Water mediated oxygen activation in NH3 SCR reaction over a Cu-SAPO-34 catalyst: a first principles study

Abstract

At low temperature, water can greatly improve the NH₃ selective catalytic reduction (SCR) performance of Cu-SAPO-34 under standard reaction conditions. During the SCR process on Cu-CHA catalysts, oxygen (O₂) activation over pairs of Cu(NH₃)₂⁺ complexes is a crucial step. The concentration of H₂O in the reaction atmosphere may affect the local environment of Cu ions and subsequent oxygen activation. In this work, first-principles calculations are used to study the adsorption of NH₃ on Cu sites and the detailed reaction paths of O₂ activation with or without water in Cu-SAPO-34. NH₃ adsorption is generally favored over H₂O on Cu ions, yet at low temperature and with partial pressure of water vapor, H₂O and NH₃ molecules could co-adsorb on the same Cu site from the phase diagram analysis. During the O₂ activation process, this kind of behavior will influence the formation of the Cu(NH₃)₂⁺ complex and a new kind of Cu(NH₃)(H₂O)⁺ complex will form during the reduction process of Cu²⁺ ions. Compared with O₂ activation over pairs of Cu(NH₃)₂⁺ complexes, Cu(NH₃)₂⁺–Cu(NH₃)(H₂O)⁺ complexes change the torsion angle of the Cu–O–O–Cu configuration and decrease the reaction energy barriers of O₂ activation in Cu-SAPO-34. Our results shed light on the water mediated NH₃ SCR reaction mechanism of Cu-SAPO-34 at the molecular level.