On the determination of the location of metal clusters supported on molecular sieves by X-ray absorption spectroscopy

Typically, smaller metal - metal contributions are found in the EXAFS of zeolite supported metals compared to metals on nonporous supports. Two factors may account for this, i.e., (i) a smaller average coordination number, which is related to smaller metal particles, and (ii) an increase in the Debye - Waller factor indicating less ordered metal clusters. Based on experimental results as well as on theoretical calculations we compare both effects and assign the decrease in the metal - metal contributions to an exponential dampening of the EXAFS oscillations resulting from contributions of the zeolite lattice atoms. This leads to an increase in the Debye - Waller factor, which is not related to a decrease in the statistical order of the metal clusters supported on the zeolite. The model discussed can be used as an indication for the location of small metal clusters on porous supports. It is shown that a large value of Δσ², in addition to the particle size, can be used to determine the location of metal particles on microporous supports.