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

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 NOx 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.

Automated summary

Carbon-based selective catalytic reduction (SCR) technology combined with reaction-regeneration process is used to reduce NOx emissions, but NH3 slip is often ignored. This study shows that although carboxyl groups on metal-free carbon catalysts have strong NH3 adsorption, they do not participate in the SCR reaction. These catalytic inactive carboxyl groups not only prolong the SCR steady state, but also increase the risk of NH3 slip. Decomposition of carboxyl groups can alleviate NH3 slip, and the presence of inactive sites with strong NH3 adsorption on carbon catalysts should be carefully considered. This work not only enhances the understanding of the NH3-SCR mechanism, but also provides opportunities for controlling ammonia emissions from industrial processes.