High-surface area CuO–CeO2 catalysts prepared by a surfactant-templated method for low-temperature CO oxidation

Enhancing the CO Oxidation Performance of Copper by Alloying with Immiscible Tantalum

Copper-tantalum (Cu-Ta) immiscible alloy nanoparticles (NPs) have been the subject of extensive research in the field of structural materials, due to their exceptional nanostructural stability and high-temperature creep properties. However, Cu is also a highly active oxidation catalyst due to its abundant valence changes. In this study, we have for the first time obtained homogeneous CuxTa1-x (x = 0.5, 0.7, 0.9, 1) nanoparticles by wet coreduction with an average particle size of approximately 30 nm. Testing verified all the CuxTa1-x/TiO2 (x = 0.5, 0.7, 0.9) showed higher CO oxidation activity than Cu/TiO2, with Cu0.7Ta0.3/TiO2 exhibiting the most promising performance. The temperature-programmed reduction with hydrogen demonstrated that Cu0.7Ta0.3/TiO2 exhibits enhanced redox properties. While kinetic studies indicated that the reaction of the Cu0.7Ta0.3/TiO2 catalyst followed the Langmuir-Hinshelwood mechanism, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) verified the introduction of Ta induced the generation of bicarbonate as an intermediate product and increased the adsorption capacity of Cu+ on CO in the catalyst, which facilitated the reaction of surface adsorbed CO with oxygen and led to the enhanced CO oxidation activity.

Hydroxyls on CeO2 Support Promoting CuO/CeO2 Catalyst for Efficient CO Oxidation and NO Reduction by CO

Transition metal catalysts, such as copper oxide, are more attractive alternatives to noble metal catalysts for emission control due to their higher abundance, lower cost, and excellent catalytic activity. In this study, we report the preparation and application of a novel CuO/CeO2 catalyst using a hydroxyl-rich Ce(OH)x support for CO oxidation and NO reduction by CO. Compared to the catalyst prepared from a regular CeO2 support, the new CuO/CeO2 catalyst prepared from the OH-rich Ce(OH)x (CuO/CeO2-OH) showed significantly higher catalytic activity under different testing conditions. The effect of OH species in the CeO2 support on the catalytic performance and physicochemical properties of the CuO/CeO2 catalyst was characterized in detail. It is demonstrated that the abundant OH species enhanced the CuOx dispersion on CeO2, increased the CuOx-CeO2 interfaces and surface defects, promoted the oxygen activation and mobility, and boosted the NO adsorption and dissociation on CuO/CeO2-OH, thus contributing to its superior catalytic activity for both CO oxidation and NO reduction by CO. These results suggest that the OH-rich Ce(OH)x is a superior support for the preparation of highly efficient metal catalysts for different applications.

Kinetic and Computational Studies of CO Oxidation and PROX on Cu/CeO2 Nanospheres

As supported CuO is well-known for low temperature activity, CuO/CeO2 nanosphere catalysts were synthesized and tested for CO oxidation and preferential oxidation of CO (PROX) in excess H2. For the first reaction, ignition was observed at 95 °C, whereas selective PROX occurred in a temperature window from 50 to 100 °C. The catalytic performance was independent of the initial oxidation state of the catalyst (CuO vs. Cu0), suggesting that the same active phase is formed under reaction conditions. Density functional modeling was applied to elucidate the intermediate steps of CO oxidation, as well as those of the comparably less feasible H2 transformation. In the simulations, various Cu and vacancy sites were probed as reactive centers enabling specific pathways.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11244-023-01848-x.

Multiple Interface Coupling in Ultrathin Mn-based Composites for Superior Catalytic Oxidation: Implications of Interface Coupling on Structural Defects

Manganese oxide has been recognized as one of the most promising gaseous heterogeneous catalysts due to its low cost, environmental friendliness, and high catalytic oxidation performance. The modulation of the interfacial coupling effect of manganese oxides by chemical means is considered a critical and effective way to improve the catalytic performance. Herein, a novel one-step synthetic strategy of highly-efficient ultrathin manganese-based catalysts is proposed through optimal regulation of metal/manganese oxide multi-interfacial coupling. Carbon monoxide (CO) and propane (C₃H₈) oxidation are employed as probe reactions to investigate the structure-catalytic mechanism - catalytic performance relationship. The ultrathin manganese (Mn)-based catalyst exhibits superior low-temperature catalytic activity with a 90% conversion of CO/C₃H₈ realized at 106℃ and 350℃. Subsequently, the effect of "interfacial effect" on the intrinsic properties of manganese oxides is revealed. The ultrathin appearance of two-dimensional (2D) manganese dioxide (MnO₂) nanosheets changes the binding force in the vertical direction, thus resulting in an increase in the average manganese-oxygen (Mn-O) bond length and exposing more surface defects. Besides, the introduction of Copper (Cu) species into the catalyst further weakens the Mn-O bond and promotes the generation of oxygen vacancies, which subsequently enhances the oxygen migration rate. This study provides new insights into the optimal design of transition metal oxide interfacial assemblies for efficient catalytic reactions.

Enhanced activation of persulfate improves the selective oxidation of alcohols catalyzed by earth-abundant metal oxides embedded on porous N-doped carbon derived from chitosan

Metal clusters oxide were embedded in an N-doped carbon and used as catalysts for the activation of peroxydisulfate or peroxymonosulfate in the selective oxidation of benzyl alcohol. Quenching tests were done to investigate the reaction mechanism.

Biomimetic Tremelliform Ultrathin MnO2/CuO Nanosheets on Kaolinite Driving Superior Catalytic Oxidation: An Example of CO

Highly efficient three-dimensional (3D) kaolinite/MnO₂-CuO (KM@CuO-NO₃) catalysts were synthesized by a mild biomimetic strategy. Kaolinite flakes were uniformly wrapped by ultrathin tremelliform MnO₂ nanosheets with thicknesses of around 1.0-1.5 nm. Si-O and Al-O groups in kaolinite hosted MnO₂ nanosheets to generate a robust composite structure. The ultrathin MnO₂ lamellar structure exhibited excellent stability even after calcination above 350 °C. Kaolinite/MnO₂ exhibited abundant edges, sharp corners, and interconnected diffusion channels, which are superior to the common stacked structure. Open channels guaranteed fast transportation and migration of CO and O₂ during CO oxidation. The synthesized KM@CuO-NO₃ achieved a 90% CO conversion efficiency at a relatively low temperature (110 °C). Furthermore, the abundant oxygen vacancies on KM@CuO-NO₃ assisted the adsorption and activation of oxygen species and thus enhanced the oxygen mobility and reactivity in the catalytic process. The mechanism results suggest that CuO introduced to the catalyst not only acted as CO active sites but also weakened the Mn-O bond, subsequently improved the mobilities of the oxygen species, which was found to contribute to its high activity for CO oxidation. This study provides new conceptual insights into rationally regulating structural assembly between transition metal oxides and natural minerals for high-performance catalysis reactions.

Probing the role of surface activated oxygen species of CeO2 nanocatalyst during the redox cycle in CO oxidation

The oxygen species of CeO₂ nanocatalysts plays a key role in the CO oxidation. In this work, nanocrystalline CeO₂ with infrared spectroscopy detectable surface superoxide (O₂⁻) species at room temperature is fabricated and CO oxidation is used as a probe reaction for the exploration of the characteristics of surface O₂⁻ species on the CeO₂ surface. We discover that the surface O₂⁻ species have ignorable influences on the overall reaction rate of CO oxidation on pure ceria by comparing P-CeO₂ (CeO₂ prepared by precipitation method) with HT-CeO₂ (CeO₂ prepared by hydrothermal method). It is concluded that the reaction between CO molecules and surface O₂⁻ species is the first and the fast step in the whole redox cycle, while the release of surface lattice oxygen is the second and the rate determining step of the catalysts. This work gives an intuitionistic exploration on the redox properties of pure nanocrystalline CeO₂ with surface O₂⁻ species and reveals the influences of these species in the whole redox circle of CO oxidation.

Pure CeO₂ nanocatalysts fabricated by different methods are obtained and the impact of surface oxygen species on their CO oxidation is compared and evaluated.

Recent advances in catalytic systems in the prism of physicochemical properties to remediate toxic CO pollutants: A state-of-the-art review

Carbon monoxide (CO) is the most harmful pollutant in the air, causing environmental issues and adversely affecting humans and the vegetation and then raises global warming indirectly. CO oxidation is one of the most effective methods of reducing CO by converting it into carbon dioxide (CO₂) using a suitable catalytic system, due to its simplicity and great value for pollution control. The CO oxidation reaction has been widely studied in various applications, including proton-exchange membrane fuel cell technology and catalytic converters. CO oxidation has also been of great academic interest over the last few decades as a model reaction. Many review studies have been produced on catalysts development for CO oxidation, emphasizing noble metal catalysts, the configuration of catalysts, process parameter influence, and the deactivation of catalysts. Nevertheless, there is still some gap in a state of the art knowledge devoted exclusively to synergistic interactions between catalytic activity and physicochemical properties. In an effort to fill this gap, this analysis updates and clarifies innovations for various latest developed catalytic CO oxidation systems with contemporary evaluation and the synergistic relationship between oxygen vacancies, strong metal-support interaction, particle size, metal dispersion, chemical composition acidity/basicity, reducibility, porosity, and surface area. This review study is useful for environmentalists, scientists, and experts working on mitigating the harmful effects of CO on both academic and commercial levels in the research and development sectors.

Bioinspired Synthesis of Ce1-x O2:x%Cu2+ Nanobelts for CO Oxidation and Organic Dye Degradation

Ce_1-x O₂:x%Cu²⁺ nanobelts were bioinspired, designed, and fabricated using commercial filter papers as scaffolds by adding Cu(NO₃)₂ in the original sol solution of CeO₂ nanobelts, which display excellent catalyst properties for CO oxidation and photocatalytic activity for organic dyes. Compared with pure CeO₂, CuO belts were synthesized using the same method and the corresponding Ce_0.5O₂:50%Cu²⁺ bulk materials were synthesized without filter paper as scaffolds; the synthesized Ce_1-x O₂:x%Cu²⁺ nanobelts, especially Ce_0.5O₂:50%Cu²⁺ nanobelts, can decrease the reaction temperature of CO to CO₂ at 100 °C with the conversion rate of 100%, much lower than the formerly reported kinds of Ce_1-x O₂:x%Cu²⁺ catalysts. Meanwhile, the synthesized Ce_1-x O₂:x%Cu²⁺ nanobelts also display better photocatalytic activity for organic dyes. All of these results provide useful information for the potential applications of the synthesized Ce_1-x O₂:x%Cu²⁺ nanobelts in catalyst fields.

Efficient Imine Formation by Oxidative Coupling at Low Temperature Catalyzed by High-Surface-Area Mesoporous CeO2 with Exceptional Redox Property

High-surface-area mesoporous CeO₂ (hsmCeO₂ ) was prepared by a facile organic-template-induced homogeneous precipitation process and showed excellent catalytic activity in imine synthesis in the absence of base from primary alcohols and amines in air atmosphere at low temperature. For comparison, ordinary CeO₂ and hsmCeO₂ after different thermal treatments were also investigated. XRD, N₂ physisorption, UV-Raman, H₂ temperature-programmed reduction, O₂ temperature-programmed desorption, EPR spectroscopy, and X-ray photoelectron spectroscopy were used to unravel the structural and redox properties. The hsmCeO₂ calcined at 400 °C shows the highest specific surface area (158 m²  g^-1 ), the highest fraction of surface coordinatively unsaturated Ce³⁺ ions (18.2 %), and the highest concentration of reactive oxygen vacancies (2.4×10¹⁵  spins g^-1 ). In the model reaction of oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the hsmCeO₂ catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol  g ceria - 1  h^-1 based on >99 % yield at 60 and 80 °C, respectively) in air with no base additives. It can also work effectively at a temperature of 30 °C and in gram-scale synthesis. These are among the best results for all benchmark ceria catalysts in the literature. Moreover, the hsmCeO₂ catalyst shows a wide scope towards primary alcohols and amines with good to excellent yield of imines. The influence of reaction parameters, the reusability of the catalyst, and the reaction mechanism were investigated.

Characterization and catalytic activity of soft-templated NiO-CeO2 mixed oxides for CO and CO2 co-methanation

Nanosized NiO, CeO2 and NiO-CeO2 mixed oxides with different Ni/Ce molar ratios were prepared by the soft template method. All the samples were characterized by different techniques as to their chemical composition, structure, morphology and texture. On the catalysts submitted to the same reduction pretreatment adopted for the activity tests the surface basic properties and specific metal surface area were also determined. NiO and CeO2 nanocrystals of about 4 nm in size were obtained, regardless of the Ni/Ce molar ratio. The Raman and X-ray photoelectron spectroscopy results proved the formation of defective sites at the NiO-CeO2 interface, where Ni species are in strong interaction with the support. The microcalorimetric and Fourier transform infrared analyses of the reduced samples highlighted that, unlike metallic nickel, CeO2 is able to effectively adsorb CO2, forming carbonates and hydrogen carbonates. After reduction in H2 at 400 °C for 1 h, the catalytic performance was studied in the CO and CO2 co-methanation reaction. Catalytic tests were performed at atmospheric pressure and 300 °C, using CO/CO2/H2 molar compositions of 1/1/7 or 1/1/5, and space velocities equal to 72000 or 450000 cm3·h−1·gcat−1. Whereas CO was almost completely hydrogenated in any investigated experimental conditions, CO2 conversion was strongly affected by both the CO/CO2/H2 ratio and the space velocity. The faster and definitely preferred CO hydrogenation was explained in the light of the different mechanisms of CO and CO2 methanation. On a selected sample, the influence of the reaction temperature and of a higher number of space velocity values, as well as the stability, were also studied. Provided that the Ni content is optimized, the NiCe system investigated was very promising, being highly active for the COx co-methanation reaction in a wide range of operating conditions and stable (up to 50 h) also when submitted to thermal stress.

A review of the catalysts used in the reduction of NO by CO for gas purification

The reduction of NO by the CO produced by incomplete combustion in the flue gas can remove CO and NO simultaneously and economically. However, there are some problems and challenges in the industrial application which limit the application of this process. In this work, noble metal catalysts and transition metal catalysts used in the reduction of NO by CO in recent years are systematically reviewed, emphasizing the research progress on Ir-based catalysts and Cu-based catalysts with prospective applications. The effects of catalyst support, additives, pretreatment methods, and physicochemical properties of catalysts on catalytic activity are summarized. In addition, the effects of atmosphere conditions on the catalytic activity are discussed. Several kinds of reaction mechanisms are proposed for noble metal catalysts and transition metal catalysts. Ir-based catalysts have an excellent activity for NO reduction by CO in the presence of O₂. Cu-based bimetallic catalysts show better catalytic performance in the absence of O₂, in that the adsorption and dissociation of NO can occur on both oxygen vacancies and metal sites. Finally, the potential problems existing in the application of the reduction of NO by CO in industrial flue gas are analyzed and some promising solutions are put forward through this review.

Engineering CuOx–ZrO2–CeO2 nanocatalysts with abundant surface Cu species and oxygen vacancies toward high catalytic performance in CO oxidation and 4-nitrophenol reduction

CuO_x–ZrO₂–CeO₂ nanocrystalline catalysts were designed and synthesized by a solvent-free synthetic strategy, and exhibited excellent catalytic performance owing to the increased oxygen vacancies and better dispersed active metal species.

High-heat treatment enhanced catalytic activity of CuO/CeO2 catalysts with low CuO content for CO oxidation

CuO/CeO₂ catalysts with low CuO content and calcined at 800 °C exhibited better catalytic performance than those calcined at 500 °C. The coordinatively unsaturated copper atoms were proved to be the main active sites in the CuO/CeO₂ catalysts.

Facile preparation of highly active and stable CuO–CeO2 catalysts for low-temperature CO oxidation via a direct solvothermal method

CuO–CeO₂ catalysts prepared by a direct solvothermal method exhibit high activity and stability for low-temperature CO oxidation.

CO Oxidation over Metal Oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2) Doped CuO-Based Catalysts Supported on Mesoporous Ce0.8Zr0.2O2 with Intensified Low-Temperature Activity

CuO-based catalysts are usually used for CO oxidation owing to their low cost and excellent catalytic activities. In this study, a series of metal oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2)-doped CuO-based catalysts with mesoporous Ce0.8Zr0.2O2 support were simply prepared by the incipient impregnation method and used directly as catalysts for CO catalytic oxidation. These mesoporous catalysts were systematically characterized by X-ray powder diffraction (XRD), N2 physisorption, transmission electron microscopy (TEM), energy-dispersed spectroscopy (EDS) mapping, X-ray photoelectron spectroscopy (XPS), and H2 temperature programmed reduction (H2-TPR). It was found that the CuO and the dopants were highly dispersed among the mesoporous framework via the incipient impregnation method, and the strong metal framework interaction had been formed. The effects of the types of the dopants and the loading amounts of the dopants on the low-temperature catalytic performances were carefully studied. It was concluded that doped transition metal oxides could regulate the oxygen mobility and reduction ability of catalysts, further improving the catalytic activity. It was also found that the high dispersion of rare earth metal oxides (PrO2, Sm2O3) was able to prevent the thermal sintering and aggregation of CuO-based catalysts during the process of calcination. In addition, their presence also evidently improved the reducibility and significantly reduced the particle size of the CuO active sites for CO oxidation. The results demonstrated that the 15CuO-3Fe2O3/M-Ce80Zr20 catalyst with 3 wt. % of Fe2O3 showed the best low-temperature catalytic activity toward CO oxidation. Overall, the present Fe2O3-doped CuO-based catalysts with mesoporous nanocrystalline Ce0.8Zr0.2O2 solid solution as support were considered a promising series of catalysts for low-temperature CO oxidation.

Ag2S-Sensitized NiO-ZnO Heterostructures with Enhanced Visible Light Photocatalytic Activity and Acetone Sensing Property

Visible light-driven Ag2S-grafted NiO-ZnO ternary nanocomposites are synthesized using a facile and cost-effective homogeneous precipitation method. The structural, morphological, and optical properties were extensively studied, confirming the formation of ternary nanocomposites. The surface area of the synthesized nanocomposites was calculated by electrochemical double-layer capacitance (C dl). Ternary Ag2S/NiO-ZnO nanocomposites showed excellent visible light photocatalytic property which increases further with the concentration of Ag2S. The maximum photocatalytic activity was shown by 8% Ag2S/NiO-ZnO with a RhB degradation efficiency of 95%. Hydroxyl and superoxide radicals were found to be dominant species for photodegradation of RhB, confirmed by scavenging experiments. It is noteworthy that the recycling experiments demonstrated high stability and recyclable nature of the photocatalyst. Moreover, the electrochemical results indicated that the prepared nanocomposite exhibits remarkable activity toward detection of acetone. The fabricated nanocomposite sensor showed high sensitivity (4.0764 μA mmol L-1 cm-2) and a lower detection limit (0.06 mmol L-1) for the detection of acetone. The enhanced photocatalytic and the sensing property of Ag2S/NiO-ZnO can be attributed to the synergistic effects of strong visible light absorption, excellent charge separation, and remarkable surface properties.