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Theoretical Study On Electronic Structures Of Polyoxometalate/g-C3N4 Composites

Posted on:2019-11-02Degree:MasterType:Thesis
Country:ChinaCandidate:Q WangFull Text:PDF
GTID:2371330563953625Subject:Materials Physics and Chemistry
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Graphitic-carbon nitride?g-C3N4?is the most stable allotrope of carbon nitride and has attracted increasing attentions.As a new catalyst,g-C3N4 is regarded as a promising nonmetallic catalyst due to a variety of advantages such as an adjustable band gap,easily-prepared by cheap feedstocks and good physico-chemical and thermo stability.However,the band gap of g-C3N4 is not narrow enough to absorb visible light,which dramatically limits its application in photocatalysis.As a result,decreasing the band gap of g-C3N4 has become an important research direction.Polyoxometalates?POMs?are a unique class of metal-oxygen clusters,which are usually formed by early-transition elements in combination of oxygen atoms.POMs functionalized g-C3N4 nano-composites as catalysts have recieved many attentions.Although there are several experimental studies in this field,the corresponding theoretical studies are relatively rare due to the complex electronic properties of POMs/g-C3N4 composites.Using first-principle method to investigate the electronic structures and catalytic abilities of POMs/g-C3N4 at micro level has great significance for deep understanding the catalytic mechanism and providing valuable informations for experiment.In this thesis,first-principle methods based on density functional theory?DFT?have been carried out to explore the geometric and electronic structures and optical properties of Lindqvist-type polyoxometalates functionalized g-C3N4 and mechanistic insights into CO2 reduction on POMs/g-C3N4.The work focuses on the following two aspects:1.The geometric and electronic structures of nano-composites based on Lindqvist-type polyoxometalates?POMs?and monolayer graphitic carbon nitride?g-C3N4?were investigated by first-principles method.We investigated the most stable adsorption site on the g-C3N4 surface and the electronic structures ofLindqvist-type POMs functionalized g-C3N4,including band structures and density of states?DOS?.Based on the conduction band?CB?and valence band?VB?levels of Lindqvist-type POMs,H2WnMo6-nO19?n=0,1,2,3,4,5,6?are suitable to generate composites with g-C3N4,whichare expected for electron and hole transfer between POMs and g-C3N4.The results indicate that the bandgap of g-C3N4 composing with Lindqvist POMs obviously decreases comparing to pristine g-C3N4 and the charge transfer is from g-C3N4 to POMs which could enhance the photocatalytic ability.Moreover,theabsorption strength of W6O19/g-C3N4 obviously increases in visible light comparing with g-C3N4.2.DFT calculations were carried out to investigate the insight mechanism of transition-metal substituted polyoxometalates functionalised carbon nitride?g-C3N4?.The geometric and electronic structures of POMs/g-C3N4 were first calculated to show the significantly change in basic properties compared to pristine g-C3N4.The results indicate that the band gap of composite decreases and the frontier molecular orbitals distributions are obviously changed due to the introduction of POMs.Moreover,two possible mechanisms for CO2reduction to CO were studied in details.The results indicate that the energy barriers of two mechanisms are reduced with the introduction of POMs.The hydrogenation-dissociation path is considered the optimal mechanism for CO2 reduction to CO at both pristine g-C3N4 and POMs/g-C3N4 due to lower energy barrier.The calculated energy barriers suggest that the energy barriers decrease with the introduction of POMs,and the substituted transition metal atoms of POMs,including Co,Mn and Fe play important role in reducing energy barrier of CO2 reduction to CO to enhance catalytic ability.What's more,the calculated absorption spectra indicate that the absorption peaks of POMs/g-C3N4 obviously red shift and adsorption strength also is enhanced in visible light region compared to pristine g-C3N4.
Keywords/Search Tags:Graphitic Carbon Nitride, Density Functional Theory, Electronic Structure, Photocatalysis, Polyoxometalates
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