Research Projects

The project is focused on two major issues- (a) Nitrate/Nitrite reduction (NER) to NH₃, a potential fuel to minimizing carbon footprint. (b) Removal of Nitrate/Nitrite from water, two common pollutant. The project involves synthesis of bioinspired catalysts for NER, establishing the mechanism, finding thermodynamic and kinetic bottleneck and application of suitable mediators to overcome it. The various facets are given below – (i) NER to NH₃, an multielectron-multiproton process, is highly challenging. Nature tackles that by employing cascade of enzymes that sequentially catalyse NER stepwise, reducing one or two electrons at each step via intermediate species formation e. g. nitrate, NO, N₂, NH₂OH etc. Since, developing such sequential catalysts are formidable task, a series of biomimetic hetero-bimetallic complexes have been proposed where one metal site will catalyze the reduction of nitrate and 2nd site will convert nitrite to NH₃. Inspired from nitrate reductase bearing Mo-thiolate cofactor, coordination geometry of 1st metal is chosen to be Mo(IV)/W(IV) thiolate moiety. 2nd site comprises of base metals (e.g. Fe, Co. Ni and Cu) within polypyridyl framework, inspired from nitrite reductase that harbor either heme or nitrogenous ligands bound to copper(T2Cu) at active site. The Mo(IV) center should be sufficiently lewis acidic even upon reduction so that nitrate remains bound and capable to polarize one of the N=O bond, leading to cleavage of it. A series of pyridine-thiolate and pyrrol-bisthiolate ligands have been targeted for Mo/W site. Once nitrite is produced, it would react with the 2nd metal center in polypyridyl ligand environment. Planner nitrogenous ligands in general has π-accepting character, pull the electron density from metal to facilitate nitrite binding, as well as can supply the electrons if needed. Eventually, both of these metal would be attached to a ligand such a way that the they are electronically coupled. Such type of cooperative catalysts along with mediator (vide infra) might be the key to overcome the inherent limitation of metal nitrosyl reduction to ammonia, leading to lower overpotential, enhancing selectivity and rates. (ii) The performances of these complexes towards NER will be tested in nonaquesous solvents by various voltammetric techniques and bulk electrolysis in presence of suitable proton donors. The products will be analyzed by GC, NMR, IR and other methods. The mechanism will be elucidated by electrochemistry coupled UV-Vis and IR spectroscopy which will provide details of rate determining step i. e. kinetic bottleneck of the process. Bench marking of these catalysts will be done using FOWA analysis. (iii) Attempts will be made to synthesize intermediates e.g. M=N, M-NO etc. and determining the thermochemical parameters of them e.g. E₀, pKa and BDFE. Suitable PCET mediators will be synthesized and applied along with catalysts to bypass bottlenecks. Such approach was found effective in CO₂RR & N₂RR.