Revealing the catalytic oxidation mechanism of CO on α-Fe2O3 surfaces: an ab initio thermodynamic study

Abstract

Significant research efforts have been devoted to improving the efficiency of catalytic carbon monoxide (CO) oxidation over α-Fe₂O₃-based catalysts, but details of the underlying mechanism are still under debate. Here we apply the ab initio thermodynamic method (AITM) within the density functional theory framework to investigate the phase diagram of α-Fe₂O₃(0001) surfaces with various terminations and the catalytic mechanism of CO oxidation on these surfaces. By extending the conventional AITM to consider the charge state of surface defects, we build the phase diagram of α-Fe₂O₃(0001) surfaces in relation to the Fermi energy as well as the oxygen chemical potential, which makes it possible to explain the influence of point defects on the surface morphology and to predict the existence of the experimentally observed functional sites such as the ferryl group (FeO) and oxygen vacancies. Our calculations show that the surface with the ferryl-termination exhibits the highest catalytic activity for CO oxidation with remarkably low activation energy (0.05 eV) and the largest exothermic reaction energy, while other surfaces with different terminations are inadequate with relatively high activation energies. Moreover, it is revealed that a Fermi energy increase by means of intrinsic or extrinsic n-type doping is beneficial to the formation of ferryl-termination under both oxygen-rich and oxygen-poor conditions, which promotes the catalytic oxidation of CO via the Eley–Rideal (E–R) mechanism only (O-rich) or in combination with the Mars–van Krevelen (MvK) mechanism (O-poor). Meanwhile, under the O-poor conditions, a Fermi energy decrease by p-type doping facilitates the formation of oxygen vacancies and the CO oxidation might be promoted via the MvK mechanism which is less effective with higher activation energy than the E–R mechanism. Our work provides new fundamental insights into CO oxidation chemistry and mechanisms, thereby contributing to the design of new catalysts with high performance and low cost.