Simultaneous oxidative and reductive reactions in one system by atomic design

Recent Advances in Carbon-Based Adsorbents for Adsorptive Separation of Light Hydrocarbons

Light hydrocarbons (LHs) separation is an important process in petrochemical industry. The current separation technology predominantly relies on cryogenic distillation, which results in considerable energy consumption. Adsorptive separation using porous solids has received widespread attention due to its lower energy footprint and higher efficiency. Thus, tremendous efforts have been devoted to the design and synthesis of high-performance porous solids. Among them, porous carbons display exceptional stability, tunable pore structure, and surface chemistry and thus represent a class of novel adsorbents upon achieving the matched pore structures for LHs separations. In this review, the modulation strategies toward advanced carbon-based adsorbents for LHs separation are firstly reviewed. Then, the relationships between separation performances and key structural parameters of carbon adsorbents are discussed by exemplifying specific separation cases. The research findings on the control of the pore structures as well as the quantification of the adsorption sites are highlighted. Finally, the challenges of carbonaceous adsorbents facing for LHs separation are given, which would motivate us to rationally design more efficient absorbents and separation processes in future.

Carbon Nitride Photocatalysts with Integrated Oxidation and Reduction Atomic Active Centers for Improved CO2 Conversion

Single-atom active-site catalysts have attracted significant attention in the field of photocatalytic CO₂ conversion. However, designing active sites for CO₂ reduction and H₂ O oxidation simultaneously on a photocatalyst and combining the corresponding half-reaction in a photocatalytic system is still difficult. Here, we synthesized a bimetallic single-atom active-site photocatalyst with two compatible active centers of Mn and Co on carbon nitride (Mn₁ Co₁ /CN). Our experimental results and density functional theory calculations showed that the active center of Mn promotes H₂ O oxidation by accumulating photogenerated holes. In addition, the active center of Co promotes CO₂ activation by increasing the bond length and bond angle of CO₂ molecules. Benefiting from the synergistic effect of the atomic active centers, the synthesized Mn₁ Co₁ /CN exhibited a CO production rate of 47 μmol g^-1  h^-1 , which is significantly higher than that of the corresponding single-metal active-site photocatalyst.

Subsurface oxygen defects electronically interacting with active sites on In2O3 for enhanced photothermocatalytic CO2 reduction

Oxygen defects play an important role in many catalytic reactions. Increasing surface oxygen defects can be done through reduction treatment. However, excessive reduction blocks electron channels and deactivates the catalyst surface due to electron-trapped effects by subsurface oxygen defects. How to effectively extract electrons from subsurface oxygen defects which cannot directly interact with reactants is challenging and remains elusive. Here, we report a metallic In-embedded In2O3 nanoflake catalyst over which the turnover frequency of CO2 reduction into CO increases by a factor of 866 (7615 h-1) and 376 (2990 h-1) at the same light intensity and reaction temperature, respectively, compared to In2O3. Under electron-delocalization effect of O-In-(O)Vo-In-In structural units at the interface, the electrons in the subsurface oxygen defects are extracted and gather at surface active sites. This improves the electronic coupling with CO2 and stabilizes intermediate. The study opens up new insights for exquisite electronic manipulation of oxygen defects.

Hydrolysis of Ammonia Borane on a Single Pt Atom Supported by N-Doped Graphene

The hydrolysis of ammonia borane (NH₃BH₃ or AB) at room temperature is a promising method to produce hydrogen, but the complete reaction mechanism is still less investigated. Herein, the full hydrolysis process of the AB molecule on single Pt atom coordinated by two carbon atoms and one nitrogen atom (Pt₁-C₂N₁) on nitrogen doped graphene is investigated using the density functional theory (DFT) method. Our results demonstrate that the rate-limiting step is the formation of *BH₂NH₃ by breaking the first B-H bond in AB with an energy barrier of 0.68 eV, implying that Pt₁-C₂N₁ is a potential room-temperature catalyst for the full hydrolysis of AB. In addition, 27 more types of M₁-C₂N₁ (M represents transiton metal atom) and Pt₁ supported on nitrogen-doped graphene with different local coordination environments (Pt₁-C_xN_y, x and y are the number of carbon and nitrogen atoms that coordinated with the platinum atom) are considered to screen out potential single-atom catalysts for AB hydrolysis. The screening results further show that Pt₁-C₁N₂ is another potential catalyst for AB hydrolysis. In particular, two hydrogen atoms precovered on Pt₁-C₁N₂, resulting in a lower energy barrier for the rate-limiting step than that on Pt₁-C₂N₁. This study provides a prototype of Pt₁-C₁N₂ and Pt₁-C₂N₁ for catalytic full hydrolysis of AB at room temperature.

In Situ Synthesis of CuN4 /Mesoporous N-Doped Carbon for Selective Oxidative Crosscoupling of Terminal Alkynes under Mild Conditions

The 1,3-conjugated diynes are an important class of chemical intermediates, and the selective crosscoupling of terminal alkynes is an efficient chemical process for manufacturing asymmetrical 1,3-conjugated diynes. However, it often occurs in homogenous conditions and costs a lot for reaction treatment. Herein, a copper catalyzed strategy is used to synthesize highly ordered mesoporous nitrogen-doped carbon material (OMNC), and the copper species is in situ transformed into the copper single-atom site with four nitrogen coordination (CuN₄ ). These features make the CuN₄ /OMNC catalyst efficient for selective oxidative crosscoupling of terminal alkynes, and a wide range of asymmetrical and symmetrical 1,3-diynes (26 examples) under mild conditions (40 °C) and low substrates ratio (1.3). Density functional theory (DFT) calculations reveal that the aryl-alkyl crosscoupling has the lowest energy barrier on the CuN₄ site, which can explain the high selectivity. In addition, the catalyst can be separated and reused by simply centrifugation or filtration. This work can open a facile avenue for constructing single-atom loaded mesoporous materials to bridge homogeneous and heterogeneous catalysis.

Tuning the local electronic structure of a single-site Ni catalyst by co-doping a 3D graphene framework with B/N atoms toward enhanced CO2 electroreduction

Various single metal sites supported on N-doped carbon materials have been demonstrated to be effective catalysts for CO₂ electroreduction. However, it remains a challenging task to gain comprehensive understanding on how the local electronic structures of single metal catalytic sites are rationally tuned, which eventually holds the key to significantly enhance the electrocatalytic performance. Herein, we implement B-N bonds into an N-doped 3D graphene framework by B doping to further stabilize the supported catalytic Ni single-sites and simultaneously tune their local electronic structure. Moreover, electrochemical in situ Fourier-transform infrared spectroscopy reveals that the B-N bonds can further facilitate the production of pivotal *COOH intermediates in comparison with only N doping. As a result, the Ni single-site catalyst on the B, N co-doped 3D graphene framework achieves excellent catalytic performance with a CO faradaic efficiency (FE) of 98% and a turnover frequency (TOF) value of 20.1 s^-1 at -0.8 V (vs. RHE), whereas the FE and TOF for the control sample without B doping are as low as 62% and 6.0 s^-1, respectively. This work highlights the superiority of modulating local electronic structures of single-site catalysts toward efficient electrocatalytic CO₂ reduction.

Identifying the impact of the covalent-bonded carbon matrix to FeN4 sites for acidic oxygen reduction

The atomic configurations of FeN_x moieties are the key to affect the activity of oxygen rection reaction (ORR). However, the traditional synthesis relying on high-temperature pyrolysis towards combining sources of Fe, N, and C often results in the plurality of local environments for the FeN_x sites. Unveiling the effect of carbon matrix adjacent to FeN_x sites towards ORR activity is important but still is a great challenge due to inevitable connection of diverse N as well as random defects. Here, we report a proof-of-concept study on the evaluation of covalent-bonded carbon environment connected to FeN₄ sites on their catalytic activity via pyrolysis-free approach. Basing on the closed π conjugated phthalocyanine-based intrinsic covalent organic polymers (COPs) with well-designed structures, we directly synthesized a series of atomically dispersed Fe-N-C catalysts with various pure carbon environments connected to the same FeN₄ sites. Experiments combined with density functional theory demonstrates that the catalytic activities of these COPs materials appear a volcano plot with the increasement of delocalized π electrons in their carbon matrix. The delocalized π electrons changed anti-bonding d-state energy level of the single FeN₄ moieties, hence tailored the adsorption between active centers and oxygen intermediates and altered the rate-determining step.

Unveiling the effect of carbon matrix adjacent to Fe-N towards oxygen reduction reaction is important yet challenging. Here the authors investigate the carbon environment covalent-connected to FeN₄ sites on their catalytic activity using models prepared by pyrolysis-free approach.

Bromine and oxygen redox species mediated highly selective electro-epoxidation of styrene

Olefin epoxidation is an essential transformation and arouses great interest among the scientific community for the key role of epoxide in the chemical industry.

Double-shell microcapsules with spatially arranged Au nanoparticles and single Zn atoms for tandem synthesis of cyclic carbonates

Tandem catalysts with multifunctional sites can achieve high-efficiency catalytic transformations for quickly converting simple raw materials into complex value-added products. The integration of highly active species of metal nanoparticles (NPs) and single-atom catalytic sites (SACs) into one tandem system promises to synthesize an ideal bifunctional catalyst on account of the synergistic effect between NPs and SACs. However, such ideas face some challenges as deactivation or loss of active species, and low efficiency or side reactions caused by the disorder of different active species. Herein, a double-shell microencapsulated nanoreactor was fabricated as a bifunctional catalyst for the one-pot synthesis of cyclic carbonates from olefins. The microcapsules consist of an inner shell of nitrogen-doped porous carbon rich in Zn SACs, an outer shell of mesoporous SiO₂, and Au NPs confined between the outer and inner shells, noted as Zn-N-C/Au@mSiO₂. Particularly, two active species are spatially compartmented within microcapsules. Furthermore, the catalyst was applied in the one-pot synthesis of styrene carbonate from styrene with CO₂ under normal pressure and showed admirable performance. The yield of cyclic carbonate reached 92.9% at 93.2% olefins conversion. Furthermore, the catalyst shows good reusability with little loss of catalytic performance (4.0%) even after using it 15 consecutive times. The unique structure used in this work can rationally integrate diverse catalytic species into one system and offering adequate protection, which provides an effective strategy for the development of multi-site catalysts.

Dual-Metal Hetero-Single-Atoms with Different Coordination for Efficient Synergistic Catalysis

Rationally tailoring the coordination environments of metal single atoms (SAs) is an effective approach to promote their catalytic performances, which, however, remains as a challenge to date. Here, we report a novel misplaced deposition strategy for the fabrication of differently coordinated dual-metal hetero-SAs. Systematic characterization results imply that the as-synthesized dual-metal hetero-SAs (exemplified by Cu and Co) are affixed to a hierarchical carbon support via Cu-C₄ and Co-N₄ coordination bonds. Density functional theory studies reveal that the strong synergistic interactions between the asymmetrically deployed CuC₄ and CoN₄ sites lead to remarkably polarized charge distributions, i.e., electron accumulation and deficiency around CuC₄ and CoN₄ sites, respectively. The obtained CuC₄/CoN₄@HC catalyst exhibits significantly enhanced capability in substrate adsorption and O₂ activation, achieving superior catalytic performances in the oxidative esterification of aromatic aldehydes in comparison with the Cu- and Co-based SA counterparts.

Carbon Nitride Supported High-Loading Fe Single-Atom Catalyst for Activating of Peroxymonosulfate to Generate 1 O2 with 100 % Selectivity

Singlet oxygen (¹ O₂ ) is an excellent active species for the selective degradation of organic pollutions. However, it is difficult to achieve high efficiency and selectivity for the generation of ¹ O₂ . In this work, we develop a graphitic carbon nitride supported Fe single-atoms catalyst (Fe₁ /CN) containing highly uniform Fe-N₄ active sites with a high Fe loading of 11.2 wt %. The Fe₁ /CN achieves generation of 100 % ¹ O₂ by activating peroxymonosulfate (PMS), which shows an ultrahigh p-chlorophenol degradation efficiency. Density functional theory calculations results demonstrate that in contrast to Co and Ni single-atom sites, the Fe-N₄ sites in Fe₁ /CN adsorb the terminal O of PMS, which can facilitate the oxidization of PMS to form SO₅ ^.- , and thereafter efficiently generate ¹ O₂ with 100 % selectivity. In addition, the Fe₁ /CN exhibits strong resistance to inorganic ions, natural organic matter, and pH value during the degradation of organic pollutants in the presence of PMS. This work develops a novel catalyst for the 100 % selective production of ¹ O₂ for highly selective and efficient degradation of pollutants.