Hydrogen Production from Semiconductor-

based Photocatalysis via Water Splitting

Pawan M Paramashetti

14CH60R29

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• Introduction/Alternative Energies and Hydrogen

• Hydrogen Production

• Hydrogen Production by Solar Energy

Thermochemical water splitting

Photobiological water splitting

Photocatalytic water splitting

• Titania (TiO2)

• Improvement of the Photoactivity of TiO2

• High-Efficient Photocatalytic System for Water Splitting

• Types of Photocatalytic Water-Splitting Reaction

• Conclution and Future Prospects of Photocatalytic Water Splitting

• Refernces

Outline

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Introduction/Alternative Energies and Hydrogen

--most of the energy that we use comes from fossil fuels,

--Problem with fossil fuel:

Production of greenhouse gases

The amount of fossil fuel on the Earth is limited

To replace or reduce the use of fossil fuels, several alternative energies have been developed.

These energy sources include

--wind.

--hydropower.

--solar.

--geothermal.

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• Hydrogen is the ideal fuel for the future.

• it is clean, energy efficient, and abundant in nature.

• While various technologies can be used to generate hydrogen, only some of them

can be considered environmentally friendly.

• solar hydrogen generated via photocatalytic water splitting

low-cost

clean hydrogen production.

Continued..

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Hydrogen Production

Steam Methane Reforming

Coal Gasification

Partial Oxidation of Hydrocarbons

Biomass Gasification

Biomass Pyrolysis

Electrolysis

Thermochemical

Photochemical

Photobiological

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Current global hydrogen production

48% from natural gas

30% from oil

18% from coal

4% from electrolysis of water

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Hydrogen Production by Solar Energy

Hydrogen production via solar water splitting generally can be categorized into 3 types:

(1) thermochemical water splitting;

(2) photobiological water splitting,

- water biophotolysis

- organic biophotolysis

(3) photocatalytic water splitting.

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Photocatalytic Water Splitting

Photocatalytic water splitting is another promising technology to produce “clean” hydrogen.

Compared with thermochemical and photobiological water-splitting techniques, it has the

following advantages:

(1) reasonable solar-to-hydrogen efficiency;

(2) low process cost;

(3) small reactor systems suitable for household applications, thus providing for a huge

market potential.

Mechanism of photocatalytic water splitting

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In a photocatalytic water splitting reaction, photocatalyst plays a crucial role.

Until now, titania (TiO2) has been a widely used photocatalyst for photocatalytic water splitting

Titania is

-stable.

-non-corrosive.

-environmentally friendly.

-abundant, and cost-effective.

-More importantly, its energy levels are appropriate to initiate the water-splitting reaction

Titania (TiO2)

Still It is difficult to achieve water splitting for hydrogen production using TiO2 photocatalyst

in pure water.

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Photocatalytic water-splitting efficiency under solar energy is still quite low,

mainly due to the following reasons:

1. The photo-generated electrons in the CB of TiO2 may recombine with the VB holes

quickly to release energy in the form of unproductive heat or photons.

2.The decomposition of water into hydrogen and oxygen is a chemical reaction with large

positive Gibbs free energy (ΔG = 237 kJ/mol).

3.The bandgap of TiO2 is about 3.2 eV, and therefore, only UV light can be utilized to

activate the photocatalyst.

What are the problems with TiO2?

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How to Improve the Photoactivity of TiO2

The techniques that have been investigated in the past include the

• addition of sacrificial agent/carbonate salts,

• metal loading,

• dye sensitization,

• ion (cation, and anion) doping.

Some of experimental have been proven to be useful for improving the

photocatalytic activity of TiO2.

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Addition of sacrificial agent/carbonate salts

--rapid recombination of photo-generated CB electrons and VB holes.

--Adding electron donors or sacrificial reagents to react with the photo-generated VB.

--organic pollutants acting as electron donors, such as oxalic acid, formic acid, and formaldehyde

--the addition of carbonate salts was found to improve photocatalytic process

However, the drawback of this technique is the need to continuously add electron donors in order to

sustain the reaction since they will be consumed during photocatalytic reaction.

Metal loading

loading of metals that act as co-catalysts on the surface of photocatalyst.

Loading of metals, including Pt, Au, Pd, Rh, Ni, Cu, and Ag, have been reported to be very

effective for improving TiO2’s activity in photocatalysis.

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Dye sensitization

--is a widely used technique to utilize visible light for energy conversion.

--Some dyes having redox property and visible-light sensitivity can be used in solar cells as

well as photocatalytic systems.

--the excited dyes can inject electrons to the CB of photocatalyst

--In order to regenerate dyes, redox systems or sacrificial agents, such as I3−/I− pair and

EDTA, can be added to the solution.

Ion (cation, and anion) doping

small percentage of metal ion(s) is incorporated into the crystal lattice of photocatalyst.

Transitional metal ion doping and rare-earth metal ion doping have been extensively used.

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High-Efficient Photocatalytic System for Water Splitting

• Most of the high-efficiency photocatalysts synthesized for H2 production via

photocatalytic water splitting are composed of two or more components that are more

complicated than TiO2.

• An example is the NiO-SrTiO3 photocatalyst prepared by Domen and his group.

• Nickel oxide (NiO) as a co-catalyst was first loaded on the surface of SrTiO3, which

then underwent reduction and oxidation by hydrogen and oxygen, respectively, to

form a core (Ni)-shell (NiO) structure.

• The co-catalyst with a core-shell structure is believed to facilitate the transport of

electrons toward the surface of the photocatalyst, hence improving the photoactivity.

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Types of Photocatalytic Water-Splitting Reaction

(1) Photochemical-cell reaction.

(2) Photoelectrochemical-cell reaction.

The mechanism basically involves 4 major steps:

(1)generation of electron-hole pairs.

(2)oxidation of water by photo-generated holes

(3)transfer of photo-generated electrons

(4) reduction of H+ by photo-generated electrons.

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--Pt/SrTiO3:Rh and WO3, used as H2-photocatalyst and O2-photocatalyst, respectively

--ion-exchange membrane allows the transport of protons, As well as the exchange of the

mediator ions (Fe2+/Fe3+) in solution

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Conclusions and Future Prospects of Photocatalytic Water Splitting

Over the past few decades, several semiconductor materials and photocatalytic systems have

been developed for the water-splitting reaction under UV and visible-light irradiation.

It has been observed that photo-generated charge separation, prevention of water-splitting

backward reaction

utilization of a large fraction of the incident energy are the essential requirements for

achieving high photo-conversion efficiency.

The development of new technologies requires collaboration of many streams with a strong

theoretical background for a better understanding of the hydrogen production mechanism in

order to come up with a low-cost and environmentally friendly water-splitting process for

hydrogen production.

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References

Chi-Hung Liao, Chao-Wei Huang, and Jeffrey C. S. Wu. Hydrogen Production from

Semiconductor-based Photocatalysis via Water Splitting. Catalysts 2012, 2, 490-516.

Midilli, A.; Ay, M.; Dincer, I.; Rosen, M.A. On hydrogen and hydrogen energy strategies I:

Current status and needs. Renew. Sustain. Energy Rev. 2005, 9, 255–271.

Parida, B.; Iniyan, S.; Goic, R. A review of solar photovoltaic technologies. Renew. Sustain.

Energy Rev. 2011, 15, 1625–1636.

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Thank you for your kind attention…