RESUMO
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.
RESUMO
Electronic metal-support interactions (EMSIs) comprise an area of intense study, the manipulation of which is of paramount importance in the improvement of heterogeneous metal nanoparticle (NP) supported catalysts. EMSI is the transfer of charge from the support to NP, enabling more effective adsorption and interaction of reactants during catalysis. Ru NPs on CuO supports show different levels of EMSI (via charge transfer) depending on their crystal structure, with multiple twinned NPs showing greater potential for EMSI. We use magnetron-assisted gas phase aggregation for the synthesis of batches of Ru NPs with different populations of single crystal and multiple twinned nanoparticles, which were deposited on CuO nanowires (NWs). The surface charging of the Ru-CuO catalysts was investigated by Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS). By doubling the population of multiple twinned NPs, the surface potential of the Ru-CuO catalysts increases roughly 4 times, coinciding with a similar increase in the amount of Ru4+. Therefore, tuning the amount of EMSI in a catalyst is possible through changing the population of multiple twinned Ru NPs in the catalyst. Increasing the amount of multiple twin NPs resulted in improved activity in the oxygen evolution reaction (a roughly 2.5 times decrease in the overpotentials when the population of multiple twinned NPs is increased) and better catalyst stability. This improvement is attributed to the fact that the multiple twin NPs maintained a metallic character under oxidation conditions (unlike single crystal NPs) due to the EMSI between the NP and support.
RESUMO
Tuning the metal support interaction (MSI) in heterogeneous catalysts is of utmost importance for various applications in different catalysis reactions. Pt-TiN systems are strong contenders for commercial catalysts, although the charge screening of Pt and non-involvement of N reduces their effective MSI and limits it to the Pt-Ti interface. Here, the bias driven landing of gas phase synthesized Pt nanoparticles (NPs) is used to change the nature of the MSI and enhance the charge transfer phenomenon. Bias driven landing of the Pt NPs translates their impact energies to the TiN surface, resulting in a weakening of the Ti-N bonds. This facilitates a new interaction between the Pt and N atoms, resulting in an electronic equilibration in the N-Pt-Ti triumvirate, nullifying the charge screening of Pt. This change in the nature of the MSI enables long range charge transfer throughout the catalyst surface and an increase in the electrocatalytic hydrogen evolution reaction (HER) activity of the Pt-TiN system.
RESUMO
Pd NP concentration guided the self-assembly of core-shell Pd@SiO2 nanoparticles (NPs) into microcubes. The Pd NPs were stacked by molten dodecyltrimethylammonium bromide (DTAB) to create the SiO2 envelope. The microcubes demonstrated improved leaching resistance in heterogeneous catalysis over a conventional porous support.