Development of photocatalytic materials for water oxidation and carbon dioxide reduction

Yusuf, Seif
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University of Delaware
Fossil fuels are currently the primary energy source of the world, but are an undesirable energy source for the future as their combustion generates CO2, a greenhouse gas and it is unlikely current reserves of fossil fuel will meet future energy demand. One alternative is to use solar energy, which provides more than enough energy to meet demand, to produce carbonaceous fuels. These solar fuels are advantageous because their generation uses H2O, which is abundant, and CO2, which needs to be removed from the atmosphere. The oxidation of H2O is needed to the electrons needed for the reduction of CO2 In this thesis, nanostructured manganese and cobalt oxide catalysts were synthesized and systematically investigated as cheap, stable, and efficient H2O oxidation catalysts. The first part of research mainly focused on the development of highly efficient manganese based oxygen evolution catalysts and investigation of effects of alkaline cations in the photocatalytic water oxidation reaction. K+ containing and Na+ containing δ-MnO2 catalysts were synthesized using a hydrothermal synthetic approach. To determine the effect of alkaline ions, K+ in δ-MnO2 was replaced by H+ ions using an ion-exchange process. The K-δ-MnO2 and Na-δ-MnO2 catalysts had TOF’s about one order of magnitude higher than their H-δ-MnO2 counterparts, even though the H-δ-MnO2 catalysts have much larger surface areas. The H-δ-MnO2 may have had lower activity because the Ru2+(bpy)3 was too large to access the new surface sites. The results suggest that the alkaline cations stabilize the MnO2 layers and are not directly involved in the H2O oxidation reaction. Turning to cobalt based oxygen evolution catalysts, it has been shown in previous reports that Co3O4 supported in mesoporous silica exhibited high oxygen evolution activities, while there is still no report on systematic study of support effect. In this thesis, Co3O4 nanoclusters supported in various materials were studied. Wetimpregnation and bi-solvent methods were used to fabricate mesoporous silica with Co3O4 clusters. Hexane/water was found to be the optimal combination yielding a catalyst with a small particle size and narrow size distribution. The results confirmed that as the Co3O4 cluster size became smaller, the water oxidation activity increased. KIT-6 was found to be a better support than SBA-15, due to its 3D porous structure offering a more accessible system. Bare Co3O4 nanoparticles and particles supported by SiO2 and Al2O3 were also studied so that the role of support could be determined. It was found that the supported particles had higher activity than the bare particle, but there were no significant differences in activity between the two supported particles. This indicates that the surface is not directly involved in the reaction and the key role of the support in photocatalytic water oxidation reaction is to physically separate and immobilize the Co3O4 nanoclusters.