Surface Reactions and Model Catalysis

In the preceding chapters we have tried to lay a useful physical basis for an understanding of a phenomenon that has been known to chemists for several hundred years, namely, heterogeneous catalysis.

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93

LU

Fig. 5.1. Potential energy diagram (sehematie) showing the uneatalyzed and the eatalyzed reaetion path of a bimoleeular reaetion A + B =Pr with reaetion energy ,1~ The direet (uneatalyzed) path towards the transition state eom~lex {AB}* is aetivated by an energy EJ,*of appreciable height, whereas the eatalyzed route (via formation of intermediate X with the aetivated eomplexes \' and *2' respectively) requires the mueh smaller aetivation energy E~: thus eausing the mueh faster reaetion rate

>,

E'

~

(1)

c: .2

13 o Cl>

'-

reoction coordinote p -----....;~. same ratio. This principle has an important consequence. It allows conclusions to be made about the path of the product decomposition re action if just the product formation reaction is known, or vice versa. In the following kinetic scheme we again discuss the simple re action in which a product Pr is formed from the reactants A and B. This reaction is assumed to proceed i) in a direct way, and ii) in a catalyzed way (catalyst symbol K, intermediate compound symbol X). The reaction energy diagram of Fig. 5.1 illustrates the situation. We have for the direct channel: (i)

A + B ~ Pr

(slow),

5.1

whose rate can be written _ d[A]

dt

=+d[Pr] =k1[A][B] . dt

5.2

This is an ordinary bimolecular reaction with an overall second-order kinetics. The catalyzed re action must be formulated: (ii)

A +K ~ X

(relatively slow)

X +B ~ Pr+K

(fast).

5.3 5.4

The reaction rate then reads

d~r]

= k 3 [X][B] ,

5.5

The unknown concentration of the intermediate species X can be obtained by applying the steady state approximation:

d~~] ~ 0 = k2 [A][K] -

194

k 3 [X][B] ;

5.6

[X]

= k2[A][K] k3 [B]

,

5.7

and insertion into Eq.5.5 yields d[Pr] = k 2 [A][K] .

dt

5.8

This means that (provided k 3 ยป k 2 ) the reaction rate can be greatly influenced by the concentration of the catalyst. It is worth mentioning that these considerations apply to homogeneous as weH as to heterogeneous catalysis in quite a similar manner; note that in heterogeneous catalysis the catalyst is represented by a surface of active material, and the decisive quantity [K] is the number 0/ active surface sites. As repeatedly mentioned before, we are only interested here in heterogeneous catalysis, and we begin our discourse with a consideration of the physical and chemical interactions of atoms and/or molecules that are simultaneously present at a surface.

5.1 Coadsorption Phenomena When we consider the simultaneous adsorption of two or more different particles on a surface, we must again distinguish (cf., Sect. 3.2) the limits of a sparsely covered surface (zero-coverage condition), where only the interaction between the individual particles A, B, C ... and the surface plays a role, and the situation where a whole ensemble of all these particles coexists on the surface, interacting with the surface and with each other (multiparticle interaction). As opposed to our preceding considerations with chemicaHy identical particles, there is now a new component in the interactions, namely, the