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

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