Role of Gallic Acid in the Synthesis of Carbon-Encapsulated Iron Nanoparticles by Hydrothermal Carbonization: Selecting Iron Oxide Composition

Abstract

In this work, the role of phenolic compounds in the hydrothermal synthesis of carbon-encapsulated iron nanoparticles (CEINs) was studied. To model phenolic compounds, gallic acid (GA) was selected, with glucose as the carbon source. Iron was found as α-Fe2O3, γ-Fe2O3, Fe3O4, and zero-valent iron (ZVI) depending on the synthesis pH and GA/Fe molar ratio. For GA/Fe = 1, the CEINs' yield increased significantly. In the samples with phenolics, increasing the initial pH increased the amount of γ-Fe2O3 and Fe3O4 and enhanced the iron oxide encapsulation due to enhanced chelating ability. Reducing the GA/Fe ratio to 0.2 resulted in CEINs with stronger magnetization due to the presence of Fe3O4. Ash weight, HCl digestion, and Raman spectroscopy were used in conjunction to characterize the composition of the CEINs. The magnetization of the samples was compared using a simple magnetic weight setup. A scheme for the reactions occurring during the hydrothermal carbonization of GA-Fe complexes was proposed.