Enhanced catalytic performance for toluene purification over Co3O4/MnO2 catalyst through the construction of different Co3O4-MnO2 interface

Regulating the mobility of lattice oxygen on hollow cobalt-manganese sub-nanospheres for enhanced catalytic oxidation of toluene and o-xylene

Promoting lattice oxygen mobility of Co-based catalysts is crucial to making progress in catalytic oxidation technology. The addition of manganese, a transition metal with similar ionic radius to cobalt and variable valence, was supposed to enhance the mobility of lattice oxygen species of Co-based oxide. A range of hollow CoMnaOx sub-nanosphere catalysts with different Mn/Co ratios was synthesized via a template-sacrificed method, and the effects of different Mn/Co ratios on the structural properties of the catalysts and their catalytic performance for benzene series volatile organic compounds (VOCs) oxidation were investigated. Hollow CoMn2Ox sub-nanosphere exhibited good catalytic activity for oxidation of toluene (T90 = 265 °C) and o-xylene (T90 = 297 °C), as well as excellent recycling ability and water resistance. By adjusting the Mn/Co ratio, metal ions enter into the different tetrahedral or octahedral active sites. Compared with Co3O4, the desorption temperature of surface lattice oxygen on CoMn2Ox decreased by 110 °C. These results demonstrate that the addition of manganese can encourage the electron transfer on CoMnaOx, indicating that the introduction of the appropriate amount of manganese accelerates the activation of gas O2 and mobility of surface lattice oxygen species, thereby expediting the oxidation of benzene series VOCs.

Catalytic oxidation of toluene by manganese oxides: Effect of K+ doping on oxygen vacancy

Alkali metal potassium was beneficial to the electronic regulation and structural stability of transition metal oxides. Herein, K ions were introduced into manganese oxides by different methods to improve the degradation efficiency of toluene. The results of activity experiments indicated that KMnO4-HT (HT: Hydrothermal method) exhibited outstanding low-temperature catalytic activity, and 90% conversion of toluene can be achieved at 243°C, which was 41°C and 43°C lower than that of KNO3-HT and Mn-HT, respectively. The largest specific surface area was observed on KMnO4-HT, facilitating the adsorption of toluene. The formation of cryptomelane structure over KMnO4-HT could contribute to higher content of Mn3+ and lattice oxygen (Olatt), excellent low-temperature reducibility, and high oxygen mobility, which could increase the catalytic performance. Furthermore, two distinct degradation pathways were inferred. Pathway Ⅰ (KMnO4-HT): toluene → benzyl → benzoic acid → carbonate → CO2 and H2O; Pathway ⅠⅠ (Mn-HT): toluene → benzyl alcohol → benzoic acid → phenol → maleic anhydride → CO2 and H2O. Fewer intermediates were detected on KMnO4-HT, indicating its stronger oxidation capacity of toluene, which was originated from the doping of K+ and the interaction between KOMn. More intermediates were observed on Mn-HT, which can be attributed to the weaker oxidation ability of pure Mn. The results indicated that the doping of K+ can improve the catalytic oxidation capacity of toluene, resulting in promoted degradation of intermediates during the oxidation of toluene.

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.

Insight into modified CeMn based catalysts for efficient degradation of toluene by in situ infrared

Trace activated carbon (AC) and diatomaceous earth (DE) were used as structural promoters to be incorporated into Ce-Mn-based solid-solution catalysts by the redox precipitation method. The modified catalysts exhibit superior reducibility, with abundant Ce3+, Mn3+and reactive oxygen species, which are facilitated to the migration of oxygen and the generation of oxygen vacancies. In particular, the catalytic combustion temperatures of 90 % toluene (3000 ppm) on Ce1Mn3Ox-AC/DE were 84 °C (dry) and 123 °C (10 vol% H2O), respectively. The role of lattice oxygen and adsorbed oxygen was revealed by in situ DRIFTS. Additionally, in situ DRIFTS was employed to verify that the degradation of toluene by Ce1Mn3Ox-AC/DE satisfied the Langmuir-Hinshelwood (L-H) mechanism and the Mars-Van Krevelen (MvK) mechanism. The possible reaction pathway was elucidated (toluene → benzyl alcohol → benzoic acid → maleic anhydride → CO2 + H2O). Furthermore, final products attributed to toluene oxidation were detected by in situ DRIFTS at 50 °C in the absence of oxygen, confirming that the catalyst possessed outstanding performance at low temperatures beyond mere adsorption.

Promotional effect of cobalt doping on catalytic performance of cryptomelane-type manganese oxide in toluene oxidation

The cryptomelane-type manganese oxide (OMS-2)-supported Co (xCo/OMS-2; x = 5, 10, and 15 wt.%) catalysts were prepared via a pre-incorporation route. The as-prepared materials were used as catalysts for catalytic oxidation of toluene (2000 ppmV). Physical and chemical properties of the catalysts were measured using the X-ray diffraction (XRD), Fourier transform infrared spectroscopic (FT-IR), scanning electron microscopic (SEM), X-ray photoelectron spectroscopy (XPS), and hydrogen temperature-programmed reduction (H₂-TPR) techniques. Among all of the catalysts, 10Co/OMS-2 performed the best, with the T_90%, specific reaction rate at 245°C, and turnover frequency at 245°C (TOF_Co) being 245°C, 1.23 × 10^-3 mol_toluene/(g_cat·sec), and 11.58 × 10^-3 sec^-1 for toluene oxidation at a space velocity of 60,000 mL/(g·hr), respectively. The excellent catalytic performance of 10Co/OMS-2 were due to more oxygen vacancies, enhanced redox ability and oxygen mobility, and strong synergistic effect between Co species and OMS-2 support. Moreover, in the presence of poisoning gases CO₂, SO₂ or NH₃, the activity of 10Co/OMS-2 decreased for the carbonate, sulfate and ammonia species covered the active sites and oxygen vacancies, respectively. After the activation treatment, the catalytic activity was partly recovered. The good low-temperature reducibility of 10Co/OMS-2 could also facilitate the redox process accompanied by the consecutive electron transfer between the adsorbed O₂ and the cobalt or manganese ions. In the oxidation process of toluene, the benzoic and aldehydic intermediates were first generated, which were further oxidized to the benzoate intermediate that were eventually converted into H₂O and CO₂.

Photothermal Catalytic Reduction of CO2 by Cobalt Silicate Heterojunction Constructed from Clay Minerals

The coupled utilization of solar and thermal energy is considered an efficient way to improve the efficiency of CO2 reduction. Herein, palygorskite (Pal) clay is as a silicon source, while Co2+ is introduced to prepare two-dimensional Co2SiO4 nanosheets, and the excess of Co2+ leads to the growth of Co3O4 on the surface of Co2SiO4 to obtain an S-scheme Co2SiO4/Co3O4−x heterojunction, which facilitates the charge transfer and maintains higher redox potentials. Benefiting from black color and a narrow band gap, the cobalt oxide on the surface can increase the light absorption and produce a local photothermal effect. Under proper thermal activation conditions, the photoelectrons captured by the abundant oxygen vacancies can obtain a secondary leap to the semiconductor conduction band (CB), suppressing the recombination of electron-hole pairs, thus favoring the electron transfer on Co2SiO4/Co3O4−x. The composites not only have abundant oxygen vacancies, but also have a large specific surface area for the adsorption and activation of CO2. The yields of CH3OH on Co2SiO4/Co3O4−5% reach as high as 48.9 μmol·g−1·h−1 under simulated sunlight irradiation. In situ DRIFTS is used to explore the photocatalytic reduction CO2 mechanism. It is found that the thermal effect facilitates the generation of the key intermediate COOH* species. This work provides a new strategy for photothermal catalytic CO2 reduction by taking advantage of natural clay and solar energy.

Efficient and recyclable Ni-Ce-Mn-N modified ordered mesoporous carbon electrode during electrocatalytic ozonation process for the degradation of simulated high-salt phenol wastewater

In this study, an efficient and stable NiO/CeO₂/MnO₂-modified nitrogen-doped ordered mesoporous carbon (NOMC) particle electrode was developed, in which the metal oxides were mosaicked within the pore channels by one-pot skeleton hybridization, and the comodification of NiO/CeO₂/MnO₂/N was found to improve the electrocatalytic activity and stability of the particle electrode. The improved stability of the ordered mesoporous carbon towards pore collapse was applied to the degradation of simulated high-salt phenol wastewater by an electrocatalytic ozonation process using simple binder pelletization. The modified ordered mesoporous carbon shows a specific surface area of 269.7 m² g^-1 and a pore size of 3.17 nm, and SEM and TEM were used to show that the mesoporous structure is well maintained and the metal nanoparticles are well dispersed. The electrochemically active area of the Ni_2%/Ce_0.5%/Mn_2.5%-NOMC particle electrode reaches 224.65 mF cm^-2, which indicates that NiO improves the capacitance of the ordered mesoporous carbon and accelerates the electron transfer efficiency. Encouragingly, the phenol removal efficiency is found to reach up to 93.0% for 60 min over a wide range of pH values, with an initial phenol concentration of 150 mg L^-1, low current (0.03 A) and fast reaction rate (0.0895 min^-1), and the presence of CeO₂ ameliorates the low activity of the particle electrode under acidic conditions. These results indicate that the presence of pyridine-N and β-MnO₂ effectively mitigates carbon corrosion and improves electrode stability, as the accumulation of large amounts of ·OH at 20 min and the maintenance of a degradation efficiency of more than 90% after eight cycles provides a viable solution for the widespread practical application of ordered mesoporous carbon particle electrodes.

Co-Co3O4 encapsulated in nitrogen-doped carbon nanotubes for capacitive desalination: Effects of nano-confinement and cobalt speciation

Capacitive deionization (CDI) has gained increasing attention as an environmentally friendly and energy-efficient technology for brackish water desalination. However, traditional CDI electrodes still suffer from low salt adsorption capacity and unsatisfactory reusability, which inhibit its application for long-term operations. Herein, we present a facile and effective approach to prepare Co and Co₃O₄ nanoparticles co-incorporating nitrogen-doped (N-doped) carbon nanotubes (Co-Co₃O₄/N-CNTs) via a pyrolysis route. The Co-Co₃O₄ nanoparticles were homogeneously in-situ encapsulated in the inner channels of the conductive CNTs to form a novel and efficient CDI electrode for the first time. The encapsulation of Co-Co₃O₄ nanoparticles in CNTs not only inhibits the Co leaching but also significantly enhances the desalination capacity. The morphology, structure, and capacitive desalination properties of the Co-Co₃O₄/N-CNTs were thoroughly characterized to illuminate the nano-confinement effects and the key roles of the interaction between cobalt species in the CDI performance. The co-existing metallic cobalt and cobalt oxides act as the roles of effective active sites in the CDI performance. As a consequence, the optimum Co-Co₃O₄/N-CNTs electrode displays an outstanding desalination capacity of 66.91 mg NaCl g^-1 at 1.4 V. This work provides insights for understanding the nano-confinement effects and the key roles of the interaction between cobalt species on the CDI performance.

The Flower-like Co3O4 Hierarchical Microspheres for Methane Catalytic Oxidation

The development of non-noble Co3O4 catalysts exposing highly active crystal planes to low-temperature methane oxidation is still a challenge. Hence, a facile solvothermal method was adapted to construe flower-like Co3O4 hierarchical microspheres (Co3O4-FL), which are composed of nanosheets with dominantly exposed {112} crystal planes. The flower-like hierarchical structure not only promotes the desorption of high levels of active surface oxygen and enhances reducibility, but also facilitates an increase in lattice oxygen as the active species. As a result, Co3O4-FL catalysts offer improved methane oxidation, with a half methane conversion temperature (T50) of 380 °C (21,000 mL g−1 h−1), which is much lower than that of commercial Co3O4 catalysts (Co3O4-C). This study will provide guidance for non-noble metal catalyst design and preparation for methane oxidation and other oxidative reactions.