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Photocatalysis

Photocatalysis. Fundamental and Applications. Environmental Pollution. Atmosphere pollution Green house effect (CO 2 ) Acid rain Water pollution Soil pollution. Air Pollution. Smog. Acid rain. Burning of fossil fuels. Water Pollution. Waste water from textile industry.

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Photocatalysis

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  1. Photocatalysis Fundamental and Applications

  2. Environmental Pollution • Atmosphere pollution • Green house effect (CO2) • Acid rain • Water pollution • Soil pollution

  3. Air Pollution Smog Acid rain Burning of fossil fuels

  4. Water Pollution Waste water from textile industry

  5. Contaminated soil Pesticides buried with strong odor Soil Pollution

  6. •OH Advanced Oxidation Technology • O3/H2O2 • O3/UV • O3/CATALYSTS • Fenton reaction (H2O2/Fe2+) • Photo-Fenton reaction (H2O2/Fe2+/UV) • H2O2/UV • O3/H2O2/UV • UV/TiO2 (Photocatalysis)

  7. Nature’s Cleaner：•OH In Atmosphere: 1) O3+ h(λ< 320 nm) →O2(1∆g) + O (1D)O (1D) + H2O →2•OH 2) HONO + h(λ< 400 nm) →NO + •OH [•OH]avg∼106radicals cm-3 (< 0.1 ppt!!) • In Water: • FeIII(OH)2+ (aq) + h(λ< 400 nm) →Fe2+ (aq) + •OH • NO3-(aq) + h→NO2+ O- • O-+ H2O →OH-+ •OH

  8. Oxidation Potentials (V vs NHE) HO• 2.80 O3 2.07 H2O2 1.78 HO2• 1.70 ClO2 1.57 HOCl 1.49 Cl2 1.36 Oxidation Potentials of Common Chemical Oxidants

  9. What is Photocatalysis? • The definition of photocatalysis is basically the acceleration of a photoreaction in the presence of a catalyst.

  10. UV light (< 387.5nm) O2 -0.5V CB O2-/O2 e- / H+ H2O2 3.2eV OH- OH./OH- +2.7V VB OH TiO2 Principle of TiO2 Photocatalysis e- O2- h+ OH • Hoffmann, M. S.; Martin, T.; Choi, W.; Bahnemann, D. W. Chem. Rev.1995, 95, 69-96.

  11. UV TiO2 h+ + e- e- + O2 O2- O2- + 2H+ + e- H2O2 •OH + OH- + O2 H2O2 + O2- •OH + H+ h+ + H2O Important Reactions during Photocatalysis

  12. Three Parameters Affecting Photocatalytic Activity • Light absorption Property • Light absorption spectrum and coefficient • Rate of reduction and oxidation of reaction substrate by e- and h+, respectively • Rate of e- and h+ recombination

  13. Enhancement of Photocatalytic Activity • Enhancing interfacial charge-transfer • Improving charge separation • Inhibiting charge carrier recombination

  14. Common Semiconductor Photocatalyst • TiO2 • Why TiO2? • Strong oxidizing power of valance band hole • Excellent chemical and photochemical stability • Availability: One of top 50 chemicals • Band gap: 3.2 eV • Only active under UV light (4%of the incoming solar energy)

  15. Anatase Rutile Brookite Crystal Structure of TiO2 Anatase is the most active one!

  16. Approaches to Improve the Activity of TiO2 • To enlarge band gap by reducing crystal sizes (quantum size effect) • To increase surface area (mesoporous structure) • To reduce crystal defects ( high crystallinity ) • To dope metal ions • To deposit noble metal nanoparticles • To couple two kinds of semiconductors

  17. Hot Research Topics of Photocatalysis • How to enhance the efficiency • Preparation of nanostructured photocatalysts • Extension of absorption of TiO2 to the visible region • Design of novel non-titania based visible Light photocatalysts

  18. Nanostructured Photocatalysts • Nanocrystals • Nanoporous materials

  19. Preparation Methods of Nanostructured TiO2 • Thermal decomposition method • Sol-gel method • Microemulsion method • Hydrothermal (or solvothermal) method • Combustion method • Other methods • microwave • nonhydrolytic • sonochemical

  20. Approaches to Improve the Activity of TiO2

  21. UV light (< 387.5nm) Au O2 -0.5V CB O2-/O2 e- / H+ H2O2 3.2eV OH- OH./OH- +2.7V VB OH TiO2 Photocatalytic Activity Enhancement by Noble Metal Deposition e- O2- h+ OH Inhibition of the recombination of h+ and e-!

  22. -0.5V CB CB +2.7V VB VB TiO2 Photocatalytic Activity Enhancement by Semiconductor Couples e- h+ Inhibition of the recombination of h+ and e-!

  23. TiO2-based Photocatalysts Responding to Visible Light • Sensitization of TiO2 • Organic dyes • Metal complexes • Narrow band gap semiconductors • Polymers • Ion-doped TiO2 • Metal ions • Non-metal ions

  24. Visible light O2 CB e- / H+ H2O2 +2.7V VB OH TiO2 Sensitization of TiO2-Dye Dye* e- O2- Dye Dye+• This is also the fundamental of dye-sensitized solar cell!

  25. Visible light O2 -0.5V e- CB CB O2-/O2 e- / H+ H2O2 O2- h+ VB OH +2.7V VB CdS band-gap:2.4eV TiO2 Sensitization of TiO2-Narrow Band-Gap Semiconductor e-

  26. Environmental Applications

  27. Water Purification Water purification (Purifics environmentaltechnologies)

  28. Air Cleaner

  29. Self-Cleaning Glass

  30. Photo-Induced Superhydrophilicity of TiO2 Coating UV

  31. 0 min 30 min 60 min Anti-Bacterial Materials

  32. Photo-ElectricityConversion

  33. Fuel Light Electricity Fuels Electricity CO H O 2 2 2 e e Sugar sc M sc M H O 2 H O 2 O 2 Semiconductor/Liquid Junctions Photosynthesis Photovoltaics Strategies of Solar Energy Conversions

  34. Traditional Silicon Solar Cell

  35. GratzelCell Dye Sensitized Solar Cell Gratzel, Nature 414, 338 (2001)

  36. Characteristics ofGratzelCell • Inexpensive • 1/10 of amorphous silicon • Flexible • Efficiency not high enough • Solid electrolyte

  37. 25 20 15 Efficiency (%) crystalline Si 10 amorphous Si nano TiO2 CIS/CIGS 5 CdTe 1980 1990 1970 1950 1960 2000 Year Efficiency of Photovoltaic Devices

  38. Water Splitting Utilizing Solar Energy -Hydrogen Production

  39. 4H+ + 4e-2H2 2H2O O2+4H+ + 4e- H2 O2 e- MSx MOx H+ cathode anode membrane Water Splitting Utilizing Solar Energy

  40. 纳米二氧化钛光催化性能研究 实验目的 1. 了解纳米光催化材料的性质； 2. 确定纳米二氧化钛光催化降解罗丹明B水溶液的反应速率常数； 3. 了解光催化剂催化性能评价的一般方法。

  41. 仪器与药品 分光光度计，离心机，电动搅拌器，光催化反应器(自制)，卤钨灯（220V 500W） 罗丹明B，纳米二氧化钛P25（德国Degussa公司产品）。

  42. 实验步骤 1. 取罗丹明B水溶液100 mL置于光催化反应器(自制)中，加入0.1 g P25，避光，开启冷凝水，搅拌。 2. 2 h后，取6 mL反应液，离心分离，测上层清液的吸光度A0 。 3. 0.5 h后，取6 mL反应液，离心分离，测上层清液的吸光度A0 ，将其与第2步测定的吸光度进行比较，判断罗丹明B在催化剂上是否达到吸附平衡。 4. 确认罗丹明B在催化剂上是否达到吸附平衡后，打开卤钨灯，每隔1 h取样6 mL反应液，离心分离，取上层清液用分光光度法测定其吸光度A。 5. 实验完毕，关闭卤钨灯，停止搅拌，清洗反应器，将仪器恢复原位，桌面擦拭干净。

  43. 注 释 1．数据处理 lnA对t作图，求出k及t1/2。 2．注意事项 实验前仔细阅读离心机说明书，使用时一定要遵守操作规程。

  44. 思考题 1．如何确定光催化剂的暗态吸附达到稳定时间？ 2．简述TiO2做为光催化剂降解有机污染物 的原理。 3．欲提高TiO2的光催化活性，你认为可采取哪些措施？

  45. 参考文献 [1] Fujishima A，Honda K. Nature[J]. 1972，37:238~239. [2] Piscopo A，Robert D，Weber J V. Journal of Photo chemistry and Photobiology A: Chemistry[J]. 2001，139 (2):253~256. [3] 李越湘，吕功煊，李树本等. 分子催化[J]. 2002，16(4):241~246. [4] 黄东升，陈朝凤，李玉花，曾人杰.无机化学学报[J]. 2007，4(4):738～742. [5] 张立德，牟季美. 纳米材料和纳米结构[M]. 北京:科学出版社，2001.

  46. Thank You!

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