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 technique we consider more appropriate is EXAFS, because it detects metal-metal bonds even before metal atoms are forming particles. Due to this capability it has become a routine technique in catalysis, although it requires an elaborate data analysis procedure. In the particular case of supported metal catalyst, this procedure can be simplified because nearly everything is known about the investigated structure, the metal particles. With the appropriate fitting strategies, the main contribution to the EXAFS spectrum, the metal-metal bonds, can be emphasized, and within it, most part of the fit parameters are known. The only unknown parameter is the coordination number for each metal shell. Once this value is obtained, the number of metal atoms per particle can be 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.