Search Results (257)

Ceria is an important redox catalyst due to the facile Ce³⁺/Ce⁴⁺ switching at its surface. Therefore, in situ determination of the oxidation state of surface cerium cations is of significant interest. Infrared spectroscopy of probe molecules such as CO holds great potential for this purpose. However, the ability of CO to reduce Ce⁴⁺ cations is an important drawback as it alters the initial cerium speciation. Dinitrogen (N₂), due to its chemical inertness, presents an attractive alternative. We recently demonstrated that low-temperature ¹⁵N₂ adsorption on stoichiometric ceria leads to the formation of complexes with Ce⁴⁺ cations on the (110) and (100) planes (bands at 2257 and 2252 cm⁻¹, respectively), while the (111) plane is inert. Here, we report results on the low-temperature ¹⁵N₂ adsorption on reduced ceria nanoshapes (cubes, polyhedra, and rods). A main band at 2255 cm⁻¹, with a weak shoulder at 2254 cm⁻¹, was observed. We attributed these bands to ¹⁵N₂ adsorbed on Ce³⁺ sites located on edges and corners as well as on {100} facets. In conclusion, ¹⁵N₂ adsorbs on the most acidic surface Ce³⁺ sites and enables their distinction from Ce⁴⁺ cations. Full article

The use of CH₄ and CO₂ as fuels in direct internal reforming solid oxide fuel cells (DIR-SOFCs) is a promising strategy for efficient power generation with reduced greenhouse gas emissions. In this study, Ni catalysts supported on Ce–Pr mixed oxides with varying Pr contents (0–80 mol%) were synthesized, calcined at 1200 °C, and tested for dry reforming of methane (DRM), aiming at their application as catalytic layers in SOFC anodes. Physicochemical characterization (XRD, TPR, TEM) showed that increasing Pr loading enhances catalyst reducibility and promotes the formation of the Pr₂NiO₄ phase, which contributes to the generation of smaller Ni⁰ particles after reduction. Catalytic tests revealed that all samples exhibited low-carbon deposition, attributed to the large Ni crystallites. The catalyst with 80 mol% Pr showed the best performance, achieving the highest CH₄ conversion (72%), a H₂/CO molar ratio of 0.89, and improved stability. These findings suggest that Ni/Ce_0.2Pr_0.8 could be a promising candidate for use as a catalyst layer of anodes in DIR-SOFC anodes. Although electrochemical data are not yet available, future work will evaluate the catalyst’s performance and durability under SOFC-relevant conditions. Full article

The Ni/SiO₂ and the ceria-promoted Ni-CeO₂/SiO₂ were prepared by the impregnation method and co-impregnation method, respectively. The performance of the carbon dioxide reforming of methane (CDR) over Ni/SiO₂ and Ni-CeO₂/SiO₂ was investigated under the conditions of CH₄/CO₂ = 1.0, T = 800 °C, and GHSV = 60,000 mL·g⁻¹·h⁻¹. As a result, a high CDR performance, especially stability, was obtained over Ni-CeO₂/SiO₂, in which the conversion of CH₄ was very similar to that of the thermodynamic equilibrium (88%), and a negligible decrease in CH₄ conversion was observed after 50 h of the CDR reaction. Ni/SiO₂ and Ni-CeO₂/SiO₂ before and after the CDR reaction were subjected to structural characterization by XRD, TEM, TG–DSC, and physical adsorption. It was found that the addition of CeO₂ into Ni/SiO₂ significantly affected its surface area, the size and dispersion of Ni, the reduction behavior, and the coking properties. Moreover, the redox property of Ce³⁺-Ce⁴⁺, which accelerates the gasification of the coke, made Ni-CeO₂/SiO₂ successfully operate for 50 h without observable deactivation. Thus, the developed catalyst is very promising for the CDR. Full article

Electrochemical formation of ceria (mixed Ce₂O₃ and CeO₂) coatings on different types of screen-printed carbon electrodes (SPCEs) (based on graphite (C110), carbon nanotubes (CNT), single-walled carbon nanotubes (SWCNT), carbon nanofibers (CNF), and mesoporous carbon (MC)) were studied. Their potential applications as catalysts for various redox reactions and electrochemical sensors were investigated. The ceria oxide layers were electrodeposited on SPCEs at various current densities and deposition time. The morphology, structure, and chemical composition in the bulk of the ceria layers were studied by SEM and EDS methods. XRD was used to identify the formed phases. The concentration, chemical composition and chemical state of the elements on the surface of studied samples were characterized by XPS. It was established that the increase of the concentration of CeCl₃ in the solution and the cathode current density strongly affected the surface structure and concentration (relation between Ce³⁺ and Ce⁴⁺, respectively) in the formed ceria layers. At low concentration of CeCl₃ (0.1M) and low values of cathode current density (0.5 mA·cm⁻²), porous samples were obtained, while with their increase, the ceria coatings grew denser. Full article

Waste biomass gasification can contribute to the production of alternative and environmentally sustainable green fuels. Research at the CREC–UWO (Chemical Reactor Engineering Center–University of Western Ontario) considers an integrated gasification process where both electrical power, biochar, and tar-free syngas suitable for alcohol synthesis are produced. In particular, the present review addresses the issues concerning tar removal from the syngas produced in a waste biomass gasifier via a catalytic post-gasification (CPG) downer unit. Various questions concerning CPG, such as reaction conditions, thermodynamics, a Tar Conversion Catalyst (TCC), and tar surrogate chemical species that can be employed for catalyst performance evaluations are reported. Catalyst performance-reported results were obtained in a fluidizable CREC Riser Simulator invented at CREC–UWO. The present review shows the suitability of the developed fluidizable Ni–Ceria γ-alumina catalyst, given the high level of tar removal it provides, the minimum coke that is formed with its use, and the adequate reforming of the syngas exiting the biomass waste gasifier, suitable for alcohol synthesis. Full article

The present study aims to investigate a Pd catalyst on a complex multi-oxide medium-entropy support interlayer La₂O₃-CeO₂-ZrO₂-Al₂O₃ and its possible use as catalysts for methane abatement applications. The low-temperature N₂-adsorption, XRD, TEM, XPS, TPD, and TPR techniques were used to characterize the catalyst. The palladium deposition on the supports leads to the formation of PdO. After the catalytic tests, the metal-Pd phase was observed. The complete oxidation of methane on Pd/La-Ce-Zr-Al catalyst takes place at temperatures above 250 °C, and in the presence of water vapor, the reaction temperature increases to about 70 °C. The careful choice of constituent oxides provides a balance between structural stability and flexibility. The alumina and lanthanum oxide ensure the high specific surface area, while the simultaneous presence of zirconia and ceria leads to the formation of a mixed-oxide phase able to interact with palladium ions by incorporating and de-incorporating them at different conditions. The mechanism of Mars–van Kerevelen was considered as the most probable for the reaction of complete methane oxidation. The possibility of the practical application of Pd-modified La-Ce-Zr-Al catalyst is evaluated. The use of a mix of multiple rare and abundant oxides makes the proposed catalyst a cost-effective alternative. Full article

Amines and hydroxylamines are essential compounds in the synthesis of pharmaceuticals and other functionalized molecules. However, the synthesis of primary amines and particularly hydroxylamines remains a challenging task. The most common way to obtain amines and hydroxylamines involves the reduction of substances containing C-N bonds, such as nitro compounds, nitriles, and oximes. Among these, oximes are the most readily accessible substrates easily derived from ketones and aldehydes. However, oximes are much harder to reduce compared to nitro compounds and nitriles. The catalytic heterogeneous hydrogenation of oximes often requires harsh conditions and catalysts with high precious metal loadings, while hydroxylamines are hard to be obtained by this method. In this work, we showed that Pt supported on a porous ceria–zirconia solid solution enables the selective and atom-efficient synthesis of both hydroxylamines and amines through the hydrogenation of oximes, achieving yields of up to 99% under ambient reaction conditions in a “green” THF:H₂O solvent system. The high activity of the 1% Pt/CeO₂-ZrO₂ catalyst (TOF > 500 h⁻¹) is due to low-temperature hydrogen activation on Pt nanoparticles with the formation of a hydride, Pt-H. The strong influence of electron-donating and electron-withdrawing groups on the hydrogenation of aromatic oximes implies the nucleophilic attack of hydridic hydrogen from Pt to the electrophilic carbon of protonated oximes. Full article

With unique 4f electronic shells, rare earth metal-based catalysts have been attracting tremendous attention in electrocatalysis, including oxygen reduction reaction (ORR). In particular, atomically dispersed Ce/CeO₂-based catalysts have been explored extensively due to several unique features. This review article provides a comprehensive understanding of (i) the significance of the effect of Ce high-spin state on ORR activity enhancement on the Pt and non-pt electrocatalysts, (ii) the spatially confining and stabilizing effect of ceria on the generation of atomically dispersed transition metal-based catalysts, (iii) experimental and theoretical evidence of the effect of Ce³⁺ ↔ Ce⁴⁺ redox pain on radical scavenging, (iv) the effect of the Ce 4f electrons on the d-band center and electron transfer between Ce to the N-doped carbon and transition metal catalysts for enhanced ORR activity, and (v) the effect of Pt/CeO₂/carbon heterojunctions on the stability of the Pt/CeO₂/carbon electrocatalyst for ORR. Among several strategies of synthesizing Ce/CeO₂ electrocatalysts, the metal–organic framework (MOF)-derived catalysts are being perused extensively due to the tendency of Ce to readily coordinate with O- and N-containing ligands, which upon undergoing pyrolysis, results in the formation of high surface area, porous carbon networks with atomically dispersed metallic/clusters/nanoparticles of Ce active sites. This review paper provides an overview of recent advancements regarding Ce/CeO₂-based catalysts derived from the MOF precursor for ORR in fuel cells and metal–air battery applications and we conclude with insights into key issues and future development directions. Full article

This work presents a study of the redox properties of CeO₂ particles with (FeCeHS) and without (CeHS) Fe₂O₃ impregnation, as possible innovative catalysts for oxidation and combustion reactions as well as CO₂ activation. The topic, therefore, is part of a broader analysis of environmental catalysis, which aims to reduce the emissions of polluting substances and improve the exploitation of energy resources, with consequent progress in the eco-friendly field. Different laboratory techniques (Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Ultraviolet–Visible (UV-Vis), and Fourier Transform–Infrared (FT-IR) spectroscopies) point out that iron oxide is deposited on the surface of ceria, which maintains its lattice structure, although the particle morphology is slightly changed. Methanol and ethanol adsorption and conversion were evaluated on these catalysts by Temperature Programmed Surface Reaction (TPSR) and by in situ FT-IR spectroscopy of the probe redox properties, evidencing the formation of surface oxidized intermediates and combustion products. The FeCeHS catalyst demonstrates, in our reaction conditions, a good combustion activity in total oxidation of oxygenated molecules, hindering the formation of formaldehyde from methanol and reducing the quantity of CO produced by the partial oxidation reaction. A cooperative effect is suggested by the mixture of these two metals in the oxidation process. Full article

Hydrogen is a priority energy vector for energy transition. Its production from renewable feedstock like ethanol is suitable for many applications. The performance of a Ni catalyst supported on samaria-doped ceria in the production of hydrogen by the reforming of ethanol is investigated, adding Sm₂O₃ to CeO₂ in molar ratios of 1:9, 2:8, and 3:7. A CeO₂-supported Ni catalyst was also evaluated for comparative purposes. The supports were prepared by the coprecipitation method and Ni was incorporated by incipient wetness impregnation to obtain catalysts with a Ni/(Ce+Sm) molar ratio of 4/6. The catalysts were characterized by a nitrogen adsorption isotherm, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Increasing Sm₂O₃ content leads to a more homogeneous distribution of Sm₂O₃ and Ni particles on the support, and higher oxygen mobility, favoring the catalytic properties. The catalyst with a Sm₂O₃/CeO₂ molar ratio of 3/7 showed outstanding behavior, with an average ethanol conversion of 97%, hydrogen yield of 68%, and great stability. The results suggest that the main route for hydrogen production is ethanol dehydrogenation, followed by steam reforming of acetaldehyde, and acetone and ethylene formation are promoted by increasing Sm content in the outer surface of the catalyst. Full article

Developing efficient strategies for VOC emission abatement is an urgent task for protection of the environment and human health. Complete catalytic oxidation exhibits advantages, making it an effective, environmentally friendly, and economically profitable approach for VOC elimination. Pd-based catalysts are known as highly active for hydrocarbon catalytic oxidation. The nature of carrier materials is of particular importance because it may affect activity by changing physicochemical properties of the palladium species. In this work, Al₂O₃, CeO₂, CeO₂-Al₂O₃, and Y-doped CeO₂-Al₂O₃ were used as carriers of palladium catalysts. Methane and benzene were selected as representatives of two types of hydrocarbons. A decisive step in complete methane oxidation is the first C–H bond breaking, while the extraordinary stability of the six-membered ring structure is a challenge in benzene oxidation. The support effect was explored by textural measurements using XRF, XRD, XPS, EPR, and TPR techniques. Three ceria-containing samples showed superior CH₄ oxidation performance, achieving 90% methane conversion at about 300 °C and complete oxidation at 320 °C. Evidence for presence of Pd²⁺ species in all samples regarded as most active was provided by XP-derived analysis. Pd/Y-Ce/Al catalysts exhibited very high activity in benzene oxidation by reaching 100% conversion at 180 °C. The contributions of higher Pd and Ce³⁺ surface concentrations, the presence of O₂⁻-adsorbed superoxo species, and Pd⁰ ↔ PdO redox transfer were considered. The potential of a simple, environmentally friendly, and less energy demanding mechanochemical preparation procedure of mixed oxides was demonstrated. Full article

How important is the support during the rational design of a catalyst? Herein, doped ceria (Zr; Pr and Tb) was used as an active support to prepare Pt catalysts (0.5 wt%) for glycerol selective oxidation. A thorough characterization of achieved catalytic systems showed that the nature of doping elements led to different physicochemical properties. The presence of surface Pr³⁺ and Tb³⁺ not only increased oxygen vacancies but also electron mobility, modifying the oxidation state of platinum particles. The redox properties of the catalyst were also affected, achieving a close interaction between the support and metal particles even in the form of Pt-O-Pr(Tb) solid solutions. Furthermore, the combination of medium-sized metal particle dispersion, strong metal–support interaction and a synergy between the amount of oxygen vacancies and Pt⁰, observed in the Pt/CeTb catalyst, led to a high turnover frequency (TOF) and increased selectivity to glyceric acid. Thus, the present study reveals how a simple structural modification of active supports, such as cerium oxide, by means of doping elements is capable of improving the catalytic performance during glycerol selective oxidation, avoiding the cumbersome methods of synthesis and activation treatments. Full article

The removal of soot particles via high-performance catalysts is a critical area of research due to the growing concern regarding air pollution. Among various potential catalysts suitable for soot oxidation, cerium oxide-based materials have shown considerable promise. In this study, CeO₂ samples obtained using a range of preparation methods (including hydrothermal synthesis (HT), sonochemical synthesis (SC), and hard template synthesis (TS)) were tested in soot combustion. They were compared to commercially available material (COM). All synthesized ceria catalysts were thoroughly characterized using XRD, RS, UV/Vis-DR, XPS, H₂-TPR, SEM, and TEM techniques. As confirmed in the current study, every tested ceria sample can be used as an effective soot oxidation catalyst, with a temperature of 50% soot conversion not exceeding 400 °C in a tight contact mode. A strong correlation was observed between the catalysts’ Ce³⁺ concentration and activity, with higher Ce³⁺ levels leading to improved performance. These findings underscore the importance of synthesis in optimizing ceria-based catalysts for environmental applications. Full article

COx-free H₂, along with uniform carbon nanotubes, can be achieved together in high yield by CH₄ decomposition. It only needs a proper catalyst and reaction condition. Herein, Fe-based catalyst dispersed over titania-incorporated-alumina (Fe/Ti-Al), with the promotional addition of lanthanides, like CeO₂ and La₂O₃, over it, is investigated for a methane decomposition reaction at 800 °C with GHSV 6 L/(g·h) in a fixed-bed reactor. The catalysts are characterized by temperature-programmed reduction (TPR), powder X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The promoted catalysts are facilitated with higher surface area and enhanced dispersion and concentration of active sites, resulting in higher H₂ and carbon yields than unpromoted catalysts. Ceria-promoted 20Fe/Ti-Al catalyst had the highest concentration of active sites and always attained the highest activity in the initial hours. The 20Fe-2.5Ce/Ti-Al catalyst attains >90% CH₄ conversion, >80% H₂-yield, and 92% carbon yield up to 480 min time on stream. The carbon nanotube over this catalyst is highly uniform, consistent, and has the highest degree of crystallinity. The supremacy of ceria-promoted catalyst attained >90% CH₄ conversion even after the second cycle of regeneration studies (against 87% in lanthanum-promoted catalyst), up to 240 min time on stream. This study plots the path of achieving catalytic and carbon excellence over Fe-based catalysts through CH₄ decomposition. Full article

A single-bed and dual-bed catalyst system was studied to maximize H₂ production from the combination of partial oxidation of CH₄ and water gas shift reaction. In addition, the different types of catalysts, including Ni, Cu, Ni-Re, and Cu-Re supported on gadolinium-doped ceria (GDC) were investigated under different operating conditions of temperature (400–650 °C). Over Ni-based catalysts, methane can easily dissociate on a Ni surface to give hydrogen and carbon species. Then, carbon species react with lattice oxygen of ceria-based material to form CO. The addition of Re to Ni/GDC enhances CH₄ dissociation on the Ni surface and increases oxygen storage capacity in the catalyst, thus promoting carbon elimination. In addition, the results showed that a dual-bed catalyst system exhibited catalytic activity better than a single-bed catalyst system. The dual-bed catalyst system, by the combination of 1%Re4%Ni/GDC as a partial oxidation catalyst and 1%Re4%Cu/GDC as a water gas shift catalyst, provided the highest CH₄ conversion and H₂ yield. An addition of Re onto Ni/GDC and Cu/GDC caused an increase in catalytic performance because Re addition could improve the catalyst reducibility and increase metal surface area, as more of their surface active sites are exposed to reactants. Full article