Mechanistic insight into the selective catalytic reduction of NO by NH3 over α-Fe2O3 (001): a density functional theory study

Abstract

The adsorption properties and the selective catalytic reduction mechanism of NO, NH₃ and O₂ molecules over the α-Fe₂O₃ (001) surface were studied by density functional theory. The SCR reaction on the α-Fe₂O₃ (001) surface mainly followed the Eley–Rideal-type mechanism and three detailed reaction routes were proposed. The reaction of NO and NH₃ consists of seven consecutive steps: (1) NH₃ adsorption, (2) NH₃ oxidation, (3) NH₂NO formation, (4) NH₂NO oxidation, (5) ONNH rearrangement, (2) ONNH dissociation and (7) H₂O formation. –NHNO undergoes rearrangement before it dissociates to form a N₂ molecule. NH₃ dissociation and H₂O generation are the rate-determining steps. NO oxidation has two reaction paths, NO₂ generation and –NO₃ formation. The reaction of NH₃, NO and O₂ mainly consists of three consecutive steps: (1) –NO₂NH₂ formation, (2) –NO₂NH formation, and (3) –NO₂NH dissociation. The combination of free NH₂ and –NO₂ forms a –NH₂NO₂ group, which releases an energy of 288 kJ mol⁻¹ and plays an important role in the overall reaction as it provides energy in the following reactions. The formation and dissociation of H₂O determine the SCR deNO_x reaction rate and the adsorbed oxygen promotes the SCR deNO_x reaction on the α-Fe₂O₃ (001) surface.