Study on the monolayer dispersion behavior of SnO2 on ZSM-5 for NOx-SCR by C3H6: the remarkable promotional effects of air plasma treatment

Abstract

Aiming to fabricate more practical catalysts for NO_x-SCR with C₃H₆, SnO₂/ZSM-5 having different SnO₂ loadings was prepared and treated with DBD air plasma. The dispersion of SnO₂ on the H-ZSM-5 support and their interactions were investigated with both experimental methods and DFT calculations. SnO₂ displays evident monolayer dispersion behavior, getting a threshold of 0.271 mmol 100 m⁻² support. Plasma treatment improves significantly the SnO₂ dispersion, hence amplifying the monolayer dispersion threshold to 0.380 mmol 100 m⁻². XPS and DFT calculations have testified that plasma treatment strengthens strongly the SnO₂–ZSM-5 support interaction, mainly through donating electrons from Sn⁴⁺ to Al³⁺ in the support, thus improving the dispersion of SnO₂ at the same loadings. Consequently, the catalytic performance is remarkably improved because of the generation of more abundant surface acid sites and superoxide species devoted to the reaction. The sample having a SnO₂ loading near the monolayer dispersion threshold shows the optimal activity in the corresponding catalyst series, demonstrating an evident threshold effect. Over SnO₂/ZSM-5, the reaction goes through a Langmuir–Hinshelwood pathway, involving the adsorption and activation of both NO and C₃H₆ molecules. Surface mono-dentate/bridged-nitrate and carbonate species are the main reaction intermediates.