摘要
The effect of the modification of an alumina support with chloride on the structure and the catalytic performance of Ag/Al_(2)O_(3)catalysts(SA)was investigated for the selective catalytic reduction(SCR)of NO using C_(3)H_(6)or H_(2)/C_(3)H_(6)as reductants.The Ag/Al_(2)O_(3)catalyst and Cl^(–)-modified Ag/Al_(2)O_(3)catalysts(SA-Cl)were prepared by a conventional impregnation method and characterized by X-ray diffraction,Brunauer-Emmett-Teller isotherm analysis,electron probe microanalysis,transmission electron microscopy,UV-Vis diffuse reflectance spectroscopy,X-ray photoelectron spectroscopy,and hydrogen temperature-programmed reduction.The catalytic activities in the C3H6-SCR and H_(2)/C3H6-SCR reactions were evaluated,and the reaction mechanism was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy and synchrotron vacuum ultraviolet photoionization mass spectroscopy(SVUV-PIMS).We found that Cl^(-)modification of the alumina-supported Ag/Al_(2)O_(3)catalysts facilitated the formation of oxidized silver species(Ag_(n)^(ᵟ+))that catalyze the moderate-temperature oxidation of hydrocarbons into partial oxidation products(mainly acetate species)capable of participating in the SCR reaction.The low-temperature promoting effect of H_(2)on the C3H6-SCR("hydrogen effect")was found to originate from the enhanced decomposition of strongly adsorbed nitrates on the catalyst surface and the conversion of these adsorbed species to–NCO and–CN species.This"H_(2)effect"occurs in the presence of Ag_(n)^(ᵟ+)species rather than the metallic Ag^(0)species.A gaseous intermediate,acrylonitrile(CH_(2)CHCN),was also identified in the H_(2)/C3H6-SCR reaction using SVUV-PIMS.These findings provide novel insights in the structure-activity relationship and reaction mechanisms of the SA-catalyzed HC-SCR reaction of NO.
以烯烃为还原剂的NO_(x)选择性催化还原(HC-SCR)是重要的环境催化反应之一.Ag/Al_(2)O_(3)催化剂(SA)因在HC-SCR反应中表现高活性、高N_(2)选择性及中等H_(2)O和SO_(2)耐受性等优点,而被广泛研究.SA催化剂中存在不同的Ag物种,包括孤立Ag^(+)离子,带部分正电荷Ag_(n)^(δ+)团簇和金属态Ag_(n)^(0)团簇.文献研究结果表明,SA催化剂中Ag_(n)^(δ+)团簇是催化HC-SCR反应的活性Ag物种,而Ag物种类型与Ag的负载量密切相关.因此文献中研究SA催化HC-SCR反应的结构-性能关系主要是通过改变Ag负载量来开展的,最优Ag负载量约为1%~2%.本文以Cl^(–)改性的γ-Al_(2)O_(3)作为载体,采用传统的浸渍法制备了Ag/Al_(2)O_(3)-Cl催化剂(SA-Cl),通过XRD、TEM、H_(2)-TPR、UV-Vis DRS以及XPS对催化剂进行了结构表征,并结合C_(3)H_(6)-SCR和H_(2)/C_(3)H_(6)-SCR活性测试,建立催化剂结构-催化性能关系;同时利用原位傅里叶变换红外光谱(DRIFTS)和在线同步辐射单光子电离质谱(SVUV-PIMS)研究了SA催化HC-SCR的反应机理.结构表征结果表明,在SA催化剂中,Ag负载量的提高主要是增加了Ag_(n)^(0)物种,而在SA-Cl催化剂中,Ag负载量的提高主要是增加了Ag_(n)^(δ+)物种,因此Cl‒改性能促进SA催化剂中Ag_(n)^(δ+)物种的形成.活性测试结果表明,在相同Ag负载量下,SA-Cl催化剂表现出比SA催化剂更好的HC-SCR催化性能.Cl‒改性对SA催化剂中Ag物种的调控作用和HC-SCR催化性能的促进作用随Ag负载量的增加变得更为明显.原位DRIFTS结果表明,γ-Al_(2)O_(3)载体(Al位点)是NO氧化形成硝酸盐物种的主要活性位点;Ag_(n)^(δ+)物种催化丙烯适度氧化主要生成乙酸盐类物种,可以还原表面硝酸盐物种;而Ag_(n)^(0)催化丙烯过度氧化主要生成羧酸盐类物种,进而生成CO_(2),不能还原表面硝酸盐物种.由此可见,Ag_(n)^(δ+)是催化HC-SCR反应的活性Ag物种,而Ag_(n)^(0)是催化烃类完全氧化反应的活性Ag物种;Cl^(–)改性能有效促进Ag_(n)^(δ+)的形成,进而提高HC-SCR催化反应活性.在线SVUV-PIMS结果检测到H_(2)/C_(3)H_(6)-SCR反应中存在气态中间物种丙烯腈(CH_(2)=CHCN).–CN和–NCO物种被认为是HC-SCR反应的关键中间物种,能够直接与气相NO+O_(2)反应生成N_(2).因此,CH_(2)=CHCN的存在说明HC-SCR反应涉及到气相反应机理.基于SA和SA-Cl催化剂,进一步研究了H_(2)对C_(3)H_(6)-SCR低温活性的促进作用.结果表明,H_(2)的促进作用是通过作用于Ag_(n)^(δ+)物种,而不是通过Ag_(n)^(0)物种来实现的;H_(2)的引入有利于低温下强吸附硝酸盐物种的脱附或分解以及中间体向–NCO和–CN物种的转化,从而提高HC-SCR低温催化活性.综上,基于Cl^(–)改性的Ag/Al_(2)O_(3)-Cl催化剂,本文成功证实了Ag_(n)^(δ+)物种是催化HC-SCR反应的活性Ag物种,并结合原位DRIFTS在线SVUV-PIMS谱分别鉴定了催化反应表面中间物种和气相中间物种.这些结果加深了对SA催化HC-SCR反应构-效关系和反应机理的基础理解.
基金
国家自然科学基金(21703227)
中国科学技术大学(KY2060000176)
中国科学院
教育部长江学者奖励计划。
