CN114733507B - 碳纳米管包覆菱形镍颗粒催化剂及其制备方法及其应用 - Google Patents
碳纳米管包覆菱形镍颗粒催化剂及其制备方法及其应用 Download PDFInfo
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Abstract
本发明提供了一种碳纳米管包覆菱形镍颗粒催化剂及其制备方法及其应用，所述催化剂以泡沫镍为基体，在泡沫镍的表面生长有碳纳米管，在碳纳米管内包覆有菱形镍颗粒，碳纳米直径约为50‑100nm，呈现出弯曲的中空结构，菱形镍颗粒均匀地分布于碳纳米管的中部或顶端，颗粒大小为50‑100nm。该催化剂在催化甲酸还原六价铬的应用上降解速率分别是Pd/C和Ni/CNTs催化剂的9倍和16倍。同时，该催化剂能够用于固定床连续流动反应，在500h连续流动测试中，能够一直保持>99％的底物转化效率和机械稳定性。是一种能够在六价铬还原领域有效替代贵金属催化剂的低成本复合材料。
Description
技术领域
本发明属于重金属废水处理领域，尤其涉及一种碳纳米管包覆菱形镍颗粒催化剂及其制备方法及其催化甲酸还原含六价铬污染物中的应用。
背景技术
目前，全世界正面临着严重的饮用水安全问题，世界水资源协会指出目前全球有超过11亿人无法获得健康的饮用水，且每年有多于200万人死于饮用水污染引起的疾病。Cr(VI)是世界上第二大重金属污染物，具有急性毒性、致癌性、高溶解性和迁移性，对人类健康危害极大。世界卫生组织(WHO)和美国环境保护署严格限制了饮用水和工业废水中Cr(VI)含量必须分别低于0.05mg/L-1和0.1mg/L-1。
Cr(VI)在平常工业生产中被广泛应用，主要体现在电镀、染料生产、皮革制造和木材防腐等工业活动中。因此，会不可避免地产生大量含铬废水和废渣，并进一步在土壤，水体和生物中迁移转化，造成严重的环境污染和生物体损害。
传统的Cr(VI)降解方法主要包括物理吸附、化学沉淀、溶剂萃取、离子交换、电化学处理和化学还原等。然而，这些方法通常成本高昂且效率低下，并且存在引起二次环境污染的风险。利用催化技术解决环境污染问题以其高效、经济的特点逐渐成为研究的热点。
镍基催化剂常用于重金属的还原降解，但其自身存在很多缺点，包括催化效率低，循环稳定性差，不耐腐蚀，反应后难分离回收等。特别是在Cr(VI)降解反应中，因Cr(VI)具有强氧化性，其接触到活性中心后极易造成镍颗粒被氧化而失活。这大大限制了其工业化应用。泡沫镍(NF)是一种高度多孔的结构材料，由于其高孔隙率(高达97-98％)、导热性和导电性，被广泛用作电极和催化剂载体。然而，比表面积不足(<1m2g-1)限制了其在传统催化领域的实际应用。
发明内容
本发明的目的在于克服现有技术的不足之处，提供一种碳纳米管包覆菱形镍颗粒催化剂及其制备方法，该催化剂在烧瓶搅拌***和固定床连续流动***中对六价铬进行连续性降解，表现出高活性，高稳定性，易分离的特点。
本发明的第一方面是提供了一种碳纳米管包覆菱形镍颗粒催化剂，所述催化剂以泡沫镍为基体，在泡沫镍的表面生长有碳纳米管，在碳纳米管内包覆有菱形镍颗粒，碳纳米直径约为50-100nm，呈现出弯曲的中空结构。菱形镍颗粒均匀地分布于碳纳米管的中部或顶端，颗粒大小约为50-100nm。
首先菱形的镍颗粒将有助于暴露出大量的台阶、角落和边缘活性位点，大大提高了催化活性；其次菱形镍颗粒被完全包覆在碳纳米管中，在反应过程中避免了与底物中六价铬分子的直接接触，有效避免了因自身被氧化而失活的可能性。最后，甲酸还原六价铬体系为强酸性体系，菱形镍颗粒包覆在碳纳米管中将有效防止镍颗粒直接暴露于酸性环境而被腐蚀流失。以上这些结构特征将大大提高其催化甲酸还原六价铬性能和长周期运行稳定性。
以催化剂为基准，其中碳纳米管重量占催化剂重量的10-60％，泡沫镍重量占催化剂重量的40-90％。
优选地，碳纳米管重量占催化剂重量的30-40％，泡沫镍重量占催化剂重量的60-70％。
本发明的第二方面是提供了该催化剂的制备方法，包括如下步骤：
先将预处理后的泡沫镍置于管式炉的石英管中，通入氢气对泡沫镍进行预还原处理，预还原氛围为纯氢气，气体流速是5-15mL/min，升温速率为5-15℃/min，还原温度为500-750℃，还原时间为1-2h；
接着切换至乙炔气体进行CVD沉积，乙炔气体流速为5-20mL/min，乙炔沉积时间为5-20min，CVD沉积结束后降温，降温气氛为纯氮气，降至室温后取出待用，得Ni@CNTs/NF。
当CVD乙炔气体流速为5-20mL/min，CVD沉积温度为500-750℃，沉积时间为5-20min可以合成碳纳米管包覆的镍颗粒结构。通过进一步精确控制乙炔气体流速为10-20mL/min，CVD沉积温度为550-570℃，沉积时间为7-10min可以合成碳纳米管包覆的菱形镍颗粒结构。优选的，CVD沉积温度为550℃，乙炔气体流速为10mL/min，乙炔沉积时间为7-8min。
进一步地，预还原气体流速是8-10mL/min，升温速率为8-10℃/min，还原温度为500-600℃，还原时间为1-2h。
优选地，预还原气体流速是10mL/min，升温速率为10℃/min，还原温度为550℃，还原时间为1.5h。
进一步地，所述预处理是将泡沫镍分别放入乙醇和去离子水中超声净化20-30min，然后置于烘箱中60-80℃干燥5-10h。
本发明的第三方面是提供了上述催化剂在催化甲酸还原废水中六价铬污染物的应用。将制备的催化剂填充在石英管反应器中，用进料泵将浓度为0.4-0.5mol/L甲酸与浓度为2-4mmol/L的六价铬污染物混合溶液从石英管上方加入反应***，进料质量空速为10-70h-1。
也可以将六价铬和甲酸水溶液置于容器中，然后加入Ni@CNTs/NF催化剂，在持续搅拌下进行催化反应。
本发明的优点和有益效果：
1.本发明通过在NF表面原位生长碳纳米管的方法将镍纳米颗粒原位包覆在碳纳米管中，大大提高了催化甲酸还原六价铬性能和长周期运行稳定性。
2.本发明合成的催化剂为片层结构，反应后易于从反应液中分离。
3.本发明制备的镍基甲酸还原有毒六价铬催化剂方便用于固定床连续流动反应，其能够在高底物浓度，高反应空速条件下长周期性运行。
附图说明
图1是实施例1的催化剂的制备流程示意图。
图2是实施例1的Ni@CNTs/NF-550催化剂的形貌图，其中A，B分别为200μm和2μm标尺范围下的扫描电镜图，C，D分别为200nm和100nm标尺范围下的透射电镜图。
图3是对比例1-3的催化剂的形貌图，
其中A为对比例1中Ni@CNTs/NF-450的200μm下的扫描电镜图；
其中D为对比例1中Ni@CNTs/NF-450的50nm下的透射电镜图；
其中B为对比例2中Ni@CNTs/NF-650的200μm下的扫描电镜图；
其中E为对比例2中Ni@CNTs/NF-650的50nm下的透射电镜图；
其中C为对比例3中Ni@CNTs/NF-750的200μm下的扫描电镜图；
其中F为对比例3中Ni@CNTs/NF-750的50nm下的透射电镜图。
图4是催化甲酸还原六价铬的性能和运行稳定性对比图，
其中A为实施例1的Ni@CNTs/NF-550催化降解六价铬过程中溶液的紫外可见吸收光谱图；
B为传统Ni/CNTs催化降解六价铬过程中溶液的紫外可见吸收光谱图；
C为传统Pd/C催化降解六价铬过程中溶液的紫外可见吸收光谱图；
D为不同样品的速率常数(k)比较图；
E为Ni@CNTs/NF-550催化剂循环稳定性测试中速率常数(k)比较图；
F传统Ni/CNTs催化剂循环稳定性测试中速率常数(k)比较图。
图5是实施例1制备的Ni@CNTs/NF-550催化还原六价铬的固定床连续流动装置示意图。
图6是实施例1制备的Ni@CNTs/NF-550催化还原六价铬的固定床连续流动测试性能。
具体实施方式
为了更清楚地说明本发明，下面结合优选实施例对本发明做进一步的说明。本领域技术人员应当理解，下面所具体描述的内容是说明性的而非限制性的，不应以此限制本发明的保护范围。凡根据本发明精神实质所作的等效变化或修饰，都应涵盖在本发明的保护范围之内。
本发明采用简单易操作的化学气相沉积(CVD)方法，以乙炔为碳源，泡沫镍(NF)为基质，制备出了一种低成本、高活性、高稳定性、易分离的催化甲酸还原有毒六价铬的新型非贵金属催化剂，在废水处理领域具有广阔的应用前景。通过精确调控乙炔沉积温度为550-570℃，能够在NF表面生长出包覆有菱形镍颗粒的碳纳米管结构。在甲酸存在条件下，该复合结构表现出优异的六价铬降解活性，将其还原为无毒的三价铬，并通过进一步调节溶液pH值将其去除。其中活性最优的Ni@CNTs/NF-550能够在2.5min内完全催化还原降解10ml(2mmol/L)的六价铬，而相同条件下传统的Ni/CNTs和Pd/C催化剂则分别需要28min和20min。通过拟合分析不同催化剂样品的速率常数(k)，发现Ni@CNTs/NF-550降解速率(k＝1.381min-1)分别是Pd/C(k＝0.158min-1)和Ni/CNTs(k＝0.087min-1)催化剂的9倍和16倍。同时，该催化剂能够用于固定床连续流动反应，在500h连续流动测试中，在高空速(70h-1)和高底物浓度(4mmol/L)条件下，Ni@CNTs/NF-550能够一直保持>99％的底物转化效率和机械稳定性。因此，Ni@CNTs/NF-550是一种能够在六价铬还原领域有效替代贵金属催化剂的低成本复合材料。
实施例1
(1)泡沫镍(NF)的预处理：将购置的厚度为1mm的NF板进行净化处理，即依次放入乙醇和去离子水中分别超声净化30min，最后置于烘箱中70℃干燥6h。
乙炔CVD沉积：将预处理后的NF置于管式炉的石英管中，通入氢气对NF进行预还原处理，以去除NF表面氧化层，氢气流速是10mL/min，升温速率为10℃/min，还原温度为550℃，还原时间为1.5h；接着切换至乙炔气体进行CVD沉积，乙炔气体流速为10mL/min，乙炔沉积时间为7min。CVD结束后进行降温，降温气氛为纯氮气。降至室温后取出待用，得Ni@CNTs/NF-550。
图2是具体实施例1的Ni@CNTs/NF-550催化剂的形貌图，图A，B分别为200μm和2μm标尺范围下的扫描电镜图，图C，D分别为200nm和100nm标尺范围下的透射电镜图。从A图中可以看出，在200μm标尺范围下，Ni@CNTs/NF-550的骨架结构仍然清晰可见，且其表面包覆了一层致密的碳纳米管结构。将标尺进一步缩小到2μm，如图B，Ni@CNTs/NF-550表面的碳纳米管清晰可见，其形貌呈弯曲状，且粗细不一，相互交织在一起。在透射电镜图中我们可以看出碳纳米管直径约为50-100nm，呈现出中空结构。且菱形结构的，大小约为50-100nm的镍颗粒均匀地分布在碳纳米管的中间或者顶端位置。
对比例1
与实施例1的区别在于，CVD沉积温度调整为450℃，得到Ni@CNTs/NF-450。从图3中的A，D图中可以看出Ni@CNTs/NF-450表面并未长出碳纳米管，仅仅是表面覆盖了一层碳层，碳层中镍颗粒形状为不规则球形，大小约20-60nm。
对比例2
与实施例1的区别在于，CVD沉积温度调整为650℃，得到Ni@CNTs/NF-650。在图3中的B，E图中，Ni@CNTs/NF-650表面同样被致密的碳纳米管结构所包覆，碳纳米管直径约为30-60nm，镍颗粒也被成功包覆于碳纳米内，大小约为20-50nm，但镍颗粒并未完全呈现出菱形结构，而是菱形与球形镍颗粒并存。
对比例3
与实施例1的区别在于，CVD沉积温度调整为750℃，得到Ni@CNTs/NF-750。在图3中的C，F图中，Ni@CNTs/NF-750表面也可以成功长出碳纳米管，但其碳纳米管直径略粗，直径约70-120nm，颗粒尺寸约50-100nm的球形镍颗粒包覆于碳纳米管中，未出现菱形结构。
应用实施例
在固定床连续流动反应中，将Ni@CNTs/NF-550切割成直径为10mm的圆片，层层叠加放置于石英管反应器中，如图5所示，含六价铬和甲酸的混合水溶液由进料泵从反应器顶端注入进行连续催化反应。六价铬的浓度范围为2-4mmol/L，甲酸浓度为0.4-0.5mol/L，进料质量空速为10-70h-1。
从图6中可以看出，在六价铬浓度为4mmol/L，甲酸浓度为0.4mol/L，反应空速为70h-1时，催化剂连续运行500h后未出现还原效率的下降，说明其具有良好的稳定性。
从图4的A,B,C中可以看出，相同反应条件下降解相同量的六价铬，Ni@CNTs/NF-550，Ni/CNTs和Pd/C分别需要2.5min,28min和20min。Ni@CNTs/NF-550明显快于Ni/CNTs和Pd/C。
从图4的D中可以看出，未长出碳纳米管的Ni@CNTs/NF-450对还原六价铬无催化活性；具有碳纳米管包覆菱形镍颗粒的Ni@CNTs/NF-550降解速率(k＝1.381min-1)明显快于碳纳米管包覆非菱形镍颗粒的Ni@CNTs/NF-650(k＝0.418)和Ni@CNTs/NF-750(k＝0.207)，且分别是传统Pd/C(k＝0.158min-1)和Ni/CNTs(k＝0.087min-1)催化剂的9倍和16倍。
从图4的E，F可以看出，经过7次循环反应后Ni@CNTs/NF-550的速率常数变化不大，说明催化剂没有明显失活，而Ni/CNTs随着循环次数的增加，其反应速率常数迅速减小，说明催化剂失活严重。
以上所述仅为本发明的优选实施例而已，并不用于限制本发明，对于本领域的技术人员来说，本发明可以有各种更改和变化。凡在本发明的精神和原则之内，所作的任何修改、等同替换、改进等，均应包含在本发明的保护范围之内。
Claims (8)
1.一种碳纳米管包覆镍颗粒催化剂的制备方法，包括如下步骤：
先将预处理后的泡沫镍置于管式炉的石英管中，通入氢气对泡沫镍进行预还原处理，预还原氛围为纯氢气，气体流速是5-15mL/min，升温速率为5-15 ℃/min，还原温度为500-750℃，还原时间为1-2h；
接着切换至乙炔气体进行CVD沉积，CVD沉积温度为550-570℃，乙炔气体流速为5-20mL/min，乙炔沉积时间为5-20 min，CVD沉积结束后降温，降温气氛为纯氮气，降至室温后取出待用，得到催化剂Ni@CNTs/NF，催化剂以泡沫镍为基体，在泡沫镍的表面生长有碳纳米管，在碳纳米管内包覆有菱形镍颗粒，碳纳米直径为50-100 nm，呈现出弯曲的中空结构，菱形镍颗粒均匀地分布于碳纳米管的中部或顶端，颗粒大小为50-100 nm。
2.根据权利要求1所述的制备方法，其特征在于：乙炔气体流速为10-20 mL/min，乙炔沉积时间为7-10 min。
3.根据权利要求2所述的制备方法，其特征在于：CVD沉积温度为550℃，乙炔气体流速为10 mL/min，乙炔沉积时间为7-8 min。
4.根据权利要求1所述的制备方法，其特征在于：预还原气体流速是8-10 mL/min，升温速率为8-10 ℃/min，还原温度为500-600 ℃，还原时间为1-2 h。
5.根据权利要求4所述的制备方法，其特征在于：预还原气体流速是10 mL/min，升温速率为10 ℃/min，还原温度为550 ℃，还原时间为1.5h。
6.根据权利要求1所述的制备方法，其特征在于：所述预处理是将泡沫镍分别放入乙醇和去离子水中超声净化20-30 min，然后置于烘箱中60-80 ℃干燥5-10h。
7.一种权利要求1-6任一项所述的方法制备的催化剂在催化甲酸还原废水中六价铬污染物的应用。
8.根据权利要求7所述的应用，其特征在于，将制备的催化剂填充在石英管反应器中，用进料泵将浓度为0.4-0.5 mol/L甲酸与浓度为2-4 mmol/L的六价铬污染物混合溶液从石英管上方加入反应***，进料质量空速为10-70 h-1。
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