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Using water as a hydrogen source is a promising strategy for alternative hydrogen peroxide (H 2 O 2 ) synthesis. By a series of ab initio molecular dynamics (AIMD) simulations and reactive molecular dynamics (RxMD) calculations, fundamental details have been revealed regarding how liquid water interacts with oxygen on a metal-free carbon nitride catalyst, and the two-step reaction mechanism of H 2 O 2 synthesis. Metal-free porous graphitic carbon nitride (g-C 5 N 2 ) catalysts are also systematically screened by using a thermodynamics approach through the ab initio density functional theory (DFT) method. Key results include: (a) pristine g-C 5 N 2 is most active to catalyze the H 2 O/O 2 reaction and produce H 2 O 2 ; (b) the adsorption and activation of water at unsaturated carbon sites of g-C 5 N 2 are critical to initiate the H 2 O/O 2 reaction, producing HOO* intermediates; (c) interfacial free water and adsorbed water at g-C 5 N 2 form a synergetic proton transfer cluster to promote HOO* intermediates to form H 2 O 2 . To the best of our knowledge, this work presents long-needed theoretical details of direct H 2 O 2 synthesis via the water/oxygen system, which can guide further optimization of carbon-based catalysts for oxygen reduction reactions.