Metal-free β zeolite used as an efficient NH3-SCR catalyst can achieve complete immunity to SO2: Unique design strategy of sulfur-resistant catalyst

Abstract

Sulfur oxides present in flue gas can deactivate metal oxide catalysts or metal cation exchange zeolite catalysts used in NH3-SCR, as the deposition of (NH4)2SO4/NH4HSO4 and metal sulfate can obstruct and damage active sites, respectively. The physical poisoning of the (NH4)2SO4/NH4HSO4 deposition can be prevented by selecting an appropriate reaction temperature range (reversible), while chemical poisoning caused by the sulfation of active sites is challenging to avoid and can permanently deactivate the catalyst (irreversible). This research addresses the issue of irreversible catalyst deactivation resulting from sulfur poisoning by employing metal-free β zeolite without redox sites, which effectively prevents redox site sulfation at its source and shields the catalyst from physical poisoning by operating at moderate to high temperatures. This metal-free β zeolite with an appropriate pore structure exhibited exceptional deNOx activity, N2 selectivity, stability, and sulfur resistance at 300 °C. The NO oxidation experiment and DFT analysis demonstrated that the specific pore structure of β zeolite functioned as a sub-nano reactor, facilitating the oxidation of NO to NO2, thereby triggering the subsequent series catalytic reaction of “fast” SCR. Even in an atmosphere with a high concentration of 300 ppm SO2, the β zeolite catalyst maintained over 80 % NOx conversion after 130 h. The slight reduction in activity was attributable to the inhibitory influence of SO2 on the collision reaction between NO and O2; however, SO2 did not have any poisonous impact on the catalyst. The unique design strategy found in present work provides valuable insights for the sulfur resistance of metal-free zeolites used in NH3-SCR process, and provides a new idea for the design of completely sulfur-resistant catalysts.