材料研究学报, 2023, 37(10): 781-790 DOI: 10.11901/1005.3093.2022.627

研究论文

g-C3N4/Ag/BiOBr复合材料的制备及其光催化还原硝酸盐氮

刘志华,1,3,4, 岳远超2, 丘一帆2, 卜湘1,3,4, 阳涛1,3,4

1.长沙理工大学水利与环境工程学院 长沙 410114

2.长沙理工大学化学化工学院 长沙 410114

3.洞庭湖水环境治理与生态修复湖南省重点实验室 长沙 410114

4.湖南省环境保护河湖疏浚污染控制工程技术中心 长沙 410114

Preparation of g-C3N4/Ag/BiOBr Composite and Photocatalytic Reduction of Nitrate

LIU Zhihua,1,3,4, YUE Yuanchao2, QIU Yifan2, BU Xiang1,3,4, YANG Tao1,3,4

1.School of Hydraulic and Environmental Engineering, Changsha University of Science & Technology, Changsha 410114, China

2.School of Chemistry and Chemical Engineering, Changsha University of Science & Technology, Changsha 410114, China

3.Key Laboratory of Dongting Lake Aquatic Eco-Environmental Control and Restoration of Hunan Province, Changsha 410114, China

4.Engineering and Technical Center of Hunan Provincial Environmental Protection for River-Lake Dredging Pollution Control, Changsha 410114, China

通讯作者: 刘志华,liuzhihua@csust.edu.cn, 研究方向为光催化材料

收稿日期: 2022-11-25   修回日期: 2022-12-27  

基金资助: 湖南省教育厅优秀青年项目(19B040)
长沙理工大学青年教师成长计划(2019QJCZ038)
长沙理工大学研究生实践创新项目(SJCX202189)

Corresponding authors: LIU Zhihua, Tel: 13574872739, E-mail:liuzhihua@csust.edu.cn

Received: 2022-11-25   Revised: 2022-12-27  

Fund supported: Outstanding Youth Program of Hunan Education Department(19B040)
Young Teacher Development Program of Changsha University of Science and Technology(2019QJCZ038)
Postgraduate Practice Innovation Program of Changsha University of Science and Technology(SJCX202189)

作者简介 About authors

刘志华,男,1979年生,博士

摘要

用高温煅烧、反应合成以及光还原等方法制备新型g-C3N4/Ag/BiOBr复合光催化材料,使用SEM、XRD、EPMA、FT-IR、XPS和UV-vis等手段对其表征,研究了这种复合材料在金卤灯照射下对硝酸盐氮(50 mg/L)的还原效果和氮气选择性。结果表明,使用1 g/L g-C3N4/Ag/BiOBr复合光催化材料,光反应180 min后硝酸盐的去除率为95.2%。用g-C3N4/Ag/BiOBr光催化硝酸盐氮的主要产物中N2的占比最高(为88.0%),氮气的选择性为92.4%。g-C3N4/Ag/BiOBr催化剂中的Ag能促进对电子的捕捉,BiOBr的光生电子经银单质转移到g-C3N4的价带上形成Z型复合光催化结构。这种复合光催化剂可将硝酸盐氮氧化,空穴清除剂甲酸在复合材料中空穴的作用下转化成强氧化性物质(COO-)并进一步将其还原成硝酸盐氮。

关键词: 无机非金属材料; g-C3N4/Ag/BiOBr; 硝酸盐氮; 复合材料; 光催化; 氮气选择性

Abstract

Nitrate as one of the water pollutants is one of the major environmental problems. Photocatalytic reduction of nitrate nitrogen has attracted a lot of attention because of its high efficiency and environmental friendliness. The g-C3N4/Ag/BiOBr composite photocatalyst was prepared by high temperature calcination, reaction synthesis and photoreduction. The photocatalysts were characterized by SEM, XRD, EPMA、FT-IR, XPS and UV-vis, and the reduction effect of the composite on nitrate nitrogen (50 mg/L) under the irradiation of metal halide lamp was studied. The results showed that when 1g/L g-C3N4/Ag/BiOBr catalyst was used, the nitrate concentration was 2.4 mg/L, and the removal rate was 95.2% after 180 min photoreaction. Compared with g-C3N4, BiOBr and g-C3N4/BiOBr photocatalysts, the removal rates increased by 38.8%, 34.6% and 13.1%, respectively. Nitrogen was the main product in the photocatalytic conversion of nitrate nitrogen. The proportion of N2 in the main products of nitrate nitrogen photocatalyzed by g-C3N4/Ag/BiOBr was the highest (88.0%), and the selectivity of nitrogen was 92.4%. Ag can be used as an electron trapping agent to effectively reduce the recombination of electron-hole pairs in photocatalytic materials. Under the action of silver, the photogenerated electrons of BiOBr are transferred to the valence band of g-C3N4 by silver elemental material, forming a Z-type composite photocatalytic structure. Nitrate nitrogen can be directly oxidized by the composite photocatalyst, and the hole scavenger formic acid can be converted into a strong oxidizing substance (COO.-) under the action of the composite hole, which can further reduce nitrate nitrogen.

Keywords: inorganic non-metallic materials; g-C3N4/Ag/BiOBr; nitrate nitrogen; composite; photocatalysis; nitrogen selectivity

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刘志华, 岳远超, 丘一帆, 卜湘, 阳涛. g-C3N4/Ag/BiOBr复合材料的制备及其光催化还原硝酸盐氮[J]. 材料研究学报, 2023, 37(10): 781-790 DOI:10.11901/1005.3093.2022.627

LIU Zhihua, YUE Yuanchao, QIU Yifan, BU Xiang, YANG Tao. Preparation of g-C3N4/Ag/BiOBr Composite and Photocatalytic Reduction of Nitrate[J]. Chinese Journal of Materials Research, 2023, 37(10): 781-790 DOI:10.11901/1005.3093.2022.627

化肥的过度使用、工业废水和生活污水的大量排放,使硝酸盐氮成为水体中的主要污染物[1~3]。水体中的硝酸盐氮产生的富营养化加速了水质的恶化[4]。因此,世界卫生组织(WHO)和中国的《生活饮用水卫生标准》(GB5749-2022)规定饮用水中硝酸盐氮的浓度不得高于10 mg/L。

生物反硝化[5]、化学还原[6]、反渗透[7]、电渗析[8]、离子交换[9]、电催化[10]、光催化[11]等技术,可去除硝酸盐氮。光催化技术的设备简单、环境友好和无二次污染,受到了极大的重视[12,13]。但是其电子-空穴对的复合和光能的利用效率较低,限制了光催化转化硝酸盐氮的应用[14]。因此,急待开发更高效的光催化剂。

TiO2是一种常见的催化剂,可用于硝酸盐氮的光催化还原[15]。为了提高硝酸盐氮的转化效率和氮气选择性,可掺杂金属(Au[16]、Ag[17]、Cu[18]等)或金属氧化物(Ag2O[19]、Cu2O[20]等);多元材料的复合也可提高TiO2光催化还原硝酸盐氮。Wang等[21]合成多金属氧化物(POM)/TiO2/Cu复合光催化材料,硝酸盐氮的去除效率为76.53%,氮气选择性为82.09%。Hou等[22]合成核壳状Ag/SiO2@cTiO(2)复合光催化材料,硝酸盐氮的去除率为95.8%,氮气选择性为93.6%。但是,由于TiO2的带隙较宽(约3.2 eV),不能直接利用太阳光[23]。g-C3N4(带隙约2.7 eV)[24]和BiOBr(带隙2.6~2.9 eV)[25]的带隙较窄,可用于光催化去除污染物 [26]。但是,光催化材料单体的比表面积小、载流子重组速度较快,影响其光催化活性[27]。多重材料的掺杂复合,是提高光催化材料效能的主要手段[28]。Zheng R等[29]采用g-C3N4可实现约50%硝酸盐氮的去除,采用TiO2/Ti3C2/g-C3N4复合材料使硝酸盐氮的去除效率提高到93.03%,氮气选择性达到96.62%。贵金属Ag具有较低的费米能级(EF=4.74 eV),可阻止光催化材料电子-空穴对的复合[30]。不同银掺杂改性的复合材料,都能明显提高g-C3N4或BiOBr的光催化性能。杨利伟等[31]研究发现,与pg-C3N4、BiOBr 单体和二元复合材料pg-C3N4/BiOBr相比,pg-C3N4/BiOBr/Ag(5%)光催化降解磺胺甲唑的效果显著提高。Shelton等[32]用Ag/g-C3N4光催化剂对比了紫外光和可见光下还原水中硝酸盐效果,发现Ag强化了还原作用。本文以g-C3N4、BiOBr和银为基质制备一系列的复合光催化材料(Ag/g-C3N4、Ag/BiOBr、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr),选择低浓度硝酸盐氮(50 mg/L以氮计)为处理对象,对比分析不同光催化材料对硝酸盐氮的去除性能及其氮气选择性。

1 实验方法

1.1 实验用主要试剂和材料

实验用药品有:三聚氰胺、硝酸铋、溴化钾、甲酸、N-(1-萘基)乙二胺二盐酸盐、乙醇和乙二醇、硝酸银、纳氏试剂、盐酸和氢氧化钠。其中三聚氰胺为化学纯,其余的均为分析纯。

1.2 催化剂的制备

1.2.1 BiOBr的制备

用反应合成法制备BiOBr:先将4.8 mmol的Bi(NO)3.5H2O和4.8 mmol的KBr溶解在12 mL的去离子水和28 mL的乙二醇中,对其超声处理0.5 h。然后将混合物加入到50 mL内衬为聚四氟乙烯的高压釜中,在110℃加热反应10 h。将产物用蒸馏水和乙醇离心洗涤3次后在60℃干燥6 h,得到BiOBr粉末。

1.2.2 g-C3N4的制备

用高温煅烧法制备g-C3N4:一定量的三聚氰胺放入带盖的氧化铝坩埚中,然后及其放入马弗炉中,以5℃/min的速率升温至550℃保持2 h,冷却至室温后取出产物,用研钵研磨后用稀盐酸和蒸馏水充分洗涤,然后及其在80℃干燥12 h,得到黄色g-C3N4粉末。

1.2.3 g-C3N4/BiOBr复合光催化剂的制备

将4.8 mmol的 Bi(NO)3.5H2O、4.8 mmol的 KBr和一定含量的g-C3N4溶解在12 mL H2O和28 mL乙二醇中并超声处理0.5 h,然后将混合物转移到50 mL内衬为聚四氟乙烯的高压釜中,在110℃加热10 h。反应后将产物用蒸馏水和乙醇离心充分洗涤,最后在60℃干燥6 h,得到g-C3N4/BiOBr复合光催化剂。

1.2.4 Ag沉积复合光催化剂的制备

用光还原法制备Ag沉积复合光催化剂:在25 mL浓度为4 mmol/L的AgNO3水溶液中加入制备出的BiOBr、g-C3N4、g-C3N4/BiOBr粉末,超声分散30 min后用300 W Xe灯辐照2 h,然后将产物用蒸馏水洗涤3遍,离心机分离后在80℃烘箱中干燥12 h,得到银负载量为5%(质量分数)的银沉积BiOBr材料(Ag/BiOBr)、5%(质量分数)银沉积g-C3N4材料(Ag/g-C3N4)、5%(质量分数)银沉积g-C3N4/BiOBr材料(g-C3N4/Ag/BiOBr)。

1.3 催化剂性能的表征

用FEI Sirion200场发射扫描电子显微镜观察催化剂的微观形貌;用JEOL JXA-8230电子探针显微分析仪分析元素的面分布;用Rigaku D/Max2500型18 kW转靶X射线衍射仪分析光催化剂的晶体结构;用Thermo Scientific K-Alpha X射线光电子能谱仪测量光催化剂的化学结构和元素价态等有关参数;用ThermoFisher FT-I傅里叶红外光谱仪测定光催化剂的化学结构;用岛津3600plus紫外-可见近红外漫反射光谱仪测量光催化剂的光吸收性能。

1.4 光催化还原硝酸盐

将预先配置好的50 mg/L(以N计)硝酸钾(KNO3)溶液倒入200 mL反应器中,加入0.2 mg上述光催化剂,通氩气30 min后开启磁力搅拌,暗反应30 min后取样测试;加入0.1 mol/L的甲酸作为空穴清除剂,调整pH值为4,在功率为300 W的金卤灯下开始计时反应。用注射器每30 min取一次水样,用0.22 μm滤头过滤,分别测定水样中硝酸盐氮(NO3--N)、亚硝酸盐氮(NO2--N)、氨氮(NH4+-N)的浓度,重复3次实验。用以下公式计算NO3N的去除率和氮气选择性[29]

RN=[NO3-]0-[NO3-]t[NO3-]0×100%
SN2=[NO3-]0-[NO3-]t-[NO2-]t-[NH4+]t[NO3-]0-[NO3-]t×100%

式中RN为NO3--N去除率(%),[NO3-]0为初始硝酸盐浓度(mg/L),SN2为N2选择性(%),[NO3-] t 、[NO2-] t 、[NH4+] t 分别为时间t时刻的硝酸盐氮(NO3--N)、亚硝酸盐(NO2--N)、氨氮(NH4+-N)的浓度(mg/L)。

1.5 测试污染物质

用紫外分光光度法(HJ/T346-2007)测量硝酸盐氮;用分光光度法(GB7493-87)测量亚硝酸盐氮;用纳氏试剂分光光度法(HJ535-2009)测量氨氮;用电极法(HJ1147-2020)测量pH值。

2 实验结果和讨论

2.1 光催化剂的物相和结构

2.1.1 光催化剂的物相组成

图1给出了不同光催化剂(g-C3N4、Ag/g-C3N4、BiOBr、Ag/BiOBr、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr)的XRD谱。可以看出,在g-C3N4的XRD谱的12.67°和27.46°处有两个明显的特征峰,对应g-C3N4标准卡片(JCPDS87-1526)的(100)、(110)的晶面特征衍射峰。BiOBr催化剂的衍射特征峰分别位于10.9°、21°、25.2°、31.7°、32.2°、39.4°、46.2°、51.8°、53.6°、57.1°、66.4°、67.5°、71.2°和76.7°,对应BiOBr标准卡片(JCPDS78-0348)的(001)、(002)、(101)、(102)、(110)、(112)、(200)、(104)、(211)、(212)、(204)、(220)、(214)和(310)晶面特征衍射峰。

图1

图1   不同光催化剂的XRD谱

Fig.1   XRD patterns of the different photocatalyst


2.1.2 光催化材料的表面化学组成和状态

为了了解光催化材料的表面化学组成及状态,采用X射线光电子能谱(XPS)对BiOBr、g-C3N4、g-C3N4/Ag/BiOBr光催化剂进行了分析(图2)。图2a中为BiOBr、g-C3N4、g-C3N4/Ag/BiOBr全谱分析图,其中元素C和元素N能在g-C3N4、g-C3N4/Ag/BiOBr光催化剂中发现,Bi、O、Br等元素能在BiOBr、g-C3N4/Ag/BiOBr光催化剂中发现,但g-C3N4/Ag/BiOBr中的Ag峰相对较弱,这可能是由于催化剂中银含量较低所致。图2b BiOBr、g-C3N4/Ag/BiOBr中Bi的结合能均为159.2和164.5 eV,峰值均相差5.3 eV,说明两者均有Bi3+的存在[33]图2c中BiOBr中Br的结合能为68.3和69.4 eV,分别对应于Br3d5/2和Br3d3/2[34]图2d中g-C3N4和g-C3N4/Ag/BiOBr均存在典型C-C键(284.8 eV)[35]以及 N=C-N键(288.2 eV),而g-C3N4中存在C-NH2(285.1 eV),这可能是g-C3N4/Ag/BiOBr中其他元素的遮蔽作用导致。图2e中g-C3N4存在C-N=C键(398.8 eV)和C-NH2键(400.7 eV),g-C3N4/Ag/BiOBr结合能为299.2 eV(C-N=C键)和400.4 eV(C-NH2键),N的结合能产生了轻微的偏移,这可能是材料杂化导致。图2f中BiOBr 中结合能为530.0 eV(Bi-O键)和531.2 eV(O-H键)[36],g-C3N4/Ag/BiOBr中结合能为530.1 eV(Bi-O键)和531.2 eV(O-H键),O的结合能也出现轻微的偏移。图2g中Ag结合能为267.9 eV和373.9 eV,两个峰间距为6.0 eV,说明有单质银的存在[37]

图2

图2   BiOBr、C3N4和C3N4-Ag-BiOBr 的XPS谱

Fig.2   XPS analysis of BiOBr, C3N4 and C3N4-Ag-BiOBr (a) survey spectrum, (b) Bi4f, (c) Br3d, (d) C1s, (e) N1s, (f) O1s, (g) Ag3d


图3可以看出,Ag/g-C3N4、Ag/BiOBr和g-C3N4/Ag/BiOBr的FT-IR谱中中未出现明显的Ag衍射峰,其原因是Ag沉积量(5%)较少[38]。同时g-C3N4/BiOBr、g-C3N4/Ag/BiOBr中g-C3N4衍射峰也不明显(在27.46°出现了较弱的g-C3N4(110)的晶面特征衍射峰),其主要原因是在催化剂制作过程中g-C3N4为主要基质,表层为BiOBr覆盖。

图3

图3   不同催化剂的FT-IR谱

Fig.3   FT-IR analysis of the different photocatalyst


2.1.3 光催化剂的结构

为了揭示光催化剂的化学组成及化学键情况,用傅里叶变换红外光谱(FT-IR)分析了不同催化剂(g-C3N4、Ag/g-C3N4、BiOBr、Ag/BiOBr、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr)(图3)。由图3可见,在1200~1700 cm-1之间出现了明显的吸收峰,对应C-N结构[39~41],是g-C3N4、Ag/g-C3N4、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr催化剂中的典型结构。与g-C3N4、Ag/g-C3N4对比表明,Ag的沉积明显强化了吸收峰的强度,而g-C3N4/Ag/BiOBr催化剂的吸收峰反而减弱,其原因可能是g-C3N4作为基质被Ag、BiOBr包裹(图12)。在3100~3600 cm-1的峰分别对应-NH基团和-OH基团[42,43],在g-C3N4、Ag/g-C3N4、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr催化剂的FT-IR光谱中较为明显。在700~800 cm-1处的峰主要涉及三嗪结构[39~41],为g-C3N4、Ag/g-C3N4、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr催化剂的典型结构,Ag的沉积同样提高了吸收峰的强度。在450~600 cm-1的吸收峰对应Bi-O键的伸缩振动[25,44,45],是BiOBr、Ag/BiOBr、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr催化剂的典型结构。

2.2 g-C3N4/Ag/BiOBr复合催化材料的形貌和元素分布

2.2.1 复合光催化材料的形貌

图4给出了不同催化剂(g-C3N4、Ag/g-C3N4、BiOBr、Ag/BiOBr、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr)的SEM照片。g-C3N4催化剂以层状结构为主(图4a),而Ag/g-C3N4催化剂只表现出一定的层状结构(图4d)。BiOBr催化剂以小型片状颗粒态为主(图4b),Ag/BiOBr催化剂颗粒粒径比BiOBr催化剂的更小(图4e),其原因可能主要是在催化剂的制作过程中进行了超声混合。g-C3N4/BiOBr催化剂以颗粒态为主(图4c)。与图4a图4b相比,由于g-C3N4/BiOBr材料是在g-C3N4的基础上采用反应合成法制作,g-C3N4材料主要起基质作用,表面细小的颗粒物主要为合成的BiOBr颗粒。g-C3N4/Ag/BiOBr催化剂颗粒比g-C3N4/BiOBr催化剂颗粒进一步细化和分散(图4f),可能也是超声分散的影响。

图4

图4   不同催化剂的SEM像

Fig.4   SEM images of BiOBr and Ag/BiOBr (a) g-C3N4, (b) BiOBr, (c) g-C3N4/BiOBr, (d) Ag/g-C3N4, (e) Ag/BiOBr, (f) g-C3N4/Ag/BiOBr


2.2.2 g-C3N4/Ag/BiOBr催化剂元素的面分布

图5给出了g-C3N4/Ag/BiOBr催化剂的二次电子图像和元素面分布。可以看出,在g-C3N4/Ag/BiOBr光催化材料中分布着元素Bi、O、Br、Ag、C、N,其中元素Bi、O、Br是g-C3N4/Ag/BiOBr光催化材料中的主要元素,它们的分布很明显且含量很高,而元素C和N较少。元素Ag均匀弥散地分布在催化剂表面,表明用反应合成法和电沉积法可将Ag颗粒沉积在催化剂的表面。

图5

图5   g-C3N4/Ag/BiOBr的元素面分布

Fig.5   Element mapping of g-C3N4/Ag/BiOBr


2.3 复合材料光吸收能力的影响因素

图6给出了不同光催化剂(g-C3N4、Ag/g-C3N4、BiOBr、Ag/BiOBr、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr)的紫外-可见漫反射光谱(UV-vis)。从图6可见,g-C3N4、Ag/g-C3N4、BiOBr、Ag/BiOBr、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr催化剂在紫外光和可见光均有光吸收,其中g-C3N4最大光吸收波长约530 nm,BiOBr最大光吸收波长约510 nm,g-C3N4/BiOBr最大光吸收波长约520 nm。这表明,Ag可明显提高光催化剂对可见光的吸收,Ag/g-C3N4、Ag/BiOBr、g-C3N4/Ag/BiOBr相对于g-C3N4、BiOBr、g-C3N4/BiOBr对可见光的吸收均有明显的提高,其中g-C3N4/Ag/BiOBr增幅最大(图6)。

图6

图6   不同催化剂的紫外可见光漫反射谱

Fig.6   UV-vis diffuse reflectance spectra of the different photocatalyst


2.4 光催化剂还原性能的比较

图7给出了不同光催化对硝酸盐氮还原的性能。使用催化剂单体(BiOBr、g-C3N4)时,催化剂还原硝酸盐的效率相对较低,其中BiOBr在光照180 min时硝酸盐浓度下降到19.7 mg/L,去除率达60.6%;g-C3N4在光照180 min时硝酸盐浓度下降到21.8 mg/L,去除率达56.4%。采用BiOBr和g-C3N4材料复合后,硝酸盐氮的去除率迅速提高,g-C3N4/BiOBr在光照180 min时硝酸盐浓度下降到9.4 mg/L,去除率达81.2%。采用Ag沉积后,催化材料的光催化效能进一步提高,g-C3N4/Ag/BiOBr在光照180 min时硝酸盐浓度最小(为2.4 mg/L),去除率达95.2%。这表明,Ag能促进光生电子和空穴的分离,提高光催化效率。

图7

图7   不同光催化剂对硝酸盐氮还原的影响

Fig.7   Influence of the different photocatalyst on nitrate reduction process


硝酸盐氮的还原产物主要有亚硝酸盐、氨氮和氮气,亚硝酸盐、氨氮为水体污染物质,在转化过程中氮气选择性是光催化还原的主要评价指标。为了分析光催化过程中氮气选择性,图8给出了使用不同光催化剂时的硝酸盐氮产物情况。从图8可见,在硝酸盐氮的光催化转化过程中亚硝酸盐氮的产率较低,仅在g-C3N4、Ag/g-C3N4光催化作用下副产物中亚硝酸盐氮的浓度较高,分别为0.95、1.3 mg/L。副产物氨的浓度较高,使用Ag/g-C3N4光催化时氨氮浓度最大(为9.0 mg/L)。氮气是光催化转化过程中的主要产物(图8),表明光催化还原硝酸盐氮的氮气选择性较好。用g-C3N4/Ag/BiOBr光催化时N2占比最高(为88.0%),氮气选择性为92.4%;使用Ag/BiOBr光催化时N2的占比为83.5%,而氮气选择性最高(为95.0%);使用Ag/g-C3N4光催化时N2占比为66.1%,氮气选择性最低(为76.0%);使用g-C3N4光催化时N2占比最低(为47.6%),氮气选择性为84.4%。

图8

图8   不同光催化剂对硝酸盐氮还原及产物的影响

Fig.8   Effect of the different photocatalyst on nitrate reduction and its products


为了评估复合光催化剂还原效果,在表1给出了相关复合光催化还原硝酸盐效果的对比。研究光催化还原硝酸盐氮,多使用甲酸作为光催化剂剂的空穴清除剂,而用草酸作为空穴清除剂时硝酸盐氮的转化率相对较低(表1)。其原因是,甲酸在捕捉空穴清除剂生成强氧化性物质(COO.-)[46,47],可进一步促进硝酸盐氮的还原。由于硝酸盐氮相对较为稳定,在光催化过程中多采用高压汞灯提供紫外光源。近年来,在硝酸盐氮的转化率和氮气选择性方面均取得了较好的效果。本文采用对可见光有较强响应的g-C3N4、BiOBr催化材料进行合成,并使用贵金属银进行掺杂以提高g-C3N4/Ag/BiOBr复合材料的性能。实验中用金卤灯光源,硝酸盐氮的去除效率仅次于Ag/SiO2@cTiO2复合材料,氮气选择性也较高,表明本文制作的复合光催化材料具有较高的性能。

表1   光催化还原硝酸盐氮的比较

Table 1  Photocatalytic reduction of nitrate in this work and previous literature studies

PhotocatalystHole scavengerLight sourceRN/%SN2/%Ref.
g-C3N4/Ag/BiOBrFormic acidHalide lamp95.292.4This work
Ag/TiO2Formic acidXe lamp9590[48]
Au/TiO2Oxalic acidHigh-pressure Hg lamp4449.9[49]
0.5TiO2/Ti3C2/g-C3N4Formic acidHigh-pressure Hg lamp93.396.62[29]
Fe-LiNbO3Formic acidHigh-pressure Hg lamp9088[50]
ZnSe/BiVO4Formic acidHigh-pressure Hg lamp89.8491.03[14]
NH2-MIL-101(Fe)/BiVO4Formic acidHigh-pressure Hg lamp94.893.5[51]
Ag/SiO2@cTiO2Formic acidHigh-pressure Hg lamp95.893.6[22]

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2.5 光催化降解机理

本文分析对比了g-C3N4及BiOBr复合光催化材料,选择硝酸盐氮(50 mg/L)为研究对象以探索光催化还原效率。经g-C3N4/Ag/BiOBr光催化后硝酸盐氮的浓度可降低到2.4 mg/L(图7),氮气选择性可达92.4%(图8),表明使用Ag强化复合材料光催化作用可有效提高光催化的还原性能。

Ag作为电子捕捉剂,能减缓光催化材料的电子-空穴对的复合[52],使光催化剂的催化性能提高(图6~8)。图6中的紫外-可见漫反射光谱分析表明,Ag强化了材料对可见光的响应,Ag/g-C3N4、Ag/BiOBr、g-C3N4/Ag/BiOBr在可见光区域光吸收能力明显加强,进一步促进复合光催化材料的光还原性能。g-C3N4/Ag/BiOBr光催化材料硝酸盐的光还原机制,如图9 所示。g-C3N4价带电势为1.52 eV,导带电势为-1.2 eV[33];BiOBr价带电势为3.18 eV,导带电势为0.30 eV[39]。这表明,g-C3N4和BiOBr导带电势接近,在银的作用下BiOBr的光生电子经银单质转移到g-C3N4的价带上形成了Z型复合光催化结构。部分硝酸盐氮可被复合光催化剂直接氧化。亚硝酸盐氮的氧化电势较低,可被迅速还原,系统中亚硝酸盐氮的浓度较低(图8)。同时,实验中使用甲酸作为空穴清除剂,甲酸在复合材料空穴作用下转化成强氧化性物质(COO.-)[46,47],其较高的电势可进一步还原硝酸盐氮,从而促进了硝酸盐氮的去除。

图9

图9   g-C3N4/Ag/BiOBr光催化材料的光还原机制

Fig.9   Photoreduction mechanism of g-C3N4/Ag/BiOBr photocatalytic materials


3 结论

(1) 用高温煅烧、反应合成以及光还原等方法可制备g-C3N4/Ag/BiOBr复合光催化剂。Ag能强化光催化剂(g-C3N4、Ag/g-C3N4、BiOBr、Ag/BiOBr、g-C3N4/BiOBr、g-C3N4/Ag/BiOBr)对可见光的响应。

(2) 用g-C3N4/Ag/BiOBr催化剂反应180 min,硝酸盐的浓度为2.4 mg/L,去除率为95.2%。银沉积和多元材料复合都能提高光催化性能,促进硝酸盐氮的还原。

(3) 硝酸盐氮光催化转化的主要产物是氮气,用g-C3N4/Ag/BiOBr光催化时产物中N2的占比最高,氮气选择性为92.4%;用Ag/BiOBr光催化时N2的占比为83.5%,而氮气选择性最高。硝酸盐氮的光催化转化中亚硝酸盐氮的占比最小,在Ag/g-C3N4光催化作用下副产物中亚硝酸盐氮浓度较高。

(4) Ag用作电子捕捉剂,可降低光催化材料的电子-空穴对的复合。在银的作用下,BiOBr的光生电子经银单质转移到g-C3N4的价带可形成Z型复合光催化结构,硝酸盐氮可被复合光催化剂氧化。在复合材料空穴作用下空穴清除剂甲酸转化成强氧化性物质(COO.-),可进一步还原硝酸盐氮。

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