FOTOKATALISIS

BAHAN KULIAH KEKHUSUSAN

OlehDr. Ir. S l a m e t , MT

Program Pascasarjana Teknik KimiaFakultas Teknik - Universitas Indonesia

Februari 2008

S i l a b u s 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

Buku Ajar :1. M. Schiavello, Heterogeneous Photocatalysis, John Wiley & Sons, 1997.2. A. Fujishima, et.al., TiO2 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

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).

Definisi-definisi

Bandgap Energy (Ebg)The energy difference between the bottom of the conduction band and 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 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.

TERMODINAMIKA & KINETIKA FOTOKATALISIS

Utk memprediksi kelayakan proses fotokatalisisUtk menjelaskan mengapa katalis ttt aktif & yg

lain tdk aktifFaktor termo & kinetika perlu dipertimbangkan

utk tentukan kondisi (eksperimen) terbaik

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

Mekanisme fotokatalisis

h

+

-

VB

CB

-

-

-

-

+

++

+

D+

D

A

A-

+

+

Rekombinasi dalam

Rekombinasi permukaan

1

3

42

e-

TiO2+h TiO2(eCB- + hVB

+)

TiO2(eCB-+hVB

+) TiO2+heat

A(ads) + eCB- A-(ads)

D(ads) + hVB+ D+(ads)

Limbah logam berat,

CO2, dll Limbah organik

PhotonThe 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

CB

VB

Band gap, energy bands

CB

VB

Band gap, energy bands

Diagram energi pada TiO2 & beberapa potensial redoks

CB

VB

Band gap, energy bands

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).

Transfer elektron

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 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.

Photocatalyst vs Chlorophyll

Function of Photocatalyst

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 oxygen vacancy • Permukaan bersifat hidrofilik

(1)

(4)(3)

(2)

Mekanisme self-cleaning

TiO2 : Self-cleaning

“TEST 9/6” markings made by orange marker on marble without photocatalyst sol coating

On 10th June, orange ink has penetrated through the marble without photocatalyst sol coating

“TEST 9/6” markings on the marble coated by E500 photocatalyst sol on 9th June.

On 10th June, the orange ink markings on the marble coated by E500 had beendecomposed.

BUILDING SELF-CLEANING SOLUTION(Organic pollutant decomposition)

Exterior wall self-cleaning

Granolith

Granite

Part 1: Increasing Contact SurfaceFirst, 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.

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.

Part 3: Preventing RecombinationIf 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 CO2 (fasa cair)

(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

(8). CH3• + H• CH4

(9). CH3• + CH3• C2H6

Pengaruh pH larutan pada reduksi CO2

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 1 2 3 4 5 6 7 8 9 10

pH

Pot

ensi

al v

s N

HE

(V

)

E-VB

E-CB (Anatase)

E-CB (Rutile)Eg = 3.2 eV

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].

24

68.25

0

50

100

150

200

250

Pro

du

k,

mo

l/(g

-ka

t.ja

m)

pH

Metanol

Etanol

Aseton

Pengaruh loading Cu pd fotoreduksi CO2 dg katalis Tembaga-TiO2 (t = 6 jam, T = 60oC), fasa cair

hv

H2OO2

CO2

CH3OH, CH4, CO,etc.

H2OCu

TiO2

Cu++

Cu0

Cu+

Mekanisme pembentukan metanol (katalis CuO/TiO2)

# 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

Mekanisme fotoreduksi Cr(VI)

Cr6+/Cr3+

VB

CB

Hg2+/Hg0

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5

Waktu (jam)Ko

ns. C

r(VI) (

mg/L)

pH 2

pH 7

pH 10

Reduksi Logam Berat Cr(VI) atau Hg(II)