General Strategy toward Hydrophilic Single Atom Catalysts for Efficient Selective Hydrogenation

Zn Single-Atom Catalysts Enable the Catalytic Transfer Hydrogenation of α,β-Unsaturated Aldehydes

Highly active nonprecious-metal single-atom catalysts (SACs) toward catalytic transfer hydrogenation (CTH) of α,β-unsaturated aldehydes are of great significance but still are deficient. Herein, we report that Zn-N-C SACs containing Zn-N3 moieties can catalyze the conversion of cinnamaldehyde to cinnamyl alcohol with a conversion of 95.5% and selectivity of 95.4% under a mild temperature and atmospheric pressure, which is the first case of Zn-species-based heterogeneous catalysts for the CTH reaction. Isotopic labeling, in situ FT-IR spectroscopy, and DFT calculations indicate that reactants, coabsorbed at the Zn sites, proceed CTH via a "Meerwein-Ponndorf-Verley" mechanism. DFT calculations also reveal that the high activity over Zn-N3 moieties stems from the suitable adsorption energy and favorable reaction energy of the rate-determining step at the Zn active sites. Our findings demonstrate that Zn-N-C SACs hold extraordinary activity toward CTH reactions and thus provide a promising approach to explore the advanced SACs for high-value-added chemicals.

Improving Magnetic Resonance Imaging and Chemodynamic Therapy Properties via Tuning the Fe(II)/Fe(III) Ratio in Hydrophilic Single-Atom Nanobowls

We developed an intrinsic hydrophilic single-atom iron nanobowl (Fe-SANB) for magnetic resonance imaging (MRI)-guided tumor microenvironment-triggered cancer therapy. Benefiting from the sufficient exposure of Fe single atoms and the intrinsic hydrophilicity of the bowl-shaped structure, the Fe-SANBs exhibited a superior performance for T1-weighted MRI with an r1 value of 11.48 mM-1 s-1, which is 3-fold higher than that of the commercial Gd-DTPA (r1 = 3.72 mM-1 s-1). After further coembedding Gd single atoms in the nanobowls, the r1 value can be greatly improved to 19.54 mM-1 s-1. In tumor microenvironment (TME), the Fe-SANBs can trigger pH-induced Fenton-like activity to generate highly toxic hydroxyl radicals for high-efficiency chemodynamic therapy (CDT). Both the MRI and CDT efficiency of these nanobowls can be optimized by tuning the ratio of Fe(II)/Fe(III) in the Fe-SANBs via controlling the calcination temperature. Furthermore, the generation of •OH at the tumor site can be accelerated via the photothermal effect of Fe-SANBs, thus promoting CDT efficacy. Both in vitro and in vivo results confirmed that our nanoplatform exhibited high T1-weighted MRI contrast, robust biocompatibility, and satisfactory tumor treatment, providing a potential nanoplatform for MRI-guided TME-triggered precise cancer therapy.

Highly Selective Hydrogenation of Unsaturated Aldehydes in Aqueous Phase

Chemoselective hydrogenation of carbonyl in unsaturated aldehydes is a significant process in the chemical industry, in which the development of aqueous-phase reaction systems as a substitution to organic ones is challenging. Herein, we report Ir atomic cluster catalysts anchored onto WO3-x nanorods via a reduction treatment at various temperatures (denoted as Ir/WOx-T, T = 200, 300, 400, and 500 °C), which accelerates the chemoselective hydrogenation of carbonyl groups in aqueous solutions. The optimal catalyst Ir/WOx-300 exhibits exceptional activity (TOF value: 1313.7 min-1) and chemoselectivity toward cinnamaldehyde (CAL) hydrogenation to cinnamyl alcohol (COL) (yield: ∼98.0%) in water medium, which is, to the best of our knowledge, the highest level compared with previously reported heterogeneous catalysts in liquid-phase reaction. Ac-HAADF-STEM, XAFS, and XPS verify the formation of interface structure (Irδ+-Ov-W5+ (0 ≤ δ ≤ 4); Ov denotes oxygen vacancy) induced by metal-support interaction and the largest concentration of interfacial Ir (Irδ+) in Ir/WOx-300. In situ studies (Raman, FT-IR), isotopic labeling measurements combined with DFT calculations substantiate that the hydrogenation of the C=O group consists of two pathways: water-mediated hydrogenation (predominant) and direct hydrogenation via H2 dissociation (secondary). In the former case, W5+-Ov site accelerates the activation adsorption of H2O, while Ir0 site facilitates the H-H bond cleavage of H2 and Irδ+ promotes the CAL adsorption. H2O molecule, as the source of hydrogen species, participates directly in the hydrogenation of the carbonyl group through a hydrogen-bonded network, with a largely reduced energy barrier relative to the H2 dissociation path. This work demonstrates a green catalytic route that breaks the activity-selectivity trade-off toward the selective hydrogenation of unsaturated aldehydes, which shows great potential in heterogeneous catalysis.

Preparation and Application of Single-Atom Cobalt Catalysts in Organic Synthesis and Environmental Remediation

The development of high-performance catalysts plays a crucial role in facilitating chemical production and reducing environmental contamination. Single-atom catalysts (SACs), a class of catalysts that bridge the gap between homogeneous and heterogeneous catalysis, have garnered increasing attention because of their unique activity, selectivity, and stability in many pivotal reactions. Meanwhile, the scarcity of precious metal SACs calls for the arrival of cost-effective SACs. Cobalt, as a common non-noble metal, possesses tremendous potential in the field of single-atom catalysis. Despite their potential, reviews about single-atom Co catalysts (Co-SACs) are lacking. Accordingly, this review thoroughly summarized various preparation methodologies of Co-SACs, particularly pyrolysis; its application in the specific domain of organic synthesis and environmental remediation is discussed as well. The structure-activity relationship and potential catalytic mechanism of Co-SACs are elucidated through some representative reactions. The imminent challenges and development prospects of Co-SACs are discussed in detail. The findings and insights provided herein can guide further exploration and development in this charming area of catalyst design, leading to the realization of efficient and sustainable catalytic processes.

Atomic-Level Regulation of Cu-Based Electrocatalyst for Enhancing Oxygen Reduction Reaction: From Single Atoms to Polymetallic Active Sites

The slow kinetics of cathodic oxygen reduction reactions (ORR) in fuel cells and the high cost of commercial Pt-based catalysts limit their large-scale application. Cu-based single-atom catalysts (SACs) have received increasing attention as a promising ORR catalyst due to their high atom utilization, high thermodynamic activity, adjustable electronic structure, and low cost. Herein, the recent research progress of Cu-based catalysts is reviewed from single atom to polymetallic active sites for ORR. First, the design and synthesis method of Cu-based SACs are summarized. Then the atomic-level structure regulation strategy of Cu-based catalyst is proposed to improve the ORR performance. The different ORR catalytic mechanism based on the different Cu active sites is further revealed. Finally, the design principle of high-performance Cu-based SACs is proposed for ORR and the opportunities and challenges are further prospected.

Enhancing Photocatalysis: Understanding the Mechanistic Diversity in Photocatalysts Modified with Single-Atom Catalytic Sites

Surface modification of heterogeneous photocatalysts with single-atom catalysts (SACs) is an attractive approach for achieving enhanced photocatalytic performance. However, there is limited knowledge of the mechanism of photocatalytic enhancement in SAC-modified photocatalysts, which makes the rational design of high-performance SAC-based photocatalysts challenging. Herein, a series of photocatalysts for the aerobic degradation of pollutants based on anatase TiO2 modified with various low-cost, non-noble SACs (vanadate, Cu, and Fe ions) is reported. The most active SAC-modified photocatalysts outperform TiO2 modified with the corresponding metal oxide nanoparticles and state-of-the-art benchmark photocatalysts such as platinized TiO2 and commercial P25 powders. A combination of in situ electron paramagnetic resonance spectroscopy and theoretical calculations reveal that the best-performing photocatalysts modified with Cu(II) and vanadate SACs exhibit significant differences in the mechanism of activity enhancement, particularly with respect to the rate of oxygen reduction. The superior performance of vanadate SAC-modified TiO2 is found to be related to the shallow character of the SAC-induced intragap states, which allows for both the effective extraction of photogenerated electrons and fast catalytic turnover in the reduction of dioxygen, which translates directly into diminished recombination. These results provide essential guidelines for developing efficient SAC-based photocatalysts.

Interface Modification Using Li-Doped Hollow Titania Nanospheres for High-Performance Planar Perovskite Solar Cells

Titania nanospheres have been utilized as building blocks of electron transporting layers (ETLs) for mesoscopic perovskite solar cells (PSCs). Nevertheless, the power conversion efficiencies (PCEs) reported so far for the mesoscopic PSCs containing titania nanospheres are generally lower than those of the state-of-the-art planar PSCs. Here, we have prepared Li-doped hollow titania nanospheres (Li-HTS) through a "cation-exchange" approach and used them for the first time to modify the SnO2 ETL/perovskite interfaces of planar PSCs. The Li-HTS-modified PSC delivered a PCE of 23.28% with a fill factor (FF) of over 80%, which is significantly higher than the PCE of the control device (20.51%). This is the best PCE achieved for PSCs containing titania nanospheres. Moreover, interfacial modification using Li-HTS greatly improves the stability of the PSCs. This work demonstrates the potential of interface modification using inorganic nanostructures for enhancing the efficiency and stability of planar PSCs.

Single-Atom Gd Nanoprobes for Self-Confirmative MRI with Robust Stability

Gadolinium (Gd)-based complexes are extensively utilized as contrast agents (CAs) in magnetic resonance imaging (MRI), yet, suffer from potential safety concerns and poor tumor targeting. Herein, as a mimic of Gd complex, single-atom Gd nanoprobes with r₁ and r₂ values of 34.2 and 80.1 mM^-1 s^-1 (far higher than that of commercial Gd CAs) at 3 T are constructed, which possessed T₁ /T₂ dual-mode MRI with excellent stability and good tumor targeting ability. Specifically, single-atom Gd is anchored on nitrogen-doped carbon matrix (Gd-N_x C) through spatial-confinement method, which is further subjected to controllable chemical etching to afford fully etched bowl-shape Gd-N_x C (feGd-N_x C) with hydrophilic properties and defined coordination structure, similar to commercial Gd complex. Such nanostructures not only maximized the Gd³⁺ site exposure, but also are suitable for self-confirmative diagnosis through one probe with dual-mode MRI. Moreover, the strong electron localization and interaction between Gd and N atoms afforded feGd-N_x C excellent kinetic inertness and thermal stability (no significant Gd³⁺ leaching is observed even incubated with Cu²⁺ and Zn²⁺ for two months), providing a creative design protocol for MRI CAs.

Recent advances in the theoretical studies on the electrocatalytic CO2 reduction based on single and double atoms

Excess of carbon dioxide (CO2) in the atmosphere poses a significant threat to the global climate. Therefore, the electrocatalytic carbon dioxide reduction reaction (CO2RR) is important to reduce the burden on the environment and provide possibilities for developing new energy sources. However, highly active and selective catalysts are needed to effectively catalyze product synthesis with high adhesion value. Single-atom catalysts (SACs) and double-atom catalysts (DACs) have attracted much attention in the field of electrocatalysis due to their high activity, strong selectivity, and high atomic utilization. This review summarized the research progress of electrocatalytic CO2RR related to different types of SACs and DACs. The emphasis was laid on the catalytic reaction mechanism of SACs and DACs using the theoretical calculation method. Furthermore, the influences of solvation and electrode potential were studied to simulate the real electrochemical environment to bridge the gap between experiments and computations. Finally, the current challenges and future development prospects were summarized and prospected for CO2RR to lay the foundation for the theoretical research of SACs and DACs in other aspects.

The synthesis of single-atom catalysts for heterogeneous catalysis

Heterogeneous catalysis is an important class of reactions in industrial production, especially in green chemical synthesis, and environmental and organic catalysis. Single-atom catalysts (SACs) have emerged as promising candidates for heterogeneous catalysis, due to their outstanding catalytic activity, high selectivity, and maximum atomic utilization efficiency. The high specific surface energy of SACs, however, results in the migration and aggregation of isolated atoms under typical reaction conditions. The controllable preparation of highly efficient and stable SACs has been a serious challenge for applications. Herein, we summarize the recent progress in the precise synthesis of SACs and their different heterogeneous catalyses, especially involving the oxidation and reduction reactions of small organic molecules. At the end of this review, we also introduce the challenges confronted by single-atom materials in heterogeneous catalysis. This review aims to promote the generation of novel high-efficiency SACs by providing an in-depth and comprehensive understanding of the current development in this research field.

Emerging materials for electrochemical CO2 reduction: progress and optimization strategies of carbon-based single-atom catalysts

The electrochemical CO₂ reduction reaction can effectively convert CO₂ into promising fuels and chemicals, which is helpful in establishing a low-carbon emission economy. Compared with other types of electrocatalysts, single-atom catalysts (SACs) immobilized on carbon substrates are considered to be promising candidate catalysts. Atomically dispersed SACs exhibit excellent catalytic performance in CO₂RR due to their maximum atomic utilization, unique electronic structure, and coordination environment. In this paper, we first briefly introduce the synthetic strategies and characterization techniques of SACs. Then, we focus on the optimization strategies of the atomic structure of carbon-based SACs, including adjusting the coordination atoms and coordination numbers, constructing the axial chemical environment, and regulating the carbon substrate, focusing on exploring the structure-performance relationship of SACs in the CO₂RR process. In addition, this paper also briefly introduces the diatomic catalysts (DACs) as an extension of SACs. At the end of the paper, we summarize the article with an exciting outlook discussing the current challenges and prospects for research on the application of SACs in CO₂RR.