Times Higher Turnover Frequency Research Articles

Noncovalent Assembly Strategy for Efficient Synthesis of Dual‐Atom Catalysts

AbstractDiatomic catalysts (DACs), as a frontier of research on atomically dispersed catalysts, have drawn great attention, especially in electrocatalysis. However, most of the current synthetic strategies for DACs suffer from poor efficiency and high cost. In this work, a novel noncovalent assembly strategy is reported for the efficient synthesis of DACs. By the aid of two kinds of noncovalent intermolecular force, the π–π stacking and Coulomb forces, a pair of metal complex ions with opposite electrical charges is spontaneously loaded onto the carbon substrate and transformed into diatomic sites via thermal treatment. This noncovalent assembly approach shows universal feasibility for a broad spectrum of DACs (Ir–Fe, Pt–Fe, Pt–Co, and Pt–Ni). To prove the practical utility, the as‐prepared PtFe‐DAC is deployed as oxygen reduction reaction catalyst, performing excellent activity with a half‐wave potential of 0.935 V (vs RHE) and a 2.3 times higher turnover frequency than that of the single‐atomic Fe‐SAC. In situ characterization and theoretical calculation indicate that the adjacent Pt sites could endow moderate activation of oxygen molecule on Fe sites by lowering the electron pairing energy as well as the modulation of the d‐band center and spin states of Fe centers propelling the desorption of the intermediates.

Stacking Fault-Enriched MoNi4/MoO2 Enables High-Performance Hydrogen Evolution.

Producing green hydrogen in a cost-competitive manner via water electrolysis will make the long-held dream of hydrogen economy a reality. Although platinum (Pt)-based catalysts show good performance toward hydrogen evolution reaction (HER), the high cost and scarce abundance challenge their economic viability and sustainability. Here, a non-Pt, high-performance electrocatalyst for HER achieved by engineering high fractions of stacking fault (SF) defects for MoNi4/MoO2 nanosheets (d-MoNi) through a combined chemical and thermal reduction strategy is shown. The d-MoNi catalyst offers ultralow overpotentials of 78 and 121mV for HER at current densities of 500 and 1000mA cm-2 in 1M KOH, respectively. The defect-rich d-MoNi exhibits four times higher turnover frequency than the benchmark 20% Pt/C, together with its excellent durability (> 100 h), making it one of the best-performing non-Pt catalysts for HER. The experimental and theoretical results reveal that the abundant SFs in d-MoNi induce a compressive strain, decreasing the proton adsorption energy and promoting the associated combination of *H into hydrogen and molecular hydrogen desorption, enhancing the HER performance. This work provides a new synthetic route to engineer defective metal and metal alloy electrocatalysts for emerging electrochemical energy conversion and storage applications.

Accelerating Hydrogen Desorption of Nickel Molybdenum Cathode via Copper Modulation for Pure‐Water‐Fed Hydroxide Exchange Membrane Electrolyzer

AbstractThe more sluggish kinetics of hydrogen evolution catalysts in base as compare to that in acid to some degree restricts hydrogen production performance of hydroxide exchange membrane electrolyzers, especially when using earth‐abundant catalysts. Here a ternary nickel–copper–molybdenum hydrogen evolution catalyst is reported that exhibits ≈5 times higher turnover frequency than without copper doping. The X‐ray absorption near‐edge structure and valence band spectrum demonstrate that the light doping of copper into nickel–molybdenum alloy modulates the electronic structure and downshifts the d‐band center, resulting in accelerated hydrogen desorption, as consolidated by H2 temperature programmed desorption and theoretical calculation. An electrolyzer employing this cathode catalyst and a nickel–iron anode, gives a current density of 1.7 A cm−2 at 2.0 V with a pure‐water feed through the anode, which outperforms the 2025 target proposed by the United States Department of Energy, and even is operated continuously for over 1000 h with a decay rate of as low as 0.5 mV h−1. Post‐mortem analysis discloses that hydroxide exchange ionomer migration is one of the key factors affecting long‐term durability. This work demonstrates the feasibility of a low‐cost, water‐fed hydroxide exchange membrane electrolyzer achieving industrial‐level performance and lifetime.

Dual-Atom Catalyst with N-Colligated Zn1Co1 Species as Dominant Active Sites for Propane Dehydrogenation.

Dual-atom catalysts (DACs) with paired active sites can provide unique intrinsic properties for heterogeneous catalysis, but the synergy of the active centers remains to be elucidated. Here, we develop a high-performance DAC with Zn1Co1 species anchored on nitrogen-doped carbon (Zn1Co1/NC) as the dominant active site for the propane dehydrogenation (PDH) reaction. It exhibits several times higher turnover frequency (TOF) of C3H8 conversion and enhanced C3H6 selectivity compared to Zn1/NC or Co1/NC with only a single-atom site. Various experimental and theoretical studies suggest that the enhanced PDH performance stems from the promoted activation of the C-H bond of C3H8 triggered by the electronic interaction between Zn1 and Co1 colligated by N species. Moreover, the dynamic sinking of the Zn1 site and rising of the Co1 site, together with the steric effect of the dissociated H species at the bridged N during the PDH reaction, provides a feasible channel for C3H6 desorption through the more exposed Co1 site, thereby boosting the selectivity. This work provides a promising strategy for designing robust hetero DACs to simultaneously increase activity and selectivity in the PDH reaction.

UiO-type MOFs catalyzed ring-opening copolymerization of epoxides and cyclic anhydrides

Ring-opening copolymerization (ROCOP) of epoxide with cyclic anhydride plays critical role in the preparation of polyesters. Herein, the UiO-type MOFs including UiO-66, UiO-66-NH2, UiO-66-NO2, UiO-66-Br and UiO-67 with octahedral morphology were synthesized by different ligands and used as heterogeneous catalysts for ROCOP of epoxide with cyclic anhydride. It was shown that the UiO-67 with ultra-high specific surface area and large pore size endows it with abundant of catalytic active sites and the highest catalytic activity, which makes UiO-67 present seven times higher turnover frequency (192.6 h−1) than that of UiO-66 (23.6 h−1) under the same conditions. Besides, the strong electron-absorbing group enhances the catalytic activity of UiO-type MOFs. The cycling test proves that the UiO-67 has good recoverability and there was no significant loss of catalytic activity after been reused for three times. The kinetic study showed that the ROCOP of epoxide with cyclic anhydride catalyzed by UiO-67 was a first-order kinetic reaction, and its apparent activation energy (Ea) was about 71.75 kJ∙mol−1 calculated by the Arrhenius equation, which was 27.5% lower than that of UiO-66 (98.94 kJ∙mol−1).

Solvent-mediated oxidative polymerization to atomically dispersed iron sites for oxygen reduction

Fe single-atom catalysts open up broad prospects for oxygen reduction reaction (ORR), but the chemical state evolution of Fe species before forming isolated sites is rarely understood. Herein, mechanistic investigation on the formation of isolated Fe sites is presented through solvent-mediated oxidative pyrrole polymerization strategy. The slow reaction kinetics of oxidative Fe3+ ions with the predesigned methanol solvent molecules can endow highly dispersed Fe sites in polypyrrole and thus Fe single-atom catalysts after pyrolysis. The Fe single-atom catalyst (Fe-SA/PNC) performs superior ORR activity with a half-wave potential of 0.90 V versus RHE and 12.8 times higher turnover frequency than that of commercial Pt/C. When assembled into Zn-air batteries, the Fe-SA/PNC cathode delivers a 1.68 V open circuit potential and ultra-long cycling stability over 9000 cycles, superior to the most reported catalysts so far. Experimental and theoretical results reveal that the rich adjacent non-coordinated graphitic nitrogen atoms can enhance electronic conductivity and promote O2 adsorption and OH- desorption, thus enabling high oxygen reduction and battery performances.

Photoactive Hexaazaphenalene‐based Coordination Network as a Two‐Pathway Photosensitizer for Oxidation of Alkanes

AbstractA tris(4‐pyridyl) hexaazaphenalenyl anion (TPHAP−) exhibited both energy and charge transfer processes to oxygen molecules under UV irradiation resulting in the production of reactive oxygen species (ROSs). The existence of ROSs was confirmed by spectroscopic characterization and photooxidation of a standard compound selective for the singlet oxygen, 1,5‐dihydroxynaphthalene (1,5‐DHN). In addition, a porous coordination network (PCN) consisting of TPHAP−, cadmium ions and fumarate co‐ligand was synthesized and used as a heterogeneous photosensitizer capable of ROSs generation. To take advantage of this ability, photooxidation of various saturated hydrocarbons including cyclohexane, cyclooctane and n‐octane was investigated. TPHAP‐based PCN demonstrated excellent photosensitizing ability achieving more than a hundred times higher turnover frequency than other reported coordination networks based on photoactive ligands.

Reaction Mechanism and Selectivity Tuning of Propene Oxidation at the Electrochemical Interface.

Electrochemical conversion of propene is a promising technique for manufacturing commodity chemicals by using renewable electricity. To achieve this goal, we still need to develop high-performance electrocatalysts for propene electrooxidation, which highly relies on understanding the reaction mechanism at the molecular level. Although the propene oxidation mechanism has been well investigated at the solid/gas interface under thermocatalytic conditions, it still remains elusive at the solid/liquid interface under an electrochemical environment. Here, we report the mechanistic studies of propene electrooxidation on PdO/C and Pd/C catalysts, considering that the Pd-based catalyst is one of the most promising electrocatalytic systems. By electrochemical in situ attenuated total reflection Fourier transform infrared spectroscopy, a distinct reaction pathway was observed compared with conventional thermocatalysis, emphasizing that propene can be dehydrogenated at a potential higher than 0.80 V, and strongly adsorb via μ-C═CHCH3 and μ3-η2-C═CHCH3 configuration on PdO and Pd, respectively. The μ-C═CHCH3 is via bridge bonds on adjacent Pd and O atoms on PdO, and it can be further oxidized by directly taking surface oxygen from PdO, verified by the H218O isotope-edited experiment. A high surface oxygen content on PdO/C results in a 3 times higher turnover frequency than that on Pd/C for converting propene into propene glycol. This finding highlights the different reaction pathways under an electrochemical environment, which sheds light on designing next-generation electrocatalysts for propene electrooxidation.

Coordination environment of Ru clusters with in-situ generated metastable symmetry-breaking centers for seawater electrolysis

Seawater electrolysis offers hope for renewable production of green hydrogen. However, its hydrogen evolution reaction (HER) is quite difficulty due to electrode corrosion and undesirable side reactions. More importantly, it is still a big challenge to regulate charge distribution of Ru clusters at atomic level for maximizing the activity. Herein, theory calculations unveil that, substituting partial Ru atoms with atomically dispersedly nonstoichiometric CdySex drives in-situ generation of metastable symmetry-breaking Ru cluster surfaces (Ru/CdySex) and more atom vacancies, which regulates the coordination environment of Ru atoms and results in optimized Ru−H bond and more channels for H migration. As expected, Ru/Cd0.02Se4 requires overpotentials as low as 4.2 and 6.3 mV at 10 mA cm−2, and has 8.1 and 9.6 times higher turnover frequency (TOF) than Ru/Sex in alkali and alkaline seawater, respectively, superior to Pt/C and most other catalysts reported so far. Also, it shows very impressive stability over 50 h. This discovery elaborates the activity origin via symmetry breaking and provides deep cognition for hydrogen product from seawater electrolysis.

A Site Distance Effect Induced by Reactant Molecule Matchup in Single-Atom Catalysts for Fenton-Like Reactions.

Understanding the site interaction nature of single-atom catalysts (SACs), especially densely populated SACs, is vital for their application to various catalytic reactions. Herein, we report a site distance effect, which emphasizes how well the distance of the adjacent copper atoms (denoted as dCu1-Cu1 ) matches with the reactant peroxydisulfate (PDS) molecular size to determine the Fenton-like reaction reactivity on the carbon-supported SACs. The optimized dCu1-Cu1 in the range of 5-6 Å, which matches the molecular size of PDS, endows the catalyst with a nearly two times higher turnover frequency than that of dCu1-Cu1 beyond this range, accordingly achieving record-breaking kinetics for the oxidation of emerging organic contaminants. Further studies suggest that this site distance effect originates from the alteration of PDS adsorption to a dual-site structure on Cu1 -Cu1 sites when dCu1-Cu1 falls within 5-6 Å, significantly enhancing the interfacial charge transfer and consequently resulting in the most efficient catalyst for PDS activation so far.

A Site Distance Effect Induced by Reactant Molecule Matchup in Single‐Atom Catalysts for Fenton‐Like Reactions

AbstractUnderstanding the site interaction nature of single‐atom catalysts (SACs), especially densely populated SACs, is vital for their application to various catalytic reactions. Herein, we report a site distance effect, which emphasizes how well the distance of the adjacent copper atoms (denoted as dCu1−Cu1) matches with the reactant peroxydisulfate (PDS) molecular size to determine the Fenton‐like reaction reactivity on the carbon‐supported SACs. The optimized dCu1−Cu1 in the range of 5–6 Å, which matches the molecular size of PDS, endows the catalyst with a nearly two times higher turnover frequency than that of dCu1−Cu1 beyond this range, accordingly achieving record‐breaking kinetics for the oxidation of emerging organic contaminants. Further studies suggest that this site distance effect originates from the alteration of PDS adsorption to a dual‐site structure on Cu1−Cu1 sites when dCu1−Cu1 falls within 5–6 Å, significantly enhancing the interfacial charge transfer and consequently resulting in the most efficient catalyst for PDS activation so far.

Regulating the electronic structures of mixed B-site pyrochlore to enhance the turnover frequency in water oxidation

This paper describes the development of mixed B-site pyrochlore Y2MnRuO7 electrocatalyst for oxygen evolution reaction (OER) in acidic media, a challenge for the development of low-temperature electrolyzer for green hydrogen production. Recently, several theories have been developed to understand the reaction mechanism for OER, though there is an uncertainty in most of the cases, due to the complex surface structures. Several key factors such as lattice oxygen, defect, electronic structure, oxidation state, hydroxyl group and conductivity were identified and shown to be important to the OER activity. The contribution of each factor to the performance however is often not well understood, limiting their impact in guiding the design of OER electrocatalysts. In this work, we showed mixed B-site pyrochlore Y2MnRuO7 catalyst exhibits 14 times higher turnover frequency (TOF) than RuO2 while maintaining a low overpotential of ~ 300 mV for the entire testing period of 24 h in acidic electrolyte. X-ray photoelectron spectroscopy (XPS) analysis reveals that this B-site mixed pyrochlore Y2MnRuO7 has a higher oxidation state of Ru than those of Y2Ru2O7, which could be crucial for improving OER performance as the broadened and lowered Ru 4d band resulted from the B-site substitution by Mn is beneficial to the OER kinetics.

Suppression of Chain Transfer and Promotion of Chain Propagation in Neutral Anilinotropone Nickel Polymerization Catalysis

Both suppression of chain transfer and promotion of chain propagation are highly important but greatly challenging to enhance desirable turnover frequency (TOF) and molecular weight (MW) in olefin polymerization, which is one of the most important chemical reactions. A transition-metal catalyst is the most important key to controlling these fundamental steps. In this contribution, we report the neutral, single-component anilinotropone nickel catalyst Ni6 via an alternative synthetic pathway, which is the isomer of the milestone salicylaldiminato nickel catalyst Ni6-iso. Compared to the classic Ni6-iso, Ni6 produces polyethylenes with elevated TOF (five times), much higher MW (102 times), narrower molecular weight distribution (PDI = 1.05, living fashion at 40 °C), and lower branching density (substantially linear microstructure) in ethylene polymerization. In particular, in the copolymerization of ethylene with a polar comonomer, Ni6 concurrently gives 4.5 times higher TOF, 34 times higher MW, and 1.6 times higher comonomer incorporation relative to Ni6-iso. Mechanistic insights using density functional theory (DFT) calculation reveal both the suppression of chain transfer and promotion of chain propagation. This evidences the anilinotropone nickel catalyst as a promising candidate.

Intermetallic Nanocatalyst for Highly Active Heterogeneous Hydroformylation.

Hydroformylation is an imperative chemical process traditionally catalyzed by homogeneous catalysts. Designing a heterogeneous catalyst with high activity and selectivity in hydroformylation is challenging but essential to allow the convenient separation and recycling of precious catalysts. Here, we report the development of an outstanding catalyst for efficient heterogeneous hydroformylation, RhZn intermetallic nanoparticles. In the hydroformylation of styrene, it shows three times higher turnover frequency (3090 h-1) compared to the benchmark homogeneous Wilkinson's catalyst (966 h-1), as well as a high chemoselectivity toward aldehyde products. RhZn is active for a variety of olefin substrates and can be recycled without a significant loss of activity. Density functional theory calculations show that the RhZn surfaces reduce the binding strength of reaction intermediates and have lower hydroformylation activation energy barriers compared to pure Rh(111), leading to more favorable reaction energetics on RhZn. The calculations also predict potential catalyst design strategies to achieve high regioselectivity.

Decarboxylation‐Induced Defects in MOF‐Derived Single Cobalt Atom@Carbon Electrocatalysts for Efficient Oxygen Reduction

AbstractDeveloping transition metal single‐atom catalysts (SACs) for oxygen reduction reaction (ORR) is of great importance. Zeolitic imidazolate frameworks (ZIFs) as a subgroup of metal‐organic frameworks (MOFs) are distinguished as SAC precursors, due to their large porosity and N content. However, the activity of the formed metal sites is limited. Herein, we report a decarboxylation‐induced defects strategy to improve their intrinsic activity via increasing the defect density. Carboxylate/amide mixed‐linker MOF (DMOF) was chosen to produce defective Co SACs (Co@DMOF) by gas‐transport of Co species to DMOF upon heating. Comparing with ZIF‐8 derived SAC (Co@ZIF‐8‐900), Co@DMOF‐900 with more defects yet one fifth Co content and similar specific double‐layer capacitance show better ORR activity and eight times higher turnover frequency (2.015 e s−1 site−1). Quantum calculation confirms the defects can weaken the adsorption free energy of OOH on Co sites and further boost the ORR process.

Decarboxylation-Induced Defects in MOF-Derived Single Cobalt Atom@Carbon Electrocatalysts for Efficient Oxygen Reduction.

Developing transition metal single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is of great importance. Zeolitic imidazolate frameworks (ZIFs) as a subgroup of metal-organic frameworks (MOFs) are distinguished as SAC precursors, due to their large porosity and N content. However, the activity of the formed metal sites is limited. Herein, we report a decarboxylation-induced defects strategy to improve their intrinsic activity via increasing the defect density. Carboxylate/amide mixed-linker MOF (DMOF) was chosen to produce defective Co SACs (Co@DMOF) by gas-transport of Co species to DMOF upon heating. Comparing with ZIF-8 derived SAC (Co@ZIF-8-900), Co@DMOF-900 with more defects yet one fifth Co content and similar specific double-layer capacitance show better ORR activity and eight times higher turnover frequency (2.015 e s-1 site-1 ). Quantum calculation confirms the defects can weaken the adsorption free energy of OOH on Co sites and further boost the ORR process.

Gold catalysts containing interstitial carbon atoms boost hydrogenation activity

Supported gold nanoparticles are emerging catalysts for heterogeneous catalytic reactions, including selective hydrogenation. The traditionally used supports such as silica do not favor the heterolytic dissociation of hydrogen on the surface of gold, thus limiting its hydrogenation activity. Here we use gold catalyst particles partially embedded in the pore walls of mesoporous carbon with carbon atoms occupying interstitial sites in the gold lattice. This catalyst allows improved electron transfer from carbon to gold and, when used for the chemoselective hydrogenation of 3-nitrostyrene, gives a three times higher turn-over frequency (TOF) than that for the well-established Au/TiO2 system. The d electron gain of Au is linearly related to the activation entropy and TOF. The catalyst is stable, and can be recycled ten times with negligible loss of both reaction rate and overall conversion. This strategy paves the way for optimizing noble metal catalysts to give an enhanced hydrogenation catalytic performance.

Size-dependent catalytic behavior of MoS2 nanoparticles in water-gas shift reaction

Well-dispersed and uniformly sized 5–84 nm Mo oxide and CoMo oxide nanoparticles were synthesized by varying the Mo and Co precursor concentrations. The water-gas shift (WGS) activity of sulfided Mo-based catalysts was found to increase with particle size and in the presence of Co. In particular, 84-nm CoMo-S nanoparticles exhibited 15 times higher effective CO consumption rate and 2.3 times higher turnover frequency (TOF) as compared to 5- nm Mo-S nanoparticles. The observed trends in size-dependent catalytic behavior correlated with the higher density of the proposed active sites present on larger (Co)Mo-S nanoparticles probed by CO chemisorption.

Ethanol as a Binder to Fabricate a Highly‐Efficient Capsule‐Structured CuO−ZnO−Al2O3@HZSM‐5 Catalyst for Direct Production of Dimethyl Ether from Syngas

AbstractThis work reports a highly efficient capsule‐structured CuO−ZnO−Al2O3@HZSM‐5 (CZA@HZSM‐5‐EtOH) core‐shell catalyst for the direct conversion of syngas to dimethyl ether by a facile physical coating method with ethanol as a binder through coating micrometer‐sized HZSM‐5 shell on the prior‐shaped millimeter‐sized CZA core, it shows 2.9 times higher CO conversion with the 2.7 times higher turnover frequency and 9.2 times higher dimethyl ether space‐time yield of the CZA@HZSM‐5‐SS catalyst prepared by a similar process but with silica sol as a binder (315.5 vs 34.3 gDME kgcat−1 h−1). The relationship between the structure and performance was explored by a variety of characterization techniques including X‐ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X‐ray diffraction (XRD), ammonia temperature programmed desorption (NH3‐TPD), nitrogen adsorption‐desorption, H2‐temperature‐programmed reduction (H2‐TPR) and H2‐TPR after oxidation of the samples by N2O. CZA@HZSM‐5‐EtOH can be considered as a highly efficient and practical catalyst for dimethyl ether synthesis from syngas. This work presents a new avenue to design other bifunctional catalysts for the cascade reactions in which the raw materials can be converted into an intermediate over the core and then the as‐formed intermediate over the core can be subsequently converted into the final product.

Xantphos‐ligated palladium dithiolates: An unprecedented and convenient catalyst for the carbonylative Suzuki–Miyaura cross‐coupling reaction with high turnover number and turnover frequency

Xantphos‐ and dithiolate‐ligated macrocyclic palladium complexes as an efficient and stable catalyst for the carbonylative Suzuki–Miyaura cross‐coupling reaction have been synthesized. The catalysts were characterized by 1H‐nuclear magnetic resonance (NMR), CHNS (carbon, hydrogen, nitrogen, and sulfur) analysis, melting point analysis, and 31P‐NMR spectroscopy. Several sensitive functional groups (e.g., –NO2, –F, –Cl, –Br, –NH2, and –CN) on the aromatic ring were well tolerated in the carbonylative Suzuki–Miyaura coupling reaction. The present palladium complexes produce six times higher turnover number (TON) and five times higher turnover frequency (TOF) compared with conventional homogeneous palladium precursors. Maximum TONs in the range of 105 to 106 and TOF in the range of 104 to 105 could be generated by a very low amount of catalyst loading (10–5 mol%).