Effect of tungsten source on dispersion and electrocatalytic property of tungsten carbide
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摘要: 碳化钨具有类铂催化性能、抗一氧化碳中毒和高导电性等特点,如能进一步改善其形貌、降低其尺寸及提高其比表面积,有望取代或部分取代贵金属催化剂.在以碳纳米管为载体,柠檬酸为络合剂的前提下,分别以偏钨酸铵、钨酸钠及仲钨酸铵为钨源,利用水热法制备了钨酸和碳纳米管复合物.采用温度控制程序,在空气气氛中于600℃对该复合物进行煅烧,得到氧化钨中间体,随之在氮气气氛中于950℃通入碳源进行碳化,得到碳化钨,发现以偏钨酸铵为钨源制备的碳化钨分散性最好,尺寸(约100 nm)最小,而以钨酸钠为钨源制备的碳化钨分散性和尺寸(约100~500 nm)次之,以仲钨酸铵为钨源制备的碳化钨分散性最差、尺寸(大于500 nm)最大.碳化钨载铂后的电催化测试结果表明,以偏钨酸铵为钨源制备的碳化钨作为载体担载铂颗粒,在硫酸体系中的峰电流密度(32.59 mA/cm2)优于钨酸钠(11.32 mA/cm2)和仲钨酸铵的(5.13 mA/cm2),在甲醇和硫酸体系中的峰电流密度(106.92 mA/cm2)优于钨酸钠(90.13 mA/cm2)和仲钨酸铵的(72.15 mA/cm2).Abstract: Tungsten carbide (WC) can be used as electrocatalyst of fuel cells for its like-Pt catalytic ability, CO anti-poisoning ability and high electric conductivity. It also can be expected to replace or partly replace noble metal catalyst because of its special morphology, reduced diameter and improved surface area. Ammonium metatungstate, sodium tungstate and ammonium paratungstate are respectively used as tungsten source, carbon nanotubes are used as carrier, citric acid is used as complexing agent, and tungsten acid and carbon nanotubes composite can be prepared after hydrothermal reaction. The tungsten source would be reduced into tungsten oxide as air pumped into reaction chamber at 600 ℃ by temperature-controlled program. And the obtained tungsten oxide can be further carbonized and changed to WC at 950℃ with nitrogen as atmosphere. The WC using ammonium metatungstate as tungsten source possesses the best dispersion and the smallest size. After the platinum nanoparticles are loaded on the surface of the obtained WC, the results demonstrated that the higher electrocatalytic activity (32.59 mA/cm2) in sulfuric acid solution of the Pt-WC prepared by using ammonium tungstate as tungsten source than those of sodium tungstate (11.32 mA/cm2) and ammonium paratungstate (5.13 mA/cm2), and higher electrocatalytic activity (106.92 mA/cm2) in methanol and sulfuric acid solution of the Pt-WC prepared by using ammonium tungstate as tungsten source than those of sodium tungstate (90.13 mA/cm2) and ammonium paratungstate (72.15 mA/cm2).
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近年来,随着钒工业的快速发展,沉钒等工序的含钒废水排放导致钒水污染严重.含钒废水处理的方法多达10余种[1-5],而应用较多的为化学沉淀法,如氯化铵沉淀法[6]、铁屑沉淀法[7],以及吸附法,如沸石吸附法[8]、活性炭吸附法[9]、阴离子树脂吸附法[10]、壳聚糖[11]等.
水滑石(Hydrotalcite,简称HT),是一种具有带正电荷层板而层间含有水和CO32-、NO3-等阴离子的层状结构化合物,通过焙烧可脱除层间水分子和阴离子并失去层状结构,再将焙烧后的产物置于含有阴离子的水溶液中,可吸附水溶液中阴离子并进行水合恢复成层状结构,这种特性也称为水滑石的“记忆效应”[12],已有报道表明焙烧后的水滑石对F-、Cl-、H2PO4-等阴离子具有高效的吸附性能[13-15].本研究合成了镁铝水滑石并研究了其对水中VO3-的吸附特性,旨在为治理水中钒污染提供参考.
1 实验方法
1.1 水滑石的合成
溶液A:将0.3 mol Mg(NO3)2 ·6H2O及0.1 molAl(NO3)3 ·9H2O完全溶解在150 mL水中;
溶液B:将50 mL 3 mol/L的Na2CO3溶液和100 mL 3 mol/L的NaOH溶液混合,搅拌均匀;
HT合成:以1滴/秒的速度向B溶液中逐滴加入溶液A,至加入A后的B溶液pH=10左右;滴加完毕后搅拌1 h,再将该溶液转入反应釜中,120 ℃反应24 h后水洗至中性;过滤,80 ℃烘干;
C-HT的合成:采用450 ℃焙烧HT 5 h,得到焙烧后的水滑石,简写为C-HT.
1.2 HT及C-HT对水中钒的吸附与脱附
1) 吸附等温线测试.分别称取0.1 g HT和0.1 g C-HT作为吸附剂,加入待吸附的100 mL含钒溶液中,pH值为7,分别于15 ℃、25 ℃、35 ℃恒温振荡24 h. VO3-在HT和C-HT上的平衡吸附量Qe可用式(1)计算:
(1) 式(1)中:C0为吸附前水溶液中VO3-的浓度(mg/L),Ce为吸附平衡后水溶液中VO3-的浓度(mg/L).
HT和C-H对TVO3-的去除效率可用式(2)计算:
(2) 式(2)中:n为吸附剂对VO3-的去除效率.
2) 吸附动力学测试.称取0.5 g C-HT作为吸附剂,分别加入待吸附的100 mL pH值为7的含钒溶液中,在25 ℃恒温振荡,间隔1~2 h进行取样测定.按照式(3)计算出不同时间C-H对VO3-的吸附量Qt.
(3) 式(3)中:Qt为不同取样时刻C-HT吸附VO3-的量(mg/g);Ct为不同取样时刻含钒溶液中的VO3-浓度(mg/L).
3) 脱附实验.称取0.5 g C-HT加入到500 mL的含VO3-100 mg/L溶液中,于25 ℃恒温震荡反应24 h后,离心,于60 ℃烘干.将吸附了VO3-的C-HT分别置于25 mL不同浓度的Na2CO3溶液中,于25 ℃恒温震荡反应24 h后,取样,测定溶液中VO3-浓度,并根据式(4)~式(6)计算脱附效率.
(4) 式(4)中:M1为Na2CO3脱附出的VO3-的质量(mg);Ce为脱附液脱附出的VO3-的浓度(mg/L);V为脱附液体积(L).
(5) 式(5)中:M2为被脱附的C-HT中所吸附的VO3-的质量(mg);Qe为被脱附的C-HT对VO3-的平衡吸附量(mg/g);m为被脱附的C-HT的质量(g).
(6) 式(6)中:η为脱附率.
1.3 分析方法
溶液中VO3-的浓度使用南京博奇分析仪器有限公司的Optima 7300DV型电感耦合等离子体发射光谱仪(ICP-OES)测定.焙烧前后及吸附后的材料的XRD图谱采用日本岛津公司XRD-6100型X射线衍射仪测定.
2 结果与讨论
2.1 吸附等温线
HT和C-HT对VO3-吸附等温线见图 1和图 2. HT对VO3-吸附量很低,35 ℃时最大吸附量仅为31.59 mg/g;而HT经过焙烧后所得的C-HT对VO3-的吸附性能大大增强,35 ℃时最大吸附量可达95.21 mg/g.
采用Langmuir方程式(7)和Freundlich方程式(8)拟合HT和C-HT对水中VO3-的吸附等温线.
(7) (8) 式(7)和式(8)中,Qe为VO3-在HT和C-HT上的平衡吸附量(mg/g);Ce为吸附后VO3-在水溶液中的平衡浓度(mg/L);n为经验常数;Qm为饱和吸附容量;KL和KF为平衡吸附系数.
吸附等温线拟合参数见表 1.采用Langmuir方程拟合VO3-吸附等温线的相关系数(R2)均大于0.98,而经验方程Freundlich方程拟合的相关系数(R2)较低,因此,焙烧前后的水滑石吸附水中VO3-的吸附等温线更符合Langmuir等温方程,表明水滑石对VO3-的吸附为单离子层吸附,被吸附的VO3-占据的位置不会再有新的VO3-吸附覆盖.
表 1 吸附等温线拟合参数Table 1. Adsorption isotherm parameters不同初始浓度条件下HT和C-HT对VO3-的去除率见图 3.随着溶液初始浓度的升高,HT的去除率仅为21.12 %;而焙烧后的镁铝水滑石对VO3-的吸附效率较高,对低浓度初始溶液中VO3-的去除率高达96.86 %,高浓度初始溶液中VO3-的去除率也可达63.66 %.
2.2 吸附动力学
C-HT在不同吸附时间条件下对的钒的吸附量见图 4.C-HT对VO3-的吸附作用主要在2 h以内完成,当VO3-初始浓度增大至150 mg/L时,吸附作用6 h内趋于平衡.
2.3 吸附机理探讨
图 5(a)为HT的XRD谱图,各衍射峰峰窄而尖,杂峰少而低,表明合成的HT晶相结构完整,结晶度高,具有2个明显的衍射峰(003)和(006),根据已有报道(003)和(006)为层状结构的特征衍射峰[16-17]. 图 5(b)为C-HT的XRD谱图,焙烧后(003)和(006)消失,出现了(400)和(440),根据标准JCPDS卡(JCPDS22-700),(400)和(440)为氧化镁和氧化铝的特征峰.
HT对VO3-的吸附主要是VO3-与层板间填充的CO32-交换吸附,常见阴离子交换能力的大致顺序为CO32->OH->SO42->HPO42->F->Cl->Br->I-[17],低价阴离子较难交换出层间高价阴离子,因此吸附量很低.而C-HT对VO3-的吸附量高,主要在于焙烧后水滑石层板间阴离子发生分解,层状结构破坏,形成结晶度较低的氧化镁和氧化铝双金属氧化物固溶体,具有“记忆效应”.对吸附了VO3-后的C-HT的XRD图谱如图 5(c)所示,吸附了VO3-后的C-HT具有和焙烧前的HT相同的层状结构(003)和(006),但特征衍射峰比HT的峰形要宽,表明吸附VO3-恢复的层状结构结晶程度下降.
2.4 C-HT的脱附性能
VO3-之所以能够被C-HT吸附,是因为VO3-进入C-HT中恢复了其层状结构.选用Na2CO3溶液为脱附剂时,CO32-与VO3-进行交换从而实现吸附剂的再生.从图 6中可以看出,脱附率大于90.0 %,而增大脱附剂的浓度,最高脱附率可达96.9 %.
3 结论
1) 采用水热法合成的镁铝水滑石具有完整的晶相结构,经450 ℃焙烧5 h形成结晶度较低的氧化镁和氧化铝双金属氧化物固溶体.
2) 焙烧前的镁铝水滑石对VO3-的吸附效率低,仅21.12 %;而焙烧后的镁铝水滑石对水中VO3-的吸附效率较高,可高达96.86 %,吸附量最大为95.21 mg/g,主要原因在于水滑石的“记忆效应”.
3) 镁铝水滑石在实验温度下对水中VO3-的吸附等温线采用Langmuir方程模拟的相关系数大于0.98.
4) 焙烧后的镁铝水滑石吸附VO3-后可以使用Na2CO3溶液进行再生,再生率达96.90 %.
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表 1 钨源、CNTs以及柠檬酸对前驱体产率的影响
Table 1 Effect of tungsten source, CNTs and citric acid on the productivity of precursor
试验序号 钨源 条件 前驱体产率/% 1 AMT 加CNTs、柠檬酸 98.1 2 APT 加CNTs、柠檬酸 83.1 3 ST 加CNTs、柠檬酸 91.3 4 AMT 不加CNTs,柠檬酸 30.0 5 AMT 加CNTs,不加柠檬酸 69.3 -
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