Atomic scale structure and surface chemistry of Fe oxides/carbonates-based composites precipitated at various temperatures via urea-supported hydrothermal synthesis

Abstract

This is the first report of the atomic-scale structure in iron oxides/carbonates based composites produced at various autoclave temperatures from divalent Fe(II) precursor. It is also one of the first works which demonstrates how EXAFS simulation can be utilized to additionally characterize metal compound-based materials to estimate their properties associated with surface reactivity. Here we examine how greatly the autoclave temperature within a practical range of 120–180 °C affects the composition, structure, surface chemistry and adsorptive anion removal of three Fe-oxides/carbonates-built composites synthesized via urea-supported hydrothermal precipitation. Divalent Fe(II) precursor was used to promote redox transformations and consequently to increase number of phases/compounds. Complementary tools EXAFS, XPS and FTIR allowed to characterize the entire composites regardless of their crystallinity. It was defined that the autoclave temperature strongly influenced both the ratios between two substances (Fe oxides and Fe carbonates) in each material and speciation in each compound. Stronger reducing conditions at higher temperature in autoclave intensified generation of FeCO3-containing substances fraction of which was growing from 20.6 to 56.2–75.6% in the materials fabricated at 120, 150 and 180 °C, respectively. In neither of the three composites, local structure around Fe simulated using EXAFS oscillations reproduced arrangement of any known individual phase of Fe oxide or Fe carbonate. The data on adsorptive performance of the samples to six aqueous anions supported the recently proposed methodological suggestion which associates an extended x-ray absorption fine structure around the main atoms (here, Fe) in outer shells simulated using EXAFS oscillations with the material anion exchange ability. In this paper, we made another observation which for the first time relates the local structure in further shells fitted solely with O atoms with physisorbed water.