DFT insights into the adsorption of NH3-SCR related small gases in Mn-MOF-74

Abstract

Mn-MOF-74 has great potential to catalyze selective catalytic reduction (SCR) reaction with NH₃ being the reductant (NH₃-SCR). However, the reaction mechanism, in particular the adsorptive properties of key reactive species in Mn-MOF-74, remains ambiguous. Besides, the effects of impurities such as H₂O and SO₂ on the process need further investigation. In this paper, based on density functional theory (DFT) calculations, we studied the adsorption characteristics of six NH₃-SCR related small gases, namely NH₃, NO₂, NO, O₂, H₂O and SO₂. DFT results show that the Mn-MOF-74 structure can bind these molecules relatively strongly in the following order: NH₃ > NO₂ > NO > O₂, allowing for subsequent NH₃-SCR reaction. In addition, a possible pathway of NO conversion to NO₂ was calculated. Investigation on competitive adsorption of NH₃ and H₂O, NH₃ and SO₂ reveals that both H₂O and SO₂ are probable to replace NH₃ under certain conditions, indicating that the two impurity gases may affect the activity of the NH₃-SCR reaction. Compared with H₂O, SO₂ can displace NH₃ more easily and should not be neglected.