Light-active Metal-Organic Frameworks for Photocatalytic Hydrogen Evolution.

The general scope of this thesis lies in the application of MOFs in photocatalysis. MOFs demonstrate inherent properties – such as high porosity, tunable optoelectronic and catalytic properties, which render them promising candidates for photocatalysis. The main research activity relative to this thesis aims at the advancement of MOF-based photocatalytic systems in terms of activity, cost and sustainability. This was achieved by employing different strategies that can lead to the favorable modification of the three major photocatalytic steps of light absorption, charge separation-migration and catalysis, which govern the overall performance of a photocatalytic system.

More specifically, Chapter 2 describes the impact of different co-catalysts on the photocatalytic activity of a MOF-based system, based on the well-known MIL-125-NH2. Variation of the co-catalysts can significantly influence the two major photocatalytic steps of charge separation and the catalytic reaction. All the metal oxide and phosphide co-catalysts investigated are found to significantly improve the activity of MIL-125-NH2, with the system using Ni2P nanoparticles (NPs) exhibiting a high H2 evolution rate (1230 ÎŒmol h-1 g-1) and an apparent quantum yield of 6.6% at 450 nm, which is comparable to the state of the art. Comparison of Ni2P with Pt showed that the noble-metal-free Ni2P/MIL-125-NH2 system significantly outperforms Pt/MIL-125-NH2. These results are attributed to the enhanced electronic interactions between MIL-125-NH2 and Ni2P, and prove that earth abundant co-catalysts can challenge the commonly used noble metals.

The low cost and high efficiency of Ni2P/MIL-125-NH2 prompted me to focus on another component used in photocatalytic systems, which is the electron donor. Chapter 3 shows that variation of the electron donors highly influences the activity of Ni2P/MIL-125-NH2, with triethylamine significantly boosting the H2 evolution rate. However, the utilization of electron donors is another factor hindering the industrialization of such systems, since these substances can be toxic and expensive. Inspired by this challenge, we then replaced the electron donor with rhodamine B (RhB) – a simulant organic pollutant – envisioning dual-functional photocatalysis for simultaneous H2 generation and organic pollutant degradation. This research project revealed the first example of a MOF-based dual-functional photocatalytic system able to generate H2 in a high rate and degrade RhB under visible-light.

Chapter 4 describes a strategy for enhancing the photocatalytic performance of MOF-based systems by favourably altering the photocatalytic steps of light harvesting and charge separation. This was achieved by developing MOF/MOF heterojunctions. The combination of MIL-125-NH2 with MIL-167 – a Ti-based MOF with complementary light absorption properties – leads to the formation of a type II heterojunction MIL-167/MIL-125-NH2, with enhanced optoelectronic properties. MIL-167/MIL-125-NH2 significantly outperforms its single components MIL-167 and MIL-125-NH2, in terms of photocatalytic H2 production. This strategy contributes to the discovery of novel MOF-based photocatalytic systems that can harvest the solar energy and exhibit high catalytic activities in the absence of co-catalysts.