Svojstva i primena solova cerijum (IV) oksida sintetisanih postupkom forsirane hidrolize

Abstract

Stabilne koloidne disperzije (solovi) cerijum(IV)-oksida dobijeni su metodom forsirane hidrolize, polazeći od cerijum(IV)-nitrata i cerijum(IV)-sulfata u prisustvu različitih količina sumporne i azotne kiseline. Posebna pažnja posvećena je ispitivanju uticaja elektrolita KNO3 i K2SO4 na stabilnost pripremljenih koloidnih disperzija. Takođe, ispitana je mogućnost primene solova CeO2 za formiranje zaštitne prevlake na površini aluminijuma i primene praha CeO2 u oblasti skladištenja vodonika. XRD analiza je pokazala da čestice CeO2 imaju kristalnu strukturu fluoritskog tipa i da je veličina kristalita sola dobijenog iz rastvora cerijum(IV)-nitrata manja od veličine kristalita sola dobijenog iz rastvora cerijum(IV)-sulfata. Prahovi CeO2 su dobre kristaliničnosti a veličina kristalita kreće se od 5,7 do 8,6 nm. Određivanjem veličine čestica metodom dinamičkog rasejanja svetlosti (eng. DLS), gde su prosečne vrednosti hidrodinamičkih prečnika čestica solova CeO2 varirale od 37 do 198 nm, pokazano je da su čestice solova dobijene iz nitrata sitnije od čestica dobijenih iz sulfata. Raspodela veličina čestica kod svih solova bila je monomodalna. XPS analiza je potvrdila prisustvo Ce3+ u kristalnoj rešetci čvrste faze sintetisanih solova CeO2. BET analiza je pokazala da su čestice solova bile mezoporozne, dok vrednost specifične površine raste sa smanjenjem veličine čestica i iznosi od 58 do 92 m2 g-1. Na osnovu UV apsorpcinih spektara, određene su vrednosti direktnog energetskog procepa CeO2. Uticaj dodataka elektrolita KNO3 i K2SO4 na stabilnost solova cerijum(IV)-oksida ispitan je određivanjem izoelektrične tačke solova, praćenjem vrednosti zeta potencijala u funkciji od pH. Utvđeno je da izoelektrična tačka CeO2 u indiferentnom elektrolitu KNO3 iznosi 7,8, a da prisustvo SO42- jona pomera izoelektričnu tačku na pH=6,7. Korozija aluminijuma sa i bez zaštitne prevlake CeO2 ispitana je spektroskopijom elektrohemijske impedancije (SEI) tokom dugotrajnog (duže od 300 h) izlaganja uzoraka 3 mas. % vodenom rastvoru NaCl. Morfologija i hemijski sastav pripremljenih i korodiranih uzoraka ispitana je skenirajućom elektronskom mikroskopijom (SEM) i rendgenskom spektroskopijom dispergovane energije (EDS). SEI podaci su pokazali znatno veću otpornost prema koroziji aluminijuma sa prevlakom CeO2 u poređenju sa aluminijumom bez prevlake. SEM i EDS analize su pokazale da je glavni produkat korozije aluminijuma sa prevlakom dobro umrežena struktura igličastih kristala AlO(OH), za razliku od poroznog sloja Al(OH)3, koji se formira na površini čistog aluminijuma. Tokom izlaganja, umrežena struktura postaje sve kompaktnija i čini da Al sa prevlakom postaje bolje zaštićen u poređenju sa čistim Al, čija površina je zaštićena klasičnim pasivnim slojem Al(OH)3.

Stable colloidal dispersions (sols) and powders of nanometer-sized cerium(IV)-oxide were obtained by forced hydrolysis of cerium(IV)- nitrate and cerium(IV) -sulfate in the presence of various amounts of sulfuric acid and nitric acid. Special attention was paid to the influence of the electrolyte KNO3 and K2SO4 on the stability of the prepared colloidal dispersions. Possible applications of CeO2 sols were also examined in order to test their ability as protective coating on the surface of aluminum and application of powder CeO2 in the field of hydrogen storage. XRD analysis showed that the particles of CeO2 have a crystal structure of fluorite type and the crystallite size of the sol solution prepared from cerium(IV)-nitrate is smaller than the crystallite size of the sol solution prepared from cerium(IV)-sulfate. CeO2 powders were of good crystallinity and the crystallite sizes range from 5.7 to 8.6 nm. XPS analysis confirms the presence of Ce3 + in a crystal lattice of the solid phase of the synthesized CeO2 sols. Determining particle size by DSL method, showed that the average value of the hydrodynamic particle diameter of CeO2 sols, ranged from 37 to 198 nm, it was shown that the particles of the sols obtained from the nitrate were smaller than the particles obtained from the sulphate. The distribution of particle sizes in all sols were monomodal. BET analysis showed that the particles contained micro- and mesopores, and the value of the specific surface areas increase with decreasing particle sizes, and ranges from 58 to 92 m2 g-1. The direct band gap of CeO2 was determined from UV –VIS absorption spectra. The influence of adding KNO3 and K2SO4 electrolytes on the stability of cerium(IV) oxide sols was examined by determining isoelectric points of the sols, by monitoring values of zeta potential as a function of pH. The value of isoelectric point of CeO2 in an indifferent electrolyte KNO3, was 7.8, and in the presence of SO42- ions the isoelectric point moves to pH = 6.7. Corrosion of aluminum with and without protective CeO2 coatings were investigated by electrochemical impedance spectroscopy (EIS) during the long-term (longer than 300 h) exposure of the samples to 3% NaCl aqueous solution The morphology and chemical composition of the initial and corroded samples were determined by scanning electron microscopy and Energy-dispersive X-ray spectroscopy (EDS). EIS data showed significantly higher corrosion resistance of aluminum coated CeO2 compared with uncoated aluminum. SEM and EDS analysis showed that the major product of the corrosion of aluminum-coated was well meshed structure of needle-like crystals of AlO(OH), in contrast to the porous layer of Al(OH)3, which is formed on the surface of the pure aluminum. During the exposure, the needle network structure became more compact and it seems that the Al coating became better protected compared to pure Al, which is protected by the typical passive layer of Al(OH)3.