Page 214 - Introduction to chemical reaction engineering and kinetics

196 Chapter 8: Catalysis and Catalytic Reactions

Two common features of catalytic rate laws are evident in this expression.

(1) Saturation kinetics: The rate is first order with respect to A at low concentra-
tions of A (such that KAcA << 1 + KBcB), but becomes zero order at higher
concentrations when KAcA >> 1 + KBcB. In the high-concentration limit, all the
catalytic sites are saturated with A(8 A = l), and the rate is given by the number
of catalytic sites times the rate constant, k.
(2) Product inhibition: If the term KBce is significant compared to 1 + KAcA, the
rate is inhibited by the presence of product. In the extreme case of KBcB >>
1 + KAcA, equation 8.4-24 becomes

(-r*) = k’C*C;l (8.4-25)

where k’ = kKAKil. Note that the inhibition of the rate by B has nothing to do
with the reversibility of the reaction (which is assumed to be irreversible).

8.4.3.2 Bimolecular Surface Reaction (Type ZZ)

For the overall reaction A + B + C, if the rds is the bimolecular surface reaction given
by equation 8.4-3, the rate of reaction is obtained by using equation 8.4-23 (applied to
A and B, with A, B, and C adsorbable) in equation 8.4-4 (for eA and 0,) to result in:

kKAK13CACB (8.4-26)
(-rA) = (1 + KAcA + KBcB + K~c,-)*

This rate law contains another widely observed feature in surface catalysis:

(3) Inhibition by one ofrhe reacrunrs: Similar to Type I kinetics, the rate is first order
in CA when KAcA << 1 + KBcB + KC+, but instead of reaching a plateau in the
other limit (KAcA >> 1 + KBcB + KC+), the rate becomes inhibited by A. The
limiting rate law in this case is

(-rA) = k’c& (8.4-27)

where k’ = kKB/KA. A maximum in the rate is achieved at intermediate values
of CA, and the ultimate maximum rate occurs when kt3, = 8, = 1/2. Many CO
hydrogenation reactions, such as the methanol synthesis reaction, exhibit rate
laws with negative effective orders in CO and positive effective orders in H,.
This reflects the fact that CO is adsorbed more strongly than H, on the metal
surface involved (KC0 > KHz).
Also apparent from equation 8.4-26 is thatproducr inhibition can have a more serious
effect in Type II kinetics because of the potential negative second-order term.

For the surface-catalyzed reaction A(g) + B(g) + products (C), what is the form of the
rate law if

(a) Both reactants are weakly adsorbed, and products are not adsorbed, and
(b) Reactant A is weakly adsorbed, B is moderately adsorbed, and products are not
adsorbed?