Perspectives on photocatalysis to the water and wastewater treatment

Prof Regina de F P M MoreiraDepartamento de Engenharia Química e Engenharia de Alimentos

Universidade Federal de Santa CatarinaFlorianópolis - SC

regina@enq.ufsc.br

Photocatalysis

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photocatalysis

Number of papers/year (www.sciencedirect.com)

Number of papers in Photocatalysis: 1975–1980: 2492000–2010: 16.757

Number of patents in photocatalysis

Air treatment

Self cleaning surfaces

Water and wastewater treatment

TiO2 - the most used photocatalyst (non-toxic, stable and not expensive)

Balkus Jr, K., New and Future Developments in Catalysis -Catalysis by Nanoparticles, 2013, Pages 213–244

Publications about “nanoparticle photocatalysts”

Photocatalysis

(1975–1985) semiconductor/solution interface under UV irradiation several semiconductors

Polycristalline materials were the most suitable.Photocatalysts in aqueous suspensions.

(1986 - 2000) Thin films;Doping of semiconductors to explore visible light;Dye sensitization (photocatalysts in aqueous suspension).

Industrial activities.

(2006–2010): Nanophotocatalysts

Photocatalysts•Semiconductors•Conduction Band (CB) electrons have a chemical potential of + 0.5 to -1.5 V vs NHE hence they can act as reductants.•Valence Band (VB) holes exhibit a strong oxidative potential + 1.0 to + 3.5 V vs NHE

Band-edge positions of semiconductor photocatalysts relative to the energy levels of various redox couples in water.

H Tong, S Ouyang, Y Bi, N Umezawa, M Oshikiri, J Ye, Nanophotocatalytic materials: possibilities and challenges, Adv Mater 2012, 24, 229-251.

Photocatalytic activity and semiconductor properties

Energy band configuration determinates the absorption of incident photons, photoexcitation of electron-hole pairs, migration of carriers, and redox capabilities of excited-state electrons and holes.

Energy bands engineering

PhotocatalystsENERGY BAND ENGINEERING

Some important aspects:- Optical absorption: direct and narrow bandgap semiconductors are more likely to

exhibit high absorbance suitable for the efficient harvesting of low energy photons.- Disadvantages:

- recombination electron/hole- Band-edge positions are frequently incompatible with the electrochemical

potential necessary to trigger specific redox reactions

- Modulate the band gap and band-edge positions in a precise manner different strategies

- Improvement of light sensitization by the inclusion of quantum dots, plasmon-exciton coupling between anchored noble metal nanoparticle co-catalysts and the host semiconductor, and photon coupling in semiconductor photonic crystals.

Energy Band Engineering

I. Modiulation of VBII. Adjustment of the CBIII. Continuous modulation of the VB and/or CB

A. Millis and S. L. Hunte J. Photochem. Photobiol. A: Chem 180 (1997) 1

• VB Redox potential should be sufficiently positive in order to the holes act as electron acceptor ; oxidation reaction

• CB: Redox potential should be sufficiently negative in order to the oxygen act as electron acceptor reduction reaction

Photocatalytic degradation of pollutants in water or wastewater oxygen as electron acceptor

Energy Band Engineering

Oxide semiconductors CB slightly negative ;VB significantly positive with respect to the oxidization of H2O (vs NHE).

Therefore

For the consideration of stability of materials, raising to top of the VB to narrow the bandgap takes precedence over all other methods of energy-band modulation.

To adjust the level of the VB: the most effective strategies:

I. Doping with 3d transition elementsII. Cations with d10 our d10s2 configurationsIII. Non-metal elements

• A) TiO2 Doping N, S, C, metals strategies to raise the VB maximum

• B) TiO2 Dye surface sensitization

• C) Surface modification to increase stability

• D) Coupled semiconductors

• E) Novel semiconductor containing 3d metals.

Energy band engineering

Miao Zhang et al, Angew. Chem. Int. Ed. 2008, 47, 9730 –9733

A) Doping with non-metal: C, N, P, B, S

Mechanism of photocatalytic activity of TiO2 doped with S

S.X. Liu, X.Y. Chen, J. Hazard. Mater. 152, 48–55 (2008)

A.1.1 Doping with sulfur

K. HASHIMOTO et al. Jpn. J. Appl. Phys., Vol. 44, No. 12 (2005)

A.1.2 Doping with nitrogen

• Doping with N, C, S narrows the bandgap by less than 0.3 V.• Significant extension of visible light absorption via anion doping remains a big challenge.

Successful example of band-edge control for the utilization of visible light mechanism under debate.

- Hybridization of the N-related states with the host VB;

- N-doping in TiO2 is accompanied by formatin of Ti3+ via donor-type deffects

Nanofio dopado com nitrogênio

Nanofio

Photocatalytic degradation of Phenol in aqueous solution using nanowires of N-doping TiO2

Ilha, José, Moreira, Degradação fotocatalítica de fenol utilizando nanofios de dióxido de titânio modificados com nitrogênio). UFSC, 2012

Photocatalyst Bandgap (eV)

TiO2 P25 3,05

Nanowire TiO2 2,62

N-doped TiO2 nanowired 2,53

Catalisador k' (10-3min-1)

P25 2,6

nanowireTiO2 0,6

N doped TiO2 nanowired 1,1

Pseudo first order kinetic constant for the phenol minearlization using different photocatalysts

Phenol initial concentration: 100 mg/ L; Photocatalyst dosage 1g/L.

Effect of nitrogen content

Decomposition of rhodamine B after 1 h using TiO2 or N- TiO2 (different N/Ti ratio) under visible light.

Ye Cong et al., J. Phys. Chem. C, Vol. 111, No. 19, 2007, 6976-6982

N doped TiO2

Theoretical studies: only 1% atomic% N (0.53 % w/w) on TiO2 is necessary to activate photocatalytic reactions under visible light.Fu, Zhang, Zhang, Zu, J Phys Chem B 2006, 110, 3061.

CBe- e- e- e- e- e- e- e- e- e- e- e-

VBh+ h+ h+ h+ h+ h+ h+ h+ h+ h+

Recombination e-/h+

e-(M) M+e-

Eg

B – Metal doping

Metal promoter: attracts the electrons to the CB recombination is inhibited.

• ionic radius of the metal similar to the Ti4+ ,

• Exhibit 2 or more oxidation states.

• Energy levels Mn+ /M(n+1) similar to Ti3+ /Ti4+ ,

• Electronegativity: higher than Ti

• incomplete/parcial electronic configuration

Ionic radius

B – Metal Doping

Effect of Pt-metal content in Pt/TiO2 (P25) catalysts on CH4 yield for photocatalytic reduction of CO2 after 7 h UV irradiation at 323 K, H2O/CO2 = 0.02.

Fotoactivity of TiO2 doped with Pt effect of the metal concentration on the production of methane by the photoreaction: CO2 + H2O CH4 + O2

Q.-H. Zhang et al. / Catalysis Today 148 (2009) 335–340

B – Noble metals doping

Capítulo 6

Copper, zinc and ChromiumDe Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44

• Photocatalyst synthesis: photodeposition by controllingl of precursor metals solubility

B – Non noble metal doping

• Photocatalytic denitrification:– Photoreduction of NO3

- to produce N2

– Hole scavanger: Formic acid (electron donor)

– Nitrate electron acceptor• Theoretical molar ratio to reduce nitrate to nitrogen CHOOH:NO3

- = 8:1

Capítulo 6

𝑁𝑂3− + 2𝑒− + 𝐻2𝑂→𝑁𝑂2− + 2𝑂𝐻− 𝑁𝑂2− + 6𝑒− + 7H+ →𝑁𝐻3 +2𝐻2𝑂 2𝑁𝐻3 →𝑁2 + 3𝐻2 6-3 2𝐻𝐶𝑂𝑂− + ℎ+ →𝐶𝑂2 + 𝐶𝑂2°− + 𝐻2

De Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44

B - Non-noble metals dopingCopper, zinc and ChromiumDe Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44

Kinetics of photocatalytic degradation of nitrate and formic acid (measured as TOC), and formation of products (ammonia and nitrite) pH 2.5. TiO2, Zn-TiO2, Cr-TiO2 e Cu-TiO2 = 1g L-1. NO3

- = 0.6 mM (9 mg N L-1); CHOOH = 9.8 mM (117.4 mg COT L-1).

Moreira., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44

B – Non-noble metal doped TiO2

Time, min

NoN-byproducts

NH4+

main byproduct

Capítulo 6

• Copper, zinc or chromium:– Zn-TiO2: higher photocatalytic activity than Cr-TiO2 or Cu-TiO2, and lower byproducts

formation.– Zn action To promote efficient charge separation (e-/h+)

Moreira et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44

B – Non noble metal doping

• Effect of dissolved oxygen on the photocatalytic activity of Zn-TiO2

– O2 competes with NO3- ions, acting as electron acceptor

Selectivity [%]Nitrate conversion after 2 h[%]

Activity [µmolNO3- (min gcatalisador) -1]

Presence of O2 (air) 77.2 87.5 4.11By purging argon (without O2

95.4 91.7 14.24

Photocatalytic nitrate reduction using 4.4% Zn–TiO2 as photocatalyst

Moreira et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44

B – Doping with non noble metals

C) Coupling semiconductors

Illustration of an electronic bond formed between (A) two atoms and (B) two nanocrystals.

Tong, Ouyang, Bi, Umezawa, Oshikiri, Ye, Adv Mater 2012, 24, 229.

Ensemble of nanoparticles may exhibit new collective properties resulting from the inter-particle coupling of surface electrons (excitons), plasmons or magnetic moments.

- induce a substantial alteration of the electronic structures of the nanoparticle ensemble bonding and anti-bonding levels are formed, yielding a new electronic structure.

The electron transference from CdS to TiO2 increase the charge separation and the photocatalytic efficiency.

Sclafani, A.; Mozzanega, M.-N.; Pichat, P. J. Photochem. Photobiol. A: Chem. 1991, 59, 81.

C) Coupling semiconductors

Interesting way to increase the efficiency of a photocatalytic process:- by increasing the charge separation- by extending the energy range of photoexcitation for the system- by extending

The potential of VB or CB of coupled semiconductors should be more negative or less positive, respectively, than pure TiO2

Hole produced in the VB remains in the CdS particle

Electron it is transferred to the CB of TiO2 particle.

Hybrid seminconductors– TiO2/graphene

Graphene increase the electric conductivity, charge transfer and chemical stability

- Decrease recombination electron/hole due to the high electronic conductivity of graphene;- High active site concentration, due to the high ratio area:volume, and bidimensional structure- High range of light absorption-TiO2/graphene composites Strong interaction aromatic rings of graphene and organic

molecules

Bond Ti-O-C graphene acts as co-catalyst (Lv et al., Procedia Engineering 27 (2012) 570-576.

TiO2 (P25)-graphene photocatalytic activity is higher than pure TiO2 P25 (Zhang et al., 4 (2010) 380)

is promising to simultaneously possess excellent adsorptivity, transparency, conductivity, and controllability, which could facilitate effective photodegradation of pollutants.

Kinetic of photocatalytic degradation of Rhodamine B

High activity results from:

• Strong coupling between TiO2 on graphene oxide facilitate interfacial change transfer;• (GO ) acts as electron acceptor and inhibits the e/h recombation.

Liang et al, Nano Res,2010.

Huimin et al., Chinese Journal of Catalysis, 33 (2012) 777-782.

TiO2/Graphene

Scheme of the Photocatalytic Degradation of methylene blue (a) TiO2 (b) TiO2/GrapheneE. Lee et al. / Journal of Hazardous Materials 219– 220 (2012) 13– 18

Kinetic constant for the photocatalytic degradation of Rhodamine B

ZnFe2O4/Magnetic graphene

Nanosheets of graphene and ZnFe2O4 nanocrystals

Comparing ZnFe2O4 and ZnFe2O4/grafeno• Composite ZnFe2O4/grafeno catalyst for photodegradation • Generation of HO* radicals via photochemical reactions of H2O2 under visible light

ZnFe2O4 – with (a) and without (b) magnetic field

The photogenerated electrons of excited ZnFe2O4 were transferred instanteously from the conduction band of ZnFe2O4 to graphene at the site of generation via a percolation mechanism, resulting in a minimized charge recombination enhanced photocatalytic activity

Spinel ZnFe2O4 (Eg= 1.90 eV)

Fu e Wang, Ind Eng Chem Res 50 (2011) 7210-7218.

Magnetic semiconductor material

Lanthanide modified semiconductor photocatalystss

General enhancement in the photocatalytic activity:- Enhanced adsorption of the organics;- Effective separation of e/h- High intrinsic absorptivity under UV irradiation due to the ability of RE metal ions to trap

electrons and minimize e/h recombination

The biggest difference between the transition metal ion and the lanthanide ions nature of the 4f orbitals

Lanthanide excellent optical properties

• Incorporation of Rare-Earths metal ions leads to the formation of multi energy levels below the conduction band edge of TiO2

• Lanthanide ions may act as electron scavenger and suppress e/h recombination;

• Lanthanite ions also can faciliate the adsorption of organics or act as electron acceptors (minimizing e/h recombination)

Photocatalytic activity of Ln3+/TiO2

Weber, Grady and Kookdali, Cat Sci & Tech 2012, 2, 683.

CeO2/TiO2

J. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 107–114

(a) UV vis absorption spectra fo undoped and Ce-doped TiO2 microspheres(b) Photographs of Ce-doped TiO2 samples

TOC removal efficiencies (Methylene blue) during visible light irradiation (t=180 min)

Effect of cerium doping the photocatalytic activity to degrade methylene blue: From 1 – 5% cerium excess Ce4+ dopants may introduce the indirect recombination of electrons and holes to reduce the photocatalytic activity.

Photocatalytic degradation of methylene blue – different catalysts and P25

J. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 107–114

CeO2/TiO2

Photocatalytic degradation of Rhodamine B– different catalysts and P25

Compósitos de TiO2 dopados com Er3+:YAlO3/Fe- e Co

•Fe and Co ions doped into TiO2 powder to restrain the recombination•Er3+:YAlO3 upconversion luminescence agent can transform the visible light into UV light more efficiently

Degradation of organic compounds in the presence of Er3+:YAlO3/(Co or Fe)/TiO2 under visible light

• Visible light is converted luz UV pelo Er3+:YAlO3.

• UV light can excite TiO2 -> electrons transfer from VB to CB

• e/h pairs no recombination due to presence of Fe or Co ions

R. Xu et al. / Solar Energy Materials & Solar Cells 94 (2010) 1157–1165

TiO2 composites doped with Er3+:YAlO3/Fe- or Co

Photocatalytic degradation of azo fuchsine int the presence of photocatalysts Fe or Co/TiO2 and different amouns of Er3+:YAlO3

Fe/TiO2 Co/TiO2

5% Er3+YAlO3/Fe/TiO2

10% Er3+YAlO3/Co/TiO2

25% Er3+YAlO3/Co/TiO225% Er3+YAlO3/Fe/TiO2

R. Xu et al. / Solar Energy Materials & Solar Cells 94 (2010) 1157–1165

Bismutum Spinels

BiWO6, Bi4Ti3O12, BIOX (X=Cl, Br, I), Bi2O3 photocatalytic activity under UV and visible lightEg = 2,9 a 3,5 eV, depending on the preparation method (Chen et al., 2012).

Bi2S3 Eg= 1,3 a 1,7 eV (Mesquita e Silva, 34ª Reunião SBQ, 2011).

* Bi2O2CO3 High activity: morphology, low band gap energy. (Chen et al., 2012)

* CdBiYO4 (Du and Juan, Solid State Sciences, 14 (2012) 1295-1305) spinel

Copper nanowires

Yu Li, Xiao-Yu Yang, Joanna Rooke, Guastaaf Van Tendeloo, Bao-Lian Su. Ultralong Cu(OH)2 and CuO nanowire bundles: PEG200-directed crystal growth for enhanced photocatalytic performance, Journal of Colloid and Interface Science 348 (2010) 303–312

Nanowires CuO e Cu(OH)2

CuO Eg ~1.2 eV

UV absorption spectra of CuO nanowires

Photocatalytic degradation of Rhodamine B using different photocatalysts under UV light

Nanowires of CuO

Efficient charge separation and increase of photocatalytic activity

FESEM images of sample

Tungstenium oxides

WO3 + co-catalyst(Pt, Cu, or Pd): high photocatocalytic efficency to degrade organics

WO3 --> Conduction Band ( +0.5 V vs NHE) is more positive than that for O2 reductionO2 + e = O2

*- (aq) 0.284 V vs NHE; O2 + H+ + e = HO2

* (aq), 0.046 V vs NHE

WO3 can act as photocatalyst sensible to visible light in the presence of an electron acceptor (ozônio +2.07 V vs NHE).

Ozone reacts with the photoexcited electrons oxidation of organic compounds

S. Nishimoto et al. / Chemical Physics Letters 500 (2010) 86–89

WO3 Eg = 2,5 ev

Photocatalytic degradation of PhenolTOC initial = 130 ppm

S. Nishimoto et al. / Chemical Physics Letters 500 (2010) 86–89

Photocatalytic degradation of PhenolTOC initial = 130 ppm

d0 e d10 Óxidos metálicos

d0

Ti4+: TiO2, SrTiO3, K2La2Ti3O10

Zr4+: ZrO2 Nb5+: K4Nb6O17, Sr2Nb2O7

Ta5+: ATaO3(A=Li, Na, K), BaTa2O6

W6+: AMWO6 (A=Rb, Cs; M=Nb, Ta)

d10

Ga3+: ZnGa2O4

In3+: AInO2 (A=Li, Na)Ge4+: Zn2GeO4 Sn4+: Sr2SnO4

Sb5+: NaSbO7

Domen et al. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. 2007

E) Photocatalysts

 Photocatalytic activity of oxides and nitrides d10 metals it is associated with the CB of the hybridized sp-orbitals, that are able to produce photoexcited eletrons with high mobility.

Generally, the band gap energy is high

Final Remarks

The function and engineering of co-catalysts is one of the most important subjects in photocatalysis.

Challenge and perspectives photocatalysts sensible to visible light and high activity

Promissor materialsGrapheneRare earthsComposites and doped co-catalyts

Reactor design is still a big challenge

Thank you

regina@enq.ufsc.br