【作者】 刘广龙；

【导师】 朱端卫；

【作者基本信息】 华中农业大学 ， 农业资源与环境， 2011， 博士

【摘要】 近年来,环境中的高/大分子有机污染物的治理已经成为环境科学技术研究的焦点之一。多相光催化作为一种深度氧化技术,已经被公认是最有前景的绿色环境净化技术之一。以TiO2为代表的半导体多相光催化技术已经在环境污染治理领域取得了较大的成效,但其较高的禁带宽度导致其仅能够在紫外光下具有光催化活性,严重限制了其推广和应用。具有较低禁带宽度的针铁矿、锰钾矿等半导体矿物则弥补了这一不足,然而,现阶段对其光催化降解有污染物的报道相对较少,降解机理尚无定论。本文针对半导体矿物光催化技术在走向应用的过程中存在的科学与技术问题,对半导体矿物及其非金属元素掺杂复合体光催化作用机理、催化材料的制备方法及催化剂的固定等方面进行了进一步的探讨。研究结果深化了对半导体矿物光催化机理的认识,为非金属元素掺杂半导体材料的制备提供了新方法,同时也为其应用奠定了理论基础。本文主要内容及结论如下：(1)采用溶胶凝胶法合成了常见的半导体矿物针铁矿和锰钾矿,为评价其对环境中高分子有机污染物的光催化降解活性,采用包埋法人工合成了负载针铁矿或锰钾矿的聚乙烯膜,进行了复合膜在紫外光及可见光照射下的光催化降解实验。利用SEM、FT-IR、XPS等技术研究复合膜及纯PE膜在光照前后的表面形貌、结构、成分的变化。研究结果表明,在紫外光照射下,添加了针铁矿或锰钾矿的复合膜降解性能显著提高,降解速率和降解程度均高于纯PE膜。FT-IR和XPS的结果表明,复合膜的光催化降解产中含有羟基、C=O和C-O,降解过程具有环境友好性。复合膜降解的主要机理为聚乙烯高分子长链在紫外光照射下与活性氧发生光解反应,产生了一定量的羰基或羧基基团,而针铁矿或锰钾矿的存在,加速了活性氧基团的生成速度,从而加剧生成的羰基或羧基在紫外光下降解过程。上述过程始于聚乙烯与催化剂的界面处,催化剂表面产生的活性氧物种扩撒至聚乙烯骨架使其降解。由于分子氧在可见光下比较稳定,故聚乙烯高分子链在可见光照射时很少受到活性氧的直接进攻,不易被破坏；另一方面,针铁矿和锰钾矿等半导体矿物在可见光照射下虽可使分子氧活性得以提高,但其量不足以打破聚乙烯的主链结构,进而没有光催化降解现象的产生。(2)本文的主要工作之一是以硼酸为B源,采用溶胶凝胶法合成了掺杂硼的针铁矿和锰钾矿等半导体矿物,利用化学组成分析、BET、XRD、TEM、FT-IR、UV-vis DRS、XPS等技术对掺杂B的针铁矿和锰钾矿进行了表征。为进一步测定非金属元素掺杂的半导体矿物的光催化降解活性,用包埋法合成了掺杂硼针铁矿(或锰钾矿)-PE复合膜,进行了紫外光和可见光照射下的薄膜降解实验。研究发现,硼的进入,改变了针铁矿和锰钾矿的基本理化性状,阻碍了针铁矿和锰钾矿的结晶生长,减小了其颗粒粒径,增加了矿物的比表面积。紫外可见漫反射图谱证明,硼的掺杂,均提高了矿物的光响应范围。尽管在XPS中未检测到硼元素的存在,然而,对矿物中其他元素的精细图谱对比可以确定,硼的掺杂使矿物中的元素价态及结构发生了改变。在紫外光照射下,掺硼铁锰氧化物-PE复合膜的降解效率明显高于普通铁锰氧化物-PE复合膜及纯PE膜；SEM和FT-IR结果表明,掺硼铁锰氧化物-PE复合膜的降解程度也均高于铁锰氧化物-PE复合膜及纯PE膜的降解程度。这一降解机理应归于掺硼铁锰氧化物更小的颗粒粒径、更大的比表面积及晶格内部中硼的缺电子特性。可见光照射下,掺硼铁锰氧化物-PE复合膜也同样没有光催化降解现象的产生,可能是由于掺硼铁锰氧化物在可见光照射下光激发产生的活性氧自由基仍较少,不足以破坏聚乙烯主链结构。(3)为进一步了解非金属元素掺杂的半导体矿物的可见光光催化降解活性,本文以油酸为表面活性剂和C源,采用基于自组装技术的改性溶胶凝胶法合成C掺杂Ti02,通过孔隙率分析、XRD、HR-TEM、XPS、FT-IR、Raman、EPR等技术对其进行了表征,并以C-TiO2为催化剂,进行了在可见光照射下多相光催化降解水体中的微囊藻毒素实验。研究发现,在煅烧温度为350℃,以油酸为碳源合成的C-TiO2呈锐钛矿相,具有较好的介孔结构。不同用量油酸合成的C-TiO2均在可见光区具有光响应,其中OA1的禁带宽度仅为2.68eV,远低于锐钛矿的3.2eV。XPS分析结果表明,实验合成的C-TiO2中含有C-Ti和C-O键,证实了C的掺杂,取代了Ti02中部分的晶格氧。EPR结果也同样证明,油酸处理的C-TiO2具有两种不同结构的含碳基团(C-Ti和C-O)。MC-LR的降解实验表明,在可见光照射下,相对于参照Ti02,C-TiO2实现了对MC-LR的可见光催化降解。(4)传统的粉末催化剂由于其纳米级粒径,极难回收,容易造成环境的二次污染,将粉末催化剂固定在特定载体上则成为了研究热点。在催化剂固定方面,本文尝试了以硅酸盐玻璃片为载体,以吐温80为表面活性剂和C源,以乙二胺为N源合成C、N共掺杂Ti02薄膜。为探讨这种半导体薄膜的可见光光催化活性,进行了其在可见光照射下对MC-LR的光催化降解实验,考察了不同薄膜制备煅烧温度对产物结构和性能的影响。研究发现,C-N-TiO2薄膜样品的比表面积、孔隙率、粒径、孔径分布均可以通过煅烧温度来控制。其中,在煅烧温度为400℃时,薄膜样品可以获得较高的比表面积、较小的粒径、较窄的孔径分布及粗糙度较高(360nm)的表面结构。紫外-可见(UV-vis)吸收光谱表明,C、N共掺杂导致了薄膜样品光响应区的红移,增强了其光响应范围。MC-LR降解实验表明,C-N-TiO2薄膜样品能够在可见光照射下光催化降解MC-LR,其中,薄膜样品经过三次循环使用后,其光催化效率并没有发生明显改变,证明了用此方法合成的薄膜催化剂样品具有较高的稳定性。C-N-TiO2半导体薄膜在可见光下增强催化降解效率可归因于C、N的共掺杂和其较高粗糙度的表观结构。更多还原

【Abstract】 Recently, the macromolecular organic pollutants have become a focus problem in the environmental remediation. As an effective advanced treatment technology, Heterogeneous phototcatalysis is a promising and innovative green purification technology. Photocatlaysis technology based on TiO2has provided an effective and promising means for remediation of environmental pollutants in air and water. However, the widespread technological used of TiO2is impaired by its wide band gap (3.2eV) which can only be activated under UV light. Iron and manganese oxide are abundant semiconductor minerals in soil and play a critical role in many chemical and biological processes due to their lower band gap energy. To our best knowledge, little has been done on the photocatalytic degradation of macromolecular organic pollutants with semiconductor minerals as catalyst and the mechanism of photocatalytic on the semiconductor minerals is not clarified. Therefore, considering the unsolved scientific and technological problems of semiconductor minerals and nonmetal doped semiconductor minerals for future application, valuable explorations have been carried out on photocatalytic mechanism, novel preparation methods of visible light photocatalytic materials, and the immobilization of catalysts in this dissertation. The study provides insights into the visible light photocatalytic mechanism, novel preparation methods for nonmetal doped semiconductor minerals photocatalytic materials and theoretical support for the application of nonmetal doped semiconductor minerals. The major research contents and results are listed as follows:(1) Goethite and cryptomelane were successfully synthesized by sol-gel method. In order to evaluate the photocatalytic activity of goethite and cryptomelane, a novel photodegradable polyethylene-goethite (cryptomelane) composite film was prepared by embedding the goethite (cryptomelane) into the commercial polyethylene. The degradation of PE-goethite (cryptomelane) composite films was investigated under ultraviolet and visible light irradiation. The photodegradation activity of the PE plastic was determined by monitoring its weight loss, scanning electron microscopic (SEM) analysis and FT-IR spectroscopy. The results show that the composite films have highly enhanced photodegradation, which decompose much faster and more completed than pure PE film. The main products of photocatalytic degradation of composite film are H2O and small molecular volatile materials with C=O and C-O under the UV light irradiation. The mechanism of the degradation of composite films could be attributed to the photodegradation and photocatalytic degradation of polyethylene. The reaction of polyethylene under UV light irradiation occurred via direct absorption of photons by the polyethylene macromolecule to create exited states, which resulted in some carbonyl and carbonxyl groups produced. The carbonyl and carbonxyl groups could be further destroyed by the reactive oxygen species generated on catalysts surface under the visible light irradiation.The degradation initially on the interface of PE and catalysis and then extended into polymer matrix. The weight loss was not detectable for the composite film and pure PE film under the visible light irradiation, which could be attributed to the stable of polyethylene under the visible light. The reactive oxygen species generated on the goethite and cryptomelane surface might not etch the polymer matrix.(2) Boron modified goethite and cryptomelane was successfully by sol-gel method using boric acid as boron source. Chemical composition analysis, BET, XRD, TEM, XPS, FT-IR and UV/Vis diffuse reflectance characterization were performed. In order to investigate the photocatalytic activity of nonmetal doped semiconductor minerals, a novel photodegradable polyethylene-boron-goethite (cryptomelane) composite film was prepared by embedding the boron doped goethite (cryptomelane) into the commercial polyethylene. The goethite (cryptomelane) catalyst was modified by boron in order to improve its photocatalystic efficiency under the ultraviolet and visible light irradiation. Solid-phase photocatalytic degradation of the PE-B-goethite (cryptomelane) composite film was carried out in an ambient air at room temperature under ultraviolet and visible light irradiation. The results showed boron doping changed the basic propersities of the goethite (cryptomelane), inhibit the grain growth, decreased the crystallize size, increased the BET surface area. UV-vis spectra revealed that the optical absorption edge of B-doped goethite (cryptomelane) was red shifted. Though there was no boron peak detected in the XPS results, the valence structure of other elements have been changed with B-doped. The photo-induced degradation of PE-B-goethite (cryptomelane) composite film was higher than that of pure films and the PE-goethite (cryptomelane) composite film under the UV light irradiation. SEM and FT-IR results showed that the PE-B-goethite (cryptomelane) composite film decompose much faster and more completed than pure PE film and PE-goethite (cryptomelane) composite film. The mechanism of degradation of PE-B-goethite (cryptomelane) composite film could be attributed to lower crystallize size, lager BET surface area, electronic characteristics of boron. There was also no degradation phenomenon occurred on the PE-B-goethite (cryptomelane) composite film, which due to the fewer reactive oxygen species generated on the B-doped goethite and cryptomelane surface under the visible light irradiation.(3) Heterogeneous photocatalytic degradation of microcystin-LR was demonstrated by visible light-activated carbon doped TiO2(C-TiO2) nanoparticles, synthesized by a modified sol-gel route based on the self-assembly technique exploiting oleic acid as pore directing agent and carbon source. The C-TiO2nanoparticles crystallize in anatase phase despite the low calcination temperature of350℃and exhibit a highly porous structure that can be optimized by tuning the concentration of the oleic acid surfactant. The carbon modified nanomaterials exhibited enhanced absorption in the broad visible-light region together with an apparent red shift of the absorption edge leading to an effective indirect bandgap of2.68eV, compared with3.18eV of a reference anatase TiO2. Carbon species were identified by XPS analysis through the formation of both Ti-C and C-O bonds, indicative of carbon substitution for oxygen atoms and the formation of carbonates, respectively. EPR spectroscopy revealed the formation of two carbon related paramagnetic centers in C-TiO2, whose intensity was markedly enhanced under visible light illumination, pointing to the formation of localized states within the anatase band gap, following carbon doping. The photocatalytic activity of C-TiO2nanomaterials was evaluated for the degradation of microcystin-LR (MC-LR) at pH3.0under visible light (λ>420nm) irradiation. The doped materials showed higher MC-LR degradation rate than reference TiO2, behavior that is attributed to the carbon incorporation into the titania lattice. (4) The powder of catalyst has has obvious drawbacks, such as agglomeration and not easy to recover, the possible mobility of suspended TiO2powders in the environment impose a health risk due to the possible toxicity of their nanoscale. Thus, it becomes a challenging to prepare materials immobilized on substrate. In this work, a carbon-based surfactant sol-gel method was employed to synthesize high surface roughness and visible-light-active C-N-TiO2films with borosilicate glass as surstrate. The enhancement of photocatalytic activity of C-N-TiO2films was evaluated for the degradation of the microcystin-LR (MC-LR) under the visible light irradiation in water. The results revealed that the physicochemical properties of the films, such as specific surface area, porosity, crystallite size and pore size distribution could be controlled by the calcination temperature. The higher surface area, smallest crystallite size and narrow pore size distribution were obtained for C-N-codoped TiO2films calcined at400℃, which exhibit very high surface roughness (360nm). UV-vis spectroscopy showed that as-prepared C-N-TiO2films exhibited stronger absorption in the visible light region and a red shift in the band gap transition due to C-N-codoping. C-N-TiO2films effectively degraded MC-LR under visible light compared to reference film. Especially, Similar MC-LR degradation rates under visible light after three cycles revealed high mechanical stability and no irreversible changes of the film during photocatalysis. The enhancement in visible light photocatalytic activities of the C-N-codoped TiO2films was attributed to the synergistic effects of carbon and nitrogen dopants, and high surface roughness of the prepared films.更多还原