Zn- and La-modified TiO2 photocatalysts for the isomerization of norbornadiene to quadricyclane

Zn-doped and La/Zn co-doped TiO₂ nanoparticles were prepared by sol–gel method and utilized as the photocatalysts for the isomerization of norbornadiene to quadricyclane that has significant potential for solar energy storage and high-energy fuel synthesis. For Zn-doped samples, Zn ions do not enter the TiO₂ lattice, but distribute on the particle surface in the form of ZnO crystallites. These crystallites inhibit the agglomeration, growth and anatase-to-rutile phase transformation of TiO₂. The prepared particles contain considerable amount of surface-bound OHs, especially for 1%Zn/TiO₂. A red shift in the optical absorption is observed due to the electron transfer between TiO₂ and ZnO. In the photocatalytic isomerization reaction, Zn-doped TiO₂ exhibits higher activity than homogenous sensitizer like Ethyl Michler's Keton, and 1%Zn/TiO₂ produces the highest yield of quadricyclane. To further enhance the activity, 1%Zn/TiO₂ was co-doped with La. La₂O₃ crystallites also distribute on the surface of TiO₂, similar to the case of ZnO. The particle size is reduced to <7 nm but the surface-bound OHs are decreased to some degree. There is a significant blue shift in the optical absorption with a sharply increased absorbance in the UV region due to the quantum-size effect. 5%La–1%Zn/TiO₂ and 3%La–1%Zn/TiO₂ exhibit higher activity compared with 1%Zn/TiO₂, but higher or lower content of La is detrimental to the reaction. It is concluded that doping Zn can significantly increase the surface-bound OHs, whereas doping La reduces the particle to quantum-size at the expense of surface-bound OHs. A good compromise between the two factors eventually provides a high activity. The isomerization reaction over semiconductors is proposed to proceed through an exciplex (charge-transfer) intermediate.