Surface Engineered Single-atom Systems for Energy Conversion

Microporous Hard Carbon Support Provokes Exceptional Performance of Single Atom Electrocatalysts for Advanced Air Cathodes

Single atom catalysts embracing metal-nitrogen (MNx) moieties show promising performance for oxygen reduction reaction (ORR). The modification on spatially confined microenvironments, which won copious attention with respect to achieving efficient catalysts, are auspicious but yet to be inspected for MNx moieties from modulating the energetics and kinetics of ORR. Here, Fe single atoms (SAs) are immobilized in microporous hard carbon (Fe-SAs/MPC), in which the microporous structure with crumpled graphene sheets serves confined microenvironment for catalysis. Fe-SAs/MPC holds a remarkable half-wave potential of 0.927 V and excellent stability for ORR. Theoretical studies unveil that hydrogen bonding between the intermediate of O* and micropore interior surfaces substantially promote its protonation and accelerate the overall ORR kinetics. Both the aqueous and quasi-solid-state zinc-air batteries driven by Fe-SAs/MPC air cathode show excellent stability with small charging/discharging voltage gaps. Importantly, when used as the air cathode for industrial chlor-alkali process, the applied voltage of Fe-SAs/MPC-based flow cell to reach 300 mA cm-2 is 1.57 V, which is 210 mV smaller than Pt/C-based one. These findings provide in-depth insights into the confined microenvironment of MNx moieties for boosted electrochemical performance, and pave the pathways for future catalyst development satisfying the requirement of industrial applications.

Transition-metal-embedded boron-doped graphene for reduction of CO2 to HCOOH

The electrochemical conversion of carbon dioxide is an excellent strategy for alleviating the greenhouse effect. Lately, single-atom catalysts have gained notable attention as emerging candidates for the CO2 reduction reaction owing to their remarkable cost-efficiency and unprecedented atomic utilization. By applying the density functional theory (DFT), our work examines the first couple of proton-coupled electron transfer steps of the CO2RR on 3d transition metal-doped B-Gr and compares the activity observed with previously studied supports. Since CO2 activation is the first step of the CO2RR, we thoroughly investigated the capability of the TM SAs in effectively activating CO2, both in the dry phase and in the presence of water. According to our calculation, except for Ti, Cu and Zn, all other TM@B-Gr systems can activate the CO2 molecule. CO2 activation on the selected SACs is further attributed to the transfer of charges from the TM SA to the CO2 molecule, as revealed by our Bader charge calculations. In addition, the Gibbs free energy changes for all the reaction intermediates are calculated to determine the most preferred pathway of the reaction. Our results indicate a preference for OCHO over COOH in the first protonation step, indicating the production of HCOOH as the preferred end product. The same trend is also observed in the presence of H2O. Our DFT-based analysis presented in this work reveals the crucial role that a support plays in determining the activity of a single-atom catalyst and paves the way for the efficient design of 2D catalysts for the CO2 reduction reaction.

Defect-Driven Atomic Engineering: Oxygen Vacancy-Stabilized Co Single Atoms on Ordered Ultrathin TiO2 Nanowires for Efficient CO2-to-Syngas Photoreduction

Single-atom catalysts (SACs) anchored on defective supports offer exceptional catalytic efficiency but face challenges in stabilizing isolated metal atoms and optimizing metal-support interactions. Here, a defect-driven strategy is reported to construct a 3D dendritic SAC comprising interwoven ultrathin TiO2 nanowires (NWs) with abundant oxygen vacancies (OVs) that stabilize atomically dispersed cobalt (Co) sites. Using hydrothermal synthesis followed by acid etching and calcination, Ti─Co─Ti motifs are engineered at OVs site. The 3D architecture provides multiscale porosity and charge transport, achieving syngas production rates of 28.4 mmol g-1·h-1 (CO) and 13.9 mmol g-1·h-1 (H2) with a high turnover frequency (TOF) of 10.6 min-1, surpassing many other state-of-the-art Co-based SACs. In situ Raman and electron paramagnetic resonance (EPR) analysis reveal OVs consumption during Co anchoring, while density functional theory (DFT) validates charge redistribution from Ti to Co, enabling efficient electron transfer and inducing strong electronic interactions that enhance CO2 adsorption and activation. The results highlight the interplay between atomic-scale coordination environments and macroscale architectural order in harnessing the catalytic potential of SACs and ultrathin 1D NWs.

Elucidating the Functional Orbital Evolution in Transition Metal-Doped Bi3O4Br Platforms for CO2 Photoreduction

Frontier orbital hybridization plays a vital role in the initial adsorption and activation process during catalysis. A formidable challenge is the precise determination of active orbitals/sites. Herein, 2D Bi3O4Br nanosheets are adopted as an operable platform for heteroatom doping of various transition metals (Fe, Ni, Zn/Cd). As the atom number of dopants increases, the capability of selective CO2 photoconversion is continuously amplified. The intrinsic nature is the variation of active functional orbital as indicated from band center distance (Δd/p-p) indicators. The calculated charge transfer of various CO2-bound geometries further demonstrates the p-p orbital interaction overwhelms d-p orbital interaction. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy results verify the charged nature of Bi sites with 6p orbitals not fully filled by electrons. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis and Gibbs free energy change profile suggest the rapid emergence of the critical *COOH intermediate in a thermodynamically preferred pathway.

Progress and perspectives of rapid Joule heating for the preparation of highly efficient catalysts

Functional catalytic materials play an important role in environmental, biological, energy, and other fields, wherein unique properties can be endowed through various synthesis strategies. However, conventional catalyst preparation methods suffer from mild conditions, prolonged treatment and low energy transfer efficiency, thus leading to limited inherent characterisation of catalysts (such as surface oxidation and agglomeration). Recently, the rapid Joule heating method, as a novel synthesis method, has attracted widespread attention owing to its controllable kinetic conditions and eco-friendly operation, while the mechanisms, advantages and recent progress of this method have been summarized in few reviews. Herein, we systematically summarize basic fundamentals and parameters of the Joule heating technique as well as recent processes in terms of effective modification strategies based on Joule heating. Meanwhile, perspective suggestions and challenges for Joule heating methods in terms of catalytic materials are put forward. This review provides an understanding for designing advanced catalytic materials.

Single-atom transition metals supported on B-doped g-C3N4 monolayers for electrochemical nitrogen reduction

Electrochemical reduction of naturally abundant nitrogen (N2) under ambient conditions is a promising method for ammonia (NH3) synthesis, while the development of a highly active, stable and low-cost catalyst remains a challenge. Herein, the N2 reduction reaction of TM@g-BC3N4 in electrochemical nitrogen reduction has been systematically investigated using density functional theory (DFT) calculations and compared with that of TM@g-C3N4. It was found that TM atoms are more stably anchored to g-BC3N4 than to g-C3N4. The adsorption free energy of N2 molecules on Fe@g-BC3N4 has the greatest change compared with that on Fe@g-C3N4, decreasing by 1.08 eV. The spin charge density around the Fe atom in Fe@g-BC3N4 increases significantly compared with that in Fe@g-C3N4, and the total magnetic moment of the system increases by 3.26μB. The limiting potential (-0.57 V) of Fe@g-BC3N4 in nitrogen reduction is decreased by 0.06 V compared with that of Fe@g-C3N4 (-0.63 V), and the desorption free energy of ammonia molecules decreases from 1.72 eV to 0.46 eV. The Fe atom has higher catalytic activity, the ammonia molecule is easier for desorption, and nitrogen reduction performance is better. This provides an important reference for the application of g-C3N4-based single atom catalysts in the field of nitrogen reduction.

Efficient Bifunctional Electrocatalysts for Oxygen Evolution/Reduction Reactions in Two-Dimensional Metal-Organic Frameworks by a Constant Potential Method

The evolution of bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts that are highly active, stable, and conductive is crucial for advancing metal-air batteries and fuel cells. We have here thoroughly explored the OER and ORR performance for a category of two-dimensional (2D) metal-organic frameworks (MOFs) called TM3(HADQ)2, and Rh3(HADQ)2 exhibits a promising bifunctional OER/ORR activity, with an overpotential of 0.31 V for both OER and ORR. The d-band center (εd) and crystal orbital Hamilton populations (COHP) are utilized to study the relationship between OER/ORR activity and the electronic structure of catalysts, and it is found that the elementary d-electron number (Ne) of the central TM for TM3(HADQ)2, as well as the electronegativity of the ligand TM-N4 and the intermediate O atom, are the main reason that affects the catalytic activity of OER/ORR. Additionally, Rh3(HADQ)2 can be proven through the constant potential method (CPM) and microkinetics method that it is an acidic OER/ORR bifunctional catalyst. Rh3(HADQ)2 has a high toxicity tolerance, making it a potential bifunctional catalyst. Our research contributes to both the rational design of SACs for various catalytic processes and the fabrication of bifunctional, cost-effective oxygen-electric catalysts.

In situ biomass-confined construction of an atomical Fe/NC catalyst towards oxygen reduction reaction

In this work, we develop an in situ biomass-confined approach to fabricate an atomically dispersed Fe/NC catalyst for ORR. The acquired Fe/NC demonstrates glorious oxygen reduction reaction (ORR) activity (E1/2: 0.858 V) in alkaline media and long-term cycling stability (∼750 h) in a homemade liquid zinc-air battery.

Ruthenium-Based Electrocatalysts for Hydrogen Evolution Reaction: from Nanoparticles to Single Atoms

Benefiting from similar hydrogen bonding energy to Pt and much lower price compare with Pt, Ru based catalysts are promising candidates for electrocatalytic hydrogen evolution reaction (HER). The catalytic activity of Ru nanoparticles can be enhanced through improving their dispersion by using different supports, and the strong metal supports interaction can further regulate their catalytic performance. In addition, single-atom catalysts (SACs) with almost 100% atomic utilization attract great attention and the coordinative atmosphere of single atoms can be adjusted by supports. Moreover, the syngenetic effects of nanoparticles and single atoms can further improve the catalytic performance of Ru based catalysts. In this review, the progress of Ru based HER electrocatalysts are summarized according to their existing forms, including nanoparticles (NPs), single atoms (SAs) and the combination of both NPs and SAs. The common supports such as carbon materials, metal oxides, metal phosphides and metal sulfides are classified to clarify the metal supports interaction and coordinative atmosphere of Ru active centers. Especially, the possible catalytic mechanisms and the reasons for the improved catalytic performance are discussed from both experimental results and theoretical calculations. Finally, some challenges and opportunities are prospected to facilitate the development of Ru based catalysts for HER.

Atomically dispersed Co-based species containing electron withdrawing groups for electrocatalytic oxygen reduction reactions

Single-atom-based catalysts are a promising catalytic system with advantages of molecular catalysts and conductive supports. In this work, a new hybrid material (CoF/NG) is produced using a low-temperature reaction between an organometallic complex (Co(C5HF6O2)2) (CoF) and N-doped reduced graphene oxide (NG). CoF contains electron-withdrawing CF3 groups in the ligand around a Co atom. Microscopic and chemical characterization studies reveal that Co-based species are coordinated to N sites of NG and molecularly dispersed on the surface of NG. The CoF/NG hybrid shows improved electrocatalytic properties, such as onset (0.91 V) and half-wave (0.80 V) potentials, for the electrochemical oxygen reduction reaction (ORR) relative to the NG material. Control experiments reveal that Co-(N)graphene acts as a major active species for ORR. CoF/NG shows moderate cycling durability and microscopy measurements of CoF/NG-after-cycle indicate the formation of nanoparticles after electrocatalytic measurements. All experimental data support that the incorporation of Co-based organometallic species containing electron-withdrawing groups around the metal center onto the graphene-based networks improves the electrocatalytic ORR performance but diminishes the electrocatalytic stability of the active species.

Metal-organic frameworks for organic transformations by photocatalysis and photothermal catalysis

Organic transformation by light-driven catalysis, especially, photocatalysis and photothermal catalysis, denoted as photo(thermal) catalysis, is an efficient, green, and economical route to produce value-added compounds. In recent years, owing to their diverse structure types, tunable pore sizes, and abundant active sites, metal-organic framework (MOF)-based photo(thermal) catalysis has attracted broad interest in organic transformations. In this review, we provide a comprehensive and systematic overview of MOF-based photo(thermal) catalysis for organic transformations. First, the general mechanisms, unique advantages, and strategies to improve the performance of MOFs in photo(thermal) catalysis are discussed. Then, outstanding examples of organic transformations over MOF-based photo(thermal) catalysis are introduced according to the reaction type. In addition, several representative advanced characterization techniques used for revealing the charge reaction kinetics and reaction intermediates of MOF-based organic transformations by photo(thermal) catalysis are presented. Finally, the prospects and challenges in this field are proposed. This review aims to inspire the rational design and development of MOF-based materials with improved performance in organic transformations by photocatalysis and photothermal catalysis.

Towards bridging thermo/electrocatalytic CO oxidation: from nanoparticles to single atoms

Proton exchange membrane fuel cells (PEMFCs), as a feasible alternative to replace the traditional fossil fuel-based energy converter, contribute significantly to the global sustainability agenda. At the PEMFC anode, given the high exchange current density, Pt/C is deemed the catalyst-of-choice to ensure that the hydrogen oxidation reaction (HOR) occurs at a sufficiently fast pace. The high performance of Pt/C, however, can only be achieved under the premise that high purity hydrogen is used. For instance, in the presence of trace level carbon monoxide, a typical contaminant during H2 production, Pt is severely deactivated by CO surface blockage. Addressing the poisoning issue necessitates for either developing anti-poisoning electrocatalysts or using pre-purified H2 obtained via a thermo-catalysis route. In other words, the CO poisoning issue can be addressed by either thermal-catalysis from the H2 supply side or electrocatalysis at the user side, respectively. In spite of the distinction between thermo-catalysis and electro-catalysis, there are high similarities between the two routes. Essentially, a reduction in the kinetic barrier for the combination of CO to oxygen containing intermediates is required in both techniques. Therefore, bridging electrocatalysis and thermocatalysis might offer new insight into the development of cutting edge catalysts to solve the poisoning issue, which, however, stands as an underexplored frontier in catalysis science. This review provides a critical appraisal of the recent advancements in preferential CO oxidation (CO-PROX) thermocatalysts and anti-poisoning HOR electrocatalysts, aiming to bridge the gap in cognition between the two routes. First, we discuss the differences in thermal/electrocatalysis, CO oxidation mechanisms, and anti-CO poisoning strategies. Second, we comprehensively summarize the progress of supported and unsupported CO-tolerant catalysts based on the timeline of development (nanoparticles to clusters to single atoms), focusing on metal-support interactions and interface reactivity. Third, we elucidate the stability issue and theoretical understanding of CO-tolerant electrocatalysts, which are critical factors for the rational design of high-performance catalysts. Finally, we underscore the imminent challenges in bridging thermal/electrocatalytic CO oxidation, with theory, materials, and the mechanism as the three main weapons to gain a more in-depth understanding. We anticipate that this review will contribute to the cognition of both thermocatalysis and electrocatalysis.

Nanostructured Flame-Retardant Layer-by-Layer Architectures for Cotton Fabrics: The Current State of the Art and Perspectives

Nowadays, nanotechnology represents a well-established approach, suitable for designing, producing, and applying materials to a broad range of advanced sectors. In this context, the use of well-suited "nano" approaches accounted for a big step forward in conferring optimized flame-retardant features to such a cellulosic textile material as cotton, considering its high ease of flammability, yearly production, and extended use. Being a surface-localized phenomenon, the flammability of cotton can be quite simply and effectively controlled by tailoring its surface through the deposition of nano-objects, capable of slowing down the heat and mass transfer from and to the textile surroundings, which accounts for flame fueling and possibly interacting with the propagating radicals in the gas phase. In this context, the layer-by-layer (LbL) approach has definitively demonstrated its reliability and effectiveness in providing cotton with enhanced flame-retardant features, through the formation of fully inorganic or hybrid organic/inorganic nanostructured assemblies on the fabric surface. Therefore, the present work aims to summarize the current state of the art related to the use of nanostructured LbL architectures for cotton flame retardancy, offering an overview of the latest research outcomes that often highlight the multifunctional character of the deposited assemblies and discussing the current limitations and some perspectives.