CeO2 Clusters Research Articles

Pushing the Performance Limit of Cu/CeO2 Catalyst in CO2 Electroreduction: A Cluster Model Study for Loading Single Atoms

Pushing the performance limit of catalysts is a major goal of CO2 electroreduction toward practical application. A single-atom catalyst is recognized as a solution for achieving this goal, which is, however, a double-edged sword considering the limited loading amount and stability of single-atom sites. To overcome the limit, the loading of single atoms on supports should be well addressed, requiring a suitable model system. Herein, we report the model system of an ultrasmall CeO2 cluster (2.4 nm) with an atomic precise structure and a high surface-to-volume ratio for loading Cu single atoms. The combination of multiple characterizations and theoretical calculations reveals the loading location and limit of Cu single atoms on CeO2 clusters, determining an optimal configuration for CO2 electroreduction. The optimal catalyst achieves a maximum Faradaic efficiency (FE) of 67% and a maximum partial current density of -364 mA/cm2 for CH4, and can maintain high CH4 FE values over 50% in a wide range of applied current densities (-50 ∼ -600 mA/cm2), exceeding those of the reported catalysts.

Grain boundary and lattice distortion enriched platinum-nickel/ceria colloidal nanoparticle clusters for highly efficient electrooxidation of liquid fuels

Clustering pony-size nanocrystals into secondary structures (colloidal nanocrystal clusters, CNCs) not only combines the properties of primary nanocrystals, but also obtains the novel functions originated from the strengthened interactions between neighboring nanocrystals. Herein, we use the PtNi clusters and CeO2 clusters as the primary subunits to construct the PtNi/CeO2 CNCs, which can enhance the interaction between PtNi and CeO2 clusters, as well as improve the electron conductivity of CeO2 originated from the strengthened attachment. Plenty of grain boundaries and lattice distortion generated after introducing the CeO2 clusters into the inner surface of PtNi CNCs, which improves the electrooxidation activity and anti-CO-poisoning ability of PtNi/CeO2 CNCs. Specifically, the PtNi/CeO2 CNCs exhibited higher specific activities than PtNi CNCs towards electrooxidation reactions of methanol, formic acid and ethylene glycol, as well as 8.5, 7.7 and 7.0 times higher than those of Pt/C, respectively, along with enhanced stability. Combined with the experiments and characterizations, the improved property is originated from the novel functions of grain boundary, subtle lattice distortion and strengthened interaction. This study realizes architectural engineering of CNCs with quite different nanocrystal subunits, and creates an effective strategy for eliminating the disadvantages of conventional Pt/CeO2 catalyst in electrooxidation of liquid fuels.

Electrochemical energy storage and hydrogen peroxide sensing using hybrid framework of CeO2-MnO2 on carbon nano fiber composite.

A hydrothermal methodology followed by calcinations employed for the formation of CeO2 - MnO2 /CNF composite to meet the requirement of superior electrochemical energy storage and sensing performance. The growth of agglomerated tiny CeO2 clusters grown with needle shaped MnO2 decorated over CNF surface gives rise to efficient hybrid sensing and electrode material. The surface structure and chemical features of as prepared nanocomposite was characterized by SEM (scanning electron microscopy), TEM (transmission electron microscopy), FTIR spectroscopy and UV- Visible spectroscopy. The crystalline phase of CeO2-MnO2 decorated over CNF substrate was analyzed by XRD (X-ray diffraction spectroscopy). The electrochemical studies of synthesized sample of CeO2-MnO2/CNF composite exhibited specific capacitance as 1453 Fg−1 at 10 mVs−1 scan rate in 1 M Na2SO4 electrolyte and provided appreciable 74% of capacitive retention after 2500 cycles. The electrode material exhibits superior energy density (ED) of 57 WhKg−1 and power density (PD) 1.8 KWKg−1. The fabricated composite CeO2- MnO2 /CNF also tested as sensing material for H2O2 and offered good sensitivity of 612 μAcm−2mM−1 with correlation coefficient (R) 0.996 up to very low detection limit of 0.1 μM.

Enhancing the thermionic electron emission performance of hafnium with nanocluster doping

Scandate cathode as a thermionic electron emission material has attracted broad interest owing to its high electron emission capability. Despite the large number of studies on the electron emission of the scandate tungsten cathode, there is a lack of understanding of the performance of scandium oxide incorporated in other refractory metal matrix. In this work, by applying first-principles calculations, we study the role that Sc2O3 nanocluster plays in enhancing the electron emission of the Hf cathode (labeled as Sc2O3@Hf). We find that Sc2O3@Hf is both thermally and mechanically stable, exhibits complex surface structures, and possesses inverse relationship between surface energy and work function. Moreover, the work function of the Hf (0001) surface can be reduced to 3.50 eV due to the change in the vacuum level when Sc2O3 nanocluster is doped, while the work function of the Hf (101¯2) surface can be reduced to 3.39, 3.39, and 3.03 eV when doped with Sc2O3, Y2O3, and CeO2 clusters, respectively.

The design of the highly active NH3-SCR catalyst Ce-W/UiO-66: Close coupling of active sites and acidic sites

Ce based catalyst is one of the most important NH3-SCR catalysts. It has been widely recognized that Ce3+ plays a crucial role in NH3-SCR reaction. However, the specific atomic sites for anchoring Ce atoms and the chemical mechanism underlying the formation of Ce3+ in CeO2 are still incompletely known. Herein, we demonstrate that there are abundant hydroxyl groups on the surface of UiO-66, which are the anchoring sites of Ce. As the loading continues to increase, the insufficiently anchored Ce species will agglomerate and induce the formation of Ce3+-rich CeO2 clusters, which served as the redox sites in 10%Ce/UiO-66. On this basis, the strong interaction between W and CeO2 was used to introduce highly dispersed acidic sites (WO3) around CeO2, and thus the redox site and the acidic site are tightly coupled. In situ DRIFTS results revealed that the NH3-SCR reaction pathway followed L-H mechanism on 10%Ce-10%W/UiO-66 catalyst, which is likely to be an important factor for the improved catalytic activity and stability.

A facile preparation of highly dispersed ultrasmall CeO2 clusters on g-C3N4 decorated TiO2 for efficient photooxidation of Hg0

Elemental mercury (Hg0) is the most difficult mercury species to remove from the flue gas. Photocatalysts of transition metal oxide modified TiO2 can effectively remove Hg0. However, the metal oxide easily aggregates into large nanoparticles, reducing the atom utilization efficiency and photocatalytic activity. We presented a facile strategy of employing g-C3N4 as an anchor to successfully load highly dispersed ultrasmall CeO2 clusters on g-C3N4/TiO2. Moreover, the mechanism of anchoring CeO2 clusters and charge transfer of the ternary photocatalyst were explored in-depth. The results demonstrated that at 250 °C, the photooxidation efficiency of Hg0 by CeO2-g-C3N4/TiO2 was several times higher than that of CeO2/TiO2, TiO2, and g-C3N4. The cavity of the tri-s-triazine ring anchored small CeO2 clusters via strong Ce-N coordination bonds. A combined type II and Z-scheme transport paths accelerated the separation of photogenerated electron holes. This research offers valuable surface engineering strategy for highly dispersed ultrasmall cluster decorated TiO2.

Mechanism and sites requirement for CO hydrogenation to CH3OH over Cu/CeO2 catalysts

This study investigated the Cu-CeO2 interactions for CO hydrogenation to methanol by applying rate measurements, chemical titration/desorption, together with in-situ/operando spectroscopy techniques under realistic reaction conditions. Ce components were enriched in the surface region of Cu/CeO2 catalysts while Ce3+ and Ce4+ atoms co-existed during CO hydrogenation. The ratio of Ce3+ to Cu0 in the surface region of catalysts increased linearly with the Ce content. The turnover rates of CH3OH formation increased as a single-valued function of the ratio, irrespective to the individual contact extent between Cu and CeO2 clusters or Ce content in the Cu/CeO2 catalysts, indicating that the Cu0-Ce3+ sites pair acts as active sites for CO hydrogenation to methanol over Ce-CeO2 samples. This study also unraveled that formyl and formate species, as reactive surface intermediates, were co-adsorbed on the Cu0-Ce3+ sites, and their hydrogenation routes occurred concurrently for CH3OH formation during CO-H2 reactions.

Engineering Dual Active Sites at the Interface between Nanoporous Pt and Nanosized CeO2 to Enhance Photo‐Thermocatalytic CO Oxidation

AbstractEffective catalytic oxidation of toxic emissions at low temperatures has great significance on environmental protection. Herein, a novel structure of Pt/CeO2 catalyst, composed of abundant CeO2 clusters captured by nanoporous Pt particles, is designed to enhance the photo‐thermocatalytic CO oxidation by boosting the amount of active sites. The nanosized CeO2 support is fabricated by vacuum thermal decomposition and then the Pt/CeO2 catalyst with a nanoporous structure is synthesized by a one‐pot solution reduction method in a mild condition. The synthesized catalyst exhibits great low‐temperature CO catalytic oxidation activity, and the temperature for 90% CO conversion is 154 °C, achieving comparable catalytic activity with the single‐atom Pt/CeO2 catalyst. Abundant dual active sites, in terms of active oxygen absorbed on the oxygen vacancies and spillover from ceria to Pt, and active Pt sites located near the perimeter of the Pt–CeO2 interface, are formed to improve the photo‐thermocatalytic activity for CO oxidation.

Nanozyme-Based Bandage with Single-Atom Catalysis for Brain Trauma.

Neurotrauma is one of the most serious traumatic injuries, which can induce an excess amount of reactive oxygen and nitrogen species (RONS) around the wound, triggering a series of biochemical responses and neuroinflammation. Traditional antioxidant-based bandages can effectively decrease infection via preventing oxidative stress, but its effectiveness is limited to a short period of time due to the rapid loss of electron-donating ability. Herein, we developed a nanozyme-based bandage using single-atom Pt/CeO2 with a persistent catalytic activity for noninvasive treatment of neurotrauma. Single-atom Pt induced the lattice expansion and preferred distribution on (111) facets of CeO2, enormously increasing the endogenous catalytic activity. Pt/CeO2 showed a 2-10 times higher scavenging activity against RONS as well as 3-10 times higher multienzyme activities compared to CeO2 clusters. The single-atom Pt/CeO2 retained the long-lasting catalytic activity for up to a month without obvious decay due to enhanced electron donation through the Mars-van Krevelen reaction. In vivo studies disclosed that the nanozyme-based bandage at the single-atom level can significantly improve the wound healing of neurotrauma and reduce neuroinflammation.

Rice Husk Derived Porous Silica as Support for Pd and CeO2 for Low Temperature Catalytic Methane Combustion

The separation of Pd and CeO2 on the inner surface of controlled porous glass (CPG, obtained from phase-separated borosilicate glass after extraction) yields long-term stable and highly active methane combustion catalysts. However, the limited availability of the CPG makes such catalysts highly expensive and limits their applicability. In this work, porous silica obtained from acid leached rice husks after calcination (RHS) was used as a sustainable, cheap and broadly available substitute for the above mentioned CPG. RHS-supported Pd-CeO2 with separated CeO2 clusters and Pd nanoparticles was fabricated via subsequent impregnation/calcination of molten cerium nitrate and different amounts of palladium nitrate solution. The Pd/CeO2/RHS catalysts were employed for the catalytic methane combustion in the temperature range of 150–500 °C under methane lean conditions (1000 ppm) in a simulated off-gas consisting of 9.0 vol% O2, and 5.5 vol% CO2 balanced with N2. Additionally, tests with 10.5 vol% H2O as co-feed were carried out. The results revealed that the RHS-supported catalysts reached the performance of the cost intensive benchmark catalyst based on CPG. The incorporation of Pd-CeO2 into RHS additionally improved water-resistance compared to solely Pd/CeO2 lowering the required temperature for methane combustion in presence of 10.5 vol% H2O to values significantly below 500 °C (T90 = 425 °C).

Influence of Matrix Nature on the Structural Characteristics of In2O3–CeO2 and SnO2–CeO2 Composites Fabricated by the Impregnation Method

The structural characteristics, valence states, and distribution of cerium ions between the components in In2O3–CeO2 and SnO2–CeO2 nanocomposites fabricated using the impregnation method were studied. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) were used to show that, during impregnation, cerium ions are not included into In2O3 crystals and are disposed only on their surface in the form of nano-sized crystallites or amorphous clusters. On the other side, under the contact of CeO2 clusters with a surface of SnO2 matrix crystals, cerium ions penetrate into the surface layer of these crystals. In contrast to an In2O3–CeO2 system, where the addition of CeO2 does not affect the conduction activation energy, where cerium oxide is added to SnO2, the observed increase in the resistance of a SnO2–CeO2 composite is accompanied by a sufficient increase in activation energy. These data and the XPS spectra confirm the modification of the surface layers of conductive SnO2 crystals as, a result of the penetration of cerium ions into these layers.

Influence of DC magnetron sputtering reaction gas on structural and optical characteristics of Ce-oxide thin films

The influence of the reaction gas composition during the DC magnetron sputtering process on the structural, chemical and optical properties of Ce-oxide thin films was investigated. X-ray diffraction (XRD) studies confirmed that all thin films exhibited a polycrystalline character with cubic fluorite structure for cerium dioxide. X-ray photoelectron spectroscopy (XPS) analyses revealed that cerium is present in two oxidation states, namely as CeO2 and Ce2O3, at the surface of the films prepared at oxygen/argon flow ratios between 0% and 7%, whereas the films are completely oxidized into CeO2 as the aforementioned ratio increases beyond 14%. Various optical parameters for the thin films (including an optical band gap in the range of 2.25–3.1 eV) were derived from the UV–Vis reflectance. A significant change in the band gap was observed as oxygen/argon flow ratio was raised from 7% to 14% and this finding is consistent with the high-resolution XPS analysis of Ce 3d that reports a mixture of Ce2O3 and CeO2 in the films. Density functional theory (DFT+U) implemented in the Cambridge Serial Total Energy Package (CASTEP) was carried out to simulate the optical constants of CeO2 clusters at ground state. The computed electronic density of states (DOSs) of the optimized unit cell of CeO2 yields a band gap that agrees well with the experimentally measured optical band gap. The simulated and measured absorption coefficient (α) exhibited a similar trend and, to some extent, have similar values in the wavelength range from 100 to 2500 nm. The combined results of this study demonstrate good correlation between the theoretical and experimental findings.

Comprehensive Understanding of the Spatial Configurations of CeO2 in NiO for the Electrocatalytic Oxygen Evolution Reaction: Embedded or Surface‐Loaded

AbstractIntroducing cerium (Ce) species into electrocatalysts has been recently developed as an effective approach to improve their oxygen evolution reaction (OER) performance. Importantly, the spatial distribution of Ce species in the hosts can determine the availability of Ce species either as additives or as co‐catalysts, which would dictate their different contributions to the enhanced electrocatalytic performance. Herein, the comprehensive investigations on two different catalyst configurations, namely CeO2‐embedded NiO (Ce‐NiO‐E) and CeO2‐surface‐loaded NiO (Ce‐NiO‐L), are performed to understand the effect of their specific spatial arrangements on OER characteristics. The Ce‐NiO‐E catalysts exhibit a smaller overpotential of 382 mV for 10 mA cm−2 and a lower Tafel slope of 118.7 mV dec−1, demonstrating the benefits of the embedded configuration for OER, as compared with those of Ce‐NiO‐L (426 mV and 131.6 mV dec−1) and pure NiO (467 mV and 140.7 mV dec−1), respectively. The improved OER property of Ce‐NiO‐E originates from embedding small‐sized CeO2 clusters into the host for the larger specific surface area, richer surface defects, higher oxygen adsorption capacity, and better optimized electronic structures of the surface active sites, as compared with Ce‐NiO‐L. Above findings provide a valuable guideline for and insight in designing catalysts with different spatial configurations for enhanced catalytic properties.

On the enhancing effect of Ce in Pd-MOR catalysts for NOx CH4-SCR: A structure-reactivity study

The effect of palladium and cerium species on the selective catalytic reduction (SCR) of NOx using methane as reductant (NOx CH4-SCR) has been investigated using Pd-HMOR and PdCe-HMOR system. The catalysts have been characterised by H2-TPR, DRS UV–Vis, TEM/EDS and FTIR using CO and pyridine as probe molecules. The oxidation of NO and CH4-SCR catalytic tests have been conducted using monometallic and bimetallic formulations.Above 0.3wt.% Pd, the increase in Pd loading leads to a decrease in NOx selectivity towards N2, with the formation of N2O, and a decrease in the CH4 selectivity towards SCR, due to CH4 direct combustion. H2-TPR and FTIR-CO studies indicate that palladium is stabilised as Pd2+ in ion-exchange position, probably in two different sites within the MOR framework.The addition of cerium to Pd-HMOR enhances its catalytic performance for NOx CH4-SCR. With 1wt.% Ce, both NOx conversion into N2 and CH4 selectivity towards SCR have increased. Small CeO2 clusters interacting with palladium are likely to play a major role in this catalytic reaction. The number of such species increases up to Ce loading of ca. 2wt.%. However, above 3wt%, NOx conversion values decrease with Ce loading, which is attributed to the formation of bulk CeO2 species not interacting with palladium.

Low‐Lanthanide‐Content CeO2/MgO Catalysts with Outstandingly Stable Oxygen Storage Capacities: An In‐Depth Structural Characterization by Advanced STEM Techniques

AbstractA novel CeO2/MgO catalyst with low ceria loading has been synthesized. This catalyst showed unique redox properties compared with conventional high and low surface area CeO2. Advanced (scanning) transmission electron microscopy techniques revealed the presence of a variety of highly dispersed ceria nanostructures: isolated CeOx entities, CeO2 clusters, as well as fairly small (<5 nm) CeO2 nanoparticles. More interestingly, this CeO2/MgO catalyst showed outstanding stability in its redox response against high temperature aging treatments. Thus, after reduction in hydrogen at 950 °C and further oxidation at 500 °C, CeO2 reduction effects took still place at low temperatures, and no significant loss of oxygen storage capacity (OSC) was detected. Unique ceria‐bilayer nanostructures were found and characterized in the aged catalyst. Their peculiar structural and chemical properties seem to be responsible for the large improvement observed in the stability of the redox response.

Cerium Oxide Nanoclusters on Graphene/Ru(0001): Intercalation of Oxygen via Spillover.

Cerium oxide is an important catalytic material known for its ability to store and release oxygen, and as such, it has been used in a range of applications, both as an active catalyst and as a catalyst support. Using scanning tunneling microscopy and Auger electron spectroscopy, we investigated oxygen interactions with CeOx nanoclusters on a complete graphene monolayer-covered Ru(0001) surface at elevated temperatures (600-725 K). Under oxidizing conditions (PO2 = 1 × 10(-7) Torr), oxygen intercalation under the graphene layer is observed. Time dependent studies demonstrate that the intercalation proceeds via spillover of oxygen from CeOx nanoclusters through the graphene (Gr) layer onto the Ru(0001) substrate and extends until the Gr layer is completely intercalated. Atomically resolved images further show that oxygen forms a p(2 × 1) structure underneath the Gr monolayer. Temperature dependent studies yield an apparent kinetic barrier for the intercalation of 1.21 eV. This value correlates well with the theoretically determined value for the reduction of small CeO2 clusters reported previously. At higher temperatures, the intercalation is followed by a slower etching of the intercalated graphene (apparent barrier of 1.60 eV). Vacuum annealing of the intercalated Gr leads to the formation of carbon monoxide, causing etching of the graphene film, demonstrating that the spillover of oxygen is not reversible. In agreement with previous studies, no intercalation is observed on a complete graphene monolayer without CeOx clusters, even in the presence of a large number of point defects. These studies demonstrate that the easily reducible CeOx clusters act as intercalation gateways capable of efficiently delivering oxygen underneath the graphene layer.

Small CeO2 clusters on the surface of semiconductor nanoparticles

Nanocomposite sensors containing CeO2 clusters on the surface of In2O3 and SnO2 crystals were synthesized. The structure of these systems was determined by Raman spectroscopy. In the CeO2 nanoclusters deposited on In2O3 crystals, the Ce-O vibration frequency was 462 cm−1 and did not depend on the CeO2 concentration. The Raman spectra of the clusters deposited on SnO2 crystals contained two peaks of Ce-O vibrations with frequencies of 462 and 470 cm−1. It was concluded that the peak at 470 cm−1 showed itself at low CeO2 concentrations in the composite (1–3 wt %) and its intensity quickly decreased as the CeO2 concentration increased; this peak was attributed to the CeO2 clusters that directly contact the SnO2 crystals and contain dissolved Sn+4. It was shown that when CeO2 was deposited on In2O3, the In+3 ions were not transferred into the deposited CeO2 clusters because of the difference between the charges and valences of the metal ions in the substrate and clusters; the mean size of the clusters was 9 nm. The relationship between the structure of the CeO2 nanoclusters and their influence on sensor effects was discussed.

Silver nanoparticles supported on CeO2-SBA-15 by microwave irradiation possess metal-support interactions and enhanced catalytic activity.

Metal-support interactions (MSIs) and particle size play important roles in catalytic reactions. For the first time, silver nanoparticles supported on CeO2-SBA-15 supports are reported that possess tunable particle size and MSIs, as prepared by microwave (MW) irradiation, owing to strong charge polarization of CeO2 clusters (i.e., MW absorption). Characterizations, including TEM, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure, were carried out to disclose the influence of CeO2 contents on the Ag particle size, MSI effect between Ag nanoparticles and CeO2-SBA-15 supports, and the strong MW absorption of CeO2 clusters that contribute to the MSIs during Ag deposition. The Ag particle sizes were controllably tuned from 1.9 to 3.9 nm by changing the loading amounts of CeO2 from 0.5 to 2.0 wt%. The Ag nanoparticle size was predominantly responsible for the high turnover frequency (TOF) of 0.41 min(-1) in ammonia borane dehydrogenation, whereas both particle size and MSIs contributed to the high TOF of 555 min(-1) in 4-nitrophenol reduction for Ag/0.5CeO2-SBA-15, which were twice as large as those of Ag/SBA-15 without CeO2 and Ag/CeO2-SBA-15 prepared by conventional oil-bath heating.

Structural and electronic properties of cubic CeO2: Unpaired electrons in CeO2

AbstractThe high performance of cerium dioxide (CeO2) as support for Au in catalysis relies on its ability to anchor Au clusters and transfer charge to the gold nanoparticles. These two processes are governed by oxygen vacancy formation. To study the localization and the mobility of the electrons left behind by the vacancy from the relaxed oxygen, an experimental and theoretical study is hereby reported. CeO2 powder was synthesized by the polyacrylamide gel method and its microstructure was characterized by X‐ray diffraction and high resolution scanning electron microscopy. We have determined the presence of only the fluorite cubic phase in our samples and the formation of acicular particles of ∼1 μm in length. In the electron spin resonance study, we observed that the unpaired electrons are quasi free. A single‐line paramagnetic signal is observed that exhibits spin characteristics with a g‐value of 2.0039 at room temperature. This compares well to the value of 2.0023 determined for free electrons. In addition, we have conducted a theoretical study using a density functional theory cluster approach and calculate the electron spin density of CeO2 clusters. We find that spin density clouds are localized around the cerium ions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Effect of Cumulative Severe Plastic Deformation on Microstructure and Properties of Ag-CeO2 Composites by RSP

Abstract In present study, Ag-CeO2 composites were prepared by reaction synthesis processing. The phases, the procedure of microstructure homogenization, and the microstructure and physical property evolution of the Ag-CeO2 composites via the severe plastic deformation were researched extensively. XRD, SEM and EDS analysis indicate that the Ag-CeO2 composites with tiny AgCeO2 clusters forming in the Ag matrix can be obtained by reactive synthesis processing, and the microstructure shows the tiny CeO2 clusters distribute in the Ag matrix which becomes homogenous and dispersed after cumulative severe plastic deformation. The tensile strength and electrical resistivity decrease with the increase of true strain, while the elongation changes contrarily.