Steps, elementary combinations

The following elementary steps in combination with (45, —45) represent a mechanism which satisfactorily describes observations at 1 atm ... 

The equations for the elementary steps must combine to give the equation for the overall reaction. 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

The cubic nature of the empirical rate law discussed in the previous section, and the representation in eqn (1.17), is not at all meant to imply that we are thinking of a single, termolecular, elementary step. There are various ways in which a combination of simple bimolecular steps can combine together to give an overall rate law with this cubic form. For instance, in the two-step mechanism involving an intermediate X... 

The rate of the last reaction, for example, is proportional to the concentration of H and the concentration of Br2, i.e. it is second order. When the rates of these elementary steps are combined into an overall rate equation, this becomes ... 

Reaction mechanism graphs may be useful in studying reaction mechanisms even when there is no species cycling through the reaction. To see this, consider the simple example of the electrochemical hydrogen evolution reaction (HER) mechanism. Here the overall reaction is 2H20 +2e / 20H + H2. The elementary reaction steps that combined to yield HER (cf. Fishtik etal., 2005)26 are... 

Case 3. In this last case, the desorption of product is assumed to be rate determining. Similar to Case 1, two elementary steps are combined into an overall equilibrated reaction ... 

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