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

Manipulating the Spin State of Spinel Octahedral Sites via a π-π Type Orbital Coupling to Boost Water Oxidation

Spin state is often regarded as the crucial valve to release the reactivity of energy-related catalysts, yet it is also challenging to precisely manipulate, especially for the active center ions occupied at the specific geometric sites. Herein, a π-π type orbital coupling of 3d (Co)-2p (O)-4f (Ce) was employed to regulate the spin state of octahedral cobalt sites (CoOh) in the composite of Co3O4/CeO2. More specifically, the equivalent high-spin ratio of CoOh can reach to 54.7 % via tuning the CeO2 content, thereby triggering the average eg filling (1.094) close to the theoretical optimum value. The corresponding catalyst exhibits a superior water oxidation performance with an overpotential of 251 mV at 10 mA cm-2, rivaling most cobalt-based oxides state-of-the-art. The π-π type coupling corroborated by the matched energy levels between Ce t1u/t2u-O and CoOh t2g-O π type bond in the calculated crystal orbital Hamilton population and partial density of states profiles, stimulates a π-donation between O 2p and π-symmetric Ce 4fyz 2 orbital, consequently facilitating the electrons hopping from t2g to eg orbital of CoOh. This work offers an in-depth insight into understanding the 4f and 3d orbital coupling for spin state optimization in composite oxides.

A Bifunctional Catalyst for Green Ammonia Synthesis from Ubiquitous Air and Water

Ammonia (NH3 ) is essential for modern agriculture and industry, and, due to its high hydrogen density and no carbon emission, it is also expected to be the next-generation of "clean" energy carrier. Herein, directly from air and water, a plasma-electrocatalytic reaction system for NH3 production, which combines two steps of plasma-air-to-NOx - and electrochemical NOx - reduction reaction (eNOx RR) with a bifunctional catalyst, is successfully established. Especially, the bifunctional catalyst of CuCo2 O4 /Ni can simultaneously promote plasma-air-to-NOx - and eNOx RR processes. The easy adsorption and activation of O2 by CuCo2 O4 /Ni greatly improve the NOx - production rate at the first step. Further, CuCo2 O4 /Ni can also resolve the overbonding of the key intermediate of * NO, and thus reduce the energy barrier of the second step of eNOx RR. Finally, the "green" NH3 production achieves excellent FENH3 (96.8%) and record-high NH3 yield rate of 145.8 mg h-1  cm-2 with large partial current density (1384.7 mA cm-2 ). Moreover, an enlarged self-made H-type electrolyzer improves the NH3 yield to 3.6 g h-1 , and the obtained NH3 is then rapidly converted to a solid of magnesium ammonium phosphate hexahydrate, which favors the easy storage and transportation of NH3 .

PO6 geometric configuration unit enhanced electrocatalytic performance of Co3O4 in acidic oxygen evolution

It is challenging to develop high-efficient and stable nonprecious metal-based electrocatalyst for oxygen evolution reaction (OER) in acid for proton exchange membrane (PEM) water splitting. Herein, P atoms were introduced into the lattice of spinel Co₃O₄ (P-Co₃O₄) to replace with octahedral coordinated Co³⁺ via a hydrothermal process following a thermal treatment. The formation of PO₆ geometric configuration unit in Co₃O₄ can trigger electron rearrangement around Co ions, which resulted in the high-active Co²⁺ site on the surface, significantly decreasing the energy barrier of rate-determining step for OER. Moreover, the weaker covalency of the Co 3d-O 2p bond and higher formation energy of oxygen vacancy around Co²⁺ in P-Co₃O₄ inhibited the participation of lattice oxygen during OER process, enabling that P-Co₃O₄ can work stably in acidic media. The obtained P-Co₃O₄ afforded satisfying stability over 30 h in a PEM electrolysis device with an overpotential of 400 mV@10 mA/cm² in 0.1 M HClO₄.

Oxygen Vacancy Promoted O2 Activation over Mesoporous Ni-Co Mixed Oxides for Aromatic Hydrocarbon Oxidation

Whether the oxygen vacancies of heterogeneous catalysts improve their catalytic activity or not has recently been the topic of intense debate in the oxidation of hydrocarbons. We designed an effective strategy to construct mesoporous Ni-Co mixed oxides via a ligand-assisted self-assembly approach. The surface oxygen vacancy concentrations of the mesoporous Ni-Co mixed oxide catalysts were regulated by changing the doping amount of Ni or the reduction method, and the relationship between oxygen vacancies and catalytic activity was studied. Controlled experiments and DFT calculations revealed that oxygen molecules were more favorably adsorbed and activated on oxygen vacancies to form active oxygen species. Increasing the oxygen vacancy concentration within a certain range can effectively enrich the active oxygen species, therefore improving the oxidation rate of ethylbenzene. The optimized mCo₃O₄-0.1NiO catalyst exhibited a remarkable catalytic activity for the solvent-free oxidation of ethylbenzene to acetophenone, typically including 68.0% conversion and 95.4% selectivity (20 mg mCo₃O₄-0.1NiO, 10 mL ethylbenzene, and 0.6 MPa O₂).

A novel electrocatalytic system with high reactive chlorine species utilization capacity to degrade tetracycline in marine aquaculture wastewater

The problems of high salinity and coexistence of antibiotics in mariculture wastewater pose a great challenge to the traditional wastewater treatment technology. Herein, an electrocatalytic system based on cathodes to sustain reactive chlorine species (RCS) in a high chlorine environment was proposed. The results show that the content of RCS is affected by cathodes. The electrocatalytic system with FeNi/NF as cathode has the largest RCS retention capacity when compared with other cathode systems (carbon felt, nickel foam, copper foam, stainless steel, and nickel-iron foam). This is related to FeNi/NF's higher hydrogen production activity, which inhibits the reduction reaction of RCS. Furthermore, the degradation of tetracycline by the proposed FeNi/NF system maintained long-term effective performance across 20 cycles. Thus, the application of high chlorine resistance electrocatalysis system provides a possibility for practical electrocatalysis treatment of mariculture wastewater.

Boosting alkaline water splitting and the urea electrolysis kinetic process of a Co3O4 nanosheet by electronic structure modulation of F, P co-doping

Designing non-precious metal electrocatalysts for accelerated electron transfer and richer active site exposure is necessary and challenging to achieve the versatility of electrocatalysts. In this research, a self-grown nanosheet array electrocatalyst on nickel foam with high structural stability is first rationally designed through suitable anionic doping. The combined experimental and theoretical calculations reveal that the F-P-Co₃O₄/NF material optimizes the adsorption energy of hydrogen/water through electron coupling, and its nanosheet structure provides abundant active sites, accelerating the mass and electron transfer in the reaction process. It is worth noting that the as-developed F-P-Co₃O₄/NF materials exhibit outstanding catalytic activity for overpotentials of 192 and 110 mV at a current density of 10 mA cm^-2 for the oxygen evolution reaction and the hydrogen evolution reaction in 1 M KOH, respectively. More notably, an assembled F-P-Co₃O₄/NF//F-P-Co₃O₄/NF alkaline electrolytic cell requires only an ultra-low cell voltage of 1.53 V to achieve a current density of 10 mA cm^-2, which is one of the best activities reported so far. Furthermore, F-P-Co₃O₄/NF also shows excellent performance for urea electrolysis. Theoretical calculations show that the superior activity of the F-P-Co₃O₄/NF catalyst is attributed to the optimal electron configuration and the lower Gibbs free energy of hydrogen adsorption due to co-doping of P and F. The work provides an alternative solution for the preparation of electrocatalysts with high structural stability, high catalytic activity and multifunction for alkaline water splitting and urea electrolysis.

Bimetallic Pt-Co Nanoparticle Deposited on Alumina for Simultaneous CO and Toluene Oxidation in the Presence of Moisture

Carbon monoxide (CO) and hydrocarbons (HCs) generally have competitive adsorption on the active site of noble-metal nano-catalysts, thus developing an effective way to reduce the passivation of competitive reaction with each other is an urgent problem. In this study, we successfully synthesized transition metal-noble metal (Pt-M) alloys via introducing inexpensive metal elements (M = Ni, Co and Cu) into Pt particles and then deposited on alumina support to form Pt-based catalysts. Subsequently, we choose CO and toluene as polluting gases to evaluate the catalytic activities of Pt-M/Al2O3 catalysts. Introducing inexpensive metal elements (M = Ni, Co, and Cu) significantly changed the physicochemical properties and catalytic activities of these Pt-based catalysts. It can be found that the Pt-Co/Al2O3 catalyst exhibited outstanding catalytic activity for CO and toluene oxidation under mixed gas atmosphere, compared with other Pt-based catalysts, which is due to the higher dispersity, more surface adsorption oxygen, and well redox ability. Surprisingly, H2O could promote the catalytic activities for CO/toluene co-oxidation over the Pt-Co/Al2O3 catalyst. Thus, the present synthetic strategy not only opens an avenue towards the synthesis of noble metal-based catalysts, but also provides an excellent tolerance to H2O in the catalytic process.

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.

Macroscopic Hexagonal Co3O4 Tubes Derived from Controllable Two-Dimensional Metal-Organic Layer Single Crystals: Formation Mechanism and Catalytic Activity

Macroscopic Co₃O₄ hexagonal tubes were successfully synthesized using hollow two-dimensional (2D) MOL (metal-organic layer) single crystals as sacrificial templates. The hollow 2D MOL single crystals were prepared under hydrothermal conditions with acetonitrile (MeCN) as an interference agent. The formation of hollow-structured 2D MOL single crystals was tracked by time-dependent experiments, and two simultaneous paths-namely, the crystal-to-crystal transformation in solution and the dissolution + migration (toward the external surface) of inner crystallites-were identified as playing a key role in the formation of the unique hollow structure. The calculated change in Gibbs free energy (ΔG = -1.18 eV) indicated that the crystal-to-crystal transformation was spontaneous at 393 K. Further addition of MeCN as an interference agent eventually leads to the formation of macroscopic hexagonal tubes. Among all of the as-synthesized Co₃O₄, Co-MeCN-O with a hexagonal tube morphology exhibited the best catalytic performance in toluene oxidation, it achieved a toluene conversion of 90% (T₉₀) at ∼227 °C (a space velocity of 60 000 mL g^-1 h^-1) and the activity energy (E_a) is 69.5 kJ mol^-1. A series of characterizations were performed to investigate the structure-activity correlation. It was found that there are more structure defects, more adsorbed surface oxygen species, more surface Co³⁺ species, and higher reducibility at low temperatures on the Co-MeCN-O than on other Co₃O₄ samples; these factors are responsible for its excellent catalytic performance. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) characterization showed that, when there is no oxygen in the atmosphere, the lattice oxygen may be involved in the activation of toluene, and the gas-phase oxygen replenished by the oxygen vacancies was essential for the total oxidation of toluene on the surface of the Co-MeCN-O catalysts, it also proves the importance of oxygen vacancies. Moreover, for the Co-MeCN-O catalysts, no obvious decrease in catalytic performance was observed after 120 h at 220 °C and it is still stable after cycling tests, which indicates that it exhibits excellent stability for toluene oxidation. This study sheds lights on the controllable synthesis of macroporous-microporous materials in single-crystalline form without an external template, and, thus, it may serve as a reference for future design and synthesis of hollow porous materials with outstanding catalytic performance.

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.

Extremely sensitive and accurate H2S sensor at room temperature fabricated with In-doped Co3O4 porous nanosheets

In-Doped Co₃O₄ porous nanosheets were synthesized and exhibited a fast response and high selectivity towards H₂S at room temperature.

In situ DRIFT spectroscopy insights into the reaction mechanism of CO and toluene co-oxidation over Pt-based catalysts

The Pt–CeO₂ catalyst with adsorption sites and oxygen-rich vacancies exhibited outstanding activity towards CO and toluene co-oxidation.

Relationships Between Crystal, Internal Microstructures, and Physicochemical Properties of Copper-Zinc-Iron Multinary Spinel Hierarchical Nano-microspheres

Rational design and fabrication of high quality complex multicomponent spinel ferrite with specific microstructures and solar light harvestings toward CO₂ reduction and antibiotic degradation to future energetic and catalytic applications are highly desirable. In this study, novel copper-zinc-iron multinary spinel hierarchical nano-microspheres (MSHMs) with different internal structures (solid nano-microspheres, yolk-shell hollow nano-microspheres, and double-shelled hollow nano-microspheres) have been successfully developed by a facile self-templated solvothermal strategy. The morphology and structure, optical, as well as photoinduced redox reactions including interfacial charge carrier behaviors and the intrinsic relationship of structure-property between intrinsic nano-microstructures and physicochemical performance of copper-zinc-iron ferrite MSHMs composites were systematically investigated with the assistance of various on- and/or off- line physical-chemical means and deeply elucidated in terms of the research outcomes. It is demonstrated that the modification of the interior microstructures can be applied to tune the catalytic properties of multinary spinel by tailoring the temperature programming to fine control the two opposite forces of contraction (Fc) and adhesion (Fa). Among various internal microstructures, the obtained double-shelled copper-zinc-iron MSHMs exhibited the superior catalytic performance toward 8.8 and 38 μmol for H₂ and CO productions as well as 80.4% removal of sulfamethoxazole antibiotics. As evidenced from primary characterizations, for example, combined steady-state PL, ns-TAS, and Mössbauer and sequential investigations, the remarkable improvements in the catalytic activity can be primarily attributed to several crucial factors, for example, the more effective e^--h⁺ spatial separations and interfacial transfers, multiple internal light scattering, higher photonic energy harvesting and effective reactive oxygen species generation with long radical lifetimes. The current research provides new insights into the molecular design of novel copper-zinc-iron multinary spinels and the intrinsic relationship of structure-property between interior structures (e.g., different crystal texture, morphologies structures) and the physicochemical performance of the aforementioned multinary spinels.