Pyrolysis of simple molecules on hot filaments

Analysis of thermal decomposition of molecules on hot surfaces of solids is of considerable interest not only for investigation of mechanisms of heterogeneous decomposition of molecules into fragments which interact actively with solid surfaces. It is of importance also for clarifying the role of the chemical nature of a solid in this process. Furthermore, pyrolysis of molecules on hot filaments made of noble metals, tungsten, tantalum, etc., is a convenient experimental method for producing active particles. Note that it allows continuous adjustment of the intensity of the molecular flux by varying the temperature of the filament [8]. 

In a number of cases, the temperature of the filament and thermodynamic parameters allow one to calculate [9] the flux intensity of free atoms produced in dissociation of molecules. Specifically, in the case of dissociation of hydrogen, oxygen, and nitrogen molecules on hot metal filaments under pressures of molecular gases higher than lO Torr, the flux intensity of atoms A originating from A2 molecules is given by 

When using a semiconductor sensor for analysis of the flux of free radicals and atoms produced on the filament, one should rewrite expression (4.1) as 

A simplest vessel used for experimental investigation of pyrolysis of hydrogen, oxygen, and nitrogen, as well as other molecules is shown in Fig.4.3. The pyrolysis Hlament (below) is separated from the adsorption chamber (above) by a plane shutter driven by a magnet, whidi permits the sensor to be exposed to the established atomic flux during a required 

H2 molecule, we can plot a graph in a (Ino -) frame and then calculate the sou t-for quantity from the slope of the straight line (Fig.4.4). Thus we And that the energy at which the bond in H2 breaks is equal to 101 kcal/mol. This value differs from flie one obtained by spectroscopic measurements (104 kcal/mol) by only several percent. Similar results were obtained by the same method for pyrolysis of oxygen [5] and nitrogen [6]. Note that in experiments with hydrogen, the 

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Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role they are acting botii as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catal rtic process. 

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