Page - 144 - in Advanced Chemical Kinetics

mechanicalbehavior [2,3]. Itsapplicationsfocusedontheuseasadditivesandadsorbents.The crystalline structure is composed of alternating silicate blocks and cavities. The presence of silanol groups (Si-OH) makes use of sepiolite as a support for metals and metal oxide nanoparticles [4]. ZnO is one of themost promisingphotocatalysts under theultraviolet radiation toprotect the environmentbydegradationoforganicpollutants inwater andair. Ingeneral, aphotocatalytic reactionstartswith thegenerationofelectronsandholesbyphotoexcitation.Then, thesecharge carriersmigrate to the surface of thephotocatalyst and reactwith adsorbed electron acceptors and donors, respectively. Thus, an efficient photocatalyst requires a suitable band-gap, facile separation and transportation of electrons andholes for the feasibility of potential redox reac- tions.ZnOwithitshighphotosensitivityandlargebandgapplaysasignificantroleforreduction andoxidationprocesses.However,usageofZnOnanoparticles incatalyticslurriesalonecreates fast chargecarrier recombinationandrequires long-timecentrifugation for the removalprocess. An effectiveway to overcome these difficulties is to immobilize theZnOnanoparticles on the inner and outer surfaces of inorganic porous supports with the formation of nanocomposite materials [5, 6]. Thesematerials induce high surface area, large pore volume, gooddispersion andstrongadsorptivity.Asynergeticeffect isexpected in thecoexistenceofZnOandasupport whicheventually transferspecies fromthesupport toZnOorviceversabythe interfacecreated betweentwophases.Hence,an increase inreactionrates isexpectedandZnO/support compos- iteshavebeenpostulatedassuitablealternativephotocatalysts inenvironmentalapplications. Theadsorptiononthecenospheresand/orplerospheresof flyashesmakesuseof themas ideal supportmaterials.Synthesis,characterization,adsorptionpropertyandphotoactivityofZnO-or TiO2-loaded fly ash composites are recently examined for anti-corrosion in coatings and also degradationof inorganicandorganicpollutants toprovideadditionalwaytoutilize thewaste fly ash [7–13]. Photoactive ZnO nanoparticles supported on sepiolite are also reported for applications in decontamination of pollutants [14–19]. The immobilization of ZnO nano- particles can be improvedwithin the sepiolite framework due to the net negative charge of the sepiolite. Thus, charge separation efficiency and high adsorption capability can enhance the removalperformance for thephotodecompositionoforganicpollutants. This chapter is focusedona ternaryphotocatalyst;ZnO-supported flyash-sepiolite (ZnO-FA- Sep) by taking advantage from the above-mentioned unique characteristics of fly ash and sepiolite. This composite can be considered as the first approximation of in situ formation of ZnOnanoparticles inpresenceofaclay (Sep)andanashmaterial (FA).With thisobjective, the as-prepared supported catalysts are characterized, structurally examined and evaluated via adsorptionability,kineticsandphotocatalyticperformance. 2.Preparationandcharacterizationof thephotocatalysts ZnOcatalystsweresynthesizedviaaco-precipitation route [20]. Ina typicalpreparationstep, 0.5M(or 0.25or 0.125M)Zn(NO3)2 6H2Owasaddedgradually to 0.5M(or 0.25or 0.125M) Advanced Chemical Kinetics144