Autocatalytic mechanisms

A review of epoxy-novolac reaction mechanisms and kinetics is provided by Biernath et al.85 Depending on the structures of the novolac and the epoxy, reactions have been reported to proceed through an nth-order mechanism or an autocatalytic mechanism.88-92... 

Quite complex kinetic behavior has been identified on some surfaces. For instance, on Ir(100), the TPD data from NO-saturated surfaces display two N2 desorption peaks, one at 346 K from the decomposition of bridge-bonded NO, and a second at 475 K from the decomposition of atop-bonded NO molecules [13], Interestingly, the first feature is quite narrow, indicating an autocatalytic process for which the parallel formation of N20 appears to be the crucial step. An additional complication arises from the fact that this Ir(100) surface undergoes a (1x5) reconstruction, and that NO adsorbed on the metastable unreconstructed (lxl) phase leads to N2 desorption at lower temperatures. In another example, on the reconstructed hexagonal Pt(100) surface, when a mixed NO + CO adsorbed layer is heated, a so-called surface explosion is observed where the reaction products (N2, C02 and N20) desorb simultaneously in the form of sharp peaks with half-widths of only 7 to 20 K. The shape of the TPD spectra suggests again an autocatalytic mechanism [14],... 

This group covers aromatic nuclei bearing one nitro group most mono-nitro benzenes can be persuaded to detonate by a tetryl booster few are an immediate danger in tbe absence of other sources of energy. Calorimetric studies [1] suggest tbat nitroaiyl compounds decompose by an autocatalytic mechanism, and thus stability may depend upon thermal history. Individually indexed compounds are ... 

In the 45 years since its proposal, Frank s autocatalytic mechanism (Section 11.3, above) has spawned numerous theoretical refinements including consideration of such factors as reversibility, racemization, environmental noise, and parity-violating energy differences. [100,101] In contrast to the above examples of stereospecific autocatalysis by the SRURC, however, none of these theoretical refinements is supported by experimental evidence. While earlier attempts to validate the Frank mechanism for the autocatalytic amplification of small e.e.s in other experimental systems have generally been unsuccessful, several recent attempts have shown more promising results. [102,104]... 

The decomposition of this intermediate on both the Ni(llO) and Ni(lOO) surfaces occurred by an autocatalytic mechanism (99) for adsorbate coverages above about one-tenth of a monolayer. In fact, the decomposition rate was observed to accelerate isothermally as the reaction proceeded on both the Ni(l 10) and Ni(lOO) surfaces (98, 99) the rate of acceleration was more pronounced on the (110) surfaces. Furthermore, the intermediates were observed to form islands, as if a two-dimensional phase condensation occurred at about one-tenth monolayer coverage. The formation of this 2D condensed phase was clear indication of attractive interactions among the adsorbed species. 

Kinetic Studies of Indium Nanoparticle Formation The Autocatalytic Mechanism 377... 

Using the well-defined system of polyoxoanion/Bu4N -stabilized iridium nanoparticles [9, 29] as a model for the studies, Finke and coworkers [30] proposed a method that attempted to explain the formation and growth of transition-metal nanoparticles. This indirect method is based on an autocatalytic mechanism that considers a nudcation step in which a precursor A is converted to a zero-valent nuclei B with a rate constant fej, and a second step that considers the autocatalytic surface growth of the metal nanoparticles where species B catalyzes its own formation with a rate constant tc2 (Scheme 15.5). 

In this autocatalytic mechanism the olefin hydrogenation step is considered faster than the nucleation and growth steps. When the olefin hydrogenation is a rapid process, the equations can be deduced in terms of the two constants, ki and lc2, the values of which can be obtained from kinetic Equation 15.1 ... 

Scheme 15.6 The accepted autocatalytic mechanism for monitoring the formation of transition-metal nanoparticles, as proposed by Finke and coworkers.

Kinetic Studies of Iridium Nanoparticle Formation The Autocatalytic Mechanism 379... 

In the same context, the autocatalytic mechanism was successfully applied to the formation of iridium nanoparticles dispersed in imidazolium-based ILs [25,... 

Figure 15.7 Experimental points ( ) and calculated curves (line) using the autocatalytic mechanism (rate constants k, and (C2) for the formation of iridium nanoclusters during 1-decene hydrogenation by the catalyst precursor [lr(COD)Cl]2 under 4atm of molecular hydrogen in (a) BMI PFs ...

Fig. 8. Concentration B in reaction mechanism for diffusion coefficient of X much less than that of Y, necessary to achieve instability in a system in which the autocatalytic mechanism occurs on a one-dimensional array of local sites, versus logarithm of site density a (solid line). Dashed and dotted lines are for the isolated and continuum site limit, respectively.

The oxidation of polymers can be represented by an autocatalytic mechanism involving the intermediate formation of hydroperoxides (Scheme 1) (B-79MIU501). The termination steps involving reactions of peroxy radicals are thought to predominate in most cases. Metal ion impurities accelerate degradation by promoting decomposition of intermediate hydroperoxides (B-79MI11502). 

In the case of the nonisothermal first-order exothermic reaction heat is auto catalytic, for it raises the temperature and provokes an increase of reaction rate, yet is itself a product of the reaction. In the Gray-Scott scheme, B is plainly autocatalytic and its degeneration by the second reaction plays the role of the direct cooling in the non-isothermal case. This reaction appears in the chemical engineering literature in 1983,16 and is the keynote reaction in Gray and Scott s 1990 monograph on Chemical Oscillations and Instabilities.17 A justification of the autocatalytic mechanism in terms of successive bimolecular reactions is the subject of Chapter 12. 

Since the early work of Kissinger (Ref 2) much work has gone into the improvement of the study of the kinetics of decompn. Table 5 lists the data of Rogers (Ref 45), and additional values can be found in Vol 7, K11, Equation (1) suggests that one should be able to vary the size of the shape at will, whereas in our experience the assumption of a size independent decompn rate is not justified. Most expls decomp by an autocatalytic mechanism, particularly at the upper temp limits so that in these cases thermo-analytically obtained kinetic data are not trustworthy... 

The presence of V3S4 crystals can only be attributed either to an autocatalytic mechanism of this type or the migration of the deposited metals. It is known that deposited Ni and V sulfides possess some catalytic activity (see Section IV). Slurry processes have been proposed which utilize Ni and V deposited from the oil onto a slurry material (Bearden and Aldridge, 1981). Studies have appeared in the literature demonstrating that nearly all of the transition metals are catalytically active for HDS reactions and presumably for HDM (Harris and Chianelli, 1984). Rankel and Rollmann (1983) impregnated an alumina catalyst base with Ni and V and concluded that these sulfides display an order of magnitude lower activity than the standard Co-Mo sulfide catalyst for HDS reactions, but exhibited similar activity for HDM reactions. 

A close look at the hydrogenolysis reaction of the Cp2 SmCH(TMS)2 complex with H2Si(SiMe3)2 shows that the two compounds do not react directly. The kinetic profile shows an S-shape form with an induction time. This shape is consistent with a second-order autocatalytic mechanism in which the reaction is catalyzed by a product or an intermediate. Indeed, the induction period was completely eliminated by addition of catalytic amounts of H2, [Cp2 SmH]2, or of the complex [Cp2 Sm (p-H) (p-C CsMe SmCp ], which is a decomposition product of the [Cp2 SmH]2. Thus, two possible mechanistic pathways (Schemes 3 and 4) have been proposed to explain the hydrogenolysis reaction. 

Even a very slow decomposition may impinge on the thermal stability of a substance decomposing by an autocatalytic mechanism. Thus, the time factor plays an important role. 

Certain classes of compounds are known to decompose following an autocatalytic mechanism. Among them are ... 

Once it is known with certainty that a decomposition follows an autocatalytic mechanism, it is recommended to proceed as follows ... 

The cooperativity of amplification, switching, and memory in synthetic helical polymers might thus be shared with ideas of a scenario for the biomolec-ular homochirality, autocatalytic mechanism in chiral chemical synthesis, and bifurcation equilibrium mechanisms in crystallization of chiral crystals. Indeed, amplification phenomena in several optical activity and helicity of synthetic polymers in isotropic solution appears to be common and are now established as sergeants and soldiers experiment and majority rules in polymer stereochemistry [17,18]. Any minute chiral forces caused by intramolecular and intermolecular systems can be detectable, when a proper model polymer system is chosen to elucidate the cooperativity of amplification, switching, and memory. 

The third covalent bond present in MPO is also thought to form by an autocatalytic process, although it has not been clearly demonstrated to do so. However, incubation of an ascorbate peroxidase mutant into which a methionine has been introduced by mutagenesis has been shown to result in covalent attachment of the Met sulfur atom to a heme vinyl group [54]. Although the link is not identical to that in MPO, this finding provides strong circumstantial evidence that the bond in MPO is also formed by an autocatalytic mechanism. 

Cyanide Ion Concentration. Increasing the cyanide ion concentration speeds up the autoreduction, as shown in Figure 5. Because of the autocatalytic nature of the reduction, the exact cyanide ion dependence has not yet been defined. Also it is not yet clear whether the cyanide ion is involved mechanistically in the autoreduction. The cyanide ion concentration can affect the observed rate via the autocatalytic mechanism or by competing with the complexed cyanide ion for hydrogen bonding with trace amounts of water. 

This autocatalytic mechanism paves the way to new processes to promote the growth of ZnO by vapor transport using the generation of an additional Zn vapor pressure and limiting the off-stoichiometric effects by controlling it. 

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