Title Napredne metode sinteze nanokristaliničnoga cerijeva (IV) oksida dopiranoga manganom Title (english) Advanced synthesis methods of nanocrystalline manganese-doped ceria Author Ivana Katarina Ivković Mentor Stanislav Kurajica (mentor)Committee member Jelena Macan (predsjednik povjerenstva)Committee member Vesna Tomašić (član povjerenstva)Committee member Ivan Brnardić (član povjerenstva)Granter University of Zagreb Defense date and country 2023-12-12, Croatia Scientific / art field, TECHNICAL SCIENCES Universal decimal classificationUDC ) 66 - Chemical technology. Chemical and related industries 54 - Chemistry. Crystallography. Mineralogy Abstract U ovom radu pripravljeni su uzorci cerijeva(IV) oksida, CeO2, koristeći tri napredne metode sinteze: hidrotermalnu, sol-gel i mehanokemijsku sintezu. Pripravljeni uzorci karakterizirani su rendgenskom difrakcijskom analizom, termogravimetrijskom analizom, transmisijskom te pretražnom elektronskom mikroskopijom, rendgenskom fotoelektronskom spektroskopijom, adsorpcijsko desorpcijskim izotermama dušika i difuznom refleksijskom spektroskopijom. Pokazano je da se svakom od tri istraživane metode sinteze može pripraviti nanokristalinični cerijev(IV) oksid te da su sve tri metode pogodne za pripravu nanokristaliničnog cerijevog(IV) oksida dopiranog s manganom. Sol-gel metoda omogućuje ugradnju iona mangana u strukturu cerijeva(IV) oksida bez pojave novih faza, dok se u uzorcima pripravljenim hidrotermalnom i mehanokemijskom sintezom uz CeO2 u uzorcima pojavljuje dodatna faza s manganom, birnesit. Produkte sol-gel i mehanokemijske sinteze potrebno je naknadno termički obrađivati kako bi se dobio CeO2. Međutim, ukoliko su takvi produkti potencijalni katalizatori u procesima koji se provode pri povišenim temperaturama, potrebno ih je prethodno termički obraditi. Ulazak mangana u kristalnu strukturu CeO2 dokazan je smanjenjem parametra jedinične ćelije kubične kristalne rešetke cerijeva(IV) oksida za uzorke dobivene primjenom prethodno navedenih metoda. Zapažen je i pozitivan utjecaj mangana na termičku stabilnost CeO2 koji se očituje u manjim veličinama kristalita dopiranih uzoraka u odnosu na nedopirane. Iako su hidrotermalnom sintezom dobiveni kristaliti najmanje veličine, sol-gel sinteza pokazala se kao metoda čiji kristaliti pokazuju najmanji rast. Isto tako, iako uzorci pripravljeni hidrotermalnom sintezom imaju najveće specifične površine, nakon termičke obrade najveće površine pokazali su uzorci pripravljeni sol-gel sintezom. Kao prednost mehanokemijske sinteze može se navesti dobivanje najvećih količina uzoraka jednom sintezom. Katalitička aktivnost pripravljenih uzoraka testirana je u procesu oksidacije toluena ili smjese aromatskih spojeva (benzen, toluen, etilbenzen i ksilen, BTEX). Svi dopirani uzorci pokazali su značajno povećanje aktivnosti u odnosu na nedopirane. Za svaku od sinteza dopirani uzorci postigli su konverzije od 95 % pri nižim temperaturama u odnosu na nedopirane uzorke, međutim, čak i nedopirani uzorak pripravljen sol-gel sintezom pokazuje bolju katalitičku aktivnost od najuspješnijeg katalizatora iz serije dobivene mehanokemijskom sintezom. Povećanje katalitičke aktivnosti svih uzoraka u kojima je dio cerija zamijenjen manganom posljedica je ulaska iona nižeg oksidacijskog stanja (Mn3+) umjesto Ce4+ u kristalnu strukturu CeO2. Zbog toga dolazi do stvaranja vakancija kisika i površinskih aktivnih (nezasićenih) mjesta kojima se kompenzira negativan naboj. Također, dopirani uzorci su toplinski stabilniji te imaju manje čestice i veće specifične površine u odnosu na nedopirane.
Abstract (english) In this paper, ceria, CeO2, samples were prepared using three advanced synthesis methods: hydrothermal, sol-gel and mechanochemical synthesis. The prepared samples were characterized by X-ray diffraction analysis, thermogravimetric analysis, transmission and scanning electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorptiondesorption isotherms and diffuse reflection spectroscopy. It is shown that each of the three researched synthesis methods can be used to prepare nanocrystalline cerium (IV) oxide and that all three methods are suitable for the preparation of nanocrystalline cerium (IV) oxide doped with manganese. The sol-gel method enables the incorporation of manganese ions into the cerium (IV) oxide structure without presence of new phases, while in hydrothermally and mechanochemically derived samples, in addition to CeO2, birnessite, a phase with manganese also appears. The products of sol-gel and mechanochemical synthesis need to be subsequently thermally processed to obtain CeO2. However, if the products are intended for use in catalysis processes that occur at elevated temperatures, thermal treatment in necessary. The entry of manganese into the crystal structure of CeO2 was proven by the reduction of the unit cell parameter of the cubic crystal lattice of cerium (IV) oxide for the samples obtained by the three methods mentioned. A positive influence of manganese on the thermal stability of CeO2 was also observed, which is manifested in the smaller crystallite sizes of the doped samples compared to the non-doped ones. Although hydrothermal synthesis produced crystallites of the smallest size, sol-gel synthesis proved to be the method whose crystallites showed the smallest growth. Likewise, although the samples prepared by hydrothermal synthesis have the highest specific surfaces, after thermal treatment the samples prepared by sol-gel synthesis showed the highest surface area. One of the advantages of mechanochemical synthesis is obtaining the largest amount of product. The catalytic activity of the prepared samples was tested in the process of toluene or the aromatic (benzene, toluene, ethylbenzene and xylene, BTEX) gas mixture oxidation. All doped samples showed a significant increase in activity compared to non-doped ones. For each of the syntheses, doped samples achieved conversions of 95 % at a lower temperature compared to undoped samples, however, even the undoped sample prepared by sol-gel synthesis shows better catalytic activity than the most successful catalyst in the series obtained by mechanochemical synthesis. The increase in the catalytic activity of all samples in which part of the cerium is replaced by manganese is a consequence of the entry of ions of a lower oxidation state (Mn3+) instead of Ce4+ into the crystalline structure of CeO2. This results in the creation of oxygen vacancies and surface-active (unsaturated) sites that compensate for the negative charge. Also, doped samples are more thermally stable and have smaller particles and larger specific surfaces compared to non-doped ones.
Keywords
cerijev(IV) oksid
dopiranje manganom
sol-gel sinteza
hidrotermalna sinteza
mehanokemijska sinteza
katalitička aktivnost
oksidacija toluena
Keywords (english)
ceria
sol-gel
hydrothermal synthesis
mechanochemical synthesis
catalytic activity
toluene oxidation
Language croatian URN:NBN urn:nbn:hr:149:424835 Promotion 2023 Study programme Title: Chemical Engineering and Applied Chemistry - Doctoral course Type of resource Text Extent 113 str. ; 30 cm File origin Born digital Access conditions Open access Terms of use Created on 2024-01-16 13:13:21
