Preparation Of Modified Cu-SAPO-34 Molecular Sieve And Study On Selective Catalytic Reduction Of NO_x By Ammonia

Large amounts of nitrogen oxides（NO_x）are present in the exhaust of fuel vehicles.Its excessive emission will lead to acid rain,photochemical smog,destruction of the ozone layer in the atmosphere,and also cause serious damage to animals and plants.With the increasing number of fuel vehicles year by year,a large number of NO_x emissions lead to increasingly serious environmental pollution.Therefore,controlling the NO_x contents in motor vehicle exhaust has become an urgent issue.Currently,NH₃-SCR technology is widely used for the removal of nitrogen oxides,where the most important factor is the SCR catalyst.Compared with the early vanadium based,noble metal,metal oxide and other molecular sieve catalysts,Cu-SAPO-34 molecular sieve with rhombic zeolite（CHA）configuration shows high catalytic performance,excellent hydrothermal durability and resistance to sulfide toxicity for SCR process,so it has attracted the interest of researchers in the research of new SCR catalysts.In order to reduce the preparation cost of the catalyst,shorten the synthesis cycle,improve its low-temperature activity and broaden the temperature window,the following studies were carried out:（1）A low cost and efficient Cu-SAPO-34 molecular sieve catalyst was prepared by a one-step hydrothermal method using a cheap commercial morpholine as the template and a new copper amine complex Cu-TEPA as the Cu source and co template.The effects of copper salt,template and aging process on the catalytic activity of Cu-SAPO-34 zeolite were investigated.XRD,XPS,BET,ICP,NH₃-TPD and H₂-TPR were used to analyze the bulk structure,surface composition,phase structure and acidity of Cu-SAPO-34 molecular sieve catalyst.The preparation process of Cu-SAPO-34 molecular sieve was designed reasonably to reduce the preparation cost and improve the SCR performance.In Cu-SAPO-34 zeolite,the amount of Cu O crystal particle is affected by Si content.With the decrease of Si ratio,the number of acidic sites and Cu O crystal particle increases gradually,which has a great impact on the catalytic performance of the catalyst.The catalyst with a silicon content of 0.2 has a wide temperature activity window（150–350 ^oC）at the three space velocities tested(40,000,80,000,and 120,000 h^-1).At low temperature and space velocity of 40,000 h^-1,the NO_xconversion of Si/Al-0.2 zeolite catalyst at 32.72–99.09%,which is 5.5 times higher than the activity of commercial V-based catalyst.The catalysts with Si content molar ratios of 0.2 and0.5 showed promising SCR activity at low temperatures at 80,000 and 120,000 h^-1 space velocities.Low Si/Al ratio is beneficial to improve the catalytic activity of SCR at low temperature.With the increase of space velocity,the SCR activity of Cu-SAPO-34 at medium and low temperature decreases,but the SCR activity at high temperature does not change significantly.With the further increase of air velocity,the medium and low temperature SCR activity of Cu-SAPO-34 decreased,while the high temperature activity did no apparent change.The lower Si/Al ratio is to some extent favorable to the improvement of low-temperature SCR catalytic activity.（2）In order to further enhance the medium and low temperature activity of the molecular sieve catalysts,bimetallic CeCu/SAPO-34 molecular sieve catalysts were synthesized by a one-step hydrothermal method.Using cerium nitrate as cerium source,doped with different content of cerium ions,adding 0.02 wt.%cerium nitrate,CeCu/SAPO-34 molecular sieve catalyst has the best catalytic activity at low temperature（150 ^oC）,the NO_x removal rate can reach more than 68%,the NO_x removal rate can reach more than 90%at 200 ^oC,and has the highest NO_x conversion rate in the whole temperature activity window;At the same time,the CeCu/SAPO-34 molecular sieve catalysts with the addition of Ce species were characterized,and the analytical results showed that the samples still maintained better crystallinity after the addition of Ce species and exhibited a regular orthotetrahedron structure.The introduction of Ce species promoted the formation of abundant acid centers,and promoted the high dispersion of Cu O crystal particle.As an effective oxidation additive,Ce species enhances the redox cycle,which is beneficial to NH₃-SCR reaction.