Design of metal-organic frameworks containing single-metal-site cocatalysts for photocatalytic hydrogen production

Abstract

The reliance of global energy production on fossil fuels leads to drastic changes in the world ́s climate. Renewable energy derived from natural sources like sunlight or wind is abundantly available, but its intermittent nature requires new solutions for storing this energy. Hydrogen represents an interesting option as a sustainable energy carrier that can be generated from water upon irradiation with sunlight by means of photocatalysis. However, current photocatalytic systems do not meet the requirements for scalable hydrogen production, with heterogeneous systems lacking activity and selectivity and homogeneous catalysts degrading quickly under reaction conditions. The incorporation of single-metal sites onto heterogeneous support materials has the possibility to bridge the two concepts, creating catalysts with well-defined active sites and maximum atom utilization efficiency. Metal-organic frameworks (MOFs) are an ideal platform for integrating such single sites due to their large surface area and high structural tunability. In this thesis, single-site cocatalysts were introduced into the MOF systems UiO-67 and COK-47 as organometallic complexes containing noble Pt or earth-abundant Ni that also function as MOF linkers. Two synthesis strategies were used to obtain such single-site-functionalized MOFs: direct one-pot synthesis where the functionalization occurs during the MOF synthesis and post-synthetic ligand exchange. The obtained materials were characterized carefully and their activity for photocatalytic hydrogen production was evaluated. Pt or Ni single-metal sites on UiO-67 showed excellent photocatalytic performances for already very low metal loadings below 0.5 wt-% and they were more active than their nanoparticle counterparts obtained by photodeposition. In contrast to this, the functionalization of COK-47 with Ni sites resulted in almost no photocatalytic activity and while Pt single siteson COK-47 did produce hydrogen, the performance was considerably worse than with Pt nanoparticles, highlighting that the concept of functionalization with single-site cocatalysts works better for UiO-67 than for COK-47. Compared to one-pot synthesis, ligand exchange increased flexibility for the synthesis and enabled more MOF-cocatalyst combinations. However, the ligand exchange procedure most likely introduced missing ligand defects in the MOF structure and electron trap states close to the conduction band which resulted in a decreased catalytic activity. This work highlights the importance of single-metal sites as cocatalyst in photocatalysis and also shows the strong influence of different implementation strategies, providing ground for future research.