Insight into the Origin of Excellent SO₂ Tolerance and de-NO_<i>x</i> Performance of quasi-Mn-BTC in the Low-Temperature Catalytic Reduction of Nitrogen Oxide

Abstract

NO x emission is a major environmental issue, and selective catalytic reduction (SCR) is the most effective method for the conversion of NO x to harmless N 2 and H 2 O. Manganese oxide has excellent low-temperature (LT) denitration (de-NO x ) activity, but poor SO 2 tolerance hinders its application. Herein, we report an interesting SCR catalyst, quasi-metal–organic-framework (MOF) nanorod containing manganese (quasi-Mn-BTC) with abundant oxygen vacancies (Vo), unique hierarchical porous structure, and half-metallic property, which successfully overcome the disadvantage of poor SO 2 tolerance of Mn-based catalysts. The NO x conversion over the Mn-BTC-335 °C only drops by 7% until SO 2 is gradually increased to 200 ppm from 100 ppm for 36 h. Furthermore, the quasi-Mn-BTC presents excellent LT de-NO x performance with above 90% NO x conversion between 120 and 330 °C at a gas hourly space velocity of 36,000 h –1 . Experimental and theoretical calculations confirm that the difficult electron transport between SO 2 and active sites can prevent it from competing adsorption with NH 3 and NO. Furthermore, the low degree of d–p hybridization and unstable p–p hybridization of SO 2 on the active sites make it difficult for adsorption and oxidation; thus, the weak adsorption of SO 2 can prevent it from sulfation on the active sites, ensuring Mn-BTC-335 °C excellent SO 2 tolerance. Additionally, the half-metallicity property, the extraordinary d–sp hybridization, and the high degree of s–p hybridization cause strong bonding and the delocalization of electrons that promote the charge transfer and adsorbed ion diffusion for NH 3 and NO adsorption, promoting the LT de-NO x performance. In situ diffuse reflectance infrared Fourier transform spectra and density functional theory calculation further reveal that the de-NO x reaction over Mn-BTC-335 °C follows both Eley–Rideal (E-R) and Langmuir–Hinshelwood (L-H) mechanisms. The “standard reaction” is more likely to occur in the E-R reaction, while the “fast reaction” is prone to occur in the L-H pathway, and HNNOH and NH 3 NO 2 are the two key intermediates. This work provides a viable strategy for augmenting the LT de-NO x and SO 2 tolerance of Mn-based catalysts, which may pave a new way in the application of MOFs in de-NO x, and the complete reaction mechanism provides a solid basis for future improvements of the LT NH 3 -SCR de-NO x reaction.

Topics & Concepts