Ceria–zirconia mixed oxides: Synthetic methods and applications

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.

Abundance of Low-Energy Oxygen Vacancy Pairs Dictates the Catalytic Performance of Cerium-Stabilized Zirconia

Cerium-stabilized zirconia (Ce1-xZrxOy, CZO) is renowned for its superior oxygen storage capacity (OSC), a key property long believed to be beneficial to catalytic oxidation reactions. However, 50% Ce-containing CZO recorded with the highest OSC has disappointingly poor performance in catalytic oxidation reactions compared to those with higher Ce contents but lower OSC ability. Here, we employ global neural network (G-NN)-based potential energy surface exploration methods to establish the first ternary phase diagram for bulk structures of CZO, which identifies three critical compositions of CZO, namely, 50, 60, and 80% Ce-containing CZO that are thermodynamically stable under typical synthetic conditions. 50% Ce-containing CZO, although having the highest OSC, exhibits the lowest O vacancy (Ov) diffusion rate. By contrast, 60% Ce-containing CZO, despite lower OSC (33.3% OSC compared to that of 50% Ce-containing CZO), reaches the highest Ov diffusion ability and thus offers the highest CO oxidation catalytic performance. The physical origin of the high performance of 60% Ce-containing CZO is the abundance of energetically favorable Ov pairs along the ⟨110⟩ direction, which reduces the energy barrier of Ov diffusion in the bulk and promotes O2 activation on the surface. Our results clarify the long-standing puzzles on CZO and point out that 60% Ce-containing CZO is the most desirable composition for typical CZO applications.

A proxy for oxygen storage capacity from high-throughput screening and automated data analysis

Oxygen storage and release is a foundational part of many key pathways in heterogeneous catalysis, such as the Mars-van Krevelen mechanism. However, direct measurement of oxygen storage capacity (OSC) is time-consuming and difficult to parallelise. To accelerate the discovery of stable high OSC rare-earth doped ceria-zirconia oxygen storage catalysts, a high-throughput robotic-based co-precipitation synthesis route was coupled with sequentially automated powder X-ray diffraction (PXRD), Raman and thermogravimetric analysis (TGA) characterisation of the resulting materials libraries. Automated extraction of data enabled rapid trend identification and provided a data set for the development of an OSC prediction model, investigating the significance of each extracted quantity towards OSC. The optimal OSC prediction model produced incorporated variables from only fast-to-measure analytical techniques and gave predicted values of OSC that agreed with experimental observations across an independent validation set. Those measured quantities that feature in the model emerge as proxies for OSC performance. The ability to predict the OSC of the materials accelerates the discovery of high-capacity oxygen storage materials and motivates the development of similar high-throughput workflows to identify candidate catalysts for other heterogeneous transformations.

Ce-Zr-based mixed oxide catalyst for oxidative depolymerization of kenaf stalk (biomass) into vanillin

Since petroleum became depleted, rapid attention has been devoted to renewable energy sources such as lignocellulosic biomass to produce useful chemicals for industry (for instance vanillin). Three primary components of lignocellulose are lignin, cellulose, and hemicellulose. This paper uses microwave-assisted technology to oxidize the kenaf stalk (lignocellulosic biomass) and extract lignin to produce vanillin. Catalysts with variable acid-base and redox properties are essential for the mentioned effective conversion, for this reason, CeO2-CA, ZrO2-CA, and CeZrO2-CA catalysts were synthesized. The citrate complexation method was used for the catalyst synthesis and the physicochemical characteristics were analyzed by XRD, FTIR, FE-SEM, TEM, BET, and TPO. The characterization results demonstrated that CeZrO2-CA shows the smallest sized crystallites with a large specific surface area among the other chosen catalysts. For vanillin production, the effect of reaction temperature, reaction time, and catalyst loading was studied. It was observed that compared to other catalysts, CeZrO2-CA produced the highest vanillin yield of 9.90% for kenaf stalk for 5 wt% of CeZrO2-CA at 160 °C for 30 min. Furthermore, vanillin production using extracted lignin is studied keeping CeZrO2-CA as a catalyst and with the same operating parameters, which yielded 14.3% of vanillin. Afterward, the change in yield with respect to pH is also presented. Finally, the recyclability of catalyst is also studied, which showed that it has a strong metal support and greater stability which may give industrial applications a significant boost.

Preparation and Characterization of Isostructural Na2MoO4 and Na2WO4 and a Study of the Composition of Their Mixed System

Na2MoO4 and Na2WO4 are isostructural semiconductors, belonging to the spinel class. They have interesting properties and find applications in numerous sectors. These properties can be tuned by controlling the composition of their solid solutions. Here, different methods to obtain these compounds are presented, both wet and solid-state synthesis. The obtained results show a possible dependence of the material properties on the chosen synthesis method. The pure compounds and their mixtures were characterized by Raman spectroscopy, scanning electron microscopy, and X-ray diffraction.

UiO66-Derived Catalyst for Low Temperature Catalytic Reduction of NO with NH3

Diesel exhaust emissions are major outdoor air pollutants. Reducing the emission of NOx by diesel commercial vehicles and related machineries is at present a great challenge. In this study, we synthesize a catalyst for low-temperature catalytic reduction of NO using calcinated UiO-66(Zr) as a host for the doping of cerium, manganese, and titanium by the incipient wetness impregnation, followed by the dispersion of 1.0 wt % platinum. A solid solution of Ce_0.15Zr_0.54Mn_0.11Ti_0.20O₂/1.0Pt (CZMTO/Pt) is synthesized as evident by the structural characterizations. The catalyst demonstrates significant NO reduction in the laboratory due to the synergistic effect of various elements, with NO conversion above 80% at 160 °C.

Effects of Hydrothermal Treatment on Mesopore Structure and Connectivity in Doped Ceria-Zirconia Mixed Oxides

Pore size and pore connectivity control diffusion-based transport in mesopores, a crucial property governing the performance of heterogeneous catalysts. In many cases, transition-metal oxide catalyst materials are prepared from molecular precursors involving hydrothermal treatment followed by heat treatment. Here, we investigate the effects of such a hydrothermal aftertreatment step, using an aqueous ammonia solution, on the disordered mesopore network of Ce_xZr_1-x-y-zY_yLa_zO_2-δ mixed oxides. This procedure is a common synthesis step in the preparation of such ceria-based oxygen storage materials applied in three-way catalysis, employed to improve the materials' thermal stability. We perform state-of-the-art Ar-physisorption analysis, especially advanced hysteresis scanning, to paint a detailed picture of the alterations in mesopore space caused by the hydrothermal aftertreatment and subsequent aging at 1050 °C. Furthermore, we investigate the network characteristics by electron tomography in combination with suitable statistical analysis, enabling a consistent interpretation of the desorption scans (physisorption). The results indicate that the hydrothermal aftertreatment enhances the mesopore connectivity of the continuous 3D network by widening pores and especially necks, hence facilitating accessibility to the particles' internal surface area and the ability to better withstand high temperatures.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

A Review of CeO2 Supported Catalysts for CO2 Reduction to CO through the Reverse Water Gas Shift Reaction

The catalytic conversion of CO2 to CO by the reverse water gas shift (RWGS) reaction followed by well-established synthesis gas conversion technologies could be a practical technique to convert CO2 to valuable chemicals and fuels in industrial settings. For catalyst developers, prevention of side reactions like methanation, low-temperature activity, and selectivity enhancements for the RWGS reaction are crucial concerns. Cerium oxide (ceria, CeO2) has received considerable attention in recent years due to its exceptional physical and chemical properties. This study reviews the use of ceria-supported active metal catalysts in RWGS reaction along with discussing some basic and fundamental features of ceria. The RWGS reaction mechanism, reaction kinetics on supported catalysts, as well as the importance of oxygen vacancies are also explored. Besides, recent advances in CeO2 supported metal catalyst design strategies for increasing CO2 conversion activity and selectivity towards CO are systematically identified, summarized, and assessed to understand the impacts of physicochemical parameters on catalytic performance such as morphologies, nanosize effects, compositions, promotional abilities, metal-support interactions (MSI) and the role of selected synthesis procedures for forming distinct structural morphologies. This brief review may help with future RWGS catalyst design and optimization.

Development of Pore Morphology During Nitrate Group Removal by Calcination of Mesoporous CexZr1-x-y-zYyLazO2-δ Powders

Here, we present a study of the development of the micro- and mesoporosity of a Ce_xZr_1-x-y-zY_yLa_zO_2-δ oxygen storage material upon treatment at temperatures up to 1050 °C. The investigated powder, obtained from nitrate-based metal oxide precursors in a specially developed hydrothermal synthesis, is highly crystalline, features a high surface area and does not show phase segregation at high temperatures. By employing an advanced methodology, consisting of state-of-the-art argon physisorption, thermogravimetric analysis coupled with mass spectrometry (TG-MS) and X-ray powder diffraction (XRD) along with Raman spectroscopy, we correlate the stability of the mesopore system to the presence of surface-bound nitrate groups introduced during synthesis, which prevent sintering up to a temperature of 600 °C. In addition, the connectivity of mesopores was further studied by hysteresis scanning within the argon physisorption measurements. These advanced physisorption experiments suggest a three-dimensionally interconnected pore system and, in turn, a 3D network of the material itself on the nanometer scale which appears to be beneficial to endow the mesopore space with enhanced stability against sintering and mesopore collapse once the removal of nitrate groups is completed.

A Label-Free Colorimetric Assay Based on Gold Nanoparticles for the Detection of H2O2 and Glucose

The significance of sensing hydrogen peroxide (H2O2) is due to its ubiquity, being a potential biomarker as well as an end-product of several oxidation reactions. Herein, based on gold nanoparticles (AuNPs) and coupled with single-stranded DNA (ssDNA) and ceria nanoparticles (CeO2), we developed a novel colorimetric method to detect H2O2 and glucose in NaCl solutions. In the presence of H2O2, ssDNA adsorbed on the surface of CeO2 could be released and subsequently decorated AuNPs, resulting in a distinct color change of the aqueous solution from purple to red, which could be observed by the naked eye. Since H2O2 can be produced in the process of glucose oxidation by glucose oxidase (GOx), this approach can also be employed to detect glucose. By employing this sensing system, the detection limits for H2O2 and glucose are about 0.21 μM and 3.01 µM, respectively. Additionally, monitoring the content of glucose in blood serum samples was successfully achieved by the proposed strategy. This work opens a potential avenue for the quantitative detection of H2O2 and glucose in clinical diagnostics.

Cerium-, Europium- and Erbium-Modified ZnO and ZrO2 for Photocatalytic Water Treatment Applications: A Review

In the last decades photocatalysis has become one of the most employed technologies for the implementation of the so-called Advanced Oxidation Processes (AOPs) for the removal of harmful pollutants from wastewaters. The materials identified as the best photocatalysts are transition metal oxides, in which the band structure allows charge carrier separation upon solar irradiation. The photoinduced charge carrier can thus cause oxidative and reductive redox reactions at the surface, inducing the formation of the radical species able to initiate the AOPs. Despite the great advantages of this process (non-toxic, cheap and environmentally clean), the main drawback lies in the fact that the most efficient semiconductors are only able to absorb UV irradiation, which accounts for only 5% of the total solar irradiation at the Earth’s surface and not enough to generate the required amount of electron-hole pairs. On the other hand, many efforts have been devoted to the sensitization of wide band gap transition metal oxides to visible light, which represents a higher percentage (almost 45%) in the solar electromagnetic spectrum. Among all the strategies to sensitize transition metal oxides to visible irradiation, doping with lanthanides has been less explored. In this regard, lanthanides offer a unique electronic configuration, consisting in 4f orbitals shielded by a 5s5p external shell. This occurrence, coupled with the different occupation of the localized 4f orbitals would provide an astounding opportunity to tune these materials’ properties. In this review we will focus in depth on the modification of two promising photocatalytic transition metal oxides, namely ZnO and ZrO2, with cerium, europium and erbium atoms. The aim of the work is to provide a comprehensive overview of the influence of lanthanides on the structural, optical and electronic properties of the modified materials, emphasizing the effect of the different 4f orbital occupation in the three considered doping atoms. Moreover, a large portion of the discussion will be devoted to the structural-properties relationships evidencing the improved light absorption working mechanism of each system and the resulting enhanced photocatalytic performance in the abatement of contaminants in aqueous environments.

Co3 O4 -CeO2 Nanocomposites for Low-Temperature CO Oxidation

In an effort to combine the favorable catalytic properties of Co₃ O₄ and CeO₂ , nanocomposites with different phase distribution and Co₃ O₄ loading were prepared and employed for CO oxidation. Synthesizing Co₃ O₄ -modified CeO₂ via three different sol-gel based routes, each with 10.4 wt % Co₃ O₄ loading, yielded three different nanocomposite morphologies: CeO₂ -supported Co₃ O₄ layers, intermixed oxides, and homogeneously dispersed Co. The reactivity of the resulting surface oxygen species towards CO were examined by temperature programmed reduction (CO-TPR) and flow reactor kinetic tests. The first morphology exhibited the best performance due to its active Co₃ O₄ surface layer, reducing the light-off temperature of CeO₂ by about 200 °C. In contrast, intermixed oxides and Co-doped CeO₂ suffered from lower dispersion and organic residues, respectively. The performance of Co₃ O₄ -CeO₂ nanocomposites was optimized by varying the Co₃ O₄ loading, characterized by X-ray diffraction (XRD) and N₂ sorption (BET). The 16-65 wt % Co₃ O₄ -CeO₂ catalysts approached the conversion of 1 wt % Pt/CeO₂ , rendering them interesting candidates for low-temperature CO oxidation.

High-temperature oxidation and reduction of the inverse ceria/Cu(111) catalyst characterized by LEED, STM, nc-AFM and KPFM

The inverse catalyst 'cerium oxide (ceria) on copper' has attracted much interest in recent time because of its promising catalytic activity in the water-gas-shift reaction and the hydrogenation of CO₂. For such reactions it is important to study the redox behaviour of this system, in particular with respect to the reduction by H₂. Here, we investigate the high-temperature O₂oxidation and H₂reduction of ceria nanoparticles (NPs) and a Cu(111) support by low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). After oxidation at 550 °C, the ceria NPs and the Cu(111) support are fully oxidized, with the copper oxide exhibiting a new oxide structure as verified by LEED and STM. We show that a high H₂dosage in the kilo Langmuir range is needed to entirely reduce the copper support at 550 °C. A work function (WF) difference of △ϕ_rCeria/Cu-Cu≈ -0.6 eV between the ceria NPs and the metallic Cu(111) support is measured, with the Cu(111) surface showing no signatures of separated and confined surface regions composed by an alloy of Cu and Ce. After oxidation, the WF difference is close to zero (△ϕ_Ceria/Cu-Cu≈ -0.1…0 eV), which probably is due to a WF change of both, ceria and copper.

Transformation of Highly Stable Pt Single Sites on Defect Engineered Ceria into Robust Pt Clusters for Vehicle Emission Control

Engineering surface defects on metal oxide supports could help promote the dispersion of active sites and catalytic performance of supported catalysts. Herein, a strategy of ZrO₂ doping was proposed to create rich surface defects on CeO₂ (CZO) and, with these defects, to improve Pt dispersion and enhance its affinity as single sites to the CZO support (Pt/CZO). The strongly anchored Pt single sites on CZO support were initially not efficient for catalytic oxidation of CO/C₃H₆. However, after a simple activation by H₂ reduction, the catalytic oxidation performance over Pt/CZO catalyst was significantly boosted and better than Pt/CeO₂. Pt/CZO catalyst also exhibited much higher thermal stability. The structural evolution of Pt active sites by H₂ treatment was systematically investigated on aged Pt/CZO and Pt/CeO₂ catalysts. With H₂ reduction, ionic Pt single sites were transformed into active Pt clusters. Much smaller Pt clusters were created on CZO (ca. 1.2 nm) than on CeO₂ (ca. 1.8 nm) due to stronger Pt-CeO₂ interaction on aged Pt/CZO. Consequently, more exposed active Pt sites were obtained on the smaller clusters surrounded by more oxygen defects and Ce³⁺ species, which directly translated to the higher catalytic oxidation performance of activated Pt/CZO catalyst in vehicle emission control applications.

Studies of Nickel/Samarium-Doped Ceria for Catalytic Partial Oxidation of Methane and Effect of Oxygen Vacancy

We investigated the performance of nickel/samarium-doped ceria (Ni/SDC) nanocatalysts on the catalytic partial oxidation of methane (CPOM). Studies of temperature-programmed surface reaction and reduction reveal that catalytic activity is determined by a synergistic effect produced by Ni metals and metal-support interaction. Catalytic activity was more dependent on the Ni content below 600 °C, while there is not much difference for all catalysts at high temperatures. The catalyst exhibiting high activities toward syngas production (i.e., a CH4 conversion >90% at 700 °C) requires a medium Ni-SDC interaction with an Sm/Ce ratio of about 1/9 to 2/8. This is accounted for by optimum oxygen vacancies and adequate ion diffusivity in the SDCs which, as reported, also display the highest ion conductivity for fuel cell applications.

Facile Fabrication of CeO2-Al2O3 Hollow Sphere with Atomically Dispersed Fe via Spray Pyrolysis

A facile spray pyrolysis method is introduced to construct the hollow CeO₂-Al₂O₃ spheres with atomically dispersed Fe. Only nitrates and ethanol were involved during the one-step preparation process using the ultrasound spray pyrolysis approach. Detailed explorations demonstrated that differences in the pyrolysis temperature of the precursors and heat transfer are crucial to the formation of the hollow nanostructure. In addition, iron species were in situ atomically dispersed on the as-formed CeO₂-Al₂O₃ hollow spheres via this strategy, which demonstrated promising potential in transferring syn-gas to valuable gasoline products.

Conversion of waste frying oil into biodiesel using recoverable nanocatalyst based on magnetic graphene oxide supported ternary mixed metal oxide nanoparticles

The magnetic graphene oxide (GO) supported with heterogeneous ternary mixed metal oxide (MMO) was used as nanocatalyst to enhance the conversion of waste frying oil (WFO) triglycerides to biodiesel via esterification process. In this regard, acidic MGO was modified with three basic metal cations of cerium, zirconium, and strontium oxides to produce heterogeneous MGO@MMO nanocatalyst. The nanocatalyst was characterized by FESEM, TEM, EDX and FTIR. The influence of different parameters such as catalyst material ratio, methanol to oil ratio, contact time, and reaction temperature was studied. Based on the results of effecting parameters, the MGO@MMO nanocatalyst converted WFO to biodiesel with a yield 94%, a reaction time of 90 min, methanol to oil ratio (8:1), and a temperature of 60 °C. Esterification mechanism indicated the MGO@MMO nanocatalyst having both binary Brønsted acid-base sites that increased the conversion yields as compared to MGO and MMO at low temperatures.

Linking the Electrical Conductivity and Non-Stoichiometry of Thin Film Ce1-xZrxO2-δ by a Resonant Nanobalance Approach

Bulk ceria-zirconia solid solutions (Ce_1−xZr_xO_2−δ, CZO) are highly suited for application as oxygen storage materials in automotive three-way catalytic converters (TWC) due to the high levels of achievable oxygen non-stoichiometry δ. In thin film CZO, the oxygen storage properties are expected to be further enhanced. The present study addresses this aspect. CZO thin films with 0 ≤ x ≤ 1 were investigated. A unique nano-thermogravimetric method for thin films that is based on the resonant nanobalance approach for high-temperature characterization of oxygen non-stoichiometry in CZO was implemented. The high-temperature electrical conductivity and the non-stoichiometry δ of CZO were measured under oxygen partial pressures pO₂ in the range of 10⁻²⁴–0.2 bar. Markedly enhanced reducibility and electronic conductivity of CeO₂-ZrO₂ as compared to CeO_2−δ and ZrO₂ were observed. A comparison of temperature- and pO₂-dependences of the non-stoichiometry of thin films with literature data for bulk Ce_1−xZr_xO_2−δ shows enhanced reducibility in the former. The maximum conductivity was found for Ce_0.8Zr_0.2O_2−δ, whereas Ce_0.5Zr_0.5O_2-δ showed the highest non-stoichiometry, yielding δ = 0.16 at 900 °C and pO₂ of 10⁻¹⁴ bar. The defect interactions in Ce_1−xZr_xO_2−δ are analyzed in the framework of defect models for ceria and zirconia.

Synergistic Effects of Multicomponents Produce Outstanding Soot Oxidation Activity in a Cs/Co/MnOx Catalyst

The control of soot emission from diesel vehicles is of extraordinary importance to the environment, and catalytic removal of soot is a highly effective and clean method. Here, we report a novel, non-noble metal catalyst for application in the catalytic combustion of soot with superb activity and resistance to H₂O and SO₂. MnO_x oxide was prepared via a hydrothermal method, and then, Cs and Co were loaded on MnO_x by impregnation. The 5%Cs/1%Co/MnO_x catalyst displayed excellent catalytic activity with values of T₁₀ (332 °C), T₅₀ (371 °C), and T₉₀ (415 °C) under loose contact. The as-prepared catalysts were investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), H₂ temperature-programmed reduction (TPR), O₂ temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS). The results suggest that, after the introduction of Cs and Co into the MnO_x oxide, more NO₂ molecules take part in soot oxidation, exhibiting higher NO₂ utilization efficiency; this is due to the synergistic effects of multiple components (Cs, Co, and Mn) promoting the generation of more surface-active oxygen and then accelerating the reaction between NO₂ and soot. This study provides significant insights into the development of high-efficiency catalysts for soot oxidation, and the developed 5%Cs/1%Co/MnO_x catalyst is a promising candidate for application in diesel particulate filters.

Synergetic effect of Cu active sites and oxygen vacancies in Cu/CeO2–ZrO2 for the water–gas shift reaction

A positive linear correlation was established between the TOF and the ratio of oxygen vacancy concentration to Cu dispersion, demonstrating the synergetic effect of Cu active sites and oxygen vacancies for WGS.

Hierarchical macroparticles of ceria with tube-like shape – synthesis and properties

The hierarchical organization of CeO2 nanoparticles into tube-like macroparticles has a great influence on the properties of the material.

Enhanced Performance of Zirconium-Doped Ceria Catalysts for the Methoxycarbonylation of Anilines

The methoxycarbonylation of anilines stands as an attractive method for the phosgene-free production of carbamates. Despite the high yields obtained for ceria catalysts, the reduction of the amount of side products and the prevention of catalyst deactivation still represent major hurdles in this chemistry. One advantage of ceria is the possibility of tuning its reactivity by doping its lattice with other metals. In the present work, a series of doped ceria-based materials, prepared by substitution with metals, are evaluated in the methoxycarbonylation of 2,4-diaminotoluene with dimethyl carbonate. Among all catalysts, containing Eu, Hf, La, Pr, Sm, Tb, Y or Zr, ceria promoted with 2 mol % Zr exhibited 96 % selectivity towards the desired carbamates, improving the pure CeO₂ catalyst. Density functional theory demonstrates that two descriptors are needed: 1) a geometric factor that governs the reduction of energy barriers for carbamate formation through ureas; 2) catalyst basicity as N-H bonds need to be activated. Assessment in subsequent reaction cycles revealed that the CeO₂ -ZrO₂ catalyst is more stable than bulk CeO₂ , along with the reduction of fouling processes.

A novel regeneration method for deactivated commercial NH3-SCR catalysts with promoted low-temperature activities

A novel route is developed for regeneration of deactivated commercial NH₃-SCR catalysts, which includes an initial in situ construction of anatase TiO₂ porous film, followed by loading of MnO_x, CeO_x, and Mn-Ce mixed oxides as active components. The regenerated catalysts present largely improved low-temperature denitrification performance due to the synergetic effect of MnO_x and CeO_x. The denitrification efficiency could reach a high value of 97% at 200 °C and 100% at 250 °C when the Ce-Mn mixed oxides are loaded at the optimized molar quantity ratio of 10:9 (Ce:Mn). Properties and reaction mechanisms of the regenerated catalysts are investigated with characterizations of X-ray photoelectron spectroscopy (XPS), NH₃ temperature-programmed desorption (NH₃-TPD), H₂ temperature-programmed reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Our results demonstrate that the adsorption and oxidation of NO plays a crucial role for these three catalysts even though a difference exists on the reaction pathways. Graphical abstract.

Influence of different preparation methods on the activity of Ce and Mo co-doped ZSM-5 catalysts for the selective catalytic reduction of NOx by NH3

The Ce-doped different MoO₃ [conventional molybdenum oxide (con-MoO₃) or nano molybdenum oxide (nano-MoO₃) and synthetic molybdenum oxide (syn-MoO₃)] modification of ZSM-5 catalyst synthesized by different preparation methods (the combination of grinding and ion-exchange method and the combination of impregnation and ion-exchange method) was studied on selective catalytic reduction (SCR) of NO_x with NH₃. The results demonstrated that the SCR performance of the prepared Ce-doped syn-MoO₃ modification of ZSM-5 catalyst [Ce(0.9%)-syn-MoO₃(6%)/ZSM-5] by the combination of impregnation and ion-exchange method was better than Ce-doped con-MoO₃ modification of ZSM-5 [Ce(0.9%)-con-MoO₃(6%)/ZSM-5] and Ce-doped nano-MoO₃ modification of ZSM-5 [Ce(0.9%)-nano-MoO₃(6%)/ZSM-5] via the combination of grinding and ion-exchange method, especially when the temperature window is 200-350 °C. That is because it is easy to form Mo-O-Al by the smaller sized MoO₃ more easily interacting well with Brønsted acid under calcining temperature, which results in the decrease of Brønsted acid sites in the catalyst. Combing with the binding energy of Mo for all the catalysts, the combination of Mo and Al (Mo-O-Al) altered the chemical environment around the Mo species. Furthermore, Ce(0.9%)-syn-MoO₃(6%)/ZSM-5 exhibited excellent sulfur resistance.

Smartphone-based Fluoride-specific Sensor for Rapid and Affordable Colorimetric Detection and Precise Quantification at Sub-ppm Levels for Field Applications

Higher levels of fluoride (F^-) in groundwater constitute a severe problem that affects more than 200 million people spread over 25 countries. It is essential not only to detect but also to accurately quantify aqueous F^- to ensure safety. The need of the hour is to develop smart water quality testing systems that would be effective in location-based real-time water quality data collection, devoid of professional expertise for handling. We report a cheap, handheld, portable mobile device for colorimetric detection and rapid estimation of F^- in water by the application of the synthesized core-shell nanoparticles (near-cubic ceria@zirconia nanocages) and a chemoresponsive dye (xylenol orange). The nanomaterial has been characterized thoroughly, and the mechanism of sensing has been studied in detail. The sensor system is highly selective toward F^- and shows unprecedented sensitivity in the range of 0.1-5 ppm of F^-, in field water samples, which is the transition regime, where remedial measures may be needed. It addresses multiple issues expressed by indicator-based metal complexes used to determine F^- previously. Consistency in the performance of the sensing material has been tested with synthetic F^- standards, water samples from F^- affected regions, and dental care products like toothpastes and mouthwash using a smartphone attachment and by the naked eye. The sensor performs better than what was reported by prior works on aqueous F^- sensing.

Template Synthesis of Porous Ceria-Based Catalysts for Environmental Application

Porous oxide materials are widely used in environmental catalysis owing to their outstanding properties such as high specific surface area, enhanced mass transport and diffusion, and accessibility of active sites. Oxides of metals with variable oxidation state such as ceria and double oxides based on ceria also provide high oxygen storage capacity which is important in a huge number of oxidation processes. The outstanding progress in the development of hierarchically organized porous oxide catalysts relates to the use of template synthetic methods. Single and mixed oxides with enhanced porous structure can serve both as supports for the catalysts of different nature and active components for catalytic oxidation of volatile organic compounds, soot particles and other environmentally dangerous components of exhaust gases, in hydrocarbons reforming, water gas shift reaction and photocatalytic transformations. This review highlights the recent progress in synthetic strategies using different types of templates (artificial and biological, hard and soft), including combined ones, in the preparation of single and mixed oxide catalysts based on ceria, and provides examples of their application in the main areas of environmental catalysis.

Potential of Ceria-Zirconia-Based Materials in Carbon Soot Oxidation for Gasoline Particulate Filters

ZrO2 and Ce0.8Zr0.2O2 mixed oxides were prepared and tested in the oxidation of carbon soot at different oxygen partial pressures and degrees of catalyst/soot contact to investigate their activity under typical gasoline direct injection (GDI) operating conditions. Under reductive atmospheres, generation of oxygen vacancies occurs in Ce0.8Zr0.2O2, while no reduction is observed on ZrO2. Both materials can oxidize carbon under high oxygen partial pressures; however, at low oxygen partial pressures, the presence of carbon can contribute to the reduction of the catalyst and formation of oxygen vacancies, which can then be used for soot oxidation, increasing the overall performance. This mechanism is more efficient in Ce0.8Zr0.2O2 than ZrO2, and depends heavily on the interaction and the degree of contact between soot and catalyst. Thus, the ability to form oxygen vacancies at lower temperatures is particularly helpful to oxidize soot at low oxygen partial pressures, and with higher CO2 selectivity under conditions typically found in GDI engine exhaust gases.

Mesoporous Ce–Zr mixed oxides for selective oxidation of styrene in liquid phase

This work reports the synthesis of mesoporous Ce1-x-ZrxO2-δ (x = 0.5 and 0.8) mixed oxides with distinct Ce/Zr mole ratio by inverse micelle template method and their catalytic exploration for epoxidation of styrene in isopropanol solvent using TBHP as the oxidant. Among various catalysts investigated, the Ce0.8Zr0.2O2 combination catalyst exhibited best catalytic activity with ~ 98% conversion and ~ 90% selectivity to styrene epoxide. The synthesized Ce–Zr mixed oxide catalysts were characterized by various state-of-the-art techniques. Characterization studies revealed that Ce/Zr mole ratio has an imperative influence on the physicochemical properties such as surface area, oxygen vacancy concentration, and redox nature. Interestingly, catalytic efficiency was significantly improved with the increase of Ce and decrease of Zr content in the Ce–Zr mixed oxides. Catalytic efficiency and distribution of the products for styrene oxidation under various conditions such as reaction time, solvent, temperature, and styrene to TBHP mole ratio were also evaluated. Reusability of the highly active Ce0.8Zr0.2O2 mixed oxide catalyst was also demonstrated.

Gas phase sulfation of ceria-zirconia solid solutions for generating highly efficient and SO2 resistant NH3-SCR catalysts for NO removal

A series of ceria-zirconia solid solutions (Ce_xZr_1-xO₂) were prepared by co-precipitation method and then sulfated with SO₂ + O₂ at 200 °C. Subsequent testing with the selective catalytic reduction of NO by NH₃ (NH₃-SCR) showed that the activity of the sulfated Ce_xZr_1-xO₂ catalysts oxide catalysts exhibited a volcano-type tendency with increasing Zr content. Furthermore, the sulfated Ce_0.6Zr_0.4O₂ catalyst showed the most desirable NH₃-SCR activity at 250-300 °C, and exhibited much better SO₂ resistance at 250 °C. Detailed characterization results demonstrated that Ce_0.6Zr_0.4O₂ could adsorb more surface sulfate species and then produce more stable acid sites than pure CeO₂ at 200 °C. After sulfation treatment, more Ce³⁺ and oxygen vacancies were formed on the surface of Ce_0.6Zr_0.4O₂. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) experiments suggested that the nitrates species deposited on the surface of as-prepared Ce_0.6Zr_0.4O₂, which showed no reactivity, could barely deposit on the same sample after sulfation. While, the sulfated Ce_0.6Zr_0.4O₂ had more reactive acid sites to participate in the NH₃-SCR and the reaction proceeded via Eley-Rideal mechanism. This work proved that sulfation treatment could be used in designing an efficient cerium-zirconium based NH₃-SCR catalyst with great application prospect.