CN116159571A - 一种三维有序大孔钴酸镧负载钯铑锰三元金属催化剂及应用 - Google Patents
一种三维有序大孔钴酸镧负载钯铑锰三元金属催化剂及应用 Download PDFInfo
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Abstract
一种三维有序大孔钴酸镧负载钯铑锰三元金属催化剂及应用,属于功能材料技术领域。制备方法为:在二苄醚中,采用油相还原法,将乙酰丙酮钯、乙酰丙酮铑和十羰基二锰热解还原,形成PdRhMn三元合金,将得到的PdRhMn纳米晶负载到三维有序大孔(3DOM)LaCoO3载体上,随后经过一步高温灼烧过程,得到PdRhMn/3DOM LaCoO3纳米催化剂。本发明所得催化剂制备过程简单,应用于天然气车尾气净化具有优异的氧化还原活性。
Description
技术领域
本发明涉及一种用于天然气车尾气净化的三效催化剂,三维有序大孔(3DOM)钴酸镧负载PdRhMn三元金属(PdRhMn/3DOM LaCoO3)催化剂及其制备方法。
背景技术
随着全球石油资源的日益短缺和油价上涨,天然气汽车(NGVs)在客运和商用汽车市场上已经得到了广泛的推广(M.I.Khan,et al.,[J].Renew.Sust.Energ.Rev.,2015,51:785-797)。截至2018年11月底,全球NGVs的数量已超过2600万辆,并保持着约20%的年均增长率,仅低于汽油和柴油汽车(D.Pan,et al.,[J].Nat.Commun.,2020,11:4588)。压缩天然气(CNG)被认为是最清洁的碳氢化合物燃料,它比汽油更便宜,产生的二氧化碳、氮氧化物含量也比柴油、汽油更低。因此,压缩天然气在针对环境污染问题和稀缺的石油资源相关问题上有较大的应用前景(D.Bounechada,et al.,Appl.Catal.B,2012,91:119-120)。NGV发动机通常作为化学计量或稀薄燃烧***运行,在化学计量***中,NOx、CO和未完全燃烧的甲烷是主要的有害污染物。然而,甲烷是一种比二氧化碳更强烈的温室气体,由于具有最高的碳氢比,所以甲烷也是烃类中最难以氧化的碳氢化合物。未完全燃烧的甲烷是NGVs尾气的主要碳氢化合物成分,因此有必要使用催化转化器从尾气中消除甲烷。
三效催化剂(TWCs)是天然气汽车排放控制的主要技术之一。目前用于天然气汽车尾气的排放控制主要使用与汽油汽车相同的三效催化剂,但其贵金属含量很高。然而,净化日益严格的废气排放限制,特别是甲烷和NOx,仍然是一个很大的挑战。因此在成本控制的前提下,NGVs尾气排放达到严格排放限值的技术是控制的一个巨大的挑战。
三效催化技术以其同时催化净化碳氢化合物、氮氧化物和一氧化碳而命名,于20世纪70年代首次应用于汽油汽车上。迄今为止,它仍是汽油汽车尾气排放控制的核心技术(M.Shelef,et al.,[J].Catal.Today,2000,62:35-50)。化学计量天然气发动机配备了三效催化剂(TWCs)转化器,用于催化净化尾气污染物:甲烷、NOx和CO(L.J.Wei,et al.,[J].Fuel Process.Technol.,2016,142:264-278)。NGVs废气排放的碳氢化合物主要是甲烷(超过90vol.%),一般来说,通过在汽油车改装的NGVs上配备的传统TWC的甲烷转化率低于15%(S.Oh,et al.,[J].J.Catal.,1991,132:287-301)。为了在发动机相同的工作条件下完成更多的甲烷净化,需要增加与传统TWC相比的3倍以上的贵金属负荷(0.003g/cm3-0.01g/cm3)(L.F.Liotta,et al.,[J].Appl.Catal.B Environ.,2006,66:217-227)。此外,甲烷和NOx之间的耦合反应比非甲烷碳氢化合物和NOx困难得多,与汽油车辆相比,这导致NOx的转化率较低。
到目前为止,含Pd的催化剂是去除NGVs中废气排放的最有效的催化剂。大多数商业公司的贵金属由Pd和少量Rh和Pt组成(M.Salaün,et al.,[J].Appl.Catal.B Environ.,2009,88:386-397),Rh被认为是最适合进行NOx还原的贵金属,它的蒸汽重整活性更强,具有更好的抗烧结性能(T.Maillet,et al.,[J].J.Catal.,2001,202:367-378)。因此,在Pd基催化剂中加入Rh有助于提高热稳定性和还原性能。Pd和Pt对CH4和CO均具有良好的氧化活性。Pd具有良好的三效催化活性,水热稳定性高于Pt,但其耐硫性较差(V.Meeyoo,etal.,[J].Appl.Catal.B Environ.,1998,16:L101-L104)。一般式为ABO3±δ的钙钛矿型氧化物比单一过渡金属氧化物具有更稳定的晶体结构、更优异的热稳定性(S.Keav,et al.,[J].Catalysts,2014,4:226-255)。其独特的自再生性能有助于抑制催化剂表面活性相的团聚和生长,保持良好的热稳定性(M.Uenishi,et al.,[J].Appl.Catal.BEnviron.,2005,57:267-273)。将贵金属掺杂入钙钛矿晶体中,具有抑制贵金属热烧结的显著优势。但是传统方法所制得的ABO3的比表面积低,不利于贵金属纳米粒子的高分散以及反应物与催化剂的充分接触,从而影响催化性能。三维有序大孔(3DOM)材料不但具有孔径尺寸单一、孔结构在三维空间内有序排列的特点,而且其孔径尺寸都在50nm以上(最大可达几个微米),可用于大分子的催化,弥补了以往小孔结构的分子筛及介孔材料难以让大分子进入空腔的缺点,因此,将ABO3制成具有较高比表面积的三维有序大孔结构,可显著改善催化性能。目前还没有文献报道过PdRhMn/3DOM LaCoO3催化剂的制备方法与其在天然气车尾气净化上的研究。
发明内容
本发明的目的是提供一种具有三维有序大孔结构的钴酸镧负载PdRhMn纳米晶催化剂及其制备方法。具体采用油相还原法制备PdRhMn三元合金,采用胶晶模板法制备三维有序大孔(3DOM)LaCoO3,采用一步高温灼烧将PdRhMn纳米粒子负载到LaCoO3上形成PdRhMn/3DOM LaCoO3。
具体包括以下步骤:
(1)制备PdRhMn三元合金纳米晶,PdRhMn三元合金纳米晶尺寸均一,分散均匀,其制备主要包括以下步骤:
称取乙酰丙酮钯、乙酰丙酮铑和十羰基二锰,加入到二苄醚中,在氮气气氛下室温搅拌溶解记为溶液A;取三正辛基膦加入到二氯苯中,加入油胺在氮气气氛下100℃保持30min,记为溶液B,每0.15mmol乙酰丙酮钯对应0.73mL三正辛基膦;将溶液B在氮气气氛中升温至200℃保持10min,此时将溶液A快速注入到溶液B中;在200℃保温30min,在氮气气氛中降至室温,用乙醇和环己烷离心分离,得到尺寸均一的PdRhMn纳米晶;将离心得到的纳米粒子分散到环己烷中保存。
按照金属钯:铑:锰的摩尔比为1:0.3:10投料。
(2)制备出平均直径约为300nm的聚甲基丙烯酸甲酯(PMMA)微球,然后采用PMMA模板法,制备出具有大孔结构的3DOM LaCoO3,制备过程如下:
分别称取La(NO3)3·6H2O和Co(NO3)2·6H2O,加入到无水甲醇中续搅拌1h;称取一定量的PMMA硬模板,加入上述溶液中浸渍约4h,之后使用布氏漏斗抽滤,除去富余溶液,在室温下干燥48h;优选每30mmol La(NO3)3·6H2O对应2-7mL无水甲醇,La(NO3)3·6H2O和Co(NO3)2·6H2O的摩尔比为1:1;
将干燥样品置于管式炉中,在氮气气氛中(50mL/min)以1℃/min的速率从室温升至300℃并在该温度下保温3h,然后降至室温,切换至空气气氛(50mL/min)以1℃/min的速率升温至300℃并在该温度下保温2h,并最终升至650℃并在该温度下保温4h,获得三维有序大孔(3DOM)LaCoO3;
(3)将得到的PdRhMn纳米晶负载到三维有序大孔LaCoO3载体上,随后经过一步高温灼烧过程,得到PdRhMn/3DOM LaCoO3纳米催化剂;具体步骤如下:
按照一定负载量计算出所需要的PdRhMn三元合金纳米晶的量,将一定量三维有序大孔LaCoO3载体加入到量取的含有PdRhMn三元合金纳米晶的环己烷溶液中;搅拌8h使PdRhMn三元合金纳米晶负载到3DOM LaCoO3载体上,离心得到负载型催化剂;将得到的催化剂装入瓷舟置于管式中炉进行焙烧,以5℃/min的速率从室温升至700-750℃,并在该温度下保持6-10h,降温得到PdRhMn/3DOM LaCoO3催化剂。
PdRhMn三元合金纳米晶负载到3DOM LaCoO3载体上,其中Pd在3DOM LaCoO3载体上的负载量为0.5wt%-1.0wt%。
本发明所得催化剂应用于甲烷和NOx的催化,具有优异的氧化还原活性。
在反应气组成为0.5vol%CH4+5vol%O2+1.12vol%CO+3800ppm H2+1250ppm NOx+N2空速为60000mL/(g h)的反应条件下,该催化剂对甲烷转化率达到90%时所需反应温度为530℃。PdRhMn/3DOM LaCoO3对NO的转化能力优于Pd/3DOM LaCoO3,在200℃下对NO转化率达到66%。
本发明具有纳米粒子粒径均一,尺寸可控,制备过程简单等特征。
本发明制备的PdRhMn/3DOM LaCoO3催化剂的贵金属在载体表面呈高分散态,且催化剂具有较好的催化活性,在催化领域具有良好的应用前景。
利用D8 ADVANCE型X射线衍射仪(XRD)、JEOL-2010型透射电子显微镜(TEM)和岛津GC-2014C气相色谱(GC)和42i-HL型高浓度氮氧化物(NO-NO2-NO)分析仪等仪器测定所得催化剂的晶体结构、粒子形貌以及对甲烷和氮氧化物的催化活性。结果表明,采用本发明的方法所制得的各样品均具有较好的结晶度,贵金属在载体表面高分散,负载型催化剂对甲烷的催化氧化活性和对氮氧化物的催化性能相比较单钯颗粒催化剂显著提高。
附图说明
图1为所制得PdRhMn/3DOM LaCoO3和Pd/3DOM LaCoO3催化剂的XRD谱图,其中曲线(a)和(b)分别为对比样品PdRhMn/3DOM LaCoO3和Pd/3DOM LaCoO3的XRD谱图;
图2为所制得PdRhMn纳米粒子的(A)-(B)TEM照片,(C)粒径分布统计;
图3为PdRhMn NPs的HAADF-STEM以及元素扫描.;
图4中(A)为所制得PdRhMn/3DOM LaCoO3和(B)Pd/3DOM LaCoO3催化剂对甲烷氧化和氮氧化物还原的催化活性曲线。
具体实施方式
下面结合实施例对本发明做进一步说明,但本发明并不限于以下实施例。
实施例1
按照金属钯:铑:锰的摩尔比为1:0.3:10投料,称取0.15mmol乙酰丙酮钯、乙酰丙酮铑和十羰基二锰,加入到5ml二苄醚中,在氮气保护下室温搅拌1h,记为溶液A;称取0.73ml三正辛基膦加入到5ml二苄醚中,加入1.084mL油胺,在氮气气氛中100℃搅拌溶解搅拌溶解,记为溶液B。将溶液B在氮气气氛中升温至200℃保持10min,此时将溶液A快速注入到溶液B中;在200℃保温30min,在氮气气氛中降至室温,用乙醇和环己烷离心分离,得到尺寸均一的PdRhMn纳米晶;将离心得到的纳米晶分散到环己烷中保存。
将30mmol La(NO3)3·6H2O和Co(NO3)2·6H2O,溶解在3mL无水甲醇中,常温下搅拌至充分溶解,随后用无水甲醇定容使总金属浓度为2mol/L,常温继续搅拌1h;称取一定量的PMMA硬模板,加入上述溶液中浸渍约4h,之后使用布氏漏斗抽滤,除去富余溶液,在室温下干燥48h;将干燥样品置于管式炉中,在氮气气氛中(50mL/min)以1℃/min的速率从室温升至300℃并在该温度下保温3h,然后降至室温,切换至空气气氛(50mL/min)以1℃/min的速率升温至300℃并在该温度下保温2h,并最终升至650℃并在该温度下保温4h,获得三维有序大孔(3DOM)LaCoO3。
将得到的PdRhMn三元纳米晶经过一步高温灼烧过程负载到载体上,得到PdRhMn/3DOM LaCoO3纳米催化剂。具体步骤如下:
按照一定负载量计算出所需要的含有PdRhMn三元纳米晶溶液的量,将一定量3DOMLaCoO3载体加入到量取的含有纳米晶的环己烷溶液。搅拌12h,离心得到负载型催化剂。将得到的催化剂装入瓷舟置于管式炉中,以5℃/min的速率从室温升至700℃,并在该温度下保持8h,降温得到PdRhMn/3DOM LaCoO3催化剂。负载后,Pd的负载量为0.5wt%。
在汽车尾气模拟气氛中(0.5vol%CH4+5vol%O2+1.12%CO+3800ppm H2+1250ppmNO+N2(平衡气)),该催化剂对甲烷的T90%(甲烷转化率达到90%时所需反应温度)为500℃。PdRhMn/3DOM LaCoO3对NO的转化能力优于Pd/3DOM LaCoO3,在200℃下对NO转化率达到66%,具有良好的催化活性。
Claims (7)
1.一种三维有序大孔钴酸镧负载钯铑锰三元金属催化剂的制备方法,其特征在于,包括以下步骤:
(1)制备PdRhMn三元合金纳米晶,PdRhMn三元合金纳米晶尺寸均一,分散均匀,其制备主要包括以下步骤:
称取乙酰丙酮钯、乙酰丙酮铑和十羰基二锰,加入到二苄醚中,在氮气气氛下室温搅拌溶解记为溶液A;取三正辛基膦加入到二氯苯中,加入油胺在氮气气氛下100℃保持30min,记为溶液B,每0.15mmol乙酰丙酮钯对应0.73mL三正辛基膦;将溶液B在氮气气氛中升温至200℃保持10min,此时将溶液A快速注入到溶液B中;在200℃保温30min,在氮气气氛中降至室温,用乙醇和环己烷离心分离,得到尺寸均一的PdRhMn纳米晶;将离心得到的纳米粒子分散到环己烷中保存;
按照金属钯:铑:锰的摩尔比为1:0.3:10投料;
(2)制备出平均直径约为300nm的聚甲基丙烯酸甲酯(PMMA)微球,然后采用PMMA模板法,制备出具有大孔结构的3DOM LaCoO3,制备过程如下:
分别称取La(NO3)3·6H2O和Co(NO3)2·6H2O,加入到无水甲醇中续搅拌1h;称取一定量的PMMA硬模板,加入上述溶液中浸渍约4h,之后使用布氏漏斗抽滤,除去富余溶液,在室温下干燥48h;
将干燥样品置于管式炉中,在氮气气氛中(50mL/min)以1℃/min的速率从室温升至300℃并在该温度下保温3h,然后降至室温,切换至空气气氛(50mL/min)以1℃/min的速率升温至300℃并在该温度下保温2h,并最终升至650℃并在该温度下保温4h,获得三维有序大孔(3DOM)LaCoO3;
(3)将得到的PdRhMn纳米晶负载到三维有序大孔LaCoO3载体上,随后经过一步高温灼烧过程,得到PdRhMn/3DOM LaCoO3纳米催化剂;具体步骤如下:
按照一定负载量计算出所需要的PdRhMn三元合金纳米晶的量,将一定量三维有序大孔LaCoO3载体加入到量取的含有PdRhMn三元合金纳米晶的环己烷溶液中;搅拌8h使PdRhMn三元合金纳米晶负载到3DOM LaCoO3载体上,离心得到负载型催化剂;将得到的催化剂装入瓷舟置于管式中炉进行焙烧,以5℃/min的速率从室温升至700-750℃,并在该温度下保持6-10h,降温得到PdRhMn/3DOM LaCoO3催化剂。
2.按照权利要求1所述的方法,其特征在于,PdRhMn三元合金纳米晶负载到3DOM LaCoO3载体上,其中Pd在3DOM LaCoO3载体上的负载量为0.5wt%-1.0wt%。
3.按照权利要求1所述的方法,其特征在于,步骤(2)优选每30mmol La(NO3)3·6H2O对应2-7mL无水甲醇,La(NO3)3·6H2O和Co(NO3)2·6H2O的摩尔比为1:1。
4.按照权利要求1-3任一项所述的方法制备得到的三维有序大孔钴酸镧负载钯铑锰三元金属催化剂。
5.按照权利要求1-3任一项所述的方法制备得到的三维有序大孔钴酸镧负载钯铑锰三元金属催化剂的应用,所得催化剂应用于甲烷和NOx的催化。
6.按照权利要求5的应用,在反应气组成为0.5vol%CH4+5vol%O2+1.12vol%CO+3800ppm H2+1250ppm NOx+N2空速为60000mL/(g h)的反应条件下,该催化剂对甲烷转化率达到90%时所需反应温度为530℃。PdRhMn/3DOM LaCoO3对NO的转化能力优于Pd/3DOMLaCoO3,在200℃下对NO转化率达到66%。
7.按照权利要求5的应用,用于天然气车尾气净化。
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