Understanding the structure-performance relationship of active sites at atomic scale

Currently, more than 86% of global energy consumption is still mainly dependent on traditional fossil fuels, which causes resource scarcity and even emission of high amounts of carbon dioxide (CO2), resulting in a severe “Greenhouse effect.” Considering this situation, the concept of “carbon neutrality” has been put forward by 125 countries one after another. To achieve the goals of “carbon neutrality,” two main strategies to reduce CO2 emissions and develop sustainable clean energy can be adopted. Notably, these are crucial for the synthesis of advanced single-atom catalysts (SACs) for energy-related applications. In this review, we highlight unique SACs for conversion of CO2 into high-efficiency carbon energy, for example, through photocatalytic, electrocatalytic, and thermal catalytic hydrogenation technologies, to convert CO2 into hydrocarbon fuels (CO, CH4, HCOOH, CH3OH, and multicarbon [C2+] products). In addition, we introduce advanced energy conversion technologies and devices to replace traditional polluting fossil fuels, such as photocatalytic and electrocatalytic water splitting to produce hydrogen energy and a high-efficiency oxygen reduction reaction (ORR) for fuel cells. Impressively, several representative examples of SACs (including d-, ds-, p-, and f-blocks) for CO2 conversion, water splitting to H2, and ORR are discussed to describe synthesis methods, characterization, and corresponding catalytic activity. Finally, this review concludes with a description of the challenges and outlooks for future applications of SACs in contributing toward carbon neutrality. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/cey2.194

Single‐atom catalysis for carbon neutrality

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single‐atom catalysts (SACs), the comprehensive understanding of SACs, dual‐atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state‐of‐the‐art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Isolating Single and Few Atoms for Enhanced Catalysis

Superhydrophobic nanohybrid sponges for separation of oil/ water mixtures.

Oil-in-water and water-in-oil emulsions formation and demulsification

Cobalt species as active sites for photocatalytic reduction of CO2 to valuable products such as methanol have received increasing attention, however, it remains a huge challenge to achieve the high activity. Herein, a pyrolysis-induced-vaporization strategy was successfully employed to fabricate Co/g-C3N4 single-atom catalysts (Co/g-C3N4 SACs) with surface Co atom loading up to 24.6 wt%. Systematic investigation of Co/g-C3N4 SACs formation process disclosed that concentrated-H2SO4 exfoliation of g-C3N4 nanosheets (g-C3N4 NSs) as the substrate followed by a two-step calcination process is essential to achieve ultrahigh metal loading. It was found that the ultrahigh-density of Co single-atom sites were anchored on the g-C3N4 substrate surface and coordinated with two nitrogen and one carbon atoms (Co-N2C). These single dispersed Co-N2C sites on the g-C3N4 surface were found to act not only as electron gathering centers but also as the sites of CO2 adsorption and activation, subsequently, boosting the photocatalytic methanol generation during light irradiation. As a result, the methanol formation rate at 4 h (941.9 μmol g−1) over Co/g-C3N4-0.2 SAC with 24.6 wt% surface Co loading was 13.4 and 2.2 times higher than those of g-C3N4 (17.7 μmol g−1) and aggregated CoOx/g-C3N4-0.2 (423.9 μmol g−1), respectively. Simultaneously, H2 (18.9 μmol g−1 h−1), CO (2.9 μmol g−1 h−1), CH4 (3.4 μmol g−1 h−1), C2H4 (1.1 μmol g−1 h−1), C3H6 (1.4 μmol g−1 h−1), and CH3OCH3 (3.3 μmol g−1 h−1) products were detected over Co/g-C3N4-0.2 SAC. Besides, the photocatalytic activity of the Co/g-C3N4-0.2 SAC for the reduction of CO2 to methanol was stable within 12-cycle experiments (~48 h). This work paves a strategy to boost the photoreduction CO2 activity via loading ultrahigh surface density single atomically dispersed cobalt active sites.

Ultrahigh surface density of Co-N2C single-atom-sites for boosting photocatalytic CO2 reduction to methanol

One of the main challenges for the treatment of oily wastewater is emulsion separation. However, emulsions are generally more stable and difficult to separate. Herein, we report a facile strategy f...

3D Superhydrophobic Sponge Coated with Magnesium Hydroxidefor Effective Oil/Water Mixture and Emulsion Separation

Demulsification is highly important for both dehydration of crude oil and environmental remediation. The demulsification performance of conventional demulsifiers is limited by the condition and type of emulsion. Herein, we demonstrated the unique advantage of a metal–organic framework (MOF), MIL-100(Fe), for effective demulsification. The demulsification efficiency (DE) of MIL-100(Fe) for oil-in-water (O/W) model emulsion exceeds 99% within 30 min under optimal conditions. For the water-in-oil (W/O) crude oil emulsion, the DE was up to 79% within 5 min. MIL-100(Fe) also exhibited excellent demulsification performance against various  pH and salinity, which were supported by astable DE among the pH values of 2.0–10.0 and a positive relationship with salinity (1–1000 mmol L−1 of NaCl solution). It was found that the amphipathicity of MIL-100(Fe) is crucial for demulsification, and the electrostatic interaction has an important role in regulating the demulsification. MIL-100(Fe) degraded sodium dodecyl sulfate by abstracting SO32−, destroying the function of the surfactant and eliminating the potential risk of emulsifying again. This study exhibited the potential of MOFs on demulsification for both O/W and W/O emulsion, and confirmed that enhancing the interaction between the demulsifier and emulsion is an effective strategy for universal demulsification against harsh conditions.

Effective demulsification for oil–water separation through metal–organic frameworks with an amphipathic micro-domain

Emulsion poses a greater challenge for the remediation of oily wastewater, which can be effectively resolved by the metal-organic framework of MIL-100(Fe). The formula Fe3O(H2O)2(OH) (BTC)2 pronounces that MIL-100(Fe) suffers from an intrinsic defect of less charged atoms, which limits its demulsification performance for oil-water separation. Herein, cations of the ionic liquid (1-allyl-3-methylimidazolium, Amim+) were encapsulated in the micropore of MIL-100(Fe) in situ to increase the positive charge density of MIL-100(Fe). Zeta potential demonstrated that the encapsulation of Amim+ increased the positive charge amount of MIL-100(Fe). N2 probe isothermal adsorption/desorption and spectral measurements (X-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflection spectroscopy, and attenuated total-reflectance infrared spectroscopy) revealed the host-guest interactions of π···Fe complexation and π···cation electrostatic attraction between Amim+ and MIL-100(Fe) for the composite materials. Amim+ encapsulation greatly enhanced the demulsification performance of MIL-100(Fe) for oil-in-water (O/W) emulsion stabilized by sodium dodecyl sulfate. Amim+-encapsulated MIL-100(Fe) with an Amim+/Fe3+ molar ratio of 1:1 [Amim@MIL-100(Fe)-3:3] showed a demulsification efficiency (DE) of 94% within 30 s, compared with MIL-100(Fe) within 30 min. The maximum DE of Amim@MIL-100(Fe)-3:3 was found to be more than 98% within 5 min. The DE lost by MIL-100(Fe) at the third run decreased from 36 to 17% after encapsulating Amim+. The analysis of surface charge and interfacial tension implied a demulsification mechanism of capturing-fusion, which could be promoted by the greater electrostatic attraction. Finally, the role of Amim+ on the outstanding demulsification performance by Amim+-encapsulated MIL-100(Fe) could be explained by the enhanced nonbonded interaction of electrostatic attraction and van der Waals based on the molecular dynamics simulation.

Enhancing Demulsification Performance for Oil-Water Separation through Encapsulating Ionic Liquids in the Pore of MIL-100(Fe).

Discharge of oily sewage causes serious environmental pollution and waste of resources, which can be solved by the process of oil/water separation. Polyurethane foam is a kind of promising candidate material for oil/water separation. In this work, a series of hydrophobic/lipophilic polyurethane foams were prepared by the in situ self-foaming method. The as-prepared polyurethane foams exhibited abundant open pore structures, excellent stability and superior hydrophobicity/lipophilicity. Thus, the as-prepared polyurethane foams can selectively absorb various oils/organic solvents from static and dynamic oil/water mixtures. The maximum absorption capacity for pump oil was up to 75.0 g g−1, and the separation fluxes were 1.5 × 106 L m−3 h−1 and 3.0 × 105 L m−3 h−1 for layered and turbulent oil/water mixtures, respectively. Moreover, the as-prepared polyurethane foams can separate the oil-in-water emulsions with a separation efficiency of >99% and a separation flux of 2.7 × 105 L m−3 h−1. The large-scale preparation of hydrophobic polyurethane foams by this method confirmed their potential in practical applications.

Rui Wang

Papers

Ultrahigh surface density of Co-N2C single-atom-sites for boosting photocatalytic CO2 reduction to methanol

3D Superhydrophobic Sponge Coated with Magnesium Hydroxidefor Effective Oil/Water Mixture and Emulsion Separation

Effective demulsification for oil–water separation through metal–organic frameworks with an amphipathic micro-domain

Enhancing Demulsification Performance for Oil-Water Separation through Encapsulating Ionic Liquids in the Pore of MIL-100(Fe).

In situ self-foaming preparation of hydrophobic polyurethane foams for oil/water separation