BAHAN KULIAH KEKHUSUSAN FOTOKATALISIS Oleh Dr Ir S

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

Silabus Ø Ø Ø Ø Konsep dasar proses foto- katalisis Termodinamika & kinetika Mekanisme proses fotokatalisis Aplikasi fotokatalisis pada produksi H 2 dari air Material fotokatalis & dopan fotokatalis Foto-reaktor untuk produksi H 2 Pemanfaatan energi surya untuk produksi H 2 Buku Ajar : 1. M. Schiavello, Heterogeneous Photocatalysis, John Wiley & Sons, 1997. 2. A. Fujishima, et. al. , Ti. O 2 Photocatalysis: Fundamentals and Applications, BKC Inc. Japan, 1999. 3. J. B. Galvez, et. al. , Solar Detoxification, Natural Sciences, Basic and Engineering Sciences, UNESCO. 4. Paper-paper & Internet

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

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 the conduction band the top of the valence band in semiconductors and insulators. Conduction Band (CB) A vacant or only partially occupied set of many closely spaced electronic levels resulting from an array of a large number of atoms forming a solid system in which electrons can move freely or nearly so. The term is usually used to describe the electrical properties (among several others) of metals, semiconductors and insulators. Valence Band (VB) The highest energy continuum of energy levels in a semiconductor (or insulator) that is fully occupied by electrons at 0 K.

Sejarah fotokatalisis q Renz (1921) fenomena fotokatalisis pd permukaan semikonduktor metal-oksida Ø q Fujishima (1972) Pemecahan H 2 O jadi hidrogen & oksigen dg kristal tunggal Ti. O 2 dg input sinar UV energi rendah Ø Ø Ø q 1921 – 1960 -an: stagnant, kurang diminati. 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.

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

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

Mekanisme fotokatalisis Rekombinasi permukaan - + h + - CB - e- 1 A- 4 2 - A Limbah logam berat, CO 2, dll + + VB Ti. O 2+h Ti. O 2(e. CB- + h. VB+) 3 - + + Rekombinasi dalam D+ + Ti. O 2(e. CB-+h. VB+) Ti. O 2+heat D Limbah organik A(ads) + e. CB- A-(ads) D(ads) + h. VB+ D+(ads)

Photon The quantum of electromagnetic energy at a given frequency. This energy (E = hν) is the product of Planck’s constant (h) and the frequency of the radiation (ν). Photocatalysis

Band gap, energy bands CB VB

Band gap, energy bands CB VB

Band gap, energy bands CB VB Diagram energi pada Ti. O 2 & beberapa potensial redoks

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

Anti-bacteria For imparting anti-bacterial properties, nano-sized silver, titanium dioxide and zinc oxide are used. Metallic ions and metallic compounds display a certain degree of sterilizing effect. It is considered that part of the oxygen in the air or water is turned into active oxygen by means of catalysis with the metallic ion, thereby dissolving the organic substance to create a sterilizing effect. With the use of nano-sized particles, the number of particles per unit area is increased, and thus anti-bacterial effects can be maximized.

Several papers have discussed the use of the photocatalytic property of Ti. O 2 in the field of textiles. It was determined that a fabric treated with nano-Ti. O 2 could provide effective protection against bacteria and the discoloration of stains, due to the photocatalytic activity of nano-Ti. O 2. 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 (Zn. O: 3. 37 e. V, Ti. O 2: 3. 2 e. V) is different from titanium dioxide. Nano-Zn. O provides effective photocatalytic properties once it is illuminated by light, and so it is employed to impart anti-bacterial properties to textiles.

Photocatalyst vs Chlorophyll

Function of Photocatalyst

Mekanisme hidrofilik • • • Elektron mereduksi kation Ti(IV) menjadi Ti(III) Hole mengoksidasi anion O 2 Atom oksigen diusir membentuk oksigen vacancy Molekul air akan terserap ke dlm oxygen vacancy Permukaan bersifat hidrofilik

Mekanisme self-cleaning (1) (3) (2) (4)

Ti. O 2 : Self-cleaning

BUILDING SELF-CLEANING SOLUTION (Organic pollutant decomposition) “TEST 9/6” markings made by orange marker on marble without photocatalyst sol coating On 10 th June, orange ink has penetrated through the marble without photocatalyst sol coating “TEST 9/6” markings on the marble coated by E 500 photocatalyst sol on 9 th June. On 10 th June, the orange ink markings on the marble coated by E 500 had been decomposed.

Exterior wall self-cleaning Granite Granolith

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 Ti. O 2, preferably in the shape of a sphere. But how small should we go? Our team visualizes that since modern technology permits, Ti. O 2 particles should be nanosized to promote the greatest amount of surface area possible.

Part 2: Reducing the Band-Gap Energy To reduce the band-gap energy, our group speculates that doping Ti. O 2 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 Ti. O 2 will be reduced so that visible light is capable of supplying enough energy to generate e-/h+ pairs.

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.

Mekanisme fotoreduksi CO 2 (fasa cair) (1). Ti. O 2 + h Ti. O 2(e- + h+) (2). H+ + e- H • (3). CO 2 + 2 H • HCOOH (4). HCOOH + 2 H • H-CO-H +H 2 O (5). H-CO-H + H • H- • C(OH)-H (6). H- • C(OH)-H + H • CH 3 OH (7). CH 3 OH + H • CH 3 • + H 2 O (8). CH 3 • + H • CH 4 (9). CH 3 • + CH 3 • C 2 H 6

Pengaruh p. H larutan pada reduksi CO 2 ü ü ü Aspek termodinamika: semakin tinggi p. H larutan reduksi CO 2 semakin efektif. Aspek kinetika: makin rendah p. H jumlah H+ ( radikal H ) bertambah produk lebih banyak. Nilai p. H optimum (= 4): keseimbangan antara aspek kinetika & termodinamika. Jika p. H sangat rendah (asam), kons. ion karbonat yg terlarut menurun produk berkurang. Nilai p. H larutan yg biasa digunakan, antara: 4 – 6 [US Patent].

CO Cu 2 Cu++ h v Cu+ H 2 OO Ti. O H 2 O 3 OH, CH CH 4, CO, etc. 2 2 Cu 0 Pengaruh loading Cu pd fotoreduksi CO 2 dg katalis Tembaga-Ti. O 2 (t = 6 jam, T = 60 o. C), fasa cair

Mekanisme pembentukan metanol (katalis Cu. O/Ti. O 2)

Mekanisme fotoreduksi Cr(VI) # Reaksi fororeduksi Cr(VI) yang terjadi pada p. H asam ( 2): Cr 2 O 72 - + 14 H+ + 6 e- → 2 Cr 3+ + 7 H 2 O E 0 = 1, 23 V # Reaksi fotoreduksi Cr(VI) yang terjadi pada p. H netral/basa: Cr. O 42 - + 4 H 2 O + 3 e- → Cr(OH)3 + 5 OH- E 0 = -0, 13 V

Reduksi Logam Berat Cr(VI) atau Hg(II) VB Cr 6+/Cr 3+ Hg 2+/Hg 0 CB