Revealing catalytic oxidation mechanism of CO on a-Fe2O3 surface: An ab initio thermodynamic study

Abstract

Significant research efforts have been devoted to improving the efficiency of catalytic carbon monoxide (CO) oxidation over a-Fe2O3-based catalysts, but detailed underlying mechanism is still under debate. Here we apply the ab initio thermodynamic method (AITM) within the density functional theory framework to investigate phase diagram of a-Fe2O3 (0001) surface with various terminations and catalytic mechanism of CO oxidation on these surfaces. By extending the conventional AITM to consider the charge state of surface defect, we build the phase diagram of a-Fe2O3 (0001) surface in relation with 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 ferryl group (Fe−−O) and oxygen vacancy. Our calculations reveal that the surface with the ferryl-termination exhibits the highest catalytic activity for CO adsorption and oxidation with remarkably low activation energy (0.05 eV) and the largest exothermic reaction energy along the Eley-Rideal mechanism, while other surface with different terminations are inadequate with relatively high activation energies and different mechanisms. Furthermore, the electronic density of states and partial atomic charges are carefully analyzed, demonstrating that some electrons are transferred from CO molecule to the substrate on the event of CO adsorption and oxidation. Our work provides new fundamental insights into CO oxidation chemistry and mechanism, thereby contributing to a design of new catalysts with high performance and low cost.