vis-active n-p-n photocatalytic structures · for wastewater treatment 1. efficient in the complete...
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Vis-active n-p-n photocatalytic structures
IC-ANMBES, 13-15 June, Brasov, Romania
A.Duta, A. Enesca, C. Bogatu Transilvania University of Brasov, Romania R&D Center: Renewable Energy Systems and Recycling
6th International Workshop
"Advanced optical and X-ray characterization techniques of multifunctional materials for information and communication technologies, health and renewable energy applications"
Diaspora in Cercetarea Stiintifica si Invatamantul Superior din Romania, Timisoara, 25-28.04.2016
Outline
Problem 1: the water stress
Problem 2: barriers in scaling up AOPs
Material (VIS-active photocatalysts)
Process (heterogeneous photocatalysis)
Integration (material – process – equipment)
Conclusions
Transilvania University of Brasov www.unitbv.ro 19000 students
18 faculties (9 technical and 9 with non-technical profile), Study Programs: B.Sc. 107; M.Sc. 70, Ph.D. 17 fields
28 departments
29 R&I Centres In the Research Institute of the Transilvania University
(ICDT)
The R&I Centre Renewable Energy Systems and Recycling
Sustainable Built
Environment
Renewable Energy
Systems
Hybrid systems
Materials for environment and energy
Wastes recycling and
reuse
Sustainable Community
Education and Training
Research and Innovation
Awareness (stakeholders,
community)
The water stress
• Sustainable built environment
• Sustainable water use
• Wastes management
Water and wastewater – sustainable use Fresh water -
sanitation Wastewater –
treatment for recycling, reuse
Using the sludge for energy production
Sanitation
Water recycling Water reuse
Sustainable (waste)water treatment
Bio-treatment Bioaccumulation
(using energy plants)
Membrane processes (UF, RO)
- mature technology - affordable - long duration
- highly efficient - allow water reuse - high cost
Solar driven advanced oxidation
processes (AOP)
- highly efficient - optimized for water reuse - high cost
IUPAC definition: „Change in the rate of a chemical reaction or its initiation: - Under the action of ultraviolet, visible, or infrared radiation - In the presence of a photocatalyst, that absorbs light and is involved in the chemical transformation of the reaction partners.” Silvia E. Braslavskyet al., Pure Appl. Chem. , Vol. 83, No. 4, pp. 931–1014, 2011.
Homogeneous photocatalysis:
UV/O2; UV/H2O2/O2; Fe2+/3+ + UV (Photo-Fenton Systems)
Large pilot plants: tertiary treatment, dissinfection
(Almeria, Valencia)
Sludge + potential polluting by-products
Photocatalysis for wastewater treatment
Almeria, Spain Solar water treatment plant
Heterogeneous photocatalysis („semiconductor-assisted photoreactions”)
UV, VIS, Semiconductor/H2O2 Small laboratory installations
Heterogeneous photocatalysis for wastewater treatment
1. Efficient in the complete removal (mineralization) of pollutants at low concentrations
2. Efficient in mineralizing recalcitrant pollutants: industrial organic by-products, natural matter, micro-organisms, innorganic pollutants
3. No solid and liquid by-products (no sludge)
Advantages
Future prospects: WATER RE-USE
Heterogeneous photocatalysis for wastewater treatment
Barriers to scaling up
The cost
Photocatalyst Photocatalytic
process
Composition Opto-electric
properties Long term stability
Radiation: UV, Vis, solar
Process conditions:
pH, H2O2, …
The installation The photoreactor
Processes based on UV activated TiO2 are about four times more expensive as RO Al-Bastaki, 2004, Chem. Eng. Proces., 43, 935-940
Efficiency
Wide band gap semiconductors: TiO2, ZnO, WO3, SnO2, ZrO2
1. Inert in the working environment - Water stable - Stable over a broad pH range - No leaching
2. Active over long operation periods 3. Easy recovareable 4. Avoiding critical materials 5. Low cost
1. High photoactivity - Reduced recombination - Wide band gap
2. High VIS activity
- Lower band gap
+
The Photocatalyst
The wish-list
Narrow band gap semiconductors: CuxS, CuInS2
Wide band gap semiconductors: TiO2, SnO2 Thin films
Vis- active thin photocatalytic films - Anion doping - Cation doping (?) - Loading with metals (Ag, Pt) Schottky diode?
The possible solutions
State of the art : Enhance the photo-response of wide band gap semiconductors. … and beyond: Associating semiconductors into VIS-active systems
- Diode type systems: n-p (n-p-n-...) semiconductor composites - Tandem systems: n-n semiconductor composites
The Photocatalytic Process
Eg, rutile = 3.02 eV (λg < 413 nm) CB = -0.6 eV VB = +2.4 eV Eg, anatase = 3.2 eV CB = - 0.4 eV VB = + 2.8 eV (λg < 387 nm) Eg, brookite = 3.54 eV (λg < 357 nm)
E hc/λg
Parallel process: recombination h+ + e-
Solution 1: unbalanced consumption of the charge carriers
H+ and HO- concentrations pH
λ < 254 nm
The Photocatalytic Process
Eg, rutile = 3.03 eV (λg < 413 nm) CB = -0.62 eV VB = +2.41 eV Eg, anatase = 3.2 eV CB = - 0.4 eV VB = + 2.8 eV (λg < 387 nm)
E hc/λg
Parallel process: recombination h+ + e-
Solution 2: insure fast charge separation (in situ)
e.g. Degussa P25 (Evonik, 71% anatase, 27% rutile, 2% amorphous)
e-
h+
Addapted from Scanlon D.O., et al., Nature Materials, 2013, 12, 798-801
2.81 eV Efficient charge flow
Lower effective Eg (2.81 eV)
n-p composite photocatalytic systems
Fig. 10. Energy levels diagram for the tandem structures (inset: Eg values and EDX
spectra).
Fig. 10. Energy levels diagram for the tandem structures (inset: Eg values and EDX
spectra).
CuInS2 – SnO2 CuInS2 – TiO2 – SnO2 (solid state solar cell: FTO/TiO2/CIS) Nanu et al., Thin solid films, 492-496, 2003
Eg = 1.45 Eg = 1.22
n-p composite photocatalytic systems
CuInS2 – SnO2 CuInS2 – TiO2 – SnO2 Mineralization: Pollutant: MB 0.0125mM Radiation: 10 W/m2 15% UV (λ= 365nm) + 85%Vis (λ= 565nm)
Stability, 3 consecutive working cycles
Stability, 3 consecutive working-rinsing cycles
Enesca A et al., , Appl. Cat. B, 69-76, 2016
n-p composite photocatalytic systems
CuInS2 – SnO2 CuInS2 – TiO2 – SnO2
Poluant: MB 0.0125mM Effect of Radiation Radiation source: 1Vis: 18W 1UV: 18W
Radiation sources 2Vis+ 1UV
4Vis+ 2UV
5Vis+ 2UV
4Vis+ 3UV
2Vis 4UV
7Vis
Irradiance [W/m2] 10 20 23 21 9 9 32
Bleaching efficiency [%] 93,5 94,8 96,4 94,2 80,2 81,5 93,6
Stability (T%)
81% 34%
Composite photocatalysts CuxS – SnO2 ZnO – CuxS(CuxO) – SnO2 TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Material design
A. Enesca et al., Appl. Cat. B., 2015, 162, 352 - 363
Sustainable n-p - composite photocatalytic systems Replacing In-based compounds
Eg: 1.47 eV 1.19 eV 1.19 eV
Sustainable n-p - composite photocatalytic systems
Pollutant: MB 0.0125mM (4ppm) Radiation (10 W/m2): 15% UV + 85%Vis
Efficiency: 77% Efficiency: 66%
pH = 6.8
Composite photocatalysts CuxS – SnO2 ZnO – CuxS(CuxO) – SnO2 TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design
Composite photocatalysts TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design: trapping the charge carriers
Pollutant: MB 0.0125mM (4ppm) Radiation: 15% UV + 85%Vis
H+ and HO- concentrations pH
Sustainable n-p - composite photocatalytic systems
Composite photocatalysts TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design: - Trapping the charge
carriers - Increasing the
efficiency
Acidic and neutral pH
pzc, TiO2 = 6.2
Thiazine dye, MB
Colourless in the absence of oxygen
Side effects: photo-corrosion
Sustainable n-p - composite photocatalytic systems
Composite photocatalysts TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design: - Trapping the charge
carriers - Increasing the
efficiency
Efficiency: 75%
Acidic and neutral pH
pzc, TiO2 = 6.2
Efficiency: 60%
Efficiency: 70%
Sustainable n-p - composite photocatalytic systems
Composite photocatalysts TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design: - Trapping the charge
carriers - Increasing the
efficiency
Alkaline pH
pzc, TiO2 = 6.2
TiO2(h+) +HO - → HO•
TiO2 -O - + MB(+) → TiO2 - O - MB
E0 = +2.53V
Sustainable n-p - composite photocatalytic systems
Composite photocatalysts TiO2 – CuxS(CuO) – SnO2 (n-type – p-type – n-type, Elecrolyte: the wastewater and the active ionic species) Process design: - Trapping the charge
carriers - Increasing the
efficiency
Alkaline pH
pzc, TiO2 = 6.2 Initial
Efficiency: 88% Efficiency: 65%
Sustainable n-p - composite photocatalytic systems
Incident solar radiation
Transmitted solar radiation
Wavelength [nm]
Wavelength [nm] Wavelength [nm] E
ffic
ien
cy [
%]
Eff
icie
ncy
[%
]
Eff
icie
ncy
[%
]
Efficiency [%]
Key component: The photoreactor Thickness Light penetration in water
Dimensions Irradiation duration
Flow Thin film stability Process efficiency Stagnation Inhomogeneous flow
List of pre-requisites - Use solar radiation - Allow pollutants mineralization - Able to treat large amounts of water - Technically feasible - Economic affordable - Minimal changes in the WWT plant
Sustainable n-p - composite photocatalytic systems The photreactor
v=0.01m/s v=0.005m/s
v=0.0025m/s v=0.001m/s
v=0.0025m/s v=0.0001m/s
Key component: The photoreactor Thickness Light penetration in water
Dimensions Irradiation duration
Flow Thin film stability Process efficiency Stagnation Inhomogeneous flow
Sustainable n-p - composite photocatalytic systems The photreactor and the thin film additional prerequisites
Conclusions
1. Heterogeneous photocatalysis has a broad range of applications (WWT,
atmospheric decontamination, surfaces decontamination, etc.)
2. Upscaling photocatalysis in WWT requires market acceptance.
3. Key-barriers need to be jointly approached :
The materials fundamentals: design and development of VIS-active
photocatalytic systems, mimicking SSSC
Photocatalytic process design and optimization
Feasible equipment
4. Interdisciplinary projects can support complex and complete results
Acknowledgements The structural founds project PRO-DD (POS-CCE, O.2.2.1., ID 123, SMIS 2637, No 11/2009) for providing the infrastructure used in this work The PNII-Cooperation project NANOVISMAT, contract no. 162/2012 financed by UEFISCDI which supported the latest research hereby presented.