An Insightful Investigation Into Catalytic Applications Of Graphite-like Carbon Nitride-based Materials In Photodegradation Of Environmental Pollutants And CO₂ Immobilization

Rapid and continuous development of domestic economy leads to excessive consumption of fossil energy as a main origin of energy shortage and environmental pollution to directly affect sustainable development of economic society.Therefore,energy and environment-related issues have been research hotspots.Clean and effective photocatalysis technology has trigger keen interest among scholars.Its centers at design and preparation of a highly active and stable photocatalytic material.Semiconductors such as g-C3N4,TiO2,CexZr1-xO2 and ZnxCd1-xS are cost-effective,easily available,structurally unique and functionally stable.Therefore,they are widely used in photogradation of environmental pollutants,photocatalytic H2 evolution and CO2 photoreduction.At present,they still suffer from some drawbacks such as low light-responsive level,prompt exciton recombination and undesirable reuse,which is far from desirable for practical application.Therefore,these materials were modified to access modified materials above including P or B doped g-C3N4/Zn0.8Cd0.2S(PCN/ZS or BCN/ZS),morphologically functionalized Ce0.4Zr0.6O2(CZO)solid solutions and Ru/TiO2.Their photocatalytic performances,physicochemical properties such as micostructures,morphologies and optical properties,chemical stability and catalytic mechanisms were studied.To begin with,a two-step method was applied to access PCN/ZS.XRD,DRS,FT-IR,PL,TEM-EDS,EDS and XPS were used to provide structure information of the as-prepared samples involving the formation of the ZS solid solution,the existence of the heterostructure between PCN and ZS,the graphite-like structure of PCN and substitution of P for carbons in the framework of CN.The comparative analysis in a whole array of MB(10-4mol/L)degradations indicates catalytic activity D(degradation efficiency,%)and rate constant k(the first order)of the composite increase up to 91.3%and 0.0203 min-I at y≤0.5wt%and go down at y>0.5wt%.The optimal catalyst PCN/ZS-0.5 exhibits enhanced photocatalytic performance(91.3%)compared to pure PCN(21.4%)and ZS(56.1%).The occurrences are attributed to favorable transport and separation efficiency of photoinduced charge carriers through the construction of the heterojunction originated from in situ growth of the ZS solid solution on PCN.In addition,PCN/ZS-0.5(91.3%)proves more photocatalytically active than that of CN/ZS-0.5(61.4%),implying P doping works well in elevated photocatalytic activity.Next,calcination of a homogeneous mixture of boric acid(B source)and urea(N source)yielded BCN,on which ZS was precipitated to obtain BCN/ZS.The layered structure of BCN,formation of ZS solid solution,B substutution for carbons in CN framework as strong L acid sites and formation of heterostructure between BCN and ZS are firmly confirmed by means of XRD,TEM-EDS,XPS,DRS,FT-IR,PL,EDS and XPS,which provided information on morphology,component and physicochemical properties.In an assay of aqueous MB(0.1 mM),the optimal catalyst BCN/ZS-0.25(84.3%)is above pristine BCN(48.1%)and ZS(56.1%)in terms of photocatalytic function,which benefits from surpressed recombination rate of photoinduced charge carriers through the construction of the heterojunction between BCN and ZS.The promoting effect by B doping on photocatalytic activity is exemplified by raised function of BCN/ZS-0.25(84.3%)relative to that of CN/ZS-0.25(70.0%).It is well-rationalized by accelerated excitonic transfer and separatation via B doping as a strong Lewis acid site.Photocatalytic mechanism indicated the accelerated transport and separation of photoinduced excitons at heterojuction.Thus,BCN modification made ZS more active and stable.Moreover,mesoporous CZO solid solutions with four different shapes including mixed nanorod and nanoparticle(RP,cubic),nanopolyhedron(PH,tetragonal),nanosphere(SP,cubic)and nanoegg(EG,tetragonal)were hydrothermally fabricated.Various physical and chemical techniques including XRD,Raman,EDS-mapping,SEM,TEM,XPS,N2 sorption,NH3 and CO2-TPD,DRS and PCR(photocurrent response)are used to reveal shape-dependent crystallographic,electronic and surface textures.Photocatalytic activity towards MB degradation(0.01 mM)followed the order:RP(70.5%)>EG(52.8%)>SP(47.1%)>PH(29.2%).RP with the top activity benefits from prompt separation of excitons and suitable textural property for MB absorption and activation.Kinetic analysis suggests MB degradation over samples as the first-order process.Also,physical mixture of CN with RP(CN-RP)with different mass ratio as a substitute of RP with equal quality as catalysts for aforementioned MB degradation.The optimal catalyst CN-RP with 20wt%CN in the mixtures(70.7%)was 1.6 times as active as that of CN(44.8%)due to some cooperation mechanism.Finally,Ru/TiO2 composites were fabricated through NaBH4 reduction.They are characterized by XRD,TEM,XPS,DRS and PL.A successive red shift of DRS curves with Ru loading is clearly observed in DRS image due to surface plasmon resonance(SPR)effect of Ru.Nevertheless,Ru loading is a double-edged sword for separation of excitons(PL results),leading to an increase in separation efficiency at x<1.0wt%and a decrease in it with Ru overloading.The occurrence is possibly associated with expanded efficient separation interface at low Ru loading or development of new recombination centers for charge carriers due to Ru overloading.The yields of CH4(main product),CO and C2H6(by-products)from CO2 methanation follow the trend of initial increase and subsequent decrease with x and reach their peaks of 59.3,10.1 and 6.6 μgmol g-1 h-1 at 1.0wt%Ru loading.The occurrence is closely related with coincidently enhanced light response and splitting efficiency of charge carrier at low Ru loading and with the further improved former and the promptly reduced and dominating latter.However,textural property such as BET surface area(SBET)play a minor role in photocatalytic performance.CN-RT(a physical mixture of CN and 1.0-Ru/TiO2)equal to pure RP in mass photocatalyzed CO2 methanation in the identical condition.CN-RT with 30wt%CN,the best promotor among mixtures,provide superior CH4 yield(62.2 μmol g-1 h-1)to that of pure CN(trace),possibly associated with synergetic mechanism that favor exciton splitting and adsorption and activation of CO2 and H2.