Synthesis Of Metal Composite Oxides And Their Applications In Environmental Catalysis

In recent decades,the deteriorating environmental issues,especially water contamination and air pollution,have triggered worldwide concerns.Hence,it is necessary to develop effective advanced techniques to remove pollutants either from wastewater or from gas phase.Advanced oxidation processes(AOPs),based on the formation of highly reactive species such as hydroxyl radicals(HO·)and sulfate radicals(SO4·-),have gained popularity due to their high oxidative capacity allowing to degrade wide variety of refractory chemical molecules in wastewater.Compared with conventional oxidant H2O2,ozone is also a good source of hydroxyl radicals.Meanwhile,the peroxymonosulfate(PMS,HSO5-)with similar structure of H2O2 can be decomposed to form sulfate radical,another kind of highly active species that can degrade organic contaminants.Classical ozonation or peroxymonosulfate catalytic system based on transition metal ions redox cycles(Fe2+,Cu+,Co2+,etc.)usually suffers from some intrinsic drawbacks,such as limited pH and potential problem of secondary pollution.To resolve these issues,heterogeneous catalytic process were applied for the removal of organic pollutants.Similar problematics are identified for gas phase depollution.Though adsorption processes are widely used in industrial plants to remove volatile organic compounds(VOCs)from exhaust gases,the relatively short lifetime of adsorbent or requirement of adsorbent regenetation is also a problem.In contrast,heterogeneous catalysis is proposed as an efficient and economically viable process for the removal of VOCs by complete combustion into CO2 and H2O.The search for low-cost catalysts based on readily available elements has brought the materials such as spinels and perovskites to the forefront.Spinel type oxides with a general formula AB2O4,and perovskite with a general formula ABO3(where A and B are metal ions),have received particular attention due to their availability and low cost.Some properties such as numerous compositions,valence states,morphologies and surface defects have demonstrated that their good catalytic performances in various research fields(i.e.energy,automotive depollution).Therefore,application of these oxides to efficiently activate ozone or PMS in water or for heterogeneous catalytic oxidation of toxic hydrocarbons in air is necessary.In this work,two kinds of spinels(CuAl2O4 and CuFe2O4)were evidenced as active catalysts for ozone and peroxymonosulfate activation into reactive oxygen species to degrade organics in water respectively,while the perovskites(raw LaMnO3 and its modified oxides)were first studied to oxidize formaldehyde(HCHO)in gas totally at mild temperatures.This PhD manuscript is constituted of several experiment results:(1)CuAl2O4 based mixed oxides were used as heterogeneous catalysts for ozone activation to degrade Acid Orange 7(AO7)in aqueous solution.The solids were thoroughly characterized by SEM/EDS,N2 phy si sorption,XRD,FTIR,Pyridine-FTIR,TEM and XPS.We demonstrated that the solid precursor calcined at 300℃ exhibited the best catalytic ozonation activity with respect to CuAl2O4 spinel phase obtained at higher temperatures.Such performance was attributed to the better textural properties and a higher density of active sites(hydroxyl groups and Lewis acidity).Then,we proposed that such catalytic performance was related to a synergistic function between ≡Cu2+ and ≡Al3+,which took part of a mechanism of radical formation.In such mechanism,present ≡Al3+ could act as a reservoir for surface active sites such as hydroxyl groups and Lewis acid sites,while ≡Cu2+ could provide the possibility of electron transfer with ozone for the enhancement of radical generation.Finally,the surface adsorbed HO· and few O2·-were determined to be the major reactive species during this oxidation process.(2)The removal of bisphenol A(BPA)in aqueous solution by an oxidation process involving peroxymonosulfate(PMS)activated by CuFe2O4 magnetic nanoparticles(MNPs)is reported herein.Results indicate that nearly complete removal of BPA(50 mg L-1)within 60 min and 84.0%TOC removal in 120 min could be achieved at neutral pH by using 0.6 g L-1 CuFe2O4 MNPs and 0.3 g L-1 PMS.The generation of reactive radicals(mainly hydroxyl radicals)was confirmed using electron spin resonance(ESR).Possible mechanisms on the radical generation from CuFe2O4/PMS system are proposed based on the results of radical identification tests and XPS analysis.The lack of inhibition of the reaction by free radical scavengers such as methanol and tert-butyl alcohol suggests that these species may not be generated in the bulk solution,and methylene blue probe experiments confirm that this process does not involve free radical generation.Surface-bound,rather than free radicals generated by a surface catalyzed-redox cycle involving both Fe(Ⅲ)and Cu(Ⅱ),are postulated to be responsible for the mineralization of bisphenol A.(3)A simple template-free method,based on a mineral acid etching process using perovskite as precursors,was successfully developed to obtain a series of 3D meso/macro-porous materials.The manganite perovskite(LaMnO3)transformation was fully investigated using XRD,N2 physisorption,SEM,TEM/EDS,ICP,XPS and TPR.This transformation proceeds through a soft-chemical process involving the dissolution of trivalent lanthanum and manganese from the perovskite structure and dismutation of Mn3+ cations into MnO2 and Mn2+ species.Strength and oxidizing properties of the acid used as modifying agent strongly impact textural and redox surface properties of the resulting materials.Specifically,extending the acid etching duration,on one hand,promotes the surface area and pore volume of the materials while developing interconnected macro-mesoporous networks,as well increases the concentration of active surface oxygen species and further enhances the redox property of the obtained material.In our case,this soft process allowed us to obtain the ε-MnO2 phase with hierarchical porosity without any templates.Superior catalytic properties of ε-MnO2 were observed toward HCHO oxidation as well as an excellent catalytic stability with respect to other macro-mesoporous counterparts.In the light of the experimental results,such performances can be related to cumulated effects associated to the formation of a meso/macro-porous morphology,surface area,surface redox ability and a higher density of active surface oxygen species compared to the perovskite precursors.(4)Alkali and alkaline earth metals-promoted LaMnO3(LMO)perovskites were prepared following addition of metal salts during the synthesis(La0.8A0.2MnO3 with A=K,Na,Sr,Ca),post-synthesis impregnation(LMO-Na)and alkali treatment(LMO-OH).TPR and XPS characterizations revealed that low temperature reducibility was promoted by the substituted elements(K,Na,Sr),while the related perovskites show higher oxidation state ion at B-site(Mn)and higher formation of adsorbed oxygen species,as well as oxygen vacanices on the surface.These results were corroborated by catalytic oxidation of formaldehyde(HCHO),with a remarkable effect of the substituted elements(K,Na,Sr)on the catalytic activity.In comparison,La0.8Ca0.2MnO3,LMO-Na and LMO-OH activities were similar to LMO.Based on the T50(corresponding to HCHO conversions of 50%),the overall catalyst ranking in terms of the catalytic activity was La0.8K0.2MnO3>La0.8Na0.2MnO3>La0.8Sr0.2MnO3>LaMnO3≈La0.8Ca0.2MnO3≈LMO-OH≈LMO-Na.Stability tests,under humid and dry conditions,were conducted on the catalysts and a gradual deactivation of the substituted perovskites(K,Na,Sr)was observed,which may due to the loss of surface active oxygen species and high-valence Mn(Mn4+).Besides,owing to the blocking effects caused by the adsorbed oxidation intermediates,the inhibition of adsorption and activation of molecule oxygen on catalyst surface might be also considered as the reason of deactivation.This phenomenon is critical to consider for the further development of alkali-modified perovskite catalysts.(5)A series of la-deficient La1-xMnO3 perovskites(La1-xMn,x=0,0.1,0.2,0.3 and 0.4)was directly synthesized to investigate the effect of cation vacancies on their physico-chemical properties,as well on the HCHO oxidation.Experimental results revealed that La-deficiency induces perovskites with La and Mn vacancies for La/Mn atomic ratio between 1.0 and 0.8,and thus increases the concentration of active oxygen species at their surface.Further deficiency of lanthanum(La0.6Mn and La0.7Mn)resulted however in the formation of La-deficient perovskites together with Mn3O4 as a secondary crystalline phase.The presence of Mn3O4 increases the concentration,the mobility and reactivity of the active(lattice)oxygen species.Additionally,both cation vacancy and the presence of Mn3O4 improved the low-temperature reducibility of the materials.These materials were first applied in catalytic oxidation of formaldehyde(HCHO),with the most La-deficient perovskite-based materials being the most active.The leading material(La0.6Mn)could retain its high activity for 63 hours,either under dry or humid air.Especially in humid air,the La0.6Mn exhibits the extremely stability.