Catalytic Oxidation Of N-hexanal By Two-dimensional Ultrathin Cobalt-based Metal Oxides

Effectively controlling volatile organic compound(VOCs)pollution and improving the atmospheric ecological environment is one of the hot issues of current social concern.The "14th Five-Year Plan" proposed that the total amount of VOCs emissions should be reduced by more than 10%.Among many VOCs removal technologies,catalytic oxidation method has attracted wide attention.The core of this technology lies in the research and development of low-cost,highperformance catalysts.Traditional bulk catalysts generally suffer from problems such as fewer active sites,poor low-temperature activity,and insufficient understanding of the reaction mechanism.Two-dimensional(2D)ultrathin nanocatalysts have atomic-level thickness,unique electronic structure,and a large number of active sites on the surface,which can further improve the lowtemperature activity and provide reference for the reaction mechanism of bulk catalysts.In this paper,taking n-hexanal,a typical VOCs in lampblack,as the research object,2D ultrathin(less than 2 nm thick)cobalt-based nanosheets were designed and synthesized for its catalytic oxidation,and its intrinsic reaction mechanism was explored.The main research contents and conclusions are as follows:Compared with the bulk catalysts,2D Co3O4 and 2D CoO nanosheet catalysts were synthesized by solvothermal method,in which 2D Co3O4 nanosheets showed up to 90%conversion rate and more than 95%CO2 selectivity to n-hexanal at 202℃.Under the same conditions,the activity is about 1.2 times and 6 times higher than that of 2D CoO and bulk Co3O4,respectively.Benefiting from the ultrathin(about 1.70 nm)thickness of the 2D Co3O4 nanosheets,the presence of a large number of active oxygen on the surface,and the existence of low-coordination Co atoms,the nanosheets are endowed with abundant active sites on the surface.The study shows that O2 enters the oxygen defect to form active oxygen components by capturing excited electrons,which changes the catalytic reaction path,reduces the activation energy to about 27.7 kJ/mol,and reduces the complete conversion temperature of n-hexanal by about 60℃.Using a single-valence Zn2+ substitution strategy to control the surface oxygen defect structure and electronic structure of 2D Co3O4 nanosheets,a 2D ultrathin(about 1.64 nm)ZnCo2O4 nanosheet catalyst was designed,and the microstructure and macroscopic structure were studied from the 2D nanoscale space.The structureactivity relationship between properties.The XPS analysis shows that the surface oxygen defects of 2D ZnCo2O4 nanosheets are about twice that of 2D Co3O4 nanosheets,in which Zn2+ does not participate in the redox reaction and does not act as an active site.XAFS,fs-TAS,and DFT data show that the further improvement of the low-temperature catalytic activity of 2D ZnCo2O4(from 49%to 77.5%at 175℃)is due to the simultaneous participation of holes in the reaction,reactive oxygen species(O*)and empty When the holes(O)exist at the same time,the gas molecules are more likely to be adsorbed,and the catalytic reaction is more likely to occur.First,the C4-C5 bond in n-hexanal is activated and broken,and short-chain substances are generated to be oxidized.Through the deep reduction treatment of the catalyst lattice structure,it has been found that the collapse and fracture of the lattice can reduce the production of active components,resulting in a significant decrease in catalyst activity.In order to further control the oxygen defect structure and active oxygen species on the surface of nanosheets,2D ultrathin(about 1.61 nm)NiCoOx nanosheet catalysts with bimetallic active sites were constructed by replacing Zn atoms with Ni atoms with variable valence.The performance test of n-hexanal shows that the 2D NiCo2O4 nanosheets have high catalytic activity,the conversion efficiency can reach about 85%at 180℃,and it has high stability and certain water resistance.XPS spectrum analysis shows that the surface oxygen defects of 2D NiCo2O4 nanosheets are 1.3 times that of 2D ZnCo2O4 nanosheets,and electron transfer occurs between Nix+ and Coy+,which participates in the redox process and constitutes the bimetallic active site of the catalyst,realizing the NiCoOx catalyst.Regulation of catalytic performance.The EPR data showed that the excellent catalytic activity of the 2D NiCo2O4 nanosheet catalyst is inseparable from the superoxide radical(·O2-)and hydroxyl radical(·OH-)generated inside.From the perspective of quantum mechanics,it is believed that in the reaction process of multi-site catalysts,the electron interference wave(superposition)has different effects on the oxygen defect structure,thereby producing strong active components.The above conclusions provide reference for the design of efficient and economical VOCs catalytic oxidation catalysts.