Pore Structure of Crystalline Ferric Hydroxides and Oxides

Abstract

Water vapour sorption isotherms at 20 °C of two series of crystalline ferric hydroxide powders (α and γ) are reported. Samples of the α-series were prepared from synthetic crystalline α-ferric hydroxide by thermal decomposition at a fixed temperature between 25 °C and 350 °C. Samples of the γ-series were prepared from synthetic crystalline γ-ferric hydroxide in the same way. It has been found that water vapour sorption isotherms on α-series samples exhibit a reproducible closed hysteresis loop located at p/p0 = 0.3–0.8. Both the shape and the location of the hysteresis loop change as the decomposition temperature of the samples increases. Initially, the hysteresis loop seems somewhat parallel to the pressure axis but later it becomes perpendicular. To explain this phenomenon, a pore structure model is proposed whereby a system with slit-shaped pores changes into one with pores open at both ends.