【作者】 徐俊英；

【导师】 李越湘；

【作者基本信息】 南昌大学 ， 工业催化， 2015， 博士

【摘要】 面对日益增长的能源需求,光解水制氢引起广泛的关注,而建立稳定、高效、低廉的多相光催化分解水制氢是当今面临的一个巨大挑战。石墨相C3N4(g-C3N4)作为一种新型的无金属层状光催化剂,由于具有较好的热稳定性和化学稳定性,资源广泛,制备成本低,成为目前光催化研究的热点。然而,由于可见光吸收较弱(Eg=2.7 eV),纯的g-C3N4的光催化活性还很低。(001)晶面占优TiO2纳米片具有典型的片层结构,由于含有高能活性面,被广泛研究。但TiO2只吸收紫外光,无可见光催化活性。活性白土是天然层状氧化物,也无可见光催化活性。染料敏化是拓宽宽带光催化剂可见光吸收的有效方法。染料敏化能增加g-C3N4的可见光吸收范围和吸收强度,(001)晶面占优TiO2纳米片和活性白土经染料敏化后,也能吸收可见光。和三维材料相比,层状结构材料具有特殊结构和性能,是值得研究的敏化基质。本论文以呫吨类染料为敏化剂,g-C3N4和TiO2纳米片及活性白土等层状氧化物为敏化基质,研究了呫吨类染料敏化层状化合物的可见光光催化制氢,主要包括以下五部分:(一)、分别采用尿素、硫脲、三聚氰胺为前驱体,在550°C煅烧制得g-C3N4。以Eosin Y(EY)为敏化剂,Pt为助催化剂,三乙醇胺为电子给体,考察了三种g-C3N4可见光光解水制氢活性。研究发现,以尿素为前驱体制备的g-C3N4因具有更大的比表面,较低的电子-空穴复合率及较高的染料吸附量,展示了较高的光催化制氢活性,分别是硫脲和三聚氰胺为前驱体制备的g-C3N4的3.1倍和4.0倍。(二)、考察了尿素热解温度效应(450-650°C)对g-C3N4组成、结构与性质的影响,同样以Eosin Y为敏化剂,考察了样品的性质与敏化制氢活性的关系。利用TG–DTA,EA(C/H/N),XRD,UV–DRS vis,BET,FT-IR,PL,XPS等手段对催化剂进行了表征。低温下(450和500°C)得到的样品为g-C3N4和杂质的混合物,而高温下(550,600和650°C)得到的样品为g-C3N4(polymeric carbon nitride)。随着尿素热解温度的升高,样品的聚合度也随着增加,600°C时的聚合度为最大,随后减少。而样品的缺陷浓度按着相反的规律变化,即600°C制备的样品有着最低的缺陷浓度。以EY为敏化剂,制备的样品为载体,三乙醇胺为电子给体,考察了其可见光光解水制氢活性。600°C制备的样品活性最高。最佳条件下(1.25×10-5 mol L-1 EY和7.0 wt%Pt),EY敏化600°C制备的g-C3N4,其制氢的最大量子效率达到18.8%。最高的活性归结于样品的组成纯度高、较高的染料吸附量及最低的缺陷浓度。(三)、以MoSx作为助催化剂,三乙醇胺(TEOA)为电子给体,研究了Erythrosin B(EB)敏化g-C3N4的可见光(λ≥420 nm)制氢行为。以(NH4)2MoS4为前驱体,MoSx在光催化反应过程中通过原位光还原方法负载到g-C3N4上。相对于EB敏化的g-C3N4(EB-g-C3N4)和EB敏化的MoSx(EB-MoSx),EB敏化的MoSx-g-C3N4(EB-MoSx-g-C3N4)显示了更高的活性和稳定性。当MoSx的负载量为0.5 wt%时,光照2小时后,EB-MoSx-g-C3N4的光催化活性比EB-g-C3N4提高了160多倍;光照10小时后,EB-MoSx-g-C3N4的光催化活性是EB-MoSx的19.3倍,且前者较后者更稳定。545 nm处制氢活性最高,量子效率达到8.3%。提高的活性和稳定性归结于优良的制氢助催化剂MoSx及EB、g-C3N4和MoSx之间有效的电子转移。本文对可能的反应机理也进行了探讨。(四)、以钛酸四丁酯和HF为原料,通过水热法合成了含(001)高能面的TiO2纳米片。光还原沉积法进行载铂,浸渍法使伊红Y(Eosin Y)负载在Pt-TiO2表面,制得系列染料敏化的Eosin Y-Pt-TiO2可见光光催化剂。所制备的催化剂通过扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线衍射(XRD)、紫外-可见漫反射(Uv-vis DRS)、比表面(BET)、荧光光谱(PL)和傅立叶红外光谱(FT-IR)等手段进行表征。实验表明:当Eosin Y浓度一定时,HF用量和载Pt量对催化剂的制氢活性有显著影响。当HF用量为3 mL时,Eosin Y敏化Pt负载(001)晶面占优TiO2纳米片的可见光制氢活性是纯TiO2纳米颗粒的2.1倍,这可能与TiO2(001)面含量高和表面氟化有关;同时,当载Pt量仅为0.02 wt%时,该催化剂也显示了相当高的可见光制氢活性。(五)、以活性白土和伊红Y(Eosin Y)为原料,通过浸滞法制备了Eosin Y-活性白土复合催化剂,并借助X射线衍射(XRD)、紫外-可见漫反射(UV–vis DRS)、比表面(BET)对其进行了表征。在光催化反应过程中,溶液的pH值及Eosin Y浓度对催化剂的制氢活性有显著影响,最佳pH值和Eosin Y的最佳初始浓度分别是7.0和1.5×10-2 mol L-1。另外,对可能的光敏化制氢机理进行了分析与讨论。更多还原

【Abstract】 The photocatalytic production of hydrogen by splitting water has attracted worldwide attention due to increasing energy demands, however, it is a dif?cult challenge to establish stable, ef?cient and affordable heterogeneous photocatalytic systems for H2 production from water splitting today. Graphitic carbon nitride(g-C3N4), as a novel and metal-free layed visible-light photocatalyst, is most employed at present because of its high thermal and chemical stability as well as rich and cheap building elements. However, pristine g-C3N4 exhibits low photocatalytic activity due to its poor ability for visible light absorption(band gap energy 2.7 eV). TiO2 nanosheets with preferential(001) facets are tipical of amellar structure and have been investigated extensively due to its high energy facets. Nevertheless, TiO2 absorbs only UV light and has no photocatalytic activity under visible light. Activated clay is a kind of nonreactive oxide with natural layed structure and also has no photocatalytic activity under visible light.Dye sensitization is a good strategy to expand the absorption in visible light region of wide-band-gap photocatalyst. Dye sensitization can increase the visible-ligh absorption region and strength of g-C3N4. Both TiO2 nanosheets with preferential(001) facets and activated clay also absorb visible light via the dye sensitization. Compared with three dimensional materials, layered materials are the sensitization matrices worthy of study because of their special structure and performance.In this thesis, photocatalytic hydrogen production using xanthene dyes sensitized g-C3N4 and layered oxides under visible light irradiation was studied and the whole work is composed of the following five parts:(Ⅰ). g-C3N4 was prepared by pyrolysis of urea, thiourea and melamine at 550 °C respectively. The photocatalytic activity for hydrogen evolution from aqueous triethanolamine solution under visible light irradiation(λ≥420 nm) was evalulated using Eosin Y(EY) as the sensitizer, Pt as the co-catalyst and the prepared sample as the matrix, respectively. The g-C3N4 from urea exhibits the higher sensitization activity which is 3.1 and 4.1 times higher than those from thiourea and melamine, respectively. The higher activity can be ascribed to the increased BET surface area, lower electron-hole recombination rate and higher dye adsorption amount.(Ⅱ). Effect of the pyrolysis temperature(450-650 oC) of urea on the composition, structure and property of g-C3N4 was investigated, moreover, the relation between the property of the prepared g-C3N4 and the activity of the EY-sensitized g-C3N4(EY-g-C3N4) was studied. The obtained-catalysts were characterized by thermogravimetric and differential thermal analysis(TG–DTA), elemental analysis(C/H/N), X-ray diffractometer(XRD), UV–vis diffuse re?ectance spectra(DRS), Brunauer-Emmett-Teller(BET), Fourier transform-infrared(FT-IR), X-ray photoelectron spectroscopy(XPS), and photoluminescence spectra(PL). The samples prepared at low temperatures(450 and 500 oC) are a mixture of g-C3N4 and impurities, whereas the samples at high temperatures(550, 600 and 650 oC) should be g-C3N4(polymeric carbon nitride). The polymerization degree of g-C3N4 for the prepared samples increases to a maximum at 600 oC with increasing the pyrolysis temperature and then decreases, whereas the defect concentration changes conversely, that is, g-C3N4 prepared at 600 oC has the lowest defect concentration. Using EY and the prepared sample as the sensitizer and matrix, respectively, the photocatalytic activity for hydrogen evolution from aqueous triethanolamine solution was investigated. The g-C3N4 prepared at 600 oC exhibits the highest sensitization activity. On optimum conditions(1.25×10-5 mol L-1 EY and 7.0 wt% Pt), the maximal apparent quantum yield of EY-sensitized g-C3N4 prepared at 600 oC for hydrogen evolution is 18.8%. The highest activity can be attributed to pure composition, higher dye adsorption amount and the lowest defect concentration.(Ⅲ). Erythrosin B(EB) sensitized g-C3N4 for photocatalytic hydrogen evolution was investigated using triethanolamine(TEOA) as an electron donor under visible light irradiation(λ > 420 nm). MoSx was loaded on g-C3N4 as a cocatalyst by an in situ photoreduction method during the photocatalytic reaction employing(NH4)2MoS4 as a precursor. EB-sensitized MoSx-g-C3N4(EB-MoSx-g-C3N4) exhibits the higher activity and stability than both EB-sensitized g-C3N4(EB-g-C3N4) and EB-sensitized MoSx(EB-MoSx). With deposition of 0.5 wt% MoSx, the photoactivity of EB-MoSx-g-C3N4 increases by more than 160 times after 2 h irradiation compared with that of EB-g-C3N4. The activity of EB-MoSx-g-C3N4 is 19.3 times as high as that of EB-MoSx after 10 h irradiation, and the former is much more stable than the latter. The highest apparent quantum yield for hydrogen evolution reaches 8.3% at 545 nm. The improved photoactivity and stability is owing to MoSx as an excellent hydrogen evolution cocatalyst and the efficient electron transfer among photoexcited EB, g-C3N4 and MoSx. The possible mechanism was discussed.(Ⅳ). TiO2 nanosheets with preferential(001) facets were prepared by a hydrothermal route using tetrabutyl titanate and HF as materials. Pt was loaded on TiO2 samples by a photodeposition method. A dye-sensitization photocatalyst was prepared by impregnation method with EY and the platinized TiO2 samples. The prepared samples were characterized by Scanning electron microscope(SEM), Transmission electron microscope(TEM), XRD, UV–vis DRS, BET, PL and FT-IR. The experiments show that the volume of HF and Pt loading content have remarkable effects on the rate of photocatalytic hydrogen production at the given concentration of EY. When the volume of added HF is 3 m L, the activity of the prepared EY Y-Pt-TiO2 nanosheets reaches a maximum which is more than two times compared to that of TiO2 nanoparticles. The high percentage of(001) and surface ?uorination of TiO2 nanosheets can be responsible for the improved photocatalytic activity. When the loading Pt content is only 0.02 wt%, the EY-Pt-TiO2 nanosheets can also exhibit higher photocatalytic activity of hydrogen production.(Ⅴ). EY-sensitized activated clay was prepared by a simple impregnation method with EY and activated clay as materials. The as-prepared photocatalyst was characterized by XRD, UV-vis DRS and BET analysis. The results showed that pH value and EY concentration of the reaction system have remarkable effects on the photocatalytic activity for hydrogen generation, the optimal pH and initial concentration of EY are 7.0 and 1.5×10-2 mol L-1, respectively. In addition, the probable mechanism of photosensitized hydrogen evolution has been discussed.更多还原