The discrepancy of NH₃ oxidation mechanism between SAPO-34 and Cu/SAPO-34.

Abstract

The difference of NH₃ oxidation mechanism over SAPO-34 and Cu-SAPO-34 was studied. XRD (X-ray diffraction), SEM (scanning electron microscopy) and H₂-TPR (H₂-temperature programmed desorption) were conducted to estimate the Cu species distribution. The quantity of individual Cu²⁺ ions escalated with the elevation of silicon content in the Cu/SAPO-34 catalysts, leading to an enhancement in the activity of the NH₃-SCR (ammonia-selective catalytic reduction) process. This augmentation in activity can be attributed to the increased presence of isolated Cu²⁺ species, which are pivotal in facilitating the catalytic reaction. In addition, the kinetic test of NH₃ oxidation indicated that the CuO species were the active sites for NH₃ oxidation. Specifically, the strong structural Brønsted acid sites were the NH₃ oxidation active sites over the SAPO-34 support, and the NH₃ reacted with the O₂ on the Brønsted acid sites to produce the NO mainly. While the NH₃ oxidation mechanism over Cu/SAPO-34 consisted of two steps: firstly, NH₃ reacted with O₂ on CuO sites or residual Brønsted acid sites to form NO as the product; subsequently, the generated NO was reduced by NH₃ into N₂ on isolated Cu²⁺ sites. Simultaneously, the isolated Cu²⁺ sites might demonstrate a significant function in the NH₃ oxidation process to form N₂. The identification of active sites and corresponding mechanism could deepen the understanding of excellent performance of NH₃-SCR over the Cu/SAPO-34 catalyst at high temperature.