New Insights into NH3 Oxidation and N2O Formation Over Cu-SSZ-13: Comparison of NH3-SCO and NH3-SCR

Cu-SSZ-13 catalyst is highly active for both NH3-SCO and NH3-SCR of NOx. NH3 conversion and N2 selectivity were over 98% at 225–500 °C during NH3-SCO, while NOx conversion and N2 selectivity remained above 91 and 98%, respectively, at 150–500 °C during NH3-SCR. N2O formation showed bimodal behavior with increasing temperature during both processes, with higher amounts of N2O produced in the NH3-SCR process. Increasing the NO feed concentration enhanced the conversion of NH3 at low temperatures while promoting the formation of N2O at both low and high temperatures. Complete removal of NOx and excessive NH3 can be achieved at low temperatures (e.g., 250 °C) by coupling NH3-SCR and NH3-SCO over the Cu-SSZ-13 catalyst. O2 is necessary for both NH3 oxidation and NOx reduction. Increasing the O2 content favored NH3 oxidation, leading to increasing NH3 conversion at low temperatures during NH3-SCO and decreasing NOx conversion at high temperatures during NH3-SCR. N2O selectivity increased with increasing O2 during both low- and high-temperature NH3-SCO and NH3-SCR processes. NH3 oxidation over Cu-SSZ-13 follows the two-step mechanism: partial NH3 is first oxidized to NO and N2O, and the formed NO is reduced by the remaining NH3 to N2 and N2O. Nonselective NH3 oxidation (NSNO) and catalytic reduction (NSCR) contribute to N2O formation at both low and high temperatures.