CN116673024A - 一种碱金属改性的负载型Pt基脱硝催化剂的制备及其应用 - Google Patents
一种碱金属改性的负载型Pt基脱硝催化剂的制备及其应用 Download PDFInfo
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Abstract
本发明制备了碱金属钾(K)改性的负载型Pt基催化剂,应用在CO还原NO反应中,能够同时去除尾气中CO和NO两种污染物。氧气处理激活了K对Pt‑CeO2催化剂金属载体相互作用的调控,K的给电子效应不仅可以通过改善金属载体相互作用缓解Pt基催化剂的CO中毒现象,而且降低了单金属Pt基催化剂合成的成本。合成方法结合了水热法、沉积沉淀法和浸渍法合成的优点,制备条件温和,未使用有毒反应原料,是一种环境友好的绿色合成催化剂。
Description
技术领域
本发明通过氧气处理激活了碱金属钾(K)对负载型Pt基催化剂金属-载体相互作用的调控,明显改善了催化剂在CO还原NO反应中的催化活性,属于大气污染控制技术领域。
背景技术
由于早期公众对环境生态的认识和重视程度相对匮乏,工业化和城市化促进经济快速发展的同时,导致了严重的以雾霾为代表的大气污染,PM2.5是形成雾霾的主要成份,空气中的氮氧化物(NOX)和SO2是PM2.5形成的重要前驱体污染物质,这对气候、生物多样性、社会文明、交通和公共卫生安全都构成了巨大威胁。因此,治理雾霾必须严格控制大气污染物的形成和排放。大气中的污染物主要包括NOX、粉尘、CO和SOX等,其中NOX是主要的环境污染物之一,人为活动排放的氮氧化物在数量上远低于自然过程所排放的氮氧化物含量,但是人为活动分布相对集中,危害更大,也是目前NOX污染严重的主要原因。目前NOX的排放控制技术中烟气脱硝技术应用较为广泛,脱硝效率高,是NOX控制的主要研究方向,其中,选择性催化还原(SCR)是最完善,实践性最强的方法,可解决与大气中NOX含量增加有关的一些环境问题。NH3作为还原剂通常是昂贵的,因为高投资成本、高能量需求以及化学处理要求。由于NH3-SCR工艺的缺点,碳氢化合物、H2和CO已成为SCR工艺中最有前途的替代还原剂,并且已经广泛研究了几种类型的催化剂和还原剂。CO是还原NOX的有效试剂,因为其价格比NH3低且易于获得。通常,CO共存于烟气中,因为它是在燃烧过程中形成的。因此,当CO用作还原剂时,可以期望为NOX减排过程提供一个相当实惠和简化的进料系统。
如今,氧化铈(CeO2)被广泛应用于催化科学,主要是由于Ce3+/Ce4+转变引起的氧化还原特性,氧储存能力,缺陷结构(包括氧空位),自身在许多环境重要反应中的催化活性以及与各种材料相互作用的倾向,尤其是贵金属。在金属中,Pt因其将各种类型的化学品转化为增值产品的出色能力而受到关注,这是由于其高效激活C-H和C-O键的能力所致。Pt基催化剂还具有较高的化学和水热稳定性,同时对某些元素(例如氯)的中毒表现出较低的耐受性。将氧化铈和Pt的优势结合为制备各种Pt-CeO2基催化活性复合材料提供了广泛机会。在这样的体系中,Pt通常起着活性组分的作用,而在某些应用中(例如,用于甲烷干重整(DRM)反应的Ni基催化剂),它被用作 CO 促进剂。二氧化铈通常被用作具有萤石结构的载体,而在一些复合材料中,它作为其它载体(例如,氧化铝、二氧化硅)的改性剂或负载在这样的载体上的Pt组分的促进剂。在催化剂中的Pt-CeOX界面构成了二氧化铈应用的很大一部分,包括但不限于甲醇,乙醇,氧还原反应,甲烷稳定化(包括DRM反应)和CO2,通过水煤气变换(WGS)反应产生H2,乙醇的重整,H2储存化合物的转化。
环境应用催化中涉及Pt-CeO2基催化剂的研究占有较大比重,关于Pt/CeO2复合材料和催化反应中Pt-CeO2界面的报道越来越多,这也反映了人们对Pt/CeO2复合材料的高度研究兴趣。Pt-CeO2界面的金属-载体相互作用(MSI)特征,包括强金属-载体相互作用(SMSI),也受到关注。目前,单原子催化剂是新的趋势,对Pt/CeO2催化剂的结构修饰改性可以围绕以下几个方面展开:(1)催化剂制备,包括Pt和Ce-MOF衍生的前体,和处理方法,(2)Pt相关因素:尺寸,包括单原子配方和状态,(3)CeO2相关因素:形态、表面缺陷和衍生特征,(4)催化剂组合物的改性,包括双金属颗粒的形成,(5)活性位点的性质和结构,(6)金属-载体相互作用的特征。
催化反应过程中,随着温度升高,反应物和产物中会存在破坏金属-氧化物界面间的化学作用,并直接导致催化金属物种团聚,催化性能下降,这一问题对于单原子分散金属催化剂可能会更为严重。因此,如何获得稳定高效的高分散金属催化剂仍然是多相催化领域的一个重要挑战。自1845年首次报道以来,碱金属在多相催化领域已成为“万能”助剂,用于改性金属催化剂,以提升系列重要催化反应(例如氧化、脱氢、加氢、费托、三效催化、合成气转化、电解水等)的性能。长期以来,碱金属的电子助剂机制主要有两个观点。一种观点认为碱金属作为电子供体,通过与催化活性金属组分间的电子转移作用,改变反应物和中间体在表面的吸附和反应行为,从而提升催化性能。但是另一种流行的观点是碱金属离子通过其周围的氧原子与活性金属位点间接相互作用。这种相互作用有助于稳定小金属纳米颗粒甚至原子级分散的金属位点,防止它们在催化反应条件下聚集。
基于上述原因,研究和开发一种通过改变金属-载体相互作用进而改善CeO2负载Pt基CO中毒问题的催化剂用于CO还原NO反应具有重要的经济和应用价值。
发明内容
本发明的目的是提供一种金属元素,结合氧气处理条件,改善CeO2负载Pt基催化剂的金属载体相互作用,缓解催化剂CO中毒问题,提高其在CO还原NO反应中的催化活性。包括以下制备步骤:
(1)称取一定量硝酸铈(Ce(NO3)3·6H2O)和一定量的聚乙烯吡咯烷酮(PVP)溶于一定量的乙二醇和去离子水中,搅拌直至完全溶解,记为溶液I;
(2)将得到的澄清溶液转移至聚四氟乙烯内衬的不锈钢反应釜中密封,置于烘箱内进行水热反应。当高压反应釜冷却至室温后,收集淡紫色产物并依次用去离子水和无水乙醇洗涂,将所得沉淀在40~100°C烘干,即得CeO2。
(3)量取一定量的H2PtCl6·6H2O溶解在水中,搅拌,滴加1 mol/L的NaOH溶液,调节pH=9~10,记为溶液II;称取一定量的CeO2粉末,添加一定量的水,通过超声使载体分散均匀,将混合物滴加到溶液II中,记为溶液III,置于60~80°C恒温搅拌2 h;
(4)对搅拌后的溶液进行洗涤,收集橘黄色沉淀,70~110°C烘干,300~400°C煅烧,即得CeO2负载Pt基纳米催化剂,记为Pt-Ce。
(5)称取一定量的硝酸钾(KNO3)置于水中搅拌溶解,记为溶液Ⅳ;称取一定量的Pt-Ce,加入到溶液Ⅳ中,室温搅拌一段时间,记为溶液Ⅴ;
(6)将溶液Ⅴ在50~80℃下搅拌蒸干,收集样品在60~110°C烘干,即得碱金属K改性的CeO2负载Pt基纳米催化剂,记为K/Pt-Ce;
本发明与现有技术相比较,具有如下显而易见的实质性特点。
1、本发明催化剂的合成结合了水热法、沉积沉淀法和浸渍法合成的优点,得到碱金属K改性的Pt-Ce基催化剂。
2、本发明合成的Pt-Ce基催化剂均为纳米球状,形貌规整,分布均匀。
3、本发明合成的K/Pt-Ce催化剂能够同时去除尾气中CO和NO两种污染物,K的引入不仅可以通过改善金属载体相互作用缓解Pt基催化剂的CO中毒现象,而且降低了单金属Pt基催化剂合成的成本。
4、本发明方法简单易行,制备条件温和,原料廉价易得,且未使用有毒反应原料,是一种环境友好的绿色合成催化剂。
附图说明
图1为本发明制备的Pt-Ce和K/Pt-Ce催化剂的X射线衍射图谱。
图2-图3为实施例2制备的Pt-Ce和K/Pt-Ce催化剂的高分辨透射电子显微镜图。
图4为本发明制备的Pt-Ce和K/Pt-Ce催化剂的H2程序升温还原图谱。
图5为本发明制备的Pt-Ce和K/Pt-Ce催化剂的O2程序升温脱附图谱。
图6-图7为本发明制备的Pt-Ce和K/Pt-Ce催化剂的CO还原NO催化图谱。
具体实施方式
下面结合具体实施例对本发明作进一步说明,本发明包括但不限于下面的实施例。
实例一:
(1)称取1.5 g硝酸铈(Ce(NO3)3·6H2O)和0.6 g聚乙烯吡咯烷酮(PVP)溶于一定量的乙二醇和去离子水中,搅拌直至完全溶解,记为溶液I;
(2)将得到的澄清溶液转移至聚四氟乙烯内衬的不锈钢反应釜中密封,置于烘箱内进行水热反应。当高压反应釜冷却至室温后,收集淡紫色产物并依次用去离子水和无水乙醇洗涂,将所得沉淀在50°C烘干,即得CeO2。
(3)量取0.8mL的H2PtCl6·6H2O溶解在水中,搅拌,滴加1 mol/L的NaOH溶液,调节pH=9,记为溶液II;称取0.3 g的CeO2粉末,添加一定量的水,通过超声使载体分散均匀,将混合物滴加到溶液II中,记为溶液III,置于70°C恒温搅拌2 h;
(4)对搅拌后的溶液进行洗涤,收集橘黄色沉淀,70°C烘干,300°C煅烧,即得CeO2负载Pt基纳米催化剂,记为Pt-Ce。
对本实施例制得的样品进行X射线衍射(XRD)和高分辨透射电子显微镜(HRTEM)测试,其结果分别见图1和图2。Pt的加入没有使CeO2晶相结构发生变化,所有样品的主晶相均为萤石结构的CeO2特征峰(JCPDS NO.34-0394),分别对应(111)、(200)、(220)、(311)、(222)、(400)、(331)、(420)晶面。没有观察到金属态Pt以及PtOx的衍射峰,这可能是由于活性组分Pt的含量少,且在CeO2表面高度分散,而导致低于XRD仪器的检测限的缘故。从电镜图2中可以看到催化剂呈现出规整的空心球状,同时可以看到负载的Pt颗粒。综上说明成功制备出了以CeO2纳米球为载体,负载贵金属Pt的催化剂。
实例二:
(1)量取1.6mL的H2PtCl6·6H2O溶解在水中,搅拌,滴加1 mol/L的NaOH溶液,调节pH=9.5,记为溶液II;称取0.6 g的CeO2粉末,添加一定量的水,通过超声使载体分散均匀,将混合物滴加到溶液II中,记为溶液III,置于70℃恒温搅拌2 h;
(2)对搅拌后的溶液进行洗涤,收集橘黄色沉淀,80°C烘干,300°C煅烧,即得CeO2负载Pt基纳米催化剂,记为Pt-Ce。
(3)称取0.37 g的硝酸钾(KNO3)置于水中搅拌溶解,记为溶液Ⅳ;称取1.0 g的Pt-Ce,加入到溶液Ⅳ中,室温搅拌一段时间,记为溶液Ⅴ;
(4)将溶液Ⅴ在70°C下搅拌蒸干,收集样品进行烘干,即得碱金属K改性的CeO2负载Pt基纳米催化剂,记为K/Pt-Ce;
对本实例制得的样品进行X射线衍射和高分辨透射电子显微镜测试,其结果分别见图1和图3。从图1XRD谱图中可以看出,Pt和K的加入没有使CeO2晶相结构发生变化,所有样品的主晶相均为萤石结构的CeO2特征峰(JCPDS NO.34-0394),同时,可以看到K2O的衍射峰,说明K成功负载,且相比于Pt-Ce催化剂,K的加入提高了催化剂的结晶度。电镜结果图3显示,K/Pt-Ce催化剂呈现出片状结构,说明K对催化剂的形貌产生了一定的影响。
实例三:
使用H2程序升温还原对Pt-Ce和K/Pt-Ce催化剂的氧化还原能力进行测试。首先,将50mg 40-80目催化剂在50ml/min的N2气氛下500°C处理0.5 h,随后冷却至室温。最后,通入5% H2,按10 °C/min的加热速率增加温度至700°C以完成还原操作。图4为测试结果,两种催化剂出现了明显的H2消耗峰,表现为PtOx、Pt-Ce界面处氧物种和CeO2表面氧的还原。对比出峰温度,负载K的催化剂降低了100°C左右,说明K的引入减弱了Pt-O和Ce-O键的键能,提高了催化剂的氧化还原能力。
实例四:
为进一步探索CO还原NO中的特定氧物种,考察催化剂表面氧的迁移率和吸附氧的脱附过程,通过O2程序升温脱附对Pt-Ce和K/Pt-Ce催化剂进行测试,首先,将100 mg 40-80目催化剂在50 ml/min的N2气氛下300°C处理1 h,随后冷却至室温。随后,通入10% O2,室温吸附1h后,N2吹扫0.5 h,按10 °C/ min的加热速率增加温度至700°C以完成脱附操作。结果如图5所示。催化剂氧气脱附过程分为2个阶段:低于400°C的信号峰O α 是由于催化剂中化学吸附氧物种的脱附以及表面PtOx的分解,与反应产生的氧空位有关,负载K后信号峰对应的温度更低,说明K的电子效应促进了氧物种的分解脱附,有利于提高催化反应活性。而温度高于400°C的信号峰O β 对应于CeO2中晶格氧的脱附。
实例五:
使用CO+NO反应程序升温测试Pt-Ce和K/Pt-Ce催化CO还原NO的能力。首先将50 mg的40-80目样品放置在催化小样评价系统中,通入1333ppm CO和1333ppm NO,以4°C /min的升温速率程序升温至600°C。使用配备有氢火焰离子化检测器(FID)的在线气相色谱仪(GC-2080)和氮氧化物分析仪(MAL3690)检测反应后的气体。根据反应前后混合气体中的CO和NO浓度计算CO和NO转化率,结果曲线图分别为图6和图7。相比Pt-Ce催化剂,K/Pt-Ce催化CO还原NO的能力从起燃温度,转化率50%和90%对应的温度来看,都有明显改善,与实例三四的结果有较好的一致性。
Claims (9)
1.一种碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:所述催化剂以CeO2为载体,负载活性组分Pt,活性组分的前驱体为氯铂酸(H2PtCl6·6H2O),Pt含量以重量计为载体质量的2%以下,碱金属K以硝酸钾(KNO3)为前驱体,含量以重量计为载体质量的6~8%,最后对催化剂进行氧气预处理,激活K对催化剂金属-载体相互作用的调控。
2.一种如权利要求1所述的碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:所述的催化剂载体CeO2的制备方法采用水热法,包括以下步骤:
(1)称取1.0 g硝酸铈(Ce(NO3)3·6H2O)和一定量的聚乙烯吡咯烷酮(polyvinylpyrrolidone,PVP)溶于一定量的乙二醇和去离子水中,搅拌,直至完全溶解,记为溶液I;
(2)将得到的澄清溶液转移至聚四氟乙烯内衬的不锈钢反应釜中密封,置于烘箱内160℃恒温8 h,进行水热反应;
(3)当高压反应釜冷却至室温时,收集淡紫色产物并依次用去离子水和无水乙醇洗涂;
(4)洗涤后将所得沉淀在干燥箱中60℃烘干过夜,即得CeO2载体。
3.根据权利要求2所述的碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:制备CeO2载体过程中所用的溶剂同时有有机溶剂乙二醇和去离子水,有机溶剂乙二醇的量为20~30 mL,去离子水的量为2~5 mL。
4.一种如权利要求1所述的碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:所述的催化剂活性组分Pt的负载方式采用沉积沉淀法,包括以下步骤:
(1)量取一定量的H2PtCl6·6H2O溶解在水中,搅拌,滴加1 mol/L的NaOH溶液,调节pH=9~10,记为溶液II;
(2)称取一定量的CeO2粉末,添加一定量的水,通过超声使载体分散均匀,将混合物滴加到溶液II中,记为溶液III;
(3)将溶液III在70℃恒温搅拌2 h;
(4)对搅拌后的溶液进行洗涤,收集淡黄色样品烘干,并于煅烧,即得负载型Pt基纳米催化剂;
(5)负载贵金属Pt的过程所用水均为超纯水。
5.根据权利要求4所述的碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:在所述步骤(1)中,H2PtCl6·6H2O和水的体积比为2:75。
6.根据权利要求4所述的碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:在所述步骤(2)中,载体CeO2的量为0.3 g,水的量为5 mL。
7.根据权利要求4所述的碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:在所述步骤(5)中,清洗方法和次数为用超纯水离心洗涤10次;烘干温度为100~120℃,烘干时间为10~12 h,煅烧温度为300℃,煅烧时间为3h。
8.一种如权利要求1所述的碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:所述的催化剂碱金属K改性的方法为过量浸渍法,包括以下步骤:
(1)称取0.15~0.2 g的硝酸钾(KNO3)置于水中搅拌溶解,记为溶液Ⅳ;
(2)称取0.5 g负载型Pt基催化剂,加入到溶液Ⅳ中,室温搅拌一段时间,记为溶液Ⅴ;
(3)将溶液Ⅴ在80℃下搅拌蒸干,收集样品进行烘干处理,即得碱金属K改性的负载型Pt基纳米催化剂。
9.根据权利要求1所述的碱金属改性的负载型Pt基脱硝催化剂的制备方法,其特征在于:碱金属K的加入可以提高负载型Pt基催化剂氧化处理后CO还原NO的能力。
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