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随着全球工业化的迅速发展,越来越多的有机污染物被排放到我们赖以生存的水环境中,这不仅严重威胁着水生态环境,还对人体健康带来危害。随着国家对环保要求的不断提高以及相关政策的不断出台,采用传统的方法处理难降解有机废水在经济和技术上都难以达标,因此,迫切需要一种绿色高效的方法来去除水中的有机污染物。以产生活性自由基(例如SO4·−和·OH)高效分解污染物的高级氧化技术(advanced oxidation processes,AOPs)[1-2]被认为是处理这类生化性较差的有机污染物的有效手段[3]。根据提供的能量和反应种类的不同,AOPs技术可分为光催化、电催化、声解、臭氧化、Fenton/类Fenton反应和硫酸盐基AOPs (SR-AOPs)等[4-5]。不同的AOPs技术各有优点和缺点,在实际运用中,通常需要根据不同的情况将不同工艺进行组合来提高有机污染物降解率和矿化率[6]。尽管一些AOPs技术(例如,UV/H2O2体系和UV/过一硫酸盐(PMS)体系)在没有催化剂的情况下也能正常工作,但如果添加催化剂,就可以显著减少能源和试剂(活性物质来源)的消耗[7]。因此,设计一种有效且稳定的催化剂是AOPs绿色可持续发展的关键策略。
近年来,石墨氮化碳(g-C3N4)备受科学家们的关注。g-C3N4作为一种非金属半导体光催化物具有诸多优点:较宽的带隙宽度、丰富的活性位点、化学稳定性高和合成原材料成本低廉[8-10]。2009年,Wang等[11]首次将g-C3N4作为非金属半导体应用于光催化分解水制H2,并呈现出良好的光催化活性。从此,掀起了g-C3N4应用于催化领域的研究热潮,尤其是在催化降解水中有机污染物方面。然而,其可见光利用效率低、比表面积小以及光生的电子-空穴对快速重组等缺点限制了其催化活性[12-13]。目前,研究人员已开发出多种改性方法提高g-C3N4光催化活性,如形貌调控、元素掺杂、半导体复合等[14]。其中,元素掺杂被证明是调节g-C3N4独特电子结构和分子结构的有效方法,通过元素掺杂氮化碳可以大幅扩展其光响应范围,并促进光生电荷分离[15]。元素掺杂包括金属元素(如铁(Fe)、铜(Cu)、钴(Co)等)和非金属元素(如氮(N)、氧(O)、硫(S)、磷(P)等)。与金属掺杂相比,非金属元素更易接近和进入到g-C3N4骨架中,实现缺陷调控。经过这十多年的努力,已有大量研究结果表明非金属掺杂g-C3N4在光催化氧化(降解有机污染物)中的应用的可行性[16-17]。由于非金属原子具有较高的电离能和不同的电负性,当非金属原子被引入时,就会从其他物质中获得形成共价键的电子,从而改变掺杂处周围的电子分布。一般来说,由于高电负性的掺杂原子会促使电子从相邻的C原子迁移到掺杂处,原有的化学惯性收到破坏,C原子会具有诱导极化,从而产生新的反应位点。而电负性较低的掺杂原子会增加相邻C原子的不对称自旋密度,从而增强了g-C3N4的给电子能力。然而,仅利用光催化技术难以实现有机污染物的深度矿化,因此利用非金属掺杂g-C3N4的氧化能力活化硫酸盐,产生高活性物种降解有机污染物的研究引起广泛关注[18-19]。在光催化中引入额外的活性物质,如过硫酸盐,可以将可见光光催化技术与基于过硫酸盐的高级氧化工艺(PS-AOP)互补,进而显著提高有机污染物的降解效率和矿化率[20-21]。过硫酸盐包括过一硫酸盐(PMS, HSO5−)和过二硫酸盐(PDS, S2O82−),在PMS与PDS活化特性相比中,PMS中的O—O键更容易被激活,因此选用PMS作为硫酸根自由基的前躯体更有利于水中有机污染物降解。
鉴于非金属掺杂g-C3N4复合材料在AOPs领域的应用日趋频繁,本文综述了非金属掺杂g-C3N4的制备方法及其在光催化AOPs和硫酸盐基AOPs降解有机污染物的最新研究进展,讨论了近年引起关注的非金属掺杂g-C3N4光催化耦合过硫酸盐降解有机污染物的基本机理和研究进展。最后,提出了非金属掺杂氮化碳复合材料在这些AOPs中所面临的挑战和机遇,以期为非金属石墨氮化碳材料日后在光催化协同过硫酸盐活化催化降解有机污染物的发展和应用提供有益的参考。
非金属掺杂石墨氮化碳复合材料光催化协同活化过一硫酸盐降解有机污染物的研究进展
Research progress of nonmetallic doped graphite carbon nitride composites in photocatalytic activation of peroxymonosulfate system for organic pollutants removal
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摘要: 石墨氮化碳(g-C3N4)是一种二维的非金属半导体聚合物,因具有较宽的带隙宽度、丰富的活性位点和原材料合成成本低廉等优点,在协同光催化和活化过一硫酸盐(PMS)降解有机污染物方面具有广阔的应用前景。非金属元素掺杂g-C3N4不仅能克服纯氮化碳可见光利用效率低、比表面积小以及光生的电子-空穴对快速复合等缺点,还能大幅扩展其光响应范围,促进激活PMS的电子迁移,提高催化活性。本文综述了非金属掺杂g-C3N4的制备方法及其耦合光催化和PMS催化降解有机污染物的最新研究进展。此外,针对非金属掺杂g-C3N4光催化协同PMS去除污染物的过程机理进行了简要讨论。最后,提出非金属掺杂g-C3N4复合材料在高级氧化技术中所面临的挑战和机遇,旨在为非金属掺杂g-C3N4耦合光催化- PMS氧化体系在可持续碳催化中的应用提供参考依据。Abstract: Graphite carbon nitride (g-C3N4), as a two-dimensional nonmetallic semiconductor polymer, has been widely used for organic pollutants removal by synergistic photocatalysis-peroxymonosulfate (PMS) oxidation coupling system due to its wide band gap, abundant active sites and low cost. Doping non-metallic elements into g-C3N4 can not only overcome the shortcomings of low visible light utilization efficiency, small specific surface area and rapid recombination of photogenerated electron-hole pairs of pure g-C3N4, and it can also expand visible light response, promote electron migration for PMS activation and improve catalytic activity. In this paper, a comprehensive overview on the preparation methods of non-metallic doped g-C3N4 and their application in the removal of organic pollutants by the synergistic effect of photocatalysis and Fenton-like catalytic/PMS activation reactions. In addition, the photocatalysis-PMS mechanisms of non-metal doped g-C3N4 for pollutant removal are also discussed. Finally, the challenges and opportunities of non-metallic doped g-C3N4 composites in advanced oxidation technology are presented, and this review helps to provide insights into the synergistic effect between non-metallic doped g-C3N4 and photocatalysis-PMS oxidation coupling system in sustainable carbocatalysis.
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表 1 非金属掺杂g-C3N4的特性及应用
Table 1. The characteristic and application of non-metallic doped g-C3N4
掺杂元素
Doping
element前躯体
Precursor合成方法
Fabrication
method应用
Application带隙能量/eV
Band gap效率
(掺杂后/未掺杂)
Efficiency
(Doped/undoped)数据来源
ReferencesO 三聚氰胺、过氧化氢 水热法 降解亚甲基蓝 2.7 50%/20%(在3 h内) [33] O 1,3,5-三氯三嗪、双氰胺 溶剂热法 降解罗丹明B 2.09 0.249 min−1/0.007 min−1 [34] O 三聚氰胺、过氧化氢 水热法 降解罗丹明B 1.98 90%/20%(在30 min内) [40] O 双氰胺 水热法 光催化析氢 2.49 37.5 μmol·h−1/15.2·μmol h−1 [25] S 硫脲、三聚氰胺 热缩合 降解亚甲基蓝 2.53 60%/36%(在2 h内) [35] P 三聚氰胺 热缩合 芳烃催化 2.55 — [41] P 磷酸腺嘌呤、尿素 热缩合 光催化析氢 2.56 9524 μmol·g−1h−1/458 μmol·g−1h−1 [42] N 双氰胺、柠檬酸、尿素 热聚合 降解吲哚美辛 2.67 91.5%/16%(在90 min内) [38] N 尿素、咪唑 热聚合 降解罗丹明B 2.26 94.5%/23%(在120 min内) [43] P 磷酸氢二铵、双氰胺 热聚合 降解罗丹明B 2.63 0.0466 min−1/0.0115 min−1 [44] P 三聚氰胺、聚磷酸盐 热聚合 光催化析氢 2.31 57 μmol·h−1/6 μmol·h−1 [45] N 柠檬酸、尿素 热聚合 光催化析氢 — 64 μmol·h−1/14.8 μmol·h−1 [37] O 尿素、二水合草酸 热聚合 降解阿特拉津 100%/65%(在20 min内) [46] P 硫氰酸铵、六氟磷酸铵 缩聚法 降解罗丹明B 2.86 0.03679 min−1/0.09856 min−1 [39] B 三聚氰胺、硼酸 缩聚法 降解四环素 2.83 68% /10%(在2 h内) [47] C 无水乙醇预处理的三聚氰胺 缩聚法 降解罗丹明B 2.65 0.0362 min−1/0.0081 min−1 [32] 表 2 非金属掺杂g-C3N4活化过硫酸盐光催化降解有机污染物
Table 2. Non-metallic doping g-C3N4 activated persulfate photocatalytic degradation of organic pollutants
催化剂
Catalyst污染物
Target pollutant光源
Light
source反应条件
Reaction conditions去除效果
Removal efficiency活化降解机理(主要的活性物种)
Active degradation mechanism (Main active species)数据来源
ReferencesOCN 卡马西平 500W
氙灯Cat = 1 g·L−1;
CBZ = 5 mg·L−1;
PMS=5.0 mmol·L−198.7%
(在3 h内)氧掺杂降低了带隙,提高了光响应能力和光致载流子分离,从而提高了OCN的光催化活性能。(1O2、O2·−、SO4·−和h+) [71] OCN 吡虫啉(IMD) 500W
氙灯Cat = 0.5 g·L−1;
IMD = 3 mg·L−1;
PMS = 5.0 mmol·L−1;
T = 25 ℃;
pH = 4.2 ± 0.3约100%
(在2 h内)OCN调制电子结构促进了PMS的吸附、电子转移和氧化还原位点的形成(1O2) [21] S-doped CN(CNS) 双酚A 150W
可见光灯Cat = 0.3 g·L−1;
BPA = 50 mg·L−1;
PMS = 0.3 g·L−1;
T = 20–40 °C;
pH = 580%
(在1 h内)硫和氮共掺杂的协同作用,这可能使相邻的碳原子更活跃,带隙更窄,从而分别提高其催化活性活化PMS和光催化活性。(SO4·–) [20] S-doped CN 双酚A 150W
可见光灯Cat = 0.3 g·L−1;
BPA = 50 mg·L−1;
PMS = 0.3 mmol·L−1;
T = 40°C;
pH=540%
(在1 h内)硫掺杂使碳原子更活跃,带隙更窄,从而提高其催化活性和光催化活性。(SO4·−) [72] VCN 罗丹明B 350W
氙灯Cat = 0.15 g·L−1;
RhB = 0.04 mmol·L−1;
MB = 0.04 mmol·L−1;
MO = 0.04 mmol·L−1;
PMS = 0.5 g·L−1;
T = 20 ℃;
pH = 5.594%
(在1 h内)N空位增强了PMS的吸附,捕获光生电子使PMS还原为·OH,而由N损失而产生的缺电子C原子促进PMS氧化为O2·−。(O2·−和·OH) [73] CQDs-(PCNC) 双酚A 300W
氙灯Cat = 0.33 g·L−1;
BPA = 20 mg·L−1;
PMS = 30 mg·L−1;
T = 20 ℃;
pH = 6.795%
(在0.5 h内)CQDs的加入使氮化碳表面积增大,光吸收增强,电荷分离改善,活性位点增多有利于PMS的光催化活化。(1O2、O2·−和 h+) [65] B-CNU 2、4-二氯苯酚 35W
LED灯Cat = 0.25 g·L−1;
2,4-DCP = 0.06 mmol·L−1;
PMS = 0.5 mmol·L−1100%
(在1.5 h内)硼掺杂拓宽并强化了催化剂表面的正静电势分布并降低电阻,使得PMS向催化剂的电子转移更容易,从而有效地生成1O2(1O2) [66] PI-g-C3N4 双酚A 300W
氙灯Cat = 1 g·L−1;
BPA = 10 mg·L−1;
PMS = 5 mmol·L−196%
(在1 h内)光生电子-空穴对的有效分离效率和PI-g-C3N4具有更高的电导率,通过光生电子促进PMS活化为活性自由基(O2·−、1O2和 h+) [70] AC/CN 阿特拉津 300W
氙灯Cat = 1g·L−1;
ATZ = 5 mg·L−1;
PMS = 3 g·L−1;
pH = 5.60.0376 min−1 光催化
(·OH、SO4·−)[69] PCN/GO 马拉硫磷农药 35W
LED灯Cat = 0.6 g·L−1;
农药 = 0.1 mmol·L−1;
PMS = 0.15 mmol·L−1;
pH = 1098%
(在5 h内)GO与PCN的耦合增加了界面电荷传输并最小化了光生载流子复合,促进PMS活化(O2·−、SO4·−、h+和·OH) [74] CO-C3N4 TCH 300W
氙灯Cat = 0.2 g·L−1;
TCH = 10 mg·L−1;
PMS = 0.2 g·L−197.77%
(在1 h内)CO-C3N4对可见光的吸收更强,光产生的载流子分离更快,这在很大程度上有助于PMS活化为活性自由基(O2·−、1O2和h+) [67] -
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