Enhanced CO oxidation on CeO2/Co3O4 nanojunctions derived from annealing of metal organic frameworks

Enhanced CO oxidation performance over hierarchical flower-like Co3O4 based nanosheets via optimizing oxygen activation and CO chemisorption

Enhancing low-temperature activity is a focus for carbon monoxide (CO) elimination by catalytic oxidation. In this work, the hierarchical flower-like silver (Ag) modified cobalt oxides (Co3O4) nanosheets were prepared by solvothermal method and applied into catalytic CO oxidation. The doped Ag species in the form of AgCoO2 induced the prolongated surface Co-O bond and weaker bond intensity. Consequently, the oxygen activation/migration ability and redox capacity of Ag0.02Co were enhanced with more oxygen vacancies. The chemisorbed CO was preferentially converted to CO2 but not carbonates. The inhibited carbonates accumulation could avoid the coverage of active sites. According to Density functional theory (DFT) calculations, the electron transfer from AgCoO2 to Co3O4 promote electron donation ability of Co3O4 layer, benefiting for oxygen activation. Moreover, the longer Co-C and C-O bond length suggest the weakened chemisorption strength and higher active of CO molecule. The Ag modified Co3O4 exhibited more satisfactory activity at lower temperature. Typically, it realized 100% CO conversion at 90 °C, and displayed 6.3-fold higher reaction rate than pristine Co3O4 at 40 °C. Moreover, the Ag0.02Co exhibited outstanding long-term stability and water resistance. In summary, the optimized oxygen activation, CO chemisorption and interfacial electron transfer synergistically boosted the CO oxidation activity on Ag modified Co3O4.

Cobalt-Ceria Binary Oxide Nanojunctions for Aqueous-Phase Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid: The Role of Interfaces

Cobalt-ceria binary oxide nanojunctions were prepared by a sol-gel method with various chelating agents. The formed interfaces among CeO2 and Co3O4 can promote the generation of nucleophilic •O2- from O2 and then tune the catalytic oxidizability of the as-prepared CoCe nanojunctions. Given the results of HMF oxidations, malic acid as a complexing agent during the preparation process of the cobalt-ceria binary oxide nanojunctions can lead to a good catalytic performance on HMF oxidations to FDCA, and a remarkable FDCA selectivity of 92.3% and almost 100% HMF conversion were obtained at 110 °C under O2 and alkali conditions. By comparing the catalytic performance of the nanojunctions and physical mixing of cobalt-ceria binary oxide on oxidations of HMF, 5-hydroxymethyl-2-furancarboxylic acid (HFCA), and 5-formyl-2-furancarboxylic acid (FFCA), the interfaces intrinsically enhanced the FDCA yield dominantly via boosting the HMF oxidation to HFCA with •O2- during the stepwise oxidation of HMF to FDCA. It can be enlightening that the introduction of the active sites for transforming O2 to •O2- to promote the transformation of HMF into HFCA is the key to boosting the selective aerobic oxidation of HMF to FDCA.

Enhanced CO oxidation with cobalt-impregnated porous single-crystal manganese oxides

In this work, Mn2O3, Mn3O4 and MnO single crystals are prepared by using MnCO3 as the precursor. In addition, Co element is loaded on the three manganese oxide single crystals for the CO oxidation reaction.

Selective aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over nanojunctions of cobalt–ceria binary oxide in water

Non-precious cobalt–ceria binary oxide nanojunctions with rich interfaces were designed to efficiently catalyze 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA).

Multishelled NiO/NiCo2O4 hollow microspheres derived from bimetal-organic frameworks as high-performance sensing material for acetone detection

Recently, tremendous research interest was stimulated to obtain advanced function materials with hierarchical structure and tailored chemical composition from metal-organic frameworks (MOFs) based precursors. Herein, Bimetal-organic frameworks of Ni-Co-BTC solid microspheres synthesized through hydrothermal method were acted as template to induce multishelled NiO/NiCo₂O₄ hollow microspheres by annealing treatment. When evaluated as gas sensing material, the optimal hybrid of NiO/NiCo₂O₄ (the molar ration of NiCo=1.5) multishelled hollow microspheres endowed a high sensitivity (17.86) to 100 ppm acetone with rapid response/recovery time (11/13 s) under low working temperature (160 °C) and the low detection limit reached 25 ppb. The enhanced mechanism was originated from the following aspects: the multishelled hollow architecture provided efficient diffusion path for gas molecules and sufficient active site for gas sensing reaction; the nanoscale p-p heterojunction created at NiO and NiCo₂O₄ nanoparticles interface amplified the resistance variation by tuning the potential barrier; the potent combination of the "chemical catalytic" effect of NiO and the "electrical catalytic" effect of NiCo₂O₄ improved the selective acetone detection.

Co3O4/CeO2 multi-shelled nanospheres derived from self-templated synthesis for efficient catalytic CO oxidation

Co3O4/CeO2 multi-shelled nanospheres with various Ce/Co ratios for low temperature CO oxidation were fabricated by using a bimetallic coordination polymer as a self-sacrificial template. The obtained Co0.9Ce0.1O2-δ catalyst exhibited superior CO oxidation performance, which reached 100% conversion at 80 °C. After five-cycle tests, the conversion could also be maintained at approximately 100% with almost no activity loss. Furthermore, the effects of the CeO2 component on catalytic performance have been investigated by various advanced physicochemical characterization methods. This indicated that the addition of cerium can significantly increase the surface area, so as to enrich the oxygen vacancies and provide a higher degree of accessible active sites. This work proposed an alternative approach to construct multi-shelled nanosphere composites.

Boosting benzene combustion by engineering oxygen vacancy-mediated Ag/CeO2-Co3O4 catalyst via interfacial electron transfer

Oxygen vacancy (O_v) engineering is a widely accepted effective strategy to manipulate the catalytic activity for volatile organic compounds (VOCs) abatement. Herein, we report the oxygen vacancy-mediated Ag/CeO₂-Co₃O₄ catalyst to boost benzene combustion. The incorporation of Ag species in Ag/CeO₂-Co₃O₄ induces the predominately exposed surface Co³⁺ sites and structural distortion of Co₃O₄ as well as rich oxygen vacancy owing to the improved interfacial electron transfer, which promote the adsorption of benzene and the dissociation of oxygen. The low-temperature reducibility and mobility of oxygen species are also improved due to the generation of oxygen vacancy. The isotopic ¹⁸O₂ exchange experiments demonstrate that abundant oxygen vacancies contribute to the rapid generation of active oxygen species, and the consumed oxygen vacancies can be compensated steadily during benzene oxidation. In-situ DRIFTS results reveal that benzene oxidation is a continuous oxidation process, and active oxygen species plays a crucial role in the deep oxidation of benzene by engineering oxygen vacancy. This work provides an efficient strategy for designing high-performance environmental catalysts for VOCs abatement.

Enhanced Acetone Oxidation over the CeO2/Co3O4 Catalyst Derived from Metal-Organic Frameworks

A novel CeO₂/Co₃O₄ catalyst with a high catalytic activity has been designed and prepared by pyrolysis of metal-organic frameworks, and its catalytic performance was evaluated by the acetone catalytic oxidation reaction. The Co₃O₄-M catalyst with T₉₀ at 194 °C was prepared by pyrolysis of the MOF-71 precursor, which was 56 °C lower than that of commercial Co₃O₄ (250 °C). By the addition of cerium to the MOF-71 precursor, an enhanced CeO₂/Co₃O₄ catalyst with T₉₀ at 180 °C was prepared. Importantly, the CeO₂/Co₃O₄ catalyst exhibited superior stability for acetone oxidation. After 10 cycle tests, the conversion could also be maintained at 97% for at least 100 h with slight activity loss. Characterization studies were used to investigate the influence of cerium doping on the property of the catalyst. The results showed that addition of cerium could facilitate the expansion of the surface area and enhance the porous structure and reducibility at low temperature. Furthermore, the surface ratio of Co³⁺/Co²⁺ and mobile oxygen obviously improved with the addition of cerium. Therefore, the metal oxides prepared by this method have potential for the elimination of acetone.

Catalytic combustion of toluene over CeO2–CoOx composite aerogels

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

Catalytic activity boost of CeO2/Co3O4 nanospheres derived from CeCo-glycolate via yolk–shell structural evolution

CeO₂/Co₃O₄ hybrid nanospheres have been successfully prepared via thermal decomposition of CeCo-glycolate precursors.

Oxygen Vacancy-rich Porous Co3O4 Nanosheets toward Boosted NO Reduction by CO and CO Oxidation: Insights into the Structure-Activity Relationship and Performance Enhancement Mechanism

Oxygen vacancy-rich porous Co₃O₄ nanosheets (OV-Co₃O₄) with diverse surface oxygen vacancy contents were synthesized via facile surface reduction and applied to NO reduction by CO and CO oxidation. The structure-activity relationship between surface oxygen vacancies and catalytic performance was systematically investigated. By combining Raman, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and O₂-temperature programmed desorption, it was found that the efficient surface reduction leads to the presence of more surface oxygen vacancies and thus distinctly enhance the surface oxygen amount and mobility of OV-Co₃O₄. The electron transfer towards Co sites was promoted by surface oxygen vacancies with higher content. Compared with the pristine porous Co₃O₄ nanosheets, the presence of more surface oxygen vacancies is beneficial for the catalytic performance enhancement for NO reduction by CO and CO oxidation. The OV-Co₃O₄ obtained in 0.05 mol L^-1 NaBH₄ solution (Co₃O₄-0.05) exhibited the best catalytic activity, achieving 100% NO conversion at 175 °C in NO reduction by CO and 100% CO conversion at 100 °C in CO oxidation, respectively. Co₃O₄-0.05 exhibited outstanding catalytic stability and resistance to high gas hour space velocity in both reactions. Combining in situ DRIFTS results, the enhanced performance of OV-Co₃O₄ for NO reduction by CO should be attributed to the promoted formation and transformation of dinitrosyl species and -NCO species at lower and higher temperatures. The enhanced performance of OV-Co₃O₄ for CO oxidation is due to the promotion of oxygen activation ability, surface oxygen mobility, as well as the enhanced CO₂ desorption ability. The results indicate that the direct regulation of surface oxygen vacancies could be an efficient way to evidently enhance the catalytic performance for NO reduction by CO and CO oxidation.

Leaf-like Co-ZIF-L derivatives embedded on Co2AlO4/Ni foam from hydrotalcites as monolithic catalysts for toluene abatement

Herein, a series of distinctively monolithic catalysts were first synthesized by decorating leaf-like Co-ZIF-L derivatives on Co₂AlO₄ coral-like microspheres from CoAl layered double hydroxides (LDHs), which were coated on three-dimensional porous Ni foam. As a proof of concept application, toluene was chosen as a probe molecule to evaluate their catalytic performances over the as-synthesized catalysts. As a result, the L-12 sample derived from Co₂AlO₄@Co-Co LDHs displayed an excellent catalytic performance, cycling stability and long-term stability for toluene oxidation (T₉₉ = 272 °C, 33 °C lower than that of Co₂AlO₄ sample), where leaf-like Co-ZIF-L served as a sacrificial template to synthesize Co-Co LDHs. The improved catalytic performance was attributed to its distinctive structure, in which leaf-like Co-ZIF-L derivatives on Co₂AlO₄ resulted in its higher specific surface area, lower-temperature reducibility, rich surface oxygen vacancy and high valence Co³⁺ species. This work thus demonstrates a feasible strategy for the design and fabrication of hybrid LDHs/ZIFs-derived composite architectures, which is expected to construct other novel monolithic catalysts with hierarchical structures for other potential applications.

Highly enhanced soot oxidation activity over 3DOM Co3O4-CeO2 catalysts by synergistic promoting effect

Three-dimensionally ordered macroporous (3DOM) Co₃O₄-CeO₂ catalysts with controllable Co/Ce molar ratios synthesized by colloidal crystal template method were developed to catalyze the NO_x-assisted soot oxidation for the first time, and the obtained 3DOM Co₃O₄-CeO₂ catalysts exhibited highly enhanced soot oxidation activity. Detailed characterizations of 3DOM Co₃O₄-CeO₂ catalysts revealed that the highly enhanced soot oxidation activity was originated from the synergistic promoting effect by combining the macroporous effect resulted from the unique 3DOM framework, the chemical nature associated with more Co³⁺ reactive sites, the surface enrichment of Ce species and the improved redox properties. Meanwhile, the high NO_x storage and oxidation capacity resulted from the integrated respective merits of Co₃O₄ and CeO₂ also accounted for the enhanced soot oxidation activity via NO_x-assisted mechanism. Furthermore, the 3DOM Co₃O₄-CeO₂ catalysts demonstrated strong stability because of the surface enrichment of Ce species improving the thermal stability and the robust 3DOM framework inhibiting the structural collapse, showing their potential applications under practical conditions.

Macroporous Ni foam-supported Co3O4 nanobrush and nanomace hybrid arrays for high-efficiency CO oxidation

Herein, we reported the synthesis of well-defined Co₃O₄ nanoarrays (NAs) supported on a monolithic three-dimensional macroporous nickel (Ni) foam substrate for use in high-efficiency CO oxidation. The monolithic Co₃O₄ NAs catalysts were obtained through a generic hydrothermal synthesis route with subsequent calcination. By controlling the reaction time, solvent polarity and deposition agent, these Co₃O₄ NAs catalysts exhibited various novel morphologies (single or hybrid arrays), whose physicochemical properties were further characterized by using several analytical techniques. Based on the catalytic and characterization analyses, it was found that the Co₃O₄ NAs-6 catalyst with nanobrush and nanomace arrays displayed enhanced catalytic activity for CO oxidation, achieving an efficient 100% CO oxidation conversion at a gas hourly space velocity (GHSV) 10,000hr^-1 and 150°C with long-term stability. Compared with the other Co₃O₄ NAs catalysts, it had the highest abundance of surface-adsorbed oxygen species, excellent low-temperature reducibility and was rich in surface-active sites (Co³⁺/Co²⁺=1.26).

Litchi-peel-like hierarchical hollow copper-ceria microspheres: aerosol-assisted synthesis and high activity and stability for catalytic CO oxidation

Copper-ceria is considered to be a promising system used in exhaust treatment due to its low cost and decent catalytic activity. Herein, we have developed novel litchi-peel-like hollow copper-ceria microspheres with varying Cu contents via an aerosol-assisted route. It is found that the dextrin in the spray solution plays a significant role as a sacrificial template and leads to the formation of this hierarchical hollow structure, in which higher surface area and active CuOx species with higher dispersion result in better catalytic activity compared to the usual hollow samples. The litchi-peel-like sample with 20% Cu exhibits the best reactivity for CO oxidation, namely 50% conversion at 83 °C and 100% conversion at 120 °C. Importantly, this novel copper-ceria sample displays outstanding catalytic stability involving cycle stability, long-term stability and thermal stability, which is attributed to step-stabilized strong interaction between CuOx species and CeO2. The superior catalytic activity and stability beyond commercial 5 wt% Pt/Al2O3 provides it with the potential to be a substitute for Pt-based catalysts in practical applications.

Morphology-controlled synthesis of 3D, mesoporous, rosette-like CeCoOx catalysts by pyrolysis of Ce[Co(CN)6] and application for the catalytic combustion of toluene

We synthesized one-dimensional (1D) nanoparticles, 2D hexagonal nanosheets and 3D rosettes of Ce[Co(CN)₆] by a hydrothermal process at different temperatures, and CeCoOx catalysts with similar shapes were obtained by the pyrolysis of Ce[Co(CN)₆]. T₉₀ of 1D nanoparticles, 2D hexagonal nanosheets and 3D rosette-like CeCoOx catalysts was 306, 275 and 168 °C, respectively. The 3D ordered mesoporous rosettes of CeCoOx had superior catalytic activity for toluene combustion, superior thermal stability and moisture resistance, and curves for three consecutive catalytic runs showed overlapping due to more oxygen vacancies, larger pore sizes, more surface Ce³⁺ species, more surface Co³⁺ species, and chemically adsorbed oxygen on the surface.

A versatile CeO2/Co3O4 coated mesh for food wastewater treatment: Simultaneous oil removal and UV catalysis of food additives

Food waste water is one of the most urgent environmental problems for the close connection between food and our daily life. Herein, we use a simple hydrothermal method to prepare a highly efficient catalyst-CeO₂/Co₃O₄ compound on the stainless steel mesh, aiming for food waste water treatment. Possessing the superhydrophilic property and catalytic ability under ultraviolet light, CeO₂/Co₃O₄ coated mesh has successfully processed three representative contaminants in food wastewater, which are soybean oil (food oil), AR (food dye) and VA (food flavor) simultaneously with an one-step filtration. Besides, the mesh is stable in a wide pH range and performs well in reusability. Therefore, such a multifunctional material with simple preparation method, high processing efficiency and facile operation shows a promising prospect for practical production and application for food wastewater treatment.

Vertically-aligned Co3O4 arrays on Ni foam as monolithic structured catalysts for CO oxidation: effects of morphological transformation

A generic hydrothermal synthesis route has been successfully designed and utilized to in situ grow highly ordered Co3O4 nanoarray (NA) precursors on Ni substrates, forming a series of Co3O4 nanoarray-based monolithic catalysts with subsequent calcination. The morphology evolution of Co3O4 nanostructures which depends upon the reaction time, with and without CTAB or NH4F is investigated in detail, which is used to further demonstrate the growth mechanism of Co3O4 nanoarrays with different morphologies. CO is chosen as a probe molecule to evaluate the catalytic performance over the synthesized Co-based oxide catalysts, and the effect of morphological transformation on the catalytic activity is further confirmed via using TEM, H2-TPR, XPS, Raman spectroscopy and in situ Raman spectroscopy. As a proof of concept application, core-shell Co3O4 NAs-8 presenting hierarchical nanosheets@nanoneedle arrays with a low density of nanoneedles exhibits the highest catalytic activity and long-term stability due to its low-temperature reducibility, the lattice distortion of the spinel structure and the abundance of surface-adsorbed oxygen (Oads). It is confirmed that CO oxidation on the surface of Co3O4 can proceed through the Langmuir-Hinshelwood mechanism via using in situ Raman spectroscopy. It is expected that the in situ synthesis of well-defined Co3O4 monolithic catalysts can be extended to the development of environmentally-friendly and highly active integral materials for practical industrial catalysis.

Low-temperature CO oxidation over integrated penthorum chinense-like MnCo2O4 arrays anchored on three-dimensional Ni foam with enhanced moisture resistance

Advanced integrated nanoarray (NA) catalysts have been designed by growing metal-doped Co₃O₄ arrays on nickel foam with robust adhesion.

Nitrogen doped RGO-Co3O4 nanograin cookies: highly porous and robust catalyst for removing nitrophenol from waste water

The fabrication of nanograins with a uniform morphology wrapped with reduced graphene oxide (RGO) in a designed manner is critical for obtaining a large surface, high porosity and efficient catalytic ability at mild conditions. Hybrid structures of metal oxides decorated on two-dimensional (2D) RGO lacked an interface and channels between the individual grains and RGO. The present work focuses on the synthesis of RGO-wrapped Co₃O₄ nanograin architecture in micron-sized polyhedrons and the ability to reduce aromatic nitro compounds. Doping N in the designed microstructure polyhedrons resulted in very large surface area (1085.6 m² g^-1) and pore density (0.47 m³ g^-1) microcages. Binding energies from x-ray photoelectron spectroscopy (XPS) and Raman intensities confirmed the presence of doped N and RGO-wrapped around Co₃O₄ nanograins. However, the morphology and microstructure was supported by FESEM and HRTEM images revealing the fabrication of high integrity RGO-Co₃O₄ microstructure hybrids composed of a 10 nm grain size with narrower grain size distribution. Ammonia treatment produced interconnected channels and dumbbell pores that facilitated ion exchange between the catalyst surface and the liquid medium at the grain boundary interfaces, and offered less mass transport resistance providing fast adsorption of reactants and desorption of the product causing surface renewal. Prepared N-RGO-Co₃O₄ shows the largest percentage reduction (96%) of p-nitrophenol (p-NP) at room temperature as compared to pure Co₃O₄ and RGO-Co₃O₄ nanograin microstructures over 10 min. Fabricated architectures can be applied effectively for fast and facile treatment of industrial waste streams with complex organic molecules.

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

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