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58 3. Heterogeneous Processes and Reactor Analysis

where:
N the moles of reactant that appear in the reaction
t time
n the number of actie sites on the catalyst surf v ace.
This rate is usually referred to as the turnover frequency and it is the number of mole-
cules reacting per actie site per unit time at the conditions of the experiment (Boudart,
v
ilkinson,
,
ogler
1985; McNaught and W 1997; F 1999). Boudart (1995) used the term
“turnover frequenc to define the number of reolutions of the catalytic cycle per unit
v
y”
olution,
time and actie site. In each re one mole of reactant is consumed. For e xample,
v
v
the revolution of a catalytic cycle for SO 2 oxidation is shown in Figure 3.1.
Frequently, the number of actie sites is expressed in mole units (the number of acti v v e
A
v sites divided by the ogadro number) and thus, turnoer frequency is found in s v -1 units.
y depends on the nature of the catalytic active For a specific reaction, the turno v er frequenc
site, the temperature, and the reactants’ concentration. The aboined catalytic rate v e-def
v el” e-site le could be described as an “acti rate. v
v
Following the reaction rate definition of the form gien in eq. (3.1), if component i is a
reaction product the rate is positie; if it is a reactant that is being consumed, the rate is
v
negative; thus, the rate of disappearance of the reactant is –r t . In environmental applica-
tions, as we are interested in the disappearance of a pollutant, the rate is expressed as –r ,
which is positiThe rate of disappearance is used in Chapters 3 and 5, where for sim-
e.
v
plicity it is referred to as the reaction rate.
Catalyst level—active site plus support
ined abo The rate of a catalytic reaction as defve exhibits a great disadvantage: the number
of the active sites is unknown and cannot be easily determined from common experiments.
f
The dificulties associated to the measurement of actie sites leads, for the time being, to
v
el rates, ” the use of “catalyst le in most practical applications. v
Specifically the most common reaction rate types used are expressed per unit vol-
,
ume of the solid phase ( r vs ), per unit surface of the solid surface ( r ) or per unit mass of
s
the solid
½ O 2 metal oxidation SO 2

SO 3 metal reduction

Figure 3.1 Revolution of a catalytic cycle for SO 2 oxidation.