Environmental Engineering Reference
In-Depth Information
TABLE 5.3
Examples of Homogeneous and Heterogeneous Catalysis
in Environmental Engineering
Catalysis Type
Reaction
Applications
Homogeneous
Oxidation of S(IV) by H
2
O
2
Atmospheric chemistry,
aquatic chemistry, and
waste treatment
Homogeneous and
heterogeneous
Acid and base hydrolysis of
pesticides and esters
Aquatic, soil, and sediment
chemistry
Homogeneous
Enzyme-catalyzed
biodegradation
Aquatic, soil chemistry, and
waste treatment
Homogeneous and
heterogeneous
NO
x
formation in combustion
reactors
Atmospheric chemistry and
hazardous waste
incinerators
Homogeneous
Ozone destruction in
gas phase
Stratospheric ozone
chemistry
Heterogeneous
Production of
hydrochlorofluoro-carbons
Manufacture of CFC
replacement chemicals
Heterogeneous
Hydroxylation of N
2
O over
zeolite catalysts
Oxidative removal of N
2
O
which contributes to
greenhouse effect
Heterogeneous
Oxidation of organics in
water on TiO
2
Removing pollutants from oil
slicks
Heterogeneous
Dehalogenation of pesticides
using membrane catalysts
and bacteria
Hazardous waste treatment of
soils
5.7.1 M
ECHANISMS AND
R
ATE
E
XPRESSIONS FOR
C
ATALYZED
R
EACTIONS
For environmental catalysis, the primary step is the formation of a complex Z between
catalyst X and substrate A.
A
k
f
X
+
k
b
Z
+
Y.
(5.114)
The complex Z further reacts with another reactant W (e.g., the solvent) to give the
desired products:
k
−→
Z
+
W
P
+
X.
(5.115)
The products consist of the regenerated catalyst and other reaction products. Note
that the second step is a nonequilibrium reaction. As we will see in the next section,
Z is a surface-adsorbed complex for heterogeneous catalysis.Y and W do not exist in
that case. Acids or bases catalyze many reactions in the environment. In these cases,
X can be H
+
or OH
−
.IfX
H
+
, then Z is the conjugate base of A and the reaction is
≡
OH
−
, then Z is the conjugate acid ofA and the reaction
acidcatalyzed
, whereas if X
≡
is said to be
base catalyzed
.
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