Heterogeneous decomposition

Kinetic studies of the decomposition of metal formates have occasionally been undertaken in conjunction with investigations of the mechanisms of the heterogeneous decomposition of formic acid on the metal concerned. These comparative measurements have been expected to give information concerning the role of surface formate [522] (dissociatively adsorbed formic acid) in reactions of both types. Great care is required,... 

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]. 

This has so far only been observed for the heterogeneous decomposition in glass or silica vessels at temperatures between 250 and 400 °C 39. However, the formation of small quantities of hydrogen in these experiments indicates that (a) is accompanied by a pyrolysis corresponding to... 

The kinetics of the thermally induced homogeneous decomposition of phosphine (PH3) have not yet been studied. The species PH2, PH and P2 are formed on flash photolysis of PH3 and could be identified by their absorption spectra63. There are proposals as to the mechanism of the consecutive process after the photochemical primary step, but nothing is known about the kinetic parameters of these reactions. With arsine and antimony hydride only the heterogeneous decomposition has been studied64,65. 

Fig. 14.18 Gasometric analysis of the heterogeneous decomposition of hydrogen peroxide at non-doped CNFs (circle) and N-doped CNFs (square) in 1M KN03 (Reprinted with permission from [180] - 2005 American Chemical Society).

Ozone decomposes on surfaces, the rate depending on the nature of the particular surface and whether it has been previously conditioned by exposure to 03. While this heterogeneous decomposition is much slower than the wall loss of OH, the homogeneous gas-phase... 

Fig. 1 d.—Influence of temperature on the heterogeneous decomposition of nitrous oxide on a platinum wire. 

The heterogeneous decomposition of nitrous oxide in contact with a heated gold wire is interesting, though undoubtedly rather exceptional. The reaction is unimole-cular with respect to nitrous oxide. The heat of activation, obtained from the temperature coefficient of the reaction velocity, is 29,000 calories. 

ACTIVATION IN HETEROGENEOUS REACTIONS 245 measurably slow even at temperatures in the region of 1,000° C., must have a heat of activation greater than 70,000 or 80,000 calories (page 97). The heterogeneous decomposition on the surface of tungsten has a heat of activation of 39,000 calories. 

The initial pyrolysis reaction of SiH4 has been explored extensively (98), and the growing consensus is that the thermal decomposition of SiH4 involves the elimination of H2 to form silylene, as shown in equation 1 (98, 101). Possibly, thermal pyrolysis is controlled by heterogeneous reactions on hot surfaces (102, 103), but this hypothesis is controversial, and considerable experimental evidence for a gas decomposition mechanism exists (98). However, at low pressures and high temperatures, heterogeneous decomposition will likely be important in the overall mechanism (104). 

Equation (6.117) shows the heterogeneous decomposition of H202 on mineral surface ... 

A high linear rate of H202 supply by the quartz tube (determined experimentally for every particular reactor) usually eliminates any possibility of its dissociation. Moreover, to prevent heterogeneous decomposition of H202, in some cases the internal walls of the tube are treated with boric acid [45], If these conditions are maintained, H202 concentration remains... 

The thermally initiated oxidation of SO2 has also been studied, Russell and Smith " found that mixtures of SO2 and O2 which were allowed to stand over certain metal catalysts produced SO 3 even at room temperature. Flint and Lindsay have reported that at temperatures of 200-900 °C no oxidation occurs except that due to heterogenous decomposition catalyzed by silica above about 700 °C. Recently, Cullis et studied the system and concluded that an ap-... 

In the heterogeneous decomposition in the rate-limiting step is the removal of the first hydrogen on the surface ... 

The heterogeneous decomposition mechanism is characterized by easy chemisorption followed by rapid decomposition. Again the presence of adsorbed organogallium species catalyses this process, thus the formation of GaAs is a synergetic, mutually catalytic process. 

During the heterogeneous decomposition of formic acid on copper, the active metaUic phase undergoes sintering, sublimation and modifications of surface texture [25], By comparison with the behaviour of copper(II) formate [13], it is concluded that copper(l) formate is formed. Thus variations in the values of A and E, reported for the catalytic process may be attributed to a dependence of kinetic behaviour on the lifetime of the volatile participant, and thus upon metal mobility and reaction conditions [5], The decompositions of the copper formates and the catalytic decomposition of formic acid on copper metal thus include the participation of common intermediates, but these different reactions each consist of a sequence of several interdependent processes. 

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