Catalytic Reduction Of Nitrogen-containing Aromatic Compounds In Aqueous Solution Over Functionalized Gold Nanoparticles/Carbon Based Composites

With the fast growth of economic industry and human production,abundant pollutants are discharged into environmental water,menacing the human health and environmental safety.Among the various pollutants,nitrophenols and azo dyes as typical representative of nitrogen-containing aromatic compounds have caused cause serious water pollution.Hence,it is of great significance to develop an efficient,simple and cost-effective method to remove these pollutants from water body.Catalytic reduction is an efficient,simple m ethod with good economic effectiveness.On the one hand,it can reduce the harm of nitrophenols and azo dyes,decolorize the wastewater.On the other hand,it can achieve certain economic benefits from the reaction products.The method requires a catalyst to act as an electron shuttle to stimulate the interaction between the reducing agent（electron donor/hydrogen donor）and the pollutant（electron acceptor/hydrogen acceptor）to achieve reduction.Hence,appropriate catalyst is vital for the catalytic reduction in pollutants removal.At present,the catalysts using gold nanoparitcles（Au NPs）as active sites and carbon based materials as supports show great prospects in the treatment of nitrogen-containing aromatic compounds in catalytic reduction technology.Thereinto,Au NPs possess thousands of low coordination number atoms on the surface,which makes it show unusual properties and differ from macroscopic metals,exhibiting super high catalytic activity.However,the high specific surface energy of Au NPs makes it easy to agglomerate and thus decrease the catalytic activity.Through fixing Au NPs on the carbon based materials with cheap and abundant resourc es properties,the catalytic activity of Au NPs can be utilized to a great extent,and thus realizing high catalytic activity by the synergistic effect of Au NPs and carbon based materials.Herein,two types of carbon based materials with cheap and widely available properties,activated coke and carbon black,were chose as supports to prepare a series of functionalized Au NPs/carbon based composites.The prepared composites were used in catalytic reduction to reduce nitrophenols and azo dyes.The performance and catalytic mechanism were deeply studied to guide the development of simple,efficient and cost-effective catalytic reduction technology.The main research contents are as follows:（1）In the first part,,Au/AC composites were synthesized via a one-step mild and simple reduction method,in which Au NPs were served as active sites and activated coke（AC）as a support,for catalytic reduction of nitrophenols and azo dyes.In detail,the effect of different Au NPs loading amount,catalyst dosage and Na BH₄dosage,as well as the stability and renewable properties of the composites were investigated.The catalytic mechanism was analyzed by exploring the kinetics of catalytic reaction and combining the characterization results.The catalytic experiments show that the Au/AC composites with average Au NPs size of 14.9±10.8nm exhibit well activity for 4-NP reduction,and the normalized reaction rate constant k_nor is 0.1277 min^-1·mg^-1.For other nitrogen-containing aromatic compounds,the catalytic efficiency of Au/AC composites follows as the order:4-NP>Methyl Orange（MO）>3-nitrophenol（3-NP）>2,4-dinitrophenol（2,4-DNP）>Eriochrome Black t（EBT）>Congo Red（CR）.In addition,Au/AC also exhibited good stability,and the catalytic efficiency remained 84%after 6 cycles.The good catalytic activity of Au/AC is mainly attributed to abundant oxygen-containing groups on the surface of AC,which can benefit Au NPs anchoring and prevent their agglomeration,so as to achieve a good catalytic activity.In additio n,the large porosity and specific surface area of AC benefit Au NPs anchoring on AC surface,avoiding the block by Au NPs,thus ensuring good adsorption ability and accelerating the catalytic reaction.（2）Inspired by the research in first part,to furthe r research the effect of surface property of carbon based materials on Au NPs supporting and their variation of catalytic performance,steam activation was used to improve the surface property of carbon black（CB）,which can bring oxygen-containing groups,adjust the channel structure and specific surface area.Then the steam activated CB（CB）was used a support for Au NPs anchoring through a one-step mild and simple reduction method.The analysis of surface property of CB on Au NPs anchoring was discussed and the comparison with previous studies was also analyzed.The experimental results show that the catalytic reduction of 4-NP over Au/SCB composites with average Au NPs size of～65 nm（a small part of Au NPs size of～5 nm）display the normalized reaction rate constant k_nor of 0.2192 min^-1·mg^-1.It also exhibits good catalytic activity for other nitrogen-containing aromatic compounds（2-NP,3-NP,2,4-DNP,MO,CR,EBT）.Combined with characterization analysis,it can be found that steam activation brings more defect structures,larger BET specific surface area,more pore structures and abundant oxygen-containing groups to SCB surface.The improvement of these surface properties is conducive to Au NPs stably anchoring,thus ensuring the effective active sites for catalytic reaction.In addition,steam activation may enhance the hydrogen adsorption capacity of the composites,thus acceler ating the generation of active hydrogen species.（3）Inspired by the above study,it can be found that AC as cheap and readily obtained carbon based materials is an excellent support for Au NPs,but the catalytic activity of Au NPs/carbon based composites needs to be further improved.In the third part,Au nanoclusters（NCs）/SO₃H-AC composites with high catalytic activity were prepared by combining ultra-small Au NCs（～2 nm）and-SO₃H modified AC composite.Taking 4-NP as the target pollutant and Na BH₄ as the reducing agent,an efficient catalytic reduction system was constructed.The catalytic performance of Au/SO₃H-AC composites was investigated under different p H conditions and combined pollution solution.The normalized reaction rate constant k _nor for 4-NP reduction is 0.3317 min^-1·mg^-1,which is much higher than that of Au/AC(0.1277min^-1·mg^-1)and Au/SCB(0.2192 min^-1·mg^-1)in first ans second part.The high catalytic activity is mainly attributed to the functionalization of SO ₃H.It makes the surface of So₃H-AC electronegative,which can adsorb Au³⁺in the synthesis process,and thus benefiting Au NCs immobilization.Moreover,the Au NCs anchoring on SO₃H-AC possess high dispersion（58.52%）and small particle size（～2 nm）,which can expose more active sites.In addition,Au/SO₃H-AC with electronegative due to the modification of SO₃H can effectively promote the adsorption of positive pollutant4-NP and desorption of negative product 4-AP.Meanwhile,Toxicity Estimation Software was used to prove that 4-NP can be transformed into low-toxicity or non-toxic 4-AP after reduction over Au NCs/SO₃H-AC.（4）In this part,considering the perspective of green chemistry,chitosan（CTS）was chose as a green reducing agent to realize an in-situ reduction without adding other chemical agent for preparing Au NPs/CTS/AC composites.CTS with abundant-OH and-NH₂ could act as tertiary functionalized agent ro reducing and stabilizing Au NPs,and adhesiving Au NPs and AC.Meanwhile,it can stabilize the oxidized Au species(Au⁺and Au³⁺),thus providing an excellent functional unit for electronic relay and hydride transfer unit.Electron paramagnetic resonance（EPR）spectra were used to used to explore the mechanism of 4-NP hydrogenation over Au NPs/CTS/AC.To better simulate the catalytic hydrogenation of 4-NP in real wastewater,the effect of environmental water samples has been taken into discussion.The p H,dissolved oxygen（DO）,and inorganic salt in different water samples were discussed in detail.In addition,the recyclability of Au NPs/CTS/AC was also studied.In addition,the intermediates of azo dyes after catalytic reduction were analyzed by liquid chromatography-mass spectrometry（LC-MS）.The results show that Au NPs/CTS/AC exhibit well catalytic activity with the normalized reaction rate constant k_nor of 0.1399min^-1·mg^-1,and possess well recyclability.After 6 cycles,the efficiency can remain90%in the same reaction condition.More importantly,the Au NPs/CTS/AC can also show well activity in the catalytic reduction of 4-NP in actual water samples.It was proved that the active species in the Au NPs/CTS/AC catalytic reduction system is the Au-H intermediate formed after the active site Au dissociating B-H,and then attacks the-NO₂ or-N=N-.LC-MS results show that the reaction pathway of pollutants is that-N=O is firstly split or-N=N-dissociated,and then forming products with-NH₂.