Stepwise Growth of Pd Clusters in USY Zeolite at Room Temperature Analyzed by QXAFS

The stepwise growth of Pd in a USY zeolite was followed by in situ quick X-ray absorption fine structure (QXAFS) spectroscopy involving repeated alternating exposures to H 2 and O 2 flows at room temperature. During the first reduction of 0.4 wt % Pd/USY with 8% H 2, Pd clusters with a coordination number (CN) of 5.1 formed within 15 min, probably in the supercage of USY. The growth followed first-order kinetics with respect to the concentration of Pd 2+ and the CN of Pd−Pd. The clusters were stable up to 443 K. The clusters were partially oxidized in less than 2 min on exposure to 8% O 2, and then quickly reduced with 8% H 2 (second reduction) to afford larger Pd clusters with CN = 6.7. The clusters continued to increase in size in a stepwise fashion on further alternate exposures to O 2 and H 2 . The cluster growth proceeded gradually when 0.8 wt % Pd was loaded on H-USY. In contrast, stable Pd clusters were not obtained when NH 4 -USY or high-silica USY were used as supports, suggesting that the clusters were stabilized on the H + ions present in H-USY. This demonstrates the potential of QXAFS in following the clustering process of Pd, and indicates the possibility for fine-tuning the sizes of Pd clusters simply by changing the number of O 2 and H 2 exposure cycles.