Density Functional Theory Study of the Interactions of OH and Cl with the Pristine and Defective Surfaces of α-Fe2O3 (0001)

Abstract

Fe (III) oxides have been identified as a dominant structure of the outer layer of the iron passive film formed in alkaline environment but chloride ion has been suggested to induce depassivation of these passive films under the same conditions. Here we use DFT+U to investigate the mechanism of the depassivation by studying the interactions of OH and Cl with the pristine and defective α-Fe2O3 (0001) surfaces and hematite (α-Fe2O3) is used to represent the Fe (III) oxides. On the pristine surface, the adsorption of both adsorbates can affect mostly the positions of the top two layers. The structural change increases with coverage and calculation of low coverages shows that the effects are localized. The largest effects on the surface were caused by the OH at high coverage and co-adsorption of OH and Cl, which suggests that OH could play an important role in the Cl-induced depassivation process. On the defective surfaces, Fe vacancy surface and O vacancy surface are studied. On Fe vacancy surface, Fe top site is the most stable site for both adsorbates, and the adsorption energy for the two adsorbates and structural effects caused by the adsorption are comparable to those on the pristine surface. The insertion of one Cl into the second layer of α-Fe2O3 is endothermic, with the reaction energy about 0.7 eV, which suggest that Cl is thermodynamically favorable to adsorb on the surface. Similarly on O vacancy surface, Cl is more stable on the Fe top site closer to the vacancy than in the O vacancy suggesting that the vacancy affects the Cl adsorption indirectly.