Fig. 3: Identification of the hydrocarbothermal reduction mechanism.
Schematic illustration of (a) previous method and (b) current method for synthesizing HEA-NPs using the continuous-flow spray pyrolysis technique. In the previous method, the aerosol droplets containing only metal salts are dried before being spray pyrolyzed at 1100 °C with H2 as the reducing agent. The resulting HEA-NPs are substrate-free and have a large diameter with a wide size distribution. In the current method, the aerosol droplets containing both carbon precursor and metal salts are directly spray pyrolyzed at a lower temperature of 850 °C with Ar as the carrier gas. The obtained HEA-NPs are supported on carbon with a small size, dense and uniform distribution, and strong metal-support interaction. c Schematic illustration of the different reaction pathways of the spray pyrolysis synthesis with varied components contained in the aerosols. STEM image and EDS elemental maps of the control sample prepared by spray pyrolysis without H2O (d) or carbon precursor (e), showing the co-presence of metal phase (highlighted by the green contour) and oxide phase (highlighted by the orange contour). f Radar plot summarizing the EDS-determined atomic percentages of constituent elements within the metallic region shown in (d) and (e). g XRD patterns of the samples prepared by hydrocarbothermal synthesis and the two control samples. Standard patterns are included for reference, including Fe3O4 (PDF#72-2303) in green, CuO (PDF#78-0428) in orange, CoO (PDF#70-2855) in purple and NiO (PDF#89-7130) in yellow. h GC analysis of the exhaust gas collected from the hydrocarbothermal synthesis and the control experiment without H2O. i Ellingham diagram that shows the oxidation potential of different elements. Tsynthesis is the spray pyrolysis temperature (850 °C).
