Branching chain reactions, Semenov

In the 1950s, Semenov and Voevodskii [148] made an attempt to apply the concepts of the branching-chain reaction theory to the kinetics of heterogeneous catalysts. They applied the concept of free valencies migrating over the catalyst surface and of "semi-chemisorbed radicals. But their attempt was criticized (see, for example, ref. 149 where Temkin, using hydrogenation of ethylene on palladium as an example, proved experimentally the inapplicability of the chain theory concepts). 

Semenov s model considers any branching chain reaction. It assumes that some initial dissociation of fuel leads to an intermediate species. This species, or some of its products, reacts with the fuel to create more of the intermediate species, implying branching reactions. If recombination, or other chain breaking reactions, are allowed one gets a rate equation for the concentration of the intermediate species [r]. 

Equation (8) undoubtedly is identical to corresponding Semenov s equation which describes kinetics of N active sites in branched chain reaction with quadratic law of chain termination and zero order of initiation [5], However the essence of processes is different. 

These observations all point clearly to the occurrence of a branched chain reaction with chain breaking at the vessel surface. The expression for the rate of a chain reaction is shown by the formal treatments of Semenov and Hinshelwood to have the general form... 

Kowalsky reported that the oxidation of phosphorus vapor occurred only between certain upper and lower limits of O2 pressure. At the lower limit [02]i varied inversely with the phosphorus pressure. The upper pressure limit was independent of T between —40 and +15° C, with [Ozh about 4 x 10" [P4]. Between the lower and upper critical pressures oxidation occurred and was accompanied by chemiluminescence. The results were interpreted in terms of Semenov s theory of branched-chain reactions. ° ° . Kowalsky s experimental work was consistent with eqn. (c) and Semenov himself discarded eqn (b). 

As described in the Introductory Chapter, attention was focused [1] prior to 1961 mainly on the morphology of the cool-flame and ignition regions, rates were followed by pressure change, and essentially chemical techniques were used for product analysis. The acceptance of free radicals, followed by the masterly and elegant Semenov theory [2], which established the principles of branched chain reactions, provided the foundation for modern interpretations of hydrocarbon oxidation. This chapter builds on these early ideas, and pioneering experiments such as those carried out by Knox and Wells [3] and Zeelenberg and Bickel [4], to provide a detailed account of the reactions, thermochemistry and detailed mechanisms involved in the gas-phase chemistry of hydrocarbon oxidation. 

The steady-state approximation applied to chain reactions by Bodenstein cannot describe several time-dependent phenomena, for instance branched chain reactions, leading to explositions. For such cases the semi (or quasi ) steady-state approach, developed by a Nobel prize winner N. Semenov, assumes that concentrations of all radicals except one (the greatest) are considered as steady state. 

This challenge was met by Russian chemist Nikolai Nikolaevich Semenov (1896-1986), working in St. Petersburg, and, independently, English chemist Cyril Norman Hinshelwood (1897-1967) at Oxford. In 1928 they each developed the concept of a branched chain reaction to account for the kinetics of these explosions and their strange dependence on pressure and temperature. 

The reaction system H2-O2 has been greatly studied by the works of HINSHELWOOD and SEMENOV in around 1930, and even more with the spatial applications and due to the fact that its mechanism is an integral part of any hydrocarbon oxidation or combustion mechanism. It will therefore be taken as an example to illustrate the theory of branched chain reactions. 

Subsequent diseoveries in this field and formulation of the fundamentals of the chain reactions theoiy, including branching chain reactions, is linked, to a large extent, with the names of N.N. Semenov [2] and C.N. Hinshelwood [3]. So far the efforts of the seientists in this area are foeused on detailed study of the chain reactions. 

The reasons of critical phenomena in the inhibited liquid-phase oxidation of organic compounds are thoroughly studied. On the basis of theoretical fundamentals of degenerate branching-chain reactions developed by N.N. Semenov [1], N.M. Emanuel and A.B. Gagarina [33] explained the features of the manifestation of critical phenomena in such systems. 

The basis of the free-radical mechanism of oxidation, which includes combustion and explosion, was created by N. Semenov as part of his fundamental work dedicated to the theory of branched chain reactions (Nobel Prize for Chemistry in 1956). Based on this (now common) theory, the oxidation of hydrocarbons takes place according to the mechanism of the free-radical chain reaction with forced branching. Let s look at the main rules of this mechanism, which can take place in the extracellnlar skin matrix. 

The Nobel Prize was not awarded for the discovery of chain reactions. However, in 1956, Sir Cyril Norman Hinshelwood from Great Britain (1897-1987, London) and Nikolai Nikolaevich Semenov from Russia (1896, Saratov, to Moscow, 1986) were jointly awarded the Nobel Prize for (mostly) developing branched chain reactions. The first monograph on chain reactions was written by Semenov. 

Although studies on methane partial oxidation have never stopped and continued beyond this period, and even attempts to introduce new industrial processes have been made (see, e.g., [29]), none of them found practical implementation. At the same time, under the influence of the theory of branched-chain reactions, developed by Semenov and his co-workers [30], a new understanding of mechanism of the oxidation of hydrocarbons began to take shape. Results of more than half a century of research in the oxidation of hydrocarbons and new concepts of hydrocarbon oxidation mechanism were summarized in a fundamental monograph by Shtern [13]. 

Hydrocarbons are oxidized without the introduction of a radical source but this oxidation occurs with autoacceleration. This autoacceleration was explained in the framework of the theory of degenerate-branched chain reactions by the formation of an intermediate product, initiator. It was proved in 1930-50 that these products are hydroperoxides (see above). The Bach-Engler peroxide theory was thus merged with Semenov s theory of degenerate branching. Soviet scientists made the decisive contribution to the development of this area. 

Figure 2.6. Semenov cycle of the branched chain reaction of water synthesis...

Semenov put forward the concept of slow hydrocarbon oxidation as the chain reaction with degenerate branching N. N. Semenov [49]... 

Semenov, Nikolay Nikolaevich (1896-1986) Russian physical chemist. Semenov studied chemical chain reactions in the 1920s. He showed how such reactions could lead to combustion and violent explosions when branching occurs in the chain. He gave an account of his work in the influential book Chemical Kinetics and Chain Reactions (1934), the English translation of which was published in 1935. He shared the 1956 Nobel Prize for chemistry with Sir Cyril hesishelwood. 

The development of studies along this line entered a new phase after N. N. Semenov formulated the theory of chain reactions with branches [3], and especially after the creation of the theory of degenerate explosions [4]. 

It is customarily believed that carbon chain polymers are oxidized according to the mechanism of chain reactions with degenerate branches, proposed by Semenov [3]. Initiation occurs as a result of attack on the RH molecule by oxygen according to the reaction... 

In the Soviet Union, the concepts of Academician N. N. Semenov on chain reactions with degenerate branches have become the starting point of theoretical studies of the stabilization and destruction of polymers. Soviet scientists have developed a theory of critical concentrations of antioxidants and have shown that the processes of stabilization have a very complex chemical character. The nature of the polymers themselves greatly affects these processes and consequently, different stabilizers are required for polymers of different structures. In addition, it has been shown that the antioxidants used thus far can not only cause chain termination, but can also initiate oxidation and give rise to degenerate branches. 

Semenov in his Nobel lecture Some problems relating to chain reactions and to the theory of combustion described research of his groups at the Leningrad Physical-Technical Institute (under the leadership of academician Abram Ioffe) and then in the Institute of Chemical Physics of the Russian Academy of Sciences in Moscow. Semenov stressed on the similarities of branched nuclear reactions discovered in the 1930s by physicists, where, like in chemical chain reactions, the size and density are the decisive factors in the transformation of safe inert conditions to explosion. 

The oxidation of hydrocarbons follows very complex ramified processes. In the case of methane, all the mechanisms proposed make use of the radicals CH3 and OH. The most probsble process (Semenov, 1960) involves a branching chain with the formation of formaldehyde, and takes account of latest kinetic data and of modern concepts of the free energy of free radical-stable molecule reactions. Its validity is confirmed by the accord between values calculated for each elementary step and the experimental results. 

To explain the kinetic behaviour of such systems Semenov [19,20] and Hinshelwood and Thompson [21,22] have proposed the concept of branching chains. When a pair of ordinary propagation steps occurs, there is no change in the number of chain carriers. When a branching chain occurs, however, there is an increase in the number of carriers. Let us consider the pair of reactions ... 

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