Small-angle scattering of heterogeneous catalysts

A heterogeneously catalyzed reaction involves the adsorption of the reactants on the surface of the catalyst. Consequently, catalytic activity is usually strongly related to the microstructural features of the catalyst. Small-angle scattering (SAS) has been extensively used to study heterogeneous catalysts and it has been shown to provide useful information about the specific surface of the active phases. In particular, the majority of the research has dealt, and still deals, with supported metal catalysts, which are three-phase systems (support, voids and metal). Several attempts have been made experimentally and theoretically to overcome this problem and to obtain values for the three surface areas. In the first part of the paper, earlier results and recent approaches will be reviewed in order to give a general picture of the problem. In the second part, some recent results on other kinds of heterogeneous catalysts, such as sulfated zirconia, with strongly acidic properties, and functionalized silicas, will be discussed. Knowledge of the degree of smoothness of the particles (angularity) and thickness of the organic films obtained by SAS experiments can be very useful in clarifying morphological aspects of these porous systems.