Title : Who gives the hydrogen? A mechanistic study through isotope labeling in photocatalytic hydrogen evolution reactions using cobalt complexes as a catalyst under neutral ph
Abstract:
The growing global demand for energy has led to the excessive exploitation of non-renewable energy sources, especially fossil fuels. This heavy dependence on such sources significantly increases CO2 emissions in the atmosphere, resulting in serious environmental repercussions. Therefore, it is essential to innovate, develop, and implement economically feasible carbon-neutral energy production systems to tackle these energy-related issues. One promising strategy involves emulating natural photosynthesis to produce solar fuels, which enables the long-term and sustainable storage of the substantial amounts of renewable energy captured from sunlight. A key aim in artificial photosynthesis is the photocatalytic generation of hydrogen through the process of water splitting. To achieve this objective, it is necessary to develop efficient and stable catalytic systems that utilize earth-abundant elements to facilitate the conversion of protons from water into molecular hydrogen. Typically, these methods rely on the catalytic properties of metal complexes and organic dyes to transform visible light into chemical energy through an electron transfer process in an excited state. A less frequently mentioned yet effective method involves the use of in-situ generated active metal complexes to promote water splitting. Experimental analysis has shown that gas evolution occurs solely at neutral pH, with no gas production observed in acidic or basic conditions. The gas evolution is facilitated by earth-abundant metal complexes, referred to as catalysts or CAT, along with an organic photocatalyst in the presence of an electron donor. The synthesized catalysts (CAT-I to IV) demonstrated varying efficiencies for the gas-evolving reaction, as indicated by their turnover number (TON) and turnover frequency (TOF) values: CAT-I (53.69 and 2.7 × 10-3 s-1), CAT-II (62.50 and 3.2 × 10-3 s-1), CAT-III (68.45 and 3.5 × 10-3 s-1), and CAT-IV (82.73 and 5.10 × 10-3 s-1) for TON and TOF, respectively. The initial rate constants ((k)H ) were also determined, yielding values of 20.38 × 10-3 min-1, 20.91 × 10-3 min-1, 24.89 × 10-3 min-1, and 25.14 × 10-3 min-1 for CAT-I through CAT-IV, respectively. A confirmatory test for gas evolution under photocatalytic conditions in an inert atmosphere was conducted using GC-MS analysis, which confirmed that the only gas produced was hydrogen, as indicated by the m/z value of 2. Additionally, understanding the sources of protons involved in these reactions is crucial, necessitating the implementation of isotope labeling studies. These studies have shown that the primary source of hydrogen evolution is the combination of water and methanol, with water playing a more significant role than methanol. More comprehensive results will be shared during the presentation.
