Single-atom catalysts have recently attracted great attention due to their ultimate metal efficiency and the promise of novel properties. However, at the atomic level, little is known about their stability, interactions with the support, and mechanisms by which they operate. Recently the Fe3O4(001) surface with subsurface cationic vacancies has been shown to stabilize single metal atoms to temperatures as high as 700 K, making it promising support for model studies of single atom catalysts. In this study, we follow the adsorption and reactions of molecular hydrogen and methanol on single Pd atoms using a combination of scanning tunneling microscopy, temperature programmed desorption, and x-ray photoelectron spectroscopy. Density functional theory investigations provide a further mechanistic understanding of elemental reaction steps. We show that H2 dissociates heterolytically at 300 K and spills over onto the Fe3O4(001). Theoretical studies reveal that H2 dissociates heterolytically, with hydroxyls being the final, energetically most stable species. Diffusion barriers for along-the-row diffusion of hydroxyl hydrogen are found to be too high to allow for facile diffusion at room temperature. With increasing H2 dose, the initially isolated hydroxyl species start to form locally ordered (√2×√2)R45° overlayer. At highest coverages, where approximately every fourth oxygen atom is hydroxylated, the reconstruction is lifted, and the Pd atoms become destabilized. demonstrate that the diffusion is mediated by coadsorbed water. For methanol, we find that a large fraction desorbs molecularly by 280 K. Above 300 K, methanol dissociates to form methoxy and hydroxyl species. At elevated temperatures, the methoxy species recombine with hydroxyl species to desorb as additional molecular methanol at 350 K or further dehydrogenate to produce formaldehyde above 500 K. On single Pd atoms on Fe3O4(001), the formation of formaldehyde above 500 K is suppressed and a new formaldehyde channel is observed at 300 K. Increasing the Pd atom concentration increases the magnitude of the low-temperature channel. The appearance of the lower temperature reaction channel for formaldehyde shows that the Pd atoms lower the reaction barrier to dehydrogenate methoxy to formaldehyde by a factor of two. These studies demonstrate unique catalytic properties of single Pd atoms supported on Fe3O4(001).

Activation of H-H and C-H bonds on oxide supported single Pd atoms

2019 Spring National Meeting

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