Showing papers on "Reaction rate published in 1978"

A molecular beam investigation of the catalytic oxidation of CO on Pd (111)

Abstract: A detailed investigation of the steady‐state and nonsteady‐state reaction CO+1/2O2→CO2 on Pd (111) has been carried out with the molecular beam technique. It could be shown conclusively that the reaction proceeds between two adsorbed species (Langmuir–Hinshelwood mechanism) throughout the temperature and pressure range investigated. For low CO coverages, the activation energy of the reaction was determined to be 25 kcal/mole, whereas at moderate CO coverages, a rearrangement of the oxygen adlayer takes place resulting in a reduction of the activation energy to 14 kcal/mole. It is not possible to formulate a simple kinetic expression for the reaction rate which is valid over the entire range of temperatures and pressures due to changes in the adsorption rate for O2, coadsorption of CO and O2, to diffusion in the adlayer, and to changes in the geometrical arrangement within the adlayer with varying coverage.

Organometallic migration reactions

Abstract: A molecular orbital study of the reaction path for the methyl migration reaction in CH3Mn(C0)5, the "carbonyl insertion reaction", is the starting point for a general analysis of organometallic migration reactions. The reaction path contains a five-coordinate intermediate and a transition state close in energy to the intermediate. Among the questions studied are the stereochemical course of the reaction, the effect of solvent on the kinetics, possible binuclear mechanisms, the vexing problem of formyl formation by the migration reaction, the effect of changing the migrating group, the migration terminus (carbene, nitrosyl, isonitrile), the effect of other ligands on the metal, and variation of the metal and coordination number. The general reaction 1, a 1,2 shift of a ligand L to a n unsaturated fragment XY, both coordinated to a metal M, is of obvious synthetic value. As such, it has drawn the attention of chemists from the early days of organometallic chemistry on, and has been the subject of numerous synthetic and mechanistic investigati~ns.l-~ The case XY = CO has been popularly called the carbonyl insertion reaction. It is a key step in many homogeneous catalytic transformation^,^-^ and possibly in some heterogeneous processes as we11.8-10 Theoretical analyses of this crucial process are lacking, with the important exception of a recent paper by Howell and Olsen." We have attempted a general analysis of a model system for carbonyl insertion, using the extended Huckel method.12 To sharpen the questions which were the challenge for our study it is perhaps worthwhile to review some of the experimental findings and mechanistic conclusions. Since the discovery of prototype reaction 2 in 1957,13 the 0 I1 RMdCO), + L R-C-Mn(CO),L ( 2 ) following points have emerged: (1) The kinetics of the reaction are first order in RMn(C0)5, though sometimes there is an incoming ligand dependence. ( 2 ) Donating solvents can accelerate the overall reaction rate. (3) A coordinatively unsaturated intermediate appears to be involved. (4) The entering ligand occupies the position previously occupied by the alkyl group, cis to the acyl. Thus the reaction is really an alkyl migration rather than a carbonyl insertion. (5) The acyl CO originates from the original coordination sphere of the metal. Further studies have extended the scope of the reaction: ( 6 ) There is in general no restriction on the transition metal involved, and many different coordination environments allow this reaction type. ( 7 ) There is some selectivity as to the nature of the migrating group. I n particular the formation of formyl derivatives by H migration has been desired, and the reasons for its apparent absence have been the object of considerable speculation. (8) Substitution patterns on the metal center influence this migration. A rationalization of the known features of the reaction was the first focus of our study. What is the nature of the postulated intermediate? What factors govern the stereochemical course of the reaction-CO insertion vs. alkyl migration? What role is played by the solvent? We felt that if we could understand these basic aspects we should be able to make predictions as 0002-7863/78/1500-7224$01 .OO/O to the effect of different metal centers and ligands of differing cr and 7 donating capability on the ease of the migration. This would help the design of systems in which this synthetically useful reaction could be facilitated in cases in which it is difficult or unobserved. W e begin our study with the search for an approximate reaction path by which a methyl group migrates to a carbonyl, eq 3, to give an acyl complex. The computational method used is the extended Huckel procedure1* with parameters specified in the Appendix. The reader should be aware that processes in which bond breaking and re-formation occur are on the borderline of what may be reliably treated by this approximate M O method. W e have nevertheless been able to study effectively such insertion reactions in the organic realm,14.'5 and in one organometallic migration related to this one, the olefin insertion reaction.16 Proper care must be taken to buttress any computational results by qualitative symmetry and bonding arguments, and this we will do. Potential Energy Surface for the Rearrangement of CH3Mn(CO)S to CH3C(O)Mn(C0)4. The migration reaction is not restricted to a single metal nor a certain coordination environment. But certainly its most common manifestation is for octahedral d6 complexes. To extract the general features of the reaction path we have chosen CH3Mn(C0)5, which is also the example most exhaustively studied experimentalA full optimization of all degrees of freedom was not carried out, nor would we expect it to lead to a good reaction path from the extended Huckel method. But extensive exploratory calculations led to the following conclusions: 1. It is reasonable to maintain C,y symmetry throughout the reaction, and indeed we will refer to this plane as the migration plane. 2. Methyl group rotation about its local threefold axis is not a significant factor in the reaction. 3. A slight tilting up, 20°, of that carbonyl group which gets the methyl, 1 -+ l', costs approximately 0.5 eV in the ly.13,17

Measurement of fast desorption kinetics of D2 from tungsten by laser induced thermal desorption

Abstract: A laser heating technique for studying fast surface processes has been applied in an initial study to the thermal desorption of D 2 from a polycrystalline tungsten sample. This technique is a means for measuring surface reactions at rates, concentrations, and temperatures that approach conditions of technical interest, but with the high degree of definition and control made possible with an ultrahigh vacuum apparatus. The method is analogous to the fast temperature jump method used for studying reactions in condensed phases, and can sort out elementary processes that have differing activation energies. The variation of total flux desorbed with maximum surface temperature reached and initial surface coverage serves, with the aid of a model kinetic rate expression, to determine the desorption rate parameters. It is shown that the desorption of D 2 from W at rates of 5 × 10 7 monolayers/sec is governed by the same kinetics as obtained by extrapolating previous measurements made at a rate about 10 5 times slower. The surface is subjected to a sufficiently fast and large temperature rise to desorb surface atoms or molecules in a time short compared to the range of flight times to a mass spectrometer detector. In this way the velocity distribution of the desorbing species may be determined. This along with the surface temperature history gives additional information on the reaction rate model and also whether the species are emerging in translational thermal equilibrium with the surface. In the present experiments a significant number of desorbatedesorbate collisions occur. Corrections are made for the collision effects in the interpretation of the data. It is shown how modifications of the technique can be made to substantially eliminate these effects. The present conditions were laser pulse width of 3 × 10 −8 sec and surface temperature rise of 300 to 3000 K.

Measurement of the rates of reaction of the ground and metastable excited states of O2+, NO+ and O+ with atmospheric gases at thermal energy

Abstract: Thermal-energy reaction rate coefficients and product ion distributions have been measured for reactions of both the ground state and metastable electronic states of O2+, NO+ and O+ with several neutral species, using a selected-ion flow tube. In general the excited-ion reaction rates are fast, frequently approaching the Langevin limit. Collisional quenching occurs for the reactions of NO+* with N2, O2 and H2 and the quenching rates have been determined. The ion source also provided a substantial yield of doubly charged O2 permitting some measurements of reaction rates of O22+.

Nitrogenase: the reaction between the Fe protein and bathophenanthrolinedisulfonate as a probe for interactions with MgATP.

Abstract: The reaction between the Fe(II) chelating agent, bathophenanthrolinedisulfonate, and the iron-sulfur cluster in the Fe protein of nitrogenase from Clostridium pasteurianum has been studied. This reaction is greatly accelerated by the presence of MgATP. Analysis of the relationship between reaction rate and concentration of MgATP supports a model in which both of two binding sites for MgATP on the Fe protein must be occupied before the protein undergoes a conformational change, allowing the iron-sulfur site to react rapidly with chelator. This model is also consistent with presently available data on equilibrium binding of MgATP to the Fe protein. MgADP inhibits the effect of MgATP on the chelator reaction in a manner which suggests that MgADP binds strongly to one of the MgATP sites and more weakly to the other. Loss of enzymic activity due to exposure to O2 or 0 degrees C is accompanied by a decrease in the ATP-specific chelator reaction. Hence, this reaction was used to estimate the concentration of active iron-sulfur centers for the purpose of computing the extinction coefficient of the Fe protein, giving the value delta epsilon 430nm(ox-red) = 6600 M-1 cm-1.

The law of additive reaction times in fluid-solid reactions

Abstract: A law governing the rate of reaction of a solid particle with a fluid is proposed and verified for a wide range of systems. It is exact for the reaction of a nonporous solid and is a useful approximation in the case of a porous solid. The law states that the time required to attain a certain conversion is the sum of the time required to reach the same conversion in the absence of resistance due to the intrapellet diffusion of fluid reactant and the time required to reach the same conversion under the control of the intrapellet diffusion. The limitations are that the solid should be isothermal, the intrinsic kinetics should be at least approximately of first order with respect to the concentration of fluid reactant, and the effective diffusivity should remain unchanged during the reaction. Furthermore, with the appropriate definition of the fluid-solid reaction modulus, numerical criteria for asymptotic regimes of chemical reaction and diffusion controls have been shown to remain identical for all systems considered. The law is valid not only in an integral (conversionvs time) form but also in a differential form: The rate of reaction at a certain conversion can be obtained from the differential form of the law. The latter is valid even when the bulk temperature and concentration vary with time, which makes it possible to apply the solution directly to multiparticle systems such as packed-bed, moving-bed and fluidized-bed reactors.

Kinetics of Sicl4 Oxidation

Abstract: The oxidation rate of SiCl4 was studied at 1100° to 1300°C. The reaction was first order in SiCl4 and zero order in O2 up to a 20-fold excess of O2. At higher O2 concentrations, the reaction becomes first order in oxygen. The activation energy for the reaction was 96 ± 5 kcal/mol which is approximately the CI3SiCI bond energy. The reaction SiCl4→ SiCl3+ Cl is proposed as the rate-determining step.

Solution of the Smoluchowski equation with a Coulomb potential. II. Application to fluorescence quenching

Abstract: The rate of fluorescence quenching is derived for the continuum model of a diffusion‐controlled reaction, using the exact solution to the Smoluchowski equation with a Coulomb potential derived earlier. The first two terms in the long‐time expansion of the reaction rate are formally the same as the result obtained by Smoluchowski for no long‐range forces. However, the sum of the hard‐core radii in Smoluchowski’s expression is replaced by an effective encounter distance. Our expression for the reaction rate with the Coulomb interaction constitutes an extension of Debye’s steady‐state result to the transient case.

Fluid-wall reactor for high temperature chemical reaction processes

Abstract: A high-temperature, fluid-wall chemical reactor can be equipped with a variable profile, counterflow heat exchanger and a reaction product control system. The heat exchanger includes two tubular walls, positioned concentric of one another, and a spiral baffle disposed between the two walls to define a spiral, annular coolant channel. The walls and baffle are made of a refractory material. The heat exchanger has an inlet and an outlet to permit a coolant to be circulated through the coolant channel. The reaction product control system includes a reaction product analyzer and means for withdrawing and transferring samples of reaction product exiting the reactor. The reaction product analyzer can receive samples of reaction product and generate a signal corresponding to deviations between the chemical composition of the product and a preselected composition. The control system also includes a reactor temperature controller to vary a temperature of the reactor in response to the signal from the reaction product analyzer to reduce the deviations.

Simulation of ascorbic acid stability during heat processing and concentration of grapefruit juice

Abstract: The kinetics of ascorbic acid loss in grapefruit juice was determined for thermal and concentration processes. The results indicate an apparent first order anaerobic reaction which is dependent on temperature and degree of product concentration but not on initial ascorbic acid content. The effect of temperature on rate of reaction follows the Arrhenius equation with energy of activation ranging from 5 to 11.3 Kcal/mole for solids content of 11 to 62†Bx, respectively. Using a polynomial curve fitting technique, an empirical kinetic equation correlating rate of reaction with temperature and degree of concentration was established. Based on this empirical equation, a prediction mathematical model was formulated and proved useful in analyzing the extent of ascorbic acid retention in grapefruit juice during commercial thermal and concentration processes.

Reactive ion etching of aluminum and aluminum alloys in an rf plasma containing halogen species

Abstract: An etching method capable of producing vertical‐walled high‐resolution patterns in aluminum and aluminum alloy films is described. The process consists of rf sputter etching in a plasma containing ion species, which react with the metal to form volatile or easily sputtered compounds. The presence of reactive species greatly enhances the etch rate, while the electric field maintains the directionality inherent in the sputtering process. Halogen ion species, as obtained in rf plasmas containing a partial pressure of Cl2, Br2, HCl, HBr, or CCl4, were used to produce reactive ion etching of Al. The etch rate in a CCl4 plasma at a power input of 0.6 W/cm2 is as high as 5000 A/min. Variations in reaction rate with rf power, reactant concentration, reactant flow rate, temperature, gas pressure, batch size, and residual gas contamination are discussed. Etch rate data for various materials found suitable for masking are also presented.

Thermal Cracking Models For Athabasca Oil Sands Oil

Abstract: The prime purpose of this work was to provide thermal cracking reaction models which can be incorporated into numerical simulators of thermal recovery processes for the Athabasca oil sands. Athabasca bitumen, free of water and minerals, was thermally cracked at constant temperatures in a closed system under an inert atmosphere. The products of cracking were separated into 6 pseudo components: coke, asphaltenes, heavy oils, middle oils, light oils, and gases. Experimental runs were made over temperature range from 303/sup 0/C to 452/sup 0/C. Three series of runs were made at 360/sup 0/C, 397/sup 0/C, and 422/sup 0/C in which the reactions were terminated at various degrees of cracking. For these runs, reaction time vs. product concentration curves were obtained for the above 6 pseudo components. Several pseudo reaction mechanisms are proposed to simulate the experimental results. The reaction rate constants were represented by an Arrehnius-type expression; the activation energies and corresponding frequency factors were determined for each reaction mechanism proposed. (23 refs.)