Synthesis and mesopore-micropore structure characterization of nanodisperse Fe3+ hydrogels (xerogels)

A preparation procedure was developed, and samples of nanodisperse Fe³⁺ hydroxide with a narrow particle-size distribution (2.5–3.5 nm) were synthesized. The occurrence of a substructure in the bulk of Fe³⁺ hydroxide nanoparticles was detected for the first time using light-field and dark-field transmission electron microscopy. It was found that structurally ordered regions with sizes of ～1.0 nm, which were disoriented with respect to each other at angles of a few degrees, occurred in the bulk of the nanoparticles. The empirical formula of nanodisperse iron hydroxide was ～Fe₂O₃ · 1.8H₂O; the structure of this hydroxide contained crystal water, OH^?, and O^2?. The coordination number of Fe³⁺ cations with respect to oxygen was 6. It was found that both structural and nonstructural water can be removed almost completely from the bulk of nanoparticles in the course of sample heating to 150–250°C in a vacuum with the retention of their amorphous character and observed sizes. In the course of dehydration, the mutual mobility of nanoparticles within aggregates was retained in Fe³⁺ xerogels; this resulted in a decrease in the total pore volume, whereas the volume of mesopores with diameters of 3.4–3.5 nm progressively increased. The micropore structure of the samples of nanodisperse iron hydroxides was studied by the molecular probe method using the low-temperature (77 K) sorption of nitrogen and molecular hydrogen. It was found that, along with micropores of volume ～0.02 cm³/g, which are accessible to both of the sorbates, the sorption of H₂ exhibited an additional specific absorption of 1.0–1.7 cm³(STP)/g, which can be interpreted as an additional ultramicropore volume accessible to only hydrogen molecules.