Pretreatments of Co3O4 at moderate temperature for CO oxidation at −80°C

Well-defined tricobalt tetraoxide's critical morphology effect on the structure-reactivity relationship

This review focuses on exploring the intricate relationship between the catalyst particle size and shape on a nanoscale level and how it affects the performance of reactions. Drawing from decades of research, valuable insights have been gained. Intentionally shaping catalyst particles makes exposing a more significant percentage of reactive facets possible, enabling the control of overactive sites. In this study, the effectiveness of Co3O4 nanoparticles (NPs) with nanometric size as a catalyst is examined, with a particular emphasis on the coordination patterns between oxygen and cobalt atoms on the surface of these NPs. Investigating the correlation between the structure and reactivity of the exposed NPs reveals that the form of Co3O4 with nanometric size can be modified to tune its catalytic capabilities finely. Morphology-dependent nanocatalysis is often attributed to the advantageous exposure of reactive crystal facets accumulating numerous active sites. However, experimental evidences highlight the importance of considering the reorganization of NPs throughout their actions and the potential synergistic effects between nearby reactive and less-active aspects. Despite the significant role played by the atomic structure of Co3O4 NPs with nanometric size, limited attention has been given to this aspect due to challenges in high-resolution characterizations. To bridge this gap, this review strongly advocates for a comprehensive understanding of the relationship between the structure and reactivity through real-time observation of individual NPs during the operation. Proposed techniques enable the assessment of dimensions, configuration, and interfacial arrangement, along with the monitoring of structural alterations caused by fluctuating temperature and gaseous conditions. Integrating this live data with spectroscopic methods commonly employed in studying inactive catalysts holds the potential for an enhanced understanding of the fundamental active sites and the dynamic behavior exhibited in catalytic settings.

Enhanced CO oxidation performance over hierarchical flower-like Co3O4 based nanosheets via optimizing oxygen activation and CO chemisorption

Enhancing low-temperature activity is a focus for carbon monoxide (CO) elimination by catalytic oxidation. In this work, the hierarchical flower-like silver (Ag) modified cobalt oxides (Co3O4) nanosheets were prepared by solvothermal method and applied into catalytic CO oxidation. The doped Ag species in the form of AgCoO2 induced the prolongated surface Co-O bond and weaker bond intensity. Consequently, the oxygen activation/migration ability and redox capacity of Ag0.02Co were enhanced with more oxygen vacancies. The chemisorbed CO was preferentially converted to CO2 but not carbonates. The inhibited carbonates accumulation could avoid the coverage of active sites. According to Density functional theory (DFT) calculations, the electron transfer from AgCoO2 to Co3O4 promote electron donation ability of Co3O4 layer, benefiting for oxygen activation. Moreover, the longer Co-C and C-O bond length suggest the weakened chemisorption strength and higher active of CO molecule. The Ag modified Co3O4 exhibited more satisfactory activity at lower temperature. Typically, it realized 100% CO conversion at 90 °C, and displayed 6.3-fold higher reaction rate than pristine Co3O4 at 40 °C. Moreover, the Ag0.02Co exhibited outstanding long-term stability and water resistance. In summary, the optimized oxygen activation, CO chemisorption and interfacial electron transfer synergistically boosted the CO oxidation activity on Ag modified Co3O4.

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.

Restructuring Co-CoOx Interface with Titration Rate in Co/Nb-CeO2 Catalysts for Higher Water-Gas Shift Performance

H2 production via water-gas shift reaction (WGS) is an important process and applied widely. Cobalt-modified CeO2 are promising catalysts for WGS reaction. Herein, a series of Co/Nb-CeO2 catalysts were prepared by varying the rate of precipitant addition during the coprecipitation method and examined for hydrogen generation through WGS reaction. The rates of precipitant addition were 1, 5, 15, and 25 mL/min. We obtained ceria supported cobalt catalysts with different sizes and morphology such as 3, 8 nm nanoclusters, 30 nm cubic nanoparticles, and 50 nm hexagonal nanoparticles. The well dispersed small cobalt particles in Co/Nb-CeO2 that was prepared at 5 mL/min titration rate exhibit strong interaction between cobalt oxide and CeO2 that retards the reduction of CoOx producing Co-CoOx pairs. In contrast, 1-Co/Nb-CeO2 and 25-Co/Nb-CeO2 result in bigger and aggregated Co particles, resulting in fewer interfaces with CeO2. The Co0, Coδ+, Ce3+, and Ov species are responsible for improved reducibility in Co/Nb-CeO2 catalysts and were quantitively measured using XPS, XAS, and Raman spectroscopy. The Co-CoOx interface assists dissociation of the H2O molecule; CO oxidation requires low activation energy and realizes a high turnover frequency of 9.8 s-1. The 5-Co/Nb-CeO2 catalyst achieved thermodynamic equilibrium equivalent CO conversion with efficient H2 production during WGS reaction at a gas hourly space velocity of 315,282 h-1. Successively, the 5-Co/Nb-CeO2 catalyst exhibited stable performance for straight 168 h attributed to stable CO-Coδ+ intermediate formation, achieving efficient inhibition of typical CO chemistry over the Co metal, suitable for hydrogen generation from waste derived synthesis gas.

Catalytic Ozonation of Polluter Benzene from -20 to >50 °C with High Conversion Efficiency and Selectivity on Mullite YMn2O5

Catalytic decomposition of aromatic polluters at room temperature represents a green route for air purification but is currently challenged by the difficulty of generating reactive oxygen species (ROS) on catalysts. Herein, we develop a mullite catalyst YMn₂O₅ (YMO) with dual active sites of Mn³⁺ and Mn⁴⁺ and use ozone to produce a highly reactive O* upon YMO. Such a strong oxidant species on YMO shows complete removal of benzene from -20 to >50 °C with a high CO_x selectivity (>90%) through the generated reactive species O* on the catalyst surface (60 000 mL g^-1 h^-1). Although the accumulation of water and intermediates gradually lowers the reaction rate after 8 h at 25 °C, a simple treatment by ozone purging or drying in the ambient environment regenerates the catalyst. Importantly, when the temperature increases to 50 °C, the catalytic performance remains 100% conversion without any degradation for 30 h. Experiments and theoretical calculations show that such a superior performance stems from the unique coordination environment, which ensures high generation of ROS and adsorption of aromatics. Mullite's catalytic ozonation degradation of total volatile organic compounds (TVOC) is applied in a home-developed air cleaner, resulting in high efficiency of benzene removal. This work provides insights into the design of catalysts to decompose highly stable organic polluters.

Effect of reduction pretreatments on PdAg/Al2O3 for HCHO and CO oxidation

PdAg/Al₂O₃ were pretreated by CO and H₂ reduction pretreatments, respectively. The reduced catalysts were tested for HCHO and CO oxidation and characterized by Brunner Emmet Teller (BET), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and oxygen temperature programmed desorption (O₂-TPD). These results indicate that the pretreatments have effect on PdAg reconstruction, PdAg particle size and active oxygen species, which are responsible for the catalytic performance. Compared with H₂ reduction method, CO reduction is more suitable for PdAg/Al₂O₃ pretreatment. PdAg/Al₂O₃-CO exhibited better catalytic performance.

Catalytically efficient Ni-NiOx-Y2O3 interface for medium temperature water-gas shift reaction

The metal-support interfaces between metals and oxide supports have long been studied in catalytic applications, thanks to their significance in structural stability and efficient catalytic activity. The metal-rare earth oxide interface is particularly interesting because these early transition cations have high electrophilicity, and therefore good binding strength with Lewis basic molecules, such as H₂O. Based on this feature, here we design a highly efficient composite Ni-Y₂O₃ catalyst, which forms abundant active Ni-NiO_x-Y₂O₃ interfaces under the water-gas shift (WGS) reaction condition, achieving 140.6 μmol_CO g_cat⁻¹ s⁻¹ rate at 300 °C, which is the highest activity for Ni-based catalysts. A combination of theory and ex/in situ experimental study suggests that Y₂O₃ helps H₂O dissociation at the Ni-NiO_x-Y₂O₃ interfaces, promoting this rate limiting step in the WGS reaction. Construction of such new interfacial structure for molecules activation holds great promise in many catalytic systems.

Developing effective and stable catalytic interfaces in the medium temperature region is a practical route to replace the existing water gas shift (WGS) process. Here the authors designed a composite Ni-Y₂O₃ catalyst achieving the highest WGS activity for Ni based catalysts.

Effect of MnO2 Polymorphs' Structure on Low-Temperature Catalytic Oxidation: Crystalline Controlled Oxygen Vacancy Formation

MnO₂ polymorphs (α-, β-, and ε-MnO₂) were synthesized, and their chemical/physical properties for CO oxidation were systematically studied using multiple techniques. Density functional theory (DFT) calculations and temperature-programmed experiments reveal that β-MnO₂ shows low energies for oxygen vacancy generation and excellent redox properties, exhibiting significant CO oxidation activity (T₉₀ = 75 °C) and stability even under a humid atmosphere. For the first time, we report that the specific reaction rate for β-MnO₂ (0.135 molecule_CO·nm^-2·s^-1 at 90 °C) is roughly approximately 4 and 17 times higher than that of ε-MnO₂ and α-MnO₂, respectively. The specific reaction rate order (β-MnO₂ > ε-MnO₂ > α-MnO₂) is not only in good agreement with reduction rates (CO-TPSR measurements) but also agrees with the DFT calculation. In combination with in situ spectra and intrinsic kinetic studies, the mechanisms of CO oxidation over various crystal structures of MnO₂ were proposed as well. We believe the new insights from this study will largely inspire the design of such a kind of catalyst.

Synergistic effects in Mn-Co mixed oxide supported on cordierite honeycomb for catalytic deep oxidation of VOCs

A series of Co-Mn mixed oxide catalyst supported on a cordierite monolith was facilely synthesized by ultrasonic impregnation. Its catalytic performance was evaluated in the combustion of toluene, ethyl acetate and its mixture. It was observed that with incorporating Mn into Co₃O₄, the formation of solid solution with spinel structure could significantly improve the catalytic activity of pure phase Co₃O₄. And the monolithic Co_0.67Mn_0.33O_x catalyst showed the best catalytic performance in the catalytic oxidation of toluene and ethyl acetate which could be completely oxidized at 220 and 180°C respectively under the reaction velocity (WHSV) about 45,000 mL/(g•hr) and pollutant concentration of 500 ppmV. The total conversion temperature of the VOCs mixture was at 230°C (500 ppmV toluene and 500 ppmV ethyl acetate) and determined by the temperature at which the most difficult molecule was oxidized. The excellent catalytic performance of monolithic Co_0.67Mn_0.33O_x was attributed to the higher content of Mn³⁺, Co³⁺, surface adsorbed oxygen and better redox ability. The prepared catalyst showed the good mechanical stability, reaction stability, and good adaptability to different reaction conditions.

Isotopic evidence for the tangled mechanism of the CO-PROX reaction over mixed and bare cobalt spinel catalysts

The catalytic performance of the bare Co3O4 and mixed cobalt-spinel catalysts (MxCo3−xO4; M = Cr, Mn, Fe, Ni, Cu, Zn) in the CO-PROX process was investigated in the temperature-programmed surface reaction (TPSR) mode using 18O2 as an oxidant.

Dynamics of Reactive Oxygen Species on Cobalt-Containing Spinel Oxides in Cyclic CO Oxidation

Reactive oxygen species (ROS) are considered to be responsible for the high catalytic activity of transition metal oxides like Co3-xFexO4 in oxidation reactions, but the detailed influences of catalyst composition and morphology on the formation of these reactive oxygen species are not fully understood. In the presented study, Co3O4 spinels of different mesostructures, i.e., particle size, crystallinity, and specific surface area, are characterized by powder X-ray diffraction, scanning electron microscopy, and physisorption. The materials were tested in CO oxidation performed in consecutive runs and compared to a Co3-xFexO4 composition series with a similar mesostructure to study the effects of catalyst morphology and composition on ROS formation. In the first run, the CO conversion was observed to be dominated by the exposed surface area for the pure Co-spinels, while a negative effect of Fe content in the spinels was seen. In the following oxidation run, a U-shaped conversion curve was observed for materials with high surface area, which indicated the in situ formation of ROS on those materials that were responsible for the new activity at low temperature. This activation was not stable at the higher reaction temperature but was confirmed after temperature-programmed oxidation (TPO). However, no activation after the first run was observed for low-surface-area and highly crystalline materials, and the lowest surface-area material was not even activated after TPO. Among the catalyst series studied here, a correlation of small particle size and large surface area with the ability for ROS formation is presented, and the benefit of a nanoscaled catalyst is discussed. Despite the generally negative effect of Fe, the highest relative activation was observed at intermediate Fe contents suggesting that Fe may be involved in ROS formation.

Mechanochemically Prepared Co3O4-CeO2 Catalysts for Complete Benzene Oxidation

Considerable efforts to reduce the harmful emissions of volatile organic compounds (VOCs) have been directed towards the development of highly active and economically viable catalytic materials for complete hydrocarbon oxidation. The present study is focused on the complete benzene oxidation as a probe reaction for VOCs abatement over Co3O4-CeO2 mixed oxides (20, 30, and 40 wt.% of ceria) synthesized by the more sustainable, in terms of less waste, less energy and less hazard, mechanochemical mixing of cerium hydroxide and cobalt hydroxycarbonate precursors. The catalysts were characterized by BET, powder XRD, H2-TPR, UV resonance Raman spectroscopy, and XPS techniques. The mixed oxides exhibited superior catalytic activity in comparison with Co3O4, thus, confirming the promotional role of ceria. The close interaction between Co3O4 and CeO2 phases, induced by mechanochemical treatment, led to strained Co3O4 and CeO2 surface structures. The most significant surface defectiveness was attained for 70 wt.% Co3O4-30 wt.% CeO2. A trend of the highest surface amount of Co3+, Ce3+ and adsorbed oxygen species was evidenced for the sample with this optimal composition. The catalyst exhibited the best performance and 100% benzene conversion was reached at 200 °C (relatively low temperature for noble metal-free oxide catalysts). The catalytic activity at 200 °C was stable without any products of incomplete benzene oxidation. The results showed promising catalytic properties for effective VOCs elimination over low-cost Co3O4-CeO2 mixed oxides synthesized by simple and eco-friendly mechanochemical mixing.

Recent advances in catalytic systems in the prism of physicochemical properties to remediate toxic CO pollutants: A state-of-the-art review

Carbon monoxide (CO) is the most harmful pollutant in the air, causing environmental issues and adversely affecting humans and the vegetation and then raises global warming indirectly. CO oxidation is one of the most effective methods of reducing CO by converting it into carbon dioxide (CO₂) using a suitable catalytic system, due to its simplicity and great value for pollution control. The CO oxidation reaction has been widely studied in various applications, including proton-exchange membrane fuel cell technology and catalytic converters. CO oxidation has also been of great academic interest over the last few decades as a model reaction. Many review studies have been produced on catalysts development for CO oxidation, emphasizing noble metal catalysts, the configuration of catalysts, process parameter influence, and the deactivation of catalysts. Nevertheless, there is still some gap in a state of the art knowledge devoted exclusively to synergistic interactions between catalytic activity and physicochemical properties. In an effort to fill this gap, this analysis updates and clarifies innovations for various latest developed catalytic CO oxidation systems with contemporary evaluation and the synergistic relationship between oxygen vacancies, strong metal-support interaction, particle size, metal dispersion, chemical composition acidity/basicity, reducibility, porosity, and surface area. This review study is useful for environmentalists, scientists, and experts working on mitigating the harmful effects of CO on both academic and commercial levels in the research and development sectors.

Surface oxygen Vacancies on Reduced Co3 O4 (100): Superoxide Formation and Ultra-Low-Temperature CO Oxidation

The activation of molecular oxygen is a fundamental step in almost all catalytic oxidation reactions. We have studied this topic and the role of surface vacancies for Co₃ O₄ (100) films with a synergistic combination of experimental and theoretical methods. We show that the as-prepared surface is B-layer terminated and that mild reduction produces oxygen single and double vacancies in this layer. Oxygen adsorption experiments clearly reveal different superoxide species below room temperature. The superoxide desorbs below ca. 120 K from a vacancy-free surface and is not active for CO oxidation while superoxide on a surface with oxygen vacancies is stable up to ca. 270 K and can oxidize CO already at the low temperature of 120 K. The vacancies are not refilled by oxygen from the superoxide, which makes them suitable for long-term operation. Our joint experimental/theoretical effort highlights the relevance of surface vacancies in catalytic oxidation reactions.

Graphene-supported 2D cobalt oxides for catalytic applications

2D materials are attracting increasing attention in many strategic applications. In particular, ultra-thin non-layered oxides have been shown to outperform their 3D counter-parts in several health and energy applications, such as the removal of toxic carbon monoxide by low temperature oxidation and the development of high performance supercapacitors. The general reason for that is the increased surface-to-volume ratio, which maximizes exposure of active species and enhances exchange between the (limited) bulk and the surface. The challenge is to synthesize such 2D configurations of 3D oxides, which generally requires quite harsh multi-step, multi-reagent chemical processes. Here we show that natural graphite can be used as a templating matrix to grow non-stoichiometric 2D transition metal oxides. We focus on highly porous, highly reduced cobalt oxides grown from cobalt nitrate and sodium borohydride under sonication. Extensive characterization, including nitrogen physisorption, thermogravimetric analysis (TGA), scanning and transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD), temperature programmed oxidation and reduction (TPO/TPR), Fourier transformed infrared (FTIR) and Raman spectroscopies, highlights the specific features of the 2D morphologies (nanosheets and nanofilms) obtained. For comparison, 3D morphologies of Co₃O₄ spinel nanocrystallites are grown from stacked 2D cobalt phthalocyanine-graphene precursors upon controlled thermal oxidation. Finally, low temperature CO oxidation catalysis evidences the superior performance of the graphene-supported CoO-like cobalt oxide 2D nanosheets.

Enhancement of catalytic toluene combustion over Pt-Co3O4 catalyst through in-situ metal-organic template conversion

A Pt-Co₃O₄ catalyst named Pt-Co(OH)₂-O was prepared by metal-organic templates (MOTs) conversion and used for catalytic oxidation of toluene. Through the conversion, the morphology of catalysts transformed from rhombic dodecahedron to nanosheet and the coated Pt nanoparticles (NPs) were more exposed. The Binding energy shift in XPS test indicates that the strong metal-support strong interaction (SMSI) has enhanced, and the physicochemical changes caused by it are characterized by other techniques. At the same time, Pt-Co(OH)₂-O showed the best catalytic performance (T₅₀ = 157 °C, T₉₀ = 167 °C, E_a = 40.85 kJ mol^-1, TOF_Pt = 2.68 × 10^-3 s^-1) and good stability. In addition, the in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies have shown that because SMSI weakened the Co-O bond, the introduction of Pt NPs can make the migration of oxygen in the catalyst easier. The change of binding energy change and the content of various species in the quasi in situ XPS experiment further confirmed that the Pt-Co(OH)₂-O catalyst has stronger SMSI, resulting in its stronger electron transfer ability and oxygen migration ability, which is conducive to catalytic reactions. This work provides new ideas for the development of supported catalysts and provides a theoretical reference for the relevant verification of SMSI.

Toward Informed Design of Nanomaterials: A Mechanistic Analysis of Structure-Property-Function Relationships for Faceted Nanoscale Metal Oxides

Nanoscale metal oxides (NMOs) have found wide-scale applicability in a variety of environmental fields, particularly catalysis, gas sensing, and sorption. Facet engineering, or controlled exposure of a particular crystal plane, has been established as an advantageous approach to enabling enhanced functionality of NMOs. However, the underlying mechanisms that give rise to this improved performance are often not systematically examined, leading to an insufficient understanding of NMO facet reactivity. This critical review details the unique electronic and structural characteristics of commonly studied NMO facets and further correlates these characteristics to the principal mechanisms that govern performance in various catalytic, gas sensing, and contaminant removal applications. General trends of facet-dependent behavior are established for each of the NMO compositions, and selected case studies for extensions of facet-dependent behavior, such as mixed metals, mixed-metal oxides, and mixed facets, are discussed. Key conclusions about facet reactivity, confounding variables that tend to obfuscate them, and opportunities to deepen structure-property-function understanding are detailed to encourage rational, informed design of NMOs for the intended application.

An efficient defect engineering strategy to enhance catalytic performances of Co3O4 nanorods for CO oxidation

Catalytic oxidation of CO at ambient temperature is an important reaction for many environmental applications. Here, we employed a defect engineering strategy to design an extraordinarily effective Sn-doped Co₃O₄ nanorods (NRs) catalyst for CO oxidation. Our combined theoretical and experimental data demonstrated that Co²⁺ in the lattice of Co₃O₄ were substituted by Sn⁴⁺. Based on a variety of characterizations and kinetic studies, this catalyst was found to combine the advantages of the nanorod-like morphology for largely exposing catalytically active Co³⁺ sites and the promotional effect of Sn dopant for adjusting the textural/redox properties. Additionally, the Sn-substituted Co₃O₄ NRs can be further activated via heat treatment to achieve low-temperature CO oxidation (T₁₀₀ ∼ -100 °C) with excellent stability at ambient temperature. This study reveals the importance of Sn-substitution of inactive Co²⁺ in Co₃O₄ and provides an ultra-efficient catalyst for CO oxidation, making this robust material one of the most powerful catalysts available up to now.

Carbon Monoxide Oxidation over rGO-Mediated Gold/Cobalt Oxide Catalysts with Strong Metal-Support Interaction

The strong interaction between Au nanoparticles and support (Au-metal oxide interface) usually governs the performance of a supported Au catalyst in heterogeneous catalysis. In this study, a series of Au/reduced graphene oxide (rGO)/three-dimensionally ordered macroporous (3DOM) Co₃O₄ catalysts with similar textural properties were prepared using the poly(methyl methacrylate)-templating and poly(vinyl alcohol)-protected reduction strategies. It was found that introducing reduced graphene oxide (rGO) as an electron-transfer bridge between Au and 3DOM Co₃O₄ could significantly strengthen the strong metal-support interaction (SMSI), thus enhancing the catalytic activity for CO oxidation. Among all of the catalysts, 1.86 wt % Au/2 wt % rGO/3DOM Co₃O₄ (1.86Au/2rGO/3DOM Co₃O₄) showed the highest catalytic activity: the CO reaction rate at 40 °C (432.8 μmol/(g_Au s)) was 2 times higher than that (208.2 μmol/(g_Au s)) over 1.87Au/3DOM Co₃O₄. The introduction of rGO could improve the activation of oxygen molecules and hence increase the low-temperature catalytic activity. The strategy for strengthening the SMSI via rGO mediation would guide the designing of highly efficient supported metal catalysts for low-temperature oxidation of CO.

On the Activity and Selectivity of CoAl and CoAlCe Mixed Oxides in Formaldehyde Production from Pulp Mill Emissions

Contaminated methanol has very good potential for being utilized in formaldehyde production instead of its destructive abatement. The activities, selectivities and stabilities of cobalt–alumina and cobalt–alumina–ceria catalysts prepared by the hydrotalcite-method were investigated in formaldehyde production from emissions of methanol and methanethiol. Catalysts were thoroughly characterized and the relationships between the characterization results and the catalytic performances were drawn. The preparation method used led to the formation of spinel-type structures in the form of Co2AlO4 based on x-ray diffraction (XRD) and Raman spectroscopy. Ceria seems to be present as CeO2, even though interaction with alumina is possible in the fresh catalyst. The same structure is maintained after pelletizing the cobalt–alumina–ceria catalyst. The cobalt–alumina–ceria catalyst was slightly better in formaldehyde production, probably due to lower redox temperatures and higher amounts of acidity and basicity. Methanol conversion is negatively affected by the presence of methanethiol; however, formaldehyde yields are improved. The stability of the pelletized catalyst was promising based on a 16 h experiment. During the experiment, cobalt was oxidized (Co2+ → Co3+), cerium was reduced (Ce4+ → Ce3+) and sulfates were formed, especially on the outer surface of the pellet. These changes affected the low temperature performance of the catalyst; however, the formaldehyde yield was unchanged.

Mesoporous cobalt monoxide-supported platinum nanoparticles: Superior catalysts for the oxidative removal of benzene

Mesoporous Co₃O₄ (meso-Co₃O₄)-supported Pt (0.53 wt.% Pt/meso-Co₃O₄) was synthesized via the KIT-6-templating and polyvinyl alcohol (PVA)-assisted reduction routes. Mesoporous CoO (meso-CoO) was fabricated through in situ reduction of meso-Co₃O₄ with glycerol, and the 0.18-0.69 wt.% Pt/meso-CoO samples were generated by the PVA-assisted reduction method. Meso-Co₃O₄ and meso-CoO were of cubic crystal structure and the Pt nanoparticles (NPs) with a uniform size of ca. 2 nm were well distributed on the meso-Co₃O₄ or meso-CoO surface. The 0.56 wt% Pt/meso-CoO (0.56Pt/meso-CoO) sample performed the best in benzene combustion (T_50% = 156 °C and T_90% = 186 °C at a space velocity of 80,000 mL/(g h)). Introducing water vapor or CO₂ with a certain concentration led to partial deactivation of 0.56 Pt/meso-CoO and such a deactivation was reversible. We think that the superior catalytic activity of 0.56 Pt/meso-CoO was intimately related to its good oxygen activation and benzene adsorption ability.

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.

Oxygen adsorption properties of small cobalt oxide clusters: application feasibility as oxygen gas sensors

Density functional theory calculations show chemical exothermic oxygen adsorption on cobalt oxide clusters with charge transfer from the clusters to oxygen.

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.