Effect of hetero-atom doping on the electrocatalytic properties of graphene quantum dots for oxygen reduction reaction.

Oxygen reduction is an important reaction involved in a diverse variety of energy storage devices and also in many chemical and biological processes. However, the high cost of suitable catalysts like platinum, rhodium, and iridium proves to be a major obstacle for its commercialization. Consequently, many new materials have emerged in recent years such as various forms of carbon, carbides, nitrides, core-shell particles, Mxenes, and transition metal complexes as alternatives to platinum and other noble metals for oxygen reduction reaction (ORR). Among these, Graphene Quantum Dots (GQDs) as metal-free alternatives have captured universal attention, since electrocatalytic properties can be tuned not only by size and functionalization but by heteroatom doping also. We discuss electrocatalytic properties of GQDs (approximate size 3-5 nm) with specific dopants such as N and S focusing on their synergistic effects of co-doping, prepared by solvothermal routes. Cyclic Voltammetry shows benefits of doping as lowering of the onset potentials while steady-state Galvanostatic Tafel polarization measurements show a clear difference in the apparent Tafel slope, along with enhanced exchange current densities, suggesting higher rate constants.

Oxygen atom transfer promoted nitrate to nitric oxide transformation: a step-wise reduction of nitrate → nitrite → nitric oxide.

Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO3 -) to nitrite (NO2 -) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO2 - from NO3 -, which further produces nitric oxide (NO) either in acid-induced NO2 - reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl3 (V3+ ion source) induced step-wise reduction of NO3 - in a CoII-nitrato complex, [(12-TMC)CoII(NO3 -)]+ (2,{CoII-NO3 -}), to a CoIII-nitrosyl complex, [(12-TMC)CoIII(NO)]2+ (4,{CoNO}8), bearing an N-tetramethylated cyclam (TMC) ligand. The VCl3 inspired reduction of NO3 - to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions, i.e., OAT-1 = NO3 - → NO2 - (r1) and OAT-2 = NO2 - → NO (r2). In these OAT reactions, VCl3 functions as an O-atom abstracting species, and the reaction of 2 with VCl3 produces a CoIII-nitrosyl ({CoNO}8) with VV-Oxo ({VV[double bond, length as m-dash]O}3+) species, via a proposed CoII-nitrito (3, {CoII-NO2 -}) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex 3 with VCl3, which showed the generation of 4 with VV-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized 3 using VCl3 as a limiting reagent, as the second-order rate constant of OAT-2 (k 2 /) is found to be ∼1420 times faster than that of the OAT-1 (k 2) reaction. Binding constant (K b) calculations also support our proposition of NO3 - to NO transformation in two successive OAT reactions, as K b(CoII-NO2 -) is higher than K b(CoII-NO3 -), hence the reaction moves in the forward direction (OAT-1). However, K b(CoII-NO2 -) is comparable to K b{CoNO}8 , and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using 15N-labeled-15NO3 - and 15NO2 - revealed that the N-atom in the {CoNO}8 is derived from NO3 - ligand. This work highlights the first-ever report of VCl3 induced step-wise NO3 - reduction (NRs activity) followed by the OAT induced NO2 - reduction and then the generation of Co-nitrosyl species {CoNO}8.