Commercial Co3O4 Research Articles

Limitations and Opportunities of Self-Supported Co-Based Anodes for Pure Water-Fed Anion Exchange Membrane Water Electrolyzers

Hydrogen has been regarded as an energy source with a potential to be used in various sectors, including transportation, power generation and industrial processes1-3. Producing hydrogen via water electrolysis using green electricity will be necessary for a sustainable future. Proton exchange membrane water electrolysis (PEMWE) is one of mature and leading electrolysis technologies, while anion exchange membrane water electrolysis (AEMWE) is regarded as a new, emerging technology4. However, PEMWE system suffers from their expensive cost and scarcity of iridium-based oxygen evolution reaction (OER) catalysts, having no alternative OER catalysts. Compared to PEMWE, AEMWE can be pursued with non-platinum group metal (non-PGM) thanks to its alkaline environment4,5.Here, non-PGM Co-based catalyst coated anodes were prepared using a simple hydrothermal reaction followed by heat treatment. Different phases and crystallinity of cobalt-based anodes were prepared and tested in pure water-fed AEMWE devices. To investigate the catalyst-ionomer/membrane interfaces at the anodes, the loadings of Co3O4 catalyst and ionomer topcoats were controlled. The cell performance in polarization curves was correlated with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements. The comparison of anodes operated with pure water feed and with supporting electrolyte was made to highlight the importance of catalyst-ionomer interface in pure water-fed AEMWE. Lastly, our self-supported Co3O4 anode outperformed a commercial Co3O4 NP ink-based anode, projecting a potential of direct growth of catalyst materials on PTLs in pure water-fed AEMWE.Reference(1) Agyekum, E. B.; Nutakor, C.; Agwa, A. M.; Kamel, S. A Critical Review of Renewable Hydrogen Production Methods: Factors Affecting Their Scale-Up and Its Role in Future Energy Generation. Membranes 2022, 12 (2).(2) Qusay, H.; Sameer, A.; Aws Zuhair, S.; Hayder, M. S.; Marek, J. Green hydrogen: A pathway to a sustainable energy future. Int. J. Hydrogen Energy 2024, 50, 310-333.(3) Joonsik, H.; Krisha, M.; HeeJin, C. A review of hydrogen utilization in power generation and transportation sectors: Achievements and future challenges. Int. J. Hydrogen Energy 2023, 48 (74), 28629-28648.(4) Lee, S. A.; Kim, J.; Kwon, K. C.; Park, S. H.; Jang, H. W. Anion exchange membrane water electrolysis for sustainable large-scale hydrogen production. Carbon Neutralization 2022, 1 (1), 26-48.(5) Juchan, Y.; Myeong Je, J.; Xiaojun, Z.; Yoo Sei, P.; Jooyoung, L.; Sung Mook, C.; Yadong, Y. Non-precious electrocatalysts for oxygen evolution reaction in anion exchange membrane water electrolysis: A mini review. Electrochem. Commun. 2021, 131, 107118.

Highly selective regulation of non-radical and radical mechanisms by Co cubic assembly catalysts for peroxymonosulfate activation

Peroxymonosulfate (PMS) activation on efficient catalysts is a promising strategy to produce sulfate radical (SO4−) and singlet oxygen (1O2) for the degradation of refractory organic pollutants. It is a great challenge to selectively generate these two reactive oxygen species, and the regulation mechanism from non-radical to radical pathway and vice versa is not well established. Here, we report a strategy to regulate the activation mechanism of PMS for the selective generation of SO4− and 1O2 with 100 % efficiency by sulfur-doped cobalt cubic assembly catalysts that was derived from the Co-Co Prussian blue analog precursor. This catalyst showed superior catalytic performance in activating PMS with normalized reaction rate increased by 87 times that of the commercial Co3O4 nanoparticles and had much lower activation energy barrier for the degradation of organic pollutant (e.g., p-chlorophenol) (18.32 kJ⋅mol−1). Experimental and theoretical calculation results revealed that S doping can regulate the electronic structure of Co active centers, which alters the direction of electron transfer between catalyst and PMS. This catalyst showed a strong tolerance to common organic compounds and anions in water, wide environmental applicability, and performed well in different real-water systems. This study provides new opportunities for the development of metal catalyst with metal-organic frameworks structure and good self-regeneration ability geared specifically towards PMS-based advanced oxidation processes applied for water remediation.

Efficient and stable neutral seawater splitting achieved via strong-proton-adsorption in Pd-O-Co collaborative coordination

Direct electrolysis of pH-neutral seawater for hydrogen generation is a promising method for storing renewable energy. However, the oxygen evolution reaction (OER) faces a selectivity challenge, competing with chlorine evolution and confronting severe corrosion issues of the electrode. Here, we have synthesized ultrathin, polycrystalline, porous Co3O4 nanosheets with Pd single atoms (PdSA-Co3O4) for efficient and stable pH-neutral seawater decomposition. The octahedral Pd-O-Co active unit, generated by the synergistic coordination of the Pd single atom strong proton adsorption (SPA) material with Co3O4 nanosheets, resulted in PdSA-Co3O4 have activities 3, 5, and 31 times higher than Co3O4, IrO2, and commercial Co3O4, respectively. It also remained stable in pH-neutral seawater for 80 h. Operando in situ Raman spectroscopy combined with density functional theory calculations showed that synergistic interactions between the strong proton adsorption of Pd single atoms and the active site Co enhanced the adsorption of intermediates and significantly reduced the free energy of adsorption at the rate-determining step (O*→OOH*). The Pd-O-Co active units inhibited the transformation of the dynamic structure of the Co3O4 substrate and reduced the corrosive effect of Cl ions during the OER process.

PAN-templated synthesis of layered Co3O4 nanosheets for rapid reduction of 4-NP to 4-AP: Improved formation of unique Co3+-rich (440) facet on the crystal lattice of Co3O4

The efficient reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using Co3O4 in NaBH4(aq) requires the precise control of hydride (Hδ-) and protic hydrogen (Hδ+) generation. Currently, this is achieved using (311) facet-dominated Co3O4 with Co2+-rich edges that favor Hδ- activation leading to poor performance. Herein, we employ the ladder-like structure of oxidized polyacrylonitrile (PAN) to fine-tune the growth of the (311)-to-(440) facets in layered Co3O4 nanosheets. This yields a balanced distribution of Co3+/Co2+ edge sites, leading to enhanced catalytic activity. Our catalyst is prepared by air-annealing a mixture of cobalt acetate (Co(OAc)2) and PAN at 500 °C. The mass ratio of Co(OAc)2 to PAN dictates the (311)/(440) facet ratio (i.e., Y). Specifically, a ratio of 0.25/0.4 (P-0.25–500) gives Y= 1.12. Meanwhile, at ratios of 0.05/0.4 (P-0.05–500) and 0.5/0.8 (P-0.5–500), Y increases to 1.25 and 1.27, respectively. XPS analysis confirms that the surface Co3+/Co2+ distribution (X) on P-0.25–50, P-0.05–500, and P-0.5–500 is 0.43, 0.41, and 0.39, respectively, highlighting a direct relationship between the (440) exposure and Co3+ concentration. The rate of 4-NP reduction (i.e., kapp) on P-0.25–500, i.e., ∼2.84 × 10−2 s−1 is 3.1x and 3.6x faster than P-0.05–500 (∼9.16 ×10−3 s−1) and P-0.5–500 (∼7.81 ×10−3 s−1), respectively. P-0.25–50's performance is comparable to that of state-of-the-art noble metal catalysts and higher than commercial Co3O4, and it maintains a decent catalytic retention of 88.84% after six recycles, demonstrating high stability. Our research presents a simple yet highly effective method for achieving balanced Hδ+ and Hδ- activation edges in Co3O4, resulting in superior 4-NP reduction activity.

Selective recovery of lithium from spent lithium-ion batteries via mild hydrothermal driven Lewis acid-base reaction in aqua solution

Selective metal recovery from spent lithium-ion batteries has attracted wide attention since a high recovery rate and minimized environmental impact can be expected. However, either high temperature or high chemical consumption in order to realize effective lithium recovery is required, as the cathode inherent with high physiochemical stability, especially when the cobalt content is high (e.g. LiCoO2). Herein, we discover that Lewis acids in cathode scraps induce phase transition from LiCoO2 to Co(CoO2)2 and Li+, thus realizing fully selective recovery of lithium in a mild environment. Since Lewis acids are produced from scraps, the chemical consumption is reduced. Meanwhile, relying on the mild phase transition caused by Lewis acids, the morphology of the waste are maintained in the Co(CoO2)2 products, resulting in a comparable electrochemical performance with commercial Co3O4 anode materials. Consequently, a closed-loop route for recycling cathode scrap is established at mild conditions with low cost and environmental impact.

High-Efficiency Degradation of Formaldehyde and Bioelectricity Generation by Microbial Fuel Cell

Formaldehyde is a common organic pollutant in water with teratogenic and carcinogenic effects. This study reports that 200 mg l−1 formaldehyde in water can be effectively degraded with generating electricity by using microbial fuel cell (MFC) technology. A novel composite anode M-Co3O4-PEDOT-GF was prepared by modifying Co3O4 nanoparticles (M-Co3O4) derived from ZIF-67 and poly (3,4-ethylenedioxythiophene) (PEDOT) on the surface of Graphite felt (GF). The results showed that the MFC loading M-Co3O4-PEDOT-GF anode exhibited excellent electricity generation performance and formaldehyde degradation. The maximum voltage of the MFC was 549 mV, 46.0% increase than that of GF anode (376 mV), and higher than N-Co3O4-PEDOT-GF anode (488 mV) modified with commercial Co3O4 (N-Co3O4). The maximum power density of the MFC loading composite anode was 4177 mW m−2, while that of MFC loading bare GF anode was 1562 mW m−2. The dominant microorganisms were Pseudomonadales and Rhizobiales at the order level. The removal efficiency of formaldehyde by MFC loading M-Co3O4-PEDOT-GF anode was 89.2% in 152 h. The high efficiency of formaldehyde degradation was still maintained after 10 cycles. The results could be attributed to the composite anode with loose porous three-dimensional structure and good biological compatibility of PEDOT.

Boosting borohydride hydrolysis for H2 generation by MOF-templated void-engineered shaggy cobalt oxide: Abundant oxygen vacancy-mediated enhancement

Designing an advantageous catalyst for enhancing the release of hydrogen (H2) from NaBH4 hydrolysis is still desirable. Herein, a void-engineered shaggy cobalt oxide (VSCO) is constructed via facile carving and calcination using the cuboid cobalt-based metal organic framework (Co-MOF) as a template. The as-prepared VSCO shows unique structural properties, such as large internal void, high specific surface area, and abundant oxygen vacancy, enabling VSCO to boost H2 production from NaBH4 hydrolysis. VSCO also exhibits an activation energy (Ea) of 28.9 kJ mol−1, which is much lower than that of commercial Co3O4 NP (62.9 kJ mol−1) and most of recent reported noble metals. VSCO also retains its outstanding catalytic activity over multiple cycles. This work sheds a light into the new approach of constructing metal oxide material with the void structure and abundant oxygen vacancies for catalyzing the hydrolysis of NaBH4.

Interface-engineered cavity-structured cobalt oxide as a boosted activator for peroxymonosulfate to degrade environmental hormones in water: Structural defects and oxygen vacancies-induced enhancement

While heterogeneous cobalt (Co) catalysts are useful peroxymonosulfate (PMS) activators, the development of a heterogeneous Co-based catalyst with intriguing physicochemical properties and outstanding catalytic activities is still of great interest. Herein, a unique cavity-structured grassy-like cobalt oxide (Co3O4) (CGCO) is proposed and fabricated through two-step modifications of partial sculpting and calcination using Co-based metal organic framework (Co-MOF) as an initial precursor. The as-prepared CGCO exhibits a cavity structure covered by grassy-like outer shells, thus possessing special physicochemical properties and enriched oxygen vacancy. These features enable CGCO a much more advantageous activator than commercial Co3O4 for activating PMS, and would efficiently remove 100% of bisphenol A (BA) rapidly. The activation energy (Ea) of BA degradation by CGCO+PMS is only 31.2 kJ/mol, which is lower than the recently-reported values. The density function theory calculation is performed to investigate the degradation pathway, and the corresponding eco-toxicity is also studied to realize the environmental implication of BA by CGCO-activated PMS as the toxicities and environmental impact of BA are significantly reduced. This work provides valuable information in term of preparation strategy of cavity structured metal oxide materials, which should be propitious for the degradation of environmental hormones via advanced oxidation reactions.

Efficient Selective Oxidation of Aromatic Alkanes by Double Cobalt Active Sites over Oxygen Vacancy-rich Mesoporous Co3O4.

The development of an efficient catalyst for selective oxidation of hydrocarbon to functional compounds remains a challenge. Herein, mesoporous Co3O4 (mCo3O4-350) showed excellent catalytic activity for the selective oxidation of aromatic-alkanes, especially for oxidation of ethylbenzene with a conversion of 42% and selectivity of 90% for acetophenone at 120 oC. Notably, mCo3O4 presented a unique catalytic path of direct oxidation of aromatic-alkanes to aromatic ketones rather than the conventional stepwise oxidation to alcohols and then to ketones. Density functional theory calculations revealed that oxygen vacancies in mCo3O4 activate around Co atoms, causing electronic state change from Co3+(Oh) → Co2+(Oh). Co2+(Oh) has great attraction to ethylbenzene, and weak interaction with O2, which provide insufficient O2 for gradual oxidation of phenylethanol to acetophenone. Combined with high energy barrier for forming phenylethanol, the direct oxidation path from ethylbenzene to acetophenone is kinetically favorable on mCo3O4, sharply contrasted to non-selective oxidation of ethylbenzene on commercial Co3O4.

Efficient Selective Oxidation of Aromatic Alkanes by Double Cobalt Active Sites over Oxygen Vacancy‐rich Mesoporous Co3O4

AbstractThe development of efficient catalyst for selective oxidation of hydrocarbon to functional compounds remains a challenge. Herein, mesoporous Co3O4 (mCo3O4‐350) showed excellent catalytic activity for selective oxidation of aromatic‐alkanes, especially for oxidation of ethylbenzene with a conversion of 42 % and selectivity of 90 % for acetophenone at 120 °C. Notably, mCo3O4 presented a unique catalytic path of direct oxidation of aromatic‐alkanes to aromatic ketones rather than the conventional stepwise oxidation to alcohols and then to ketones. Density functional theory calculations revealed that oxygen vacancies in mCo3O4 activate around Co atoms, causing electronic state change from Co3+(Oh)→Co2+(Oh). Co2+(Oh) has great attraction to ethylbenzene, and weak interaction with O2, which provide insufficient O2 for gradual oxidation of phenylethanol to acetophenone. Combined with high energy barrier for forming phenylethanol, the direct oxidation path from ethylbenzene to acetophenone is kinetically favorable on mCo3O4, sharply contrasted to non‐selective oxidation of ethylbenzene on commercial Co3O4.

Hydrogen production from hydrolysis of NaBH4-NH3BH3 composite catalyzed by porous spherical Co3O4

NaBH4-NH3BH3 composite (xSB-AB, x is the molar ratio of SB to AB) has better hydrolysis performance than its monomer, but the hydrogen generation rate (HGR) and hydrogen yield (HY) are still not ideal at room temperature. In this work, a low cost and easily available commercial porous spherical Co3O4 was successfully used to catalyze the hydrolysis of xSB-AB composite. It was found that Co3O4 showed good catalytic performance for the hydrolysis of xSB-AB, and the HY and hydrogen release efficiency (HRE) of 4SB-AB/10 wt%Co3O4 at 40 ℃ reached 2,279.71 mL·g-1 and 89.13%, respectively.

A simple room–temperature acid capture strategy to controllably tune oxygen vacancy in Co3O4 for oxygen evolution reaction

Oxygen vacancy (Vo) engineering is considered as a general but effective way to improve the oxygen evolution reaction (OER) performance of transition metal oxides (TMOs). However, the deep understanding of Vo is still limited. In this work, a simple room–temperature acid capture strategy is employed to create Vo in TMOs. As discovered in the case of commercial Co3O4 microspheres, the Vo can be tuned by the acid capture time. As the acid capture time is growing, the concentration of Vo increases. Moderate Vo concentration created by controlled acid capture time (less than 32 h) can elevate the electrical conductivity and optimize the electronic structure. With overlong acid capture time (reaching 64 h), excessive Vo will present, leading to enlarged energy band gap, declined electrical conductivity as well as collapsed structure of Co3O4 microspheres, which is detrimental to the exposure of catalytic sites and accessibility of electrolyte. Consequently, the resultant Co3O4 −Vo− 32 (32 represents acid capture time) obtains a balanced Vo concentration while retaining structure integrity, thus displaying a better OER performance than the counterparts.

Petal-like hierarchical Co3O4/N-doped porous carbon derived from Co-MOF for enhanced peroxymonosulfate activation to remove tetracycline hydrochloride

Developing hierarchical and efficient catalysts with multiple active sites to activate peroxymonosulfate (PMS) for the degradation of persistent organic contaminants is crucial, whereas it's still a challenge. Herein, a novel petal-like N-doped porous carbon embedded with ultrafine cobalt oxide nanoparticles (NPs) (named as Co3O4/NC) was unprecedentedly fabricated via the pyrolysis of N-enriched Co-MOF (named as Co-TA), where the unique hierarchical structure, rich active sites and large specific area were contributed to improve the excellent catalytic activity toward tetracycline hydrochloride (TCH). As expected, the removal efficiency of TCH in Co3O4/NC/peroxymonosulfate system was 96.4 % within 30 min, extraordinary superior to direct air-calcining nano-Co3O4, commercial Co3O4 and Co-TA, in which the kinetic constant was 10.4 times higher than pure Co-TA. Besides, above 97 % removal efficiency of TCH was maintained over a wide pH range of 5–9, and 71 % of total organic carbon (TOC) could be eliminated. The TCH degradation was mainly characterized by the coexistence of multiple active substances using EPR analysis, where SO4·− and 1O2 were the dominant contributors. The degradation pathways of TCH were also clarified based on theoretical calculation and HPLC-TOF-MS2 data. This work can extend the application of metal–organic frameworks (MOFs)-derived nanocatalysts in the environment field, and may offer a promising metal oxide heterogeneous catalyst for the rapid elimination of persistent organic pollutants by sulfate radical-based advanced oxidation processes (SR-AOPs).

Co3O4 Nanopetals Grown on the Porous CuO Network for the Photocatalytic Degradation.

Designing a novel photocatalytic composite for the efficient degradation of organic dyes remains a serious challenge. Herein, the multi-layered Co3O4@NP-CuO photocatalyst with unique features, i.e., the self-supporting, hierarchical porous network as well as the construction of heterojunction between Co3O4 and CuO, are synthesized by dealloying-electrodeposition and subsequent thermal treatment techniques. It is found that the interwoven ultrathin Co3O4 nanopetals evenly grow on the nanoporous CuO network (Co3O4@NP-CuO). The three-dimensional (3D) hierarchical porous structure for the catalyst provides more surface area to act as active sites and facilitates the absorption of visible light in the photodegradation reaction. Compared with the commercial CuO and Co3O4 powders, the newly designed Co3O4@NP-CuO composite exhibits superior photodegradation performance for RhB. The enhanced performance is mainly due to the construction of heterojunction of Co3O4/CuO, greatly promoting the efficient carrier separation for photocatalysis. Furthermore, the possible photocatalytic mechanism is analyzed in detail. This work provides a promising strategy for the fabrication of a new controllable heterojunction to improve photocatalytic activity.

Electron-Deficient Pd clusters induced by spontaneous reduction of support defect for selective phenol hydrogenation

A facile and green synthesis route to fabricate the stable and highly-dispersed Pd catalyst was reported by utilizing the reducibility and defect structure of Co3O4. Specifically, nanoshaped Co3O4 with different Co and O vacancies was prepared. Taking these as support, Pd2+ precursors were introduced and preferentially anchored in isolated Co vacancies, confirmed by density functional calculations. Notably, Pd2+ species were reduced by support without any reducing agent. More interestingly, as-obtained Pd catalysts exhibited different product distribution in selective phenol hydrogenation, whereas Pd/hexagonal Co3O4 only generated the cyclohexanol as product, while the cyclohexanone was dominant in rod and commercial Co3O4 supported Pd catalysts. This phenomenon was ascribed to electron-deficient Pd clusters influenced by Co vacancy in hexagonal Co3O4, facilitating further hydrogenation of reaction intermediate rather than desorption. This work provides a feasible way to construct well-distributed and electron-deficient Pd clusters, offering an opportunity to control product distribution in selective hydrogenation.

Unraveling spongy Co3O4 mediated activation of peroxymonosulfate: Overlooked involvement of instantaneously produced high-valent-cobalt-oxo

Peroxymonosulfate (PMS) activation induced by tricobalt tetroxide spinel (Co3O4) has been confirmed as a typical Haber-Weiss reaction, while free radicals were once considered as the dominated reactive species in the previous studies. However, the catalytic mechanism of the spongy Co3O4 driven PMS activation was surprisingly found as a radical/nonradical mixed process rather than a pure radical process in the present work. The important role of sulfate radical (SO4−) was confirmed through the quenching experiments. Despite the inhibition of furfuryl alcohol (FFA) and 1,4-benzoquinone (BQ) on degradation was generally accepted as the evidence to support the existence of 1O2 and O2−, additional experiments using methyl phenyl sulfoxide (PMSO) as the indicator indeed verified high-valent-cobalt-oxo rather than 1O2 and O2− dominated the very early reaction stage. Notably, instead of homogeneous Co3+, heterogeneous Co(IV) = O on catalyst surface was believed to be responsible for the oxidation of organics. Spongy Co3O4 not only possessed stronger catalytic ability than commercial Co3O4 (k[spongy Co3O4] = 0.74 min−1, k[Co3O4] = 0.08 min−1), but also owned preferable stability. The performance of catalytic system was barely affected by the solution pH under the near neutral condition. Besides, little suppression of the widely existing anions on the degradation indicated the potential application of spongy Co3O4/PMS system. This study provides a reliable oxidation technology for the removal of organic pollutants, and sheds new light on the cobalt oxide triggered PMS activation process.

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.

Insight into the degradation of tetracycline hydrochloride by non-radical-dominated peroxymonosulfate activation with hollow shell-core Co@NC: Role of cobalt species

Cobalt species often play a crucial role in the sulfate radical (SO4−) generation during peroxymonosulfate (PMS) activation, but their roles in the formation of non-radical-dominated PMS activation are largely unclear. Here, shell-core hollow Co@NC with exposed two different cobalt species (i.e., Co nanoparticles and Co-Nx site) was prepared and used to activate PMS for tetracycline hydrochloride (TCH) degradation. The experimental results and theoretical calculations showed cobalt species play a crucial role in the 1O2-dominated TCH degradation in which Co-Nx directly served as active sites to adsorb PMS (*PMS) to facilitate PMS decomposition to form SO5−, and thereafter evolved into 1O2 by the rapid self-reaction of SO5−, while Co nanoparticles indirectly promote 1O2 generation via electron transfer due to their excellent conductivity. With exposed cobalt species and unique structure, Co@NC showed a remarkable catalytic activity for TCH degradation, outperforming the synthesized NC, Co-NC, and commercial Co3O4, Fe3O4 and MnO2. Meanwhile, the good stability and reusability, high environmental robustness and universal adaptability of Co@NC were demonstrated. The TCH degradation pathways including aniline ring oxidation, the cleavage of functional groups and ring-opening reactions were also proposed. The improved understanding on the roles of cobalt species in the non-radical-dominated PMS activation may inspire the development of efficient, selective, and robust cobalt-based catalysts.

Bulk Co3O4 for Methane Oxidation: Effect of the Synthesis Route on Physico-Chemical Properties and Catalytic Performance

The synthesis of bulk pure Co3O4 catalysts by different routes has been examined in order to obtain highly active catalysts for lean methane combustion. Thus, eight synthesis methodologies, which were selected based on their relatively low complexity and easiness for scale-up, were evaluated. The investigated procedures were direct calcination of two different cobalt precursors (cobalt nitrate and cobalt hydroxycarbonate), basic grinding route, two basic precipitation routes with ammonium carbonate and sodium carbonate, precipitation-oxidation, solution combustion synthesis and sol-gel complexation. A commercial Co3O4 was also used as a reference. Among the several examined methodologies, direct calcination of cobalt hydroxycarbonate (HC sample), basic grinding (GB sample) and basic precipitation employing sodium carbonate as the precipitating agent (CC sample) produced bulk catalysts with fairly good textural and structural properties, and remarkable redox properties, which were found to be crucial for their good performance in the oxidation of methane. All catalysts attained full conversion and 100% selectivity towards CO2 formation at a temperature of 600 °C while operating at 60,000 h−1. Among these, the CC catalyst was the only one that achieved a specific reaction rate higher than that of the reference commercial Co3O4 catalyst.

Hollow porous cobalt oxide nanobox as an enhanced for activating monopersulfate to degrade 2-hydroxybenzoic acid in water

As 2-hydroxybenzoic acid (HBA) represents a typical pharmaceutical and personal care product (PPCP), constant releasing of HBA into the environment poses threats to the ecology, and thus it is critical to develop effective techniques to remove HBA from water. Recently, sulfate radical (SO4‒)-based advanced oxidation processes involved with monopersulfate (MPS) activation are proven as effective approaches for eliminating PPCPs from water, and Co3O4 is recognized as a capable catalyst for activating MPS. Therefore, great interests have arisen to develop Co3O4-based catalysts with advantageous morphologies and characteristics for enhancing catalytic activities. Therefore, a special Co3O4-based material is proposed in this work. Through a surfactant-assisted strategy, a cubic Co-MOF is prepared and used as a precursor, which is etched to afford hollow structure, and then transformed into hollow porous Co3O4 nanobox (PCNB). PCNB can exhibit distinct reactive surface with abundant surface oxygen vacancy as well as physical properties in comparison to the commercial Co3O4 NPs (com-Co3O4 NP), thereby leading to the outstanding catalytic activity of PCNB for activating MPS to degrade HBA. The activation energy (Ea) of 46.2 kJ/mol is also calculated using PCNB + MPS system, which is much lower than most of recent reported studies for activating MPS. PCNB could be also reusable over 5 consecutive HBA degradation cycles. The activation mechanism of MPS by PCNB and HBA degradation pathway are also comprehensively elucidated via experimental evidences and the theoretical calculation to offer insightful information of development of Co3O4 for HBA degradation.