Parameters affecting electron transfer dynamics from semiconductors to molecular catalysts for the photochemical reduction of protons

Poster, Anna Reynal, 009
Publication date: 31st March 2013

The photochemical reduction of water into H₂ offers the possibility of harvesting sunlight and storing this energy in the form of chemical bonds. Most of such approaches combine a photoactive element to harvest solar energy with a catalytic component to catalyze the two electron reduction of protons to molecular hydrogen.^[1]

In this conference, we aim to present the kinetics of electron transfer processes that take place in three different molecule-functionalized systems for the photochemical H₂ evolution, with special emphasis on the parameters influencing the efficiency of the electron transfer from the TiO₂ to a molecular catalyst.^[2,3] All three approaches employ nanocrystalline TiO₂ films functionalized by a molecular cobalt electrocatalyst (CoP) for the production of hydrogen in aqueous solution buffered at pH 7 under N₂. The first approach is based upon these materials alone, excluding any sacrificial electron donor, and therefore relying upon the ability of TiO₂ holes to oxidize water to molecular oxygen as the source of electrons for proton reduction. In the second approach, a sacrificial electron donor (triethanolamine) is used as a hole scavenger to remove photogenerated TiO₂ holes. Finally, the third system involves the functionalisation of the TiO₂ nanoparticles with a molecular ruthenium dye (RuP) and a cobalt catalyst anchored onto the surface, again using TEOA to regenerate the photosensitizer.

Our transient absorption spectroscopy (TAS) measurements indicate that, in the absence of a hole scavenger, the use of high excitation light densities increase the electron/hole recombination rate and decrease the electron transfer efficiency to the molecular catalyst. Nevertheless, removing the photoholes from the semiconductor by adding a sacrificial agent or a photosensitizer can attenuate this recombination reaction. The catalyst loading also plays a crucial role in the H₂ evolution efficiency, since the electron transfer from the semiconductor to the catalyst shows a non-linear dependence upon surface coverage. We have assigned this behavior to the two-electron transfer reactions associated with H₂ evolution and their thermodynamic properties. We believe that the control of electron transfer kinetics to reduce the molecular catalyst is specially important in the design of systems with two co-attached molecules such as a molecular catalyst and a photosensitizer.

Scheme 1. Illustration of the three experimental systems studied herein, and the functional processes underlying their function comprising (a) a TiO2 functionalized by a CoP proton reduction catalyst in water under UV excitation, with, (b), the addition of a sacrificial electron donor, and (c) the addition of a photosensitizer dye to enable visible light activity. The two reduction potentials of the catalyst (CoIII/II and CoII/I) relevant for the H2 evolution reaction are shown. Forward electron transfer processes are represented with solid black arrows, while re-combination reactions are drawn with dashed grey arrows.
[1] Tran, P. D.; Wong, L. H.; Barber, J.; Loo, J. S. C. Recent advances in hybrid photocatalysts for solar fuel production. Energy Env. Sci. 2012, 5, 5902-5918. [2] Lakadamyali, F.; Reynal, A.; Kato, M.; Durrant, J. R.; Reisner, E. Electron Transfer in Dye-Sensitised Semiconductors Modified with Molecular Cobalt Catalysts: Photoreduction of Aqueous Protons. E. Chem. Eur. J. 2012, 18, 15464-15475. [3] Reynal, A.; Lakadamyali, F.; Gross, M.; Reisner, E.; Durrant, J. R. Parameters affecting electron transfer dynamics from semiconductors to molecular catalysts for the photochemical reduction of protons. Manuscript in preparation.

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