Green fabrication of hierarchically-structured Pt/bio-CeO2 nanocatalysts using natural pollen templates for low-temperature CO oxidation

Natural Biogenic Templates for Nanomaterial Synthesis: Advances, Applications, and Environmental Perspectives

This review explores the use of biogenic templates in nanomaterial synthesis, emphasizing their role in promoting environmentally sustainable nanotechnology. It categorizes various biogenic templates, including agricultural byproducts and microorganisms, stating their suitability for forming nanostructures due to their distinct properties. A comparative analysis of monostep and multistep synthesis methods is provided, focusing on their efficiencies and outcomes when using biogenic templates. Further, this review also highlights how these templates can generate complex nanostructures and hybrid materials with enhanced functionalities. Applications of biogenic templates across biomedicine, biotechnology, environmental science, and energy are discussed along with their utilization scope in agriculture and electronics. Benefits from nanostructures from biotemplates include sustainability, low cost, and reduced toxicity, but challenges like scalability, reproducibility, and regulatory compliance persist. Future research focuses on improving synthesis techniques, discovering new templates, and evaluating environmental and cytotoxic impacts, especially for biomedical uses. In conclusion, the review reaffirms the potential of biogenic templates in sustainable nanomaterial synthesis while highlighting the ongoing challenges that need to be addressed for broader adoption.

Non-Stoichiometric BaxMn0.7Cu0.3O3 Perovskites as Catalysts for CO Oxidation: Optimizing the Ba Content

In this work, a series of BaxMn0.7Cu0.3O3 samples (x: 1, 0.9, 0.8, and 0.7, BxMC) was synthesized, characterized, and used as catalysts for CO oxidation reaction. All formulations were active for CO oxidation in the tested conditions. A correlation between the electrical conductivity, obtained by impedance spectroscopy, and the reducibility of the samples, obtained by H2-TPR, was observed. The Ba0.8Mn0.7Cu0.3O3 composition (B0.8MC) showed the best catalytic performance (comparable to that of the 1% Pt/Al2O3 reference sample) during tests conducted under conditions similar to those found in the exhaust gases of current gasoline engines. The characterization data suggest the simultaneous presence of a high Mn(IV)/Mn(III) surface ratio, oxygen vacancies, and reduced copper species, these two latter being key properties for ensuring a high CO conversion percentage as both are active sites for CO oxidation. The reaction temperature and the reactant atmosphere composition seem to be the most important factors for achieving a good catalytic performance, as they strongly determine the location and stability of the reduced copper species.

Enhancing the Performance of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) Perovskites as Catalysts for CO Oxidation by Decreasing the Ba Content

Mixed oxides featuring perovskite-type structures (ABO3) offer promising catalytic properties for applications focused on the control of atmospheric pollution. In this work, a series of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) samples have been synthesized, characterized and tested as catalysts for CO oxidation reaction in conditions close to that found in the exhausts of last-generation automotive internal combustion engines. All samples were observed to be active as catalysts for CO oxidation during CO-TPRe tests, with Ba0.7MnO3 (B0.7M) being the most active one, as it presents the highest amount of oxygen vacancies (which act as active sites for CO oxidation) and Mn (IV), which features the highest levels of reducibility and the best redox properties. B0.7M has also showcased a high stability during reactions at 300 °C, even though a slightly lower CO conversion is achieved during the second consecutive reaction cycle. This performance appears to be related to the decrease in the Mn (IV)/Mn (III) ratio.

Construction of Immobilized Enzymes with Yeast and Metal-Organic Frameworks for Enhanced Biocatalytic Activities

Metal-organic frameworks (MOFs) have become promising host materials for enzyme immobilization and protection. Herein, ZIF-8 nanocubes were successfully self-assembled onto yeast as a biological template to obtain hybrid Y@ZIF-8. The size, morphology, and loading efficiency of ZIF-8 nanoparticles assembled on yeast templates can be well-regulated by adjusting the various synthetic parameters. Particularly, the amount of water significantly affected the particle size of ZIF-8 assembled on yeast. Through using a cross-linking agent, the relative enzyme activity of Y@ZIF-8@t-CAT could be greatly enhanced and remained the highest even after seven consecutive cycles, with improved cycling stability, as compared to that of Y@ZIF-8@CAT. In addition to the effect of the physicochemical properties of Y@ZIF-8 on the loading efficiency, the temperature tolerance, pH tolerance, and storage stability of Y@ZIF-8@t-CAT were also systematically investigated. Importantly, the catalytic activity of free catalase was decreased to 72% by 45 days, while the activity of the immobilized catalase remained above 99%, suggesting good storage stability. The present work demonstrates that yeast-templated ZIF-8 nanoparticles have a high potential to be used as biocompatible immobilization materials and are promising candidates for the preparation of effective biocatalysts in biomedicine applications.

Fabrication of supported Pt/CeO2 nanocatalysts doped with different elements for CO oxidation: theoretical and experimental studies

Supported Pt/CeO₂ catalysts have been widely used in carbon monoxide (CO) oxidation; however, the high oxygen vacancy formation energy (E_vac) in the process leads to the poor performance of these catalysts. Herein, we explored different element (Pr, Cu, or N) doped CeO₂ supports using Ce-based metal-organic frameworks (MOFs) as precursors via calcination treatment. The obtained CeO₂ supports were used to load Pt nanoparticles. These catalysts were systematically characterized by various techniques, and they showed superior catalytic activity for CO oxidation compared to undoped catalysts which could be attributed to the formation of Ce³⁺, and high amounts of O_ads/(O_ads + O_lat) and Pt^δ+/Pt_total. Moreover, density functional theory calculations with on-site Coulomb interaction correction (DFT+U) were performed to provide atomic-scale insights into the reaction process by the Mars-van Krevelen (M-vK) mechanism, which revealed that the element-doped catalysts could simultaneously reduce the adsorption energies of CO and lower reaction energy barriers in the *OOCO associative pathway.

Effects of the Crystalline Properties of Hollow Ceria Nanostructures on a CuO-CeO2 catalyst in CO Oxidation

The development of an efficient and economic catalyst with high catalytic performance is always challenging. In this study, we report the synthesis of hollow CeO2 nanostructures and the crystallinity control of a CeO2 layer used as a support material for a CuO-CeO2 catalyst in CO oxidation. The hollow CeO2 nanostructures were synthesized using a simple hydrothermal method. The crystallinity of the hollow CeO2 shell layer was controlled through thermal treatment at various temperatures. The crystallinity of hollow CeO2 was enhanced by increasing the calcination temperature, but both porosity and surface area decreased, showing an opposite trend to that of crystallinity. The crystallinity of hollow CeO2 significantly influenced both the characteristics and the catalytic performance of the corresponding hollow CuO-CeO2 (H-Cu-CeO2) catalysts. The degree of oxygen vacancy significantly decreased with the calcination temperature. H-Cu-CeO2 (HT), which presented the lowest CeO2 crystallinity, not only had a high degree of oxygen vacancy but also showed well-dispersed CuO species, while H-Cu-CeO2 (800), with well-developed crystallinity, showed low CuO dispersion. The H-Cu-CeO2 (HT) catalyst exhibited significantly enhanced catalytic activity and stability. In this study, we systemically analyzed the characteristics and catalyst performance of hollow CeO2 samples and the corresponding hollow CuO-CeO2 catalysts.