Oxygen vacancy-rich nitrogen-doped Co3O4 nanosheets as an efficient water-resistant catalyst for low temperature CO oxidation

Constructing surface oxygen defects at CuO-Co3O4 interface to boost toluene oxidation over CuO/Co3O4 catalysts

As a typical heterogeneous catalytic process, the catalytic combustion of toluene over Co3O4-based catalysts is strongly depends on the surface properties of catalysts, especially the concentration of surface oxygen defects. Here, a novel way was proposed to construct chemically bonded CuO-Co3O4 interface by chemical deposition of CuO onto Co3O4 nanoflowers. The interfacial refinement effect between CuO and Co3O4 support disrupted the ordered atomic arrangement and created countless unsaturated coordination sites at CuO-Co3O4 interface, inducing a significant generation of surface oxygen defects. Surface-rich oxygen vacancies enhanced the capacity of 20%CuO/Co3O4-R to adsorb and activate oxygen species. Benefiting from this, 90 % toluene conversion was reached at 228 °C over 20%CuO/Co3O4-R, which was much lower than that over 20%CuO/Co3O4-S prepared by impregnation method and CuO/Co3O4-mix obtained by mechanically mixing way. In-situ DRIFTS analysis revealed that toluene could be directly decomposed into benzaldehyde at the highly defective CuO-Co3O4 interface, leading to toluene oxidation following the path of toluene → benzaldehyde → benzoate → maleic anhydride → water and carbon dioxide over 20%CuO/Co3O4-R, which was significantly different from decomposition mechanism over 20%CuO/Co3O4-S. Additionally, 20%CuO/Co3O4-R displayed terrific recyclability and outstanding stability, showing good application potential.

Plasma-Engineered CeOx Nanosheet Array with Nitrogen-Doping and Porous Architecture for Efficient Electrocatalysis

Surface engineering has been proved efficient and universally applicable in improving the performance of CeO2 in various fields. However, previous approaches have typically required high-temperature calcination or tedious procedures, which makes discovery of a moderate and facile modification approach for CeO2 an attractive subject. In this paper, porous CeO2 nanosheets with effective nitrogen-doping were synthesized via a low-temperature NH3/Ar plasma treatment and exhibited boosted hydrogen evolution reaction performance with low overpotential (65 mV) and long-term stability. The mechanism of the elevated performance was investigated by introducing Ar-plasma-treated CeO2 with no nitrogen-doping as the control group, which revealed the dominant role of nitrogen-doping by providing abundant active sites and improving charge transfer characteristics. This work illuminates further investigations into the surface engineering methodologies boosted by plasma and the relative mechanism of the structure-activity relationship.

Highly Active and Water-Resistant Cu-Doped OMS-2 Catalysts for CO Oxidation: The Importance of the OMS-2 Synthesis Method and Cu Doping

Porous cryptomelane-type Mn oxide (OMS-2) has an outstanding redox property, making it a highly desirable substitute for noble metal catalysts for CO oxidation, but its catalytic activity still needs to be improved, especially in the presence of water. Given the strong structure-performance correlation of OMS-2 for oxidation reactions, herein, OMS-2 is synthesized by solid state (OMS-2S), reflux (OMS-2R), and hydrothermal (OMS-2H) methods, aiming to improve its CO oxidation performance through manipulating synthesis parameters to tailor its particle size, morphology, and crystallinity. Characterization shows that OMS-2S has the highest CO oxidation activity in the absence of water due to its low crystallinity, high specific surface area, large oxygen vacancy content, and good redox property, but the presence of water can greatly reduce its CO oxidation activity. Doping Cu into an OMS-2 can not only improve its CO oxidation activity but also greatly improve its water tolerance. The Cu-doped OMS-2S catalyst with ∼4 wt % Cu can achieve a T90 of 49 °C (1% CO/10% O2/N2 and WHSV = 60,000 mL·g-1·h-1), ranking among the lowest reported T90 values for Mn oxide-based CO oxidation catalysts, and it can maintain nearly 100% CO conversion in the presence of 5 vol % water for over 50 h. In situ DRIFTs characterization indicates that the good water resistance of Cu-doped OMS-2S can be attributed to the significantly suppressed surface hydroxyl group generation because of Cu doping. This work demonstrates the importance of the synthesis method and Cu doping in determining the CO oxidation activity and water resistance of OMS-2 and will provide guidance for synthesizing highly active and water-resistant CO oxidation catalysts.

Surface adsorbed and lattice oxygen activated by the CeO2/Co3O4 interface for enhancive catalytic soot combustion: Experimental and theoretical investigations

Metal oxide-oxide interface on supported catalyst has been rarely studied due to the complex interfacial structure and synthetic challenge. Herein, different Ag-supported CeO₂/Co₃O₄ samples with various covered-state of CeO₂ were prepared for catalytic soot oxidation. In comparison, catalytic activity was significantly improved by grafting CeO₂ on Co₃O₄, in which the best performing Ag/CoCe-2 exhibited remarkable catalytic performance towards soot oxidation with a T₅₀ of 290.5 ℃ under 10 % O₂/N₂. Catalyst characterization investigated by Scanning Electron Microscope (SEM), quasi in-situ X-ray Photoelectron Spectroscopy (XPS), in-situ Raman, etc. revealed that this outstanding promotion in catalytic activity can be principally ascribed to the formation of the CeO₂/Co₃O₄ interface. An appropriate CeO₂ dosage maximized the contact and interaction between Co₃O₄ and CeO₂, resulting in the largest CeO₂/Co₃O₄ interface featured with abundant generated superoxide species and activated surface lattice oxygen. Density functional theory (DFT) calculations were also carried out for the oxygen vacancy formation energy, Gibbs free energy, etc. In presence of the CeO₂/Co₃O₄ interface, a charge density redistribution around the adsorbed reactants at oxygen vacancies could be formed, owing to the efficient charge transfer enhanced by the electron-appealing effect. The change in electronic structure favored reducing the oxygen vacancy formation energy and boosting the lattice oxygen activation induced by the hybridized Co-O-Ce bonds, finally lowering the adsorption and activation barriers for reactive species and accelerating the reaction kinetics.

Enhanced water-resistance of Mn-based catalysts for ambient temperature ozone elimination: Roles of N and Pd modification

Cryptomelane-type MnO₂ catalysts own excellent ozone (O₃) decomposition performance. However, it is urgent to improve their long-term stability at ambient temperature, especially under the presence of water. In the present study, a modification strategy was proposed by N-doping and the successive Pd introduction. The N-doping of MnO₂ by NH₄Cl (NH₄-MnO₂) can increase its activity for O₃ decomposition. And almost 100% O₃ decomposition was achieved within 24 h under water-free atmosphere at ambient temperature (25 °C). Successive Pd addition further promoted the water-resistance of NH₄-MnO₂ catalyst under high humidity (RH > 90%). In combination with detailed characterizations, it indicated that the enhancements on stability and water-resistance were attributed to synergistic effect among acid sites, oxygen defects and Pd clusters. Finally, the decomposition mechanism of gaseous O₃ was proposed based on three decisive active sites above.

Plasma-Engineered N-CoOx Nanowire Array as a Bifunctional Electrode for Supercapacitor and Electrocatalysis

Surface engineering has achieved great success in enhancing the electrochemical activity of Co₃O₄. However, the previously reported methods always involve high-temperature calcination processes which are prone to induce agglomeration of the nanostructure, leading to the attenuation of performance. In this work, Co₃O₄ nanowires were successfully modified by a low-temperature NH₃/Ar plasma treatment, which simultaneously generated a porous structure and efficient nitrogen doping with no agglomeration. The modified N-CoO_x electrode exhibited remarkable performance due to the synergistic effect of the porous structure and nitrogen doping, which provided additional active sites for faradic transitions and improved charge transfer characteristics. The electrode achieved excellent supercapacitive performance with a maximum specific capacitance of 2862 mF/cm² and superior cycling retention. Furthermore, the assembled asymmetric supercapacitor (N-CoO_x//AC) device exhibited an extended potential window of 1.5 V, a maximum specific energy of 80.5 Wh/kg, and a maximum specific power of 25.4 kW/kg with 91% capacity retention after 5000 charge-discharge cycles. Moreover, boosted hydrogen evolution reaction performance was also confirmed by the low overpotential (126 mV) and long-term stability. This work enlightens prospective research on the plasma-enhanced surface engineering strategies.

Effect of Sn on the CO Catalytic Activity and Water Resistance of Cu-Mn Catalyst

In view of the problem that excessive CO in underground coal mine space can easily lead to a large number of casualties, Cu-Mn-Sn water-resistant eliminators with different Sn contents were prepared by a co-precipitation method. The activity of the eliminators was analyzed by using an independently developed activity testing platform, N₂ adsorption and desorption, XRD, SEM, XPS, and FTIR to characterize the activity factors and water resistance. The results showed that Cu-Mn-Sn-20 with 20% Sn content had the highest activity, which was 3.23 times that of Cu-Mn. The main reason for the increased activity is that Cu-Mn-Sn-20 doped with 20% Sn provides a larger specific surface area and more active sites and reduces the pore size, so that the crystallization degree of Cu_1.4Mn_1.5O₄ is lower. The doping of 20% Sn reduces the absorption of lattice water and coordination water and improves the water resistance of Cu-Mn-Sn-type eliminators. The Cu-Mn-Sn-20 water-resistant eliminator is used to quickly eliminate CO in underground coal mines, which is of great significance for the rescue workers in underground coal mines after disasters.

Fabrication of MnCoOx composite oxides for catalytic CO oxidation via a solid-phase synthesis: The significant effect of manganese precursor

A series of Mn3Co16Ox composite oxides catalysts were fabricated via a solid-phase synthesis using different manganese precursors (namely as manganese acetate (A), nitrate (N), and sulfate (S)). It has been...

Insight into the Improvement Effect of Nitrogen Dopant in Ag/Co3O4 Nanocubes for Soot Oxidation: Experimental and Theoretical Studies

Different doping amounts of N-doped Ag/Co₃O₄ nanocubes were synthesized for the first time for catalytic soot oxidation. The N-doped sample exhibited remarkably improved catalytic activity, of which the maximum decrease in temperature for 90% soot conversion was almost 40 ℃. Characterization results analyzed by TEM, XPS, EPR, H₂-TPR, O₂-TPD, etc. revealed that the incorporation of N atoms can alter the electronic structure, leading to the generation of more oxygen vacancies and enhancement of lattice oxygen mobility. Meanwhile, larger surface area, rugged morphology and promoted reducibility also contribute to the performance improvement. DFT calculations on the differential charge density, Gibbs free energy, etc. were performed to investigate the intrinsic reasons on an atomic level. Due to the relatively higher electronegativity, N dopant could be an electron-appealing center to promote efficient electron transfer, resulting in the redistribution of charge density and formation of conductive Co-N bonds. This variation in electronic structure favors lowering the formation energy of oxygen vacancies and facilitating the activation of the lattice oxygen originated from the highly hybridized Co-O bonds, which ultimately reduces the activation barriers for reactants/intermediates and accelerates the reaction kinetics. This study evidenced that N doping could be an effective strategy to promote catalytic soot oxidation.

Surface Lattice Oxygen Activation by Nitrogen-Doped Manganese Dioxide as an Effective and Longevous Catalyst for Indoor HCHO Decomposition

Oxygen vacancy plays an important role in catalytic oxidation of formaldehyde (HCHO), but the inherent drawback of its thermodynamic instability causes the deactivation of catalysts. Hence, improving the thermodynamic stability of oxygen vacancy is a crux during HCHO oxidation. Here, a novel and simple nitrogen doping of MnO₂/C catalyst is designed for HCHO oxidation at room temperature. The surface lattice oxygen of MnO₂ will be activated by nitrogen-doping, which acts as active sites for HCHO oxidation and solves the thermodynamic instability issue of oxygen vacancy. Furthermore, carbon is doped with nitrogen to promote electron transfer and accelerate the HCHO oxidation process. Therefore, the catalytic activity and stability of the catalyst can be significantly promoted, which can completely remove ∼1 ppm HCHO in the tank within 3 h, and remains highly active after 5 cycles at room temperature (RH = 55%). In addition, the excellent removal performance over the prepared catalyst is also attributed to abundant surface oxygen species, amorphous crystallinity, and low reduction temperature. In situ diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) and density functional theory (DFT) calculations reveal the reaction mechanism of HCHO. This strategy provides crucial enlightenment for designing novel Mn-based catalysts for application in the HCHO oxidation field.

Insight into the Crystal Structures and Surface Property of Manganese Oxide on CO Catalytic Oxidation Performance

α-MnO₂ nanorods and flower-like γ-MnO₂ microspheres were synthesized by facile and mild methods to illustrate the effect of crystal structures and surface features on catalytic performance with the help of carbon monoxide (CO) oxidation. It is revealed that the flower-like γ-MnO₂ microspheres possess better catalytic oxidation performance (CO complete conversion temperature at 120 °C and long-time stability for 50 h) than α-MnO₂ nanorods, which can be attributed to the obvious differences in the chemical bonds and linking modes of [MnO₆] octahedra due to the different crystal structures. γ-MnO₂ possesses lower Mn-O bond strength that enables γ-MnO₂ to present a large amount of surface lattice oxygen and superior oxygen mobility. The disordered random intergrowth tunnel structure can adsorb effectively CO molecules, resulting in excellent catalytic performance for CO catalytic oxidation. In addition, the MnO₂ catalyst probably occurred via a Mars-van Krevelen mechanism for CO oxidation. This work provides an insight into the effect of crystal structures and surface property of manganese oxide on catalytic oxidation performance, which presents help for the future design of promising catalysts with excellent catalytic performance.

Interfacial sites in platinum-hydroxide-cobalt hybrid nanostructures for promoting CO oxidation activity

Metal-oxide/hydroxide hybrid nanostructures provide an excellent platform to study the interfacial effects on tailoring the catalysis of metal catalysts. Herein, a hybrid nanostructure of Pt@Co(OH)2 supported on SiO2 was synthesized by incipient wetness impregnation of Co(OH)2 with the aid of H2O2 and successive urea-assisted deposition-precipitation of platinum nanoparticles. The Fenton-like reaction between Co2+ and H2O2 during the impregnation process facilitates the formation of active interfacial sites. This hybrid nanostructure exhibits much higher catalytic activity towards CO oxidation than Pt/SiO2 nanoparticles with a similar Pt loading and particle size. In situ diffuse reflectance infrared Fourier transform spectroscopy was used to track the CO adsorption processes and to identify the reaction intermediates during CO oxidation. It shows that the OH species at the Pt-OH-Co interfacial sites could readily react with CO adsorbed on neighboring Pt to yield CO2 by forming *COOH intermediates and oxygen vacancies. Under the CO + O2 oxidation conditions, O2 molecules are activated by the oxygen vacancy and react with the CO molecules adsorbed on Pt to generate CO2, via forming the highly active *OOH intermediates as observed by DRIFTS.

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.

The solid-state in situ construction of Cu2O/CuO heterostructures with adjustable phase compositions to promote CO oxidation activity

Cu₂O/CuO heterojunctions were fabricated via in situ solid-state technology. Tuning the ratio of reactants enables optimization of the components of the Cu₂O/CuO heterostructures and their catalytic activities for CO oxidation.