PARTICLE-SIZE AND CATALYSIS

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.