文章信息
- 谈玲华, 徐建华, 寇波, 杭祖圣, 石丽丽, 王钧
- TAN Ling-hua, XU Jian-hua, KOU Bo, HANG Zu-sheng, SHI Li-li, WANG Jun
- g-C3N4/NiO复合材料的制备及其对AP热分解的影响
- Preparation of g-C3N4/NiO Composites and Its Effect on Thermal Decomposition of Ammonium Perchlorate
- 材料工程, 2016, 44(11): 96-100
- Journal of Materials Engineering, 2016, 44(11): 96-100.
- http://dx.doi.org/10.11868/j.issn.1001-4381.2016.11.016
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文章历史
- 收稿日期: 2015-01-19
- 修订日期: 2016-03-13
2. 南京工程学院 材料工程学院, 南京 211167
2. School of Materials Science and Engineering, Nanjing Institute of Technology, Nanjing 211167, China
g-C3N4是类石墨结构的氮化碳材料,以三嗪环(C3N3环)或3-s-三嗪环(C6N7环)为结构单元,C, N原子均发生sp2杂化,通过pz轨道上的孤对电子形成一个类似于苯环结构的大π键,组成高度离域的共轭体系,层与层之间存在大量自由移动的电子[1, 2]。由于g-C3N4具有活性中心点多、化学稳定性好、耐高温、导电性能高、环境友好等特点[3, 4],作为新型的无金属催化剂备受关注,在环境、能源和化工等领域都有较好的应用前景[5, 6]。g-C3N4不仅可作为光催化剂用于光催化分解水制氢[7]、降解有机污染物[8]等方面,还可作为多相催化剂用于CO2活化反应[9]、Knoevenagel缩合反应[10]等方面[11]。与g-C3N4的光催化制氢及降解等方面的大量研究相比,在多相催化方面的研究报道相对较少。
据报道,为了提高g-C3N4的催化活性,可采用微观结构及形貌控制[12, 13]、N原子缺陷[14]、化学掺杂改性[15, 16]、物理复合改性[17-20]等方法。其中,g-C3N4与TiO2[17], WO3[18], SmVO4[19], ZnWO4[20]等金属氧化物材料复合能显著提高其光催化活性。
高氯酸铵(AP)是端羟基丁二烯(HTPB)复合固体推进剂中的高能组分,在推进剂中占60%~80%的比例,其热分解特性与推进剂的燃烧性能密切相关,通过研究催化剂对AP热分解的影响可推测推进剂的燃烧性能[21]。经前期研究,g-C3N4对AP的热分解表现出良好的催化作用,为了进一步提高g-C3N4的催化效果,将其与对AP有良好作用效果的NiO复合[22],探讨g-C3N4/NiO复合材料对AP热分解的催化效果,相关文献鲜见报道。
本工作拟采用混合煅烧法制备出g-C3N4/NiO复合材料,利用XRD, FT-IR, FESEM和EDS等对其进行表征,采用DTA和TG研究g-C3N4/NiO复合材料对AP热分解的影响,并探讨催化作用机理。
1 实验材料与方法三聚氰胺,国药化学试剂有限公司,分析纯;无水乙醇,国药化学试剂有限公司,分析纯;纳米NiO,南京艾普瑞纳米科技有限公司,40nm。
采用半封闭一步热解法[2]制备g-C3N4。取一定量的三聚氰胺放入陶瓷坩埚中(盖上坩埚盖),在马弗炉中以50℃/min升温到500℃,焙烧1h;5min内快速升温到520℃,保温焙烧1h,冷却研磨得g-C3N4粉末。
采用混合煅烧法[19]制备g-C3N4/NiO。取0.05g纳米NiO在乙醇中超声分散10min,然后加入0.95g g-C3N4继续超声分散10min,完成后在研钵中研磨至物体呈糊状,放入50℃真空烘箱中4h后,取出放入管式炉中,在300℃下焙烧1h得到g-C3N4/NiO复合材料。
将AP分别与g-C3N4,NiO,g-C3N4/NiO按照质量比为97:3的比例在一定量的乙醇溶液中混合、研磨,待乙醇挥发,干燥处理后得待测复合物(g-C3N4/NiO+AP)。
采用Ultima-IV型X射线衍射仪(XRD)分析样品的晶体结构,Kα辐射,波长为0.15406nm;采用NICOLET IS10型红外吸收光谱分析仪(FT-IR)进行红外分析,扫描范围400~4000cm-1;采用SU8010型场发射扫描电镜(FESEM)观察样品形貌,操作电压为30kV;采用GENESIS2000XMS60型X射线能谱仪(EDS)进行样品成分分析。
采用HTG-1型热分析仪(TGA)进行热失重分析,升温速率10℃/min,氮气流速20mL/min,试样量8mg左右,氧化铝样品池;采用404 PC型热分析仪(DTA)进行差热分析,升温速率10℃/min,氩气流速20mL/min,试样量10mg左右,氧化铝样品池。
2 结果与分析 2.1 物相分析对所制备的g-C3N4和g-C3N4/NiO复合材料进行XRD分析,结果如图 1所示。
由图 1可知,所制备的g-C3N4在2θ为13.2°和27.4°处出现两个较强的特征衍射峰,结合JCPDS 87-1526[23],分别对应于g-C3N4的(100)和(002)面。其中13.2°是melon类物质的特征峰,对应的晶面间距为0.675nm;而27.4°是典型的层状结构堆积衍射峰,对应的晶面间距为0.326nm[2]。g-C3N4/NiO复合材料既出现了g-C3N4的特征衍射峰,还在37.5°, 43.4°, 63.4°, 75.6°, 79.7°出现纳米NiO的衍射峰(JCPDS 47-1049) [24],说明所得的材料为g-C3N4/NiO复合材料。
图 2为g-C3N4和g-C3N4/NiO的FT-IR曲线。由图 2可知,纯g-C3N4在1200~1650cm-1之间出现吸收峰,1645cm-1处的吸收峰主要对应共轭CN的伸缩振动,1240, 1321, 1411, 1564cm-1处的吸收峰对应g-C3N4芳环结构的C-N旋转振动,807cm-1处的吸收峰则对应s-三嗪环的面外弯曲振动[25]。纳米NiO在646cm-1处出现红外吸收峰,对应Ni-O键的伸缩振动[26]。g-C3N4/NiO的FT-IR曲线中可以看出g-C3N4吸收峰较强,而NiO含量较低,其吸收峰相对较弱[27]。FT-IR也说明所制备的材料为g-C3N4/NiO复合材料。
采用场发射扫描电子显微镜(FESEM)进一步观察g-C3N4/NiO微观形貌及结构,结果如图 3所示。
由图 3可知,通过半封闭一步热解法制备得到的g-C3N4样品具有明显的层状结构,比较疏松[16]。图 3(b)为g-C3N4/NiO的FESEM图片,其中亮点为纳米NiO,比较均匀地分散于g-C3N4的表面。对其进行EDS分析,结果如图 3(c)所示,出现C,N,Ni,O的特征峰,由于g-C3N4/NiO中NiO的含量很少,所以Ni元素的特征峰比较低。FESEM和EDS结果说明NiO与g-C3N4复合,并均匀分布于g-C3N4的表面。
2.2 对AP热分解的催化性能研究采用DTA和TG研究g-C3N4, NiO, g-C3N4/NiO对AP热分解的影响,结果如图 4所示。
由图 4可知,DTA曲线(如图 4(a)所示)有1个吸热峰和2个放热峰,248.3℃的吸热峰为AP由斜方晶系转变为立方晶系,326.3℃和425.1℃分别对应于AP的低温分解阶段高温分解和高温分解阶段[28]。添加g-C3N4, NiO, g-C3N4/NiO对AP的晶型转变没有影响,但却均能使AP的高温分解温度降低,对AP的热分解有促进作用。单独添加g-C3N4和NiO时,高温分解温度分别降低30.9℃和42.0℃,而加入g-C3N4/NiO后,AP的高温分解峰和低温分解峰合并,在362.6℃急剧分解,分解温度比纯AP降低了62.5℃,说明g-C3N4/NiO复合材料对AP的热分解起到较强的催化作用[29]。g-C3N4/NiO催化效果均比g-C3N4或NiO单独使用时强,说明g-C3N4和NiO具有协同催化作用[30]。纯AP的TG曲线(如图 4(b)所示)中出现两个失重平台,说明纯AP的热分解过程分两步进行。分别加入g-C3N4,NiO或g-C3N4/NiO后,AP的完全分解温度均有一定降低。根据图 4(c)的DTG曲线可知,纯AP在332.6℃和447.1℃出现失重速率极值。单独添加g-C3N4和NiO后,失重仍然是两个阶段,第二阶段失重对应温度降低,说明单一的g-C3N4和NiO对AP的热分解也具有催化作用。加入g-C3N4/NiO仅出现一个较大的失重峰,说明在这一阶段内快速分解,分解温度比纯AP的第二分解阶段降低了87.7℃,显示出较强的催化效果,其效果优于单独使用g-C3N4或NiO,也说明g-C3N4和NiO具有协同催化作用[30]。
在低温分解过程中,AP经质子转移离解生成气相的NH3和HClO4,HClO4(g)进一步分解生成氧化性中间产物C1O3, ClO, O, H2O等,氧化性中间产物如自由O与部分NH3发生氧化反应[22]。g-C3N4具有类石墨型层状结构,比表面积较大,有利于吸附NH3,HClO4等反应分子。但g-C3N4在AP表面覆盖程度大,阻碍了AP的离解与升华,抑制AP的低温分解,使低温分解温度增加。高温分解阶段是主要的分解阶段,该阶段不仅在气相中进行ClO4-氧化NH3的反应,也在凝聚相表面发生AP的分解过程。由于g-C3N4是离域的π共轭电子结构,具有非常优异的导电性能,有利于电子的转移和传导[31];NiO为P型半导体,d轨道可以提供良好的电子转移轨道,对AP热分解的电子转移过程起到桥接作用,有利于电子的转移[32];在g-C3N4/NiO复合材料中,g-C3N4与NiO形成异质结[33],具有更强的电子转移和传导能力,在氧化还原循环中进一步加速电子转移,使AP在更低的温度下分解。
3 结论(1) 采用混合煅烧法制备出g-C3N4/NiO复合材料,NiO均匀分散于g-C3N4的表面。
(2) g-C3N4/NiO复合材料使AP的高低温分解峰合并,高温分解温度降低了62.5℃,对AP的热分解表现出良好催化作用。g-C3N4/NiO的催化效果优于单独使用g-C3N4或NiO,g-C3N4和NiO具有协同催化作用。
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