CN111298789A - 一种Au/RGO复合气凝胶及其制备方法和应用 - Google Patents
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Abstract
本发明提供了一种Au/RGO复合气凝胶及其制备方法和应用,属于催化材料制备和环境净化的技术领域。本发明通过原位还原水热工艺制备了Au/RGO复合气凝胶,在该复合气凝胶中可见尺寸均一的单分散超小Au纳米颗粒(NPs)(平均粒径为1.3nm)均匀分散在RGO纳米薄片上。在水热过程中,前驱物氧化石墨烯(GO)和HAuCl4被还原,L‑半胱氨酸(L‑cys)作为交联剂和还原剂被氧化成二硫键,同时RGO凝胶化,使得RGO纳米薄片表面嵌有二硫键可以保护单分散超小Au NPs。Au/RGO复合气凝胶显示了优异的催化性能,能将4‑硝基苯酚(4‑NP)迅速还原生成4‑氨基苯酚(4‑AP)。该方法简单、操作方便,减少贵金属的使用量,降低其使用成本,拓宽实际应用,有利于大规模的工业生产,具有显著的经济和社会效益。
Description
技术领域
本发明属于材料制备和环境净化的技术领域,具体涉及一种高效催化还原4-NP废水的单分散超小Au NPs负载的RGO复合气凝胶催化材料及其制备方法。
背景技术
近年来,Au NPs由于具有丰富的活性中心,在各种化学转化中成为一种有应用前景的活泼贵金属催化剂。公认的是,Au NPs的催化性能与其粒径分布和分散程度密切相关。但是,Au纳米粒子通常不稳定,并且由于尺寸效应小和比表面积高而易于团聚,最终限制了它在催化领域的适用性。由于Au NPs具有极高的表面能,合成超小Au NPs仍然是一项非常艰巨的任务。为了避免不可逆的聚集,通常将Au NPs固定在用于多相催化的各种载体上,例如碳材料,聚合物,金属氧化物等,以进一步保持其活性和延长使用寿命。
碳基材料如富勒烯,碳纳米管和石墨烯都被认为是在非均相催化中稳定金属纳颗粒的极佳载体。其中,RGO因其独特的理化特性而受到广泛关注,包括超高电导率,大比表面积,出色的热和电性能。此外,基于RGO生成的气凝胶不仅具有大的开孔,良好的机械性能,易于回收,还有从RGO纳米片中转化而来的特殊性质。RGO气凝胶的独特特性使其成为固定金属纳米颗粒的理想载体。目前,尽管已经对负载金属纳米颗粒的二维RGO纳米材料进行了广泛的研究,但是二维催化剂的再循环使用不方便,催化剂易流失等问题都限制其实际应用,尤其是对于昂贵的金属催化剂在使用过程的流失(例如Au)将造成更大的经济损失。但是,关于易回收、性能稳定的三维Au/RGO复合气凝胶的制备及其在催化还原中的实际应用的研究受到限制。
发明内容
鉴于上述现有技术的不足,本发明旨在提供一种新型的Au/RGO复合气凝胶及其制备方法和应用,拟解决传统Au催化剂在使用过程中易团聚、易失活、易流失、不易回收等问题。在本发明中使用GO作为RGO的前驱物,HAuCl4作为Au NPs(金纳米颗粒)的前驱物,L-cys既作为交联剂又作为还原剂,通过绿色、简便的水热工艺在RGO气凝上制备了平均粒径为1.3nm的单分散超小Au NPs,得到Au/RGO复合气凝胶。在水热过程中,GO和HAuCl4被还原,L-半胱氨酸被氧化生成二硫键,以及同时发生的RGO气凝胶自组装,使得RGO纳米薄片表面嵌有二硫键,可以保护单分散超小Au NPs。通过此方法获得的Au/RGO复合气凝胶对于将4-NP还原为4-AP的催化反应表现出优异的催化性能。引入RGO气凝胶作为Au NPs的载体可以稳定超小Au NPs,加速Au表面的电子转移,同时还能提高对4-NP的吸附,这些都将提高Au/RGO复合气凝胶的催化活性。此外,在催化反应后,可以用镊子轻松移除整块复合气凝胶,并以不变的性能多次循环使用。对于昂贵的Au NPs而言,这种由超小Au NPs嵌入RGO复合气凝胶组成的易于回收的三维材料将扩展其实际应用。
本发明的技术方案如下:
一种Au/RGO复合气凝胶是使用GO作为RGO的前驱物,HAuCl4作为Au NPs的前驱物,L-cys既作为交联剂又作为还原剂,通过绿色、简便的水热工艺在RGO气凝上制备了平均粒径为1.3nm的单分散超小Au NPs,该复合气凝胶的直径为12mm,高为30mm。在水热过程中,GO和HAuCl4被还原,L-半胱氨酸被氧化生成二硫键,同时RGO气凝胶发生自组装,使得RGO纳米薄片表面嵌有二硫键可以保护单分散超小Au NPs。制备得到的Au/RGO复合气凝胶对于将4-NP还原为4-AP的催化反应表现出优异的催化活性,因为RGO气凝胶的引入可以稳定超小AuNPs,加速Au表面的电子转移,同时还能提高对4-NP的吸附,这些都将提高Au/RGO复合气凝胶的催化活性。此外,在反应后,可以用镊子轻松移除整块复合气凝胶,不损失贵金属催化剂,并以不变的性能多次循环使用。
所述的Au/RGO复合气凝胶的制备方法为原位还原水热法。在1g/L半胱氨酸溶液中加入GO分散液,接着加入适量的HAuCl4溶液搅拌30min,于160oC下保温10h,自然冷却,产物经去离子水多次透析、冷冻干燥,制得Au/RGO复合气凝胶,其中Au的负载量为4.8%。
所述的Au/RGO复合气凝胶用于催化还原含4-NP废水。
本发明具有如下有益效果:
(1)本发明采用原位还原水热法,通过水热工艺将单分散超小Au NPs原位负载在RGO气凝胶上,制备得到Au/RGO复合气凝胶,该复合气凝胶的直径12mm,高30mm。RGO气凝胶上负载的Au NPs分散均匀、尺寸均一,平均粒径为1.3nm。本发明制备方法简单、操作方便,减少贵金属催化剂的使用量,降低其使用成本,拓宽其实际应用,有利于大规模的工业生产,具有显著的经济和社会效益。
(2)本发明使用绿色无毒的合成前驱物,以GO作为RGO的前驱物,HAuCl4作为AuNPs的前驱物,L-cys作为交联剂和还原剂。在水热过程中,GO和HAuCl4被还原,L-半胱氨酸被氧化生成二硫键,以及同时发生的RGO气凝胶自组装,使得RGO气凝胶表面生成二硫键以保护的超小Au NPs。由于Au NPs和RGO的协同作用,所制备的Au/RGO复合气凝胶对将4-硝基苯酚(4-NP)还原为4-氨基苯酚(4-AP)的催化反应表现出优异的催化性能。不仅如此,它还显示出高稳定性并且可以整块从反应体系中分离出来,多次循环使用。
(3)本发明涉及的Au/RGO复合气凝胶能高效地催化还原含4-NP废水,具有良好的催化活性及稳定性,使用后易分离回收,重复利用率高,具有很高的实用价值和应用前景。
附图说明
(1)图1是本发明Au/RGO复合气凝胶的(a)SEM图像(插图为Au/RGO复合水凝胶和相应气凝胶的照片);(b)TEM图像(插图为负载的Au NPs的尺寸分布);(c)HRTEM图像;(d)图(b)矩形区域对应的Au/RGO复合气凝胶的EDS谱图。
(2)图2(a)是本发明Au/RGO复合气凝胶、GO和RGO的XRD图谱;Au/RGO复合气凝胶(b)Au 4f区域,(c)S 2p区域的XPS光谱;(d)是Au/RGO复合气凝胶、GO和RGO的拉曼光谱图。
(3)图3是本发明Au/RGO复合气凝胶的(a)C 1s区域,(b)N 1s区域,(c)O 1s区域的XPS光谱。
(4)图4是本发明Au/RGO复合气凝胶和GO的红外光谱图。
(5)图5是本发明Au/RGO复合水凝胶和对应气凝胶的形成过程示意图。
(6)图6是本发明Au/RGO复合气凝胶的N2吸附-脱附曲线图(插图为Au/RGO复合气凝胶的BJH解吸孔径分布曲线图)。
(7)图7是RGO气凝胶的BJH解吸孔径分布曲线图。
(8)图8是本发明Au/RGO复合气凝胶所承重的照片。
(9)图9是NaBH4存在时,(a)本发明Au/RGO气凝胶和(b)巯基修饰的Au NPs催化还原4-NP时,4-NP的吸光度随时间变化的紫外-可见吸收光谱图。
(10)图10是本发明Au/RGO复合气凝胶通过NaBH4催化还原4-NP的Ct与时间的对比图。
(11)图11(a)是本发明Au/RGO复合气凝胶循环5次的活性对比图;(b)是该复合气凝胶在5次循环催化反应后的TEM图像(插图为相对应的XRD图谱)。
具体实施方式
下面结合较佳实施例对本发明作进一步的说明。
实施例1
单分散超小Au NPs负载的RGO复合气凝胶的制备
以HAuCl4为Au NPs的前驱体,GO为RGO的前驱体,L-半胱氨酸既作为交联剂又作为还原剂,水热合成了Au/RGO复合水凝胶,该复合水凝胶上均匀分散着含有零价金的超小且尺寸均一的Au NPs。根据我们先前的研究表明,制备带有L-半胱氨酸的RGO气凝胶的最佳条件是GO与L-半胱氨酸的质量比为1:2(已发表文献:Tuning of surface wettability of RGO-based aerogels for various adsorbates in water using different amino acids,Chem. Commun., 2014, 50, 10311-10314和授权专利:一种TiO2/RGO气凝胶及其制备方法和应用,授权公告号:CN104226290B)。因此,我们将GO与L-半胱氨酸的质量比设定为1:2。所获得的Au/RGO复合水凝胶的直径约为12mm,高度约为30mm,比以前报道的利用L-半胱氨酸制备的RGO气凝胶(以下简称cys-RGO气凝胶)体积更大。这种现象可能与超小Au NPs的引入有关。通过水凝胶的冻干脱水可以得到几乎没有体积收缩的三维圆柱气凝胶(如图1a插图所示)。通过SEM和TEM进一步观察此气凝胶的形态。圆柱体的SEM图像显示,由RGO纳米薄片形成了高度互连且多孔的网络结构,其孔径范围为3μm至10μm(如图1a所示)。尽管在SEM图像中未观察到纳米颗粒,但在该复合气凝胶的TEM图像中可以看到大量尺寸均一的超小纳米颗粒均匀地分散在RGO纳米薄片上(如图1b所示)。超过95%纳米颗粒的大小在1-2nm内(如图1b插图所示),平均粒径为1.3nm。该复合气凝胶的HRTEM图像显示的清晰晶格条纹(d=0.235nm)与金属Au(111)晶面能够很好地匹配,这表明通过此原位还原水热法已经成功合成了Au NPs(如图1c所示)。对TEM图像(图1b)所选定的区域对Au元素进行EDS分析,证实该复合气凝胶中Au的负载量为4.8 wt%(如图1d所示)。负载后的单分散超小Au NPs可以经受长期的超声处理,这表明一步自组装过程可以在RGO气凝胶中产生均匀且坚固的超小AuNPs。
Au/RGO复合气凝胶的XRD图谱在2θ值为24.3o,38.2o和44.3o处显示出特征衍射峰(如图2a所示)。在24.3o处的峰对应于RGO纳米薄片的(002)晶面,而其余两个峰分别对应Au的(111)和(200)晶面(fcc,JCPDS 04-0783)。从图上可以看出,Au的衍射峰相对宽化,表明得到的Au NPs粒径很小,这与TEM的表征结果相符(图1b)。Au/RGO复合气凝胶XPS光谱中Au4f区域的峰可以对应于Au0的4f7/2和4f5/2(87.7 eV和84.0 eV),Au+的4f7/2和4f5/2(88.5 eV和84.8 eV)(如图2b所示),通过峰面积比可以确定Au0与Au+的含量比例为3:2,表明该方法合成的Au NPs是由Au0和Au+组成。 S 2p区域的XPS光谱显示了两对峰,分别对应于S-Au(162.0和163.1 eV)和S-S(163.8和165.0 eV),表明体系中的S是分别以与Au NPs结合和形成二硫键两种形式存在(如图2c所示)。与我们先前研究的cys-RGO气凝胶相比,C 1s、N 1s和O 1s区域的XPS光谱没有发生明显变化(如图3所示),这表明Au NPs的嵌入对RGO基质的性能影响很小。此外,FT-IR光谱还可以证实在水热反应过程中GO还原生成了RGO。与原始GO相比,在所得复合气凝胶的FT-IR光谱中可以观察到在1720和3300cm-1处的透射峰强度大幅减弱,这表明在水热过程中,GO纳米薄片上的-OH和C=O基团大量减少(如图4所示)。此外,该复合气凝胶的拉曼光谱图可以解释Au NPs与RGO基质之间的电子相互作用。GO的拉曼光谱在1572cm-1和1315cm-1处表现出两个特征峰,分别归因于石墨的sp2键合碳(G谱带)和sp2碳呼吸模式(D谱带)(如图2d所示)。对于RGO和Au/RGO复合气凝胶,两者的D波段和G波段均位移到更高的波长到1345cm-1和1587cm-1。Au/RGO复合气凝胶的D波段和G波段的强度比(ID/IG)为1.33,高于在GO和RGO强度比数值(ID/IG=0.95和1.29)。该现象表明sp2轨道的数量增加,并且证实了Au NPs和RGO纳米薄片之间存在很强的电子相互作用,与该复合气凝胶的S2p区的XPS光谱非常吻合(图2c)。
实施案例2
Au/RGO复合气凝胶的形成过程
我们先前的工作表明(已发表文献:Tuning of surface wettability of RGO-basedaerogels for various adsorbates in water using different amino acids, Chem. Commun., 2014, 50, 10311-10314),由于L-cys和GO之间存在静电吸引和氢键以及GO之间的π-π堆积,对L-cys和GO进行水热处理会使GO凝胶化,GO纳米薄片相互交联形成3D水凝胶,最后形成cys-RGO气凝胶。在此基础上,我们提出了Au/RGO复合气凝胶的形成过程(如图5所示)。由于L-cys和GO的混合物的pH值约为3.8,因此在溶液中L-cys主要以带有质子化氨基的+1价阳离子以及两性离子存在。由于GO的等电点低于2.8,因此GO表面上的大多数羧基在该pH值下被去质子化,即GO此时带负电荷。带正电的L-cys会吸附在带负电的GO表面,同时暴露出大量巯基(第一步)。在Au前驱物(即HAuCl4)的存在下,由于S对Au的超强亲和力,L-cys上悬空的巯基可以快速锚链Au(第二步)。在水热过程中,GO和AuCl4 -被还原,L-cys被氧化产生二硫键,以及GO之间的π-π堆积使RGO气凝胶的自组装同时发生,这些都使得RGO气凝胶嵌有二硫键可以保护单分散超小Au NPs(第三步)。最后,通过冷冻干燥将所得到的水凝胶直接脱水形成Au/RGO复合气凝胶(第四步)。
实施案例3
Au/RGO复合气凝胶的机械性能
由于负载了单分散超小的Au NPs,该复合气凝胶测得的比表面积(SBET=160.9m2/g)远大于以类似方式制备的cys-RGO气凝胶(35.6m2/g)(如图6所示)。这是因为负载的单分散超小Au NPs充当RGO气凝胶中的间隔物,可以有效防止RGO纳米薄片的聚集和重新堆叠,而加入的RGO纳米薄片使Au NPs粒径变得更小,分散地更加均匀。与cys-RGO气凝胶相比,RGO纳米薄片在复合气凝胶上的聚集较少,这也可以从该气凝胶地SEM图像中得到证实(图1a)。Au/RGO复合气凝胶的BJH平均孔径为37 nm(如图6插图所示),表明形成均匀的介孔结构。该复合气凝胶的BJH平均孔径远大于cys-RGO气凝胶的平均孔径(3.8 nm)(如图7所示),再次表明单分散超小Au NPs在RGO纳米薄片之间的嵌入可以有效防止RGO的聚集。与纯RGO的气凝胶相比,Au/RGO复合气凝胶拥有更高的机械强度。Au/RGO复合气凝胶可承受质量为50g的砝码而没有发生尺寸和形状的变化,其承重是自重的833倍以上(如图8所示),这表明AuNPs在RGO气凝胶中的嵌入可显著提高RGO气凝胶的机械强度。
实施案例4
Au/RGO复合气凝胶的催化性能
评估Au NPs催化性能最常用的模型反应是将4-NP催化还原为4-AP。通常,在存在NaBH4的情况下,4-NP在400nm处显示出特征吸收峰。在催化还原过程中,在400nm处的峰强度将逐渐降低,同时在300nm处出现一个对应于4-AP的新吸收峰。图9a显示了以Au/RGO复合气凝胶为催化剂,催化还原反应中4-NP的吸光度随时间变化的UV-vis吸收光谱。如图所示,在400nm处的吸收峰强度迅速下降,而在300nm处的吸收峰逐渐增大。在4min的反应后,400nm处的吸收峰完全消失,仅保留在300nm处的吸收峰,这表明Au/RGO复合气凝胶在NaBH4的存在可以将4-NP还原为4-AP,并且没有任何副产物产生。然而,观察到在只有NaBH4存在情况下,4-NP在400nm处的峰强度没有显著降低,这表明我们几乎可以忽略4-NP在仅有NaBH4存在下的还原。同样,在无载体的巯基封端的Au NPs下4-NP的还原速度更缓慢,即使在10min反应后4-NP也无法完全转化为4-AP(如图9b所示)。未负载的巯基修饰的Au NPs还原4-NP的不良催化性能可能是由于Au NPs在催化反应中的大量团聚所致。Au/RGO复合气凝胶优异的催化性能可得益于以下原因。(i)RGO作为对超小Au NPs的载体,其具有高电导率,促进了Au表面的电子从BH4 -转移到4-NP上;(ii)由于RGO的BET比表面积大,气凝胶的海绵性质以及4-NP苯环和RGO之间的π-π堆积相互作用的存在,在Au/RGO复合气凝胶表面可以吸附更高浓度的4-NP。通常认为,对催化底物的良好吸附是高效催化反应的先决条件。(iii)小尺寸的AuNPs也能有助于Au/RGO复合气凝胶提高催化性能,因为超小Au NPs会暴露出更多的活性位点;(iv)与无载体支撑的巯基修饰的Au NPs相比,RGO气凝胶中嵌入的二硫键可与Au表面形成牢固的S-Au-S键,从而防止单分散超小Au NPs的聚集,这将进一步帮助复合气凝胶保持其高效催化活性和延长使用寿命。
由于4-NP和NaBH4的浓度大大超过了催化剂的浓度,因此可以假定该催化还原反应是一级反应。催化速率常数(k)可以根据4-NP的Ct浓度的变化曲线确定,即吸光度对反应时间(t)的关系。图10显示出Ct和t之间建立的线性关系,对于Au/RGO复合气凝胶,从相应的线性斜率可以计算出的k值为0.5mM/min。
实施案例5
Au/RGO复合气凝胶的的可重复使用性测试
整块Au/RGO复合气凝胶可以轻松地用镊子从反应系统中分离出来,进行循环使用,而不会损失任何金属催化剂。如图11a所示,在5次循环使用后,仍未观察到Au/RGO气凝胶的催化还原活性有明显降低。此外,与未使用的催化剂相比,5次循环催化反应后的复合气凝胶的XRD图谱没有发生明显变化,表明Au的金属状态仍然保持良好(如图11b插图所示)。回收的复合气凝胶的TEM图像也显示Au NPs仍可以在RGO纳米薄片上保持其良好的分散性和均匀的粒径分布(如图11b所示)。所有以上表征都表明Au/RGO复合气凝胶是一种可用于催化还原4-NP的高活性,坚固和可重复使用的复合催化剂。
上述实施例为本发明较佳的实施方式,但本发明的实施方式并不受上述实施例的限制,其他的任何未背离本发明的精神实质与原理下所作的改变、修饰、替代、组合、简化,均应为等效的置换方式,都包含在本发明的保护范围内。
Claims (5)
1.一种Au/RGO复合气凝胶,其特征在于:单分散Au NPs负载在RGO气凝胶上,该复合气凝胶直径为12mm,高为30mm,其中Au的负载量为4.8%。
2.如权利要求1所述的一种Au/RGO复合气凝胶,其特征在于,所述Au NPs平均粒径为1.3nm。
3.一种制备如权利要求1或2所述的Au/RGO复合气凝胶的方法,其特征在于:采用原位还原水热法制备Au/RGO气凝胶。
4.如权利要求3所述的方法,其特征在于:在1g/L的L-半胱氨酸溶液中加入GO分散液,后快速加入HAuCl4溶液搅拌30 min,于160oC下保温10h,自然冷却,产物经去离子水至少2次透析、冷冻干燥,制得所述的Au/RGO复合气凝胶。
5.一种如权利要求1或2所述的Au/RGO复合气凝胶的应用,其特征在于:所述的Au/RGO复合气凝胶用于催化还原4-NP废水。
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