Research

Jump to: Review

17 pages, 3687 KiB

Open AccessArticle

Impact of Hydrothermally Prepared Support on the Catalytic Properties of CuCe Oxide for Preferential CO Oxidation Reaction

by Christos Papadopoulos, Konstantinos Kappis, Joan Papavasiliou, John Vakros, Aspasia Antonelou, Wojciech Gac, Haibin Li and George Avgouropoulos

Catalysts 2022, 12(6), 674; https://doi.org/10.3390/catal12060674 - 20 Jun 2022

Cited by 6 | Viewed by 1941

Abstract

CuCe mixed oxide is one of the most studied catalytic systems for preferential CO oxidation (CO-PrOx) for the purification of hydrogen-rich gas stream. In this study, a series of ceria supports were prepared via a citrates-hydrothermal route by altering the synthesis parameters (concentration [...] Read more.

CuCe mixed oxide is one of the most studied catalytic systems for preferential CO oxidation (CO-PrOx) for the purification of hydrogen-rich gas stream. In this study, a series of ceria supports were prepared via a citrates-hydrothermal route by altering the synthesis parameters (concentration and temperature). The resulting supports were used for the preparation of CuCe mixed-oxide catalysts via wet impregnation. Various physicochemical techniques were utilized for the characterization of the resulting materials, whereas the CuCe oxide catalysts were assessed in CO-PrOx reaction. Through the proper modification of the hydrothermal parameters, CeO₂ supports with tunable properties can be formed, thus targeting the formation of highly active and selective catalysts. The nature of the reduced copper species and the optimum content in oxygen vacancies seems to be the key factors behind the remarkable catalytic performance of a CO-PrOx reaction. Full article

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures

Figure 1

21 pages, 3943 KiB

Open AccessFeature PaperArticle

Advanced Synthesis and Characterization of Vanadia/Titania Catalysts through a Molecular Approach

by Eleni Tella, Antonios Trimpalis, Athanasios Tsevis, Christos Kordulis, Alexis Lycourghiotis, Soghomon Boghosian and Kyriakos Bourikas

Catalysts 2021, 11(3), 322; https://doi.org/10.3390/catal11030322 - 02 Mar 2021

Cited by 4 | Viewed by 1812

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

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures

Figure 1

12 pages, 2628 KiB

Open AccessFeature PaperArticle

Precursor Effect on Mn-Fe-Ce/TiO₂ Catalysts for Selective Catalytic Reduction of NO with NH₃ at Low Temperatures

by Siva Sankar Reddy Putluru, Leonhard Schill, Anker Degn Jensen, Bernard Siret, Frank Tabaries and Rasmus Fehrmann

Catalysts 2021, 11(2), 259; https://doi.org/10.3390/catal11020259 - 15 Feb 2021

Cited by 7 | Viewed by 2187

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 5Mn_Ace/Ti, 25Mn_0.75AceFe_0.25Nit/Ti, and 25Mn_0.75AceFe_0.20NitCe_0.05Ace/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

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures

Figure 1

22 pages, 6943 KiB

Open AccessArticle

Microwave-Assisted Solvothermal Synthesis of Fe₃O₄/CeO₂ Nanocomposites and Their Catalytic Activity in the Imine Formation from Benzyl Alcohol and Aniline

by Antonino Rizzuti, Maria C. Dipalo, Ignazio Allegretta, Roberto Terzano, Nicola Cioffi, Piero Mastrorilli, Matilda Mali, Giuseppe Romanazzi, Angelo Nacci and Maria Michela Dell’Anna

Catalysts 2020, 10(11), 1325; https://doi.org/10.3390/catal10111325 - 14 Nov 2020

Cited by 10 | Viewed by 3042

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

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures

Graphical abstract

18 pages, 1621 KiB

Open AccessArticle

Characterization of Sulfated SnO₂-ZrO₂ Catalysts and Their Catalytic Performance on the Tert-Butylation of Phenol

by Justin Marlowe, Shreyas Acharya, Adam Zuber and George Tsilomelekis

Catalysts 2020, 10(7), 726; https://doi.org/10.3390/catal10070726 - 29 Jun 2020

Cited by 11 | Viewed by 4179

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

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures

Figure 1

10 pages, 3481 KiB

Open AccessArticle

Synthesis of Ce_1−xPd_xO_2−δ Solid Solution in Molten Nitrate

by Hideaki Sasaki, Keisuke Sakamoto, Masami Mori and Tatsuaki Sakamoto

Catalysts 2020, 10(6), 640; https://doi.org/10.3390/catal10060640 - 08 Jun 2020

Cited by 5 | Viewed by 2043

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 (Ce_1−xPd_xO_2−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 (Ce_1−xPd_xO_2−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

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures

Figure 1

17 pages, 1602 KiB

Open AccessFeature PaperArticle

In Situ Raman Spectroscopy as a Tool for Discerning Subtle Structural Differences between Commercial (Ce,Zr)O₂-Based OSC Materials of Identical Composition

by Chrysanthi Andriopoulou, Deb Harris, Hazel Stephenson, Angelos M. Efstathiou and Soghomon Boghosian

Catalysts 2020, 10(4), 462; https://doi.org/10.3390/catal10040462 - 24 Apr 2020

Cited by 9 | Viewed by 2977

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 Ce_x [...] 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 Ce_xZr_1−xO_2−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

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures

Figure 1

Review

Jump to: Research

50 pages, 13680 KiB

Open AccessReview

Recent Advances on Fine-Tuning Engineering Strategies of CeO₂-Based Nanostructured Catalysts Exemplified by CO₂ Hydrogenation Processes

by Georgios Varvoutis, Maria Lykaki, George E. Marnellos and Michalis Konsolakis

Catalysts 2023, 13(2), 275; https://doi.org/10.3390/catal13020275 - 26 Jan 2023

Cited by 8 | Viewed by 2194

Abstract

Ceria-based oxides have been extensively involved in a wide range of catalytic applications due to their intriguing properties, related mostly to their superior redox features in conjunction with peculiar metal-support interaction phenomena. Most importantly, the fine-tuning of key interrelated factors, such as the [...] Read more.

Ceria-based oxides have been extensively involved in a wide range of catalytic applications due to their intriguing properties, related mostly to their superior redox features in conjunction with peculiar metal-support interaction phenomena. Most importantly, the fine-tuning of key interrelated factors, such as the size, morphology and electronic state of the catalyst’s counterparts, can exert a profound influence on the intrinsic characteristics and interfacial reactivity with pronounced implications in catalysis. The present review, while also elaborating our recent efforts in the field, aims to provide key fundamental and practical aspects in relation to the rational design and functionalization strategies of ceria-based catalysts, exemplified by the CO₂ hydrogenation processes, namely, CO₂ methanation and reverse water–gas shift (rWGS) reactions. Firstly, a description of the most prominent catalytically relevant features of cerium oxide is provided, focusing on reducibility and metal-support interaction phenomena, followed by a brief overview of the current status of ceria-based catalysts for various energy and environmental applications. Then, the main implications of fine-tuning engineering via either appropriate synthesis routes or aliovalent doping on key activity descriptors are thoroughly discussed and exemplified by state-of-the-art ceria-based catalysts for CO₂ hydrogenation. It is clearly revealed that highly active and cost-efficient ceria-based catalytic materials can be obtained on the grounds of the proposed functionalization strategy, with comparable or even superior reactivity to that of noble metal catalysts for both the studied reactions. In a nutshell, it can be postulated that the dedicated fabrication of CeO₂-based systems with augmented redox capabilities and, thus, oxygen vacancies abundance can greatly enhance the activation of gas-phase CO₂ towards CO or CH₄. Besides, the morphology-engineering of CeO₂-based catalysts can notably affect the CO₂ hydrogenation performance, by means of an optimum metal-ceria interphase based on the exposed facets, whereas doping and promotion strategies can effectively shift the reaction pathway towards the selective production of either CO or CH₄. The conclusions derived from the present work can provide design and fine-tuning principles for cost-efficient, highly active and earth-abundant metal oxide systems, not only for the CO₂ hydrogenation process but for various other energy and environmental applications. Full article

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures

Graphical abstract

20 pages, 28647 KiB

Open AccessReview

Advances in Designing Efficient La-Based Perovskites for the NO_x Storage and Reduction Process

by Dongyue Zhao, Haitao Song, Jun Liu, Qiuqiao Jiang and Xingang Li

Catalysts 2022, 12(6), 593; https://doi.org/10.3390/catal12060593 - 30 May 2022

Viewed by 1671

Abstract

To overcome the inherent challenge of NO_x reduction in the net oxidizing environment of diesel engine exhaust, the NO_x storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNO_x technique over the past [...] Read more.

To overcome the inherent challenge of NO_x reduction in the net oxidizing environment of diesel engine exhaust, the NO_x storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNO_x technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NO_x abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NO_x spillover and contributes to efficient NO_x storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NO_x storage, and NO_x reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNO_x efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application. Full article

(This article belongs to the Special Issue Structure and Function of Ceria-Based Mixed Metal Oxides and Supported Transition Metal Oxide Catalysts)

► Show Figures