Design Of Ag Modified Cr(Ⅵ) Nanocomposite And Their Performance For Photocatalytic Reduction Of Cr(Ⅵ)

In a water system,hexavalent chromium（Cr（Ⅵ））pollutants pose a serious threat to organisms.The photocatalytic technology that uses solar-chemical energy conversion can realize the reduction of Cr（Ⅵ）to low-toxic trivalent chromium Cr（III）.Due to its high efficiency,cleanliness,environmentally friendliness and other characteristics,it is a very promising method for the treatment of wastewater that contains Cr（Ⅵ）.Graphite carbon nitride（g-C₃N₄）is a new metal-free semiconductor material with good chemical stability,low preparation cost,suitable bandgap width（2.7eV）,visible light response-ability,which makes it a popular material in the field of photocatalysis.However,the photocatalytic activity of pure g-C₃N₄ is low due to its high carrier recombination rate and small specific surface area,so it is necessary to improve the photocatalytic performance by further modification.Among the many modification methods,the supported Ag-based cocatalysts such as single atom Ag,Ag nanoclusters（NCs）and Ag nanoparticles have attracted much attention.Ag nanoparticles have excellent light absorption due to the localized surface plasmon resonance（LSPR）,so it is expected to improve the photocatalytic performance of g-C₃N₄.Metal nanoclusters with precise structures and protected by organic ligands have attracted great interest due to their unique optical properties,environmental friendliness,biocompatibility,and excellent catalytic properties.It is a novel research aspect to load metal nanoclusters onto g-C₃N₄to form nanocomposites for catalysis.On the other hand,with the application of low-field nuclear magnetic resonance（LF-NMR）in the detection of paramagnetic ions,it is expected that LF-NMR will be applied to the process of photocatalytic reduction of Cr（Ⅵ）to Cr（III）to realize the evaluation of photocatalytic performance and make up for the detection deficiency of traditional UV-visible absorption spectrometer and atomic absorption spectrometer.The present study addresses this issue by loading Ag₉（H₂MSA）₇ and Ag₃₂（MPG）₁₉nanoclusters（NCs）onto graphitic carbon nitride（g-CN）using a simple impregnation method to form Ag₉NCs/g-CN and Ag₃₂NCs/g-CN hybrid nanocomposites.The performance of the prepared photocatalyst samples investigated for the photocatalytic reduction of aqueous Cr（Ⅵ）to Cr（III）under visible-light irradiation is systematically investigated.The proposed nanocomposites exhibit excellent optical absorption properties and efficient separation and migration of photogenerated electron-hole pairs.As a result,the Ag₉NCs/g-CN and Ag₃₂NCs/g-CN nanocomposites provide improved photocatalytic performances relative to that of pristine g-CN photocatalysts with reduction rate constants that are greater than that of pristine g-CN by factors of 2.3 and2.0,respectively.Ag/g-C₃N₄ nanocomposites with differing Ag content have been designed and prepared via a simple NaBH₄ chemical reduction method using the AgNO₃ as metal nanoparticle source and g-C₃N₄ as support material.The performance of Ag/g-C₃N₄photocatalysts with different loadings investigated for the photocatalytic reduction of aqueous Cr（Ⅵ）to Cr（III）under visible-light irradiation is systematically investigated.The results showed that the addition of Ag cocatalysts（loading amount:1 wt%,2 wt%,5 wt%,and 10 wt%）exhibited significantly enhanced photocatalytic performance of g-C₃N₄ photocatalysts,while the optimal content of Ag cocatalysts was determined to be5 wt%,which is greater than that of pristine g-C₃N₄ by factors of 4.0.Meanwhile,operando low-field NMR（LF-NMR）technology was used to quantitatively study the photocatalytic reduction of Cr（Ⅵ）in a real solid-liquid reaction environment.We for the first time demonstrate the viability of employing operando low-field NMR relaxometry for monitoring the photocatalytic Cr（Ⅵ）reduction performance by quantifying the concentration of paramagnetic Cr（III）ions in solution using the transverse relaxation times（T₂）.