Understanding the roles of Brønsted/Lewis acid sites on manganese oxide-zeolite hybrid catalysts for low-temperature NH_(3)-SCR

Although metal oxide-zeolite hybrid materials have long been known to achieve enhanced catalytic activity and selectivity in NO_(x)removal reactions through the inter-particle diffusion of intermediate species,their subsequent reaction mechanism on acid sites is still unclear and requires investigation.In this study,the distribution of Brønsted/Lewis acid sites in the hybrid materials was precisely adjusted by introducing potassium ions,which not only selectively bind to Brønsted acid sites but also potentially affect the formation and diffusion of activated NO species.Systematic in situ diffuse reflectance infrared Fourier transform spectroscopy analyses coupled with selective catalytic reduction of NO_(x)with NH_(3)(NH_(3)-SCR)reaction demonstrate that the Lewis acid sites over MnO_(x)are more active for NO reduction but have lower selectivity to N_(2)than Brønsted acids sites.Brønsted acid sites primarily produce N_(2),whereas Lewis acid sites primarily produce N_(2)O,contributing to unfavorable N_(2)selectivity.The Brønsted acid sites present in Y zeolite,which are stronger than those on MnO_(x),accelerate the NH_(3)-SCR reaction in which the nitrite/nitrate species diffused from the MnO_(x)particles rapidly convert into the N_(2).Therefore,it is important to design the catalyst so that the activated NO species formed in MnO_(x)diffuse to and are selectively decomposed on the Brønsted acid sites of H-Y zeolite rather than that of MnO_(x)particle.For the physically mixed H-MnO_(x)+H-Y sample,the abundant Brønsted/Lewis acid sites in H-MnO_(x)give rise to significant consumption of activated NO species before their inter-particle diffusion,thereby hindering the enhancement of the synergistic effects.Furthermore,we found that the intercalated K+in K-MnO_(x)has an unexpected favorable role in the NO reduction rate,probably owing to faster diffusion of the activated NO species on K-MnO_(x)than H-MnO_(x).This study will help to design promising metal oxide-zeolite hybrid catalysts by identifying the role of the acid sites in two different constituents.

Chemical deactivation of Cu-SAPO-18 deNO_x catalyst caused by basic inorganic contaminants in diesel exhaust

Contaminants(K,Na,Ca,and Mg)were introduced into Cu-SAPO-18 via incipient wetness impregnation to investigate their effect on the selective catalytic reduction of NOx with NH3(NH3-SCR)over Cu-SAPO-18.After the introduction of contaminants into Cu-SAPO-18,the quantity of acidic sites and Cu^2+ species in catalyst decreases owing to the replacement of H^+ and Cu^2+ by K^+,Na^+,Ca^2+,and Mg^2+.Furthermore,the loss of isolated Cu^2+ induces the generation of CuO and CuAl2O4-like phases,which causes further loss in the Brunauer-Emmett-Teller surface area of the catalyst.Consequently,the deNOx performance of the contaminated Cu-SAPO-18 catalysts drops.Such decline in NH3-SCR performance becomes more pronounced by increasing the contaminant contents from 0.5 to 1.0 mmol/gcatal.In addition,the deactivation influence of the contaminants on Cu-SAPO-18 is presented in the order of K>Na>Ca>Mg,which is consistent with the order of reduction of acidic sites.To a certain degree,the effect of the acidic sites on the deactivation of Cu-SAPO-18 might be more significant than that of isolated Cu2+ and the catalyst framework.Moreover,kinetic analysis of NH3-SCR was conducted,and the results indicate that there is no influence of contaminants on the NH3-SCR mechanism.

Dual single-atom Ce-Ti/MnO_(2)catalyst enhances low-temperature NH_(3)-SCR performance with high H_(2)O and SO_(2)resistance

Mn-based catalysts have exhibited promising performance in low-temperature selective catalytic reduction of NOx with NH_(3)(NH_(3)-SCR).However,challenges such as H_(2)O-or SO_(2)-induced poisoning to these catalysts still remain.Herein,we report an efficient strategy to prepare the dual single-atom Ce-Ti/MnO_(2)catalyst via ball-milling and calcination processes to address these issues.Ce-Ti/MnO_(2)showed better catalytic performance with a higher NO conversion and enhanced H_(2)O-and SO_(2)-resistance at a lowtemperature window(100−150°C)than the MnO_(2),single-atom Ce/MnO_(2),and Ti/MnO_(2)catalysts.The in situ infrared Fourier transform spectroscopy analysis confirmed there is no competitive adsorption between NOx and H_(2)O over the Ce-Ti/MnO_(2)catalyst.The calculation results showed that the synergistic interaction of the neighboring Ce-Ti dual atoms as sacrificial sites weakens the ability of the active Mn sites for binding SO_(2)and H_(2)O but enhances their binding to NH_(3).The insight obtained in this work deepens the understanding of catalysis for NH_(3)-SCR.The synthesis strategy developed in this work is easily scaled up to commercialization and applicable to preparing other MnO_(2)-based single-atom catalysts.