Research On The Mechanism Of Nucleophile Oxidation Reaction Based On Nickel-based Catalyst

A variety of energy crisis and increasing environmental pollution have prompted chemists to develop environmentally friendly and economical synthetic techniques.Among them,electrocatalytic energy conversion technique is one of the greenest and cheapest since it is possible to perform redox reactions without the need for any chemical reagents.Recently,organic molecule electrooxidation coupling with hydrogen production has been attracted more and more attention.Thereinto,nucleophile oxidation reaction（NOR）is a potential alternative oxidation reaction coupling with hydrogen production,which has important industrial application potential.The substrate scope of NOR system is broad,such as the electrooxidation of primary amine to nitrile,electrooxidation of secondary amine to imine,electrooxidation of alcohol/aldehyde to carboxylic acid,electrooxidation of polyhydric alcohols to formic acid,etc.On the other hand,researchers have developed a series of transition metal-based materials as efficient NOR catalysts,and thereinto,nickel-based catalysts have outstanding NOR performances.Given that active organic molecules are involved,NOR is a complex reaction involving electrochemical and non-electrochemical steps;hence,it is extremely challenging to identify its complex reaction mechanism.Electrochemists tend to pay too much attention to electrochemical steps and ignore the role of non-electrochemical steps.Besides,the lack of interdisciplinary knowledge integration further aggravate the recognizing and understanding deviations of NOR mechanism,which results in development dilemma.At present,the design strategies of efficient NOR catalysts mainly focus on increasing the NOR apparent activity through increasing the number of active sites;however,how to improve the intrinsic activity of NOR catalysts remained a problem to be solved.On the other hand,the existing NOR system is uncompetitive in organic synthesis,so the expansion of NOR substrate and product system is urgent.These two dilemmas are rooted in the lack of a clear understanding of NOR mechanism.Therefore,a comprehensive understanding of NOR mechanism is crucial for the development of NOR field.In this paper,we systematically researched the catalyst function mechanism of NOR,and comprehensively understood the structure-effective relationship between nucleophile oxidation pathways and non-electrochemical steps,and thus establishing a unified theoretical basis for the NOR field.Based on the comprehensive understanding of NOR mechanism,this paper proposed a catalyst design strategy to enhance the intrinsic electrochemical activity of NOR.Besides,the catalyst design strategy can regulate the synergy of electrochemical and non-electrochemical steps in the NOR system,and thus determining the nucleophile oxidation pathway.The specific research contents of this paper are as follows:（1）The catalyst function mechanisms ofβ-Ni（OH）₂ and NiO model catalysts in the NOR system were studied.The NOR based on nickel-based catalysts is an indirect electrooxidation reaction with electrooxidation-induced electrophilic oxygen species as the redox medium,and the redox medium ofβ-Ni（OH）₂ or NiO catalyst function mechanism is electrophilic lattice oxygen species or electrophilic adsorbed oxygen species,respectively.This study deciphered the catalyst function mechanisms ofβ-Ni（OH）₂ and NiO in NOR,and provided guidance for designing high-efficient NOR catalysts.（2）Based on theβ-Ni（OH）₂ catalyst function mechanism during NOR,we proposed a universal design strategy for high-efficient NOR catalysts,and the key is to facilitate the electrochemical generation of the redox medium,i.e.,electrophilic oxygen species.Adjusting the lattice oxygen coordination structure ofβ-Ni（OH）₂catalyst（e.g.,cation substitution）could optimize the thermodynamics and kinetics of the electrooxidation of lattice electrophilic hydroxyl to lattice oxygen species,and thus regulating the electrochemical activity of NOR.For example,via doping cobalt cation inβ-Ni（OH）₂,the onset potential of NOR is reduced to 1.29 V,which breaks the performance bottleneck of nickel-based catalysts,and the current density reaches 92.3m A cm^-2 at the potential of 1.36 V.This study provides theoretical basis and technical guidance for the high-efficiency NOR catalyst design in the industrial application.（3）By combining a variety of in-situ characterization techniques and isotope tracing experiments,we figured out how the synergy between electrochemical and non-electrochemical steps determined the nucleophile electrooxidation pathway in different NOR systems.In the NOR system,the electrophilic oxygen-mediated hydrogen atom transfer mechanism causes the electrooxidation of primary amine to nitrile;primary alcohol can be electrochemically oxidized to carboxylic acid under the synergy of electrophilic oxygen-mediated hydrogen atom transfer mechanism and hydration of aldehyde intermediate;urea undergoes electrophilic oxygen-mediated hydrogen atom transfer,hydration of carbonyl and complex intramolecular rearrangement reactions to generate nitrogen and carbon dioxide.This study lays a basic theoretical foundation for the NOR field and provides a comprehensive theoretical guidance for the development of NOR field.（4）The electrooxidation of polylol（e.g.,ethylene glycol,glycerinum,etc.）to formic acid based on the NOR system has been extensively studied,but the mechanism reaction is still unclear,especially for the mechanism of carbon-carbon bond cleavage.This study investigates the electrooxidation of vicinal diol to carboxylic acid and formic acid based on NiO catalyst.We identify two different NiO function mechanism,i.e.,electrophilic adsorbed oxygen-mediated hydrogen atom transfer and electrophilic adsorbed oxygen-mediate carbon-carbon bond cleavage.Via intermediate verification experiments,this study revealed the reaction pathway of vicinal diol electrooxidation based on NiO.This study establishes a basic theoretical basis for alcohols electrooxidation reaction based on nickel-based catalysts.（5）Due to special electronic structure of lattice oxygen,the electrophilic lattice oxygen-mediated mechanism could not catalyze the cleavage of carbon-carbon bond;hence,β-Ni（OH）₂ catalyst could not drive the electrooxidation of vicinal diol to carboxylic acid and formic acid,resulting in the passivation of electrode accompanied with few electrochemical activity.In this study,we regulated the vicinal diol electrooxidation reaction pathway via the defect engineering ofβ-Ni（OH）₂ catalyst.Vacancy-richβ-Ni（OH）₂ can efficiently catalyze the electrooxidation of o-diol and produce carboxylic acid and formic acid,because oxygen vacancy-induced adsorbed oxygen-mediated mechanism can realize the cleavage of carbon-carbon bond.This findings deepen the understanding of vacancy-induced catalytic mechanism in the NOR based on nickel-based catalysts.