【作者】 徐鑫；

【导师】 陈培榕；

【作者基本信息】 华南理工大学 ， 环境科学与工程， 2024， 硕士

【摘要】 燃料燃烧产生的氮氧化物（NO_x）是典型的大气污染物,也是PM_2.5和近地面臭氧（O₃）污染的重要前体物。将NO氧化为NO₂可以提高NO_x减排效率。铂族金属催化剂（PGMs,包括Pt、Pd、Rh、Ru等）因具有优异的催化活性而被广泛应用于催化氧化反应中。但PGM催化剂价格昂贵,因而迫切需要寻找低成本、高效、稳定的非PGM催化剂。钙钛矿氧化物（ABO₃）具有氧化还原性能优异、水热稳定性好、成本低廉等优势,被认为是有前途的PGM催化剂替代品。钙钛矿的制备通常需要经历高温处理过程,因而其比表面积较低、低温氧化活性差等,不适合直接应用于催化消除低浓度大气污染物。针对这些挑战,本论文基于两步沉淀法得到CaMnO₃钙钛矿催化剂（CMO）,通过酸诱导表面歧化改性,调控Mn在催化剂表面的平均氧化态,制备具有高NO氧化活性的CaMnO₃钙钛矿催化剂（CMO-H）。进一步通过Pd掺杂提升CMO-H的催化活性和抗硫中毒能力。结合多种表征方法,研究了酸诱导表面歧化改性和Pd掺杂对催化剂NO氧化活性的影响机制,以及Pd掺杂提升催化剂抗硫中毒能力的机制。得到的主要结论如下:与CMO相比,硝酸诱导表面歧化改性所得CMO-H的NO氧化活性明显提升,最高NO转化率从43%提升至55%(500 ppm NO,10 vol.%O₂,N₂平衡;GHSV=120,000m L·h^-1·g^-1),与文献报道的反应活性相当。物化表征研究表明,表面歧化改性后CMO晶体结构保持不变,但其比表面积会增大,可以提供提多NO吸附位点。酸诱导表面歧化改性还会使处于钙钛矿结构A位的Ca溶出,处于B位的Mn则发生歧化反应,生成更高氧化态的Mn⁴⁺。结合催化性能评价可知,CMO-H催化剂表面Mn⁴⁺含量增加是其NO氧化活性提升的重要原因。以NO或NO+O₂为探针分子的持续升温脱附（TPD）研究进一步发现,CMO-H具有更多的NO吸附储存位点和表面活性氧物种,因而有利于NO氧化反应。使用微量Pd对CMO-H进行掺杂改性（Pd/CMO-H）不但可提升NO氧化活性,还可提升其CO氧化活性和CO/C₃H₆协同氧化活性。结合多种表征手段对比研究了Pd掺杂改性前后CMO-H的物理化学性质。结果表明,Pd掺杂未改变CMO-H催化剂的形貌,但是会导致其晶格发生畸变,并诱导CMO-H表面生成大量氧空位。这些氧空位不但可以作为活性氧物种直接参与氧化反应,还能够储存更多的硝酸盐和亚硝酸盐中间产物,从而显著促进NO催化氧化反应（最高NO转化率从55%提升至71%）。进一步模拟实际工况,对Pd掺杂前后的CMO-H催化剂进行SO₂中毒处理。结果表明,Pd掺杂可以提升催化剂的抗硫中毒能力。结合物理化学性质表征和催化性能评价可知,SO₂易和CMO-H表面的Mn⁴⁺反应生成非活性Mn SO₄物种,导致催化活性下降;Pd掺杂可降低CMO-H表面Mn⁴⁺的相对含量,减弱对SO₂的敏感性,抑制催化剂表面非活性Mn SO₄物种的形成。对NO氧化反应机制的原位DRIFTS研究表明,SO₂中毒导致CMO-H催化剂表面活性位点数量显著减少,但并不影响其催化NO氧化的反应路径。本论文研究解决了高效NO_x减排问题,有望为理性设计和定向制备廉价、高活性、高稳定性、抗SO₂中毒的复合氧化物NO氧化催化剂提供新思路。更多还原

【Abstract】 Nitrogen oxides（NO_x）from fuel combustion belong to a category of key air pollutants in the atmosphere,and are long known as the important precursor to PM_2.5 and ground-level ozone pollution.The oxidation of NO to NO₂ favors higher NO_x abatement efficiencies.Platinum-group metals（PGMs,including Pt,Pd,Rh,Ru）are widely used in catalytic oxidation reactions because of their excellent catalytic activities.For economic considerations,continuous efforts have been put in the search of low-cost non-PGM catalysts with good efficiency and stability.Perovskite oxides（ABO₃）,which have excellent redox properties,hydrothermal stability and low cost,are considered as promising alternatives to PGM-based catalysts.The pristine perovskites often have low specific surface area and lower temperature oxidation activity,and are thus not suitable for direct application in the catalytic abatement of low-concentration air pollutants.In response to these challenges,a CaMnO₃ perovskite catalyst（CMO）with special crystal structure obtained by two-step precipitation method was prepared.By acid-initiated disproportionation modification,the average oxidation state of Mn on the catalyst surface was regulated to obtain the CMO-H catalyst with a higher NO oxidation activity.Doping with a tiny amount of Pd further increased the NO oxidation activity and,surprisingly,the resistance against sulfur poisoning.Many characterization methods,were combined to study the effects of acid-initiated disproportionation modification and Pd doping on the NO oxidation activity of the CMO catalysts.The reasons for enhanced sulfur resistance by Pd doping was also discussed.The main conclusions are as follows:Compared with CMO,the NO oxidation activity of CMO-H modified by surface-disproportionation induced by nitric acid was significantly increased,and the maximum NO conversion rate was increased from 43%to 55%(500 ppm NO,10 vol.%O₂,N₂ balance;GHSV=120,000 m L·h^-1·g^-1),which was comparable to that reported in the literature.The CMO and CMO-H catalysts were characterized extensively by physicochemical characterization methods.After the acid-initiated disproportionation modification,the structure of the catalysts remained unchanged,but the specific surface area increased,which could provide more NO adsorption sites.Acid treatment can dissolve Ca at the A site and Mn at the B site to produce Mn⁴⁺with higher oxidation state.The increase of Mn⁴⁺content on CMO-H catalyst is the main reason for the improvement of NO oxidation activity.Meanwhile,temperature programmed desorption（TPD）using NO or NO+O₂ as probe molecules further showed that CMO-H had more NO adsorption storage sites and surface reactive oxygen species,which was conducive to the reaction.Doping with a tiny amount of Pd not only increased the oxidation activity of NO,but also increased the oxidation activity of CO and CO/C₃H₆ co-oxidation activity.The effects of Pd doping on catalyst structure,physicochemical properties and NO oxidation pathway were studied via a combination of characterization methods.Pd doping can result in lattice distortion and induce oxygen vacancies in CMO-H.These oxygen vacancies can not only directly participate in the oxidation reaction as active oxygen species,but also store more nitrates and nitrites,thus significantly promoting NO catalytic oxidation reaction（the maximum NO conversion rate was increased from 55%to 71%）.In simulated operating conditions,CMO-H and Pd-doped CMO-H were treated with SO₂exposure.Pd doping can improve sulfur resistance ability.A combination of physicochemical characterization and catalytic performance showed that SO₂ can easily react with Mn⁴⁺on the catalyst surface to form inactive Mn SO₄ species,resulting in a decrease in catalytic activity.Pd doping reduced the relative content of SO₂-sensitive Mn⁴⁺on surface,and thus inhibited the formation of inactive Mn SO₄ species.Meanwhile,in situ DRIFTS showed that SO₂ poisoning only reduced the number of active sites on the catalyst surface,but did not affect the NO oxidation pathways.This dissertation study solves the problem of efficient NO_xemission reduction,and is expected to provide a new idea for rational design and directional preparation of cheap,high activity,high stability,sulfur resistance composite oxide NO oxidation catalyst.更多还原