Effects of precipitants and surfactants on catalytic activity of Pd/CeO<Subscript>2</Subscript> for CO oxidation

A series of precipitants and commercial surfactants (soft templates) were employed to synthesize mesoporous/nano CeO₂ by a hydrothermal method. As-prepared CeO₂ was impregnated with palladium and employed for low-temperature catalytic oxidation of CO. It was found that both soft templates and precipitants had significant effects on the morphology, particle size, crystallinity, and porous structure of the CeO₂, having a significant effect on the surface palladium abundance, molar ratios of surface species, and catalytic activity of the final impregnated Pd/CeO₂. Using ammonia as precipitant could facilitate increased surface palladium abundance and surface molar ratios of PdO/Pd _SMSI , Ce³⁺/(Ce³⁺ + Ce⁴⁺), and O_surface/O_lattice. The catalytic activity of the final Pd/CeO₂ catalysts could be enhanced as well. The optimal P123-assisted ammonia-precipitated Pd/CeO₂ catalyst exhibited over 99% catalytic conversion of CO at 50 °C.     

低于300℃时两种氧化铈材料对稀燃阶段NOx存储性能的影响

研究了低于300 ℃时两种氧化铈对稀燃阶段NO_x存储性能的影响,催化剂由2%（w）Pt/Al₂O₃（PA）与CeO₂-X（X=S,I）机械混合制备. X射线衍射（XRD）,BET表面积和扫描电子显微镜（SEM）用于表征材料的物理结构. X射线光电子能谱（XPS）和H₂程序升温还原（H₂-TPR）用于表面Ce³⁺和活性氧定量. 原位漫反射傅里叶变换红外光谱（in-situ DRIFTS）用于分析表面NO_x吸附物种. 相比于CeO₂-I,CeO₂-S 具有优良的物理化学性能,包括高比表面积、丰富的空隙结构、较高的抗老化能力及表面Ce³⁺浓度. 因而,Pt/Al₂O₃+CeO₂-S 表现出优异的NO_x存储能力. 此外,PA+CeO₂-X（X=S,I）上存在Pt 与CeO₂之间的相互作用,可提高表面氧物种的活性进而促进NO氧化及NO_x存储. PA+CeO₂-S上的这种相互作用要强于PA+CeO₂-I. 研究表明,表面Ce³⁺浓度和活性氧含量对NO_x存储起到重要作用. 然而经过水热处理后,Pt 与老化的氧化铈（ACS,ACI）之间的相互作用降低,并且两种氧化铈NO_x存储性能显著下降. 另外,与PA+ACS（ACI）相比,PA+PACS（PACI）样品NO_x存储能力得到改善,这归因于表面氧物种活性增加能促进硝酸盐的形成.     

纳米CexFe1-xO2固溶体的模板法制备及其乙醇水蒸气重整催化性能

以超高比表面炭材料为模板,硝酸盐为氧化物前体,通过改进的模板路线制备了具有较高比表面积的纳米CexFe1-xO2固溶体.采用X射线衍射、拉曼光谱、物理吸附和透射电镜对制备的样品进行了表征.结果表明,α-Fe2O3,CexFe1-xO2固溶体和CeO2的粒子尺寸为5~15nm,CeO2中部分Ce4 离子被Fe3 离子取代,从而形成了CexFe1-xO2固溶体.乙醇水蒸气重整反应结果显示,CexFe1-xO2固溶体比相应的α-Fe2O3和CeO2具有更高的催化活性和对氢气的选择性.     

A Detailed Insight into the Effects of Morphologies of Cerium Oxide on Fenton-like Reactions for Different Applications

As an exceptional Fenton-like reagent, cerium oxide (CeO₂) finds applications in biomedical science and organic pollutants treatment. The Fenton-like reaction catalyzed by CeO₂ typically encompasses two distinct processes: one resembling the classical Fenton reaction, wherein cerium (Ce³⁺) triggers the decomposition of hydrogen peroxide (H₂O₂) to yield reactive oxygen species (ROS), and the other involves the complexation of H₂O₂ on the Ce³⁺ surface, leading to the formation of peroxides. However, the influence of diverse CeO₂ morphologies on these two reaction pathways has not been comprehensively explored. In this study, CeO₂ exhibiting three typical morphologies, rods, cubes, and spheres, were prepared. The generation of ROS and peroxides was evaluated using the 3,3,5,5-tetramethylbenzidine (TMB) oxidation reaction and the reduction current of H₂O₂, respectively. Moreover, the impacts of pH variations and CeO₂/H₂O₂ concentrations on the production and conversion of these two reaction products were investigated. To corroborate the distinctions between the resultant products and their applicability, apoptosis assays and acid orange 7 (AO7) degradation analyses were performed. Notably, CeO₂ rods exhibited the highest proportion of Ce³⁺, predominantly engaging in complexation with H₂O₂ to foster peroxide formation, thereby facilitating the robust degradation of AO7. However, the generated peroxides appeared to occupy Ce³⁺ sites, thereby impeding the H₂O₂ decomposition process. Conversely, Ce³⁺ species on the surface of CeO₂ cubes were primarily involved in H₂O₂ decomposition, leading to heightened ROS production, and thus showcasing substantial potential for damaging A549 tumor cells. It is worth noting that the ability of these Ce³⁺ species to form peroxides through complexation with H₂O₂ was comparatively reduced. In summation, this study sheds light on the intricate interplay between distinct CeO₂ morphologies and their divergent impacts on Fenton-like reactions. These findings expand our comprehension of the influences on its reactivity of CeO₂ morphologies and open new insights for applications in diverse domains, from organic dye degradation to tumor therapy.     

合成方法对Rh/CeO2-ZrO2-La2O3催化剂的结构、表面性能及DeNOx活性的影响

用沉积沉淀法合成两种不同系列的CeO2-ZrO2-La2O3混合氧化物(ZrO2和La2O3沉积CeO2粒子(标记为A-x)以及CeO2和La2O3沉积ZrO2粒子(标记为B-x)),并用作Rh催化剂的载体。XRD、拉曼、TPR、XPS和O2脉冲等表征结果显示出不同的沉积顺序将导致不同的结构和氧化还原性能,且B-x具有更高的氧迁移性、储氧能力和表面Ce浓度。当其负载Rh后,Rh/B-x催化剂具有更高的NO和CO转化率及N2选择性,且Ce的最佳含量为50at%。这可能归因于Rh负载于富铈表面形成更多有利于NO分解的表面Ce3+活性位。     

Role of Ceria in the Design of Composite Materials for Elemental Mercury Removal

The necessity to drastically act against mercury pollution has been emphatically addressed by the United Nations. Coal‐fired power plants contribute a great deal to the anthropogenic emissions; therefore, numerous sorbents/catalysts have been developed to remove elemental mercury (Hg⁰) from flue gases. Among them, ceria (CeO₂) has attracted significant interest, due to its reversible Ce³⁺/Ce⁴⁺ redox pair, surface‐bound defects and acid‐base properties. The removal efficiency of Hg⁰ vapor depends among others, on the flue gas composition and temperature. CeO₂ can be incorporated into known materials in such a way that the abatement process can be effective at different operating conditions. Hence, the scope of this account is to discuss the role of CeO₂ as a promoter, active phase and support in the design of composite Hg⁰ sorbents/catalysts. The elucidation of each of these roles would allow the integration of CeO₂ advantageous characteristics to such degree, that tailor‐made environmental solution to complex issues can be provided within a broader application scope. Besides, it would offer invaluable input to theoretical calculations that could enable the materials screening and engineering at a low cost and with high accuracy.     

共沉淀法和模板法制备均匀负载CeO2/ SiO2的研究

CeO2是三效催化剂(简称TWC)中被广泛应用的涂层材料[1],其优良的储放氧能力(OSC)可以扩大TWC的工作窗口,并可以在与γ-Al2O3的相互作用中提高Al2O3的高温稳定性[2]。在高温下,CeO2会因晶粒迅速长大而失去储放氧能力。为了提高CeO2的高温抗烧结能力,以及进一步提高其氧化还原能力,大部分研究者选择了在CeO2晶格中掺入其他离子的方法,如:Zr4 、Pr3 、La3 等[3￣5],这些离子在CeO2晶格中引入了晶格缺陷,不但稳定了结构,而且提高了氧传输能力。SiO2具有很高的化学稳定性、高比表面及高热稳定性,是载体的理想选择。研究表明,CeO2负载…     

Interaction of Hydrogen with Ceria: Hydroxylation,Reduction, and Hydride Formation on the Surface and in the Bulk

The study reports the first attempt to address the interplay between surface and bulk in hydride formation in ceria (CeO₂) by combining experiment, using surface sensitive and bulk sensitive spectroscopic techniques on the two sample systems, i.e., CeO₂(111) thin films and CeO₂ powders, and theoretical calculations of CeO₂(111) surfaces with oxygen vacancies (O_v) at the surface and in the bulk. We show that, on a stoichiometric CeO₂(111) surface, H₂ dissociates and forms surface hydroxyls (OH). On the pre-reduced CeO_2−x samples, both films and powders, hydroxyls and hydrides (Ce−H) are formed on the surface as well as in the bulk, accompanied by the Ce³⁺ ↔ Ce⁴⁺ redox reaction. As the O_v concentration increases, hydroxyl is destabilized and hydride becomes more stable. Surface hydroxyl is more stable than bulk hydroxyl, whereas bulk hydride is more stable than surface hydride. The surface hydride formation is the kinetically favorable process at relatively low temperatures, and the resulting surface hydride may diffuse into the bulk region and be stabilized therein. At higher temperatures, surface hydroxyls can react to produce water and create additional oxygen vacancies, increasing its concentration, which controls the H₂/CeO₂ interaction. The results demonstrate a large diversity of reaction pathways, which have to be taken into account for better understanding of reactivity of ceria-based catalysts in a hydrogen-rich atmosphere.     

Growth and Electronic Properties of Ag Nanoparticles on Reduced CeO2-x(111) Films

Ag nanoparticles grown on reduced CeO_2-x thin films have been studied by X-ray photoelec-tron spectroscopy and resonant photoelectron spectroscopy of the valence band to understand the effect of oxygen vacancies in the CeO_2-x thin films on the growth and interfacial elec-tronic properties of Ag. Ag grows as three-dimensional particles on the CeO_2-x(111) surface at 300 K. Compared to the fully oxidized ceria substrate surface, Ag favors the growth of smaller particles with a larger particle density on the reduced ceria substrate surface, which can be attributed to the nucleation of Ag on oxygen vacancies. The binding energy of Ag3d increases when the Ag particle size decreases, which is mainly attributed to the final-state screening. The interfacial interaction between Ag and CeO_2-x(111) is weak. The resonant enhancement of the 4f level of Ce³⁺ species in RPES indicates a partial Ce⁴⁺→Ce³⁺ re-duction after Ag deposited on reduced ceria surface. The sintering temperature of Ag on CeO_1.85(111) surface during annealing is a little higher than that of Ag on CeO₂(111) surface, indicating that Ag nanoparticles are more stable on the reduced ceria surface.     

Enhanced activity and SO2 resistance of Co-modified CeO2-TiO2 catalyst prepared by facile co-precipitation for elemental mercury removal in flue gas

The Co-modified CeO₂-TiO₂ catalyst prepared by facile co-precipitation was used for efficient elemental mercury oxidation in flue gas. Results indicated that Co doping greatly enhanced the activity and SO₂ resistance of the CeO₂-TiO₂ catalyst. In the presence of 5% O₂, 500 ppm NO, 800 ppm SO₂ and 3% H₂O at 200 °C, the Hg⁰ removal efficiency of CeCo₃/Ti could maintain at about 87% for a relatively long time. Characterizations of catalysts (BET, XRD, Raman spectroscopy, TEM, H₂-TPR, O₂-TPD, XPS, TG-MS and SO₂-DRIFTS) were carried out to reveal the mechanism of Co modification on the redox ability, SO₂ resistance and resultant mercury oxidation removal performance of catalyst. It was found that an interaction of Ce with Co promoted the dispersion of CeO₂, increased chemisorbed oxygen concentration, and improved the oxygen storage capacity and the reducibility of catalyst, which was beneficial to the improvement of Hg⁰ oxidation removal. Hg⁰ would adsorb onto the catalyst and react with surface active oxygen species replenished by gas-phase O₂ to be oxidized via Mars-Maessen mechanism. SO₂ consumed the surface active oxygen species and resulted in the reduction of Ce⁴⁺ to Ce³⁺, which induced the deactivation of catalyst. The introduced Co in CeO₂-TiO₂ catalyst exerted the function of protecting Ce⁴⁺ from being poisoned by SO₂ and thus promoted the sulfur resistance and Hg⁰ removal performance of the catalyst in the presence of SO₂.     

Mechanochemical Activation of Cu–CeO<Subscript>2</Subscript> Mixture as a Promising Technique for the Solid-State Synthesis of Catalysts for the Selective Oxidation of CO in the Presence of H<Subscript>2</Subscript>

A new ecologically clean method for the solid-phase synthesis of oxide copper–ceria catalysts with the use of the mechanochemical activation of a mixture of Cu powder (8 wt %) with CeO₂ was developed. It was established that metallic copper was oxidized by oxygen from CeO₂ in the course of mechanochemical activation. The intensity of a signal due to metallic Cu in the X-ray diffraction analysis spectra decreased with the duration of mechanochemical activation. The Cu¹⁺, Cu²⁺, and Ce³⁺ ions were detected on the sample surface by X-ray photoelectron spectroscopy. The application of temperature-programmed reduction (TPR) made it possible to detect two active oxygen species in the reaction of CO oxidation in the regions of 190 and 210–220°C by a TPR-H₂ method and in the regions of 150 and 180–190°C by a TPR-CO method. It is likely that the former species occurred in the catalytically active nanocomposite surface structures containing Cu–O–Ce bonds, whereas the latter occurred in the finely dispersed particles of CuO on the surface of CeO₂. The maximum conversion of CO (98%, 165°C) reached by the mechanochemical activation of the sample for 60 min was almost the same as conversion on a supported CuO/CeO₂ catalyst.     

Corrosion protective ability of electrodeposited ceria layers

It has been ascertained that the electrochemically deposited thin films of cerium oxides, containing mainly CeO₂ and also some insignificant amount of Ce₂O₃, are acting as an effective cathodic coating, leading to restoration of the passive state of the studied stainless steel (OC 404) samples. This effect is associated with a strong shifting of the stationary corrosion potential of the steel in positive direction, moving over from potentials characteristic of corrosion in active state to potentials falling within the zone of passivity. In this respect, another basic purpose of the investigations was the elucidation of the mechanism of action of the cerium oxide film and in particular collecting experimental evidence for the supposition about the occurrence of an efficient depolarization reaction of CeO₂ reduction (resulting in a state of passivity—improved ability of self-passivation) instead of hydrogen depolarization reaction. For this purpose, we considered also the decrease in the surface concentration of ceria in the passive layer under the conditions of the actual corrosion process (self-dissolution) of the stainless steel by means of XPS, SEM, ICP-AES, and gravimetric analyses. A decrease in the surface concentration of CeO₂ (Ce⁴⁺) has been observed, which is known to be chemically inert in acidic media. The obtained results prove the occurrence of an effective cathodic process of Ce⁴⁺ (CeO₂) reduction into Ce³⁺ (soluble in acids Ce₂O₃ ) in the superficial oxide film.     

Catalytic NO Reduction by NO Pre-Adsorbed RhCeO2NO− Clusters

A fundamental understanding on the dynamically structural evolution of catalysts induced by reactant gases under working conditions is challenging but pivotal in catalyst design. Herein, in combination with state-of-the-art mass spectrometry for cluster reactions, cryogenic photoelectron imaging spectroscopy, and quantum-chemical calculations, we identified that NO adsorption on rhodium-cerium bimetallic oxide cluster RhCeO₂⁻ can create a Ce³⁺ ion in product RhCeO₂NO⁻ that serves as the starting point to trigger the catalysis of NO reduction by CO. Theoretical calculations substantiated that the reduction of another two NO molecules into N₂O takes place exclusively on the Ce³⁺ ion while Rh behaves like a promoter to buffer electrons and cooperates with Ce³⁺ to drive NO reduction. Our finding demonstrates the importance of NO in regulating the catalytic behavior of Rh under reaction conditions and provides much-needed insights into the essence of NO reduction over Rh/CeO₂, one of the most efficient components in three-way catalysts for NO_x removal.     

Structure and surface properties of ZrO<Subscript>2</Subscript>, CeO<Subscript>2</Subscript>, and Zr<Subscript>0.5</Subscript>Ce<Subscript>0.5</Subscript>O<Subscript>2</Subscript> prepared by a microemulsion method according to X-ray diffraction,TPR, and EPR data

It was established by X-ray diffraction, TPR, and EPR that microemulsion (m.e.) synthesis yields the binary oxides ZrO₂(m.e.) and CeO₂(m.e.) and the mixed oxide Zr_0.5Ce_0.5O₂(m.e.) in the form of a tetragonal, cubic, and pseudocubic phase, respectively, having crystallite sizes of 5–6 nm. The bond energy of surface oxygen in the (m.e.) samples is lower than in their analogues prepared by pyrolysis. Hydrogen oxidation on the oxides under study occurs at higher temperatures than CO oxidation. ZrO₂(m.e.) and CeO₂(m.e.) are active in O₂⁻ formation during NO + O₂ adsorption, while CeO₂ is active during CO + O₂ adsorption, too. However, its amount here is one-half to one-third its amount in the pyrolysis-prepared samples, signifying a reduced number of active sites, which are Zr⁴⁺ and Ce⁴⁺ coordinatively unsaturated cations and Me⁴⁺-O²⁻ pairs. O₂⁻ radical anions are stabilized in the coordination sphere of Zr⁴⁺ coordinatively unsaturated cations via ionic bonding, and in the sphere of Ce⁴⁺ cations, via covalent bonding. Ionic bonds are stronger than ionic-covalent bonds and do not depend on the ZrO₂ phase composition. Zr_0.5Ce_0.5O₂ is inactive in these reactions because of the strong interaction of Zr and Ce cations. It is suggested that Ce^{(4 + β)+} coordinatively unsaturated cations exist on its surface, and their acid strength is lower than that of Zr⁴⁺ and Ce⁴⁺ cations in ZrO₂ and CeO₂, according to the order ZrO₂ > CeO₂ ≥ Zr_0.5Ce_0.5O₂. Neither TPR nor adsorption of probe molecules revealed Zr cations on the surface of the mixed oxide.     

Analysis of Dopant Atom Distribution and Quantification of Oxygen Vacancies on Individual Gd‐Doped CeO2 Nanocrystals

This work reports the analysis of the distribution of Gd atoms and the quantification of O vacancies applied to individual CeO₂ and Gd‐doped CeO₂ nanocrystals by electron energy‐loss spectroscopy. The concentration of O vacancies measured on the undoped system (6.3±2.6 %) matches the expected value given the typical Ce³⁺ content previously reported for CeO₂ nanoparticles. The doped nanoparticles have an uneven distribution of dopant atoms and an atypical amount of O vacant sites (37.7±4.1 %). The measured decrease of the O content induced by Gd doping cannot be explained solely by the charge balance including Ce³⁺ and Gd³⁺ ions.     

Oxidation of Reduced Ceria by Incorporation of Hydrogen

The interaction of hydrogen with reduced ceria (CeO_2?x) powders and CeO_2?x(111) thin films was studied using several characterization techniques including TEM, XRD, LEED, XPS, RPES, EELS, ESR, and TDS. The results clearly indicate that both in reduced ceria powders as well as in reduced single crystal ceria films hydrogen may form hydroxyls at the surface and hydride species below the surface. The formation of hydrides is clearly linked to the presence of oxygen vacancies and is accompanied by the transfer of an electron from a Ce³⁺ species to hydrogen, which results in the formation of Ce⁴⁺, and thus in oxidation of ceria.     

(CeO2)x-(La-Al2O3)1-x复合氧化物对柴油车尾气中可溶性有机成分的催化氧化

采用共沉淀法制备了一系列(CeO₂)_x-(La-Al₂O₃)_1-x复合氧化物(其中x为质量分数, x=0.00, 0.25, 0.50, 0.75, 1.00), 以其为催化剂. 采用热重-差热分析(TG-DTA)法模拟测试该系列催化剂对柴油车尾气中可溶性有机成分(SOF)的催化氧化性能. 结果表明, 当x=0.75时, 催化剂活性最高, 可使SOF在125℃起燃, 355℃完全转化. 并用低温N₂吸附-脱附, 储氧量(OSC)测试, X射线衍射(XRD), X射线光电子能谱(XPS)和H₂程序升温还原(H₂-TPR)等手段对催化剂进行了表征. 研究结果表明, 适量La 和Al 共掺杂增大了氧化铈的比表面积和催化剂中Ce³⁺所占的比例, 从而提高了催化剂对SOF的低温吸附能力和催化剂的储释放氧能力, 因而提高了催化剂对SOF的氧化活性. 所制备的(CeO₂)_0.75-(La-Al₂O₃)_0.25复合氧化物适合应用到柴油车尾气净化催化剂中.     

CeO<Subscript>2</Subscript>-catalyzed ozonation of phenol

Three different cerium citrate-based precursors were used for synthesizing CeO₂ through thermal treatment. Three morphological types of CeO₂ were obtained. Characterization of these oxides was carried out by XRD patterns, SEM microscopy, N₂ adsorption isotherms, Raman spectroscopy, zeta potential, and UV/Vis luminescence. Ozonation of phenol catalyzed by CeO₂ was studied as a representative reaction of environmental interest. The differences on the catalytic activity showed by these three oxides could be correlated to amounts of Ce³⁺ on CeO₂ surface and, consequently, to the demand for oxygen needed to burn each precursor.     

Construction of Cu-Ce interface for boosting toluene oxidation: Study of Cu-Ce interaction and intermediates identified by in situ DRIFTS

A facile hydrothermal method was applied to gain stably and highly efficient CuO-CeO₂ (denoted as Cu1Ce2) catalyst for toluene oxidation. The changes of surface and inter properties on Cu1Ce2 were investigated comparing with pure CeO₂ and pure CuO. The formation of Cu-Ce interface promotes the electron transfer between Cu and Ce through Cu²⁺ + Ce³⁺ ↔ Cu⁺ + Ce⁴⁺ and leads to high redox properties and mobility of oxygen species. Thus, the Cu1Ce2 catalyst makes up the shortcoming of CeO₂ and CuO and achieved high catalytic performance with T₅₀ = 234 °C and T₉₉ = 250 °C (the temperature at which 50% and 90% C₇H₈ conversion is obtained, respectively) for toluene oxidation. Different reaction steps and intermediates for toluene oxidation over Cu1Ce2, CeO₂ and CuO were detected by in situ DRIFTS, the fast benzyl species conversion and preferential transformation of benzoates into carbonates through C=C breaking over Cu1Ce2 should accelerate the reaction.