Selective Catalytic Reduction Of NO By C₃H₆ Over Cu-SSZ-13 Under Lean-burn Conditions

In our country,the total amount of NOx emitted by diesel vehicles accounts for about 70%of the total amount of mobile sources.It seriously affects the quality of the atmospheric environment and human health.There are many NOx purification control technologies applied to the exhaust denitrification of diesel vehicles.Among them,hydrocarbon selective catalytic reduction（HC-SCR）technology is a promising diesel vehicle denitrification technology.In the ammonia selective catalytic reduction（NH₃-SCR）technology,copper-based molecular sieve catalysts used for diesel vehicle exhaust treatment usually include Cu-β,Cu-ZSM-5,Cu-SAPO-34,Cu-SSZ-13,etc.Among them,Cu-SSZ-13 has higher catalytic activity,selectivity,and hydrothermal stability than the other three catalysts.However,the SCR of NO_xwith hydrocarbons over Cu-SSZ-13 has hardly been reported.In this paper,Cu-SSZ-13 was prepared by one-step hydrothermal synthesis method.The catalysts were tested for C₃H₆-SCR activity in the gas atmosphere simulating actual diesel vehicle exhaust conditions.The influence of n _Si/n _Al,n _Cu/n _Aland copper species on the physicochemical properties of the catalysts were investigated by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,N₂ adsorption-desorption test,temperature programmed reduction of H₂（H₂-TPR）,temperature programmed desorption of ammonia（NH₃-TPD）,ultraviolet-visible spectroscopy（UV-Vis）and other characterization methods.In addition,the water resistance,sulfur resistance,and durability of the catalyst were also investigated.In this work,in situ infrared spectroscopy（in situ DRIFTS）technology was used to study the catalytic mechanism of the HC-SCR reaction over Cu-SSZ-13 and SSZ-13 catalysts,and the possible reaction pathways for reducing NO by propylene over the two catalysts were proposed.The main research conclusions are as follows:（1）To investigate the effect of n _Cu/n _Alon the performance of catalyst for selective catalytic reduction of NO with C₃H₆,Cu-SSZ-13 with different n _Cu/n _Alwere prepared by one-step hydrothermal synthesis method using Cu²⁺-tetraethylenepentamine（Cu-TEPA）as a structure directing agent.Studies have shown that when n _Cu/n _Al=2.0,the catalyst has the best low-temperature activity,which could reach over 80%NO conversion at 200℃,and 100%NO conversion and～100%N₂selectivity at 250～300℃.However,a further increase of n _Cu/n _Alwould cause the migration and aggregation of copper species,which lead the formation of Cu O clusters on the surface of the catalysts and Cu O would reduce the catalytic activity at high temperatures.The isolated Cu²⁺ions in the 8-membered rings of SSZ-13 had great redox performance,which were the main active sites for the C₃H₆-SCR reaction.The conversion rate of NO on the Cu-SSZ-13 showed a positive correlation with the number of isolated Cu²⁺ions in the8-membered rings.（2）The n _Si/n _Alratio had a great influence on the crystal structure of Cu-SSZ-13 catalyst.Because the Cu-SSZ-13 catalyst synthesized by one-step hydrothermal synthesis method with Cu-TEPA as the template has high copper loading,the double Al sites in the 6-membered rings are favorable positions for accommodating isolated Cu²⁺ions.For high-silicon SSZ-13 catalysts,the ion-exchange sites that can be provided are quite limited,and the increase of Cu content would inevitably lead to the accumulation of copper species.Therefore aluminum-rich SSZ-13 catalysts are required to provide a large number of ion-exchange sites.The range of n _Si/n _Alsuitable for this method is 4≤n _Si/n _Al<7.5.When n _Si/n _Alexceeds this range,the crystallinity of catalysts is not well or amorphous products are formed.Increasing n _Si/n _Alwithin a certain range would enhance the acidity of the catalyst.（3）Cu-SSZ-13 catalysts had excellent water resistance.After adding 5%H₂O in the reaction,the catalytic activity at low temperature of 150～250℃was even improved.However,in a gas atmosphere with SO₂,the activity of the catalyst was slightly inhibited in a part of the temperature range.It may be that SO₂formed sulfate species on the surface of Cu-SSZ-13,which hindered the denitration reaction of the catalyst.In the durability test,Cu-SSZ-13 performed well.After a 10h durability test at 300℃,the NO conversion remained above 86%.（4）In situ infrared spectroscopy experiments show that copper species and acid sites on the Cu-SSZ-13 catalysts can promote the adsorption and activation of NO to form active nitrate species or adsorbed NO₂,while C₃H₆would be activated to acetate species at the acid sites.These species interact to form an important intermediate species isocyanate,which then generated the final product N₂+CO₂ in a NO+O₂ atmosphere.