Fenton-Like Oxidation Of As(Ⅲ)Over Ce-Ti Binary Oxide:Performance,Mechanism And Activity Tuning

Arsenic pollution in groundwater is a long-term environmental issue of global concern.The dominant species of As in groundwater is usually As（III）,which is more toxic and mobile than As（V）.Furthermore,being an electroneutral molecule under natural p H conditions,As（III）is difficult to be removed by conventional treatment techniques.Therefore,it is regarded as an effective pretreatment strategy to oxidize As（III）to As（V）.H₂O₂ is a mild and benign oxidant with green oxidation process free of harmful byproducts generation.Nevertheless,the homogeneous oxidation of As（III）by H₂O₂ is efficient only at alkaline p H（above 9）.To overcome such limitation,it is particularly urgent to develop highly efficient Fenton-like catalysts.Ce O₂ typically induces a non-radical Fenton-like oxidation of As（III）,which enabled a high efficiency of stoichiometric H₂O₂ utilization.Whereas,on account of the lower catalytic activity of Ce O₂,it is still necessary to deveolop more efficient Fenton-like catalysts for As（III）oxidation and the corresponding activity-tuning strategies.Ce-Ti binary oxide has shown superior activity over Ce O₂ in many catalytic processes such as photocatalysis and gas-phase catalytic reactions.However,the performance and mechanism of Fenton-like oxidation of As（III）catalyzed by Ce-Ti binary oxide are still unclear,and the corresponding activity-tuning methods remain to be established.A series of Ce_xTi_1-xO₂ catalysts（including Ce O₂ and Ti O₂）with 10 different Ce/Ti molar ratios were prepared via co-precipitation followed by calcination.The fabricated materials were characterized by BET,XRD,XPS,SEM-EDS,TEM-SAED,and STEM-EELS,while their catalytic performances for Fenton-like oxidation of As（III）were investigated.The results showed that both the catalytic activity and the surface area normalized catalytic activity exhibited a volcano-shape dependency on Ce molar fraction（x）,peaking at Ce_0.25Ti_0.75O₂（x=0.25）,which showed 6.32 or 6.36 times higher activity and 2.94 or 2.67 times higher specific activity compared with Ce O₂ and Ti O₂.The catalytic activity increased with the amorphous degree of the materials as shown in their XRD patterns,and the optimal catalyst Ce_0.25Ti_0.75O₂ was completely amorphous.Moreover,Ce_0.25Ti_0.75O₂ featured a wide applicable p H window of 3-9 and a high efficiency of stoichiometric H₂O₂ utilization（99.1%）.The catalytic activity was still almost 100%after five runs showing no noticeable decline,demonstrating the reusability of the Ce_0.25Ti_0.75O₂ catalyst for sustainable utilization.The mechanism of catalytic Fenton-like oxidation of As（III）over Ce_0.25Ti_0.75O₂was proved to be a non-radical interfacial process through experiments including radicals scavenging,enzymatic scavenging,pre-oxidation experiment,as well as DR-UVS spectroscopy and XPS study.Moreover,the predominant oxidant species for As（III）oxidation in the Ce_0.25Ti_0.75O₂/H₂O₂ system were the surface complexed hydroperoxo species（≡M-OOH）anchored on the Ce or Ti atoms.The effects of the initial As（III）and H₂O₂ concentrations on the catalytic oxidation rate were investigated,confirming that the Fenton-like oxidation of As（III）over Ce_0.25Ti_0.75O₂ observed the Langmuir–Hinshelwood mechanism.Under the Langmuir–Hinshelwood mechanism,the microkinetic model for the catalytic oxidation was established to elucidate the quantitative relationship between catalytic activity and adsorption energies for bimolecular chemisorption reactions.A catalyst with identical adsorption energies towards both chemisorbed reactants tends to have the highest activity.Otherwise,the catalytic activity of the catalyst would decline by one order of magnitude for every increase of the difference in bimolecular adsorption energies(ΔE_ad)by 0.0592 e V at298 K.Through density functional theory（DFT）calculation,the highest activity of Ce_0.25Ti_0.75O₂ was rationally attributed to the subequal adsorption energies towards As（III）and H₂O₂(ΔE_ad=0.01 e V).Based on the projected density of states（PDOS）analysis of the bonding configuration in the Ti-O-As and Ti-OOH linkage anchored on Ce_0.25Ti_0.75O₂,the delocalized bonding between Ti 3d and O 2p shifted to a lower energy region on Ce_0.25Ti_0.75O₂ with the corresponding adsorption energy significantly increased in comparison with anatase and amorphous Ti O₂,thus improving the catalytic activity.The stabilized adsorption could be explained by the deficient coordination numbers and the accompanying oxygen vacancies induced by Ce doping and amorphizations.This study provides a potent strategy to tune the catalytic activity of bimolecular chemisorption reactions,shedding new light upon the design principle for efficient environmental catalysts.