Surface catalysed reactions

H2Se (like H2O and H2S) can be made by direct combination of the elements (above 350°), but H2Te and H2P0 cannot be made in this way because of their thermal instability. H2Se is a colourless, offensive-smelling poisonous gas which can be made by hydrolysis of Al2Se3, the action of dilute mineral acids on FeSe or the surface-catalysed reaction of gaseous Se and H2 ... 

There was therefore a clear need to assess the assumptions inherent in the classical kinetic approach for determining surface-catalysed reaction mechanisms where no account is taken of the individual behaviour of adsorbed reactants, substrate atoms, intermediates and their respective surface mobilities, all of which can contribute to the rate at which reactants reach active sites. The more usual classical approach is to assume thermodynamic equilibrium and that surface diffusion of reactants is fast and not rate determining. 

Cardiff using surface-sensitive spectroscopies and which provided a different insight into surface reactivity, particularly of the role of transient and precursor states in the dynamics of surface-catalysed reactions. 

Takoudis, C. G., Schmidt, L. D., and Aris, R. (1981). Multiple steady states in reaction controlled surface catalysed reactions. Chem. Eng. Sci., 36,377-86. 

In chapter 12 we discussed a model for a surface-catalysed reaction which displayed multiple stationary states. By adding an extra variable, in the form of a catalyst poison which simply takes place in a reversible but competitive adsorption process, oscillatory behaviour is induced. Hudson and Rossler have used similar principles to suggest a route to designer chaos which might be applicable to families of chemical systems. They took a two-variable scheme which displays a Hopf bifurcation and, thus, a periodic (limit cycle) response. To this is added a third variable whose role is to switch the system between oscillatory and non-oscillatory phases. 

The general principles of the kinetics of surface-catalysed reactions have been discussed in detail elsewhere [12,13] and in this section we shall confine our attention to a discussion of the kinetics and rate expressions which are applicable to hydrogenation reactions. 

POMs/HPA themselves are usually non-porous solids, with surface area less than 10 m2/g and low decomposition temperatures. Therefore, they have limited surface sites for surface-catalysed reactions. A number of attempts have been made to disperse POMs on inert supports, with the intention of effectively increasing the number of accessible active catalytic sites. A number of materials have been used as solid supports for the dispersion of HPA, for example silica, carbon, zirconia, alumina, and porous silica. 

C). While H3CS1H3 decomposes under these conditions primarily by geminal elimination of molecular H2 from the silicon moiety and secondarily by a slow surface-catalysed reaction yielding H-and CH3SiH2 radicals [27], the significantly lower decomposition temperatures of (fluoromethyl)silanes require different low energy decomposition pathways. 

Eiswirth, M., Chaos in surface-catalysed reactions, in Chaos in Chemistry and Biochemistry, Fields, R.J., Gyorgi, L., Eds., World Scientific, Singapore, 1993, Chap. 6, 141-174. 

The ammonia formafion reaction is involved by many factors. As a surface-catalysed reaction, the adsorption of the gaseous reactants, especially nitrogen, on the catalyst surface or into the structure of the catalyst is concerned. There are a few metal catalysts that can induce chemisorption of N2 on their surfaces. A classification of metals according to their abilities in chemisorption on various gases is presented in Table 18.1. 

In surface chemistry, adsorption isotherms describe the equiUhrium situation. However, just as in the consideration of the gas-phase chemistry in the interstellar medium, it is the kinetics of surface processes which are more relevant. Two mechanisms for surface-catalysed reactions can be distinguished and are illustrated by the cartoons in Fig. 1.6. In the Eley-Rideal mechanism, it is assumed that reaction occurs when a species (say, A) from the gas-phase impacts on a species (say, B) that is adsorbed on the surface. At significant surface coverage, the rate of reaction will be proportional to the product of the fraction of the surface covered in B (6b) and the pressure (p ) of the species A, which will be proportional to the rate of collisions of A with unit area of the solid surface. An alternative picture is encapsulated in the Langmuir-Hinshelwood mechanism. Here it is assumed that reaction occurs in encounters between species both of which are adsorbed on the surface. Then the rate of reaction will be proportional to the product of the fractions of the surface covered by A and by B that is proportional to 0a b-... 

Simple surface-catalysed reactions that have been well studied include isotopic exchange reactions. 

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