NOVEL PHOTOCATALYSTS FOR HYDROGEN GENERATION
Prof. Hermenegildo Garcia Instituto de Tecnologia Quimica
Universidad Politecnica de Valencia
46022 Valencia
E‐mail:[email protected]
OUTLINE: General remarks about hydrogen technology
Future sources of hydrogen
Fundamentals of photocatalysis
Examples of novel photocatalysts
Conclusions
Should we move out from fossil fuels?
Limited resources of fossil fuels
Pollution and climate change
Increasing energy demands
Major atmospheric pollutants_
Greenhouse gases: CO2
NOx and SOx: acid rain
Hydrocarbons
Besides direct pollution from fuels
N2 + O2
2 NO
Temperature above 1700 oC
Photochemical smog
2011
World Energy Demand
2011
2011
Hydrogen is the final solution!
H2O→2H2+O2
∆V=1.23V, ∆ G=238kJ/mol
 Combustion of hydrogen gives about three times more energy per mass than the combustion of hydrocarbons  Clean – no greenhouse gases (CO2, NOx, SOx, etc)
 Energy security – can be produced from abundant sources
 Keeping pace with economic growth
 Efficient – fuel cells ~75% efficiency
 Ideal transportation fuel: Car tanks, micro fuel cells…
2011
Hydrogen Technology
Production Costs
Centralized vs.distributed
Current Hydrogen production
Extra cost 29.2 $/GJ
Total cost allowed 60 $/GJ
Coal
Natural Gas
Oil/naphtha
2011
Hydrogen from Natural Gas
•
•
Steam Reforming – convert methane in syn gas using steam
Water Gas Shift – produces extra hydrogen amount
• Steam reforming of natural gas or syngas sometimes referred to as steam methane reforming (SMR) is the most common method of producing commercial bulk hydrogen. At high temperatures (700 – 1100 °C) and in the presence of a metal‐based catalyst (nickel), steam reacts with methane to yield carbon monoxide and hydrogen. These two reactions are reversible in nature.
• The efficiency of the process is approximately 65% to 75%.
• CO2 must be sequestered.
• CH4 + H2O → CO + 3 H2
• CO + H2O → CO2 + H2
Bayswater Power Station (New South Wales, Australia)
2011
What is missing in hydrogen technology
• Production of hydrogen from renewable energy
resources
– It has to be produced from Sunlight
– Production has to be efficient and competitive
• Hydrogen storage
– Hydrogen boiling point is too low that has to be stored as compressed gas (normal conditions: 22.4 L only 2 g) – 100 atms: 224 mL for 2 g
• Efficient and cost effective fuel cells
– High efficiency
– Low temperature operation
– Affordable components (no Pt or Ru)
2011
Hydrogen production from water using
renewable energy
2011
Electrolysis of water
• Electrolysis ‐ split water with electricity
• 2H2O(l) → 2H2(g) + O2(g) E0 = ‐
1.229 V • Needs catalyst to lower overpotential
2011
Thermolysis of water
Direct Solar Thermal Water Splitting – split water with heat
Use multiple reflection devices and concentrate sunlight.
Water breaks down at 1700 C.
Need ZrO containers at high temperature.
No sunlight, no energy production.
Thermochemical, Hydrosol II plant, Almeria, Spain
2011
Natural water splitting: Photosynthesis
There is a large interest in understanding and mimicking natural photosynthesis, BUT
Artificial systems must be: more robust
less complex
focused on hydrogen
2011
Photoelectrochemical
• Honda‐
Fujishima Effect
– Water splitting by TiO2 by UV light
– Theoretical efficiency of 10% by using expensive materials
Honda–Fujishima effect‐water splitting using a TiO2 photoelectrode
Fujishima & Honda, Nat., 1972, 238, 37.
"Energy & Nano" ‐ Top Master Symposium in Nanoscience
17 June 2009
12
2011
Photocatalysts are challenging
materials
Common issues with catalysts:
• Large surface area, porosity
• well defined single sites: for hydrogen evolution
Specific issues:
•Light absorption: solar light and visible light
•Efficiency: mobility and lifetime of charge separation
•Efficient photoreactors
•Photocatalyst durability and costs
2011
Solar Energy Distribution
2011
Estimation costs of photocatalytic
hydrogen production
• Sun Power 1000 W m‐2 • Harvesting a reduced portion of sunlight 50 W m‐2 (5 % of total solar light)
• 2 photons per hydrogen molecule
• Hydrogen molecules 1.8 1023 h‐1 m‐2
• Overall photocatalytic efficiency 10 % 1.8 1022 H2
molecules h‐1 m‐2
• Mols of H2 3 10‐3 mol H2 h‐1 m‐2
• One day 10 light hours: 0.03 mol H2 m‐2
• For one year: 10.95 mol H2 m‐2
• Photocatalyst cost (one year durability) 50 € Kg‐1
• Cost per H2 (1 g per m2) 0.01 € mol H2
2011
Semiconductor as photocatalyst
O2∙‐
CB
ROS (HOO∙, H2O2)
reduction
Photocatalysis for water splitting
O2
h
H2O
VB
oxidation
∙OH + H+
2011
Developing durable visible light photocatalyst based on TiO2
O2∙‐
reduction
CB
h
oxidation
O2
APPROACHES
a) Doping with metals: Pt, Fe, Cr
b) Doping with non metals: N, C, S
Problem of reproducibility, homogeneity
and corrosion
H2O
VB
USE OF GOLD NANOPARTICLES
Easy to be prepared
No photocorrosion
Stable photocatalysts
Surface Plasmon band
∙OH + H+
0 D
1 D
2 D
2011
Visible light photocatalytic activity of Au/TiO2
UV light:
Visible light:
h
532 nm)
18
Au/TiO2 as Visible Light Photocatalyst
•Deposition/precitipation on Degussa P25
•Loading between 0.25 to 2 wt%
•This material is an excellent catalyst for aerobic oxidations and hydrogenations
(b)
Number of particles
40
30
20
10
0
2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10
dp (nm)
100 nm
TiO2 Dyesol
TiO2 + Au 2% + MeOH
TiO2 + Au 2%
photocurrent (a.u.)
1.0
0.8
0.6
0.4
0.2
200
300
400
500
Wavelength (nm)
Hydrogen evolution
 7.5 % at 560 nm
2011
Visible light Hydrogen Generation
Au/TiO2 (Au 1.5%, Sued Chemie) 2.00 g/L; TiO2 (Aeroxide P25)
EDTA (Aldrich) 0.02 M
Laser  532 nm 200 W Xe-doped Hg Lamp (Hamamatsu Lightningcure LC8)
with cutoff filter for VIS irradiation (> 420 nm)
Au/TiO2_Laser_532 nm
Au/TiO2_200W_filter_400 nm
TiO2_P25_Laser_532 nm
14
TiO2_P25_200W_filter_400 nm
12
evolved H2 (mol)
10
8
6
4
2
0
0
30
60
90
120
Time (min)
150
180
NO H2 WITH TiO2!
210
20
Photocatalysts for Hydrogen generation: Metal Organic Frameworks
• Metal Organic Frameworks: More recent type of crystalline
porous materials that are made up by a 3D extended network
of metal ions or clusters connected through multidentate
organic spacers
Isoreticular with zeolites
•Large surface area
•Large pore volume
•Lowest framework density
•Functionality
Metal ion or Cluster
Organic Linkers
2011
Metal Organic Frameworks as Photocatalysts
2011
MOF‐5 as electron donor and acceptor
upon photoexcitation
h
MOF-5
+
H3C N
MOF-5
+
N CH3
MOF-5
Me
Me
N
Me
N
MOF-5
Me
+
H3C N
N CH3
(blue)
Me
N
Me
Me
N
Me
(blue)
2011
Metal Organic Frameworks as Photocatalysts
UiO‐66: stable in boiling water
Influence of amino groups
2011
Graphene Oxide as Photocatalyst for
Hydrogen Generation
•Graphene is the most conductive material
•In contrast, depending on the degree of oxidation, graphene
oxides are semiconductors
•Graphene oxides can act as photocatalyst for hydrogen generation
•Optimization of photocatalytic properties
•Hybrid graphene oxide/inorganic oxide photocatalyst
Layered Double Hydroxides as Visible‐Light Photocatalysts
MII
MII
MIII/IV
A-
MIII/IV
A-
MII
Ti/Zn‐LDH
Ce/Zn‐LDH
Cr/Zn‐LDH
2011
Oxygen evolution using Cr/Zn LDH
2011
CONCLUSIONS
•There is an urgent need of developing alternative energy
resources
• Hydrogen is considered to be the perfect fuel for
transportation
•There is a real potential market for photocatalytic cost effective
hydrogen production.
• Target: 10 % overall efficiency using solar light without
sacrificial agents
References
Feature Article: Aprile, C., Corma, A., & Garcia, H., Enhancement of the photocatalytic activity of TiO2 through spatial structuring and particle size control: from subnanometric to submillimetric length scale. Physical Chemistry Chemical Physics 10 (6), 769‐783 (2008).
TiO2 Clusters:
•
Alvaro, M., Carbonell, E., Fornes, V., & Garcia, H., Enhanced photocatalytic activity of zeolite‐encapsulated TiO2 clusters by complexation with organic additives and N‐doping. ChemPhysChem 7 (1), 200‐205 (2006).
•
Atienzar, P., Valencia, S., Corma, A., & Garcia, H., Titanium‐containing zeolites and microporous molecular sieves as photovoltaic solar cells. Chemphyschem 8 (7), 1115‐1119 (2007). Chemical Warfare Agents:
•
Cojocaru, B. et al., Sensitizers on inorganic carriers for decomposition of the chemical warfare agent yperite. Environmental Science & Technology 42 (13), 4908‐4913 (2008).
Structured TiO2:
•
Alvaro, M., Aprile, C., Benitez, M., Carbonell, E., & Garcia, H., Photocatalytic Activity of Structured Mesoporous TiO2 Materials. J. Phys. Chem. B 110 (13), 6661‐6665 (2006). •
Maldotti, A., Molinari, A., Amadelli, R., Carbonell, E., & Garcia, H., Photocatalytic activity of MCM‐organized TiO2 materials in the oxygenation of cyclohexane with molecular oxygen. Photochemical & Photobiological Sciences 7 (7), 819‐825 (2008).
Bidimensional TiO2: •
Aprile, C., Teruel, L., Alvaro, M., & Garcia, H., Structured Mesoporous Tin Oxide with Electrical Conductivity. Application in Electroluminescence. J. Am. Chem. Soc. 131 (4), 1342‐1343 (2009).
•
C. Gomez, Y. Bousen, V. Fornes, H. García, Layered double hydroxides as highly efficient photocatalysts for visible light oxygen generation from water, J. Am. Chem. Soc. (2009) asap.
Photonic crystals
•
Carbonell, E. et al., Enhancement of TiO2 photocatalytic activity by structuring the photocatalyst film as photonic sponge. Photochemical & Photobiological Sciences 7 (8), 931‐935 (2008).
•
Ramiro‐Manzano, F. et al., Apollony photonic sponge based photoelectrochemical solar cells. Chem. Commun. (3), 242‐244 (2007).
Acknowledgements
Dr. Cláudia Gomes and Tiziana Marino Financial support: MICINN (CTQ2009‐11568)
Abengoa
Antecys
2011