Displacement of metal atoms from salts by hydrogen and the role of this reaction in chain processes

The interaction of hydrogen atoms with a variety of alkali metal and alkaline-earth metal salts results not only in the recombination of these atoms but also in the displacement, into the gas phase, of free radicals (CaCl^·(A ¹ P _1/2, B ² S ⁺) and CaF^·(A ² P)) and metal atoms, including their excited species, which are detected spectroscopically. Transmission spectra indicate that the NaCl surface undergoes metallization when treated with a high-frequency discharge and a rarefied hydrogen flame. Combustion is affected by the gas-phase hydrogen atoms involved in the chain reaction and by the varying composition and properties of the surface. The concentration of Na atoms over the NaCl surface at 770 K is 10⁹?10¹¹ cm^?3 in a stream of H atoms at 1 Torr and in the 2H₂ + O₂ flame at 4 Torr. The concentration of sodium atoms in the ² P _3/2 and ² P _1/2 excited states is ～5 × 10⁶?5 × 10⁸ cm^?3. The role of the discovered reactions in combustion, pyrolysis, and plasma chemistry is discussed.     

The chain character of the third self-ignition limit of hydrogen-oxygen mixtures and flame propagation at atmospheric pressure

It is shown that the characteristic time of the hydrogen-oxygen reaction without the participation of reaction chains is thousands of times longer than the characteristic time of heat removal from the reactor under third self-ignition limit conditions. As a result, reaction mixture self-heating does not exceed several degrees and, contrary to commonly accepted views, cannot cause thermal ignition. It is also shown that the reason for self-ignition under these conditions is the excess rate of chain branching compared with chain termination responsible for the formation of chain avalanches. For the same reason, layer-by-layer ignition during laminar flame propagation is also a chain process. Self-heating arises as a result of the development of chain combustion and strengthens chain avalanches. Explosion and detonation inhibition shows that chain avalanches play an important role in these processes.     

Features of the temperature dependence for the rates of gas-phase reactions of combustion

The specific character of the dependence for the rate of most gas-phase reactions of combustion on temperature and reagent concentrations is found to be determined by the ratio of the rates of the multiplication and death of active intermediate particles. It is shown that the stepwise acceleration of the process and the transition to the combustion mode upon raising the initial temperature by 1–2 K and crossing over a critical value is due to a change in the character of the rate’s temperature dependence as a result of the sign changing from negative to positive to reflect the difference between the rates of chain branching and termination. It is concluded that this difference’s change in sign upon varying the content of reagents imposes limits on concentration.     

Role of Heterogeneous Chain Termination in Flame Propagation

Kinetics and Catalysis - The kinetic characteristics of propagating flame were shown to depend on the chemical properties of the contacting solid surface and on the rates of heterogeneous...     

Specific features of the combustion of hydrogen-oxygen mixtures near the lower concentration flammability limit

The combustion of hydrogen-air mixtures near the lower composition flammability limit in cylindrical reactors of various diameters is studied. The speed of flame propagation in the horizontal direction is lower than that in the upward direction. Chromatographic analysis showed that, during the combustion of these mixtures, hydrogen is consumed only partially. That the flame propagates despite a low adiabatic temperature can be explained by the formation of the well-known cellular structure.     

The role of molecular-kinetic and thermal factors in inhibition of steady detonation

It was experimentally shown that the reaction of the combustion of hydrogen–air mixtures in a detonation wave virtually completely (with an accuracy of 1%) occurs by a branched-chain mechanism; it is this mechanism that determines the characteristics of the detonation wave.