Research On Controllable Synthesis,Structure-Performance And Deactivation Mechanism Of Transition Metal Modified CHA/AEI Small-pore Molecular Sieve NH₃-SCR Catalyst

As the main mobile emission source of nitrogen oxides?NO_x?,the abatement of NO_x under lean condition from diesel vehicles has drawn increasing attention both for scholars and engineers.With the "most severe in history" National-Six diesel vehicle emission regulations approaching,the highly efficient and economical NO_x removal technology has been attracted widespread attention.Currently,ammonia selective catalytic reduction?NH₃-SCR?of NO_x has been proven to be the most economical and efficient technique for the removal of NO_x from diesel engine exhaust.The catalyst is the key factor of the NH₃-SCR technology.This thesis develops a series of simple preparation,economical and efficient NH₃-SCR small pore molecular sieve catalysts based on national conditions.To obtain a deeper understanding of the catalytic reaction and theory over small pore molecular sieve catalysts,the structure-function relationship or deactivation mechanism were well established by a series characterization techniques.The detailed conclusions are listed as follows:Firstly,the combination of copper-tetraethylenepentamine?Cu-TEPA?and morpholine?MOR?as templates allowed the direct synthesis of highly active and hydrothermally stable submicron Cu-SAPO-34 catalyst for SCR of NO_x with NH₃?NH₃-SCR?.It exhibited much better catalytic activity and hydrothermal stability than the reference catalyst synthesized with Cu-TEPA,diethylamine and tetraethylammonium bromide as templates.The origin of superior SCR performance of the submicron Cu-SAPO-34 catalyst was investigated by a series of characterization techniques including XRD,N₂ sorption,NMR,NH₃-TPD,H₂-TPR,UV-vis-DRS and XPS.Consequently,the structure-performance relationship of the submicron Cu-SAPO-34 catalyst was well established.The isolated Cu²⁺ions exist predominantly in submicron Cu-SAPO-34 with a plenty of Si?nOAl??n=1-3?structures.These Si coordination structures could generate a large number of Br?nsted acid sites,which could be beneficial for NO reduction.The typical cubic crystal and isolated Cu²⁺ions remain stable even after steaming at 800?for 12 h.Furthermore,the amount of isolated Cu²⁺ions increase and copper oxides decrease and redistribute after steaming.Secondly,a series of small pore molecular sieve catalysts Cu-SAPO-18 with different Si content were prepared by the combination of copper-triethylenetetramine?Cu-TETA?and N,N-diisopropylethylamine?DIPEA?as templates.Although the as-prepared catalysts exhibited similar Cu loading,they showed different NO conversions from 150?to 575?before and after hydrothermal aging treatment at 800?for 12 h.The ²⁹Si MAS NMR,NH₃-TPD and EPR results indicated that varying the proportion of Si could affect the Si coordination structures,and further affect the acid properties and Cu distributions in Cu-SAPO-18 catalysts.The amount of acid sites and isolated Cu²⁺ions firstly increases and then decreases with increasing Si content.The NO conversion results indicated that catalytic activities are jointly decided by acidity and amount of isolated Cu²⁺ions.The Cu-SAPO-18 with Si/Al+P=0.132 shows the best NH₃-SCR performance.On the other hand,the more Si?xOAl??x=1³?structures,the more strong acid sites could be generated in Cu-SAPO-18 catalysts.Furthermore,these strong acid sites are beneficial for the stabilization of isolated Cu²⁺ions,and thus the stabilities of isolated Cu²⁺ions and zeolite framework are enhanced during the harsh hydrothermal aging process.Subsequently,the discrepancies of physicochemical and catalytic properties between Cu-SSZ-13 and Fe-SSZ-13 were studied in NH₃-SCR reaction.It was found that Cu-SSZ-13 have a better low-temperature activity and broader active temperature window with NO conversion above 90%from 250?to 500?even after hydrothermal aging.Fe-SSZ-13 have a better high-temperature activity with NO conversion above 95%from350?to 600?.Additionally,Cu-SSZ-13 is more robust during hydrothermal aging treatment than Fe-SSZ-13,which may be due to the more stable of Cu²⁺ions than Fe³⁺in the catalysts.Interestingly,the NH₃ storage capacity of Cu-SSZ-13 is higher than Fe-SSZ-13 under 100?.While it is opposite when NH₃ adsorption temperature above200?.In addition,the NH₃-SCR reaction on Cu-SSZ-13 and Fe-SSZ-13 at high temperatures are mainly performed by the E-R mechanism.Finally,The influence of phosphorus on the catalytic properties,durability,sulfur resistance and kinetics of Cu-SSZ-13 for NO_x reduction by ammonia selective catalytic reduction?NH₃-SCR?were systematically investigated to obtain a deeper understanding of the deactivation of Cu-SSZ-13 in the presence of phosphorus-containing impurities in diesel exhausts.An enhanced effect on NO conversion at low phosphorus loading was found above 450?,probably due to the reduction of copper oxides which promotes NH₃oxidation at high temperatures.As the phosphorus loading reached 0.4 mmol/g_catal,the framework dealumination and decline of BET surface areas,acidity,and isolated Cu²⁺ions accounted for the decrease in activity and hydrothermal stability in the whole temperature range.Simultaneously,the activity of phosphorus impregnated samples tested in the presence of SO₂ indicates that phosphorus accelerates the deactivation of Cu-SSZ-13 in the presence of SO₂ at low temperatures,mainly attributing to the severe decline of Cu²⁺ions content and partial coverage of the acid sites and active sites by phosphate and sulfate species.Additionally,the kinetic analysis further demonstrated that phosphorus has an inhibitory effect on SCR reaction rates in the low temperature region and high phosphorus addition changes the NH₃-SCR reaction kinetic parameters.Compared to our previous findings,the Cu-SSZ-13 catalyst is more sensitive to the deactivation by phosphorous than alkali and alkaline earth.