Comparative study on K‐tolerance performance of Fe/ZrO₂ and Fe/ZrO₂‐W catalysts for NH₃‐SCR of NO_x

Abstract

BACKGROUND: Selective catalytic reduction (SCR) of nitrous oxides (NO x ) with ammonia (NH 3 ) as reductant is used worldwide in mobile and stationary sources to reach strict emission standards. It is a feasible strategy to modify support with acidic metal oxides to improve the alkali metal potassium (K) tolerance of SCR catalysts. Herein, a comparative investigation was conducted based on iron/zirconium dioxide (Fe/ZrO 2 )and Fe/ZrO 2 -tungsten (W) catalysts to reveal the correlation of support modification with W and K-tolerance performance. RESULTS: The NO x conversion for K-Fe/ZrO 2 catalyst was <80% across the whole temperature range, and the catalyst was completely deactivated at ≈400 °C. As expected, the Fe/ZrO 2 -W catalyst exhibited a much superior anti-K-performance in comparison to the Fe/ZrO 2 catalyst. The active temperature window for K-Fe/ZrO 2 -W catalyst was 285-485 °C (NO x conversion of >80%).CONCLUSION: According to the characterization results, it was found that K-species impose a negative impact on NH 3 adsorption on the surface of the Fe/ZrO 2 catalyst, especially drastically preventing the adsorption of NH 3 species on Brønsted acid sites, thus inhibiting the occurrence of SCR reactions via the Langmuir-Hinshelwood (L-H) mechanism. By contrast, W modification resulted in more chemisorbed oxygen, stronger redox capacity and an increased Fe 3+ /(Fe 3+ +Fe 2+ ) ratio on the surface of the Fe/ZrO 2 -W catalyst. More importantly, W modification brought about abundant Brønsted acid sites, significantly promoting NH 3 adsorption and activation. W modification also weakened the adsorption stability of NO x species to a certain extent. As a result, SCR reactions over the Fe/ZrO 2 -W catalyst could proceed via both Eley-Rideal (E-R) and L-H pathways.