Adsorption of CO2, CH3OH, and H2O on Fe(III)-Substituted Calcium Hydroxyapatites

Calcium hydroxyapatites (CaHAP) substituted with Fe3+ ions of different atomic ratios Fe/(Ca + Fe) = XFe from 0 to 0.23 were synthesized and characterized by various methods. The number of Ca2+ ions in CaHAP substituted by one Fe3+ ion was higher than one, implying that the formation of cation vacancies and the protonation of PO43− ions are caused by Fe(III)-substitution. XFe of the surface layer of the particles determined by XPS was less than the total XFe. IR spectra of Fe(III)-substituted CaHAP contain a band at 3707 cm−1 assigned to the surface Fe–OH groups in addition to the surface P–OH bands at 3688, 3677, and 3658 cm−1. With an increase in XFe, the amount of irreversible adsorption of CO2 decreased whilst those of CH3OH and H2O first decreased and then increased. IR results showed that the surface Fe–OH groups act as stronger irreversible adsorption sites for CO2 and CH3OH compared to the surface P–OH groups.