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FOTOKATALISIS

FOTOKATALISIS. BAHAN KULIAH KEKHUSUSAN. Oleh Dr. Ir. S l a m e t , MT Program Pascasarjana Teknik Kimia Fakultas Teknik - Universitas Indonesia Februari 2008. Konsep dasar proses foto- katalisis Termodinamika & kinetika Mekanisme proses fotokatalisis

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FOTOKATALISIS

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  1. FOTOKATALISIS BAHAN KULIAH KEKHUSUSAN Oleh Dr. Ir. S l a m e t , MT Program Pascasarjana Teknik Kimia Fakultas Teknik - Universitas Indonesia Februari 2008

  2. Konsep dasar proses foto- katalisis Termodinamika & kinetika Mekanisme proses fotokatalisis Aplikasi fotokatalisis pada produksi H2 dari air Material fotokatalis & dopan fotokatalis Foto-reaktor untuk produksi H2 Pemanfaatan energi surya untuk produksi H2 S i l a b u s • Buku Ajar : • M. Schiavello, Heterogeneous Photocatalysis, John Wiley & Sons, 1997. • A. Fujishima, et.al., TiO2 Photocatalysis: Fundamentals and Applications, BKC Inc. Japan, 1999. • J.B. Galvez, et.al., Solar Detoxification, Natural Sciences, Basic and Engineering Sciences, UNESCO. • Paper-paper & Internet

  3. FOTOKATALISIS Suatu proses transformasi kimia yang dibantu oleh adanya CAHAYA dan material KATALIS

  4. Definisi-definisi Fotokatalisis (menurut IUPAC) Suatu reaksi katalitik yg melibatkan absorpsi cahaya oleh katalis atau substrat tertentu. Dapat juga didefinisikan sbg suatu proses kombinasi antara fotokimia dan katalis, yaitu suatu proses transfor-masi kimiawi dg melibatkan cahaya sbg pemicu dan katalis sbg pemercepat proses transformasi tsb (Serpone, 2002). Bandgap Energy (Ebg) The energy difference between the bottom of theconduction bandand the top of the valence bandinsemiconductors and insulators. Conduction Band (CB) A vacant or only partially occupied set of manyclosely spaced electronic levels resulting from an arrayof a large number of atoms forming a solid systemin which electrons canmovefreely or nearly so. Theterm is usually used to describe the electrical properties(among several others) of metals, semiconductorsand insulators. Valence Band (VB) The highest energy continuum of energy levels in asemiconductor (or insulator) that is fully occupied byelectrons at 0K.

  5. Sejarah fotokatalisis • Renz (1921) fenomena fotokatalisis pd permuka-an semikonduktor metal-oksida • 1921 – 1960-an: stagnant, kurang diminati. • Fujishima (1972) Pemecahan H2O jadi hidrogen & oksigen dg kristal tunggal TiO2 dg input sinar UV energi rendah • Fotokatalisis mulai POPULER (majalah Nature), karena • Isu krisis energi, Hidrogen: bhn bakar ramah lingkungan • Kendala: efisiensi rendah (<1%)  belum feasible • ERA FOTOKATALISIS (> ‘80)  pengembangan ‘fenomena’ fotokatalisis yg lebih feasible utk tataran aplikasi keseharian  pengembangan TEKNOLOGI.

  6. TERMODINAMIKA & KINETIKA FOTOKATALISIS • Utk memprediksi kelayakan proses fotokatalisis • Utk menjelaskan mengapa katalis ttt aktif & yg lain tdk aktif • Faktor termo & kinetika perlu dipertimbangkan  utk tentukan kondisi (eksperimen) terbaik

  7. Termodinamika pada proses fotokatalitik heterogen • Proses-proses foto-reaksi (reduksi, oksidasi) dpt dikelompokkan dlm 2 golongan: • Reaksi spontan (G < 0)  proses fotokatalitik atau exergonic reactions, atau catalytic photoassisted reactions (CPR). Contoh: foto-oksidasi senyawa organik • Reaksi tdk spontan (G > 0)  proses fotosintetik atau endergonic reactions, atau catalytic photoassisted synthesis (CPS). Contoh: photo-splitting H2O; foto-reduksi CO2; dll • Pengaruh iluminasi cahaya • G < 0  laju rekasi naik jika katalis SC diiluminasi dg cahaya • G > 0  rekasi terjadi jika katalis SC diiluminasi dg cahaya

  8. Rekombinasi permukaan + h + - - CB e- - 1 + VB + TiO2+h TiO2(eCB- + hVB+) A- 4 2 TiO2(eCB-+hVB+) TiO2+heat 3 - D+ A(ads) + eCB-A-(ads) + A - + + Rekombinasi dalam D D(ads) + hVB+D+(ads) Mekanisme fotokatalisis Limbah logam berat, CO2, dll Limbah organik

  9. Photon The quantum of electromagnetic energy at a givenfrequency. This energy (E = hν) is the product ofPlanck’s constant (h) and the frequency of theradiation(ν). Photocatalysis

  10. Band gap, energy bands CB VB

  11. Band gap, energy bands CB VB

  12. Band gap, energy bands CB VB Diagram energi pada TiO2 & beberapa potensial redoks

  13. Transfer elektron • Kemampuan semikonduktor untuk mentransfer elektron pada molekul yang teradsorbsi tergantung pada posisi pita energinya (pita konduksi dan pita valensi) dan potensial redoks molekul tersebut. • Potensial reduksi yang relevan untuk molekul penerima elektron adalah keharusan mempunyai potensial reduksi lebih positif yaitu terletak dibawah potensial reduksi pita konduksi semikonduktor (CB). • Potensial reduksi molekul pendonor elektron harus lebih negatif yaitu terletak diatas potensial reduksi pita valensi semikonduktor(VB).

  14. Several papers have discussed the use of the photocatalytic property of TiO2 in the field of textiles. It was determined that a fabric treated with nano-TiO2 could provide effective protection against bacteria and the discoloration of stains, due to the photocatalytic activity of nano-TiO2. On the other hand, zinc oxide is also a photocatalyst, and the photocatalysis mechanism is similar to that of titanium dioxide; only the band gap (ZnO: 3.37eV, TiO2: 3.2eV) is different from titanium dioxide. Nano-ZnO provides effective photocatalytic properties once it is illuminated by light, and so it is employed to impart anti-bacterial properties to textiles.

  15. Photocatalyst vs Chlorophyll

  16. Function of Photocatalyst

  17. Mekanisme hidrofilik • Elektron mereduksi kation Ti(IV) menjadi Ti(III) • Hole mengoksidasi anion O2- • Atom oksigen diusir membentuk oksigen vacancy • Molekul air akan terserap ke dlm oxygenvacancy •  Permukaan bersifat hidrofilik

  18. Mekanismeself-cleaning (2) (1) (4) (3)

  19. TiO2 : Self-cleaning

  20. BUILDING SELF-CLEANING SOLUTION (Organic pollutant decomposition) “TEST 9/6” markings made by orange marker on marble without photocatalyst sol coating “TEST 9/6” markings on the marble coated by E500 photocatalyst sol on 9th June. On 10th June, orange ink has penetrated through the marble without photocatalyst sol coating On 10th June, the orange ink markings on the marble coated by E500 had been decomposed.

  21. Exterior wall self-cleaning Granite Granolith

  22. Part 1: Increasing Contact Surface First, there needs to be an increase in contact surface.  Since spheres possess the largest surface area given a specified volume, we need to have small particles of TiO2, preferably in the shape of a sphere.  But how small should we go?  Our team visualizes that since modern technology permits, TiO2 particles should be nanosized to promote the greatest amount of surface area possible.

  23. Part 2: Reducing the Band-Gap Energy To reduce the band-gap energy, our group speculates that doping TiO2 with other appropriate metal molecules via vapordeposition will be promising.  Doping can either add an energy level filled with electrons in the band gap which can be easily excited into the conduction band (n-type) or add a level of extra holes in the band gap to allow the excitation of valence band electrons, to create mobile holes in the valence band (p-type).  Hence, by doping with certain appropriate metal molecules, the band-gap energy of TiO2 will be reduced so that visible light is capable of supplying enough energy to generate e-/h+ pairs.

  24. Part 3: Preventing Recombination If recombination of pairs occurs, all photocatalytic capabilities disappear and all advantages from doping are cancelled out.  Yet, with nanosized particles, current methods of preventing recombination, e.g. through flowing a current through the bulk material or through inserting positively charged holes and negatively charged electrons, cannot be implemented.  Here, we propose that doping be used with an additional material that can attract electrons or holes which are generated upon being exposed to visible light.  To find that material, more research must be done.

  25. (1). TiO2 + h TiO2(e- + h+) (2). H+ + e- H• (3). CO2 + 2H• HCOOH (4). HCOOH + 2H• H-CO-H +H2O (5). H-CO-H + H• H-•C(OH)-H (6). H-•C(OH)-H + H• CH3OH (7). CH3OH + H• CH3• + H2O CH4 (8). CH3• + H• (9). CH3• + CH3• C2H6 Mekanisme fotoreduksi CO2 (fasa cair)

  26. Pengaruh pH larutan pada reduksi CO2 • Aspek termodinamika: semakin tinggi pH larutan  reduksi CO2 semakin efektif. • Aspek kinetika: makin rendah pH  jumlah H+ ( radikal H) bertambah  produk lebih banyak. • Nilai pH optimum (= 4): keseimbangan antara aspek kinetika & termodinamika. • Jika pH sangat rendah (asam), kons. ion karbonat yg terlarut menurun  produk berkurang. • Nilai pH larutan yg biasa digunakan, antara: 4 – 6 [US Patent].

  27. CO2 Cu H2O H2O CH3OH, CH4, CO, etc. O2 hv TiO2 Cu++ Cu+ Cu0 Pengaruh loading Cu pd fotoreduksi CO2 dg katalis Tembaga-TiO2 (t = 6 jam, T = 60oC), fasa cair

  28. Mekanisme pembentukan metanol (katalis CuO/TiO2)

  29. Mekanisme fotoreduksi Cr(VI) # Reaksi fororeduksi Cr(VI) yang terjadi pada pH asam (2): Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O E0 = 1,23 V # Reaksi fotoreduksi Cr(VI) yang terjadi pada pH netral/basa: CrO42- + 4 H2O + 3e- → Cr(OH)3 + 5OH- E0 = -0,13 V

  30. VB Cr6+/Cr3+ Hg2+/Hg0 CB Reduksi Logam Berat Cr(VI) atau Hg(II)

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