In situ investigation of oxidation across a heterogeneous nanoparticle–support interface during metal support interactions

Abstract

Interactions between oxide supports and noble metal nanoparticles (NPs) is an area of intense research interest across all fields of catalysis. Oxygen spillover, metal support interactions (MSIs) and charge transfer are among many mechanisms observed and proposed as to how NP–support interfaces assist and enhance catalysis. This work studies the migration of oxygen across the Pd NP–CuO nanowire (NW) interface and beyond. X-ray photoelectron spectroscopy (XPS) and Kelvin probe force microscopy (KPFM) found an interaction between the Pd NP and CuO NW support, via the formation of PdO at the Pd–CuO interface. It was found, through in situ irradiation at high vacuum transmission electron microscopy (TEM), that oxygen enters the Pd NP lattice from the Pd–CuO interface via amorphization of the NP. Varying the amount of irradiation highlighted the different rates of amorphization of NPs, with full amorphization of a NP leading to the formation of an epitaxially driven PdO across the NPs. Interestingly, in situ heating in XPS observed a reduction to metallic Pd, found to be similarly amorphous during TEM investigation. On comparison with Pd supported on a non-reducible substrate – in which oxidation was found to proceed from the outer surface in, rather than the support interface (resulting in a PdO shell) – it is theorized that the oxidation and reduction of Pd on CuO forms a PdO NP surface full of Pd–PdO sites allowing for synergistic effects, of great use in the oxidation and hydrogenation of organic species.