Potential Risk of NH₃ Slip Arisen from Catalytic Inactive Site in Selective Catalytic Reduction of NO_x with Metal-Free Carbon Catalysts

Ammonia emissions from industrial processes have rapidly increased in the past years. Recent advances have used carbon-based selective catalytic reduction (SCR) technology combined with a reaction–regeneration process to reduce NO_x from sintering flue gas, while NH₃ slip is seldom accounted for in this process. This study demonstrates that although the electrophilic carboxyl groups (−COOH) on metal-free carbon catalysts exhibit strong adsorption toward NH₃, they do not participate in the SCR reaction. As a result of the competitive adsorption of NH₃ in the reaction step, these catalytic inactive carboxyl groups not only prolong the time to the SCR steady state, but also result in the potential risk of NH₃ slip. A linear relationship with the equimolar ratio between carboxyl groups and slipped NH₃ was established in the regeneration steps. The slip of NH₃ could be alleviated by the decomposition of carboxyl groups, and special attention should be paid to the presence of inactive sites with strong NH₃ adsorption on industrial-employed carbon catalysts. In addition to advancing the understanding of the NH₃–SCR mechanism, this work also provides valuable opportunities for the control of ammonia emissions from industrial processes.