What Causes Ammonia Oxidation During NH3-SCR?

This article examines simultaneous reactions that compete with the selective catalytic reduction of nitrogen oxides by ammonia (NH₃-SCR). Specifically, the described study investigates ammonia oxidation (AMO) and the generation of unwanted byproducts N₂O and NO₂ over Cu/SSZ-13 and Fe–Cu/SSZ-13 zeolite catalysts.

Special attention is given to how the technique of iron introduction and reaction conditions (dry versus wet) impact catalytic activity, selectivity, and the nature of active sites.

Bimetallic Fe–Cu/SSZ-13 catalysts were prepared with a two-stage strategy in which copper was integrated into the SSZ-13 framework using one-pot synthesis. Iron was subsequently introduced through either impregnation or ion exchange.

The experiments took place in a quartz fixed-bed reactor together with a QMS detector (Hiden Analytical HPR-20) and an FTIR spectrometer equipped with a multiple-reflection gas cell.

Additionally, density functional theory (DFT) coupled with first-principles thermodynamics (FPT) modeling was employed to determine the speciation and stability of Cu–Cu, Fe–Fe, and mixed Cu–Fe active sites under relevant reaction conditions. Isotopic experiments employing ¹⁴NH₃/¹⁵NH₃ enabled the separation and quantification of SCR and AMO pathways under steady-state conditions.

Figure 1. Hiden Analytical HPR-20 R&D. Image Credit: Hiden Analytical

Although all catalysts retained the CHA structure of SSZ-13, the preparation technique heavily impacted metal distribution and speciation. Ion exchange resulted in partial copper leaching and the generation of segregated FeOx and CuOx species on the external surface, while impregnation maintained an elevated intrazeolite copper content, leading to more favorable iron dispersion.

These structural variations significantly affected catalytic performance. Catalysts prepared using impregnation featured a wider operational temperature window, elevated apparent turnover frequencies, and superior selectivity toward N₂ relative to their ion-exchanged counterparts.

The competition between SCR and AMO was demonstrated to be highly sensitive to both catalyst composition and temperature. At reduced temperatures, isolated Cu²⁺ sites dominated SCR activity, whereas excessive NH₃ adsorption resulted in inhibition impacts, especially in Cu-depleted ion-exchanged samples.

At elevated temperatures, ammonia oxidation grew in importance, particularly on segregated metal oxide species, leading to a decline in NO conversion and improved N₂ formation unrelated to SCR.

It should be noted that Fe addition, when introduced by impregnation, was shown to mitigate these negative impacts, substantially suppressing NH₃ oxidation and minimizing N₂O formation by up to a factor of five.

Water vapor played an advantageous role by shifting the active site equilibrium from oxo to hydroxo forms, as verified by DFT/FPT modeling and operando IR spectroscopy. This transformation minimized the accumulation of nitrate and ammonium nitrate within the zeolite pores, thus reducing N₂O formation and enhancing overall N₂ selectivity, especially for Fe–Cu catalysts prepared by ion exchange.

Overall, the research shows that meticulous control of metal speciation via synthesis strategy is critical for SCR performance optimization. Impregnation-derived Fe–Cu/SSZ-13 catalysts provide an effective method for expanding the operational temperature window, suppressing parasitic ammonia oxidation, and reducing N2O emissions.

This could deliver valuable insights for the design of next-generation NH₃-SCR catalysts.

Acknowledgments

Produced from materials originally authored by Monika Fedyna, Jagiellonian University.

References and Further Reading

This information has been sourced, reviewed, and adapted from materials provided by Hiden Analytical.

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