While heterogeneous catalysis, and especially catalysis by metals, is
concerned with the size of the particles and hence with the developed
surface area, this is not only to prepare an effective product at mini
mum cost. The study of the relationship between the catalyst character
istics and their reaction properties has helped in some situations to
make a virtually atomic description of the specific active site of eac
h reaction. The various authors have all taken a different approach to
the problem. One approach considered the crystallographic properties
of the mass metals, while another considered that small particles have
neither the properties nor the structure of the mass metal. The cryst
allographic approach first led to the consideration of the crystal par
ameters and to a distinction between the different atoms located at th
e corners, kinks and faces. However, it was found very soon that the p
articles did not have the expected structures. It then became necessar
y to calculate the energy of the particles considered to identify the
way in which they could grow from a small nucleus. These calculations
considered neither the atmosphere in contact with the particle, no the
presence of a support, nor the disturbances caused by the reaction. D
epending on the reaction investigated, `easy' reactions, unaffected by
the structure, were distinguished from `demanding' reactions, of whic
h the intrinsic rate varies with the surface structure. Many investiga
tions were conducted on monocrystals, whose exposed surfaces had diffe
rence indexes, and others attempted to analyze these relationships wit
h supported metals, with all the artefacts that this could imply. It w
as difficult to find any unity in the overall results obtained, especi
ally since self-poisoning by the reactants (hydrocarbons or hydrogen)
could readily explain the mechanisms observed. Moreover, a metal depos
ited on silica and the same metal deposited on alumina can display com
pletely different behavior. This shows that some interpretations are t
oo simplistic, and that it is inadequate to vary the particle size by
any available means and to analyze the consequences on the catalytic p
rocess. The two complementary approaches, that of the crystallographer
, who tries to describe the small particles from the parameters of the
mass metal, and that of the chemist, who tries to determine the struc
ture from the behavior of the catalyst observed in the reaction invest
igated, do not truly merge to provide a totally acceptable description
of the particle structure. On the one hand, the physicochemist uses o
utrageous simplifications when he tries to describe his structures on
the basis of functions of state which do not always have clear solutio
ns. And, on the other, the chemist handles real objects but has diffic
ulty in isolating the parameter that he wants to investigate. His conc
lusions are never safe from the artefacts generated by the operating c
onditions or the support effects. This dilemma is faced by the physici
st, who tries to synthesize aggregates that are clearly defined in a g
as stream, but far from the reality of catalysis, and likewise for the
chemist, who wants to reduce the structural effects to simple compari
sons between the faces exposed by monocrystals.