Environmental Transmission Electron Microscopy Study of the Origins of Anomalous Particle Size Distributions in Supported Metal Catalysts

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• 作者：Angelica D. Benavidez ; Libor Kovarik ; Arda Genc ; Nitin Agrawal ; Elin M. Larsson ; Thomas W. Hansen ; Ayman M. Karim ; Abhaya K. Datye
• 刊名：ACS Catalysis
• 出版年：2012
• 出版时间：November 2, 2012
• 年：2012
• 卷：2
• 期：11
• 页码：2349-2356
• 全文大小：489K
• 年卷期：v.2,no.11(November 2, 2012)
• ISSN：2155-5435

文摘

In this Environmental Transmission Electron Microscopy (ETEM) study we examined the growth patterns of uniform distributions of nanoparticles (NPs) using model catalysts. Pt/SiO₂ was heated at 550 掳C in 560 Pa of O₂ while Pd/carbon was heated in vacuum at 500 掳C and in 300 Pa of 5%H₂ in Argon at temperatures up to 600 掳C. Individual NPs of Pd were tracked to determine the operative sintering mechanisms. We found anomalous growth of NPs occurred during the early stages of catalyst sintering wherein some particles started to grow significantly larger than the mean, resulting in a broadening of the particle size distribution (PSD). The abundance of the larger particles did not fit the log-normal distribution. We can rule out sample nonuniformity as a cause for the growth of these large particles, since images were recorded prior to heat treatments. The anomalous growth of these particles may help explain PSDs in heterogeneous catalysts which often show particles that are significantly larger than the mean, resulting in a long tail to the right. It has been suggested previously that particle migration and coalescence could be the likely cause for such broad size distributions. We did not detect any random migration of the NPs leading to coalescence. A directed migration process was seen to occur at elevated temperatures for Pd/carbon under H₂. This study shows that anomalous growth of NPs can occur under conditions where Ostwald ripening is the primary sintering mechanism.

Keywords:

catalyst sintering; Ostwald ripening; particle size distributions; Pt/SiO₂; Pd/carbon; environmental TEM