There is a critical need for reducing emissions of methane, the second most prevalent greenhouse gas. Homogeneous combustion results in the release of toxic gases such as NOx, SOx and CO, and current methods of catalytic emission control are still ineffective at low temperatures (300°C) and in the presence of large amounts of steam. Improving methane combustion catalysts would affect several technologies that have to deal with methane emissions. Palladium is widely accepted as one of the best metal catalysts to date. Unfortunately, it is unclear which phase of palladium is most active; furthermore, there is a need to utilize Pd to the best possible extent given its precious nature. To gain a fundamental understanding of this system and provide insights into the active sites to further improve the activity and stability of Pd-based catalysts, control over the size, shape and composition of the metal nanocrystals is essential. In this study, highly monodisperse palladium nanocrystals, prepared via high-temperature colloidal synthesis, are used to investigate the structure-activity relationships of palladium-catalyzed methane complete combustion in the low size regime (2-8nm). By deposition of the nanocrystals onto high-surface area supports and activation using a fast thermal annealing process, highly monodisperse, stable catalysts are obtained. These materials are used to draw structure-activity relationships under realistic methane complete combustion conditions. It is shown that methane combustion activity has a strong size-depedency on the Pd phase, and the maximum rates are obtained at intermediate (~3nm) Pd sizes (Figure). These findings are related to sites on the Pd nanocrystals that correspond to particular Pd coordination geometries. Furthermore, kinetic characterization demonstrates that the activity is dependent on the oxidation state of the Pd phase, which is also affected by the size of the Pd nanocrystals. These new observations are made possible through the use of tightly controlled Pd nanocrystals. Our work provides clear elements for improving the activity of Pd-based combustion catalysts and in general a framework for understanding structure-activity relationships using highly uniform catalysts under realistic reaction conditions.

Structure-property relationships of palladium-catalyzed methane complete combustion using uniform nanoparticles

251st National Meeting (2016)

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