Photolysis of Water

Prepared By : Yaseen Aziz

Roll No : 23609 Dept. Of Chemistry, Kashmir University

Water splitting-a general introduction :

• Water splitting is the general term for a chemical reaction in which water is separated into oxygen and hydrogen.

• This can be achieved through following ways:

a) Electrolysis

b) Thermal decomposition of water

c) Photobiological water splitting

d) Photoelectrochemical water splitting

e) Photocatalytic water splitting

• Need for photosplitting of water:

Hydrogen economy Hydrogen-a clean, pure fuel with very high calorific value (150 kJ/mol).

• “We are at the peak of the oil age but the beginning of the hydrogen age. Anything else is an interim solution. The transition will be very messy, and will take many technological paths .....but the future will be hydrogen fuel cells.”- Herman Kuipers

Redox stability field of water:

• Photosplitting of water:

Three methods:a) Photocatalytic splitting of waterb) Photoelectrochemical splitting of water andc) Photobiological splitting of water

Photocatalytic splitting of water:• A single step reaction assisted by photocatalysts suspended

directly in water, using sunlight to produce H and O2.

• Photocatalysts mainly used are Metal oxide semiconductors e.g. Sr2M2O7 (M = Nb and Ta),  MGa2O4(M = Mg, Sr, Ba) etc.

Basic principle of overall water splitting on a semiconductor particle.

Processes involved in photocatalytic overall water splitting on a semiconductor particle.

Basic principle of photocatalytic reactions in the presence of sacrificial reagents.

The band gap in Metal oxide plays a crucial role in using them as photocatalysts. This can be illustrated by the following diagram:

• Photoelectrochemical splitting of water:

• Water is broken down into H2 and O2 by electrolysis, but the electrical energy is obtained by a photoelectrochemical cell (PEC) process, based on semiconductors.

• This is also named as ‘artificial photosynthesis.’

• The photoelectrochemical (PEC) path to water splitting involves separating the oxidation and reduction processes into half-cell reactions.

• Three methods are possible: PV approach i.e. using photovoltaic cells SCLJ approach i.e. semiconductor-liquid junctions Combination of the two (PV/SCLJ approach)

Schematic representation of a PV approach

Schematic representation of a SCLJ approach

Schematic representation of a PV/SCLJ approach

Photobiological water splitting:• Produces H2 called as biological hydrogen, in algae bioreactors.

• Discovered in 1990’s

• When algae are deprived of sulphur they produce H2 instead of O2 i.e. normal photosynthesis is inhibited.

• Efficiency is 7-10 %, producing 10-12 ml H2 per culture litre per hour.

Conclusion:• Hydrogen as fuel is a promising substitute to the non-renewable

sources of energy.

• It is clean, pure and eco-friendly fuel.

• It can be obtained in large amounts provided a viable procedure of water splitting is made available and the problem of H2 storage is overcomed.

• Photosplitting of water is a pure, clean, less energy consuming and easily available source of H2 production.

• The research field is wide open and challenging.

THANKS A LOT for

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