Chemical Reactions on Solid Surfaces - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 472

(iv) Molecules of C adsorb on type 1 sites and molecules of D adsorb on type 2 sites
Derive an LHHW type rate expression that is consistent with this mechanism.
9.14. The solid-catalyzed exothermic reaction

catalyst

2 D
is virtually irreversible at low temperatures and low product concentrations, and the
rate law is given by

A 2 þ 2 B

%

kC 1=2

A 2 C B

r ¼

p

K A 2 C A 2

2

ð 1 þ þ K B C B Þ

(a) How many active sites are involved in the rate-limiting step?

C A p appearing in denominator of the rate law? How many species are
adsorbed on the catalyst surface?
(c) Suggest a rate law that is valid for a wide range of temperature and concentrations.
(d) Suggest a mechanism for the rate law you have suggested. Identify the rate-limiting
step.
(e) Show that your mechanism is consistent with the rate law.
9.15. Synthesis of ammonia from nitrogen and hydrogen over Ru/C catalyst is found to
follow the following steps:

(b) Why is

N 2 þ s % N 2 $s (1)

N 2 $s þ s % 2 N $s (2)

H 2 þ 2s % 2 H $s (3)

H $s þ N $s % NH $s þ s (4)

H $s þ NH $s % NH 2 $s þ s (5)

H $s þ NH 2 $s % NH 3 $s þ s

(6)

NH 3 $s % NH 3 þ s

(7)

þÞ ¼ þ 3 H 2 % 2 NH 3 (8)

overall N

The surface reaction step (4) is found to be the rate-limiting step. Derive a rate expres-
sion that is consistent with the mechanism based on nonideal surfaces with ExLan
isotherm.
9.16. The rate law for the hydrogenation (H) of ethylene (E) to form ethane (A) over a cobalt-
molybdenum catalyst is found to be
r 0 A ¼ kP E P H ln 1

K E
P E

þ

Suggest a mechanism and rate-limiting step consistent with the rate law. Describe the
feature of the catalysts surface.