Special Issue "Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts"

A special issue of Catalysts (ISSN 2073-4344). This special issue belongs to the section "Catalytic Materials".

Deadline for manuscript submissions: closed (15 December 2020).

Special Issue Editor

Prof. Dr. Soghomon Boghosian
E-Mail Website
Guest Editor
Department of Chemical Engineering, University of Patras, GR 265 04 Patras, Greece
Interests: heterogeneous catalysis; molecular structure; In-situ & operando Raman spectroscopy in catalysis; thermodynamics; inorganic coordination complexes.

Special Issue Information

Dear Colleagues,

Ceria -based mixed metal oxides constitute a formidable class of oxygen storage and release materials and their structure/function properties are of topical interest. Pertinent applications include inter alia their function as TWCs (Three Way Catalysts), catalysts for WGS (Water Gas Shift), PROX and reforming reactions, as well as their use in solar reactors for CO2 conversion etc. The art of synthesis and preparation of ceria-based mixed metal oxide solid solutions greatly affects the structure and the defect topology of their anionic sublattices that are -in turn- affecting their behavior.

The structure/activity relationships pertaining to supported transition metal oxide catalysts (e.g., VOx, MoOx,WOx, ReOx etc) on oxidic carriers (e.g., TiO2, Al2O3, ZrO2, SiO2 as well as mixed carriers) continues to attract focused research interest due to their importance for a great number of industrial processes (environmental catalysis, production of chemical commodities etc). Ideas and thoughts that continually have preoccupied or intruded on researchers’ minds are recently being questioned and controverted. Advanced synthesis routes based on molecular level approaches combined with meticulous structural characterization under controlled conditions have recently marked the dawn of a period with new unprecedented findings on structural and structure/function relationship grounds. 

This Special Issue aims to gather a collection of articles highlighting some of the recent progress on the selected subjects addressed above.

Prof. Dr. Soghomon Boghosian
Guest Editor

Manuscript Submission Information

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Keywords

  • Spectroscopy in catalysis
  • Supported transition metal oxide catalysts
  • Vanadia, molybdena, tungsta, rhenia, chromia
  • Support effects
  • Ceria-based mixed oxides
  • OSC
  • Advanced synthesis routes for tailored catalysts
  • Structure and function of supported and mixed metal oxides
  • In-situ and operando characterization
  • Transient kinetics and isotope exchange

Published Papers (6 papers)

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Research

Article
Advanced Synthesis and Characterization of Vanadia/Titania Catalysts through a Molecular Approach
Catalysts 2021, 11(3), 322; https://doi.org/10.3390/catal11030322 - 02 Mar 2021
Viewed by 403
Abstract
Vanadia/titania catalysts were synthesized by the equilibrium deposition filtration (EDF) method, which is a synthesis route that follows a molecular-level approach. The type of interfacial deposition as well as the interfacial speciation of the deposited oxo-V(V) species were determined by means of a [...] Read more.
Vanadia/titania catalysts were synthesized by the equilibrium deposition filtration (EDF) method, which is a synthesis route that follows a molecular-level approach. The type of interfacial deposition as well as the interfacial speciation of the deposited oxo-V(V) species were determined by means of a model that takes into account experimental “proton-ion” curves and “adsorption edges”. It is shown that at pH ≥ 9.5, the deposition proceeds exclusively through the formation of mono-substituted inner sphere monomeric species in an “umbrella”-like Ti–OV(OH)2O configuration, whilst with lowering of the pH, a second species, namely the disubstituted inner sphere quadrameric species in a (Ti-O)2V4O10 configuration possessing two mono-oxo V=O and two di-oxo V(=O)2 terminations gradually prevails, which is in co-existence with the monomeric species. Raman spectroscopy is used for verifying the solution speciation, which is different compared to the interfacial speciation of the deposited oxo-V(V) species. Furthermore, in situ Raman spectroscopy was used to verify the model-predicted interfacial speciation of the deposited oxo-V(V) species and to monitor the temperature-dependent evolution up to 430 °C. Hence, a controlled formation of a specific vanadia species on a titania surface is enabled, which, depending on the synthesis conditions, can result in specific catalyst characteristics and thus possibly different catalytic behavior for a specific reaction. Full article
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Article
Precursor Effect on Mn-Fe-Ce/TiO2 Catalysts for Selective Catalytic Reduction of NO with NH3 at Low Temperatures
Catalysts 2021, 11(2), 259; https://doi.org/10.3390/catal11020259 - 15 Feb 2021
Cited by 2 | Viewed by 516
Abstract
Preparation of Mn/TiO2, Mn-Fe/TiO2, and Mn-Fe-Ce/TiO2 by the deposition-precipitation (DP) method can afford very active catalysts for low-temperature NH3-SCR (selective catalytic reduction of NO with NH3). The effect of precursor choice (nitrate vs. acetate) [...] Read more.
Preparation of Mn/TiO2, Mn-Fe/TiO2, and Mn-Fe-Ce/TiO2 by the deposition-precipitation (DP) method can afford very active catalysts for low-temperature NH3-SCR (selective catalytic reduction of NO with NH3). The effect of precursor choice (nitrate vs. acetate) of Mn, Fe, and Ce on the physiochemical properties including thermal stability and the resulting SCR activity were investigated. The resulting materials were characterized by N2-Physisorption, XRD (Powder X-ray diffraction), XPS (X-ray photoelectron spectroscopy), H2-TPR (temperature-programmed reduction with hydrogen), and the oxidation of NO to NO2 measured at 300 °C. Among all the prepared catalysts 5MnAce/Ti, 25Mn0.75AceFe0.25Nit/Ti, and 25Mn0.75AceFe0.20NitCe0.05Ace/Ti showed superior SCR activity at low temperature. The superior activity of the latter two materials is likely attributable to the presence of amorphous active metal oxide phases (manganese-, iron- and cerium-oxide) and the ease of the reduction of metal oxides on TiO2. Enhanced ability to convert NO to NO2, which can promote fast-SCR like pathways, could be another reason. Cerium was found to stabilize amorphous manganese oxide phases when exposed to high temperatures. Full article
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Article
Microwave-Assisted Solvothermal Synthesis of Fe3O4/CeO2 Nanocomposites and Their Catalytic Activity in the Imine Formation from Benzyl Alcohol and Aniline
Catalysts 2020, 10(11), 1325; https://doi.org/10.3390/catal10111325 - 14 Nov 2020
Cited by 1 | Viewed by 674
Abstract
Fe3O4/CeO2 nanocomposites were synthetized by coating magnetite seeds of different morphologies (hexagonal, spheroidal, quasi-spherical) with ceria, in ethylene glycol as solvothermal solvent. The synthesis was performed in the presence of microwave irradiation aiming to overcome the common disadvantages [...] Read more.
Fe3O4/CeO2 nanocomposites were synthetized by coating magnetite seeds of different morphologies (hexagonal, spheroidal, quasi-spherical) with ceria, in ethylene glycol as solvothermal solvent. The synthesis was performed in the presence of microwave irradiation aiming to overcome the common disadvantages proper of the classic solvothermal/hydrothermal procedure. The obtained nanocomposites were calcined at the optimum temperature of 550 °C. The structure of the new nanomaterials was carefully investigated by IR, XRD, SEM, EDS and TEM analyses. The nanocomposites resulted to be constituted by CeO2 nanoparticles distributed onto Fe3O4 seeds, that kept their pristine morphology. The new materials were used as catalysts for imine synthesis from benzyl alcohol and aniline. The highest imine conversion rate was obtained with Fe3O4/CeO2, which was synthesized from Fe3O4 nanoparticles (hexagonal) obtained by microwave hydrothermal procedure in the absence of any organic additive (polyvinylpyrrolidone, trisodium citrate dihydrate or oleic acid). The catalyst could be easily removed from the reaction mixture with the help of an external magnet, and it was recycled for at least five runs with increasing catalytic activity. Full article
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Article
Characterization of Sulfated SnO2-ZrO2 Catalysts and Their Catalytic Performance on the Tert-Butylation of Phenol
Catalysts 2020, 10(7), 726; https://doi.org/10.3390/catal10070726 - 29 Jun 2020
Cited by 4 | Viewed by 1343
Abstract
Understanding the catalytic behavior of sulfated metal oxides has been the topic of several research studies in the past few decades. Their apparent super-acidic behavior has been correlated with the molecular structure of the surface sulfate species. Herein, we couple FTIR and Raman [...] Read more.
Understanding the catalytic behavior of sulfated metal oxides has been the topic of several research studies in the past few decades. Their apparent super-acidic behavior has been correlated with the molecular structure of the surface sulfate species. Herein, we couple FTIR and Raman spectroscopies to study the molecular structural evolution of surface sulfate species on mixed metal hydroxides as well as calcined oxides. We show that on the surface of hydroxides, monodentate and possibly bidentate species are dominant, while for SnO2-rich samples, clusters of polymeric sulfate species may also be present. After calcination, sulfate species bind strongly on the surface of mixed oxides, and different configurations can be seen with a range of S=O functionalities of varying strength. Through comparison of the catalytic performance of all sulfate oxides in the tert-butylation of phenol, it was found that SnO2-rich samples show high TBA conversion, with monoalkylated phenols as the primary product. Full article
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Article
Synthesis of Ce1−xPdxO2−δ Solid Solution in Molten Nitrate
Catalysts 2020, 10(6), 640; https://doi.org/10.3390/catal10060640 - 08 Jun 2020
Viewed by 642
Abstract
CeO2-based solid solutions in which Pd partially substitutes for Ce attract considerable attention, owing to their high catalytic performances. In this study, the solid solution (Ce1−xPdxO2−δ) with a high Pd content (x ~ 0.2) [...] Read more.
CeO2-based solid solutions in which Pd partially substitutes for Ce attract considerable attention, owing to their high catalytic performances. In this study, the solid solution (Ce1−xPdxO2−δ) with a high Pd content (x ~ 0.2) was synthesized through co-precipitation under oxidative conditions using molten nitrate, and its structure and thermal decomposition were examined. The characteristics of the solid solution, such as the change in a lattice constant, inhibition of sintering, and ionic states, were examined using X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM−EDS), transmission electron microscopy (TEM)−EDS, and X-ray photoelectron spectroscopy (XPS). The synthesis method proposed in this study appears suitable for the easy preparation of CeO2 solid solutions with a high Pd content. Full article
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Article
In Situ Raman Spectroscopy as a Tool for Discerning Subtle Structural Differences between Commercial (Ce,Zr)O2-Based OSC Materials of Identical Composition
Catalysts 2020, 10(4), 462; https://doi.org/10.3390/catal10040462 - 24 Apr 2020
Cited by 1 | Viewed by 760
Abstract
In situ Raman spectroscopy was used at temperatures in the 50–480 °C range under oxidizing (20% O2/He) and reducing (5% H2/He) flowing gas atmospheres to compare the spectra obtained for a series of industrial rare earth doped Cex [...] Read more.
In situ Raman spectroscopy was used at temperatures in the 50–480 °C range under oxidizing (20% O2/He) and reducing (5% H2/He) flowing gas atmospheres to compare the spectra obtained for a series of industrial rare earth doped CexZr1−xO2−δ oxygen storage capacity (OSC) mixed metal oxide materials of identical at % composition, which were prepared by the same chemical synthesis route, in which one synthesis parameter of the aqueous chemistry was slightly varied. The Raman fingerprint of the anionic sublattice is very sensitive to O atom relocations within the bulk of the material matrix and to the pertinent defect topology in each case. A protocol of sequential Raman measurements and analysis was proposed to discern subtle differences between the oxygen vacancy and defect topologies of the examined materials. It can be concluded that for two materials under comparison for their structures, identical Raman spectra are obtained only if the procedures followed for their preparation are identical; a slight variation of one single parameter (e.g., in the aqueous chemistry stage) results in discernible differences in the Raman spectra. The proposed procedure can serve as a tool for proving or disproving infringement of IPR (Intellectual Property Rights) protected preparation methods of ceria-based mixed metal oxide materials. Full article
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