Page 200 - Introduction to chemical reaction engineering and kinetics

182 Chapter 8: Catalysis and Catalytic Reactions

CO dissociation
+
P d
Hydrogenation J\
* / \
I-\ I \
I \ \
L-// \
: \ Hydrogenation
I \ \ +
; \ c.
\ ,I Ni ’ \
\ /
/ /--,\
’ /-\ ,/;i-‘>=, - ,/ pd \\
‘1
1 / \ \ \\

’ I ‘A- -A /I \\
Energy ‘I \\
11 I ‘>-
;: CH,OH
I/
/I
I/ C O H H H H P C H H H H
:L’
Hz0 M M M M M M M M M M
CH4
Figure 8.4 Hypothetical reaction coordinate diagrams for CO hydrogena-
tion on Pd and Ni; the dissociation of CO is more difficult on Pd, making
methanol synthesis more favorable than methane formation, which requires
C-O dissociation, and is the preferred pathway on Ni
8.2 MOLECULAR CATALYSIS
8.2.1 Gas-Phase Reactions

An example of a catalytic gas-phase reaction is the decomposition of diethyl ether cat-
alyzed by iodine (I,):

(W,MW) + qH6 + CH4 + co
For the catalyzed reaction

(-I-*) = kc,c,,; EA = 142 kJ mol-’
and for the uncatalyzed reaction

(-r-J = kc*; EA = 222 kJ mol-’
Another example of gas-phase catalysis is the destruction of ozone (0,) in the strato-
sphere, catalyzed by Cl atoms. Ultraviolet light in the upper atmosphere causes the dis-
sociation of molecular oxygen, which maintains a significant concentration of ozone:

0, + hv + 20.
0. + 0, + M -+ O3 + M

Ozone in turn absorbs a different band of life-threatening ultraviolet light. The rate of
ozone destruction in the pristine atmosphere is slow and is due to a reaction such as

0. + 0 , -+ 202
Chlorine-containing organic compounds, which are not destroyed in the troposphere,
are photolyzed in the stratosphere:

RCl+hv+ Cl’+R