Understanding Catalytic Reactions over Zeolites: A Density Functional Theory Study of Selective Catalytic Reduction of NOx by NH3 over Cu-SAPO-34

Metal-exchanged CHA-type (SAPO-34 and SSZ-13) zeolites are promising catalysts for selective catalytic reduction (SCR) of NOx by NH3. However, an understanding of the process at the molecular level is still limited, which hinders the identification of its mechanism and the design of more efficient zeolite catalysts. In this work, modeling the reaction over Cu-SAPO-34, a periodic density functional theory (DFT) study of NH3-SCR was performed using the hybrid functional (HSE06) with the consideration of van der Waals (vdW) interactions. A mechanism with a low N-N coupling barrier is proposed to account for the activation of NO. The redox cycle of Cu2+ and Cu+, which is crucial for the SCR process, is identified with detailed analyses. In addition, the decomposition of NH2NO is shown to readily occur on the Bronsted acid site by a hydrogen push-pull mechanism, confirming the collective efforts of Bronsted acid and Lewis acid (Cu2+) sites. The special electronic and structural properties of Cu-SAPO-34 are demonstrated to play an essential role in the reaction, which may have general implications on the understanding of zeolite catalysis.