scholarly journals An investigation of the combustion of platinum

The attack of metal surfaces by gases at low pressures under conditions suet that the products of reaction volatilise too rapidly to affect the reaction velocity is one of the simplest heterogeneous actions and presents many analogies to a simple catalytic action at a surface. Langmuir studied the rate of attack of tungsten by oxygen under these conditions and in this communication the interaction of platinum with oxygen is described. This element was chosen, not only on account of the importance of reactions involving the "clean up" of gases in contact with hot filaments, but also because the electron work functions of both tungsten and platinum are known, so that their relation ship to the reaction rate could be studied. That some connection may exist between the reactivity of a surface and the ease of electron emission has been suggested previously and is to be noted particularly in the investigations of Sir Humphry Davy on the oxidation on zinc and copper, of Langmuir, and of Thompson, Bone, Thomas, and Brine on the surface combustion of gases. From a somewhat different standpoint others, notably Brewer, and Finch and Stimson, suggest that surface reactions take place only after formation of ions on the surface has occurred, the former postulating that ions are formed by molecules entering the intrinsic field of the metal surface and react only after emergence from the field.

The relation between reaction rate and potential (or time) for electrochemical surface processes occurring under potentiodynamic control (linear potential-time programme) has been investigated with particular reference to the behaviour of thin surface oxide films on noble metals. The kinetics of processes involving adsorbed electroactive species are treated for several model cases; the rate equations are developed for mechanisms involving various reaction orders or for processes involving adsorbed reactant interactions and surface heterogeneity effects. By examination of the dependence of the reaction rate (current) with time and the effect of potential scan rate, v , on the maximum reaction velocity and the potential at which it occurs, the models may be distinguished. In this manner, the inter­dependence of v and the reaction velocity constants k a and k c for the anodic oxidation and the cathodic reduction processes respectively, can be quantitatively established. The relation between quasi-equilibrium situations where the reverse reaction is significant and irreversible situations where it is not can be demonstrated. Heterogeneity terms introduced into the kinetic relations express deviations from Langmuir adsorption behaviour and may be an intrinsic property of the substrate surface or a property of the adsorbed reactant (induced heterogeneity). Applications of the treatment are made to reduction of surface oxide species at the noble metals and the significance of hysteresis and time effects in the processes of electrochemical formation and reduction of surface oxide at platinum, rhodium, iridium and palladium is investigated.


Unimolecular reactions possess a unique interest in that, as Perrin (‘Ann. Physique,’ vol. 11, p. 5, 1919) first pointed out, for the occurrence of such, some type of interaction between radiation and matter must take place. Although such reactions appear to be extremely rare, many physical processes such as evaporation, ionisation in gases at high temperatures and radio-active decay, proceed at rates conforming to a unimolecular law; true chemical reactions which are definitely unimolecular and not pseudo-unimolecular in character are, on the other hand, stated by many ( e. g ., Lowry, ‘Trans. Farad. Soc.,’ vol. 17, p. 596 (1922) ) to be non-existent. In order to substantiate this statement, it is clearly necessary to prove the more complex character of any reaction which satisfies the usual criteria of unimolecular change. The thermal decomposition of gaseous nitrogen pentoxide apparently fulfils these conditions, for Daniels and Johnston (‘J. Am. C. S.,’ vol. 43, p. 53 (1921)) showed that the reaction proceeded according to a unimolecular law over wide ranges of variation of pressure, and Lueck ( ibid ., vol. 44, p. 757 (1922)) obtained practically identical unimolecular constants for the decomposition in solution in carbon tetrachloride and chloroform. On the other hand, Daniels, Wulf and Karrer ( ibid ., vol. 44, p. 2402 (1922) ) suspected the reaction to be autocatalytic, owing to the apparent retardation of the reaction velocity in the presence of ozone, but the experiments of one of us (Hirst, ‘J. C. S.,’ vol. 127, p. 657 (1925), and of White and Tolman (‘J. Am. C. S.’ vol. 47, p. 1,240 (1925)) proved this to be erroneous. In addition, it has been shown that the reaction proceeds uniformly according to the unimolecular law even in the presence of extensive glass surfaces, or of gases which may be either indifferent, such as argon and nitrogen, or the products of reaction, such as nitrogen tetroxide or dioxide or oxygen. The rate of reaction may be expressed in the form - d C/ dt = 4·98 × 10 13 e -24.700/RT . C. Attempts have been made to interpret the experimental results on the hypothesis that the reaction is in reality bimolecular, and only apparently unimolecular in character; but owing to the abnormally large value of the energy of activation, namely, 24,700 calories per gram. molecule, the number of molecules which could be activated per second by inelastic collision, calculated according to the kinetic theory, falls far short of the observed reaction rate, being, in fact, some 10 5 times smaller.


1938 ◽  
Vol 16b (5) ◽  
pp. 176-193 ◽  
Author(s):  
E. W. R. Steacie ◽  
I. E. Puddington

The kinetics of the thermal decomposition of n-butane has been investigated at pressures from 5 to 60 cm. and temperatures from 513 to 572 °C. The initial first order rate constants at high pressures are given by[Formula: see text]The results are in good agreement with the work of Frey and Hepp, but differ greatly from that of Paul and Marek. The reaction rate falls off strongly with diminishing pressure; this is rather surprising for a molecule as complex as butane. The first order constants in a given run fall rapidly as the reaction progresses. The last two facts suggest that chain processes may be involved.A large number of analyses of the products of reaction have been made at various pressures, temperatures, and stages of the reaction, the method being that of low-temperature fractional distillation. The products are virtually independent of temperature and pressure over the range investigated. The initial products, obtained by extrapolation to zero decomposition, are:—H2, 2.9; CH4, 33.9; C3H6, 33.9; C2H4, 15.2; C2H6, 14.1%. The mechanism of the reaction is discussed, and the results are compared with those of the other paraffin decompositions.


Author(s):  
Kristine Töglhofer ◽  
Friedrich Aumayr ◽  
Hannes Kurz ◽  
Hannspeter Winter ◽  
Paul Scheier ◽  
...  

1960 ◽  
Vol 33 (4) ◽  
pp. 1180-1187 ◽  
Author(s):  
L. P. Morozova ◽  
N. A. Krotova

Abstract 1. The nature of the adhesion bonds in different cases can be determined by investigations of the mechanical characteristics of adhesion, of electrical effects observed on destruction of the bond, and microscopical investigation of the separation boundaries in various systems consisting of polymer pairs, and polymer-metal and polymer-glass systems held together by forces of adhesion. 2. The adhesion bonds between polymer and metal and polymer and glass are electrical in character, as shown by the form of the adhesiogram, the occurrence of electron emission on separation, and the existence of electric charges on the separated surfaces. 3. After separation, the polymer film continues to emit electrons and carries a negative residual charge. The substrate (glass, metal) does not emit electrons and has a positive charge. 4. The breakdown of the adhesion bond between two polar polymers of different structure, or a polar and nonpolar polymer, is accompanied by the same characteristic effects as the separation of a polymer from glass or metal. A sharp boundary is observed in microscopic specimens. 5. Determinations of the velocities of electrons emitted during separation show that breakdown of a firm adhesion bond is accompanied by emission of electrons with higher velocities than those emitted in the breakdown of a weak bond. These results arc in good agreement with the electrical theory of adhesion. 6. The reaction of the substrate (glass) has a strong influence on the adhesion of a polymer (chlorinated polyvinyl chloride) to it. The maximum adhesion is found in the neutral region. The detached polymer film shows a reversal of residual charge in the strongly acid and strongly alkaline regions, accompanied by a sharp decrease of the work of adhesion; this can only be attributed to a decrease of the surface electrification density of the layers of the electric double layer in the charge reversal region. 7. Mechanical treatment of the metal surface increases the adhesion of polymers to it and intensifies electron emission from the regions of the polymer film which were attached to the treated regions of the metal surface. 8. The formation of an adhesion bond between two nonpolar polymers of similar structure is caused by diffusion processes in the contact zone. In such cases no electrical effects are observed during separation, the boundary in microscopic specimens is diffuse, and the work of separation depends relatively little on the rate of separation. 9. The systems studied can be subdivided into two groups: the adhesion bond in systems of the first group is the result of formation of an electric double layer at the boundary; in systems of the second group the adhesion bond is produced by diffusion processes at the boundary.


The chemical electron emission studied is that of the electrons emitted from the liquid alloy of sodium and potassium, K 2 Na, when it is acted an by different chemically active gases, such as the halogens, at very low pressures of the order of 10 -5 mm of Hg. The electron emission is looked upon as an immediate result of a bombardment, by free metallic electrons, of the unstable (excited) chemical bonds formed on the metal surface during collisions of the gas molecules with the metal. When an electron collides with an excited bond, the latter reverts spontaneously through an electronic rearrangement to the normal polar state and the available energy of the electronic rearrangement is simultaneously transferred to the electron which is thus enabled to escape from the metal. The fundamental physical problems underlying the observed phenomena, on this view, relate to (1) collisions of the second kind between free electrons and the electronically excited chemical bonds, and (2) the escape of electrons from the metal surface. Problem (2) is well known is connection with the thermionic and photo-electric effects. Problem (1) is treated by introducing, a priori , a mathematical expression, as general as possible, for the electron de-excitation, or excitation, function which represents the chance of a successful collision. An application of the theory to the experiments has enabled us to determine accurately the values of the constants included in the electron de-excitation, or excitation, function and also the value of an important constant—the total potential barrier of the alkali metal.


Author(s):  
Vishal Chopra ◽  
Vance Robinson ◽  
Frank Pfefferkorn ◽  
T. S. Fisher

Unique features of electron emission from carbon-based nanostructures are studied to explore direct energy conversion materials and processes. These structures have been shown to produce highly efficient field emission (i.e., high electrical current at low applied fields) for electrical device applications. However, the commensurate transport and conversion of thermal energy has not been previously explored in detail. Understanding these concepts could enable development of devices with efficiencies that exceed those of other direct energy conversion technologies. The electron emission behavior at elevated temperatures, of two diamond-based devices with nanoscale features, is characterized. Bulk current measurements reveal work functions of 1 and 1.3 eV for a boron-doped diamond pyramid array and a phosphorus-doped diamond surface, respectively. An electron energy spectrometer has been designed in order to understand the emission behavior in more detail. The analyzer is integrated with a heat source that enables substrate temperatures up to 1000°C. Spectral measurements indicate higher work functions for both samples.


1999 ◽  
Vol 567 ◽  
Author(s):  
Wayne L. Gladfelter ◽  
Charles J. Taylor ◽  
David C. Gilmer ◽  
Daniel G. Colombo ◽  
G. D. Wilk ◽  
...  

ABSTRACTA side-by-side comparison of the TiO2 deposition kinetics and the corresponding film microstructures using titanium(IV) isopropoxide and anhydrous titanium(IV) nitrate was conducted at low pressures (< 10−4 Torr) in an ultrahigh vacuum chemical vapor deposition reactor. Titanium(IV) nitrate exhibited a lower activation energy of reaction (Er= 98 kJ/mol) which allowed deposition at lower temperatures compared to titanium(IV) isopropoxide (Er= 135 kJ/mol). Comparison of the microstructures of films deposited at similar temperatures revealed significant differences in the reaction rate limited kinetic regime. As the growth rates of the two precursors converged in the flux-limited regime, the respective microstructures became indistinguishable.


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