Tungstosilicate Modified Porous Carbon Nitride For Photocatalytic Production Of Hydrogen Peroxide

Hydrogen peroxide（H₂O₂）is an important oxidant,which has been widely used in energy technology,biological engineering and environmental remediation because it only generates pollution-free water in the process of oxidation.Oxygen reduction reaction（ORR）is the most studied reaction in photocatalytic H₂O₂production.In ORR reaction,H₂O₂can be generated through two routes:one is the two-step 1e^–reduction route,also known as the indirect reduction process（O₂→·O₂^–→H₂O₂）;The other is a one-step 2e^–reduction route,also known as the direct reduction process（O₂→H₂O₂）.In this thesis,three tungstosilicate modified porous carbon nitride composites were designed and prepared to improve the photocatalytic production efficiency of H₂O₂.The specific research contents are as follows:1.Three kinds of tungstosilicate modified ultra-thin porous carbon nitride（PCN）composites(SiW₉/PCN,SiW₁₁/PCN,SiW₁₂/PCN)were successfully prepared by impregnation method.SEM and XPS characterizations showed that polyoxometalates were successfully modified onto PCN surface.Photocatalytic H₂O₂production showed that under visible light（λ≥420 nm）the yield of H₂O₂over different catalysts within 6 hours is as follows:SiW₁₁/PCN（215μM）>SiW₉/PCN（114μM）>PCN（107μM）>SiW₁₂/PCN（74μM）,indicating that both SiW₉/PCN and SiW₁₁/PCN have enhanced photocatalytic activity for production of H₂O₂than PCN.SiW₁₁/PCN has the highest activity,and the yield of H₂O₂is twice that of PCN.H₂O₂decomposition experiment showed that the decomposition rate constant k of different catalysts is:SiW₁₁/PCN(0.3043 h^-1)9/PCN(0.3129 h^-1)-1)12/PCN(0.587 h^-1),which indicated that SiW₁₁/PCN and SiW₉/PCN can effectively inhibit the decomposition of H₂O₂.The radical（·O₂^–）capture experiment and electrochemical rotating disk electrode test（RDE）were used to analyze the mechanism of photocatalytic H₂O₂production.The results showed that the average number of transferred electrons in the ORR reaction over SiW₉/PCN and SiW₁₂/PCN was 1.51 and 1.93,respectively.Combined with the results of free radical capture experiment,it is proved that SiW₉/PCN is dominated by a two-step single electron transfer process,that is O₂generates H₂O₂through the indirect reduction route（O₂→·O₂^–→H₂O₂）,while SiW₁₁/PCN is dominated by a one-step two-electron transfer process,that is O₂generates H₂O₂through the direct reduction route（O₂→H₂O₂）.2.In the previous section,SiW₉/PCN has photocatalytic activity for H₂O₂production,and the H₂O₂generation rate is 1.2μmol L^-1min^-1higher than that of SiW₁₁/PCN(1.1μmol L^-1min^-1)in the first hour,indicating that SiW₉/PCN is more efficient in photocatalytic production of H₂O₂.In order to further improve the photocatalytic activity of H₂O₂production to change the selectivity of 2e^-ORR process,SiW₉was modified on the spongy porous carbon nitride（Pg-C₃N₄）surface by impregnation-calcination method to obtain SiW₉/Pg-C₃N₄photocatalyst.TPC and EIS tests showed that the composite has excellent photoelectric chemical properties.Under simulated sunlight,the photocatalytic H₂O₂yield of SiW₉/Pg-C₃N₄reaches 1600μM within 120 min,which is 2.9 times of SiW₉/Pg-C₃N₄（impregnation）（546.6μM）and 3.6 times of Pg-C₃N₄（451.56μM）,8.64 times of g-C₃N₄（185.14μM）and 57 times of SiW₉（28.09μM）,respectively.The photocatalytic H₂O₂decomposition experiment showed that the decomposition rate constant k of SiW₉/Pg-C₃N₄(0.0214 min^-1)was less than that of Pg-C₃N₄(0.0238 min^-1),indicating that modification of Pg-C₃N₄by SiW₉could effectively inhibit H₂O₂decomposition.Cyclic experiments showed that SiW₉/Pg-C₃N₄had good stability.The transfer electron number of SiW₉/Pg-C₃N₄is 1.26by·O₂^–radical capture and RDE test.Compared with SiW₉/PCN n=1.51,the optimized SiW₉/Pg-C₃N₄can effectively improve the selectivity of indirect 2e^-ORR process.