CN107010674A - α-Fe2O3纳米棒和Au/α-Fe2O3催化剂及其合成和应用 - Google Patents
α-Fe2O3纳米棒和Au/α-Fe2O3催化剂及其合成和应用 Download PDFInfo
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Abstract
本发明公开了α‑Fe2O3纳米棒和Au/α‑Fe2O3催化剂及其合成和应用。本发明α‑Fe2O3维持多孔纳米棒状结构,Au粒子的平均尺寸从2.2nm增加到8.6nm。同时,α‑Fe2O3纳米棒状结构还可调变Au纳米粒子的形貌,经300,400,500℃焙烧后的Au/α‑Fe2O3催化剂上Au纳米粒子的形貌分别为二维薄层状、三维截角八面体和片状截角八面体,体现出α‑Fe2O3棒状结构在分散和稳定Au纳米粒子结构上的独特性。所制备的Au/α‑Fe2O3催化剂可在室温下催化CO氧化,其反应速率达0.8047molCO/gAuh。
Description
技术领域
本发明涉及一种多孔α-Fe2O3纳米棒及其合成方法。
本发明涉及一种α-Fe2O3负载Au纳米粒子催化剂及调控Au/α-Fe2O3催化剂上Au纳米粒子形貌的的合成方法。
本发明涉及一种Au/α-Fe2O3催化一氧化碳室温氧化反应的应用。
背景技术
α-Fe2O3具有优异的氧化还原性能和晶相稳定性,被广泛作为负载金属纳米粒子的催化剂载体。比如,Au/α-Fe2O3催化剂通过金属与载体的相互作用呈现出优异的催化CO氧化反应性能[Chem.Lett.,1987,16,405;J.Catal.,1989,115,301;Science,2008,321,1331;Catal.Sci.Technol.,2013,3,2881]。大量研究表明α-Fe2O3的形貌和尺寸是影响Au/α-Fe2O3催化剂性能的关键因素之一;如G.H.Wang等将Au纳米粒子负载在不同尺寸和形貌的α-Fe2O3纳米材料上,发现缺陷位较多的纳米纺锤体α-Fe2O3所制备的Au/α-Fe2O3催化CO氧化的活性远高于菱形α-Fe2O3纳米粒子负载的金催化剂[Appl.Catal.A,2009,364,42]。A.Gedanken等利用含有1-5nm孔状结构的α-Fe2O3纳米棒负载金纳米粒子时,发现其CO氧化的活性高于以球形纳米粒子负载的金催化剂,其原因在于这种孔状的α-Fe2O3可以很好的分散和稳定1-5nm的金纳米粒子[Chem.Mater.,2007,19,4776]。影响Au/α-Fe2O3催化剂性能的另一个关键因素是Au纳米粒子的形貌。如A.A.Herzing等发现Au/α-Fe2O3催化CO氧化的活性位为具有双层结构的Au纳米簇,而单层结构的Au纳米簇催化CO氧化的活性则较差[Science,2008,321,1331]。但由于α-Fe2O3载体尺寸和形貌的不均一,Au活性物种的比例小于20%,导致Au原子利用效率较低。到目前为止,在调控α-Fe2O3载体上Au物种形貌方面仍然缺乏有效的技术路线。
此外,Au/α-Fe2O3催化剂在焙烧处理过程中,普遍存在着Au纳米粒子易烧结变大,导致催化剂性能的大幅度下降。如J.Hua等人利用共沉淀法制备的Au/α-Fe2O3催化剂于350-550℃焙烧2h后,Au纳米粒子尺寸从5.7nm增长到8.5nm,而且其尺寸分布范围明显宽化(3.0-18.0nm)[Appl.Catal.A,2004,259,121]。P.Landon等人利用共沉淀法制备的Au/α-Fe2O3催化剂于400-600℃焙烧3h后,近球形Au纳米粒子尺寸从3.7nm增加到16.1nm[J.Mater.Chem.,2006,16,199]。从上述研究进展中可以看出,如何抑制Au/α-Fe2O3催化剂上Au纳米粒子在焙烧过程中的烧结,尤其是在α-Fe2O3载体上选择性调控Au物种的形貌仍然是当前纳米材料和纳米催化研究领域的挑战。
发明内容
本发明的目的在于提供一种多孔α-Fe2O3纳米棒的制备方法。
本发明的目的提供利用上述α-Fe2O3纳米棒调控Au纳米粒子尺寸和形貌,解决Au纳米粒子高温易烧结,分散度下降的问题。
本发明的目的提供上述α-Fe2O3纳米棒负载Au纳米粒子为催化剂应用于室温CO氧化反应。
本发明的目的是通过如下技术方案来实现的:
一种α-Fe2O3纳米棒合成方法,包括以下步骤:
(1)将无机铁盐、NaCl、聚乙二醇(PEG-400)溶解于一定量的水中,形成铁离子浓度为0.05~0.2mol/L的溶液,搅拌并升温加热至100~140℃;
(2)将一定量的0.1~0.5mol/L的Na2CO3溶液缓慢加入到上述溶液中,搅拌老化1小时,体系的pH为8.0~12.0;
(3)将老化的产物经过滤、洗涤、干燥后,得到β-FeOOH纳米棒;
(4)在β-FeOOH前驱体在300~600℃℃空气焙烧3~6小时,得到α-Fe2O3纳米棒。
所述铁盐为FeCl3·6H2O;所述的溶剂的体积比为PEG/H2O=1/19;
所述体系中NaCl的质量浓度为58.5g/L。
所述的体系的铁离子浓度0.05~0.2mol/L为优选。
所述体系的Na2CO3浓度0.1~0.5mol/L为优选。
所述的体系pH=8.0~12.0为优选。
所述的反应温度以100~140℃为优选。
采用Rigaku D/MAX-2500/PC型X射线粉末衍射仪表征产物结构,其XRD测试结果如图1a所示,表明产物为晶相单一的α-Fe2O3。采用Hitachi HT7700型透射电镜观察α-Fe2O3的形貌,测试结果如图2,直径为40-50nm,长度为300-500nm,同时具有多孔结构。
一种α-Fe2O3负载活性Au纳米粒子催化剂合成方法,包括以下步骤:
(1)利用所述的α-Fe2O3纳米棒作为载体,分散到(2~5)×10-4mol/L金盐水溶液中,并加热至60~100℃;
(2)将一定量的0.005~0.02mol/L的Na2CO3溶液缓慢加入到上述溶液中,调节pH为5.0~7.0,搅拌老化1小时;
(3)将固体产物经过滤、洗涤、干燥后,在300~500℃空气中焙烧,得到产品Au-T催化剂(T表示焙烧温度)。
所述金盐为HAuCl4水溶液,浓度以(2~5)×10~4mol/L为优选。
所述沉淀剂为Na2CO3水溶液,浓度以0.005~0.02mol/L为优选。
所述的体系以pH=5.0~7.0为优选。
所述的反应温度以300~500℃为优选。
采用Rigaku D/MAX-2500/PC型X射线粉末衍射仪表征产物结构,其XRD测试结果如图1b-d所示,表明产物为晶相单一的α-Fe2O3,峰形尖锐,表明产物结晶度良好。采用JEM–ARM200F型透射电镜观察Au/α-Fe2O3的形貌,Au粒子的尺寸在2.2~8.6nm的范围内可调;Au纳米粒子的形貌分别为二维薄层状、三维截角八面体和片状截角八面体。
利用α-Fe2O3纳米棒负载Au纳米粒子催化剂用于室温CO氧化反应,包括以下步骤:
(1)利用所述的α-Fe2O3纳米棒负载Au纳米粒子催化剂15~200mg,在300℃、20vol.%O2/N2气氛下预处理1~2h,降至室温(25~30℃);
(2)在室温(25~30℃)、通入反应气体1.0vol.%CO/2.5vol.%O2/He,流速50~200mL/min,反应12h;
所述催化剂用量以15~200mg为优选。
所述反应预处理时间以1~2h为优选
所述反应流速以50~200mL/min为优选。
采用气相色谱在线分析产物,其CO氧化反应结果如图6所示,Au-300和Au-400催化剂的CO的转化率分别为95%和24%,而Au-500催化剂则几乎没有活性,体现出明显的尺寸敏感性。相应的反应动力学数据见表1,其中可以看出当Au粒子的平均尺寸从2.2nm(Au-300)增加到3.5nm(Au-400),反应速率从0.8074减低为0.0392molCO/(gAuh);而当金粒子平均尺寸增大到8.6nm时,则几乎没有催化活性。
本发明的效果及优点是:(1)使用多元醇法合成α-Fe2O3纳米棒,工艺简单,环境友好;(2)使用无机金源,沉积沉淀法制备Au/α-Fe2O3催化剂,Au纳米粒子的热稳定性和分散度得到提高;(3)Au/α-Fe2O3可室温催化CO氧化反应,并表现出明显的尺寸效应。
附图说明
图1为由采用本发明方法的实施例1-4所制备样品的XRD谱图。横坐标为角度2θ,单位为°(度),纵坐标为衍射强度,单位为a.u.(绝对单位)。
图2是本发明实施例1所制备样品的透射电镜图(TEM)。
图3是本发明实施例2所制备样品的透射电镜图(TEM)。
图4是本发明实施例3所制备样品的透射电镜图(TEM)。
图5是本发明实施例4所制备样品的透射电镜图(TEM)。
图6是本发明实施例2-4所制备样品催化CO氧化的反应性能。
具体实施方式
表1是本发明实施例2-4制备所得样品的反应动力学数据。
实施例1
将5.408g FeCl3·6H2O、11.6g NaCl、10mL PEG加入190mL水中,搅拌并升温至于120℃;然后将200mL 0.2mol/L的Na2CO3溶液于以0.55mL/min的速度(6个小时)加入到上述溶液后,继续搅拌并老化1小时;调节pH为8.0~12.0,将老化的产物经过滤、洗涤、干燥后,于500℃下焙烧5小时,得到固体粉未。经XRD分析为α-Fe2O3晶相,其XRD谱图见图1a;TEM图像分别见图2。结果显示所合成的α-Fe2O3样品具有纳米棒状结构,直径为40-50nm,长度为300-500nm,其比表面积为79m2/g,平均孔径约20nm。
实施例2
取实施案例1中得到的1gα-Fe2O3纳米棒分散于300mL含有HAuCl4(4.9×10-4mol/L)的水溶液中,加热到80℃后,再加入60mL0.01mol/L Na2CO3溶液,调节pH为5.9。在该温度下老化1h后,经过滤、洗涤后,120℃干燥12h,得到固体前体。在300℃空气中焙烧5h所得Au/α-Fe2O3催化剂标记为Au-300,其中Au的担载量为1.6wt.%。其XRD谱图和TEM图片分别见图1b和图3a;载体纳米棒的直径为40-50nm,长度为300-500nm;Au纳米粒子的平均尺寸为2.2nm,进一步通过HRTEM(图3b)分析发现Au纳米粒子主要为二维薄层状结构。
实施例3
取实施案例2中固体前体于400℃焙烧5h,所得样品标记为Au-400,其XRD谱图和TEM图片分别见图1c和图4a;α-Fe2O3载体的形貌和晶相保持不变;Au纳米粒子的平均尺寸为3.5nm,其形貌主要为三维截角八面体(图4b)。
实施例4
取实施案例2中固体前体于500℃焙烧5h,所得样品标记为Au-500,其XRD谱图和TEM图片分别见图1d和图5a。α-Fe2O3载体的形貌和晶相保持不变;Au纳米粒子的平均尺寸为8.6nm,其形貌主要为片状截角八面体(图5b)。
实施例5
在室温(25~30℃)、反应气1.0vol.%CO/2.5vol.%O2/He,流速50mL/min条件下,考察了所制备的Au/α-Fe2O3催化剂的CO氧化性能,结果如图6所示。Au-300和Au-400催化剂的CO的转化率分别为95%和24%,而Au-500催化剂则几乎没有活性,体现出明显的尺寸敏感性。相应的反应动力学数据见表1,可以看出当Au粒子的平均尺寸从2.2nm(Au-300)增加到3.5nm(Au-400),反应速率从0.8074减低为0.0392molCO/(gAuh);而当金粒子平均尺寸增大到8.6nm时,则 几乎没有催化活性。同时还可看到,反应速率与金粒子形貌也密切相关;其中暴露较多的二维结构的薄层状的Au纳米粒子与氧化铁形成的界面周长最大,是获得CO氧化高活性的关键因素。
表1
本发明以NaCl为结构导向剂,聚乙二醇(PEG)为形貌控制剂,Na2CO3为沉淀剂,在水溶液中通过控制铁离子的水解动力学,合成了直径为40-50nm,长度为300-500nm,且具有多孔结构的α-Fe2O3纳米棒。以此α-Fe2O3纳米棒为载体,利用沉积沉淀法制备了Au/α-Fe2O3催化剂。于300~500℃空气中焙烧5h后,α-Fe2O3维持多孔纳米棒状结构,Au粒子的平均尺寸从2.2nm增加到8.6nm。同时,α-Fe2O3纳米棒状结构还可调变Au纳米粒子的形貌,经300,400,500℃焙烧后的Au/α-Fe2O3催化剂上Au纳米粒子的形貌分别为二维薄层状、三维截角八面体和片状截角八面体,体现出α-Fe2O3棒状结构在分散和稳定Au纳米粒子结构上的独特性。所制备的Au/α-Fe2O3催化剂可在室温下催化CO氧化,其反应速率达0.8047molCO/gAu h。
Claims (8)
1.一种α-Fe2O3纳米棒,直径为40-50nm,长度为300-500nm,比表面积为79g/m2。
2.一种权利要求1所述α-Fe2O3纳米棒的合成方法,其工艺步骤为:
(1)将可溶性无机铁盐、NaCl、聚乙二醇(PEG-400)溶解到水溶液中,形成铁离子浓度为0.05~0.2mol/L的溶液,搅拌并升温加热至100~140℃;
(2)将0.1~0.5mol/L的Na2CO3溶液缓慢加入到上述溶液中,搅拌老化1小时以上,控制体系的pH为8.0~12.0;
(3)将老化的产物经过滤、洗涤、干燥后,得到β-FeOOH纳米棒;
(4)在300~600℃空气中焙烧3~6小时,得到α-Fe2O3纳米棒。
3.根据权利要求2所述的α-Fe2O3纳米棒的合成方法,其特征在于所述铁盐为FeCl3·6H2O。
4.根据权利要求1所述的α-Fe2O3纳米棒的合成方法,其特征在于:所述体系中聚乙二醇与H2O的体积比为1/19。
5.根据权利要求1所述的α-Fe2O3纳米棒的合成方法,其特征在于:所述体系中NaCl的质量浓度为58.5g/L。
6.一种Au/α-Fe2O3催化剂,其特征在于:载体为权利要求1所述的α-Fe2O3纳米棒,Au与载体的质量比(1.0~4.0)/100,即金的负载量为载体质量的1.0%~4.0wt.%。
7.一种权利要求6所述Au/α-Fe2O3催化剂合成方法,其特征在于包括以下步骤:
(1)利用权利要求1所述的α-Fe2O3纳米棒作为载体,分散到(2~5)×10-4mol/L的金盐水溶液中,加热到60~100℃;
(2)将0.005~0.2mol/L Na2CO3溶液加入到上述溶液中,调节体系pH为5.0~7.0,搅拌老化1小时以上;
(3)将老化的产物经过滤、洗涤、干燥后,在300~500℃空气中焙烧3~6小时,得到Au-T催化剂(T表示焙烧的温度)。
8.一种权利要求6所述Au/α-Fe2O3催化剂用于一氧化碳室温氧化反应中。
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CN109731582A (zh) * | 2019-02-21 | 2019-05-10 | 北京工业大学 | 一种高效催化氧化苯的AuMnOx/介孔Fe2O3催化剂的制备 |
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