Ceria Particles Research Articles

Synergistic Effect of Simultaneous Doping of Ceria Nanorods with Cu and Cr on CO Oxidation and NO Reduction.

Ceria particles play a key role in catalytic applications such as automotive three‐way catalytic systems in which toxic CO and NO are oxidized and reduced to safe CO2 and N2, respectively. In this work, we explore the incorporation of Cu and Cr metals as dopants in the crystal structure of ceria nanorods prepared by a single‐step hydrothermal synthesis. XRD, Raman and XPS confirm the incorporation of Cu and Cr in the ceria crystal lattices, offering ceria nanorods with a higher concentration of oxygen vacancies. XPS also confirms the presence of Cr and Cu surface species. H2‐TPR and XPS analysis show that the simultaneous Cu and Cr co‐doping results in a catalyst with a higher surface Cu concentration and a much‐enhanced surface reducibility, in comparison with either undoped or singly doped (Cu or Cr) ceria nanorods. While single Cu doping enhances catalytic CO oxidation and Cr doping improves catalytic NO reduction, co‐doping with both Cu and Cr enhances the benefits of both dopants in a synergistic manner employing roughly a quarter of dopant weight.

Contamination Mechanism of Ceria Particles on the Oxide Surface after the CMP Process

The contamination behavior of ceria particles (50 and 100 nm) with oxide surfaces at pH 4 and 8 was studied using dipping and polishing conditions. Higher contamination at pH 4 than pH 8 was observed for dipping cases. In contrast, pH 8 conditions produced higher contamination than pH 4 for polishing cases. The difference in the contamination between dipping and polishing could be attributed to electrostatic attraction and chemical bonding. During polishing, weak or no chemical bonding may occur at pH 4, whereas the Ce–O–Si bond may form due to the surface charges of ceria and silica at pH 8. XPS analysis revealed that strong additional peaks related to Ce–O–Si bonding were detected for O 1s spectra at a binding energy of around 532 eV for surfaces polished at pH 8. Similar results were observed for 50 nm as well as 100 nm ceria particles. Based on the results, we concluded that ceria particles would have different adhesion behaviors for dipping and polishing at the same pH value. Also, higher ceria adhesion was observed at higher pH compared to lower pH during the CMP process, which was correlated to strong Ce–O–Si chemical bond formation due to the CMP conditions.

Comprehensive permeation analysis and mechanical resistance of electroless pore-plated Pd-membranes with ordered mesoporous ceria as intermediate layer

H2-selective composite membranes, particularly those based on palladium films deposited onto porous stainless-steel supports, represent a promising technology to be practically included in both independent devices and membrane reactors. To reach thin H2-selective films and hence high permeance values, the use of a wide variety of intermediate layers is usually adopted in the literature. However, an agreement about the best solution is not found up to now. In this context, the current study presents the use of Ordered Mesoporous Ceria (OMC) particles as intermediate layer for the improvement of permeation properties of Electroless Pore-Plated (ELP-PP) Pd-composite membranes. OMC was obtained by nanocasting from SBA-15 as temporary template and cerium nitrate (III) hexahydrate as metal precursor. Resultant OMC particles have around 100 nm spherical diameter, an average pore-size diameter of 10–12 nm, and a total BET surface of around 134 m2/g. This material was next deposited onto the external surface of tubular Porous Stainless-Steel (PSS) supports by vacuum-assisted dip-coating (VA-DC) to form an intermediate layer that makes the preparation of a defect-free and thin Pd-film easier. This procedure allows the preparation of Pd composite membranes (OMC-Pd) with Pd thicknesses around 10 μm, H2 permeances of 1.03·10−3 mol m−2 s−1 Pa−0.5 at 400 °C, and high ideal selectivity αH2/N2 ≥ 24,000. It should be noted that H2 permeance has been increased up to 6 times in comparison with other ELP-PP membranes without any intermediate layer and 2 times in contrast to membranes containing dense CeO2 particles instead of the mesoporous ones for the intermediate layer. Moreover, Pd-membranes so prepared (OMC-Pd) have shown excellent mechanical resistance in a wide variety of operating conditions such as temperature, pressure, and permeate flux direction, maintaining a high H2-selectivity without delamination or peeling. These properties were also maintained in case of feeding different H2/N2 mixtures, where the concentration-polarization effect seems to stabilize for lower H2 concentration values in the feed stream, not being noticeably influenced by temperature.

Atomically dispersed copper species on ceria for the low-temperature water-gas shift reaction

The structure of copper species, dispersed on nanostructured ceria (particles, rods and cubes), was analyzed by scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS). It was interestingly found that the density of surface oxygen vacancies (or defect sites), induced by the shape of ceria, determined the geometrical structure and the chemical state of copper species. Atomically dispersed species and monolayers containing few to tens of atoms were formed on ceria particles and rods owing to the enriched anchoring sites, but copper clusters/particles co-existed, together with the highly dispersed atoms and monolayers, on cubic ceria. The atomically dispersed copper sites and monolayers interacted strongly with ceria, involving a remarkable charge transfer from copper to ceria at their interfaces. The activity for the low-temperature water-gas shift reaction of the Cu/CeO2 catalysts was associated with the fraction of the positively-charged copper atoms, demonstrating that the active sites could be tuned by dispersing Cu species on shape-controlled ceria particles.

Study on the effect of ceria concentration on the silicon oxide removal rate in chemical mechanical planarization

The effect of ceria concentration on the removal rate of silicon oxide via chemical mechanical planarization was investigated. The removal rate decreased with increasing abrasive concentration. Surface analysis was used to confirm adsorption of ceria particles on silicon oxide wafers. A wide adsorption layer was formed on the wafer surface with ceria abrasive of both positive and negative charges. This adsorption layer was widened with increasing ceria concentration. Silica CMP did not show a decrease of removal rate and showed only a small area of adsorption. It was confirmed that adsorption of these ceria particles is caused by applied pressure as well as the effect of electrostatic force. It was confirmed through particle size distribution analysis of the slurry waste that the largest number of activated particles was achieved at 0.5 wt% concentration. The mechanism of effect for ceria concentration vs. removal rate is suggested with a schematic. Understanding of adsorption between ceria and silicon oxide, understood from various perspectives, will be helpful in further investigation of post CMP cleaning processes as well as CMP processes.

Tuning Single‐atom Pt1−CeO2 Catalyst for Efficient CO and C3H6 Oxidation: Size Effect of Ceria on Pt Structural Evolution

AbstractTo reveal the effect of ceria particle size on dispersion and structure of supported Pt catalysts during preparation, activation, and reaction testing, a unique CeO2/Al2O3 support (CA−c) with smaller CeO2 particle size and more surface defects was prepared using a colloidal CeO2 precursor, comparing with a conventional CeO2/Al2O3 support (CA−n) using cerium nitrate as precursor. More atomically dispersed Pt and abundant Pt−O−Ce structures were observed in the Pt/CA−c catalyst than in the Pt/CA−n catalyst. Both parent catalysts received significant enhancement on their catalytic CO oxidation activities if activated by 10% hydrogen at 400 °C before reaction. Between the two representative catalysts, the extent of activity enhancement upon activation was more pronounced for Pt/CA−c. We found that smaller Pt clusters with more active ionic Pt sites were generated on the activated Pt/CA−c catalyst, while agglomerated larger Pt particles with more metallic sites were formed on the activated Pt/CA−n. The facile formation of Ce3+ was also indicative of more active metal‐support interfaces in the activated Pt/CA−c catalyst. These results highlight the importance of regulating ceria support particles to enable a controlled anchoring and subsequent activation of Pt single atoms for low‐temperature CO oxidation reaction.

3D trajectories and diffusion of single ceria particles near a glass surface and their removal

Abstract

Space charge layer induced superionic conduction and charge transport behaviour of “alkali carbonates and tri-doped ceria nanocomposites” for LT-SOFCs applications

Our investigations highlight the importance of alkali carbonates addition on influencing the superionic conduction via composite interfaces in doped ceria based nanocomposites Ce0.76Pr0.08Sm0.08Gd0.08O2-δ/(Li0.52Na0.48)2CO3 (CPSG/LNC). X-ray diffraction of composite confirms single-phase cubic CeO2 like structure as major phase with few weak reflections due to (Li0.52Na0.48)2CO3 suggesting that the major part of the carbonates transform into amorphous phase during synthesis. The CPSG/LNC nanocomposites show improved ionic conductivity in the temperature range 200–650 °C with maximum ionic conductivity σ650°C = 6.02 × 10−2 S-cm−1 for composition having 35 wt% of (Li0.52Na0.48)2CO3. The TEM analysis reveals that the amorphous layer of carbonates forms on the nanocrystalline codoped ceria particles to develop a close shell-layer enclosure, stimulating space charge layer as a preferred conduction paths in the composite electrolytes. Dielectric and complex ac-conductivity measurements confirm the existence of low frequency interfacial/space-charge polarization along the grain boundary regions and multi-ion long-range conduction along the interface regions.

The Cycle Characteristics of Slurries in Chemical Mechanical Polishing (CMP) of Fused Silica

AbstractThe role of the polishing slurry is crucial in chemical mechanical polishing (CMP) of fused silica (FS). Researching on cycle characteristics of slurry contributes to exploring the deterioration mechanism and prolonging the service time. In this paper, cycle characteristics of colloidal silica slurry and ceria slurry applied in CMP of FS were investigated when the two slurries were recycled for 53 and 42 hours, respectively. The changes of the material removal rate (MRR) and surface roughness (Rq) of FS were analyzed, and the material removal mechanism of both slurries had been discussed. Scanning electron microscopy (SEM) and X‐ray photoelectron spectroscopy (XPS) were employed to interpret the changes of the slurry characteristics. The results indicate that with the increase of recycling time, decrease of pH value and increase of solid content are two common factors which affected MRR improvement for both two kinds of slurries. For colloidal silica slurry, the Rq value fluctuates probably due to unchanged sizes and deformed shapes of SiO2. For ceria slurry, the Rq value increases probably due to the size reduction of ceria particles.

Synergistic effect of dealumination and ceria impregnation to the catalytic properties of MOR zeolite

MOR zeolite has been extensively employed as a catalyst in industries. However, high Brønsted acidity in MOR leads to rapid deactivation due to coke deposition on the pore mouths; thus, the surface acidity of MOR needs to be moderated. Herein, we report a modification of MOR chemical composition via acid treatment and deposition of ceria nanoparticles using a wet impregnation method. The acid treatment successfully increases the Si/Al ratio of MOR from 8.39 to 11.58 and reduces the total acid site concentration of MOR from 990 μmol/g to 752 μmol/g. The acidity of MOR is decreased when the Si/Al ratio is increased since the quantity of Brønsted acid sites is proportional to the number of Al framework. In addition, the acid treatment also improves the external surface area of MOR. Furthermore, ceria particles were successfully deposited on the MOR surface using wet impregnation method. The ceria content of parent MOR sample is lower compared to that of preceded by the acid treatment, which may be attributed to the formation of more terminal silanol groups. Finally, catalytic test on Friedel–Crafts alkylation of toluene with benzyl alcohol shows that the synergy between dealumination and the impregnation of ceria significantly improves the activity of MOR zeolite.

Ag–CeO2/SBA-15 composite prepared from Pluronic P123@SBA-15 hybrid as catalyst for room-temperature reduction of 4-nitrophenol

The catalytic activity of the SBA-15-supported silver and/or ceria catalysts in the 4‑nitrophenol reduction under mild conditions is described. The catalysts are prepared by wetness impregnation of Pluronic P123@SBA-15 hybrid applied to stabilize small Ag and CeO2 particles. The bare SBA-15 and the catalysts on the basis thereof are characterized by N2 sorption, XRD, SAXS, TEM, UV–vis DRS, and H2-TPR. The catalysts possess high surface area (594−754 m2/g) and narrow pore size distributions (5.5–7.8 nm). Silver and ceria particles are stabilized in a highly dispersed state (predominant sizes below 5 nm) in the ordered SBA-15 structure. The Ag–CeO2/SBA-15 catalyst demonstrates the presence of the developed Ag–CeO2 interface that enhances the catalytic activity in 4-nitrophenol reduction to 4‑aminophenol (k = 0.016 s−1) at room temperature and ambient pressure due to the cooperation of silver and ceria active sites.

Stability of nanocrystalline Ce-Gd mixed oxide on Al2O3 support

In various technological applications (solid oxide fuel cells, heterogeneous catalysis) ceria based mixed oxides are in contact with alumina used as support or additive. It is thus important to gain knowledge on the possible interactions between the mixed oxide and the alumina at temperatures and atmospheres, corresponding to real processing or working conditions. In this work, structure and chemical stability of Ce0.5Gd0.5O1.75 nanoparticles supported on a high surface γ-Al2O3 were studied in oxidizing and reducing atmosphere by XRD, TEM, SEM-EDS, STEM-EDS, Raman spectroscopy and H2-TPR. Application of a reverse microemulsion method enabled the synthesis of highly Gd-doped, homogeneous ceria particles with mean size ∼2 nm, ideal for the studies on effect of the interaction with Al2O3 support on chemical and structural stability od doped ceria. Poor tolerance of Ce0.5Gd0.5O1.75 nanoparticles to sintering is greatly improved by their dispersion on a high surface alumina support. In oxidizing atmosphere the particles were chemically stable on the alumina up to 1100 °C, undergoing little sintering with increase of the mean crystallite size to 7 nm. In hydrogen atmosphere, the stability is limited by complex, chemical interaction between Gd doped ceria and Al2O3 support, which has been studied in detail for the first time. Ce0.5Gd0.5O1.75 nanoparticles started to spread over the support at 600 °C into a nanometer thick amorphous layer, and at 900 °C crystallized into a mixed (Gd,Ce)4Al2O9 aluminate. The (Gd,Ce)4Al2O9 aluminate, which is an intermediate phase at the low ceria/alumina molar ratio used, decomposed at 1000 °C into tetragonal CeAlO3 and hexagonal GdAlO3. When deposited on Al2O3, Ce0.5Gd0.5O1.75 nanoparticles showed much improved reducibility, especially in low-temperature range (300–600 °C). This effect is further enhanced by the spreading of ceria over Al2O3 occurring during treatment in hydrogen atmosphere at elevated temperature.The nanocrystalline Ce-Gd-O/Al2O3 system prepared by the impregnation with an aqueous solution of Ce and Gd nitrates appeared to be chemically inhomogeneous and less stable in both oxidizing and reducing atmospheres.d

Dispersion of copper oxide species on nanostructured ceria.

Copper oxides species deposited on ceria rods, particles, and cubes were examined for low-temperature oxidation of CO. It was found that the shape of ceria altered the dispersion and chemical state of copper species considerably. CuOx monolayers and bilayers were formed on ceria rods and particles, while multilayers and faceted particles co-existed on ceria cubes. The formation of Cu+ species at the copper-ceria interface involved a significant charge transfer from copper oxides to the ceria surface via a strong electronic interaction, which was more pronounced on ceria rods. The concentrations of surface Cu+ and oxygen vacancies followed the order rods > particles > cubes, in line with their catalytic activity for CO oxidation at 343 K.

Trajectories, diffusion, and interactions of single ceria particles on a glass surface observed by evanescent wave microscopy

Abstract

A comparative study of hydrothermally derived Mn, Fe, Co, Ni, Cu and Zn doped ceria nanocatalysts

The catalytic activity and properties of hydrothermally derived nanocrystalline ceria, as well as ceria doped substitutionally by 10 mol. % of Mn, Fe, Co, Ni, Cu and Zn, were investigated.It was determined that in the course of synthesis and under the synthesis conditions applied Mn, Cu and Zn enter the ceria crystal lattice, while for other investigated metals additional crystal phases appear. The presence of a foreign cation did not affect the ceria crystallite size, particle size or specific surface area. Nanocatalysts with crystallite and particle sizes close to 5 nm and with specific surface area around 200 m2 g-1 were prepared. However, except for Mn, a negative influence of dopants on coarsening resistance has been observed. Doping with Cu and Mn significantly enhanced the ceria catalytic activity, slight improvement was observed for samples with Zn and Fe, while samples with Ni and Co showed a decreased catalytic activity.

Cleaning Solutions for Removal of ∼30 nm Ceria Particles from Proline and Citric Acid Containing Slurries Deposited on Silicon Dioxide and Silicon Nitride Surfaces

A previously developed aqueous cleaning solution (4.2 mol l−1 each of H2O2 and NH4OH) was found to be ineffective in cleaning oxide/nitride surfaces after contamination with ceria particles from slurries containing proline or citric acid. However, a cleaning solution consisting of 1 wt% ascorbic acid, 1 wt% ammonium carbonate and 50 ppm triton X-100 at pH 12, aided by ultrasonic cleaning, removed these ceria particles, even those as small as ∼30 nm, from both oxide and nitride surfaces with efficiencies >99% as determined by AFM imaging. Fourier transform infrared (FTIR) spectroscopy results indicated that ceria particles treated with these additives can also bind with oxide/nitride surfaces through Si–O–C and Si–O–H bonds, in addition to any Ce–O–Si, where the C and H atoms are from the additives adsorbed on the ceria particles. All these bonds are broken effectively by the nucleophilic attack of hydroxyl anions in the cleaning solution while triton X-100 in the cleaning solution reduces adhesion between the particles and the film surface and facilitates cleaning via a wetting mechanism. More importantly, ascorbic acid and ammonium carbonate prevent particle redeposition by complexing with the removed particles and blocking the active Ce3+ species on their surface.

Communication—Effect of Hydrogen Water on Ceria Abrasive Removal in Post-CMP Cleaning

In the present study, hydrogen water was applied to ceria abrasive removal in post-CMP cleaning. The surface of the ceria abrasive was reduced by the hydrogen water from the Ce4+ to Ce3+ state. Reduction of the ceria abrasive can weaken the bonding between ceria and the SiO2 wafer surface. X-ray photoelectron spectroscopy (XPS) and UV–visible observations were utilized to reveal the reduction from Ce4+ to Ce3+ by hydrogen water. Thus, the remaining ceria particles and Ce-ion concentrations were reduced by 70% and 63%, respectively.

Chemical Mechanical Polishing (CMP) of Fused Silica (FS) Using Ceria Slurry Recycling

Due to its high optical permeability, excellent high temperature resistance, and chemical inertness, fused silica (FS) has been widely used in astronomical telescopes, laser systems, and optical communication. Based on the smooth surface polishing of fused silica using ceria slurry, the variations of polishing performance of fused silica in the recycling polishing were studied. Meanwhile, the variations of the ceria slurry characteristics in chemical mechanical polishing (CMP) of fused silica were investigated. The variations of the average size and the morphology of the abrasive ceria particles were measured. X-Ray photoelectron spectroscopy (XPS) measurement was used to evaluate the chemical state of the ceria slurry in different recycling polishing time. Then, elastic moduli via atomic force microscope (AFM) force curves measurement of the abrasive ceria particles were measured and analyzed. And the change of shear viscosity of the slurry was monitored. In addition, the relative removal mechanism of fused silica using ceria slurry recycling was discussed.

Synthesis of Ni–Co–ZrO2 nanocomposites doped with ceria particles via electrodeposition as highly protective coating

A novel Ni–Co–ZrO2 nanocomposites doped with ceria particles has been synthesized by pulse electrodeposition as highly protective coating. The effect of current density and duty cycle on its structure and properties were evaluated. The composite coatings are compact with hill-valley like structure. The average roughness (Sa) was about 73–98 nm and the duty cycle of 70% benefits low roughness and fine-grained structure. The crystallite size of these coatings is 17–20 nm. The nanocrystalline coating exhibits the preferred orientation of Ni (111) texture. AFM and XPS were utilized to analyze the surface properties. EIS results indicated that the electrodeposition parameters greatly affect the corrosion behavior of the nanocomposites. The electrochemical behavior varied with immersing time. Duty cycle of 30%, current density of 2 A dm−2 were the appropriate parameters for the best corrosion and wear resistance and optimal long-term electrochemical stability in 3.5 wt% NaCl corrosive solution. This coating shows good potential for engineering application in aggressive medium.

Pd-thickness reduction in electroless pore-plated membranes by using doped-ceria as interlayer

This work presents the use of doped CeO2 particles with palladium as intermediate barrier for the preparation of fully dense Pd films by Electroless Pore-Plating. The use of doped ceria particles instead of non-doped ones clearly helps to reduce the final palladium thickness required to prepare a fully dense membrane over porous stainless steel supports from 15 to 9 μm (average values by gravimetric analyses), thus saving around 40% of total palladium required in the process. Pure hydrogen permeation tests reveal a consequent increase in the H2 flux in the range 15–30%, depending on the operation mode. Thus, a H2 permeance of 6.26·10−4 mol m−2 s−1 Pa−0.5 at 400 °C and ΔP = 1 bar is reached, maintaining a really high H2/N2 ideal separation factor (≥10,000) and an activation energy within the typical range for these type of membranes, Ea = 13.1 kJ mol−1. Permeation of binary H2/N2 gas mixtures and the effect of feeding the mixture from the inner or the outer side of the membrane have been also studied. A significant concentration-polarization effect was observed, being higher when the gas is fed from the inner to the outer side of the membrane. This effect becomes more relevant for the membrane prepared with doped CeO2, instead of raw CeO2, due to its lower Pd thickness and higher relative influence of the surface processes. However, it should be emphasized that higher H2 permeance values were obtained for the entire set of experiments when using the Pd-membranes containing doped ceria. Finally, long-term permeation tests for more than 850 h with pure gases at T = 400 °C and ΔP = 1 bar were also carried out, demonstrating a suitable mechanical stability of membranes at these operating conditions.