Ceria and its related materials for VOC catalytic combustion: A review

State of the Art and Challenges in Complete Benzene Oxidation: A Review

Increased levels and detrimental effects of volatile organic compounds (VOCs) on air quality and human health have become an important issue in the environmental field. Benzene is classified as one of the most hazardous air pollutants among non-halogenated aromatic hydrocarbons with toxic, carcinogenic, and mutagenic effects. Various technologies have been applied to decrease harmful emissions from various sources such as petrochemistry, steel manufacturing, organic chemical, paint, adhesive, and pharmaceutical production, vehicle exhausts, etc. Catalytic oxidation to CO2 and water is an attractive approach to VOC removal due to high efficiency, low energy consumption, and the absence of secondary pollution. However, catalytic oxidation of the benzene molecule is a great challenge because of the extraordinary stability of its six-membered ring structure. Developing highly efficient catalysts is of primary importance for effective elimination of benzene at low temperatures. This review aims to summarize and discuss some recent advances in catalyst composition and preparation strategies. Advantages and disadvantages of using noble metal-based catalysts and transition metal oxide-based catalysts are addressed. Effects of some crucial factors such as catalyst support nature, metal particle size, electronic state of active metal, redox properties, reactivity of lattice oxygen and surface adsorbed oxygen on benzene removal are explored. Thorough elucidation of reaction mechanisms in benzene oxidation is a prerequisite to develop efficient catalysts. Benzene oxidation mechanisms are analyzed based on in situ catalyst characterization, reaction kinetics, and theoretical simulation calculations. Considering the role of oxygen vacancies in improving catalytic performance, attention is given to oxygen defect engineering. Catalyst deactivation due to coexistence of water vapor and other pollutants, e.g., sulfur compounds, is discussed. Future research directions for rational design of catalysts for complete benzene oxidation are provided.

Toluene catalytic oxidation over gold catalysts supported on cerium-based high-entropy oxides

A series of cerium-based high-entropy oxide catalysts (the ratio of CeO2 and HEO is 1:1) was prepared by a solid-state reaction method, which exploit their unique structural and performance advantages. The Ce-HEO-T samples can achieve 100% toluene conversion rate above 328°C when they were used as catalysts directly. Subsequently, the Ce-HEO-500 exhibited the lowest temperature for toluene oxidation was used as a support to deposit different amounts of Au for a further performance improvement. Among all of prepared samples, Au/Ce-HEO-500 with a moderate content of Au (0.5 wt%) exhibited the lowest temperature for complete combustion of toluene (260°C), which decreased nearly 70°C compared with Ce-HEO-500 support. Moreover, it also showed excellent stability for 60 h with 98% toluene conversion rate. Most importantly, under the condition of 5 vol.% H2O vapour, the toluene conversion rate remained unchanged and even increased slightly compared with that in dry air, exhibiting excellent water resistance. Combined with the characterizations of XRD, SEM, TEM, BET, Raman, H2-TPR and XPS, it was found that the high dispersion of active Au NPs, the special high-entropy structure and the synergistic effect between Au and Ce, Co, Cu are the key factors when improving the catalytic performance in the Au/Ce-HEO-500 catalyst.

Lattice oxygen activation of MnO2 by CeO2 for the improved degradation of bisphenol A in the peroxymonosulfate-based oxidation

The structure of MnO2 was modified by constructing the composites CeO2/ MnO2 via a facile hydrothermal method. The catalytic performance of optimal composite (Mn-Ce10) in peroxymonosulfate (PMS) activation for the degradation of bisphenol A (BPA) is approximately three times higher than that of MnO2 alone. The average valence of manganese in CeO2/MnO2 is lowered compared to MnO2, which induces the generation of more free radicals, such as OH and SO4•-. In addition, the composite exhibits a higher concentration of oxygen vacancies than MnO2, facilitating bondingwith PMS to produce more singlet oxygen (1O2). Moreover, the incorporation of CeO2 activates the lattice oxygen of MnO2, improving its oxidative ability. Consequently, approximately 48% of BPA decomposition in 10min is attributed to direct oxidation in the Mn-Ce10/PMS system, whereas only 36% occurs in 30min for the MnO2/PMS system. Simulation results confirm weakened Mn-O covalency and elongated Mn-O bonds due to the activation of lattice oxygen in CeO2/MnO2, demonstrating that PMS tends to be adsorbed on the composite rather than on MnO2. This work establishes a relationship between lattice oxygen and the degradation pathway, offering a novel approach for the targeted regulation of catalytic oxidation.

Recent advances in catalytic oxidation of VOCs by two-dimensional ultra-thin nanomaterials

Catalytic oxidation, an end-of-pipe treatment technology for effectively purifying volatile organic compounds (VOCs), has received widespread attention. The crux of catalytic oxidation lies in the development of efficient catalysts, with their optimization necessitating a comprehensive analysis of the catalytic reaction mechanism. Two-dimensional (2D) ultra-thin nanomaterials offer significant advantages in exploring the catalytic oxidation mechanism of VOCs due to their unique structure and properties. This review classifies strategies for regulating catalytic properties and typical applications of 2D materials in VOCs catalytic oxidation, in addition to their characteristics and typical characterization techniques. Furthermore, the possible reaction mechanism of 2D Co-based and Mn-based oxides in the catalytic oxidation of VOCs is analyzed, with a special focus on the synergistic effect between oxygen and metal vacancies. The objective of this review is to provide valuable references for scholars in the field.

Recent advances in simultaneous removal of NOx and VOCs over bifunctional catalysts via SCR and oxidation reaction

NOx and volatile organic compounds (VOCs) are two major pollutants commonly found in industrial flue gas emissions. They play a significant role as precursors in the formation of ozone and fine particulate matter (PM2.5). The simultaneous removal of NOx and VOCs is crucial in addressing ozone and PM2.5 pollution. In terms of investment costs and space requirements, the development of bifunctional catalysts for the simultaneous selective catalytic reduction (SCR) of NOx and catalytic oxidation of VOCs emerges as a viable technology that has garnered considerable attention. This review provides a summary of recent advances in catalysts for the simultaneous removal of NOx and VOCs. It discusses the reaction mechanisms and interactions involved in NH3-SCR and VOCs catalytic oxidation, the effects of catalyst acidity and redox properties. The insufficiency of bifunctional catalysts was pointed out, including issues related to catalytic activity, product selectivity, catalyst deactivation, and environmental concerns. Subsequently, potential solutions are presented to enhance catalyst performance, such as optimizing the redox properties and acidity, enhancing resistance to poisoning, substituting environment friendly metals and introducing hydrocarbon selective catalytic reduction (HC-SCR) reaction. Finally, some suggestions are given for future research directions in catalyst development are prospected.

Catalysts Prepared from Atomically Dispersed Ce(III) on MgO Rival Bulk Ceria for CO Oxidation

Atomically dispersed cerium catalysts on an inert, crystalline MgO powder support were prepared by using both Ce(III) and Ce(IV) precursors. The materials were used as catalysts for CO oxidation in a once-through flow reactor and characterized by atomic-resolution scanning transmission electron microscopy, X-ray absorption near-edge structure spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction, among other techniques, before and after catalysis. The most active catalysts, formed from the precursor incorporating Ce(III), displayed performance similar to that reported for bulk ceria under comparable conditions. The catalyst provided stable time-on-stream performance for as long as it was kept on-stream, 2 days, increasing slightly in activity as the atomically dispersed cerium ions were transformed into ceria nanodomains represented as CeOx and having increased reducibility on the MgO support. The results suggest how highly dispersed supported ceria catalysts with low cerium loadings can be prepared and may pave the way for improved efficiencies of cerium utilization in oxidation catalysis.

The enhancement of benzene total oxidation over RuxCeO2 catalysts at low temperature: The significance of Ru incorporation

Catalytic oxidation is considered to be the most efficient technology for eliminating benzene from waste gas. The challenge is the reduction of the catalytic reaction temperature for the deep oxidation of benzene. Here, highly efficient RuxCeO2 catalysts were utilized to turn the number of surface oxygen vacancies and Ce-O-Ru bonds via a one-step hydrothermal method, resulting in a preferable low-temperature reducibility for the total oxidation of benzene. The T50 of the Ru0.2CeO2 catalyst for benzene oxidation was 135 °C, which was better than that of pristine CeO2 (239 °C) and 0.2Ru/CeO2 (190 °C). The superior performance of Ru0.2CeO2 was attributed to its large surface area (approximately 114.23 m2·g-1), abundant surface oxygen vacancies, and Ce-O-Ru bonds. The incorporation of Ru into the CeO2 lattice could effectively facilitate the destruction of the CeO bond and the facile release of lattice oxygen, inducing the generation of surface oxygen vacancies. Meanwhile, the bridging action of Ce-O-Ru bonds accelerated electron transfer and lattice oxygen transportation, which had a synergistic effect with surface oxygen vacancies to reduce the reaction temperature. The Ru0.2CeO2 catalyst also exhibited high catalytic stability, water tolerance, and impact resistance in terms of benzene abatement. Using in situ infrared spectroscopy, it was demonstrated that the Ru0.2CeO2 catalyst can effectively enhance the accumulation of maleate species, which are key intermediates for benzene ring opening, thereby enhancing the deep oxidation of benzene.

Effect of microstructure in mesoporous adsorbents on the adsorption of low concentrations of VOCs: An experimental and simulation study

This study evaluated the adsorption of five volatile organic compounds (VOCs) on Opoka, precipitated silica, and palygorskite, to elucidate the effect of their pore size on VOCs adsorption. The adsorption capacity of these adsorbents is not only highly correlated with their surface area and pore volume, but also notably improved by the presence of micropores. The variation in adsorption capacity for different VOCs was primarily influenced by their boiling point and polarity. Palygorskite, which had the smallest total pore volume (0.357 cm3/g) but the largest micropore volume (0.043 cm3/g) among the three adsorbents, exhibited the highest adsorption capacity for all tested VOCs. Additionally, the study constructed slit pore models of palygorskite with micropores (0.5 and 1.5 nm) and mesopores (3.0 and 6.0 nm), calculated and discussed the heat of adsorption, concentration distribution, and interaction energy of VOCs adsorbed on different pore models. The results revealed that the adsorption heat, concentration distribution, total interaction energy, and van der Waals energy decrease with increasing pore size. The concentration of VOCs in 0.5 nm pore was nearly three times that in 6.0 nm pore. This work can also provide guidance for further research on using adsorbents with mixed microporous and mesoporous structures to control VOCs.

Simultaneous Generation of Ammonia during Nitrile Waste Gas Purification over a Silver Single-Atom-Doped Ceria Catalyst

Catalytic elimination of toxic nitrile waste gas is of great significance for preserving the atmospheric environment, but achieving resource utilization during its destruction has been less explored. Herein, this study proposed a universal strategy for nitrile waste gas purification and NH3 generation simultaneously. The developed silver single-atom-doped ceria nanorod (Ag1/R-CeO2) was endowed with near complete mineralization and around 90% NH3 yield at 300-350 °C for the catalytic oxidation of both acetonitrile and acrylonitrile. The introduction of the Ag single atom created more surface oxygen vacancies, thereby promoting water activation to form abundant surface hydroxyl groups. As a benefit from this, the hydrolysis reaction of nitrile to generate NH3 was accelerated. Meanwhile, the electron transfer effect from the Ag atom to Ce and hydroxyl species facilitated NH3 desorption, which inhibited the oxidation of NH3. Moreover, the increased surface oxygen vacancies also promoted the mineralization of hydrolysis carbonaceous intermediates to CO2. In contrast, the Ag nanoparticle-modified sample possessed stronger reducibility and NH3 adsorption, leading to the excessive oxidation of NH3 to N2 and NOx. This work provided a useful guidance for resourceful purification of nitrile waste gas.

Experimental study and kinetic model analysis on photothermal catalysis of formaldehyde by manganese and cerium based catalytic materials

Modern people spend more and more time in cars in their daily lives, and the pollution of formaldehyde in the car may directly affect people's health. Thermal catalytic oxidation technology by solar light is a potential way to purify formaldehyde in cars. MnO_x-CeO₂ was prepared by the modified co-precipitation method as the main catalyst, and the basic characteristic (SEM, N₂ adsorption, H₂-TPR, UV-visible absorbance) were also analyzed in detail. The experimental study was set up to simulate the solar photothermal catalysis of formaldehyde in-car environment. The results showed that the higher the temperature in the experimental box (56.7 ± 0.2°C, 62.6 ± 0.2°C, 68.2 ± 0.2°C), the better the formaldehyde degradation by catalytic effect (formaldehyde degradation percentage: 76.2%, 78.3%, 82.1%). With increase of the initial formaldehyde concentration (200 ppb, 500 ppb, 1000 ppb), the catalytic effect first increased and then decreased (formaldehyde degradation percentage: 63%, 78.3%, 70.6%). The catalytic effect risen gradually with the increase of load ratio (10g/m2, 20g/m2, and 40g/m2), and the formaldehyde degradation percentages were 62.8%, 78.3%, and 81.1%, respectively. According to the expressions of the Eley-Rideal (ER) model, the Langmuir-Hinshelwood (LH) model, and the Mars-Van Krevelen (MVK) model, the experimental results were fitted and verified, and it was found that the ER model had a high degree of fit. It is more suitable to explain the catalytic mechanism of formaldehyde by MnO_x-CeO₂ catalyst in the experimental cabin, where formaldehyde is in the adsorption state and oxygen is in the gas phase.Implications: Judging from the current research status, vehicles have become an indispensable mode of travel for people, and the air quality in the vehicle is not optimistic. Most vehicles generally have the phenomenon of excessive formaldehyde. The characteristic of formaldehyde in the car is the continuous release, especially in the hot summer, the temperature inside the car rises sharply under the sun radiation. At this time, the formaldehyde concentration exceeds the standard by 4 to 5 times, which can cause great damage to the health of the passengers. In order to improve the air quality in the car, it is necessary to adopt the correct purification technology to degrade formaldehyde. The problem brought by this situation is how to effectively use solar radiation and high temperature in the car to degrade formaldehyde in the car. Therefore, this study uses the thermal catalytic oxidation technology to catalyze the degradation of formaldehyde in the high temperature environment of the car in summer. The selected catalyst is MnO_x-CeO₂, mainly because manganese oxide (MnO_x) itself is the most effective catalyst for volatile organic compounds (TCO) among transition metal oxides, and CeO₂ has excellent oxygen storage and release capacity and Oxidation activity, which helps to improve the activity of MnO_x. Finally, the effects of temperature, initial concentration of formaldehyde and catalyst loading on the experiment were explored, and the kinetic model of thermal catalytic oxidation of formaldehyde with MnO_x-CeO₂ catalyst was analyzed to provide technical support for the future application of this research in practice.

Photo-Activated Direct Catalytic Oxidation of Gaseous Benzene with a Cu-Connected Serial Heterojunction Array of Co3 O4 /Cux O/Foam Cu Assembled via Layer upon Layer Oxidation

The foam copper (FCu) has been first used as a promising supporting material to prepare a photo-activated catalyst of Co₃ O₄ /Cu_x O/FCu, in which the fine Co₃ O₄ particles are inlayed on the Cu_x O nanowires to form a Z-type heterojunction array connected by substrate Cu. The prepared samples have been used as a photo-activated catalyst to directly decompose gaseous benzene and the optimized Co₃ O₄ /Cu_x O/FCu demonstrates a 99.5% removal efficiency and 100% mineralizing rate within 15 min in benzene concentration range from 350 to 4000 ppm under simulate solar light irradiation. To track the reactive mechanism, a series of MO_x /Cu_x O/FCu (M = Mn, Fe, Co, Ni, Cu, Zn) is prepared and a novel photo-activated direct catalytic oxidation route is proposed based on the comparative investigation of material properties. Moreover, the approach grew in situ via layer upon layer oxidation on FCu dedicates to the extra lasting reusability and the easy accessibility in the diverse situations. This work provides a novel strategy for the preparation of Cu connected series multidimensional heterojunction array and a promising application for the quick abatement of the high-leveled concentration gaseous benzene and its derivatives from the industrial discharged flow or the accident scene's leakage.

Mechanistic Insight into Catalytic Combustion of Ethyl Acetate on Modified CeO2 Nanobelts: Hydrolysis-Oxidation Process and Shielding Effect of Acetates/Alcoholates

In this study, based on the comparison of two counterparts [Mn- and Cr-modified CeO₂ nanobelts (NBs)] with the opposite effects, some novel mechanistic insights into the ethyl acetate (EA) catalytic combustion over CeO₂-based catalysts were proposed. The results demonstrated that EA catalytic combustion consisted of three primary processes: EA hydrolysis (C-O bond breakage), the oxidation of intermediate products, and the removal of surface acetates/alcoholates. Rapid EA hydrolysis typically occurs on surface acid/base sites or hydroxyl groups, and the removal of surface acetates/alcoholates resulting from EA hydrolysis is considered the rate-determining step. The deposited acetates/alcoholates like a shield covered the active sites (such as surface oxygen vacancies), and the enhanced mobility of the surface lattice oxygen as an oxidizing agent played a vital role in breaking through the shield and promoting the further hydrolysis-oxidation process. The Cr modification impeded the release of surface-activated lattice oxygen from the CeO₂ NBs and induced the accumulation of acetates/alcoholates at a higher temperature due to the increased surface acidity/basicity. Conversely, the Mn-substituted CeO₂ NBs with the higher lattice oxygen mobility effectively accelerated the in situ decomposition of acetates/alcoholates and facilitated the re-exposure of surface active sites. This study may contribute to a further mechanistic understanding into the catalytic oxidation of esters or other oxygenated volatile organic compounds over CeO₂-based catalysts.

Capture-bonding Super Assembly of Nanoscale Dispersed Bimetal on Uniform CeO2 Nanorod for the Toluene Oxidation

Elimination of VOCs by catalytic oxidation is an important technology. Here, a general synergistic capture-bonding superassembly strategy was proposed to obtain the nanoscale dispersed 5.8% PtFe₃ -CeO₂ catalyst, which showed a high toluene oxidation activity (T₁₀₀ =226 °C), excellent catalytic stability (125 h, >99.5%) and a good water resistance ability (70 h, >99.5%). Through the detailed XPS analysis, oxygen cycle experiment, hydrogen reduction experiment, and in-situ DRIFT experiment, we could deduce that PtFe₃ -CeO₂ had two reaction pathways. The surface adsorbed oxygen resulting from PtFe₃ nanoparticles played a dominant role, due to the fast cycling between the surface adsorbed oxygen and oxygen vacancy. In contrast, the lattice oxygen resulting from CeO₂ nanorods played an important role due to the relationship between the toluene oxidation activity and the metal-oxygen bonding energy. Furthermore, DFT simulation verified Pt sites were the dominant reaction active sites during this reaction.

Insights into the Redox and Structural Properties of CoOx and MnOx: Fundamental Factors Affecting the Catalytic Performance in the Oxidation Process of VOCs

Volatile organic compound (VOC) abatement has become imperative nowadays due to their harmful effect on human health and on the environment. Catalytic oxidation has appeared as an innovative and promising approach, as the pollutants can be totally oxidized at moderate operating temperatures under 500 °C. The most active single oxides in the total oxidation of hydrocarbons have been shown to be manganese and cobalt oxides. The main factors affecting the catalytic performances of several metal-oxide catalysts, including CoOx and MnOx, in relation to the total oxidation of hydrocarbons have been reviewed. The influence of these factors is directly related to the Mars–van Krevelen mechanism, which is known to be applied in the case of the oxidation of VOCs in general and hydrocarbons in particular, using transitional metal oxides as catalysts. The catalytic behaviors of the studied oxides could be closely related to their redox properties, their nonstoichiometric, defective structure, and their lattice oxygen mobility. The control of the structural and textural properties of the studied metal oxides, such as specific surface area and specific morphology, plays an important role in catalytic applications. A fundamental challenge in the development of efficient and low-cost catalysts is to choose the criteria for selecting them. Therefore, this research could be useful for tailoring advanced and high-performance catalysts for the total oxidation of VOCs.

Template and interfacial reaction engaged synthesis of CeMnO x hollow nanospheres and their performance for toluene oxidation

A series of well-dispersed CeMnO x hollow nanospheres with uniform diameter and thickness were synthesized by a novel approach combining the template method and interfacial reaction. A SiO2 template was used as a hard template for preparation of SiO2@CeO2 nanospheres by solvothermal reaction. SiO2@CeMnO x could be formed after KMnO4 was reacted with SiO2@CeO2 by interfacial reaction between MnO4 - and Ce3+. Among all the prepared catalysts, CeMnO x -3 with a moderate content of Mn (15 wt%) exhibited the lowest temperature for complete combustion of toluene (280 °C). Moreover, it showed high stability for 36 h with toluene conversion above 97.7% and good water tolerance with 5 vol% H2O. With characterization, we found that the reaction between Ce and Mn in the Ce-Mn binary oxides gave rise to increased Ce3+ and oxygen vacancies, which led to the formation of enhanced reducibility and more surface-absorbed oxygens (O2 2-, O2- and O-), and improved the catalytic performance further.

The Emergence of the Ubiquity of Cerium in Heterogeneous Oxidation Catalysis Science and Technology

Research into the incorporation of cerium into a diverse range of catalyst systems for a wide spectrum of process chemistries has expanded rapidly. This has been evidenced since about 1980 in the increasing number of both scientific research journals and patent publications that address the application of cerium as a component of a multi-metal oxide system and as a support material for metal catalysts. This review chronicles both the applied and fundamental research into cerium-containing oxide catalysts where cerium’s redox activity confers enhanced and new catalytic functionality. Application areas of cerium-containing catalysts include selective oxidation, combustion, NOx remediation, and the production of sustainable chemicals and materials via bio-based feedstocks, among others. The newfound interest in cerium-containing catalysts stems from the benefits achieved by cerium’s inclusion, which include selectivity, activity, and stability. These benefits arise because of cerium’s unique combination of chemical and thermal stability, its redox active properties, its ability to stabilize defect structures in multicomponent oxides, and its propensity to stabilize catalytically optimal oxidation states of other multivalent elements. This review surveys the origins and some of the current directions in the research and application of cerium oxide-based catalysts.

L-asparagine Assisted Synthesis of Pt/CeO2 Nanospheres for Toluene Combustion

Pt1/CeO2 nanospheres (Pt/CeO2-NS) were synthesized by the bath oiling method with L-asparagine as a necessary additive. Owing to the morphology control effect and coordination interaction of L-asparagine, CeO2 nanospheres can retain their nanosphere structure and show stronger electronic metal-support interaction with highly dispersed Pt. Moreover, the toluene catalytic combustion performance of Pt/CeO2-NS was investigated. The structure-performance relationship is analyzed according to the coordination state of Pt. The Pt/CeO2-NS catalyst exhibited superior catalytic activity than the commercial CeO2-supported Pt catalyst, which is attributed to its higher oxygen vacancy and Pt4+.

A critical review on plasma-catalytic removal of VOCs: Catalyst development, process parameters and synergetic reaction mechanism

It is urgent to control the emission of volatile organic compounds (VOCs) due to their harmful effects on the environment and human health. A hybrid system integrating non-thermal-plasma and catalysis is regarded as one of the most promising technologies for VOCs removal due to their high VOCs removal efficiency, product selectivity and energy efficiency. This review systematically documents the main findings and improvements of VOCs removal using plasma-catalysis technology in recent 10 years. To better understand the fundamental relation between different aspects of this research field, this review mainly addresses the catalyst development, key influential factors, generation of by-products and reaction mechanism of VOCs decomposition in the plasma-catalysis process. Also, a comparison of the performance in various VOCs removal processes is provided. Particular emphasis is given to the importance of the selected catalyst and the synergy of plasma and catalyst in the VOCs removal in the hybrid system, which can be used as a reference point for future studies in this field.

Noble Metal Single-Atom Catalysts for the Catalytic Oxidation of Volatile Organic Compounds

Volatile organic compounds (VOCs) are detrimental to the environment and human health and must be eliminated before discharging. Oxidation by heterogeneous catalysts is one of the most promising approaches for the VOCs abatement. Precious metal catalysts are highly active for the catalytic oxidation of VOCs, but they are rare and their high price limits large-scale application. Supported metal single-atom catalysts (SACs) have a high atom efficiency and provide the possibility to circumvent such limitations. This Review summarizes recent advances in the use of metal SACs for the complete oxidation of VOCs, such as benzene, toluene, formaldehyde, and methanol, as well as aliphatic and Cl- and S-containing hydrocarbons. The structures of the metal SACs used and the reaction mechanisms of the VOC oxidation are discussed. The most widely used SACs are noble metals supported on oxides, especially on reducible oxides, such as Mn₂ O₃ and TiO₂ . The reactivity of most SACs is related to the activity of surface lattice oxygen of the oxides. Furthermore, several metal SACs show better reactivity and improved S and Cl resistance than the corresponding nanocatalysts, indicating that SACs have potential for application in the oxidation of VOCs. The deactivation and regeneration mechanisms of the metal SACs are also summarized. It is concluded that the application of metal SACs in catalytic oxidation of VOCs is still in its infancy. This Review aims to elucidate structure-performance relationships and to guide the design of highly efficient metal SACs for the catalytic oxidation of VOCs.

Cerium-Copper Oxides Synthesized in a Multi-Inlet Vortex Reactor as Effective Nanocatalysts for CO and Ethene Oxidation Reactions

In this study, a set of CuCeOx catalysts was prepared via the coprecipitation method using a Multi-Inlet Vortex Reactor: the Cu wt.% content is 5, 10, 20, 30 and 60. Moreover, pure CeO2 and CuO were synthesized for comparison purposes. The physico-chemical properties of this set of samples were investigated by complementary techniques, e.g., XRD, N2 physisorption at −196 °C, Scanning Electron Microscopy, XPS, FT-IR, Raman spectroscopy and H2-TPR. Then, the CuCeOx catalysts were tested for the CO and ethene oxidation reactions. As a whole, all the prepared samples presented good catalytic performances towards the CO oxidation reaction (1000 ppm CO, 10 vol.% O2/N2): the most promising catalyst was the 20%CuCeOx (complete CO conversion at 125 °C), which exhibited a long-term thermal stability. Similarly, the oxidative activity of the catalysts were evaluated using a gaseous mixture containing 500 ppm C2H4, 10 vol.% O2/N2. Accordingly, for the ethene oxidation reaction, the 20%CuCeOx catalyst evidenced the best catalytic properties. The elevated catalytic activity towards CO and ethene oxidation was mainly ascribed to synergistic interactions between CeO2 and CuO phases, as well as to the high amount of surface-chemisorbed oxygen species and structural defects.

Construction of nanorod structure confined Pt@CeO2 catalyst by in-situ encapsulation strategy for low temperature catalytic oxidation of toluene

In this work, the Pt@CeO2 catalyst with the nanorod structure (Pt@CeO2-R) and the bunch structure (Pt@CeO2-B) were synthesized through in situ encapsulation strategy of Pt species in Ce-MOFs, respectively. It is discovered that the Pt@CeO2-R catalyst owned the best catalytic performance for toluene catalytic combustion, and this situation was mainly caused by the confinement of Pt nanoparticles in Ce-MOFs, which was related to the chemical state of Pt species, redox ability, and the amount of active oxygen species. Among them, the Pt@CeO2-R catalyst owns more Ce3+ species, rich Pt4+ species, and abundant active oxygen species due to the existence of confined structure, which were conducive to promote catalytic oxidation of toluene. In addition, the Pt@CeO2-R catalyst also exhibited more redox ability, which may speed up the catalytic reaction rates. On the contrary, the Pt/CeO2-R catalyst was synthesized through a simple impregnation method, and exhibited the poor activity for toluene catalytic combustion due to poor Pt4+ species and active oxygen species. Therefore, this work provides a feasible experimental basis for the study of different morphologies and encapsulated metal particles.

Mn(CeZr)Ox chelation-induced synthesis and its hydrothermal aging characteristics for catalytic abatement of toluene

In this work, Mn(CeZr)O_x was synthesized by using chelation-induced synergistic self-assembly strategy for the combustion of toluene. The physicochemical properties of the synthesized catalysts were characterized by XRD, ICP-MS, SEM, TEM, XPS and N₂ sorption. The Mn(CeZr)O_x catalyst with T₉₀ = 225 °C exhibited improved catalytic performance than the original MnO_x catalyst (T₉₀ = 260 °C) and had significant low-temperature activity. The relationship between catalyst activity and structure was analyzed. By substituting Ce and Zr elements into the hollow microspheres of MnO₂, oxygen vacancies were produced. The main factors affecting the catalytic activity of the catalyst and the reason why it remained high catalytic activity after a long period of hydrothermal treatment were discussed. After hydrothermal aging, the original pore structure of Mn(CeZr)O_x catalyst collapsed and the specific surface area decreased, but the overall crystallinity of the catalyst increased and the content of oxygen species in the lattice increased. The distribution of Mn and oxygen species on the catalyst surface changed significantly after hydrothermal treatment. The appropriate ratio of Mn⁴⁺ to Mn³⁺ and the ratio of lattice oxygen to adsorbed oxygen species are beneficial to the redox reaction cycle.

The lanthanide doping effect on toluene catalytic oxidation over Pt/CeO2 catalyst

The present work was undertaken to know the lanthanide doping effect on the physicochemical properties of Pt/CeO₂ catalysts and their catalytic activity for toluene oxidation. A series of lanthanide ions (La, Pr, Nd, Sm and Gd) were incorporated into ceria lattice by hydrothermal method, and the Pt nanoparticles with equal quality were successfully loaded on various ceria-based supports. Their catalytic performance toward toluene oxidation shows a remarkable lanthanide-doping effect, and the activity is much dependent on the ion radius and valence state of dopants. Owing to smaller ion radius and low valence state, the dopant of Gd would form more Gd-Ce complex and less GdO₈-type complex, generating more oxygen vacancies and then promoting oxygen replenishment. Furthermore, the high concentration of oxygen vacancy would drive electrons to transfer from support to metal, and thus electron-rich and under-coordinated Pt particles that are favorable for toluene adsorption and dissociation are obtained. Attributing to above positive factors, the doping of Gd would effectively enhance the catalytic oxidation of toluene over Pt/CeO₂ catalyst. In addition, the Pt/CeGdO₂ sample exhibits an excellent reaction stability and resistance of concentration impact.

Confinement of nano-gold in 3D hierarchically structured gadolinium-doped ceria mesocrystal: synergistic effect of chemical composition and structural hierarchy in CO and propane oxidation

Gadolinium-doped ceria hierarchical gold catalyst shows four-fold TOF increase compared to undoped non-hierarchical system, proving the synergistic effect of doping and structural hierarchy in propane oxidation.

The Influence of Precursor on the Preparation of CeO2 Catalysts for the Total Oxidation of the Volatile Organic Compound Propane

CeO2 catalysts were prepared by a precipitation method using either (NH4)2Ce(NO3)6 or Ce(NO3)3, as CeIV or CeIII precursors respectively. The influence of the different precursors on catalytic activity was evaluated for the total oxidation of propane with water present in the feed. The catalyst prepared using the CeIV precursor was more active for propane total oxidation. The choice of precursor influenced catalyst properties such as surface area, reducibility, morphology, and active oxygen species. The predominant factor associated with the catalytic activity was related to the formation of either CeO2.nH2O or Ce2(OH)2(CO3)2.H2O precipitate species, formed prior to calcination. The formation of CeO2.nH2O resulted in enhanced surface area which was an important factor for controlling catalyst activity.

Modifying Y zeolite with chloropropyl for improving Cu load on Y zeolite as a super Cu/Y catalyst for toluene oxidation

Developing an efficient catalyst is desirable when for example moving from a noble metal-based catalyst to a transition metal-based one for VOC removal. In this work, the chloropropyl-modified NaY zeolite (NaY-CPT) was first synthesized in an extremely dense system through introducing 3-chloropropyl-trimethoxysilane (CPT) in the aluminosilicate sol. Then the Cu/Y-CPT catalyst was fabricated by impregnating Cu species on the NaY-CPT zeolite and the highly effective Cu/Y based catalyst has been achieved for catalytic toluene oxidation. The structure evolution of CPT modified sol and its effect on texture properties of NaY-CPT and thereby reduction ability of Cu/Y catalyst were systematically investigated by synchrotron radiation small angle X-ray scattering (SR-SAXS), EXAFS and other characterization. The CPT modified sol can promote the formation of more active aluminosilicate species, greatly accelerating crystal growth and improving framework Si/Al ratio of NaY zeolite. Due to the presence of the CPT group, the Cu/Y-CPT catalyst enhanced the interaction between Cu species and the zeolite matrix, resulting in small sized CuO nanoparticles (2.0-4.0 nm) anchoring to NaY-CPT. The Cu/Y-CPT catalyst renders more isolated Cu²⁺ species and adsorbed oxygen species, which are reactive in the oxidation reaction due to their high reducibility and mobility. Finally, the Cu/Y-CPT catalyst exhibits 90% toluene conversion at 296 °C (T ₉₀), lower than the value of 375 °C on the conventional Cu/Y-con catalyst. Meanwhile, the optimal Cu/Y-CPT catalyst also gives higher toluene conversion and stability in moisture conditions.

Modified Red Mud Catalyst for Volatile Organic Compounds Oxidation

Red mud waste from the aluminium industry was modified by leaching using hydrochloric acid or oxalic acid with additives, followed by precipitation or evaporation. The prepared catalysts were characterized in detail and tested for toluene total oxidation. The samples prepared by precipitation of the leachate by adding a base gave a much better performance in catalytic oxidation than the ones prepared by just evaporating the leachate. These improved performances can be correlated to the enhanced textural and redox properties of the catalysts due to the better dispersion and higher enrichment of Fe oxides at their surface. The best performing catalyst had a light-off temperature of around 310 °C and complete oxidation took place at around 380 °C.

Toward an Atomic-Level Understanding of Ceria-Based Catalysts: When Experiment and Theory Go Hand in Hand

ConspectusBecause ceria (CeO₂) is a key ingredient in the formulation of many catalysts, its catalytic roles have received a great amount of attention from experiment and theory. Its primary function is to enhance the oxidation activity of catalysts, which is largely governed by the low activation barrier for creating lattice O vacancies. Such an important characteristic of ceria has been exploited in CO oxidation, methane partial oxidation, volatile organic compound oxidation, and the water-gas shift (WGS) reaction and in the context of automotive applications. A great challenge of such heterogeneously catalyzed processes remains the unambiguous identification of active sites.In oxidation reactions, closing the catalytic cycle requires ceria reoxidation by gas-phase oxygen, which includes oxygen adsorption and activation. While the general mechanistic framework of such processes is accepted, only very recently has an atomic-level understanding of oxygen activation on ceria powders been achieved by combined experimental and theoretical studies using in situ multiwavelength Raman spectroscopy and DFT.Recent studies have revealed that the adsorption and activation of gas-phase oxygen on ceria is strongly facet-dependent and involves different superoxide/peroxide species, which can now be unambiguously assigned to ceria surface sites using the combined Raman and DFT approach. Our results demonstrate that, as a result of oxygen dissociation, vacant ceria lattice sites are healed, highlighting the close relationship of surface processes with lattice oxygen dynamics, which is also of technical relevance in the context of oxygen storage-release applications.A recent DFT interpretation of Raman spectra of polycrystalline ceria enables us to take account of all (sub)surface and bulk vibrational features observed in the experimental spectra and has revealed new findings of great relevance for a mechanistic understanding of ceria-based catalysts. These include the identification of surface oxygen (Ce-O) modes and the quantification of subsurface oxygen defects. Combining these theoretical insights with operando Raman experiments now allows the (sub)surface oxygen dynamics of ceria and noble metal/ceria catalysts to be monitored under the reaction conditions.Applying these findings to Au/ceria catalysts provides univocal evidence for ceria support participation in heterogeneous catalysis. For room-temperature CO oxidation, operando Raman monitoring the (sub)surface defect dynamics clearly demonstrates the dependence of catalytic activity on the ceria reduction state. Extending the combined experimental/DFT approach to operando IR spectroscopy allows the elucidation of the nature of the active gold as (pseudo)single Au⁺ sites and enables us to develop a detailed mechanistic picture of the catalytic cycle. Temperature-dependent studies highlight the importance of facet-dependent defect formation energies and adsorbate stabilities (e.g., carbonates). While the latter aspects are also evidenced to play a role in the WGS reaction, the facet-dependent catalytic performance shows a correlation with the extent of gold agglomeration. Our findings are fully consistent with a redox mechanism, thus adding a new perspective to the ongoing discussion of the WGS reaction.As outlined above for ceria-based catalysts, closely combining state-of-the-art in situ/operando spectroscopy and theory constitutes a powerful approach to rational catalyst design by providing essential mechanistic information based on an atomic-level understanding of reactions.

Solvothermal Synthesis Routes to Substituted Cerium Dioxide Materials

We review the solution-based synthesis routes to cerium oxide materials where one or more elements are included in place of a proportion of the cerium, i.e., substitution of cerium is performed. The focus is on the solvothermal method, where reagents are heated above the boiling point of the solvent to induce crystallisation directly from the solution. This yields unusual compositions with crystal morphology often on the nanoscale. Chemical elements from all parts of the periodic table are considered, from transition metals to main group elements and the rare earths, including isovalent and aliovalent cations, and surveyed using the literature published in the past ten years. We illustrate the versatility of this synthesis method to allow the formation of functional materials with applications in contemporary applications such as heterogeneous catalysis, electrodes for solid oxide fuel cells, photocatalysis, luminescence and biomedicine. We pick out emerging trends towards control of crystal habit by use of non-aqueous solvents and solution additives and identify challenges still remaining, including in detailed structural characterisation, the understanding of crystallisation mechanisms and the scale-up of synthesis.

Antisolvent Route to Ultrathin Hollow Spheres of Cerium Oxide for Enhanced CO Oxidation

Cerium(IV) oxide (CeO₂), or ceria, is one of the most abundant rare-earth materials that has been extensively investigated for its catalytic properties over the past two decades. However, due to the global scarcity and increasing cost of rare-earth materials, efficient utilization of this class of materials poses a challenging issue for the materials research community. Thus, this work is directed toward an exploration of making ultrathin hollow ceria or other rare-earth metal oxides and mixed rare-earth oxides in general. Such a hollow morphology appears to be attractive, especially when the thickness is trimmed to its limit, so that it can be viewed as a two-dimensional sheet of organized nanoscale crystallites, while remaining three-dimensional spatially. This ensures that both inner and outer shell surfaces can be better utilized in catalytic reactions if the polycrystalline sphere is further endowed with mesoporosity. Herein, we have devised our novel synthetic protocol for making ultrathin mesoporous hollow spheres of ceria or other desired rare-earth oxides with a tunable shell thickness in the region of 10 to 40 nm. Our ceria ultrathin hollow spheres are catalytically active and outperform other reported similar nanostructured ceria for the oxidation reaction of carbon monoxide in terms of fuller utilization of cerium. The versatility of this approach has also been extended to fabricating singular or multicomponent rare-earth metal oxides with the same ultrathin hollow morphology and structural uniformity. Therefore, this approach holds good promise for better utilization of rare-earth metal elements across their various technological applications, not ignoring nano-safety considerations.