Atomic Aerogel Materials (or Single-Atom Aerogels): An Interesting New Paradigm in Materials Science and Catalysis Science

Enhancing activity and stability of FeNC catalysts through co incorporation for oxygen reduction reaction

FeNC single atom catalysts (SACs) have attracted great interest due to their highly active FeN4 sites. However, the pyrolysis treatment often leads to inevitable metal migration and aggregation, which reduces the catalytic activity. Moreover, due to the Fenton reaction caused by FeNC in alkaline and acidic solutions, the presence of Fe and peroxide in electrodes may generate free radicals, resulting in serious degradation of the organic ionomer and the membrane. Herein, we report an original strategy of introducing Co single atoms into FeNC catalysts, forming atomically dispersed bimetallic active sites (FeCoNC) and improving the activity and stability of the catalyst. Benefiting from this strategy, FeCoNC catalyst exhibits excellent oxygen reduction reaction (ORR) activity in alkaline media (E1/2 = 0.88 V) and in acidic media (E1/2 = 0.77 V). As the cathode of Zn-air battery (ZAB), FeCoNC shows an excellent peak power density of 142.8 mW cm-2 and a specific capacity of 806.6 mAh/gZn. This work provides a novel avenue to optimize and enhance the ORR performance of atomic dispersed FeNC catalysts.

Atomic Engineering of Single-Atom Nanozymes for Biomedical Applications

Single-atom nanozymes (SAzymes) showcase not only uniformly dispersed active sites but also meticulously engineered coordination structures. These intricate architectures bestow upon them an exceptional catalytic prowess, thereby captivating numerous minds and heralding a new era of possibilities in the biomedical landscape. Tuning the microstructure of SAzymes on the atomic scale is a key factor in designing targeted SAzymes with desirable functions. This review first discusses and summarizes three strategies for designing SAzymes and their impact on reactivity in biocatalysis. The effects of choices of carrier, different synthesis methods, coordination modulation of first/second shell, and the type and number of metal active centers on the enzyme-like catalytic activity are unraveled. Next, a first attempt is made to summarize the biological applications of SAzymes in tumor therapy, biosensing, antimicrobial, anti-inflammatory, and other biological applications from different mechanisms. Finally, how SAzymes are designed and regulated for further realization of diverse biological applications is reviewed and prospected. It is envisaged that the comprehensive review presented within this exegesis will furnish novel perspectives and profound revelations regarding the biomedical applications of SAzymes.

The Contact Interface Electronic Coupling of Cobalt and Zirconia Enables Stable and Highly Efficient 4e- Oxygen Reduction Reaction Catalysis

Cobalt (Co) is an efficient oxygen reduction reaction (ORR) catalyst but suffers from issues of easy deactivation and instability. Here, it shows that ZrO2 can stabilize Co through interface electron coupling and enables highly efficient 4e- ORR catalysis. Porous carbon nanofibers loaded with dispersed Co-nanodots (≈10 nm, 9.63 wt%) and ZrO2 nanoparticles are synthesized as the catalyst. The electron transfer from the metallic Co to ZrO2 causes interface-oriented electron enrichment that promotes the activation and conversion of O2, improving the efficiency of 4e- transfer. Moreover, the simulation results show that ZrO2 acts like an electron reservoir to store electrons from Co and slowly release them to the interface, solving the easy deactivation problem of Co. The catalyst exhibits a high half-wave potential (E1/2) of 0.84 V, which only decreases by 3.6 mV after 10 000 cycles, showing great stability. Particularly, the enhanced spin polarization of Co in a magnetic field reinforces the interface electron coupling that increases the E1/2 to 0.864 V and decreases the energy barrier of ORR from 0.81 to 0.63 eV, confirming that the proposed strategy is effective for constructing efficient and stable ORR catalysts.

Ultralight and Robust Covalent Organic Framework Fiber Aerogels

Shaping covalent organic frameworks (COFs) into macroscopic objects with robust mechanical properties and hierarchically porous structure is of great significance for practical applications but remains formidable and challenging. Herein, a general and scalable protocol is reported to prepare ultralight and robust pure COF fiber aerogels (FAGs), based on the epitaxial growth synergistic assembly (EGSA) strategy. Specifically, intertwined COF nanofibers (100-200 nm) are grown in situ on electrospinning polyacrylonitrile (PAN) microfibers (≈1.7 µm) containing urea-based linkers, followed by PAN removal via solvent extraction to obtain the hollow COF microfibers. The resultant COF FAGs possess ultralow density (14.1-15.5 mg cm-3 ) and hierarchical porosity that features both micro-, meso-, and macropores. Significantly, the unique interconnected structure composed of nanofibers and hollow microfibers endows the COF FAGs with unprecedented mechanical properties, which can fully recover at 50% strain and be compressed for 20 cycles with less than 5% stress degradation. Moreover, the aerogels exhibit excellent capacity for organic solvent absorption (e.g., chloroform uptake of >90 g g-1 ). This study opens new avenues for the design and fabrication of macroscopic COFs with excellent properties.

Au-CeO2 composite aerogels with tunable Au nanoparticle sizes as plasmonic photocatalysts for CO2 reduction

Tuning the size of Au nanoparticles is always an interesting task when constructing Au/semiconductor heterojunctions for surface plasmon resonance-enhanced photocatalysis. In particular, the size of Au nanoparticles in the newly emerging "plasmonic aerogel" photocatalyst concept could approach the size of the semiconductor phase. This work provides an alternative route to realize the size tuning of Au nanoparticles in Au-CeO2 composite aerogels to some extent, within the framework of the well-established epoxide addition sol-gel method. The size tuning is achieved by exploiting the multi-functionalities of a mixed organic acid additive containing a thiol group in the gelation step. The obtained aerogel photocatalysts are composed of a porous backbone of interconnected CeO2 nanoparticles and Au nanoparticles, and the size of Au nanoparticles ranges from ∼30 nm to sub-10 nm, while the size of CeO2 remains at ∼15-10 nm. The surface plasmon resonance peak position and intensity contributed by the Au nanoparticles then vary accordingly. Photocatalytic CO2 reduction at the gas-solid interface is chosen as a model reaction to study the effect of Au nanoparticle size on the photocatalytic activity of composite aerogel photocatalysts. The addition of Au nanoparticles undoubtedly enhances the overall activity of the CeO2 aerogel photocatalyst, while the degree of enhancement (in terms of total charge consumption) and product selectivity (CH4 or CO) are different and correlated with the size of the Au nanoparticles. The best performance can be achieved in a composite in which the Au sizes are the smallest.

Surface-Immobilized ZnNx Sites as High-Performance Catalysts for Continuous Flow Knoevenagel Condensation in Water

By immobilizing the metal complex on the substrate surface, our previous results have demonstrated that heterogeneous catalysts with well-dispersed active MNC (metal-nitrogen-carbon) sites can be prepared in a rational and efficient manner. In this study, we employed agarose aerogel (AA) as the substrate to illustrate a straightforward strategy for immobilizing ZnNx sites on the surface. Under relatively low temperatures, the amine group of the ligand condenses with the surface carbonyl group generated in situ, resulting in the surface immobilized Zn sites. This can be supported by the IR, PXRD, and XPS data. Comprehensive characterization methods, including synchrotron powder XRD and spherical aberration-corrected TEM, confirmed the absence of ZnNx site aggregation in the surface immobilization process, even with a high Zn content (up to 8 wt %). The immobilized ZnNx sites exhibited high catalytic performance in Knoevenagel condensation, and α,β-unsaturated compounds were obtained with high yield in both batch and continuous flow reactions. AA-ZnNx-200 showed the best catalytic activity, which was processed under 200 °C with a Zn content of 4.62 wt %. The immobilized ZnNx sites activated both the aldehyde and nitrile substrates, which were quantitatively converted into the corresponding α,β-unsaturated compounds, with water as the solvent at room temperature. In continuous flow reaction conditions, a conversion rate up to 99% can be achieved with malononitrile. This heterogeneous catalyst can be facilely produced with quantitative yield in a large scale from cheap starting material under mild conditions. No catalyst deactivation was observed after seven batch reaction cycles or 80 h of continuous flow reaction, indicating its high robustness under catalytic reaction conditions. This catalyst enables a separation-free, energy-saving, and environment-friendly production process, offering a practical way for the industrial production.

Scalable Synthesis of Flexible Single-Atom Monolithic Catalysts for High-Efficiency, Durable CO Oxidation at Low Temperature

The creation of single-atom catalysts in a large-size, high-yield, and stable form represents an important direction for high-efficiency industrial catalysis in the future. Herein, we report a strategy to synthesize flexible single-atom monolithic catalysts (SAMCs) based on the hierarchical 3D assembly of single-atom-loaded oxide ceramic nanofibers. The nanofibers, which can be produced in a continuous and scalable manner, serve as an ideal support for single atoms spontaneously and almost completely exposed at the surface through the Kirkendall effect-enabled in situ ion migration during the spinning process, resulting in both high yield and large loading quantity. Moreover, the hierarchical 3D assembly of these nanofibers into a porous, flexible structure endows the SAMCs with the advantages of sufficient infiltration and oscillation tolerance when faced with high-throughput gaseous media, leading to both high catalytic efficiency and excellent durability. As a proof-of-concept demonstration, a Pt SAMC is synthesized, which exhibits 100% CO oxidation at low temperature (∼170 °C), excellent invariance toward high-frequency (10 Hz) oscillation, and high structural stability from 25 to 300 °C. This work is beneficial for the large-scale production of SAMCs in broad industrial applications.