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

Hydrogen is regarded as one of the most promising energy carriers because of its renewable and clean nature. Among various routes to generate hydrogen, solar water splitting has received increasing...

Single-atom catalysts for high-efficiency photocatalytic and photoelectrochemical water splitting: Distinctive roles, unique fabrication methods and specific design strategies

In recent years, single-atom catalysts (SACs) have attracted the interest of researchers owing to their suitability for various catalytic applications. For instance, their optoelectronic features, site-specific activity, and cost-effectiveness make SACs ideal for photocatalytic CO2 reduction. The activity, product selectivity, and photostability of SACs depend on various factors such as the nature of the metal/support material, the interaction between the metal atoms and support, light-harvesting ability, charge separation behavior, CO2 adsorption ability, active sites, and defects. Consequently, it is necessary to investigate these factors in depth to elucidate the working principle(s) of SACs for catalytic applications. Herein, the recent progress in the development of SACs for photocatalytic CO2 reduction with H2 O is reviewed. First, a brief overview of CO2 photoreduction and SACs for CO2 conversion is provided. Several synthesis strategies and useful techniques for characterizing SACs employed in heterogeneous catalysis are then described. Next, the challenges of SACs for photocatalytic CO2 reduction and related optimization strategies, in terms of activity, product selectivity, and stability, are explored. The progress in the development of noble metal- and transition metal-based SACs and dual-SACs for photocatalytic CO2 reduction is discussed. Finally, the prospects of SACs for CO2 reduction are considered.

Single-Atom Catalysts (SACs) for Photocatalytic CO2 Reduction with H2 O: Activity, Product Selectivity, Stability, and Surface Chemistry.

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.

Wenjun Fa

Papers

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