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

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• 作者：Kazu Okumura ; Tetsuo Honma ; Sayaka Hirayama ; Takashi Sanada ; Miki Niwa
• 刊名：Journal of Physical Chemistry C
• 出版年：2008
• 出版时间：October 30, 2008
• 年：2008
• 卷：112
• 期：43
• 页码：16740-16747
• 全文大小：313K
• 年卷期：v.112,no.43(October 30, 2008)
• ISSN：1932-7455

文摘

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 H₂ and O₂ flows at room temperature. During the first reduction of 0.4 wt % Pd/USY with 8% H₂, 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 Pd²⁺ 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% O₂, and then quickly reduced with 8% H₂ (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 O₂ and H₂. The cluster growth proceeded gradually when 0.8 wt % Pd was loaded on H-USY. In contrast, stable Pd clusters were not obtained when NH₄-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 O₂ and H₂ exposure cycles.