Perspectives on photocatalysis to the water and wastewater treatment Prof Regina de F P M Moreira Departamento de Engenharia Química e Engenharia de Alimentos Universidade Federal de Santa Catarina Florianópolis - SC firstname.lastname@example.org
Photocatalysis Number of papers in Photocatalysis: 1975–1980: 249 2000–2010: 16.757 TiO2 - the most used photocatalyst (non-toxic, stable and not expensive) Air treatment Waterandwastewatertreatment Self cleaningsurfaces Numberofpapers/year (www.sciencedirect.com) Number of patents in photocatalysis Publications about “nanoparticle photocatalysts” BalkusJr, K., New and Future Developments in Catalysis -Catalysis by Nanoparticles, 2013, Pages 213–244
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 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. Band-edge positions of semiconductor photocatalystsrelative to the energy levels of various redox couples in water. Energy bands engineering H Tong, S Ouyang, Y Bi, N Umezawa, M Oshikiri, J Ye, Nanophotocatalytic materials: possibilities and challenges, AdvMater 2012, 24, 229-251.
Photocatalysts ENERGY 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 Modiulation of VB Adjustment of the CB Continuous modulation of the VB and/or CB Photocatalytic degradation of pollutants in water or wastewater oxygen as electron acceptor • 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 A. Millis and S. L. Hunte J. Photochem. Photobiol. A: Chem 180 (1997) 1
Energy Band Engineering CB slightly negative ; VB significantly positive with respect to the oxidization of H2O (vs NHE). Oxide semiconductors 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: Doping with 3d transition elements Cations with d10 our d10s2 configurations Non-metal elements
Energy band engineering • 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. Miao Zhang et al, Angew. Chem. Int. Ed. 2008, 47, 9730 –9733
A) Doping with non-metal: C, N, P, B, S A.1.1 Doping with sulfur A.1.2 Doping with nitrogen 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 Mechanism of photocatalytic activity of TiO2 doped with S S.X. Liu, X.Y. Chen, J. Hazard. Mater. 152, 48–55 (2008) K. HASHIMOTO et al. Jpn. J. Appl. Phys., Vol. 44, No. 12 (2005) • 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.
PhotocatalyticdegradationofPhenol in aqueoussolutionusingnanowiresof N-doping TiO2 Ilha, José, Moreira, Degradação fotocatalítica de fenol utilizando nanofios de dióxido de titânio modificados com nitrogênio). UFSC, 2012 Pseudofirstorderkineticconstant for thephenolminearlizationusingdifferentphotocatalysts Phenolinitialconcentration: 100 mg/ L; Photocatalystdosage 1g/L.
N doped TiO2 Effect of nitrogen content 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 PhysChem B 2006, 110, 3061. 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
B – Metal doping CB e- e- e- e- e- e- e- e- e- e- e- e- e-(M) M+e- e-/h+ Eg Recombination VB h+ h+ h+ h+ h+ h+ h+ h+ h+ h+ Metal promoter: attracts the electrons to the CB recombination is inhibited.
B – Metal Doping • ionic radius of the metal similar to the Ti4+ , • Exhibit 2 or more oxidation states. • Energy levels Mn+ /M(n+1) similar toTi3+ /Ti4+ , • Electronegativity: higher than Ti • incomplete/parcial electronic configuration Ionic radius
B – Noble metals doping Fotoactivity of TiO2 doped with Pt effect of the metal concentration on the production of methane by the photoreaction: CO2 + H2O CH4 + O2 Effect of Pt-metal content in Pt/TiO2 (P25) catalysts on CH4 yield for photocatalyticreduction of CO2 after 7 h UV irradiation at 323 K, H2O/CO2 = 0.02. Q.-H. Zhang et al. / Catalysis Today 148 (2009) 335–340
B – Non noble metal doping Capítulo 6 Copper, zincandChromium De Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44 • Photocatalystsynthesis: photodepositionbycontrollinglof precursor metalssolubility
B - Non-noble metals doping Capítulo 6 Copper, zincandChromium De Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44 • Photocatalyticdenitrification: • Photoreductionof NO3-toproduce N2 • Holescavanger: Formicacid (electrondonor) • Nitrate electronacceptor • Theoretical molar ratiotoreducenitratetonitrogenCHOOH:NO3-= 8:1 De Bem Luiz et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
B – Non-noble metal doped TiO2 NH4+main byproduct No N-byproducts Time, min Kineticsofphotocatalyticdegradationofnitrateandformicacid (measured as TOC), andformationofproducts (ammoniaandnitrite) 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 doping Capítulo 6 • Copper, zincorchromium: • Zn-TiO2: higherphotocatalyticactivitythan Cr-TiO2 or Cu-TiO2, andlowerbyproductsformation. • Zn action Topromoteefficient charge separation (e-/h+) Moreira et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
B – Doping with non noble metals • Effectofdissolvedoxygenonthephotocatalyticactivityof Zn-TiO2 • O2 competes with NO3-ions, acting as electronacceptor Photocatalytic nitrate reduction using 4.4% Zn–TiO2as photocatalyst Moreira et al., Journal of Photochemistry and Photobiology A: Chemistry 246 (2012) 36– 44
C) Coupling semiconductors • 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. 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.
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 CdSparticle Electron it is transferred to the CB of TiO2 particle. 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.
Hybridseminconductors– TiO2/graphene is promising to simultaneously possess excellent adsorptivity, transparency, conductivity, and controllability, which could facilitate effective photodegradation of pollutants. • 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 grapheneacts 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)
TiO2/Graphene Scheme of the Photocatalytic Degradation of methylene blue (a) TiO2 (b) TiO2/Graphene E. Lee et al. / JournalofHazardousMaterials219– 220 (2012) 13– 18 • 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. Kineticconstant for thephotocatalyticdegradationofRhodamineB KineticofphotocatalyticdegradationofRhodamineB Liangetal, Nano Res,2010. Huiminet al., ChineseJournalofCatalysis, 33 (2012) 777-782.
ZnFe2O4/Magneticgraphene • 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 Spinel ZnFe2O4 (Eg= 1.90 eV) Magnetic semiconductor material 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 Fu e Wang, IndEngChemRes 50 (2011) 7210-7218.
Lanthanidemodifiedsemiconductorphotocatalystss • The biggestdifferencebetweenthetransition metal ionandthelanthanideions natureofthe 4f orbitals • Lanthanide excellentopticalproperties • IncorporationofRare-Earths metal ions leads totheformationofmultienergylevelsbelowtheconductionbandedgeof TiO2 • Lanthanideionsmayact as electronscavengerandsuppress e/hrecombination; • Lanthaniteionsalsocanfaciliatetheadsorptionoforganicsoract as electronacceptors (minimizing e/hrecombination) Photocatalytic activity of Ln3+/TiO2 Weber, Grady and Kookdali, Cat Sci & Tech 2012, 2, 683. • 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
CeO2/TiO2 TOC removal efficiencies (Methylene blue) during visible light irradiation (t=180 min) (a) UV vis absorption spectra foundoped and Ce-doped TiO2 microspheres (b) Photographs of Ce-doped TiO2 samples Effectofcerium doping thephotocatalyticactivityto degrade methylene blue: From 1 – 5% cerium excess Ce4+ dopants may introduce the indirect recombination of electrons and holes to reduce the photocatalytic activity. J. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 107–114
CeO2/TiO2 Photocatalytic degradation of methylene blue – different catalysts and P25 Photocatalytic degradation of Rhodamine B– different catalysts and P25 J. Xie et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 107–114
Compósitos de TiO2dopados com Er3+:YAlO3/Fe- e Co • Fe andCoionsdopedinto TiO2powder torestraintherecombination • Er3+:YAlO3 upconversionluminescenceagent cantransformthevisible light into UV light more efficiently Degradationoforganiccompounds in thepresenceof Er3+:YAlO3/(Coor Fe)/TiO2 undervisible light • Visible light isconverted luz UV pelo Er3+:YAlO3. • UV light can excite TiO2-> electronstransferfrom VB to CB • e/hpairs no recombinationduetopresenceof Fe orCoions R. Xuet al. / Solar EnergyMaterials & Solar Cells 94 (2010) 1157–1165
TiO2 composites doped with Er3+:YAlO3/Fe- or Co 25% Er3+YAlO3/Fe/TiO2 25% Er3+YAlO3/Co/TiO2 10% Er3+YAlO3/Co/TiO2 5% Er3+YAlO3/Fe/TiO2 Fe/TiO2 Co/TiO2 Photocatalyticdegradationof azo fuchsineintthepresenceofphotocatalysts Fe or Co/TiO2anddifferentamounsof Er3+:YAlO3 R. Xuet al. / Solar EnergyMaterials & Solar Cells 94 (2010) 1157–1165
BismutumSpinels BiWO6, Bi4Ti3O12, BIOX (X=Cl, Br, I), Bi2O3 photocatalyticactivityunder UV andvisible light Eg = 2,9 a 3,5 eV, dependingonthepreparationmethod (Chen et al., 2012). Bi2S3 Eg= 1,3 a 1,7 eV(Mesquita e Silva, 34ª Reunião SBQ, 2011). * Bi2O2CO3 High activity: morphology, lowband gap energy. (Chen et al., 2012) * CdBiYO4 (Du and Juan, SolidStateSciences, 14 (2012) 1295-1305) spinel
Coppernanowires CuO Eg ~1.2 eV Nanowires CuO e Cu(OH)2 FESEM images of sample Nanowires of CuO Efficient charge separation and increase of photocatalytic activity UV absorption spectra of CuO nanowires Photocatalytic degradation of Rhodamine B using different photocatalysts under UV light Yu Li, Xiao-Yu Yang, Joanna Rooke, Guastaaf Van Tendeloo, Bao-Lian Su. Ultralong Cu(OH)2 and CuOnanowire bundles: PEG200-directed crystal growth for enhancedphotocatalytic performance, Journal of Colloid and Interface Science 348 (2010) 303–312
Tungstenium oxides WO3 + co-catalyst(Pt, Cu, or Pd): high photocatocalyticefficency to degrade organics WO3 --> Conduction Band ( +0.5 V vs NHE) is more positive than that for O2 reduction O2 + 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 WO3 Eg = 2,5 ev S. Nishimotoet al. / ChemicalPhysicsLetters 500 (2010) 86–89
PhotocatalyticdegradationofPhenol TOC initial = 130 ppm S. Nishimotoet al. / ChemicalPhysicsLetters 500 (2010) 86–89 PhotocatalyticdegradationofPhenol TOC initial = 130 ppm
E) Photocatalysts d0e d10Óxidosmetálicos Domen et al. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. 2007 • d10 • Ga3+: ZnGa2O4 • In3+: AInO2 (A=Li, Na) • Ge4+: Zn2GeO4 • Sn4+: Sr2SnO4 • Sb5+: NaSbO7 • 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) • Generally, the band gap energy is high Photocatalyticactivityof oxides andnitridesd10metals it isassociatedwiththe CB ofthehybridizedsp-orbitals, that are abletoproducephotoexcitedeletronswith high mobility.
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 materials • Graphene • Rare earths • Composites and doped co-catalyts • Reactor design is still a big challenge