Insight into the Origin of Excellent SO2 Tolerance and de-NOx Performance of quasi-Mn-BTC in the Low-Temperature Catalytic Reduction of Nitrogen Oxide

NOx emission is a major environmental issue, and selective catalytic reduction (SCR) is the most effective method for the conversion of NOx to harmless N2 and H2O. Manganese oxide has excellent low-temperature (LT) denitration (de-NOx) activity, but poor SO2 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 SO2 tolerance of Mn-based catalysts. The NOx conversion over the Mn-BTC-335 degrees C only drops by 7% until SO2 is gradually increased to 200 ppm from 100 ppm for 36 h. Furthermore, the quasi-Mn-BTC presents excellent LT de-NOx performance with above 90% NOx conversion between 120 and 330 degrees C at a gas hourly space velocity of 36,000 h-1. Experimental and theoretical calculations confirm that the difficult electron transport between SO2 and active sites can prevent it from competing adsorption with NH3 and NO. Furthermore, the low degree of d-p hybridization and unstable p-p hybridization of SO2 on the active sites make it difficult for adsorption and oxidation; thus, the weak adsorption of SO2 can prevent it from sulfation on the active sites, ensuring Mn-BTC-335 degrees C excellent SO2 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 NH3 and NO adsorption, promoting the LT de-NOx performance. In situ diffuse reflectance infrared Fourier transform spectra and density functional theory calculation further reveal that the de-NOx reaction over Mn-BTC-335 degrees 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 NH3NO2 are the two key intermediates. This work provides a viable strategy for augmenting the LT de-NOx and SO2 tolerance of Mn-based catalysts, which may pave a new way in the application of MOFs in de-NOx, and the complete reaction mechanism provides a solid basis for future improvements of the LT NH3-SCR de-NOx reaction.