Sustainable Chemistry Lab
. . . towards green and environmentally benign innovations
(With 3d transition metal Catalysts)
Artificial Photosynthesis
Electrocatalyzed and Photocatalyzed Hydrogen Evolution Reaction (HER),
Carbon-dioxide Reduction (CO2RR), Reduction of Nitrogen Oxides & Oxyanions
1. The global energy demand and the rising threat of greenhouse gases due to the burning of fossil fuels led to the development of renewable energy technologies and innovations for energy storage as well as transportation. In this regard, hydrogen evolved as a clean, carbon-neutral energy source and energy carrier. Among several strategies, the hydrogen produced through water electrolysis using renewable energy sources is in the cleanest form and thus termed green hydrogen. However, the hydrogen evolution reaction (HER) is kinetically sluggish involving multiple electron transfer processes, and requires a catalyst to propel forward. Presently, platinum is the most efficient catalyst for HER from an acidic aqueous medium, but the high cost and low abundance restrict its widespread usage. In nature, the hydrogenase enzymes containing earth-abundant transition metal (Ni, Fe) based active sites are very efficient catalysts for hydrogen evolution. Therefore, the low abundance and high price of platinum and the high efficiency of hydrogenase enzymes motivated towards the development of earth-abundant transition metal-based HER catalysts.
2. Over the last century, significant developments in science and technology have created an adverse effect on the environment by emitting large amounts of carbon dioxide (CO2), nitrogen oxides (NOx), and sulfur oxides (SOx). Carbon-based fossil fuels are major contributors to this excessive liberation of CO2, NOx and SOx. Also, a substantial amount of NOx is produced during combustion processes and different microbial processes in soil. These atmospheric nitrogen oxides come to the earth's surface through acid rain in the form of nitric acid. Additionally, the exhaustive use of nitrogen-based fertilizers during crop cultivation pollutes the earth's crust by producing nitrates in the soil. Notably, nitrate pollution has become a major concern to mankind as it causes potentially serious threats to human health like blue baby syndrome, non-Hodgkin's lymphoma, and other cancers as well as damage the ozone layer. Therefore, the climate change associated with global warming majorly caused by CO2 and the harmful effects due to elevated nitrate levels on the earth's surface has led the scientific community to discover sustainable approaches for the fixation and conversion of CO2 to carbon monoxide (CO), formic acid (HCO2H), methane and other valuable chemical feedstock.
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Similarly, developments of sustainable methods for nitrogen oxyanion reduction and conversion to hydroxylamine (NH2OH) and ammonia (NH3), etc. have drawn considerable attention from the scientific community. Importantly, ammonia is the most important chemical component for the fertilizer industry and it can act as a hydrogen carrier; thereby providing an alternative source of renewable hydrogen energy. Hence, the development of efficient catalysts/ materials for these small molecule/anion reductions is of paramount importance. In Nature, nickel-iron co-factor-based carbon monoxide dehydrogenase (CODHs) and formate dehydrogenase perform selective CO2 reduction to CO and formic acid respectively. Also, iron-based Cytochrome c nitrite reductase enzyme (CcNir) plays a crucial role in reducing the nitrite to ammonia during the final step of dissimilatory nitrite reduction. Therefore, the development of bio-inspired transition metal catalysts for reductive conversion of small molecules/anions to produce value-added chemicals such as CO, formic acid, methanol, ammonia, etc. has become one of the prime goals for the synthetic chemist for the societal benefit of the mankind.
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Electro-chemical C-H Functionalization : Electro-organic Synthesis
1. Development of Electrochemical Synthetic Methodology
Development of any mild and efficient methodologies for direct functionalization of C-H bonds has paramount importance in fundamental research as well as in industrial applications. The activation of the unactivated C(sp3)-H bond is one of the most challenging tasks because of its low polarity and high thermodynamic stability. This challenge has been addressed through the development of bio-mimetic transition metal dependent catalytic system in presence of chemical oxidants in the last two decades. However, these site-selective methodologies for the functionalization aliphatic C-H bond is comprise of several limitations.
These drawbacks intrigued our group to ponder for a mild, green, sustainable alternative to chemical procedures. In this quest for a sustainable substitute, we opt for the electrochemical synthetic methodology that uses electrons as a traceless redox reagent as a replacement of toxic and hazardous chemical oxidants. Therefore, our primary focus lies on the development of some bio-inspired catalysts based on first row transition metal that will electrochemically generate high valent putative intermediates. Thereafter, this putative intermediate will help in selective electrochemical aliphatic C-H bond transformation methodologies such as hydroxylation, azidation, fluorination, chlorination, and trifluoromethylation reaction.