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3.1 Introduction to Heterogeneous Processes 65

A look into turnover fr equency
Beyond the difficulties met in the determination of the number of active sites
,
v
in order to estimate the turnoer frequenc there are more problems to
y
be solved. The turnover frequency, een in the case that it is measured v
xperiment,
by employing an ideal e might be only an aerage one. v
This is partly because the actie sites could exhibit different reac- v
tivities, i.e. the y are not identical in respect to the catalytic reaction.
An interesting question, expressed by Boudart (1985), is the fol-
lowing: As particle size gro ws from that of a small cluster to infinite
value for a single macroscopic crystal, how does the value of
turnover frequency change for a gien reaction on a gi v v en metal?
Several experimental results reealed that turnoy could be v v er frequenc
e”
v
different for a class of reactions called “structure-sensiti (Boudart, 1981; 1981). ates, Y
An operational definition of structure sensitivity (Bouldart, 1995) is that “the turno er fre- v
quency depends on surface crystalline anisotropy revealed by working on different faces of
a single crystal or on clusters of varying size between 1 and 10 nm.” The surface crystalline
anisotropy is directly related to the crystallographic orientation (planes) of the supported
catalytically active atoms, which in turn, is af1981). It fected by the particle size (Bouldart,
should be noted that the term “particle” here has the meaning of a pure acti e agent struc- v
ture. Thus, by increasing particle size, the number of acti v e sites is increased too.
In contrast, structure-insensiti e reactions are those for which turno y under v er frequenc v
fixed conditions does not depend or depends slightly on the surface crystalline anisotrop y
xposing dif of clusters of varying size or of single crystals eferent faces. For these kinds of
reactions, all accessible surface atoms can be considered as equally actie sites (Boudart, v
1981 and 1995).
In order to avoid any confusion, the “surf used in sensiti ace structure” v e v e and insensiti
reaction analysis has nothing to do with the “surface arrangement” used in the catalyst
level rates analysis—the first refers to the microscopic level of the active site, whereas the
latter to the catalyst le el. v
The usage of turnoy exhibits v 1995): er frequenc v antages (Budart, arious adv

v
• The value of the turnoer frequency can be reproduced in different laboratories, if the
method of measurement of the rate and the counting of sites are kept the same.
Moreover, the use of turnoy allows the comparison between two catalysts er frequenc v
that differ in metal or size for a specific reaction. The great advantage of such a com-
parison is that the activity of different catalysts is compared at active site level without
the considerations of catalyst arrangement. To be more specif using turnoer fre- v
ic,
v
quency, we can compare the activity of the pure actie site, ignoring the specific area
of the catalyst.
• catalytic By definition, the turnover frequency is expressed per number of active sites. So,
samples that differ only in the amount active sites must exhibit the same values of turnover
frequency. If not, heat and mass transfer phenomena are present. Specifically, the correct
measurement of intrinsic kinetic data in heterogeneous catalysis is dificult due to the f
effect of heat and mass transfer, especially inside the pores of high specific-area materials.
The turnover frequency reveals these phenomena. In other words, in the case of supported