Fe₂O₃–CeO₂@Al₂O₃ Nanoarrays on Al-Mesh as SO₂-Tolerant Monolith Catalysts for NO_<i>x</i> Reduction by NH₃

Abstract

Currently, selective catalytic reduction of NO _x with NH₃ in the presence of SO₂ is still challenging at low temperatures (<300 °C). In this study, enhanced NO _x reduction was achieved over a SO₂-tolerant Fe-based monolith catalyst, which was originally developed through in situ construction of Al₂O₃ nanoarrays (na-Al₂O₃) on the monolithic Al-mesh by a steam oxidation method followed by anchoring Fe₂O₃ and CeO₂ onto the na-Al₂O₃@Al-mesh composite by an impregnation method. The optimum catalyst delivered more than 90% NO conversion and N₂ selectivity above 98% within 250-430 °C as well as excellent SO₂ tolerance at 270 °C. The strong interaction between Fe₂O₃ and CeO₂ enabled favorable electron transfers from Fe₂O₃ to CeO₂ while generating more oxygen vacancies and active oxygen species, consequently accelerating the redox cycle. The improved reactivity of NH₄⁺ with nitrates following the Langmuir-Hinshelwood mechanism and active NH₂ species that directly reacted with gaseous NO following the Eley-Rideal mechanism enhanced the NO _x reduction efficiency at low temperatures. The preferential sulfation of CeO₂ alleviated the sulfation of Fe₂O₃ while maintaining the high reactivities of NH₄⁺ and NH₂ species. Especially, the SCR reaction following the Eley-Rideal mechanism largely improved the SO₂ tolerance because NO does not need to compete with sulfates to adsorb on the catalyst surface as nitrates or nitrites. This work paves a way for the development of high-performance SO₂-tolerant SCR monolith catalysts.