CoOx-decorated CeO2 heterostructures: effects of morphology on their catalytic properties in diesel soot combustion

Sub-nanometric High-Entropy Alloy Cluster: Hydrogen Spillover Driven Synthesis on CeO2 and Structural Reversibility

High-entropy alloy (HEA) nanoparticles (NPs) have attracted significant attention as promising catalysts owing to the various unique synergistic effects originating from the nanometer-scale, near-equimolar mixing of five or more components to produce single-phase solid solutions. However, the study of sub-nanometer HEA clusters having sizes of less than 1 nm remains incomplete despite the possibility of novel functions related to borderline molecular states with discrete quantum energy levels. The present work demonstrates the synthesis of CeO2 nanorods (CeO2-NRs) on which sub-nanometer CoNiCuZnPd HEA clusters were formed with the aid of a pronounced hydrogen spillover effect on readily reducible CeO2 (110) facets. The CoNiCuZnPd HEA sub-nanoclusters exhibited higher activity during the reduction of NO by H2 even at low temperatures compared with the corresponding monometallic catalysts. These clusters also showed a unique structural reversibility in response to repeated exposure to oxidative/reductive conditions, based on the sacrificial oxidation of the non-noble metals. Both experimental and theoretical analyses established that multielement mixing in quantum-sized regions endowed the HEA clusters with entirely novel catalytic properties.

Study on the Catalytic Characteristics of Precious Metal Catalysts with Different Pt/Pd Ratios for Soot Combustion

Soot particles in engine exhaust seriously pollute the atmosphere and endanger human health. For soot oxidation, Pt and Pd precious metal catalysts are widely used and are effective. In this paper, the catalytic characteristics of catalysts with different Pt/Pd mass ratios for soot combustion were studied through X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller analysis, scanning electron microscopy, transmission electron microscopy, the temperature-programmed oxidation reaction, and thermogravimetry. Besides, the adsorption characteristics of soot and O₂ on the catalyst surface were explored by density functional theory (DFT) calculations. The research results showed that the activity of catalysts for soot oxidation from strong to weak is Pt/Pd = 10:1, Pt/Pd = 5:1, Pt/Pd = 1:0, and Pt/Pd = 1:1. XPS results showed that the concentration of oxygen vacancies in the catalyst is the highest when the Pt/Pd ratio is 10:1. The specific surface area of the catalyst increases first and then decreases with the increase of Pd content. When the Pt/Pd ratio is 10:1, the specific surface area and pore volume of the catalyst reach the maximum. The following are the DFT calculation results. With the increase of Pd content, the adsorption energy of particles on the catalyst surface decreases first and then increases. When the Pt/Pd ratio is 10:1, the adsorption of C on the catalyst surface is the strongest, and the adsorption of O₂ is also strong. In addition, this surface has a strong ability to donate electrons. The theoretical simulation results are consistent with the activity test results. The research results have a guiding significance for optimizing the Pt/Pd ratio and improving the soot oxidation performance of the catalyst.

Prussian blue modified CeO2 as a heterogeneous photo-Fenton-like catalyst for degradation of norfloxacin in water

The heterogeneous photo-Fenton-like process is emerging as a promising treatment of antibiotics-containing wastewater. The preparation of new efficient and stable catalysts is one of the research fields. A composite catalyst, prussian blue (PB) modified CeO₂ was prepared, characterized, and applied for photo-Fenton oxidation of norfloxacin (NOR) in this study. It was found that chemical doping of PB leaded to more oxygen vacancies and increased the surface area of CeO₂ obviously. PB/CeO₂ with more Ce³⁺ facilitated electron transfer between Fe³⁺/Fe²⁺ with Ce³⁺/Ce⁴⁺. PB could also improve the separation rate of photoexcited electron-hole pairs in CeO₂ nanostructures. When the doping ratio of PB and CeO₂ was 10%, PB/CeO₂ show the highest catalytic degradation ability and 88.93% of NOR could be degraded within 30 min. PB/CeO₂ composite showed well reactivity at the wide pH value range of 3-8. The reusable experiments and low iron dissolution with less than 1 mg/L indicated that PB/CeO₂ could be employed as an efficient heterogeneous photo-Fenton-like catalyst in organic contaminants degradation.

An Insight into Geometries and Catalytic Applications of CeO2 from a DFT Outlook

Rare earth metal oxides (REMOs) have gained considerable attention in recent years owing to their distinctive properties and potential applications in electronic devices and catalysts. Particularly, cerium dioxide (CeO2), also known as ceria, has emerged as an interesting material in a wide variety of industrial, technological, and medical applications. Ceria can be synthesized with various morphologies, including rods, cubes, wires, tubes, and spheres. This comprehensive review offers valuable perceptions into the crystal structure, fundamental properties, and reaction mechanisms that govern the well-established surface-assisted reactions over ceria. The activity, selectivity, and stability of ceria, either as a stand-alone catalyst or as supports for other metals, are frequently ascribed to its strong interactions with the adsorbates and its facile redox cycle. Doping of ceria with transition metals is a common strategy to modify the characteristics and to fine-tune its reactive properties. DFT-derived chemical mechanisms are surveyed and presented in light of pertinent experimental findings. Finally, the effect of surface termination on catalysis by ceria is also highlighted.

Boosting Catalytic Purification of Soot Particles over Double Perovskite-Type La2-xKxNiCoO6 Catalysts with an Ordered Macroporous Structure

The catalytic performances for soot purification over the perovskite-type ABO3 oxides, as one of the most potential non-noble metal catalysts, are closely correlated with the substitution of A-site and B-site ions. Herein, three-dimensional ordered macroporous (3DOM) structural catalysts of double perovskite-type La2-xKxNiCoO6 were prepared by a method of colloidal crystal template. The contact efficiency between the catalyst and soot particles is significantly promoted by the 3DOM structure, and the partial substitution of A-site (La) with low-valence potassium (K) ions in La2-xKxNiCoO6 catalysts boosts the increasing surface density of coordinatively unsaturated active B-sites (Co and Ni) and active oxygen. 3DOM La2-xKxNiCoO6 catalysts exhibited superior performance during the purification of soot particles, and the 3DOM La1.80K0.20NiCoO6 catalyst exhibited the highest activity, that is, the values of T50, SCO2, and turnover frequency are 346 °C, 99.3%, and 0.204 h-1 (at 300 °C), respectively. According to the results of multiple experimental characterizations and density functional theory calculations, the mechanism of the samples during soot removal is proposed: the increase in surface oxygen density induced by the doping of K ions significantly promotes the critical step of the oxidation from NO to NO2 in catalyzing soot purification. It is one new strategy to develop the high-efficient non-noble metal catalysts for soot purification in practical application.

Electrochemical Ce(III)/Ce(IV) Redox Behavior and Ce Oxide Nanostructure Recovery over Thio-Terpyridine-Functionalized Au/Carbon Paper Electrodes

Understanding the electrochemical behaviors of Ce(III)/Ce(IV) ions is essential for better treatment, separation, and recycling of lanthanide (Ln) and actinide (An) elements. Herein, electrochemical redox behavior and interconversion of Ce(III)/Ce(IV) ions and their recoveries were demonstrated over newly developed thio-terpyridine-functionalized Au-modified carbon paper electrodes in acidic and neutral electrolytes. Cyclic voltammetry and amperometry were performed for the electrodes with and without thio-terpyridine functionalization. Ce oxide nanostructure recovery was successfully conducted by amperometry, and the electrodeposited nanostructured Ce materials were fully characterized by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction crystallography, and X-ray photoelectron spectroscopy. Geometry optimization and the electronic energy state calculations were conducted by density functional theory at the B3LYP/GENECP level for the complexes of Ce(III) and Ce(IV) ions with the thio-terpyridine in an aqueous state. The present unique results provide valuable information on understanding redox behaviors of Ln and An ions for their recycling and treatment processes.

How the Morphology of NiOx-Decorated CeO2 Nanostructures Affects Catalytic Properties in CO2 Methanation

Effects of morphology and exposed crystal planes of NiO_x-decorated CeO₂ (NiCeO₂) nanostructured catalysts on activity during CO₂ methanation were examined, using nanorod (NR), nanocube (NC), and nanooctahedron (NO) structures. The NiCeO₂ nanorods (NiCeO₂-NR) showed superior activity to NiCeO₂-NC and NiCeO₂-NO along with excellent selectivity for CH₄. This material also demonstrated exceptional durability, with no significant loss of catalytic activity or structural change after use. Comprehensive physicochemical characterization as well as density functional theory calculations determined that the high performance of the NiCeO₂-NR was closely related to the large quantity of surface oxygen vacancies and the high degree of reversibility associated with the Ce⁴⁺ ↔ Ce³⁺ redox cycle of the support. These effects originate from the enhanced reactivity of oxygen atoms on the (110) surfaces of the oxide compared with the (100) and (111) surfaces. This information is expected to assist in the rational design of practical catalysts for the activation of CO₂ molecules and other important transformations.

Surface engineering on porous perovskite-type La0.6Sr0.4CoO3-δ nanotubes for an enhanced performance in diesel soot elimination

The porous perovskite-type La_0.6Sr_0.4CoO_3-δ nanotubes are synthesized by sol-gel method combined with electrospinning technique following the calcination, while the porous nanotubular structure can increase the utilization of active sites related to the catalytic activity in soot oxidation. In order to further improve the catalytic activity, porous La_0.6Sr_0.4CoO_3-δ nanotubes are further treated with nitric acid to obtain a larger specific surface area in this work. The as-prepared catalysts are characterized by different techniques to study their physical and chemical properties. The soot catalytic activity is evaluated by the temperature programmed oxidation tests and the values of activation energy. Based on the characterizations and catalytic activity evaluation, the correlation between the specific surface area and catalytic activity is well revealed by the isothermal kinetic measurements. The higher specific surface area (more than 150.0 m² g^-1) contributes to a larger amount and a better dispersion of the active oxygen species, thence improving the catalytic activity of soot oxidation. As a result, porous perovskite-type La_0.6Sr_0.4CoO_3-δ nanotubes after nitric acid treatment for 4 h have the best activity and a good stability, with the T₅₀ of 442 °C (5% O₂) and 415 °C (5% O₂ + 500 ppm NO), and the E_a of 93.6 kJ mol mol^-1.

Oxygen vacancy enhancing Fenton-like catalytic oxidation of norfloxacin over prussian blue modified CeO2: Performance and mechanism

To develop an efficient heterogeneous Fenton-like catalyst is of great importance for degrading organic pollutants. CeO₂ was selected as the catalyst carrier. Prussian blue (PB) was chose as the iron resource for its sensitivity to H₂O₂ and low toxicity. PB modified CeO₂ composite was successfully fabricated and used for Fenton-like oxidation of norfloxacin (NOR) in this study. The characteristics of the catalysts demonstrated that the doping of PB distorted the lattice locally and increased the surface area of CeO₂ obviously. The XPS analysis also indicated that chemically supported catalysts PB/CeO₂ with more Ce³⁺ was beneficial to Fenton-like catalytic reaction. The degradation tests showed that the PB/CeO₂ significantly enhanced the removal of NOR which indicated a synergistic effect between PB and CeO₂. The reason should be mainly attributed to the synergetic catalysis of H₂O₂ by Fe³⁺/Fe²⁺ and Ce³⁺/Ce⁴⁺ redox couples. At the same time, PB/CeO₂ composite showed well reusability and wide pH value range of 2-9 with fairly low concentration of iron ions. The reaction mechanisms were identified to be OH oxidation and improvement of oxygen vacancies (OVs).

Ce-Mn coordination polymer derived hierarchical/porous structured CeO2-MnOx for enhanced catalytic properties

Catalytic performance is largely dependent on how the structures/compositions of materials are designed. Herein, CeO2-MnOx binary oxide catalysts with a hierarchical/porous structure are prepared by a facile and efficient method, which involves the preparation of the hierarchical Ce-Mn coordination polymer (CPs) precursor, followed by a thermal treatment step. The obtained CeO2-MnOx catalysts not only well inherit the hierarchical structure of Ce-Mn CPs, but also possess porous and hollow features due to the removal of organic ligands and heterogeneous contraction during the calcination process. In addition, the effect of the Mn/Ce ratio is also studied to optimize catalytic performance. Specifically, the as-prepared CeO2-MnOx (5 : 5) catalyst exhibits excellent catalytic performance toward CO oxidation and selective catalytic reduction (SCR) of NO with NH3 at low temperatures. Based on the characterization results, we propose that the special hierarchical structure, high surface area, strong synergistic interaction between CeO2 and MnOx, and high content of active Ce3+, Mn4+ and Osurf are collectively responsible for its remarkable catalytic performance.