Concepts from Chemical Reaction Kinetics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 231

Activated complex

E a - q ads

E a

A

q ads

Adsorbed A

P

Reaction co-ordinate

FIGURE 5.13 Potential energy surface for a heterogeneous reaction where the pressure of
the reactant in the gas phase is low.

At low P A , only a few A are adsorbed and these need an energy E a − q ads to cross

the barrier. At high P A , most of the A is on the surface and the system needs to over-
come a larger barrier to form products. Most environmental reactions in the natural
environment fall in the category of low pressure or concentration reactions and have
potential energy diagrams such as those depicted in Figure 5.13.

E XAMPLE 5.14 H ETEROGENEOUS C ATALYSIS OF E STER H YDROLYSIS IN W ATER

An example of heterogeneous catalysis in the environment is the metal oxide (min-
eral surface)-catalyzed hydrolysis of esters. Stone (1989) discussed an environmentally
important reaction, namely, the influence of alumina on the base hydrolysis of
monophenyl terephthalate (MPT) in aqueous solution. Phthalate esters are ever-present
pollutants in wastewater and atmospheric moisture. The hydrolysis in a homogeneous
system without alumina follows the reaction MPT − + OH − → PhT + PhOH, where
PhT is phenyl phthalate and PhOH is phenol. The rate is given by

d [ MPT − ]
d t

= k base [ OH − ][ MPT − ] , r =

with the base hydrolysis rate constant k base = 0.241 L/mol s at 298 K in buffered
solutions (pH 7.6-9.4). At pH > p K a = 3.4, MPT exists mostly as MPT − , which is
the species of interest throughout the pH range of hydrolysis. The addition of a small
concentration of a heterogeneous surface (alumina) increases the rate of hydrolysis sub-
stantially (Figure 5.14). The rate increased with increasing pH and [Al 2 O 3 ], whereas
the ionic strength adversely affected the rate. It was shown that the adsorption of MPT −
continued