Electrochemical and catalytic applications of cerium(IV) oxide

Cerium(IV) oxide is one of the most important rare earth metal oxides owing to its high oxygen storage and release capacity, as well as thermal and mechanical stability. Its various applications range from sensors, solid oxide fuel cells and supercapacitors to the most important catalytic application in three-way catalytic converters, oxidation of volatile organic molecules, water-gas shift reaction, etc. This short review article gives a brief introduction to rare earth metals, then describes in detail the properties of cerium(IV) oxide and ways to improve them, as well as presents an overview of the most important and current applications of cerium(IV) oxide. Finally, it gives an overview of the results obtained by our group regarding pure and doped cerium(IV) oxide for various applications

Peeling the Layers of Caddisfly Diversity on a Longitudinal Gradient in Karst Freshwater Habitats Reveals Community Dynamics and Stability

Freshwater biodiversity is facing a severe crisis due to many human impacts, yet the diversity dynamics of freshwater communities and possibilities of assessing these are vastly unexplored. We aimed at emphasizing different aspects of portraying diversity of a species-rich, aquatic insect group (caddisflies ; Trichoptera) across four different habitats in an anthropogenically unimpacted, connected karst barrage lake/riverine system. To define diversity, we used common indices with pre-set sensitivity to species abundance/dominance ; i.e., sensitivity parameter (species richness, Shannon, Simpson, Berger-Parker) and diversity profiles based on continuous gradients of this sensitivity parameter: the naïve and non-naïve diversity profiles developed by Leinster and Cobbold. The non-naïve diversity profiles show diversity profiles with regard to the similarity among species in terms of ecological traits and preferences, whereas the naïve diversity profile is called mathematically “naïve” as it assumes absolute dissimilarity between species that is almost never true. The commonly used indices and the naïve diversity profile both ranked the springs as least diverse and tufa barriers as most diverse. The non-naïve diversity profiles based on similarity matrices (using feeding behavior and stream zonation preferences of species), showed even greater differences between these habitats, while ranking stream habitats close together, regardless of their longitudinal position. We constructed the Climate Score index (CSI) in order to assess how diversity and species’ vulnerability project the community’s resistance and/or resilience to climate change. The CSI ranked the springs as most vulnerable, followed by all habitats longitudinally placed below them. We highlight the importance of integrating ecological information into biodiversity and vulnerability assessment of freshwater communities

Humidity sensing ceria thin-films

Lowering the constitutive domains of semiconducting oxides to the nano-range has recently opened up the possibility of added benefit in the research area of sensing materials, in terms both of greater specific surface area and pore volume. Among such nanomaterials, ceria has attracted much attention ; therefore, we chemically derived homogeneous ceria nanoparticle slurries. One set of samples was tape-casted onto a conducting glass substrate to form thin-films of various thicknesses, thereby avoiding demanding reaction conditions typical of physical depositions, while the other was pressed into pellets. Structural and microstructural features, along with electrical properties and derivative humidity-sensing performance of ceria thin-films and powders pressed into pellets, were studied in detail. Particular attention was given to solid- state impedance spectroscopy (SS-IS), under controlled relative humidity (RH) from 30%– 85%, in a wide temperature and frequency range. Moreover, for the thin-film setup, measurements were performed in surface-mode and cross-section- mode. From the results, we extrapolated the influence of composition on relative humidity, the role of configuration and thin-film thickness on electrical properties, and derivative humidity- sensing performance. The structural analysis and depth profiling both point to monophasic crystalline ceria. Microstructure analysis reveals slightly agglomerated spherical particles and thin-films with low surface roughness. Under controlled humidity, the shape of the conductivity spectrum stays the same along with an increase in RH, and a notable shift to higher conductivity values. The relaxation is slow, as the thickness of the pellet slows the return of conductivity values. The increase in humidity has a positive effect on the overall DC conductivity, similar to the temperature effect for semiconducting behavior. As for the surface measurement setup, the thin-film thickness impacts the shape of the spectra and electrical processes. The surface measurement setup turns out to be more sensitive to relative humidity changes, emphasized with higher RH, along with an increase in thin-film thickness. The moisture directly affects the conductivity spectra in the dispersion part, i.e., on the localized short-range charge carriers. Moisture sensitivity is a reversible process for thin-film samples, in contrast to pellet form samples

Advanced synthesis methods of nanocrystalline manganese-doped ceria

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.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

Advanced synthesis methods of nanocrystalline manganese-doped ceria

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.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