X-ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in Aqueous Solutions. 3. Stabilization of Ammonium Species by Surface (Hydr)oxo Groups

The speciation of ammonium at the hematite/water interface was probed by cryogenic X-ray photoelectron spectroscopy. Wet pastes of colloidal hematite spheroids equilibrated in aqueous solutions of 50 mM NH4Cl exhibit distinctive pH-sensitive N 1s peaks for both NH4+ (401.7 eV) and NH3 (400.1 eV), yet total N/Fe ratios remain relatively invariant (0.029 ± 0.006) throughout the pH 2.2−10.5 range. Both NH4+ and NH3 species coexist throughout most of the tested pH range. NH4+ is most likely stabilized at the interface by hydrogen bonding with surface (hydr)oxo groups. A cationic sorption edge for NH3 is driven by proton abstraction of NH4+ by (hydr)oxo groups, forming surface complexes of the type ≡Fe−OH···NH3. These interactions shift the NH4+/NH3 equilibrium from pKa = 9.3 in water to 8.4 at the interface. Removal of excess water by vacuum dehydration induces, on the other hand, formation of NH2 directly bound to surface Fe atoms. These results underscore distinct ammonium species in contact with mineral surfaces and should be considered in understanding environmental and catalytic reactions in this medium.