Facile synthesis of Co₃O₄@CNT with high catalytic activity for CO oxidation under moisture-rich conditions

Recent progress of catalytic methane combustion over transition metal oxide catalysts

Methane (CH4) is one of the cleanest fossil fuel resources and is playing an increasingly indispensable role in our way to carbon neutrality, by providing less carbon-intensive heat and electricity worldwide. On the other hand, the atmospheric concentration of CH4 has raced past 1,900 ppb in 2021, almost triple its pre-industrial levels. As a greenhouse gas at least 86 times as potent as carbon dioxide (CO2) over 20 years, CH4 is becoming a major threat to the global goal of deviating Earth temperature from the +2°C scenario. Consequently, all CH4-powered facilities must be strictly coupled with remediation plans for unburned CH4 in the exhaust to avoid further exacerbating the environmental stress, among which catalytic CH4 combustion (CMC) is one of the most effective strategies to solve this issue. Most current CMC catalysts are noble-metal-based owing to their outstanding C–H bond activation capability, while their high cost and poor thermal stability have driven the search for alternative options, among which transition metal oxide (TMO) catalysts have attracted extensive attention due to their Earth abundance, high thermal stability, variable oxidation states, rich acidic and basic sites, etc. To date, many TMO catalysts have shown comparable catalytic performance with that of noble metals, while their fundamental reaction mechanisms are explored to a much less extent and remain to be controversial, which hinders the further optimization of the TMO catalytic systems. Therefore, in this review, we provide a systematic compilation of the recent research advances in TMO-based CMC reactions, together with their detailed reaction mechanisms. We start with introducing the scientific fundamentals of the CMC reaction itself as well as the unique and desirable features of TMOs applied in CMC, followed by a detailed introduction of four different kinetic reaction models proposed for the reactions. Next, we categorize the TMOs of interests into single and hybrid systems, summarizing their specific morphology characterization, catalytic performance, kinetic properties, with special emphasis on the reaction mechanisms and interfacial properties. Finally, we conclude the review with a summary and outlook on the TMOs for practical CMC applications. In addition, we also further prospect the enormous potentials of TMOs in producing value-added chemicals beyond combustion, such as direct partial oxidation to methanol.

Hierarchical N-Doped CuO/Cu Composites Derived from Dual-Ligand Metal-Organic Frameworks as Cost-Effective Catalysts for Low-Temperature CO Oxidation

Development of multi-ligand metal-organic frameworks (MOFs) and derived heteroatom-doped composites as efficient non-noble metal-based catalysts is highly desirable. However, rational design of these materials with controllable composition and structure remains a challenge. In this study, novel hierarchical N-doped CuO/Cu composites were synthesized by assembling dual-ligand MOFs via a solvent-induced coordination modulation/low-temperature pyrolysis method. Different from a homogeneous system, our heterogeneous nucleation strategy provided more flexible and cost-effective MOF production and offered efficient direction/shape-controlled synthesis, resulting in a faster reaction and more complete conversion. After pyrolysis, they further transformed to a unique metal/carbon matrix with regular morphology and, as a hot template, guided the orderly generation of metal oxides, eliminating sintering and agglomeration of metal oxides and initiating a synergistic effect between the N-doped metal oxide/metal and carbon matrix. The prepared N-doped CuO/Cu catalysts held unique water resistance and superior catalytic activity (100% CO conversion at 140 °C).

Precise design and synthesis of Pd/InOx@CoOx core-shell nanofibers for the highly efficient catalytic combustion of toluene

In this work, Pd/InO_x@CoO_x core-shell nanofibers, CoO_x@Pd/InO_x core-shell nanofibers and Pd/InO_x/CoO_x nanofibers with different morphologies have been successfully synthesized for the catalytic combustion of toluene. Among them, the Pd/InO_x@CoO_x core-shell sample is novel and composed of Pd/InO_x nanotube cores, CoO_x nanocubes and CoO_x nanoparticle shells derived from ZIF-67. On the contrary, the CoO_x@Pd/InO_x core-shell catalyst is assembled by CoO_x nanocube cores and Pd/InO_x nanotube shells. Finally, the Pd/InO_x/CoO_x nanofibers as references are synthesized by a method similar to the synthesis of the CoO_x@Pd/InO_x core-shell sample. Interestingly, the Pd/InO_x@CoO_x core-shell sample displayed the best activity for toluene oxidation with T₉₀ = 253 °C, good thermal stability and good cyclic stability during three runs. Through some characterizations, it was verified that the Pd/InO_x@CoO_x core-shell sample exhibited the best performance for toluene oxidation reactions due to a larger specific surface area, higher reducibility, more abundant structural defects and oxygen vacancies, higher proportion of Pd⁰ and Co³⁺ species and higher lattice oxygen species than others. Simultaneously, the Pd/InO_x@CoO_x core-shell sample exhibited good thermal stability and cyclic stability, which might be due to the layer of the CoO_x shell to protect the stability of the Pd nanoparticle core.

Self-assembly of a highly stable and active Co3O4/H-TiO2 bulk heterojunction with high-energy interfacial structures for low temperature CO catalytic oxidation

Self-assembly of a highly stable and active Co₃O₄/H-TiO₂ bulk heterojunction with high-energy interfacial structures was realized for low temperature CO catalytic oxidation.

Template-free Synthesis of Stable Cobalt Manganese Spinel Hollow Nanostructured Catalysts for Highly Water-Resistant CO Oxidation

Development of spinel oxides as low-cost and high-efficiency catalysts is highly desirable; however, rational synthesis of efficient and stable spinel systems with precisely controlled structure and components remains challenging. We demonstrate the design of complex nanostructured cobalt-based bimetallic spinel catalysts for low-temperature CO oxidation by a simple template-free method. The self-assembled multi-shelled mesoporous spinel nanostructures provide high surface area (203.5 m2/g) and favorable unique surface chemistry for producing abundant active sites and lead to the creation of robust microsphere configured by 16-nm spinel nanosheets, which achieve satisfactory water-resisting property and catalytic activity. Theoretical models show that O vacancies at exposed {110} facets in cubic spinel phase guarantee the strong adsorption of reactive oxygen species on the surface of catalysts and play a key role in the prevention of deactivation under moisture-rich conditions. The design concept with architecture and composition control can be extended to other mixed transition metal oxide compositions.

Aligned N-doped carbon nanotube bundles with interconnected hierarchical structure as an efficient bi-functional oxygen electrocatalyst

The fabrication of cost effective and efficient electrocatalysts with functional building blocks to replace noble metal ones is of great importance for energy related applications yet remains a great challenge. Herein, we report the fabrication of a hierarchical structure containing CNTs/graphene/transition-metal hybrids (h-NCNTs/Gr/TM) with excellent bifunctional oxygen electrocatalytic activity. The synthesis was rationally designed by the growth of shorter nitrogen-doped CNTs (S-NCNTs) on longer NCNTs arrays (L-NCNTs), while graphene layers were in situ generated at their interconnecting sites. The hybrid material shows excellent OER and ORR performance, and was also demonstrated to be a highly active bifunctional catalyst for Zn–air batteries, which could be due to rapid electron transport and full exposure of active sites in the hierarchical structure.

A hierarchical structure containing aligned CNTs/graphene/transition-metal was fabricated and worked as a highly active bifunctional catalyst for Zn–air batteries.

Synthesis of Mesoporous CoS2 and NixCo1-xS2 with Superior Supercapacitive Performance Using a Facile Solid-Phase Sulfurization

Synthesis of nanostructured transition metal sulfides is of particular interest in providing new methods to control their porosity and improve their surface area because those sulfides hold promising applications in high-energy density devices. Significant challenges remain currently to prepare metal sulfides having three-dimensional (3-D) continuous mesoporous structures, known to be critical for increasing their active surface sites and enhancing ion transport. We herein present a facile solid-phase sulfurization method to synthesize 3-D continuous mesoporous CoS₂, NiS₂, and their binary sulfides in a two-step nanocasting using bicontinuous KIT-6 as hard templates. The solid-phase sulfurization taking place at 400 °C yields mesoporous sulfides with highly crystalline frameworks and a stoichiometric ratio of metal-to-sulfur, 1:2 (mol), within 30 min. Elemental sulfur as an inexpensive sulfur source can be directly used for the solid-phase sulfurization of mesoporous oxides of Co₃O₄, NiO, and their binary oxides. This facile synthetic method is highly efficient to prepare mesoporous sulfides in the gram-scale production at a very low cost. Mesoporous sulfides are demonstrated to be superior electrode materials for pseudo-supercapacitors, given their high surface area and accessible bicontinuous mesopores, the suitable crystalline sizes, and the enhanced ion transport capability. The use of binary mesoporous sulfides presents interesting synergetic effect where the doping of metal ions can significantly enhance the capacitive performance of single-component sulfides. The binary sulfides of mNi_0.32Co_0.68S₂ show a specific capacitance up to 1698 F g^-1 at a current density of 2 A g^-1. The supercapacitor device of mNi_0.32Co_0.68S₂ has a high energy density of 37 Wh kg^-1 at a power density of 800 W kg^-1. We believe that the reported solid-phase synthesis offers a universal method toward the conversion of mesoporous oxides materials into various useful and functional forms for energy conversion and storage applications.

In-situ polymerization and multifunctional properties of surface-modified multiwalled carbon nanotube-reinforced polyimide nanocomposites

In this study, strong multiwalled carbon nanotube (MWNT)–polyimide (PI) matrix interfaces were designed and constructed to obtain high-performance nanocomposites via in-situ polymerization. MWNTs with reactive amino groups were produced by the covalent linking of phenylenediamine to the surface of MWNTs by amide bonds; this material exhibited excellent dispersibility and compatibility with the PI matrix. The incorporation of amine-functionalized MWNT (MWNT-NH2) significantly improved the macroscopic properties of the PI-based nanocomposites. A 50.5% increase in the tensile strength and an 83.1% increase in the Young’s modulus were achieved by 3.0 wt% MWNT-NH2 loading. Furthermore, the storage modulus, thermal stability, and glass transition temperature of the nanocomposite clearly increased by adding MWNT-NH2. The success of this method provides a good rational for developing high-performance polymer-based nanocomposites.

Solvent-Free Chemical Approach to Synthesize Various Morphological Co3O4 for CO Oxidation

Co₃O₄ nanomaterials with diverse morphologies were usually synthesized in liquid phase accompanied by the template or surfactant under harsh conditions, which further restricted their practical application. Herein, we reported an extremely simple and practical solid-state chemical method to synthesize Co₃O₄-octahedrons, -plates, and -rods. Among these, the shape control of Co₃O₄-octahedrons and Co₃O₄-plates involve variation of the amount of reactant, and the formation of Co₃O₄-rods with {110} facet can be achieved by replacing the reactant. The formation of the Co₃O₄ nanomaterials with different morphologies originated from the different microenvironments of reaction and the structure of reactants. The catalytic activity of Co₃O₄ samples for CO oxidation was evaluated in normal feed gas. The as-prepared Co₃O₄-rods exposed {110} facet exhibited superior catalytic activity for CO oxidation, which can be attributed to more oxygen defects on Co₃O₄-rods surface. Additionally, Co₃O₄-rods exhibited excellent durablility (without pretreatment) in normal feed gas, even in the presence of moisture, comparable or better than that reported in the literature. The practical and environmental friendly solvent-free strategy provided a new promising route for large-scale preparation of (metal) oxide with remarkable CO oxidation performance for practical application.

Ultrafine Co-based Nanoparticle@Mesoporous Carbon Nanospheres toward High-Performance Supercapacitors

A general synthetic methodology is reported to grow ultrafine cobalt-based nanoparticles (NPs, 2-7 nm) within high-surface-area mesoporous carbon (MC) frameworks. Our design strategy is based on colloidal amphiphile (CAM) templated oxidative self-polymerization of dopamine. The CAM templates consisting of a hydrophobic silica-like core and a hydrophilic PEO shell can coassemble with dopamine and template its self-polymerization to form polydopamine (PDA) nanospheres. Given that PDA has rich binding sites such as catechol and amine to coordinate metal ions (e.g., Co²⁺), PDA nanospheres containing Co²⁺ ions can be converted into hierarchical porous carbon frameworks containing ultrafine metallic Co NPs (Co@MC) using high-temperature pyrolysis. The CAM templates offer strong "nanoconfinements" to prevent the overgrowth of Co NPs within carbon frameworks. The yielded ultrafine Co NPs have an average size of <7 nm even at a very high loading of 65 wt %. Co@MC can be further converted into various oxides and sulfides, e.g., CoO, Co₃O₄, CoS₂ and transition-metal doped bimetallic Co_xM_1-xS₂, without significantly changing the size of NPs. As a proof-of-concept application, the porous Co-based NPs@MC hybrids were used as electrode materials for supercapacitors, which exhibit excellent supercapacitive performance with outstanding long-term cycling stability, due to the features such as ultrafine size, controllable chemical compositions, hierarchical porous structures, and full coverage of conductive carbons.

Catalytically active ceria-supported cobalt–manganese oxide nanocatalysts for oxidation of carbon monoxide

The crystallinity of the surface of the two-dimensional Co₃O₄ phase governs the catalytic performance of ceria-supported cobalt–manganese oxide nanostructures.

Ni- and Mn-Promoted Mesoporous Co3O4: A Stable Bifunctional Catalyst with Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction

Efficient bifunctional catalysts for electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable due to their wide applications in fuel cells and rechargeable metal air batteries. However, the development of nonprecious metal catalysts with comparable activities to noble metals is still challenging. Here we report a one-step wet-chemical synthesis of Ni-/Mn-promoted mesoporous cobalt oxides through an inverse micelle process. Various characterization techniques including powder X-ray diffraction (PXRD), N2 sorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) confirm the successful incorporation of Ni and Mn leading to the formation of Co-Ni(Mn)-O solid solutions with retained mesoporosity. Among these catalysts, cobalt oxide with 5% Ni doping demonstrates promising activities for both ORR and OER, with an overpotential of 399 mV for ORR (at -3 mA/cm(2)) and 381 mV (at 10 mA/cm(2)) for OER. Furthermore, it shows better durability than precious metals featuring little activity decay throughout 24 h continuous operation. Analyses of cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), Raman, and O2-temperature-programmed desorption (O2-TPD) reveal that redox activity of Co(3+) to Co(4+) is crucial for OER performance, while the population of surface oxygen vacancies and surface area determine ORR activities. The comprehensive investigation of the intrinsic active sites for ORR and OER by correlating different physicochemical properties to the electrochemical activities is believed to provide important insight toward the rational design of high-performance electrocatalysts for ORR and OER reactions.

Functionalized graphene oxide quantum dot-PVA hydrogel: a colorimetric sensor for Fe²⁺, Co²⁺ and Cu²⁺ ions

Functionalized graphene oxide quantum dots (GOQDs)-poly(vinyl alcohol) (PVA) hybrid hydrogels were prepared using a simple, facile and cost-effective strategy. GOQDs bearing different surface functional groups were introduced as the cross-linking agent into the PVA matrix thereby resulting in gelation. The four different types of hybrid hydrogels were prepared using graphene oxide, reduced graphene oxide, ester functionalized graphene oxide and amine functionalized GOQDs as cross-linking agents. It was observed that the hybrid hydrogel prepared with amine functionalized GOQDs was the most stable. The potential applicability of using this solid sensing platform has been subsequently explored in an easy, simple, effective and sensitive method for optical detection of M(2+) (Fe(2+), Co(2+) and Cu(2+)) in aqueous media involving colorimetric detection. Amine functionalized GOQDs-PVA hybrid hydrogel when put into the corresponding solution of Fe(2+), Co(2+) and Cu(2+) renders brown, orange and blue coloration respectively of the solution detecting the presence of Fe(2+), Co(2+) and Cu(2+) ions in the solution. The minimum detection limit observed was 1 × 10(-7) M using UV-visible spectroscopy. Further, the applicability of the sensing material was also tested for a mixture of co-existing ions in solution to demonstrate the practical applicability of the system. Insight into the probable mechanistic pathway involved in the detection process is also being discussed.

Ethylene diamine mediated cobalt nanoparticle studded graphene oxide quantum dots with tunable photoluminescence properties

The present study demonstrates ethylene diamine mediated in situ synthesis of cobalt oxide nanoparticles (Co₃O₄NPs) studded on graphene oxide quantum dots (GOQDs) showing reversible on/off fluorescence switching.

Highly water-resistant carbon nanotube supported PdCl2–CuCl2 catalysts for low temperature CO oxidation

PdCl₂–CuCl₂/CNT was prepared by the two-step impregnation method, and the effects of the Pd and Cu loadings on its physicochemical properties and catalytic performance for low-temperature CO oxidation in the presence of H₂O were investigated.

Ultra-tiny Co(OH)2 particles supported on graphene oxide for highly efficient electrocatalytic water oxidation

The TOF value of the ultra-tiny Co(OH)₂/GO nanocomposite electrocatalyst is 2.7 times higher than that of hydrothermally treated Co₃O₄/GO.

Biomorphic synthesis of mesoporous Co₃O₄ microtubules and their pseudocapacitive performance

A novel meosoporous tubular Co3O4 has been fabricated by a simple and cost-effective biomorphic synthesis route, which consists of infiltration of cotton fiber with cobalt nitrate solution and postcalcination at 673 K for 1 h. Its electrochemical performance as a supercapacitor electrode material is investigated by means of cyclic voltammetry and chronopotentiometry tests. Compared with bulk Co3O4 prepared without using cotton template, biomorphic Co3O4 displays 2.8 fold enhancement of pseudocapacitive performance because of the unique tubular morphology, relative high specific surface area (3 and 0.8 m(2)/g for biomorphic Co3O4 and bulk Co3O4, respectively), and mesoporous nature.