Two decades of ceria nanoparticle research: structure, properties and emerging applications

Cerium oxide nanoparticles (CeNPs) are versatile materials with unique and unusual properties that vary depending on their surface chemistry, size, shape, coating, oxidation states, crystallinity, dopant, and structural and surface defects. This review encompasses advances made over the past twenty years in the development of CeNPs and ceria-based nanostructures, the structural determinants affecting their activity, and translation of these distinct features into applications. The two oxidation states of nanosized CeNPs (Ce3+/Ce4+) coexisting at the nanoscale level facilitate the formation of oxygen vacancies and defect states, which confer extremely high reactivity and oxygen buffering capacity and the ability to act as catalysts for oxidation and reduction reactions. However, the method of synthesis, surface functionalization, surface coating and defects are important factors in determining their properties. This review highlights key properties of CeNPs, their synthesis, interactions, and reaction pathways and provides examples of emerging applications. Due to their unique properties, CeNPs have become quintessential candidates for catalysis, chemical mechanical planarization (CMP), sensing, biomedical applications, and environmental remediation, with tremendous potential to create novel products and translational innovations in a wide range of industries. This review highlights the timely relevance and the transformative potential of these materials in addressing societal challenges and driving technological advancements across these fields.

Restructuring Co-CoOx Interface with Titration Rate in Co/Nb-CeO2 Catalysts for Higher Water-Gas Shift Performance

H2 production via water-gas shift reaction (WGS) is an important process and applied widely. Cobalt-modified CeO2 are promising catalysts for WGS reaction. Herein, a series of Co/Nb-CeO2 catalysts were prepared by varying the rate of precipitant addition during the coprecipitation method and examined for hydrogen generation through WGS reaction. The rates of precipitant addition were 1, 5, 15, and 25 mL/min. We obtained ceria supported cobalt catalysts with different sizes and morphology such as 3, 8 nm nanoclusters, 30 nm cubic nanoparticles, and 50 nm hexagonal nanoparticles. The well dispersed small cobalt particles in Co/Nb-CeO2 that was prepared at 5 mL/min titration rate exhibit strong interaction between cobalt oxide and CeO2 that retards the reduction of CoOx producing Co-CoOx pairs. In contrast, 1-Co/Nb-CeO2 and 25-Co/Nb-CeO2 result in bigger and aggregated Co particles, resulting in fewer interfaces with CeO2. The Co0, Coδ+, Ce3+, and Ov species are responsible for improved reducibility in Co/Nb-CeO2 catalysts and were quantitively measured using XPS, XAS, and Raman spectroscopy. The Co-CoOx interface assists dissociation of the H2O molecule; CO oxidation requires low activation energy and realizes a high turnover frequency of 9.8 s-1. The 5-Co/Nb-CeO2 catalyst achieved thermodynamic equilibrium equivalent CO conversion with efficient H2 production during WGS reaction at a gas hourly space velocity of 315,282 h-1. Successively, the 5-Co/Nb-CeO2 catalyst exhibited stable performance for straight 168 h attributed to stable CO-Coδ+ intermediate formation, achieving efficient inhibition of typical CO chemistry over the Co metal, suitable for hydrogen generation from waste derived synthesis gas.

Boosting reactivity of water-gas shift reaction by synergistic function over CeO2-x/CoO1-x/Co dual interfacial structures

Dual-interfacial structure within catalysts is capable of mitigating the detrimentally completive adsorption during the catalysis process, but its construction strategy and mechanism understanding remain vastly lacking. Here, a highly active dual-interfaces of CeO2-x/CoO1-x/Co is constructed using the pronounced interfacial interaction from surrounding small CeO2-x islets, which shows high activity in catalyzing the water-gas shift reaction. Kinetic evidence and in-situ characterization results revealed that CeO2-x modulates the oxidized state of Co species and consequently generates the dual active CeO2-x/CoO1-x/Co interface during the WGS reaction. A synergistic redox mechanism comprised of independent contribution from dual functional interfaces, including CeO2-x/CoO1-x and CoO1-x/Co, is authenticated by experimental and theoretical results, where the CeO2-x/CoO1-x interface alleviates the CO poison effect, and the CoO1-x/Co interface promotes the H2 formation. The results may provide guidance for fabricating dual-interfacial structures within catalysts and shed light on the mechanism over multi-component catalyst systems.

Unveiling the Role of In Situ Sulfidation and H2O Excess onH2S Decomposition to Carbon-Free H2 over Cobalt/Ceria Catalysts

The emerging energy and environmental concerns nowadays are highlighting the need to turn to clean fuels, such as hydrogen. In this regard, hydrogen sulfide (H2S), an abundant chemical compound found in several natural sources and industrial streams, can be considered a potential carbon-free H2 source through its decomposition. In the present work, the H2S decomposition performance of Co3O4/CeO2 mixed oxide catalysts toward hydrogen production is investigated under excess H2O conditions (1 v/v% H2S, 90 v/v% H2O, Ar as diluent), simulating the concentrated H2S-H2O inflow by the Black Sea deep waters. The effect of key operational parameters such as feed composition, temperature (550–850 °C), and cobalt loading (0–100 wt.%) on the catalytic performance of Co3O4/CeO2 catalysts was systematically explored. In order to gain insight into potential structure-performance relationships, various characterization studies involving BET, XRD, SEM/EDX, and sulfur elemental analysis were performed over the fresh and spent samples. The experimental results showed that the 30 wt.% Co/CeO2 catalyst demonstrated the optimum catalytic performance over the entire temperature range with a H2 production rate of ca. 2.1 μmol H2∙g−1·s−1 at 850 °C and a stable behavior after 10 h on stream, ascribed mainly to the in-situ formation of highly active and stable cobalt sulfided phases.

Promotion effects of halloysite nanotubes on catalytic activity of Co3O4 nanoparticles toward reduction of 4-nitrophenol and organic dyes

Nanosized clay minerals have been widely used as efficient supports to immobilize catalyst nanoparticles. However, clay support-induced interactions and their influences on the catalyst structure and performance currently have not been fully understood. Here, Co₃O₄ nanoparticles supported on halloysite nanotubes (HNTs) were synthesized by a facile deposition-precipitation approach followed by thermal treatment. A series of characterization methods were employed for the Co₃O₄/HNTs hybrid nanostructure to identify its crystal phase, chemical composition, morphology, specific surface area, surface chemical states, and redox property. Characterization results showed that HNTs not only impacted the particle size of Co₃O₄ nanoparticles, but also modified surface chemical surface states of the later, which ultimately promoted the effective catalytic reduction of 4-nitrophenol (4-NP) and azo dyes with sodium borohydride. The interaction between HNTs and Co₃O₄ nanoparticles was found to shorten the induction period of the 4-NP reduction. Meanwhile, the Co₃O₄/HNTs catalyst for the 4-NP reduction achieved an apparent rate constant of 0.265 min^-1 and an activity parameter of 1.63 × 10⁴ min^-1 g^-1 as well as a turnover frequency of 4.37 min^-1. In addition, Co₃O₄/HNTs showed an improvement in reduction efficiency of the azo dyes when compared to bare Co₃O₄ nanoparticles.

Co3O4-Impregnated NiO-YSZ: An Efficient Catalyst for Direct Methane Electrooxidation

Co₃O₄-impregnated NiO-YSZ (yttria-stabilized zirconia) is a possible electrocatalyst for direct methane electrooxidation with both high catalytic activity and the ability to mitigate coking. The physical and electrochemical properties of Co₃O₄-impregnated NiO-YSZ anodes are investigated and benchmarked against NiO-YSZ and CeO₂-impregnated NiO-YSZ anodes. The following methane electrooxidation activity trend: Co₃O₄-impregnated NiO-YSZ > CeO₂-impregnated NiO-YSZ > NiO-YSZ with i_o (exchange current density) values of 88, 83, and 2 mA cm^-2, respectively, is obtained in the high overpotential region. The high activity of Co₃O₄-impregnated NiO-YSZ is attributed to the changes in the electronic structure and microstructure with the incorporation of nickel into the lattice of Co₃O₄ as observed using X-ray photoelectron spectroscopy, temperature-programmed reduction, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. Co₃O₄-impregnated NiO-YSZ also demonstrated the least coking during operation, confirming its utility as a methane electrooxidation catalyst.

Conversion of Co Nanoparticles to CoS in Metal-Organic Framework-Derived Porous Carbon during Cycling Facilitates Na2S Reactivity in a Na-S Battery

Room-temperature sodium-sulfur batteries have attracted wide interest due to their high energy density and high natural abundance. Polysulfide dissolution and irreversible Na₂S conversion are challenges to achieving high battery performance. Herein, we utilize a metal-organic framework-derived Co-containing nitrogen-doped porous carbon (CoNC) as a catalytic sulfur cathode host. A concentrated sodium electrolyte based on sodium bis(fluorosulfonyl)imide, dimethoxyethane, and bis(2,2,2-trifluoroethyl) ether is used to mitigate polysulfide dissolution. We tune the amount of Co present in the CoNC carbon host by acid washing. Significant improvement in reversible sulfur conversion and capacity retention is observed with a higher Co content in CoNC, with 600 mAh g^-1 and 77% capacity retention for CoNC and 261 mAh g^-1 and 56% capacity retention for acid-washed CoNC at cycle 50 at 80 mAh g^-1. Post-mortem X-ray photoelectron spectroscopy, transmission electron microscopy, and selected area electron diffraction suggest that CoS is formed during cycling in place of Co nanoparticles and CoN₄ sites. Raman spectroscopy suggests that CoS exhibits a catalytic effect on the oxidation of Na₂S. Our findings provide insights into understanding the role Co-based catalysts play in sulfur batteries.

Efficient Waste to Energy Conversion Based on Co-CeO2 Catalyzed Water-Gas Shift Reaction

Waste to energy technology is attracting attention to overcome the upcoming environmental and energy issues. One of the key-steps is the water-gas shift (WGS) reaction, which can convert the waste-derived synthesis gas (H2 and CO) to pure hydrogen. Co–CeO2 catalysts were synthesized by the different methods to derive the optimal synthetic method and to investigate the effect of the preparation method on the physicochemical characteristics of Co–CeO2 catalysts in the high-temperature water-gas shift (HTS) reaction. The Co–CeO2 catalyst synthesized by the sol-gel method featured a strong metal to support interaction and the largest number of oxygen vacancies compared to other catalysts, which affects the catalytic activity. As a result, the Co–CeO2 catalyst synthesized by the sol-gel method exhibited the highest WGS activity among the prepared catalysts, even in severe conditions (high CO concentration: ~38% in dry basis and high gas hourly space velocity: 143,000 h−1).

Catalytic combustion of toluene over CeO2–CoOx composite aerogels

The dispersion of active species and redox cycle of Co³⁺/Co²⁺ in cobalt based aerogels have an important influence on catalytic performance for toluene oxidation.

Metal-Support Interactions in CeO2- and SiO2-Supported Cobalt Catalysts: Effect of Support Morphology, Reducibility, and Interfacial Configuration

With the increasing demand for highly efficient and durable catalysts, researchers have been doing extensive research to engineer the shape, size, and even phase (e.g., hcp or fcc Co) of individual catalyst nanoparticles, as well as the interface structure between the catalyst and support. In this work, cobalt oxides were deposited on ceria with rod-like morphology (CeO₂NR) and cube-like morphology (CeO₂NC) and silica with sphere-like morphology (SiO₂NS) via a precipitation-deposition method to investigate the effects of support morphology, surface defects, support reducibility, and the metal-support interactions on redox and catalytic properties. XRD, Raman, XPS, BET, H₂-TPR, O₂-TPD, CO-TPD, TEM, and TPR/TPO cycling measurements have been mainly employed for catalysts characterization. Compared with CeO₂NC and SiO₂NS supports, as well as CeO₂NC- and SiO₂NS-supported cobalt catalysts, CeO₂NR counterparts exhibited enhanced reducibility and CO oxidation performance at a lower temperature. Both the apparent activation energy and CO conversion demonstrated the following catalytic activity order: 10 wt % CoO _x/CeO₂NR > 10 wt % CoO _x/CeO₂NC > 10 wt % CoO _x/SiO₂NS. These results showed a strong support-dependent reducibility, CO oxidation, and redox cycling activity/stability of the as-prepared catalysts. Moreover, the significantly enhanced catalytic CO oxidation of the 10 wt % CoO _x/CeO₂NR catalyst indicated the vital role of CeO₂NR support with rich surface oxygen vacancies, superior oxygen storage capacity and mobility, and excellent adsorption/desorption behavior of CO and O₂ species.

Facet-Dependent Photodegradation of Methylene Blue Using Pristine CeO2 Nanostructures

This work comprises the shape- and facet-dependent catalytic efficacies of different morphologies of CeO₂, namely, hexagonal, rectangular, and square. The formation of different shapes of CeO₂ is controlled using polyvinyl pyrrolidone as a surfactant. The surface reactivity of formation of differently exposed CeO₂ facets is thoroughly investigated using UV-visible, photoluminescence, Raman, and X-ray photoelectron spectroscopies. A correlation between the growth of a surface-reactive facet and the corresponding oxygen vacancies is also established. Considering the tremendous contamination, caused by the textile effluents, the present study articulates the facet-dependent photocatalytic activities of pristine CeO₂ for complete degradation of methylene blue within 175 min. The observed degradation time deploying pristine CeO₂ as a catalyst is the shortest to be reported in the literature to our best knowledge.

Superior catalytic performance of a CoOx/Sn–CeO2 hybrid material for catalytic diesel soot oxidation

A Co₃O₄/Sn–CeO₂ hybrid catalyst exhibited superior soot oxidation activity due to the existence of synergism among the multivalent cations and the stepped surface of the hybrid catalyst.

Co3O4@CeO2 hybrid flower-like microspheres: a strong synergistic peroxidase-mimicking artificial enzyme with high sensitivity for glucose detection

In recent years, the development of artificial nanostructured enzymes has received enormous interest in nanobiotechnology due to their advantages over natural enzymes. In the present work, different amounts (5, 10, and 20 wt%) of Co₃O₄ nanoparticle decorated CeO₂ hybrid flower-like microspheres (Co₃O₄@CeO₂) have been investigated for peroxidase-like activity and it was found that 10 wt% of Co₃O₄@CeO₂ exhibited excellent peroxidase-like activity for the catalytic oxidation of the 3,3',5,5'-tetramethylbenzidine (TMB) substrate in the presence of H₂O₂. The formation of more Ce³⁺ ions associated with the oxygen vacancies and a strong synergistic interaction between CeO₂ and Co₃O₄ may be responsible for the enhanced peroxidase-like activity. Based on their peroxidase activity, Co₃O₄@CeO₂ hybrid microspheres were used for the colourimetric detection of glucose. It was found that Co₃O₄@CeO₂ hybrid microspheres showed a substantial enhancement in the detection selectivity. The limit of detection (LOD) was also improved with a limit as low as 1.9 μM. Thus, we believe that Co₃O₄@CeO₂ hybrid flower-like microspheres with high peroxidase-like activity can be exploited for biosensing applications.

Low-temperature CO oxidation over CeO2 and CeO2@Co3O4 core–shell microspheres

The excellent CO catalytic activity and stability of CeO₂@Co₃O₄ composite were ascribed to the synergistic interactions between Co₃O₄ and CeO₂.

Low-temperature CO oxidation over manganese, cobalt, and nickel doped CeO2 nanorods

Transition metal doped ceria nanorods exhibit a better CO oxidation activity at lower temperatures.

Highly efficient continuous-flow oxidative coupling of amines using promising nanoscale CeO2–M/SiO2 (M = MoO3 and WO3) solid acid catalysts

Nanoscale CeO₂–MoO₃/SiO₂ solid acid shows an outstanding catalytic performance in the oxidative coupling of amines under industrially-favourable conditions.