Preparation Of Cobalt-containing Catalyst With Graphene As Support And Their Utilization For PMS Activation In Acid Orange 7 Degradation

Recently sulfate radical(SO₄^-·)-based advanced oxidation processes（SR-AOPs）have attracted increasing attention in the field of environmental catalysis.The radical with high redox potential is a powerful oxidant for degradation of recalcitrant organic pollutants.Peroxymonosulfate（PMS）activation using cobalt containing materials as heterogeneous catalysts（Co/PMS）is an efficient way to produce reactive sulfate radicals.Beside the way is less complex and easy to recycle,it can be effective for wide range of pH.Graphene with many unique properties such as ultra large specific surface area,excellent electrical conductivity and extraordinary mechanical strength could be an ideal support for catalyst to improve its performances in PMS activation.In this study,we combined the good catalytic properties of cobalt catalysts and the excellent features of the reduced graphene oxides（rGO）together to develop three heterogeneous cobalt-cotaining catalysts,and successfully applied them for the removal of Acid orange 7（AO7）from aqueous media.Synthesis,characterization,PMS activation,stability and possible catalytic activation mechanism of graphene supported cobalt catalyst were systematically discussed as follows:（1）Co（OH）₂/r GO was prepared and have good performance in AO7 degradationGO was first prepared from natural graphite according to the Hummers method.Cobalt hydroxide-reduced graphene oxide composites（Co（OH）₂/r GO）was then prepared through a simple one-step hydrothermal synthesis method with hydrazine hydrate as the reducer.The structure and morphology of the catalyst were systematically investigated by X-ray diffraction（XRD）,Raman spectroscopy,transmission electron microscopy（TEM）,energy dispersive X-ray spectroscopy（EDS）and energy-dispersive X-ray spectrometer（XPS）.XRD and TEM results showed that amorphous hydrotalcite α-Co（OH）₂ was distributed on rGO.TG data showed the catalyst with a good thermal stabilities have about 26.87wt% Co（OH）₂ in it.The catalytic performance in activation of PMS for AO7 degradation in aqueous solution was investigated.Experimental results showed that Co（OH）₂/rGO exhibits a high catalytic activity in the degradation of AO7 while little dye adsorption was observed.100% decoloration of 70 mg/L AO7 can be achieved in less than 6 min with 0.05 g/L catalyst and 0.2 mmol/L PMS.Solution p H greatly affect the degradation rate and optimum decoloration of AO7 can be reached at p H= 5.Result of TOC experiment indicated a higher mineralization degree with increasing PMS dose and the reached at maximum when [PMS]/[AO7]≥ 60.The stability of Co（OH）₂/rGO hybrids was obtained in multiple runs.Dye degradation efficiency and time（20 min）were found to be identical up to 3rd recycling runs,thus indicated the good catalytic stability performance.Little Co²⁺ dissolved under neutral condition.Furthermore,AO7 degradation rate in Co（OH）₂/r GO/PMS is faster than in Co²⁺/rGO/PMS,demonstrate the heterogeneous catalysis process.Study on the prepare condition of the catalyst show that,highest catalytic activity could be achieved when the quantity of hydrazine is 7/10 times of the graphite oxide.The best synergistic catalysis between r GO and Co（OH）₂ is observed when graphite oxide and cobalt nitrate are equal in concentration.Longer heating time in catalyst preparation have little effect on either structure or catalytic activity.（2）CoMn₂O₄/rGO was prepared and used for AO7 degradationReduced graphene oxide embedded with bimetallic nano particles of cobalt-manganese oxide（CoMn₂O₄/rGO）were fabricated by solvothermal method use graphite oxide as the precursor.The obtained product was characterized and applied for the heterogeneous activation of PMS to degrade AO7 for the first time.The characterization results revealed that 10-20 nm nanosized CoMn₂O₄ were homogenously decorated on the surface of r GO.The loading of CoMn₂O₄ in the CoMn₂O₄/rGO material is about 62 wt%.CoMn₂O₄/rGO demonstrates a very efficient catalytic performance for the degradation of AO7.In a system containing 2 mM PMS and 0.05 g/L CoMn₂O₄/rGO,100% conversion and 43% mineralization of 70 mg/L AO7 could be achieved within 12 and 60 min respectively.Moreover the catalyst exhibited stable performance after five rounds with low cobalt leaching.The effect of different CoMn₂O₄ loading showed that catalyst with a CoMn₂O₄ contents higher than 20% perform stronger capability for catalytic degradation of AO7 compared to pure CoMn₂O₄,indicating a synergistic effect between graphene and CoMn₂O₄.Recycling experiment along with XRD data demonstrate good stability of the catalyst for five successive runs.Inhibition confirmation results suggest that surface bound SO4·-and HO· radicals played a key role in degradation of AO7 while free radicals in the bulk liquid have little contribution for the AO7 removal.Based on the XPS data of the catalyst before and after use,mechanism of radical’s generation was proposed.Beside,main degradation intermediates of AO7 were identified by GC/MS and a proposed degradation pathways were elucidated.（3）Co₃O₄/rGO/TOCNs was prepared and used for AO7 degradationIn order to improve the strengthen of the catalyst,and make the recycle process easily.O xidized cellulose（TOCNs）and Co₃O₄/rGO were separately prepared by TEMPO method and solvothermal method.They were then doped into each other by inorganic/organic blending modification to prepare the new composite catalyst Co₃O₄/rGO/TOCNs.The prepared materials with a diameter of about 5 mm were characterized by XRD,FT-IR,SEM and TEM.The results show that the Co₃O₄/rGO/TOCNs composite catalyst has been successfully prepared and the nanometer Co₃O₄ well-distributed on the support.The degradation test showed that operational parameters such as catalyst amount,oxidant amount,p H,and temperature greatly affected the degradation rate of AO7.Complete decoloration can be achieved within 6 minutes under a best condition and catalyst can quickly precipitated after reaction.The degradation kinetics was investigated and activation energy of Co₃O₄/rGO/TOCNs/PMS is lower than that of the traditional Fenton oxidation technology.The catalyst exhibited a stable performance for 6 rounds of regeneration,AO7 degradation can be finished within 12 min in the sixth round and the cobalt dissolution kept under 0.05 mg/L during the recycling process.The effect of inorganic ions on degradation of AO7 in Co₃O₄/rGO/TOCNs/PMS system showed that H₂PO₄^-promoted the degradation of AO7;HCO₃^-showed a positive effect at low concentration but a negative effect on AO7 degradation at high concentration;while C l-have an opposite trend with a negative at low concentration but a positive at high concentration;the NO₃^-had a slight inhibitory effect on the degradation rate of AO7.