The rational design of heterogeneous photocatalysts for the tandem photocatalytic hydrogen evolution and reduction of organic compounds in water

The rapid increase in the global population has spurred a concurrent rise in industrial activities and technological advancements, intensifying the demand for reliable energy resources. Currently, most of the global energy needs are met by fossil fuels. However, the extensive use of fossil fuels over the past century has significantly contributed to global warming, posing severe environmental and sustainability challenges. These challenges underscore the urgent need to transition toward renewable energy sources. Among the alternatives for meeting energy needs from renewable sources, the use of hydrogen as an energy carrier stands out because of its high energy density and environmentally benign nature, which emits zero greenhouse gases. One promising method for sustainable hydrogen production is photocatalytic water splitting, which harnesses the inexhaustible and free energy of the sun. This method has recently garnered considerable attention as a potential solution to the global energy crisis. In photocatalytic water splitting, semiconductor materials with an appropriate band gap facilitate the hydrogen evolution reactions (HER) under solar light irradiation. The efficiency of this process depends on the development of advanced semiconductor materials capable of optimizing the HER under solar illumination. One of the advanced strategies to enhance the photocatalytic activity of semiconductor materials is the construction of their heterojunctions with other semiconductors or metals. Literature indicates that heterojunctions composed of semiconductor materials and noble metal nanoparticles (NPs) such as platinum (Pt), and palladium (Pd)achieve high efficiency in HER due to their unique electronic properties and the heterojunction (Schottky junction) formed between the semiconductor and conductor. To form efficient Schottky junctions, support material should be cost-effective, non-toxic, have a large surface area and proper band gap that can be activated by the large portion of solar light. Among the various candidates, red phosphorus (RP) and black phosphorus (BP), two stable allotropes of elemental phosphorus, have attracted significant attention due to their exceptional properties. These semiconductor materials have been increasingly used in photocatalytic HER applications. However, the photocatalytic activity of a single semiconductor is often limited by the accumulation behavior of charge carriers and inefficient solar light absorption. To address this limitation, the formation of heterostructures with these semiconductor materials has been shown to significantly enhance photocatalytic activity, providing a more efficient pathway for charge separation and transfer, thereby improving overall HER performance. On the other hand, hydrogenation is an important chemical reaction in which unsaturated molecules are converted into saturated counterparts by addition of hydrogen (H2) in the presence of a metal catalyst. This process plays a critical role in synthesizing valuable compounds in organic chemistry, producing pharmaceuticals in the pharmaceutical industry, and refining processes in the petrochemical industry. Commonly referred to as conventional catalytic hydrogenation, hydrogenation reactions are predominantly conducted under severe conditions requiring the presence of external H₂ gas and elevated reaction temperatures. As a mild and safer alternative to this method, the transfer hydrogenation technique facilitates hydrogenation reactions without needing external H₂ gas. Instead, it employs hydrogen carriers such as isopropanol, formic acid, ammonia borane and sodium borohydride to provide the necessary hydrogen. The photocatalytic hydrogenation method allows hydrogenation reactions to be carried out under milder conditions and offers a more sustainable and environmentally friendly approach. Advancing a strategy by integrating photocatalytic HER with transfer hydrogenation methods can be achieved by selecting greener, non-toxic, and cost-effective hydrogen sources. In this context, the utilization of hydrogen generated photocatalytically from water in hydrogenation reactions presents a highly influential approach, potentially yielding impactful outcomes in the field of organic chemistry. In this thesis, the initial section details the synthesis of RP-BP/Pt-PtP2 heterojunctions using a wet-chemical synthesis method from red phosphorus (RP) with an appropriate solvent and a platinum precursor. This unique protocol simultaneously generates BP nanosheets from RP under mild conditions and in-situ produces Pt and PtP2 NPs for the first time. The RP-BP/Pt-PtP2 heterojunctions were first tested as photocatalysts for the hydrogen evolution reaction (HER) using triethanolamine (TEOA) as a hole scavenger and Eosin Y as a photosensitizer under visible light irradiation, achieving a high HER activity of 47.46 mmol H2.g–1 catalyst in 8 hours, which is 40 and 16 times higher than pristine RP and BP, respectively. Encouraged by these results, a tandem photocatalytic HER and hydrogenation of nitroarenes were designed using RP-BP/Pt-PtP2 heterojunctions as photocatalysts, with water as the hydrogen donor under visible light irradiation. This one-pot RP-BP/Pt-PtP2 catalyzed process exhibited high selectivity in reducing nitrobenzene derivatives to their corresponding azoxy compounds, with conversion reaching up to 99%. Deuterium labeling experiments confirmed that water served as the hydrogen generator. In the subsequent section, Pt NPs were synthesized in situ on RP via the chemical reduction method, resulting in an easy and effective method to obtain the RP/Pt heterojunction photocatalysts. Delightfully, RP/Pt catalysts outperformed the RP/BP-Pt-PtP2 , demonstrating a photocatalytic HER activity of 70.57 mmol H2.g–1 in 8 hours. The activity of the RP/Pt in the tandem photocatalytic HER and transfer hydrogenation of nitroarene compounds was also investigated. Additionally, Pd metal is known for its hydrogenation efficiency, were synthesized in situ on RP. The performance of RP/Pt and RP/Pd catalysts in photocatalytic HER and the reduction of nitroarene compounds were compared. Due to the varying electronic properties of the metals, notable results were obtained that emphasize their distinct impacts on the reaction outcomes. While RP/Pt was identified as the catalyst with the highest photocatalytic HER activity, the RP/Pd catalyst exhibited superior activity in photocatalytic hydrogenation of nitrobenzene compounds. The conversion and selectivity of products formed were determined by using GC-MS and NMR. The structural, chemical, and optical properties of as-synthesized materials were elucidated using various advanced characterization techniques, including TEM, HR-TEM, XPS, XRD, UV-Vis, PL, TRPL, and EIS.