Thermal Aging Behavior of Pt-only TWC Converters Under Simulated Exhaust Conditions: Effect of Rare Earths (CeO2, La2O3) and Alkali (Na) Modifiers

In the present work, the feasibility to design monometallic (Pt-only), low metal loading (0.1 wt% Pt) three-way catalytic converters (TWCs), with comparable catalytic efficiency and thermostability to that of commercial bimetallic Pt/Rh TWCs has been explored. It is shown that this can be accomplished by modifying Pt/??-??l₂O₃ washcoat of TWC via two different methods of promotion: support-mediated promotion by modifying the ??-Al₂O₃ support with rare earth structure-modifiers (CeO₂?CLa₂O₃) and surface-induced promotion by directly modifying the Pt surface with alkalies (e.g., Na), producing a doubly-promoted Pt(Na)/Al₂O₃-(CeO₂?CLa₂O₃) TWC. The catalytic performance of as prepared TWCs in comparison to that of a commercial bimetallic (Pt?CRh) TWC, under simulated exhaust conditions at the stoichiometric point appears to be superior, even after severe thermal treatment at 900 °C for 5 h in air and despite the fact that the latter contain 4.5-fold higher noble metals loading. Moreover, the evolution of structural, textural and surface behavior of aged catalysts was identified by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy and diffuse reflectance infrared Fourier transform spectroscopy and appropriately correlated with the catalytic performance and thermostability of TWCs.     

Effect of CexZryLazOδ Mixed Oxides on the Structural and Catalytic Behavior of Monometallic Catalytic Converters Under Simulated Exhaust Conditions

The effect of CeO₂, La₂O₃ and ZrO₂ structural stabilizers/promoters, in the form of Ce_xZr_yLa_zO_δ mixed oxides, on the catalytic performance and thermal stability of monometallic (Pt), monolithic-type, Al₂O₃-washcoated three-way catalytic converters (TWCs) was investigated under simulated exhaust conditions. The evolution of textural, structural and surface behavior of Ce_xZr_yLa_zO_δ modified TWCs were identified by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance infrared spectroscopy (DRIFTS) and appropriately correlated with the catalytic behavior and thermal stability of TWCs.     

Novel electropositively promoted monometallic (Pt-only) catalytic converters for automotive pollution control

Novel, Pt-only, electropositively promoted by Na, monolithic catalytic converters are developed and tested under simulated exhaust conditions within 150–500 °C. The effects of CeO₂ and La₂O₃ additives on the novel TWC washcoat are also studied. The performance of these simple in constitution, with one precious metal TWCs, was found to be well compared to that of a commercial bimetallic (Pt/Rh)-TWC.     

Characterisation of Three-Way Automotive Aftertreatment Catalysts and Related Model Systems

The techniques applied to the characterisation of three-way automotive aftertreatment catalysts, and some of the most relevant results obtained from those, are briefly overviewed. Studies dealing with both, commercial converters and NM/CeO₂ (CeO₂-ZrO₂) model systems are considered. The analytical and structural characterisation studies on commercial TWCs reported here are mainly devoted to the identification of some of the most relevant factors causing their deactivation. Regarding the reviewed literature on model systems, some more fundamental aspects are considered. Among them, the use of HRTEM, spectroscopic, and chemical techniques in studies on metal sintering and re-dispersion, metal/support interaction phenomena, and redox characterisation of ceria-based catalytic materials are discussed.     

Crystal structure,microstructure, thermal expansion and electrical conductivity of CeO2–ZrO2 solid solution

CeO₂–ZrO₂ solid solution was synthesised by mechanical activation solid-state chemical reaction method and characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal dilatometer, Hebb–Wagner method and DC van der Pauw method. The effects of CeO₂ content on the crystal structure, microstructure, thermal expansion coefficient (TEC), electronic conductivity and total conductivity were investigated. XRD analysis showed that (25 and 75?mol-%) CeO₂–ZrO₂ solid solutions corresponded to tetragonal and cubic phase, and 50?mol-% CeO₂–ZrO₂ belonged to the mixture of tetragonal and cubic phases. SEM analysis showed that doping CeO₂ was helpful to the sinterability of CeO₂–ZrO₂ samples. The TECs increased from 13.27?×?10^?6 to 14.72?×?10^?6?K^?1 with increasing CeO₂ content. The electronic and total conductivities of 75?mol-% CeO₂–ZrO₂ were largest, reaching 1.02?×?10^?4?S?cm^?1 and 1.02?×?10^?2?S?cm^?1 at 850°C, respectively.     

The Effect of CeO2 on the Hydrodenitrogenation Performance of Bulk Ni2P

Bulk Ni₂P and CeO₂-containing bulk Ni₂P (Ce?CNi₂P(x), where x represents the Ce/Ni atomic ratio) were prepared by a co-precipitation method followed by an in situ H₂ temperature-programmed reduction procedure. The catalysts were characterized by XRD, CO chemisorption, TEM, N₂ adsorption?Cdesorption, XPS and X-ray absorption spectroscopy (XAS). Their hydrodenitrogenation performances were studied using quinoline (Q) and decahydroquinoline as the model compounds. Both the hydrogenation and C?CN bond cleavage activities of Ni₂P were improved by the introduction of CeO₂. CeO₂ mainly accelerated the denitrogenation of Q to propylcyclohexane rather than to propylbenzene. XRD and XPS measurements revealed that the Ce species in Ce?CNi₂P(x) were mainly in the oxide form and both Ce⁴⁺ and Ce³⁺ species coexisted on the surface of the catalysts. Addition of CeO₂ significantly decreased the particle size of Ni₂P, resulting in increased specific surface areas and CO uptakes, possibly due to the strong interaction between the Ce species and Ni₂P. At a Ce/Ni atomic ratio higher than 0.25, segregation of CeO₂ took place. XAS results of the passivated catalysts showed that CeO₂ not only affected the oxidability of Ni₂P but also led to the formation of metallic Ni. The promoting effect of CeO₂ was discussed by considering the electronic interactions between Ce species and Ni₂P as well as the presence of the amorphous Ni and low valence Ce³⁺ species.     

Influence of CeO2 on NOx emission during iron ore sintering

The evolution of NO_x during coke combustion in the presence and absence of CeO₂ was studied in a quartz fixed bed reactor. The distribution of CeO₂ in the coke was examined by SEM, and the effects of CeO₂ loading and CeO₂ particle size on NO_x emission were discussed. NO_x emission was also investigated by sintering pot tests with CeO₂ modified coke as sintering fuel. The results showed that CeO₂ was catalytically active in promoting not only coke combustion but also NO_x reduction. SEM examination indicated that the CeO₂ particles were well distributed on the surface and in pore canals of coke. In coke combustion experiments, NO_x and CO emission decreased with increasing CeO₂ loading up to 2.0 wt.% and decreasing CeO₂ particle size (28–150 µm), while sintering pot tests showed that NO_x emission decreased by 18.8% with 2.0 wt.% CeO₂ modified coke as sintering fuel.     

Cytotoxic activity of greener synthesis of cerium oxide nanoparticles using carrageenan towards a WEHI 164 cancer cell line

Carrageenan hydrogel as a “greener” and a vegetable-based stabilizing agent has the potential for many biosyntheses of different nanoparticles by sol-gel method. Herein, we describe for the first time an economic and eco-friendly preparation of cerium oxide nanoparticles (CeO₂-NPs) using carrageenan. When carrageenan hydrogel comes in contact with a cerium nitrate solution, cerium ions anchor themselves to the –SO₃^- groups into the carrageenan and after the gelation process, have fewer opportunities escaping from the polymeric network. The CeO₂-NPs were well-prepared and successfully characterized by PXRD, FTIR, FESEM, UV–Vis, and TGA-DTA. The calcined CeO₂-NPs showed strong UV absorption (λ_max = 328?nm) with the calculated band gap of 2.69?eV. The results obtained from FESEM images indicate that CeO₂-NPs obtained at 600?°C ranges from 18 to 60?nm and have a mean diameter of ~34?nm. The in vitro cytotoxicity study on WEHI 164 cell line has mentioned low toxic and non-toxic CeO₂-NPs in a range of concentrations (0.97–250?μg/ml), thus, we reckon that the greener synthesized CeO₂-NPs will have persistent utilization in various fields of medical applications.     

Facile fabrication of porous hollow CeO2 microspheres using polystyrene spheres as templates

Porous hollow CeO₂ microspheres were fabricated using negative-charged PS microspheres as templates by a facile method. The hollow CeO₂ microspheres were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and N₂ adsorption?Cdesorption. The results showed that the as-synthesized hollow CeO₂ microspheres are well monodisperse and uniform in size. The porous shells of hollow microspheres are relatively rough and composed of tiny nanoparticles. The external diameter, internal diameter, and shell thickness of hollow CeO₂ microspheres are about 190, 160, and 15?nm, respectively. A possible mechanism for the formation of hollow CeO₂ spheres was also discussed.     

Nanostructured CeO2 for selective-sensing and smart photocatalytic applications

Well crystalline CeO₂ nanoparticles have been successfully synthesized via solution combustion synthesis (SCS) using (NH₄)₂[Ce(NO₃)₆] and C₄H₆O₆ as oxidizer and fuel. The structural characteristics of as-synthesized material were investigated in terms of FESEM, HRTEM, EDS, XRD, FTIR and UV–Vis spectroscopy techniques. The surface area of synthesized CeO₂ nanoscale material was obtained from BET plot. Results showed a pure, well-crystallized, flake-like mesoporous material to be formed with crystallite size of 18.86?nm. The focus of this study was to investigate the application of as-synthesized CeO₂ nanomaterial for sensing and photocatalytic degradation of picric acid (PA) in its aqueous solution. It was found to be highly selective for PA detection in aqueous solution when compared with other aromatic compounds. Detection limit (0.52?µM) for PA when compared with earlier studies was found to be much better. In addition, 0.05?gm of as-synthesized CeO₂ nanomaterial is found to be optimum amount ensuring maximum catalytic photodegradation of 10?ppm PA in aqueous solution. These experimental findings point out that as-synthesized CeO₂ nanomaterial can be efficiently used as an effective chemical sensor and photocatalyst.     

Effect of Co substitution and magnetic field on the morphologies and magnetic properties of CeO2 nanoparticles

Pure and Co-doped CeO₂ nanoparticles were synthesized successfully by the solvothermal method. The effect of Co substitution and external magnetic field on the morphologies and magnetic properties of nanoparticles was investigated. Results showed that synthesized Co-doped CeO₂ had the face-centered cubic structure and no other impurities existed in the samples with the increase of Co concentration from 5 to 75?wt%. The increasing Co concentration made the morphologies of Co-doped CeO₂ nanoparticles vary from the hollow sphere, solid sphere to rod-like shape. The applied external magnetic field of 5T decreased the nanoparticle size effectively including the diameter of hollow sphere with low Co concentration and rod-like particles with high Co concentration. Moreover, the wall thickness of hollow sphere particles was also decreased from 35?nm to 18?nm for pure CeO₂. The Co-doped CeO₂ nanoparticles showed the weak ferromagnetic behavior. With the increase of Co concentration, the saturation magnetization (M_s) value increased first and then decreased. The Co-doped CeO₂ with 30?wt% showed the highest value of 3.65?×?10^?2 emu/g (M_s). The M_s value of Co-doped CeO₂ prepared in 5T showed an increasing trend with the Co concentration. The highest value (M_s) reached 4.21?×?10^?2 emu/g for doped CeO₂ with 75?wt% Co.     

Effect of CeO2 addition on the sintering behavior of pre-synthesized magnesium aluminate spinel ceramic powders

In this work, the effects of 1?wt%, 2?wt%, and 3?wt% CeO₂ as an additive on the sintering behavior of alumina-rich spinel and magnesia-rich spinel powders subjected to sintering at temperatures of 1600?°C, 1650?°C, 1700?°C, and 1750?°C were investigated. The sintering behavior of the ceramics was investigated according to dilatometry measurements, linear shrinkage, bulk density, phase analysis, and microstructure. It was demonstrated that CeO₂ hindered the sintering process in alumina-rich spinel by reacting with Al₂O₃ exsolved from the spinel to form platelet-shaped particles of CeAl₁₁O₁₈ interspersed between the spinel grains. Meanwhile, the presence of CeO₂ promotes the sintering process in magnesia-rich spinel by being distributed in an isolated form among the spinel grains.     

Highly stable mesoporous CeO2/CeS2 nanocomposite as electrode material with improved supercapacitor electrochemical performance

The performance of a pseudocapacitor electrode relies largely on the conductivity, cyclic stability, specific surface area and the mesoporosity of the nanomaterials. The CeO₂ is highly stable oxide but poor conductor, on the other hand, CeS₂ is highly conductive but its stability is questionable. Herein, we report the synthesis of CeO₂/CeS₂ nanocomposite, and exploit the properties of both the constituent materials and demonstrates that CeO₂/CeS₂ nanocomposite electrode exhibits an improved capacitance and energy density than CeO₂ nanomaterial. It encompasses large number of pores with a mean size of ～17?nm. The mesoporous nature of the CeO₂/CeS₂ nanocomposite electrode increases its activity, rapid diffusion and transportation of ions and facilitates surface-dependent reversible redox reactions. The nanocomposite electrode demonstrates high stability and its specific capacitance increases almost linearly up to 1000 cyclic voltammetry (CV) cycles. At a current density of 1?A/g it achieves a specific capacitance of 420?F/g. These findings evidently suggest the practical use of CeO₂/CeS₂ nanocomposite as electrode material for future supercapacitors.     

Promoting Effect of CeO2 on NO x Reduction with Propene over SnO2/Al2O3 Catalyst Studied with In situ FT-IR Spectroscopy

The mechanistic cause of the promoting effect of CeO₂ on the activity of SnO₂/Al₂O₃ catalyst for the SCR of NO _x by propene was investigated using X-ray photoelectron spectra (XPS) and in situ Fourier transform infrared (FT-IR) spectroscopy. FT-IR measurements have revealed that the role of CeO₂ on the CeO₂–SnO₂/Al₂O₃ catalyst is to contribute to the formation of formate, acetate and nitrate species, and to promote the reaction between nitrates and hydrocarbon-derived species to form isocyanate (–NCO), which is a reaction intermediate for NO _x reduction.     

Water�CGas Shift and CO Methanation Reactions over Ni�CCeO2(111) Catalysts

X-ray and ultraviolet photoelectron spectroscopies were used to study the interaction of Ni atoms with CeO₂(111) surfaces. Upon adsorption on CeO₂(111) at 300 K, nickel remains in a metallic state. Heating to elevated temperatures (500?C800 K) leads to partial reduction of the ceria substrate with the formation of Ni²⁺ species that exists as NiO and/or Ce_1?xNi_xO_2?y. Interactions of nickel with the oxide substrate significantly reduce the density of occupied Ni 3d states near the Fermi level. The results of core-level photoemission and near-edge X-ray absorption fine structure point to weakly bound CO species on CeO₂(111) which are clearly distinguishable from the formation of chemisorbed carbonates. In the presence of Ni, a stronger interaction is observed with chemisorption of CO on the admetal. When the Ni is in contact with Ce⁺³ cations, CO dissociates on the surface at 300 K forming NiC_x compounds that may be involved in the formation of CH₄ at higher temperatures. At medium and large Ni coverages (>0.3 ML), the Ni/CeO₂(111) surfaces are able to catalyze the production of methane from CO and H₂, with an activity slightly higher than that of Ni(100) or Ni(111). On the other hand, at small coverages of Ni (<0.3 ML), the Ni/CeO₂(111) surfaces exhibit a very low activity for CO methanation but are very good catalysts for the water?Cgas shift reaction.     

Ultrathin CeO2 coating for improved cycling and rate performance of Ni-rich layered LiNi0.7Co0.2Mn0.1O2 cathode materials

In this study, we have successfully coated the CeO₂ nanoparticles (CeONPs) layer onto the surface of the Ni-rich layered LiNi_0.7Co_0.2Mn_0.1O₂ cathode materials by a wet chemical method, which can effectively improve the structural stability of electrode. The X-ray powder diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) are used to determine the structure, morphology, elemental composition and electronic state of pristine and surface modified LiNi_0.7Co_0.2Mn_0.1O₂. The electrochemical testing indicates that the 0.3?mol% CeO₂-coated LiNi_0.7Co_0.2Mn_0.1O₂ demonstrates excellent cycling capability and rate performance, the discharge specific capacity is 161.7?mA?h?g^?1 with the capacity retention of 86.42% after 100 cycles at a current rate of 0.5?C, compared to 135.7?mA?h?g^?1 and 70.64% for bare LiNi_0.7Co_0.2Mn_0.1O₂, respectively. Even at 5?C, the discharge specific capacity is still up to 137.1?mA?h?g^?1 with the capacity retention of 69.0%, while the NCM only delivers 95.5?mA?h?g^?1 with the capacity retention of 46.6%. The outstanding electrochemical performance is assigned to the excellent oxidation capacity of CeO₂ which can oxidize Ni²⁺ to Ni³⁺ and Mn³⁺ to Mn⁴⁺ with the result that suppress the occurrence of Li⁺/Ni²⁺ mixing and phase transmission. Furthermore, CeO₂ coating layer can protect the structure to avoid the occurrence of side reaction. The CeO₂-coated composite with enhanced structural stability, cycling capability and rate performance is a promising cathode material candidate for lithium-ion battery.     

An unusual promotion of the redox behaviour of CeO2-ZrO2 solid solutions upon sintering at high temperatures

The reduction behaviour of a high surface area CeO₂-ZrO₂ solid solution is compared with that of a high surface area CeO₂. It is shown that, upon sintering induced by repetitive reduction/oxidation processes, the temperature of reduction of the solid solution decreases from 900 to 700 K. In contrast, the reduction at low temperatures of the CeO₂ sample is strongly retarded after such treatments. The role of ZrO₂ in promoting the reduction at low temperatures is discussed.     

Catalytic Efficiency of Ceria–Zirconia and Ceria–Hafnia Nanocomposite Oxides for Soot Oxidation

The catalytic oxidation of soot particulates has been investigated over CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ nanocomposite oxides. These oxides were synthesized by a modified precipitation method employing dilute aqueous ammonia solution. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and BET surface area methods. The soot oxidation has been evaluated by a thermogravimetric method under ‘tight contact’ conditions. The XRD results revealed formation of cubic CeO₂, Ce_0.75Zr_0.25O₂ and Ce_0.8Hf_0.2O₂ phases in case of CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ samples, respectively. TEM studies confirm the nanosized nature of the catalysts. Raman measurements suggest the presence of oxygen vacancies, lattice defects and oxide ion displacement from normal ceria lattice positions. UV-Vis DRS studies show presence of charge transfer transitions Ce³⁺←O^2? and Ce⁴⁺←O^2? respectively. The catalytic activity studies suggest that the oxidation of soot could be enhanced by incorporation of Zr⁴⁺ and Hf⁴⁺ into the CeO₂ lattice. The CeO₂–HfO₂ combination catalyst exhibited better activity than the CeO₂–ZrO₂. The observed high activity has been related to the nanosized nature of the composite oxides and the oxygen vacancy created in the crystal lattice.     

Plant-based synthesis of cerium oxide nanoparticles as a drug delivery system in improving the anticancer effects of free temozolomide in glioblastoma (U87) cells

Nowadays, nanocarriers were proven to contain the potential of improving cancer treatments and are utilized to carry anticancer medications to tumors. In this study, cerium oxide nanoparticles (CeO₂-NPs) were synthesized by using Caccinia macranthera leaf extract as the stabilizer and reducer agent, as well as cerium nitrate salt as the supplier source of cerium. The synthesized CeO₂-NPs were analyzed through different procedures such as UV–Vis, FTIR, FESEM/EDX/PSA, XRD, XPS, and TGA/DTA. The outcomes of XRD and FTIR analyses con?rmed the synthesis of pure and crystalline structures of CeO₂-NPs. The average size and zeta potential of our nanoparticles were about 30 nm and ?18.5 mV, respectively. According to the results of XPS analysis, the percentage of Ce⁴⁺ was more than that of the Ce³⁺ oxidation state in synthesized NPs. The CeO₂-NPs were loaded with Temozolomide (TMZ) as an anti-cancer drug through electrostatic interaction and the produced nano-drug (CeO₂-TMZ) was delivered to glioblastoma multiforme (GBM) tumor cells. In conformity to the observations, the drug loading content (DLC) and drug loading efficiency (DLE) of CeO₂-TMZ were about 89.10 and 20.29, respectively. In comparison to the TMZ drug, the in vitro assay exhibited the exertion of higher antiproliferative activities, cell cycle arrest, apoptosis, and expression of p53 by CeO₂-TMZ, which proves the promising capability of this drug as a remedial factor for cancer treatment.     

Selective acetylation of glycerol over CeO2–M and SO42−/CeO2–M (M = ZrO2 and Al2O3) catalysts for synthesis of bioadditives

The feasibility of acetylation of glycerol with acetic acid was investigated employing CeO₂–ZrO₂, CeO₂–Al₂O₃, SO₄^2?/CeO₂–ZrO₂, and SO₄^2?/CeO₂–Al₂O₃ solid acid catalysts to synthesize monoacetin, diacetin and triacetin having interesting applications as bioadditives for petroleum fuels. Intensive physicochemical and surface characterization of the prepared catalysts were performed using XRD, BET surface area, ammonia-TPD and Raman spectroscopy techniques. Characterization results revealed that impregnated sulfate ions strongly influence the physicochemical characteristics of the investigated mixed oxide catalysts. Among various catalysts investigated, the SO₄^2?/CeO₂–ZrO₂ combination catalyst exhibited superior catalytic activity under mild conditions. The effect of various parameters such as reaction temperature, molar ratio of acetic acid to glycerol, catalyst wt% and time-on-stream were studied over the SO₄^2?/CeO₂–ZrO₂ catalyst to optimize the reaction conditions. Catalyst reusability was also carried out to understand its stability.