In situ studies on ceria promoted cobalt oxide for CO oxidation

Heterojunction-Mediated Co-Adjustment of Band Structure and Valence State for Achieving Selective Regulation of Semiconductor Nanozymes

Improving reaction selectivity is the next target for nanozymes to mimic natural enzymes. Currently, the majority of strategies in this field are exclusively applicable to metal-organic-based or organic-based nanozymes, while limited in regulating metal oxide-based semiconductor nanozymes. Herein, taking semiconductor Co3O4 as an example, a heterojunction strategy to precisely regulate nanozyme selectivity by simultaneously regulating three vital factors including band structure, metal valence state, and oxygen vacancy content is proposed. After introducing MnO2 to form Z-scheme heterojunctions with Co3O4 nanoparticles, the catalase (CAT)-like and peroxidase (POD)-like activities of Co3O4 can be precisely regulated since the introduction of MnO2 affects the position of the conduction bands, preserves Co in a higher oxidation state (Co3+), and increases oxygen vacancy content, enabling Co3O4-MnO2 exhibit improved CAT-like activity and reduced POD-like activity. This study proposes a strategy for improving reaction selectivity of Co3O4, which contributes to the development of metal oxide-based semiconductor nanozymes.

Resource utilization of high-concentration SO2 for sulfur production over La-Ce-Ox composite oxide catalyst

Sulfur dioxide is one of the main causes of air pollution such as acid rain and photochemical smog, and its pollution control and resource utilization have become important research directions. La₂O₃ was incorporated into CeO₂ to enhance the surface basicity of La-Ce-O_x catalyst and increase the concentration of chemisorbed oxygen, thereby promoting the improvement of catalytic performance of SO₂ reduction by CO. Results have showed that the incorporation of La₂O₃ would not only increase the concentration of chemisorbed oxygen and hydroxyl on the catalyst surface, but also increase the basicity of the catalyst, thereby facilitating the adsorption of SO₂ on the catalyst surface. The 12%La-Ce-O_x was the optimal catalyst, and its SO₂ conversion at 350-400 ℃ reached close to 100%, and the sulfur yield at this temperature range was higher than 93%. Finally, according to the in situ infrared diffuse reflectance spectrum, it was found that the main reaction intermediates of 12%La-Ce-O_x in the catalytic reduction of SO₂ were weakly adsorbed sulfate, SO₃^2-, non-coordinating CO₃^2-, monodentate carbonate, and CO, so the catalytic reaction followed the L-H and E-R mechanisms simultaneously.

Electronic metal-support interaction constructed for preparing sinter-resistant nano-platinum catalyst with redox property

Generally, support materials with particular structural properties could effectively anchor metal nanoparticles and provide lower activation barriers in heterogeneous catalysis. To tailor the structure of stable iron oxide, NiFe₂O₄ of inverse spinel structure was obtained by combining nickel with iron element under an alkaline environment and high-temperature calcination. The p-type conductivity of NiFe₂O₄ provides the possibility of constructing electronic interfacial interaction with Pt nanoparticles by electron transfer. The constructed metal-support interaction could effectively stabilize Pt nanoparticles and be further enhanced during long-term harsh calcination (700 °C for 48 h) even under an O₂ atmosphere. Meanwhile, the abundant structural defects of NiFe₂O₄ are beneficial for constructing low-temperature redox centers with the aid of Pt nanoparticles. Pt/NiFe₂O₄ exhibited not only excellent activity in room-temperature oxidation (CO and HCHO) and reduction reactions (chemo-selective hydrogenation of nitroarenes), but also high stability even after storage for more than 6 months. A self-adjusting mechanism triggered by structural defects is disclosed by in situ characterization and systematic reaction results. This work demonstrates an alternative concept to construct sinter-resistant and highly-effective nano-platinum catalysts robust for oxidation and reduction reactions.

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.

Exsolution of Iron Oxide on LaFeO3 Perovskite: A Robust Heterostructured Support for Constructing Self-Adjustable Pt-Based Room-Temperature CO Oxidation Catalysts

Constructing highly active and stable surface sites for O₂ activation is essential to lower the barrier of Pt-based catalysts for CO oxidation. Although a few active Pt-metal oxide interfaces have been reported, questions about the stability of these sites under the long-term storage and operation remain unresolved. Here, based on developing a robust FeO_x/LaFeO₃ heterostructure as a support, we constructed stable Pt-support interfaces to achieve highly active CO oxidation at room temperature. Even after it is kept in the air for more than 6 months, the catalyst (without pretreatment) still maintains the high activity like a fresh one, which is superior to metal hydroxide-Pt interfaces, and meets the requirements of long-term storage for emergency use. In situ characterizations and systematic reaction results showed that CO oxidation occurs through an alternative mechanism, which is triggered by intrinsic reactants and self-adjusted to a more active interface in the reaction process. Theoretical calculations and ⁵⁷Fe Mössbauer spectra revealed that abundant cation vacancies significantly increase the activity of surface oxygen species and should be responsible for this unique process. This work demonstrates an alternative concept to fabricate robust and highly active Pt-based catalysts for catalytic oxidation.

A highly active Rh1/CeO2 single-atom catalyst for low-temperature CO oxidation

Rh₁/CeO₂ SAC is highly active for CO oxidation, which exhibits a high TOF of 0.41 s⁻¹via the Mars–van Krevelen mechanism.