Preparation Of Porous Transition Metal-Based Catalysts And Their Photocatalytic/Electrocatalytic Properties

With the rapid development of social economy and the rapid growth of population since20^thcentury,industrial technology and social economy,the global energy consumption has been increasing.While developing and utilizing fossil energy-based energy resources to develop industrial civilization,human beings have brought unprecedented changes to the earth’s atmospheric and water environment.The burning of a large number of fossil fuels has caused a sharp increase in global carbon emissions and the concentration of carbon dioxide（CO₂）in the atmosphere has continued to increase,resulting in an imbalance in carbon cycle,occurrence of global extreme climate and huge losses to the economy.At the same time,industrial production and human daily activities discharge a large amount of refractory pollutants into water environment,such as organic dyes,phenolic wastewater,surfactants,antibiotics,etc.,posing a serious threat to water resources security.Heterogeneous catalysis technology with mild reaction conditions and low energy consumption is a technical solution with great application prospects that can deal with the above-mentioned environmental problems.At present,electrocatalytic CO₂reduction technology and photocatalytic oxidation degradation of pollutants in water have become hot topics in the field of environmental heterogeneous catalysis.Both of these two technologies have technical advantages such as energy saving,environmental friendliness and easy operation,which are in line with the current trend of green chemistry.However,there are still problems such as cumbersome preparation process,short life,and low catalytic conversion efficiency,which hinders industrial application.Therefore,the development of electrocatalysts and photocatalysts with high catalytic activity,strong stability and simple preparation is a breakthrough to solve the above problems.Various transition metal-based materials,including transition metals and their alloys,oxides,hydroxides,sulfides,nitrides,carbides,etc.,are widely used as the catalysts or cocatalyst for heterogeneous reactions due to their low cost and excellent catalytic properties.At the same time,catalysts with a porous structure can improve the specific surface area and the accessibility of catalytic active centers,and expose more catalytic active sites,thereby enhancing the catalytic performance.Based on the above analysis,this paper takes transition metal-based materials as the main research object and designs transition metal-based electrocatalysts,photocatalysts and photocatalytic coupled Fenton reaction catalysts with porous structures,which are applied to the electrocatalytic CO₂reduction process and degradation of organic dyes and phenolic pollutants in water.Furthermore,various characterization methods were used to analyze and study the composition,structure,catalytic performance and catalytic mechanism of the catalysts.The main research contents of this paper are as follows:（1）Self-supporting hollow fibers with different copper-tin alloy crystal phases were designed and fabricated to serve as catalysts for electrocatalytic CO₂reduction.The hollow fibers were formed by a phase inversion method and the polymer was removed by oxidation-reduction step,forming a large number of finger-like pores on the outer wall of the fibers.Dynamic comparison of different gas-inlet way proved that the finger-like pores,on one hand,can act as a CO₂gas diffuser and CO₂can press excessive electrolyte out of the pores,which help achieve excellent inhibition effect of hydrogen evolution reaction.On the other hand,the pores promote the solid-liquid-gas contact,providing more active sites.Different alloy crystal phases realized the tuning of the selectivity for carbon monoxide（CO）and formic acid production from hollow fibers,among which Cu-Sn 45%HF achieves over90%formic acid Faradaic efficiency under optimal potential conditions and has a 6 h stability,the Faradaic efficiency of the hydrogen evolution is significantly suppressed and its Faradaic efficiency is less than 4%.（2）ZnO microspheres with hierarchical porous structure（HPZ）were constructed by nano-component self-assembly method with transition metal oxide as substrate.The Zn O microspheres were doped with four transition metal ions,manganese,iron,cobalt and copper and used for visible light catalytic degradation of methyl orange（MO）.After doping the transition metal ions,the two-dimensional nanosheets on the surface of Zn O microspheres remained completely,and the transition metal ions were doped into the lattice.The effect of different transition metal doping on Zn O substrate was studied.It is found that the d-d transition occurs when the tetrachronal cobalt ions were doped into Zn O lattice,and three new light absorption bands and new energy levels were introduced.Cobalt-doping greatly improves the light absorption performance of Zn O microspheres in the visible region,and the absorption edge showed a sharp red shift,thus the band gap of the photocatalyst was reduced.In photocatalytic degradation experiment,the cobalt-doped hierarchical porous Zn O microspheres achieved the best degradation effect of MO.ESR tests and trapping experiments showed that hydroxyl radical and superoxide radical were generated on the catalyst,and hydroxyl radical was dominant.（3）Based on the Co-HPZ microspheres prepared in the previous chapter,a visible light catalytic system,Co-HPZ microspheres/g-C₃N₄（CZG）,with unique p-n-n heterojunction was constructed.The introduction of g-C₃N₄lamella further narrowed the band gap of the photocatalysts,improved the separation efficiency of photo-generated carriers and reduced the charge transfer resistance.The results of Mott-Schottky tests proved the p-n-n heterojunction characteristics of CZG composite photocatalyst.The results showed that the formation of the heterojunction can induce the internal electric field between the components of the catalyst and promote the separation of photo-generated electron-hole pairs,so that the photocatalytic performance was significantly improved.The optimized catalyst CZG-2 has a degradation rate of more than 96%for methyl orange within 1 h,and the cycle stability is significantly improved.By capture experiments and ESR tests,the main reactive oxygen species of the composite photocatalyst were proved to be hydroxyl radicals and the oxidation mechanism of CZG photocatalyst was proposed.（4）NH₂-MIL-53（Fe）/Ag@g-C₃N₄ternary heterojunction photo-fenton catalyst system was constructed by hydrothermal method for photo-Fenton oxidative degradation of phenol and methyl orange.NH₂-MIL-53（Fe）,a kind of metal-organic frameworks with iron as metal node,was introduced to promote the adsorption of pollutants on photo-fenton catalyst,greatly enhancing the visible light absorption and providing immobilized Fe species for Fe cycling in Fenton process.As an effective electronic conductor between NH₂-MIL-53（Fe）and g-C₃N₄,Ag can transfer photo-generated electrons fast and inhibit the photo-generated electron-hole recombination process.Under the optimal reaction conditions,FMAG-2 with the best performance can achieve complete degradation of methyl orange and phenol within 40 min and 100 min,respectively.Experimental results showed that the optimized photo-fenton catalyst,FMAG-2,effectively formed heterojunction inside with stable oxidative degradation activity,good thermal stability and low iron leaching amount.The degradation mechanism of FMAG-2 was clarified by liquid mass spectrometry analysis.According to the degradation intermediates presented by mass spectrometry,the degradation route of phenol was proposed.