Thermal emission reaction

Fig. 8-28. Cathodic polarization curves for several redox reactions of hydrated redox particles at an n-type semiconductor electrode of zinc oxide in aqueous solutions (1) = 1x10- MCe at pH 1.5 (2) = 1x10 M Ag(NH3) atpH12 (3) = 1x10- M Fe(CN)6 at pH 3.8 (4)= 1x10- M Mn04- at pH 4.5 IE = thermal emission of electrons as a function of the potential barrier E-Et, of the space charge layer. [From Memming, 1987.]...

Clyne, Thrush, and Wayne107 reexamined the chemiluminescence from the nitric oxide-ozone reaction and found its spectrum to be similar to that of the thermal emission of N02 at 1200°K. They concluded that the spectra represented transitions from similar low-lying vibration levels of the same excited electronic state of NOa to the ground state. By measuring the decay in chemiluminescence down a flow tube, they obtained the value of the rate constant between 216 and 322°K. The partial pressures of ozone and nitric oxide were 5 x 10-3 and 2 x 10 2 torr, respectively, in an argon carrier at a total pressure of 2 torr. In the presence of excess nitric oxide, they assumed the logarithmic disappearance of ozone proportional to [NO], so that... 

Chemiluminescence can occur when a thermal (dark) reaction is so exothermic that its energy exceeds that of the electronically excited state of one of the product molecules. The major pathway for these reactions is the decomposition of cyclic peroxides, and this is at the basis of most bioluminescence processes. There are some other physico-chemical processes which can lead to the formation of excited states and thereby to the emission of light these are based on the bimolecular recombination of high-energy species such as free radicals and radical ions. 

In the study of chemiluminescence in elementary transfer reactions the behavior of the new-bond molecule has been emphasized over the past decade. This is understandable. The initial successful experiments showed new-bond excitation. Also, the work was conditioned by the observation of a highly non-thermal emission from OH in the upper atmosphere59 and the demonstration60 that a likely cause was the reaction H+03 -+ OH +Oj. Indeed, early work aimed at a test of the possibility that the principal excitation channel was via the highest accessible level. This hypothesis has not been proven. The available facts contradict it. Many levels of the new-bond molecule are populated by reaction. Ample evidence for this is provided by the reactions listed in Table 1. 

Fig. 5. (a) Bulk electronic concentration at the metal—oxide interface and electron-hole concentration at the oxide—oxygen interface associated with equilibrium interfacial reactions, (b) Electronic energy-level diagram illustrating the dielectric (or semiconducting) nature of the oxide, with the possibility of electron transport (e.g. by tunneling or thermal emission) from the metal to fill O levels at the oxide—oxygen interface to create a potential difference, VM, across the oxide. 

In all cases of electron transport, whether it be hopping, thermal emission, or quantum tunneling, the effect of the electric field in the oxide film is extremely important. In fact, the electric field effect on ion motion is the primary reason the electronic species must be considered at all in most real metal oxidation reactions. This can be understood better when we discuss the coupled-currents approach [10,11] in Sect. 1.15. 

The properties of the Thermal DeNOx reaction define the situations/applications in which it can be used to control NOx emissions. It must be possible to rapidly and completely mix NH3 with the gas being treated. When this is done the gas must be at an average temperature in the range of 1300°F to 2000°F and the more uniform the gas s temperature the better. 

In flames with lower final flame temperatures where the thermal emission from added metal atoms is less, a chemiluminescent effect [134] may occur. Here, there is a rapid rise of intensity in the reaction zone followed by a steady decay towards the thermal level. The chemiluminescence is due to excitation of the metal (in this case sodium) by the reactions... 

Clyne et re-examined this reaction in a flow system and found that the emission was similar to the thermal emission from NO2 at 1200 °K. It was concluded that both spectra were due to transitions from similar low-lying vibrational levels of the same excited electronic state of NO2 to the ground state. The mechanism was shown to be ... 

Carbon dioxide and water are the main products of this reaction. However, incomplete combustion causes some emissions of unbiuned hydrocarbons, as well as intermediate oxidation products such as alcohols, aldehydes and carbon monoxide. As a result of thermal cracking reactions that take place in the flame, especially with incomplete combustion, hydrogen is formed and emitted, as well as hydrocarbons that are different from the ones present in the fuel. 

The results discussed so far correspond mainly to reactions in the electronic ground state. Some information is also available for the contribution from excited electronic states. Observation of thermal emission of electronically excited NOg during dissociation provided information about the population of vibrational levels of electronically excited NOg. Marked deviations from a Bolt2mann distribution were found at levels near the dissociation limit of the excited state. This observation is in accord with arguments presented in section 1.7 on the population of vibrational states in the low pressure limit of dissociation reactions. Dissociation out of the electronically excited state contributes only to a small extent to the overall rate of dissociation (a few % at 2000 K). 

The thermal emission coefficients of a reaction mixture flow in the profiled channel... 

The central problem in the study of ionic conduction is to discover the details of the atomic transport processes involved in the growth of films. This will be discussed in the first part of this review. The electronic conductivity is of considerable theoretical interest and is of great practical importance for microelectronic devices. We discuss the system in which a thin metal counterelectrode replaces the electrolyte solution in which the oxide was made. Thermionic and field assisted emission, tunneling processes, impurity band conduction, and space-charge limited currents, have to be considered. We shall draw on results for oxide films made by other processes, such as evaporation and thermally promoted reaction with oxygen. 

Experimental determination of the excitation factor shows that with chemical luminescence excitation (chemiluminescence) in flames the f value is, as a rule, much higher than the thermal emission factor. This stems from the non-equilibrium nature of this kind of emission, directly related to the energy liberated by some or other elementary chemical process. This shows the high importance of chemiluminescence both for the identification of labile intermediates and for the elucidation of certain fine details of the chemical reaction mechanism. 

FT-Raman studies of thermally and photochemically induced epoxy curing reactions have been reported [135]. In some instances pulsed sources may be required to overcome thermal emission problems. 

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