【作者】 蒋丽；

【导师】 上官文峰；

【作者基本信息】 上海交通大学 ， 动力工程及工程热物理， 2010， 博士

【摘要】 化石能源的大规模开采与使用所造成的环境问题,及其日趋枯竭所引发的能源危机,促使人类必须开发出清洁、无污染、可持续利用的替代能源。氢能来源广泛,利用形式多样,燃烧性能好,燃烧产物只有水,且燃烧生成的水可以继续制氢,能够循环利用,因此氢能被许多国家的学者认为是21世纪最有希望的替代能源。在众多的制氢方法中,由“Fujishima-Honda”效应发展而来的太阳能多相光催化分解水技术因其能直接利用太阳能,且体系简单,日益受到广泛关注。然而,目前制约光解水制氢技术进一步向前发展的瓶颈仍然是较低的光催化效率。为了提高光解水效率,现阶段研究者采用的主要策略包括:(1)开发具有可见光性能的光催化剂,从而能够更有效的利用占太阳光中43%能量的可见光;(2)通过各种修饰改性手段提高现有光催化剂的活性;(3)设计和制备新型光催化剂材料,寻找具有优异性能的光催化剂。本文从材料设计的思想出发,提出了一种新型光解水催化剂的设计思路:首先,在现阶段光解水催化剂研究成果的基础上,根据电子结构与光解水性能的构效关系设计一种可能的目标材料;然后,借助第一性原理方法对该材料进行电子结构的理论计算,据此预测其是否具有光解水能力;最后,对通过计算筛选具有光解水潜力的材料进行实验验证及改性研究。根据该思路,本文成功开发了新型Zr-W基光解水催化剂ZrW2O8,并利用XRD、DRS、XPS、SEM、TEM、BET、TG-DTA以及元素分析等技术对ZrW2O8的制备、表征、光解水性能以及可见光化修饰改性进行了详细研究。研究中还发现作为水热合成ZrW2O8的前驱体,ZrW2O7(OH)2(H2O)2也具有光解水能力。本文具体研究内容及重要结论摘示如下:(1)依据本文提出的光解水催化剂的设计开发思路选取ZrW2O8做为目标材料,利用第一性原理方法对其电子结构进行理论计算,发现其具有一般金属氧化物半导体能带结构的特征,导带主要由Zr4d和W5d的杂化轨道构成,价带主要由O2p轨道构成。依据理论计算结果以及ZrO2与WO3的实际能带结构对ZrW2O8的能带结构进行了理论预测,认为ZrW2O8的能带结构符合光解水要求,非常有潜力成为光解水材料。(2)利用水热反应法制备了ZrW2O8样品,并详细研究了Zr:W原料配比、HCl浓度、水热温度和水热时间四个制备参数对样品结构和理化性质的影响。研究表明,这四个参数主要影响合成样品的晶相纯度和结晶度。当采用Zr源过量的方式投料(Zr:W高于1:2),HCl浓度高于4 mol/L,水热温度高于120°C,水热反应时间超过3 h,能够合成具有单一晶相、结晶良好的ZrW2O8样品。合成样品的吸收边处于280-360 nm范围内,不同合成条件导致吸收边存在细微差别。这四个制备参数对合成的ZrW2O8样品的比表面积没有明显影响,比表面积都在1-10 m2/g范围内。(3)对ZrW2O8的晶体结构、光吸收性能、表面化学态、形貌特征、比表面积等理化性质及其光解水性能进行了详细考察。研究表明利用水热反应法制备的样品具有立方晶相,空间群为P213,晶粒形貌呈竹叶状,吸收边为300 nm,带隙为4.0 eV,比表面积为3.58 m2/g。在紫外光(260nm<λ<390nm)照射下,以CH3OH为电子给体进行产氢半反应时,0.3wt%Pt/ZrW2O8(0.5 g)催化剂上的光催化产氢平均速率达23.4μmol/h;以AgNO3为电子受体进行产氧半反应时,ZrW2O8(0.5 g)催化剂上的光催化产氧平均速率达9.8μmol/h;进行光催化分解纯水反应时,0.3wt%Pt/ ZrW2O8(0.5 g)催化剂上的光催化产氢平均速率达5.2μmol/h,未检测到氧气的生成。实验结果证实了第一性原理计算对ZrW2O8具有光解水潜力的预测:ZrW2O8能带结构符合光解水要求,具有光解水能力,是一种新型的光解水催化剂。(4)研究了助催化剂负载对ZrW2O8光解水性能的影响。研究表明:利用几种常见的贵金属助催化剂(Pt, Au, Ru, Rh, Pd)进行紫外光响应性能修饰改性时,原位光沉积负载的Pt和Au表现出了较好的修饰改性作用;以贵金属Pt为助催化剂对ZrW2O8光催化剂的进行修饰改性时,原位光沉积法负载优于浸渍法,且Pt的最佳负载量为0.3wt%;利用几种常见的氧化物助催化剂(RuO2, Ni-NiO, Pt-RuO2)进行修饰改性时,RuO2和Pt-RuO2表现出了较好的修饰改性作用。(5)研究了制备条件对ZrW2O8光解水性能的影响。在结晶程度、比表面积、晶相纯度等多种因素的综合影响下,不同条件下合成的ZrW2O8样品性能存在一定差别。ZWO-(6 h)样品表现出了最优的产氢产氧活性,以CH3OH为电子给体进行产氢半反应时,0.3wt%Pt/ZrW2O8(0.5 g)催化剂上的光催化产氢平均速率达61.2μmol/h;以AgNO3为电子受体进行产氧半反应时,ZrW2O8(0.5 g)催化剂上的光催化产氧平均速率达53.1μmol/h。(6)为了扩展ZrW2O8对可见光的吸收利用,采用Bi掺杂、N掺杂和S掺杂三种技术路线对ZrW2O8进行可见光化修饰改性,并对掺杂样品的晶体结构、光吸收性能、表面化学态以及光催化分解水性能进行了详细考察,同时考察了S掺杂样品的制备条件(灼烧温度与原料配比)对合成样品的结构与性能的影响。研究表明,利用浸渍灼烧法制备Bi掺杂ZrW2O8时,吸收边扩展至450 nm左右,但ZrW2O8分解成ZrO2和WO3,Bi未能成功掺入ZrW2O8,利用高温氨解法制备N掺杂ZrW2O8样品时,高温条件下NH3的碱性会破坏ZrW2O8的骨架结构,无法实现N掺杂修饰改性ZrW2O8以获得可见光响应性能的目的。利用硫脲混合灼烧法能够成功制备S掺杂ZrW2O8样品。ZrW2O8的骨架结构能够保持,并且S能够以S2-的形态掺入晶格,取代O2-位置。S的掺杂改性能够明显扩展ZrW2O8的吸收性能,最大吸收波长可达600 nm左右。ZrW2O8经过S掺杂修饰改性处理后,在全光谱照射下基本保持了与未掺杂样品接近的光解水能力,并且S掺杂改性后的样品能够利用波长更长的光进行光解水产氢产氧。具体而言:S掺杂ZrW2O8样品能够在直到360nm的光激发下产氢,能够在直到510nm的光激发下产氧。(7)研究中发现ZrW2O7(OH)2(H2O)2表现出与ZrW2O8类似的光吸收特性,即在紫外光区域也具有陡峭的吸收边,预示ZrW2O7(OH)2(H2O)2具备光催化分解水活性的可能。因此进一步对ZrW2O7(OH)2(H2O)2的热稳定性、晶体结构、光吸收性能、比表面积及其光催化分解水性能进行了详细考察。研究表明:制备的样品为结晶良好且晶相单一的四方相ZrW2O7(OH)2(H2O)2粉体,空间群为I41cd,吸收边为310 nm,带隙为3.9 eV,比表面积为5.9 m2/g,不具有中孔或微孔结构。在紫外光(260nm<λ<390nm)照射下,以CH3OH为电子给体进行产氢半反应时,0.3wt%Pt/ZrW2O7(OH)2(H2O)2(0.5 g)催化剂上的光催化产氢平均速率达3.7μmol/h;以AgNO3为电子受体进行产氧半反应时,ZrW2O7(OH)2(H2O)2(0.5 g)催化剂上的光催化产氧平均速率达27.8μmol/h。ZrW2O7(OH)2(H2O)2的能带结构符合光催化分解水要求,是一种新型的光催化分解水材料。在同等实验条件下,ZrW2O7(OH)2(H2O)2的光催化分解水活性低于ZrW2O8,晶体结构(晶体场,堆隙率)的差异可能是造成两者活性差异的主要原因。本文在新型光解水催化剂的设计开发方面做了新的尝试。文中所提出的设计开发思路有望能为日后新型光催化剂的研发提供新的途径;同时Zr-W基光催化剂的研究不仅丰富了现有的光解水材料体系,而且为着眼于提高该体系光解水性能的后续研究提供了重要的参考。更多还原

【Abstract】 Nowadays, it is very urgent for human beings to develop clean, non-polluting and sustainable alternative energy resources, since the environmental problems caused by the large-scale exploitation and use of fossil energy, as well as the energy crisis caused by the incoming depletion of fossil fules are becoming more and more serious. Hydrogen energy has many advantages, such as: it could be produced from many hydrogen-containing resources and utilized in various forms; it has excellent combustion performance and water is the only combustion product; moreover, the water produced from the burning of hydrogen can be reused to generate hydrogen. Considering above merits, hydrogen energy has been regarded as the most promising alternative energy resource in 21st century by the majority of scholars around the world. Among various hydrogen production paths, the heterogeneous photocatalytic water splitting technique developed from“Fujishima-Honda”effect is increasingly under the spotlight, due to the fact that the system is simple and sustainable, and solar energy could be utilized directly.However, the low photocatalytic efficiency is still the bottleneck which constrains the further development of photocatalytic water splitting technique currently. In order to improve the efficiency, key strategies used by researchers at this stage includes: (1) to develop visible light responsive photocatalysts, so as to utilize the visible light in the solar irradiation which accounts for 43% of energy; (2) to improve the performance of existing photocatalysts by varieties of modification methods; (3) to develop novel photocatalysts with excellent catalytic performance.In this paper, based on the materials design concept a new method for developing novel photocatalysts was put forward. Firstly, design a possible target material based on the relationship of electronic structure-photocatalytic property. Secondly, calculate the electronic structure of the target material by using the first-principles calculation, and predict whether the target material possesses the photocatalytic water splitting ability. Finally, conduct experimental verification and further follow-up study of the target material which has passed the screening step. According to the above idea, a new type of Zr-W-based catalyst ZrW2O8 for photocatalytic water splitting was successfully developed in this paper. The preparation, characterization, photocatalytic water splitting properties and the visible light sensitization of ZrW2O8 were studied in detail by using XRD, DRS, XPS, SEM, TEM, BET, TG-DTA and elemental analysis techniques. The study also found that, as the precursor for preparing ZrW2O8 by hydrothermal reaction method, ZrW2O7(OH)2(H2O)2 possessed the photocatalytic water splitting ability as well.The content of this study and important conclusions were summarized as follows:(1) ZrW2O8 was selected as the target material based on the idea for designing and developing novel photocatalysts proposed in this paper, and its electronic structure was calculated by using the first-principles method. It was found ZrW2O8 possessed the characteristic band structure of general metal-oxide semiconductors, that the conduction band was mainly constituted by Zr4d and W5d hybrid orbitals, while the valence band was mainly constituted by O2p orbitals. The band structure of ZrW2O8 was predicted based on the theoretical calculations and the actual band structure of ZrO2 and WO3. It was predicted ZrW2O8 had suitable band structure for photocatalytic water splitting, and was quite possible to be developed as a water splitting photocatalyst.(2) ZrW2O8 samples were prepared by hydrothermal reaction method, and the effect of Zr:W mole ratio, HCl concentration, hydrothermal temperature and hydrothermal time on the structure and physicochemical properties were studied in detail. It was found the four parameters mainly affected the crystalline phase purity and crystallinity of synthesized samples. When excessive Zr was used (Zr:W is higher than 1:2), HCl concentration was higher than 4 mol/L, hydrothermal temperature was higher than 120°C and hydrothermal time was longer than 3 h, ZrW2O8 samples with single phase and good crystallinity could be synthesized. The absorption edges of synthesized samples were in the range of 280-360 nm, showing slight differences between samples prepared under different synthesis conditions. The four parameters didn’t show any significant effect on the specific surface area of synthesized samples, which were in the range of 1-10 m2/g. (3) The crystal structure, photon absorption property, surface chemical state, morphology, and specific surface area of ZrW2O8 as well as its photocatalytic water splitting properties were examined in detail. It was shown the prepared ZrW2O8 was crystallized in cubic phase (P213), with the crystalline grain shape similar as a bamboo-leaf. The absorption edge was 300 nm and the surface area was 3.58 m2/g. Under the UV irradiation (260nm<λ<390nm), the average rate of H2 evolution over 0.3wt%Pt/ZrW2O8(0.5 g) in the presence of CH3OH as electron donor (ED) was 23.4μmol/h, and the average rate of O2 evolution over ZrW2O8(0.5 g) in the presence of AgNO3 as electron scavenger (ES) was 9.8μmol/h. Moreover, H2 was evolved over 0.3wt%Pt/ZrW2O8(0.5 g) from pure water splitting at a rate of 5.2μmol/h. Oxygen evolution was not detected. The experimental results confirmed the prediction by first-principle calculation: the band structure of ZrW2O8 was suitable for reducing H+ to H2 and oxidizing H2O to O2, and was a new type of photocatalyst for water splitting.(4) The effects of co-catalyst loading on the photocatalytic properties of ZrW2O8 were studied. When several common nobel metal co-catalysts (Pt, Au, Ru, Rh, Pd) were used for improving UV responsive acitivity of ZrW2O8, Pt and Au loaded by in situ photochemical deposition showed better modification effect. When Pt was used as co-catalyst, the in-situ photochemical deposition method was superior to the impregnation method, and the optimal loading amount of Pt was 0.3wt%. When several common oxide co-catalysts (RuO2, Ni-NiO, Pt-RuO2) were used, RuO2 and Pt-RuO2 showed relative better modification effect.(5) The effects of preparation conditions on the photocatalytic properties of ZrW2O8 were studied. Under the combined effects of crystallinity, specific surface area and crystal phase purity, samples prepared under different conditions showed slight differences in performance. ZWO-(6 h) sample was found to show the optimal properties. Under the UV irradiation (260nm<λ<390nm), the average rate of H2 evolution over 0.3wt%Pt/ZrW2O8(0.5 g) in the presence of CH3OH as electron donor (ED) reached 61.2μmol/h, and the average rate of O2 evolution over ZrW2O8(0.5 g) in the presence of AgNO3 as electron scavenger (ES) reached 53.1μmol/h.(6) In order to expand the light absorption range of ZrW2O8, Bi-doping, N-doping and S-doping techniques were attempted. The crystal structure, photon absorption property, surface chemical state of doped ZrW2O8 samples as well as their photocatalytic water splitting properties were examined in detail. Also the effect of preparation conditions (calcination temperature and the ratio of raw materials) on the structure and properties of synthesized S-doped ZrW2O8 samples were studied. Through Bi-doping, the absorption edge of ZrW2O8 could be extended to 450 nm. However, ZrW2O8 was found to decompose into ZrO2 and WO3, indicating that introducing Bi into to the lattice of ZrW2O8 by impregnation method was not achieved in present study. When N-doped ZrW2O8 was prepared by the high-temperature ammonia decomposition method, the alkaline property of NH3 would destroy the crystal structure of ZrW2O8, thus the purpose of visible light sensitization of ZrW2O8 by N-doping could not be achieved. The S-doped ZrW2O8 samples could be successfully prepared by burning the mixture of ZrW2O8 and thiourea under Ar atmosphere. The crystal structure of ZrW2O8 could be maintained, and S was able to be doped into the O2- position in the state of S2-. S-doping could significantly extend the absorption properties of ZrW2O8, and the maximum absorption wavelength could reach up to 600 nm. After S-doping, the sample maintained similar photocatalytic water splitting activity with that of un-doped one under the full arc irradiation. Morever, S-doped sample could utilize light with longer wavelength for photocatalytic water splitting. Specifically, S-doped ZrW2O8 sample can use light with wavelength up to 360 nm to produce hydrogen, and can use light with wavelength up to 510 nm to produce oxygen.(7) It was found ZrW2O7(OH)2(H2O)2 showed similar absorption property with that of ZrW2O8, that is, it possessed a steep absorption edge in the UV light region, suggesting ZrW2O7(OH)2(H2O)2 might also possess the ability of photocatalytic water splitting. Therefore, the thermal decomposition property, crystal structure, photon absorption property and specific surface area as well as the photocatalytic water splitting properties of ZrW2O7(OH)2(H2O)2 were studied in detail. The results showed that ZrW2O7(OH)2(H2O)2 was crystallized well in tetragonal phase, with absorption edge of 310 nm, band gap energy of 3.9 eV, and specific surface area of 5.9 m2/g. Under the UV irradiation (260nm<λ<390nm), the average rate of H2 evolution over 0.3wt%Pt/ ZrW2O7(OH)2(H2O)2(0.5 g) in the presence of CH3OH as electron donor (ED) was 3.7μmol/h, and the average rate of O2 evolution over ZrW2O(0.5 g) in the presence of AgNO3 as electron scavenger (ES) was 27.8μmol/h. It was concluded that the hydroxy group containing ZrW2O7(OH)2(H2O)2 had suitable band structure and possessed the photocatalytic ability to split water. Under the same experimental conditions, the photocatalytic water splitting properties of ZrW2O7(OH)2(H2O)2 was lower than that of ZrW2O8. The differences in crystal structure (crystal field and crystal packing factor) might cause the differences in performance.In this paper, a new method for designing and developing novel photocatalysts was attempted. The idea proposed in this paper was expected to provide a new approach for the development of novel photocatalysts in future. At the same time, the study of Zr-W-based photocatalysts not only enriched the existing photocatalytic water splitting material systems, but also provided important references for the follow-up study which may focus on improving the performance of ZrW2O8 photocatalyst.更多还原