PNNL

Abstract

Electric potentials of the (001) plane of hematite were measured as a function of pH and ionic strength in solutions of sodium nitrate and oxalic acid using the single-crystal electrode approach. The surface is predominantly charge-neutral in the pH 4-14 range, and develops a positive surface potential below pH 4 due to protonation of ?-OH0 sites (pK1,1,0 int = -1.34). This site is resilient to deprotonation up to at least pH 14 (-pK-1,1,0 int >> 21). The associated Stern layer capacitance of 0.62 F/m2 is smaller than typical values of powders, and possibly arises from a lower degree of surface solvation. Acid-promoted dissolution under elevated concentrations of HNO3 etches the (001) surface, yielding a convoluted surface populated by OH20.5+ sites. The resulting surface potential was therefore larger under these conditions than in the absence of dissolution. Oxalate ions also promoted (001) dissolution. Associated electric potentials were strongly negative, with values as large as -0.5 V, possibly from metal-bonded interactions with oxalate. The hematite surface can also acquire negative potentials in the pH 7-11 range due to surface complexation and/or precipitation of iron species (0.003 Fe/nm2) produced from acidic conditions. Oxalate-bearing systems also result in negative potentials in the same pH range, and may include ferric-oxalate surface complexes and/or surface precipitates. All measurements can be modeled by a thermodynamic model that can be used to predict inner-Helmholtz potentials of hematite electrodes.