| Summary: | One of the main factors in the deterioration of automobile three-way catalysts is the sintering of platinum group metals (PGMs). In this study, we used in situ tunneling electron microscopy (TEM) to examine the sintering of Rh particles as the temperature increases. Two types of environmental conditions were tested, namely, vacuum atmosphere with heating up to 1050 °C, and N<sub>2</sub> with/without 1% O<sub>2</sub> at 1 atm and up to 1000 °C. Under vacuum, Rh particles appeared to be immersed in ZrO<sub>2</sub>. In contrast, at 1 atm N<sub>2</sub> with or without 1% O<sub>2</sub>, the sintered Rh particles appeared spherical and not immersed in ZrO<sub>2.</sub> The latter trend of Rh sintering resembles the actual engine-aged catalyst observed ex situ in this study. In the N<sub>2</sub> atmosphere, the sintering of support material (ZrO<sub>2</sub> or Y-ZrO<sub>2</sub>) was first observed by in situ TEM, followed by Rh particle sintering. The Rh particle size was slightly smaller on Y-ZrO<sub>2</sub> compared to that on ZrO<sub>2</sub>. To better understand these experimental results, density functional theory was used to calculate the systems’ junction energies, assuming three layers of Rh(111) 4 × 4 structures joined to the support material (ZrO<sub>2</sub> and Y-ZrO<sub>2</sub>). The calculated energies were consistent with the in situ TEM observations in the N<sub>2</sub> atmosphere.
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