THE INVISIBLE METAL PARTICLES IN CATALYSIS

An easy, reliable and straightforward method to determine the sizes of small metal particles in supported metal catalyst which are invisible for most techniques (chemisorption, XRD, HRTEM) is presented. The tec hnique we consider more appropriate is EXAFS, because it detects metal -metal bonds even before metal atoms are forming particles. Due to thi s capability it has become a routine technique in catalysis, although it requires an elaborate data analysis procedure. In the particular ca se of supported metal catalyst, this procedure can be simplified becau se nearly everything is known about the investigated structure, the me tal particles. With the appropriate fitting strategies, the main contr ibution to the EXAFS spectrum, the metal-metal bonds, can be emphasize d, and within it, most part of the fit parameters are known. The only unknown parameter is the coordination number for each metal shell. Onc e this value is obtained, the number of metal atoms per particle can b e calculated and from that, metal particle diameter can be deduced. An example of this procedure for a Pt/Al2O3 catalyst is shown. (C) 1997 Elsevier Science B.V.