Adsorption of Energy in Photocatalytic Reactors

Abstract

The dissociation of water to hydrogen and oxygen requires energy ∆G₂₉₈ = 228 kJ/mole. By irradiating a semiconductor with light of energy greater than this amount, one may product electrons in the excited state and electron vacancies at the surface that can perform the photochemical reduction (2H⁺ + 2e⁻ → 2H → H₂) and oxidation (2OH⁻ + 2+ → H₂O₂→ H₂O + (1/2)O₂).

There are several semiconductors, SrTiO₃, TiO₂, CdS, and Fe₂O₃ among them, that can photodissociate water. Some possess sites for both reduction and oxidation, while others carry out the two processes at different surfaces. A reversible solid state reaction that involves changes in the transition metal and ion oxidation state must accompany the splitting of water. Platinum, rhodium, and ruthenium oxide, when deposited on the semiconductor, serve as catalysts that accelarate the water photodissociation. These additives accelerate the recombination of hydrogen and oxygen atoms, shift the semiconductor Fermi level to a more favorable position that improves the thermodynamic feasibility for the process, accelerate electron transport, and inhibit side reactions like the photoreduction of oxygen. Many of the elementary reaction steps leading to photoproduction of hydrogen and oxygen over SrTiO₃ and Fe₂O₃ have been identified and will be discussed.