Autocatalytic, solid reactions

Section 3 deals with reactions in which at least one of the reactants is an inorganic compound. Many of the processes considered also involve organic compounds, but autocatalytic oxidations and flames, polymerisation and reactions of metals themselves and of certain unstable ionic species, e.g. the solvated electron, are discussed in later sections. Where appropriate, the effects of low and high energy radiation are considered, as are gas and condensed phase systems but not fully heterogeneous processes or solid reactions. Rate parameters of individual elementary steps, as well as of overall reactions, are given if available. 

Figure 12.2 Comparison of an autocatalytic (solid line) and an nth-order reaction (dashed line) under adiabatic conditions starting from 150°C. Both reactions have the same adiabatic induction time orTMRad of 10 hours. If an alarm level is set at 160°C,...

In this case, no specific problem in quantitative terms has been posed and solved. For the first step, which is a solid-solid reaction, experimental details have been given because it is a unique solid-solid reaction, perhaps the only known autocatalytic one. Having clearly established this fact, general procedures are described for the two reactors involved, since no experimental data are available for attempting a specific design. The thrust here is the description of a procedure for calculating the outlet solid size distribution given the inlet size distribution. This is an important factor in solid-phase reactions. 

Silicon reacts with copper chloride catalytically to form a solid product known as q-phase, which then catalyzes the reaction between silicon and methyl chloride to form methyl chlorosilanes. The formation of the q-phase is a unique example of an autocatalytic solid-solid reaction (we are not aware of any other example of such a reaction). Procedures for determining the kinetics of this reaction and the size distribution of the final products of the second reaction are described in this case study. The most important feature of the study is the unexpected discovery of autocatalysis in a solid-solid reaction, which leads us to the important lesson that one should vigorously pursue any such observation that falls outside the comfort zone of research, for that is where true discoveries lie. This advice is particularly relevant to engineers, since they generally do not accept the unexpected and would prefer to tread on safer ground. 

Methyl chlorosilanes, used in the manufacture of a variety of resins, elastomers, and silicone oils, are manufactured by reacting silicon (a solid) with methyl chloride (a gas) in the presence of an alloy Cu3Si (called the 7 -phase) as catalyst. Although strictly a gas-solid reaction, and hence mote appropriate to Chapter 15, we illustrate the procedure here by noting all mass transfer effects and focusing only on the two reactions involved (one homogeneous and the other autocatalytic). 

Figure 1. Typical a vs. time curves for the thermal decomposition of P HMX at 3.6 GPa for the various temperatures indicated. Sigmoid curves such as those shown here are characteristic of autocatalytic decomposition reactions of a single solid.

These sigmoid-type curves can be identified with thermal decomposition of a single solid in an autocatalytic-type reaction having an initial (a<0.2) induction period and an intermediate (0.2normal growth stage with the final (a>0.9) decay or deceleratory stage absent [8,9]. 

The various types of heterogeneous reactions are shown in Table 3.3. They are broadly grouped as solid-gas, solid-liquid, solid-solid, liquid-gas, and liquid-liquid reactions. The different types included in each group are also shown in the compilation. Some representative processes have been indicated as examples. It may be pointed out that in the group of solid-liquid reactions a specific mention of what is known as autocatalytic reactions has not been made. The autocatalytic processes occur when the liquid product reacts further with the solid undergoing reaction. The dissolution of copper in dilute sulfuric acid (or aqueous ammonia) in the presence of oxygen may be cited as an example ... 

In addition, the results of such reactions have suggested plausible models for the mechanism of abiotic generation of optical activity, including an autocatalytic feedback mechanism (261). The latter involves random development of chiral crystals from achiral starting material, and solid-state reaction leading to products in which one enantiomer is in excess and thus can bias subsequent further crystallization (262). 

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

Kinetic curves relative to polymerization reactions in the solid state commonly show a sigmoidal shape with a slow initiation step followed by a steep increase, even by two orders of magnitude, of the reaction rate. A reaction with this kind of kinetic curve is said to have an autocatalytic behavior. 

This can be accomplished by means of two different processes (1) an electrodeposition process in which z electrons (e) are provided by an external power supply, and (2) an electroless (autocatalytic) deposition process in which a reducing agent in the solution is the electron source (no external power supply is involved). These two processes, electrodeposition and electroless deposition, constitute the electrochemical deposition. In this book we treat both of these processes. In either case our interest is in a metal electrode in contact with an aqueous ionic solution. Deposition reaction presented by Eq. (1.1) is a reaction of charged particles at the interface between a solid metal electrode and a liquid solution. The two types of charged particles, a metal ion and an electron, can cross the interface. 

Autocatalytic decomposition of [PtMe2(COD)] on platinum black, under dihydrogen has already been pointed out in Uquid solution [44]. Indeed, the platinum atoms present in the starting complex are incorporated into the surface of the solid platinum catalyst, thus becoming the reactive sites for further cycles of chemisorption and reaction (Scheme 1). 

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.

Fig. 8.1. Indication of local stability or instability for the simple cubic autocatalytic step without decay solid curves indicate branches of stable stationary-state solutions, broken curves correspond to unstable states, (a) Stationary-state locus with no autocatalyst inflow, fl0 = 0, with one stable solution, 1 - = 0, corresponding to zero reaction (b) stationary-state locus...

Figure 12.11 Adiabatic temperature course of an autocatalytic reaction (solid line) compared to the zero-order approximation (dashed line). Both reactions have a maximum heat release rate of lOOWkg-1 at 200°C and an energy of500)g . ...

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