in This symposium will chemical kinetics involving feature the presentation of Chemical Reactivity of Solids, concepts, theories, and 12 papers, drawn from responsored by the Division of Inmodels carried over from search carried out in indusfrial a n d Engineering Chemistry the kinetic theory of gases. dustrial, government, and Understanding the reacademic laboratories. The of the ACS at fhe Universify of activity is achieved through topics covered will include M ichiaa n, Novern ber 12-1 3. 1964. application of kinetic analthe surfaces of solids, the ysis to rate processes, taking influence of defects on solid into account the peculistate reactivity, gas-solid rearities of solid state reactions. Such peculiarities result actions, including exchange and decomposition reactions, from special properties of the solid associated with the influence of the atmosphere on the transport propordered arrangements of atoms and molecules in erties of solids, and organic reactions in solids. both crystalline and amorphous states. In both states The reactivity of the solid surface is, of course, of either short or long range periodicity of the constituent critical importance for many processes. The correct atoms or molecules is present. Different arrangments interpretation of the surface effects can only be based and degrees of order produce varying energy contents on a clear understanding of the nature of solid surfaces. which are reflected in any kinetic process involving At the present time the application of low energy the solid. Characterizing solid state reactivity, conelectron diffraction promises to add interesting insequently, involves a thermodynamic-kineticduality. formation about the structure, chemistry, and surface The thermodynamic aspect of the duality is related reactivity of solids. J. J. Lander of the Bell Telephone to the study of the energy content of different structural Laboratories will discuss the most recent information in arrangements, including crystalline, partly crystalline, this area and show how a variety of reactions at surfaces strained and amorphous zones, boundaries between can be studied continuously through production of crystallites, dislocations, stacking faults, and point fractional monolayer phases, and through the formation defects. In each case, theoretical calculations should of films many layers thick. Examples studied include permit the determination of the energy content of the silicon and nickel substrates. I t has been observed solid which will be manifested in a typical chemical e.g., that about a monolayer of aluminum deposited reactivity whenever the different solid state arrangeon a clean and “atomically smooth” (100) surface of ments react with surrounding phases. silicon followed by heat treatment to about 700” C. Thus, the field of the chemical reactivity of the solid converts the entire surface to edged pits with (111) state has benefited greatly from investigations carried faces exposed. The low energy electron diffraction out during the past 30 years on the chemistry and technique has also shown how transitions of various physics of the solid state. Kinetic studies of solid state types occur frequently in the surface structure of both reactivity have also been gfeatly aided by investigations dean and “contaminated” surfaces. of transport processes in solids and in gases. All The influence of solid state defects on the reactivity of these studies have given a dearer picture of the basic solids will be discussed by G. R. Henning, Argonne laws and relations for the movement of atoms in a National Laboratories. His presentation will center solid. upon results obtained from single crystals of graphite. The subject of solid state reactivity covers a vast Characteristic surface patterns have been produced by area, a fact reflected in the variety of applications means of colloidal metallic catalysts placed on dear that solid state reactivity has at the present time. The surfaces. Catalyzed surface reactions and the unreactivity of solids influences processes involving the catalyzed growth of the channels and pits produced by production of new solid phases by reactions between the catalysts have been investigated. solids, liquids, and gases, the decomposition of solids, The influence of defects on solid state reactivities is the transformation of solids into different phases (crystalalso the subject of a paper concerned with the formation line and amorphous), the aggregation and sintering of energy of Frenkel defects in calcium fluoride. A. D. solids, the formation of porous and surface active solids Franklin, National Bureau of Standards, will report on (adsorption and heterogeneous catalysis), and polymcalculations of the formation energy of a fluorine vacancy erization. This variety of applications cannot be and an interstitial ion in calcium fluoride. It takes 4.3 “
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ev. for the formation of' the vacancy, while - 1.6 ev. are needed for the production of interstitial ions. The sum of the two energies is in satisfactory agreement with experimental results on the formation of Frenkel defects in calcium fluoride. T h e area of reaction between solids and gases will be represented at the symposium by four papers. L. 0. Brockway, University of Michigan, will discuss some surface features of the reaction between copper films and oxygen at 525" C. J. D. McKinley, National Bureau of Standards, will present results on the systems nickel-bromine, nickel-chlorine, and yttrium-chlorine at pressures of about 10-6-10-4 torr and temperatures between 300" K. and the melting point of the metal. These conditions produce a surface concentration of absorbed halogen less than a few per cent of a monolayer. I n the same area of the gas-metal reactions, E. A. Gulbransen, Westinghouse Research Laboratories, will discuss his results on the high temperature interaction between gases, metals, and salts. Thus, the reactions between oxygen and molybdenum, tungsten, uranium, carbon, and boron nitride will be discussed. A different type of gas-solid reaction will be discussed by E. A. Secco, St. Francis Xavier University. T h e exchange reactions of zinc vapor with zinc oxide, and of zinc vapor with zinc sulfide, in both single crystal and polycrystalline states, affords an interesting tool for reactivity studies. These reactions were found to be dependent upon temperature, zinc vapor pressure, and mole fraction of zinc in solid. T h e study was carried out by means of the radio exchange technique, which is provinq quite fruitful for investigations of this type. The exchange results have revealed that two processes are involtred. An initial exchange, consisting of surface adsorption fdlowed by an exchange, which is first order and not limited to the surface but persists to a depth of up to 25 molecular diameters, and a second, slower process which appears controlled by solid state diffusion. At the lower temperatures studied, the activation energy obtained for the exchange from polycrystalline samples agrees with that calculated from the results on single crystals. At higher temperatures the activation energy for exchange was found to be lower for the polycrystalline solid. The field of solid state decomposition will be represented a t the symposium by two contributions. One involves the decomposition of complex salts, barium and strontium trioxalatoferrates, by P. K. Gallagher, Bell Telephone Laboratories, and a second one on the decomposition of simpler salts such as calcium oxide by R. E. Carter, General Electric Company. The former investigation was aimed at finding the influence of various parameters (temperature, pressure, gas atmosphere) on the mechanism of thermal decomposition of barium and strontium salts. T h e stoichiometry of the reaction and the variation of valence of the metal ions during AUTHOR G. Parravano is Professor of Chemical and Metallurgical Engineering at the University of Michigan. He is Chairman of the symposium previewed here. 38
INDUSTRIAL A N D ENGINEERING CHEMISTRY
decomposition have been recorded as a function of various reaction parameters. T h e latter paper of this group is concerned with the decomposition of calcium oxide stabilized by zirconium oxide. I n this instance the decomposition did not produce a new phase, but was accompanied by order of magnitude changes in ionic conductiL7ity. It will be shown that the decomposition reaction follows the kinetic expressions for normal precipitation. The rate reached a maximuin a t about 1000° C . ; below this temperature the rate was decreased by the slowness of the ionic diffusion, while above 1000° C., the rate dropped because of the decreased thermodynamic driving force for the decomposition reaction. I n the same field of thermal decomposition of solids, J. Jach, State University of New York, will contribute a paper on the influence of irradiation on the thermal decomposition of solids. T h e discussion will review the influence of pre-irradiation on the reactivity of solids. This effect is an enhancement in overall reaction rate due to the formation of nuclei from which the decomposition may proceed. This is particularly marked in reactions which normally begin at discrete nuclei on the surface. A different aspect of solid state reactivity is discussed in a paper by L. F. Korris, University of LMichigan. He reports on studies of the influence of the surrounding atmosphere on the solid state transport in metals. I n particular, he will relate experiments on the rate of formation of bridges between polycrystalline, high purity, platinum microspheres of about 150 to 20011 in diameter, in the temperature range between 800 and 1 4 O O O C. The work has shown that the rate of bridge formation is increased by several orders of magnitude whenevcr hydrogen or hydrogen rich mixtures of hydrogen and oxygen are present in the surrounding atmosphere. Thus, it was possible to obtain the ready formation of bridges, corresponding to a relatively rapid sintering of platinum particles at temperatures 800' C. below the melting point of platinum. In addition, the presence of hydrogen produced several surface features (pits, groves, steps). In the area of organic solids, H. Morawetz, Polytechnic Institute of Brooklyn, will present results on reactions in organic crystalline solids. These studies have shown that the relative reactivity of a series of related organic compounds in the crystalline state differ markedly from their relative reactivity when in disordered phases. This is, in brief, a summary of the relevant topics to be discussed at the symposium. Several areas of interest in solid state have been purposely left out since these are generally presented a t other symposia. I t is hoped that the presentation at the same meeting of different aspects of solid state reactivity of interest to chemists, physicists, and engineers will produce a crossfertilization of ideas among research workers, thus permitting a more balanced and comprehensive approach to the complex problems of the reactivity of solids.
