Abstract
Methane in the form of natural gas is increasingly used as a transportation fuel, but the treatment of methane in the exhaust is a challenge since methane is a potent greenhouse gas. Pd is one of the most active catalysts for methane oxidation. Previous work has shown that transformation of Pd into the oxide, and decomposition of the oxide to metallic Pd can occur as temperature is raised in an oxidizing atmosphere, causing profound changes in catalytic reactivity. Equilibrium thermodynamics predict that the phases Pd and PdO must be in equilibrium at a well-defined temperature and oxygen pressure, since the two phases are immiscible and do not form solid solutions. But catalytic data suggests the existence of metallic Pd under conditions where only PdO should be thermodynamically stable. In this study we have explored the Pd ↔ PdO transition at high temperature using in situ XRD, TGA and from TEM examination of Pd catalysts that were quenched in liquid nitrogen or in a heating TEM holder to prevent any changes in microstructure during cooling. Corresponding data was obtained during methane oxidation, helping shed light on the nature of the working catalyst. The results show that the oxidation of metallic Pd to PdO is kinetically-controlled at high temperatures, allowing Pd to co-exist along with PdO. We refer to these as metastable Pd ↔ PdO structures. TEM shows that Pd and PdO domains can co-exist within a single particle, forming a phase boundary but allowing both Pd and PdO to be exposed to the gas phase. This kinetically controlled oxidation of Pd explains why we do not see core–shell PdO–Pd structures at elevated temperatures.
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Acknowledgements
Financial support from NSF GOALI Grant CBET-1438765 and General Motors Global R&D and from the Center for Biorenewable Chemicals (CBiRC) supported by NSF under Grant EEC-0813570 is gratefully acknowledged. Microscopy research sponsored in part by the U. S. DOE, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, Propulsion Materials Program, at Oak Ridge National Laboratory.
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Xiong, H., Lester, K., Ressler, T. et al. Metastable Pd ↔ PdO Structures During High Temperature Methane Oxidation. Catal Lett 147, 1095–1103 (2017). https://doi.org/10.1007/s10562-017-2023-7
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DOI: https://doi.org/10.1007/s10562-017-2023-7