Catalytic function of ferric oxide and effect of water on the formation of sulfur trioxide.

Sulfur trioxide (SO) is not only environmentally harmful but also highly corrosive, taking a great threat to the safe operation of coal-fired power plants. A dominant pathway of SO formation in coal-fired power plant is through the catalytic oxidation of SO (SO+1/2O→SO) on the surfaces of ash particles containing FeO The catalytic formation of SO could be affected by complex atmosphere, where the effect from HO is still debatable. In this paper, density functional theory (DFT) is employed to explore the reaction pathway of SO formation catalyzed by α-FeO in complex atmosphere containing O, O, SO and HO. In order to get the stable adsorption sites of these species, the adsorption energy of potential adsorption configurations on the α-FeO (001) surface is calculated. The dissociations of O molecule on complete and defect α-FeO (001) surfaces with O vacancy are calculated, and the Langmuir-Hinshelwood and Eley-Rideal mechanisms for the O(ads) reaction with SO(ads) or SO are compared. The effect of HO besides of SO and O on the formation of SO is especially discussed. The DFT calculation results show that for the formation of SO in gas phase, the energy barrier of 'SO+1/2O→SO' is 436.75 kJ mol, in contrast, for the catalytic formation of SO on α-FeO surfaces, this energy barrier becomes an order of magnitude smaller, 24.82 kJ mol. O molecules can dissociate on the defect α-FeO (001) surface with O vacancy spontaneously, indicating that the defect α-FeO is favorable for the dissociation of O, thereby promotes the formation of SO. The energy barrier of 'SO(ads)+O(ads)→SO(ads)' through Langmuir-Hinshelwood mechanism is much higher than that of 'SO+O(ads)→SO(ads)' through Eley-Rideal mechanism. The adsorption energy on the α-FeO (001) surface of HO is much smaller than that of SO and O, indicating that HO has little effect on the adsorption of O, O, SO and eventually the heterogeneous formation of SO. The DFT analysis results in this study provide a deep understanding on the reaction pathway of SO catalytic formation by FeO.