【作者】 朱林；

【导师】 仲兆平；

【作者基本信息】 东南大学 ， 热能工程， 2018， 博士

【摘要】 低温选择性催化剂还原（SCR）脱硝技术在降低固定源燃烧烟气氮氧化物排放方面具有广阔的应用前景。作为该技术的核心环节,低温SCR脱硝催化剂仍面临低温活性及抗硫性能差的问题,制约其工业化应用。本文从新型过渡金属氧化物催化剂开发和钒基脱硝催化剂改性两条路径出发,对过渡金属氧化物低温脱硝性能进行系统分析,筛选出适合工业应用环境的低温脱硝催化剂,并借助密度泛函（DFT）理论计算从分子层面对催化剂表面脱硝反应过程及H₂O、SO₂失活原因进行深入分析。首先对比了常规新型过渡金属元素为活性组分的金属氧化物及硫酸盐催化剂脱硝性能。相同活性元素摩尔含量情况下,Mn、Ce金属氧化物催化剂表现出优异的中低温活性,但对应的硫酸盐催化剂只在较窄的高温区间具有一定的脱硝活性。研究认为硫酸根对活性元素氧化还原性能的抑制是导致催化剂低温活性减弱的主要原因。相比之下,硫酸根存在显著增加了 Cu、Fe基催化剂的表面酸性,但表面吸附NH₃组分的反应活性并未得到增强。催化剂表面宏观硫酸铵盐沉积行为可能源自于表面活性组分硫酸盐化后吸附的还原剂组分未能及时被反应消耗。综合对比认为,铁基催化剂在中高温区域具有优异的脱硝性能。从脱硝机理认识出发对铁氧化物催化剂的改性研究中,通过引入Cu、Co得到的负载型Cu-Fe/TiO₂、Co-Fe/TiO₂催化剂及体相掺杂型CuFeO_x与CoFeO_x复合氧化物催化剂低温脱硝活性均得到有效提升。负载改性铁基催化剂活性受高浓度水蒸汽影响显著,Cu-Fe/TiO₂催化剂脱硝性能150～300℃温度范围都受到抑制,Co-Fe/TiO₂催化剂温度窗口向高温区域迁移。催化剂表面吸附物种热稳定性及自身氧化还原性能的差异导致了有无水蒸汽环境下不同的SCR反应性能。SO₂接触会造成体相掺杂催化剂低温活性的不可逆损失,由表征分析推测活性组分硫酸盐化是造成催化剂SO₂中毒的主要原因。新型过渡金属氧化物脱硝催化剂虽在洁净气氛下具有优异的低温脱硝活性,但H₂O和SO₂引起的失活与中毒仍然是限制其实践应用的关键障碍。钒基催化剂的改性研究发现,助剂MoO₃和Nb₂O₅的添加均可以有效提高钒基催化剂低温脱硝性能。钒氧化物分散状态的改善与活性位点反应性能的提升是改性催化剂性能提升的主要因素。抗中毒性能研究表明,H₂O、NH₃与SO₂对改性钒基催化剂的失活具有显著的协同加速效应。应用温度低于225℃时,失活催化剂表面观察到了硫酸铵盐类物质的宏观沉积。表面硫酸铵盐的反应及热分解特性显示400℃的热处理可使失活催化剂活性完全恢复。相比于颗粒催化剂,成型样品H₂O与SO₂环境下的失活速率变缓,225℃时3V6Nb/WTi蜂窝催化剂可在H₂O、SO₂环境下长时间保持稳定的脱硝效率。钒基催化剂理论计算结果显示:还原剂在表面L酸位点的吸附较弱,B酸位点的吸附更为稳定。不同活性位点反应途径的对比分析表明L酸位催化途径最大能垒较高,达到57.83kcal/mol,而B酸位催化过程的最大反应能垒较低,约44kcal/mol。研究发现由于表面情况的差异,B酸位可能有不同的反应途径。计算结果从分子层面论证了B酸位点对脱硝反应的重要性。同时,理论计算研究认为高浓度的H₂O在催化剂表面的竞争吸附导致的对活性位点占据、表面空间压缩是造成含水环境下催化剂活性下降的原因。SO₂在催化剂表面的单独吸附很弱,而H₂O、NH₃与SO₂共存时通过气相催化水解生成的亚硫酸盐团簇与催化剂表面V=O基团具有较强的结合能力,研究推测该水解过程可能是导致钒基催化剂H₂O和SO₂环境下加速失活的主要原因。更多还原

【Abstract】 Low-temperature selective catalytic reduction（SCR）of NO_x with NH₃ is new technology with the broad application prospects in reducing the NO_x emission of stationary sources.As the key element of this technology,the low-temperature SCR catalysts still face some obstacles such as the unsatisfactory low-temperature activity and SO₂ resistance restricting its real application.The system investigations of transition metal oxide catalysts low temperature SCR performance were performed in this paper,combining two research paths including the development of novel transition metal oxides catalysts and the modification of traditional V-W-Ti catalysts,in order to obtain the practical catalyst under the real industrial condition.The density function theory（DFT）calculations were used to deepen the understanding of surface reaction process and the deactivation reasons of H₂O and SO₂ in the molecular level.The SCR activity and physical-chemical characterstics of different transition metal oxide and sulfate catalysts were compared firstly.With the same molar content of active elements,the oxide catalysts of Mn and Ce exhibited the excellent SCR activity at the low and medium temperatures,while the corresponding sulfate catalysts only showed some deNO_x effiency in the narrow high temperature range.The inhibition redox capacity of active elements by the existence of surface sulfate radical was concluded to be the main reason for the lost of low temperature activity.The surface sulfate radical increased the acidities of Fe/Cu catalysts,while the low-temperature reactivity of adsorbed NH₃ species was inhibited.The macroscopic deposition of ammonium sulfate species might resulte from the fact that the ammonia species adsorbed on the sulfated active sites fail to be consumed by the SCR reaction in time at low temperatures.In comparison,the influence of sulfate radical on Fe/Cu based samples was weaker.Fe based catalysts showed the good SCR activity in the medium temperature range.The modifications of iron oxide catalysts were carried out by introducing Cu and Co elements according to the reported reaction mechanism.The low-temperature SCR activities of the novel supported catalysts Cu-Fe/TiO₂,Co-Fe/TiO₂ and bulk phase doping catalysts CuFeO_x,CoFeO_x were all improved distinctly.However,the performance of supported catalysts was affected by high concentration water vapor.The SCR activity of Cu-Fe/TiO₂ between 150℃ and 300℃ was inhibited seriously and the temperature window for Co-Fe/TiO₂ shifted towards the higher temperature.The different thermal stability of surface adsorbed species and redox capacities of catalysts materials leaded to the diverse SCR performance under the dry and wet conditions.The contact of SO₂ resulted in the deactivation of bulk phase doping catalysts at low temperatures irreversibly.The deep research concluded that the sulfation of active species was the main reason accounting for the catalysts poisoning.Even though these novel transition metal oxide catalysts own the excellent SCR activity at low temperatures,the deactivation and poisoning caused by H₂O and SO₂ are still the obvisou obstacle for their real application.The study of vanadium based catalysts found that the modification by MoO₃ and Nb₂O₅ could improve the SCR activity of vanadium based catalysts at low temperatures.The adjustment of vanadium oxide dispersed state and the enhancement of active species reactivity accouted for the performance elevation.The synergy effect of H₂O,NH₃ and SO₂ on accelerating the catalyst deactivation was found during the SO₂ resistance test.The deposition of ammonium sulfate species on deactivated catalysts surface was observed below 225℃.The decomposition and reactivity investigation of surface ammonium sulfate species indicated that the thermal treatment at 400℃ could regenerate the deactivated catalysts totally.Compared with the particle sample,the H₂O and SO₂ deactivation rate of monolith catalyst decreased.3V6Nb/WTi honeycombed catalyst showed the stable NO_x conversion during the long time test at 225 ℃ in the presence of H₂O and SO₂.The density functional theory（DFT）calculation of vanadium based catalyst showed the adsorption of NH₃ on Br（?）nsted acid sites was more stable than that on Lewis acid sites.Similarly,the energy barrier of Br（?）nsted acid site reaction paths was smaller than that of Lewis acid site reaction path.Due to the diversity of surface condtions,there might be different Br（?）nsted acid site reaction paths.The calculation result proved the importance of Br（?）nsted acid sites for the SCR reaction of vanadium based catalysts.The occupancy of active species and surface reaction space by competitive adsorption of high concentration water vapor could explain the activity decline phonemonen under the wet condition.The adsorption of SO₂ alone on the catalyst surface was very weak.But the sulfurous acid salt clusters formed through gas phase catalytic reaction with the coexistence of NH₃,H₂O and SO₂ have strong binding capacity with the surface V=O structure.This behavior was deduced to lead to the synergy deactivation of H₂O and SO₂ for vanadium based catalysts.更多还原