Turnover Frequency Research Articles

Unraveling the Harmonious Coexistence of Ruthenium States on a Self‐Standing Electrode for Enhanced Hydrogen Evolution Reaction

The development of cost‐effective, highly efficient, and durable electrocatalysts has been a paramount pursuit for advancing the hydrogen evolution reaction (HER). Herein, a simplified synthesis protocol was designed to achieve a self‐standing electrode, composed of activated carbon paper embedded with Ru single‐atom catalysts and Ru nanoclusters (ACP/RuSAC+C) via acid activation, immersion, and high‐temperature pyrolysis. Ab initio molecular dynamics (AIMD) calculations are employed to gain a more profound understanding of the impact of acid activation on carbon paper. Furthermore, the coexistence states of the Ru atoms are confirmed via aberration‐corrected scanning transmission electron microscopy (AC‐STEM), X‐ray photoelectron spectroscopy (XPS), and X‐ray absorption spectroscopy (XAS). Experimental measurements and theoretical calculations reveal that introducing a Ru single‐atom site adjacent to the Ru nanoclusters induces a synergistic effect, tuning the electronic structure and thereby significantly enhancing their catalytic performance. Notably, the ACP/RuSAC+C exhibits a remarkable turnover frequency (TOF) of 18 s−1 and an exceptional mass activity (MA) of 2.2 A mg−1, surpassing the performance of conventional Pt electrodes. The self‐standing electrode, featuring harmoniously coexisting Ru states, stands out as a prospective choice for advancing HER catalysts, enhancing energy efficiency, productivity, and selectivity.

Reversible hydrogen spillover induced by Brønsted acid for accelerating direct synthesis of hydrogen peroxide

Supported PdAu bimetallic nanoparticles have been developed for the direct synthesis of H2O2 (DSHP). However, to date, the dilute H2 streams necessary to avoid potential risks of explosion limits the attainable product concentrations of H2O2. Here, a series of silicon-modified Al2O3 (SixAl) were synthesized for loading PdAu bimetallic nanoparticles. The H2 consumption turnover frequency (TOF) and H2O2 formation TOF show a volcano trend with the silica content, where PdAu/Si5.6Al exhibits the best H2 consumption TOF and H2O2 formation TOF (11259.3 and 1995.2 h−1). Silica on the surface of Al2O3 is identified as Brønsted (B) acid site. The increase of B acid sites density accelerates the H2 conversion into H atoms, thereby boosting H2O2 formation process. The adsorption and dissociation of H2 into atomic H at B acid sites would spill across the support to PdAu nanoparticles reversely. The reversed hydrogen spillover effect induced by B acid site accounts for enhanced H2 conversion and H2O2 productivity for PdAu/SixAl.

A Multipeak Solar Flare with a High Turnover Frequency of the Gyrosynchrotron Spectra from the Loop-top Source

The origin of multiple peaks in light curves of various wavelengths remains illusive during flares. Here we discuss the flare of SOL2023-05-09T03:54M6.5 with six flux peaks as recorded by a tandem of new microwave and hard X-ray (HXR) instruments. According to its microwave spectra, the flare represents a high-turnover-frequency (>15 GHz) event. The rather-complete microwave and HXR spectral coverage provides a rare opportunity to uncover the origin of such an event together with simultaneous EUV images. We concluded that (1) the microwave sources originates around the top section of the flaring loops with a trend of source spatial dispersion with frequency; (2) the visible movement of the microwave source from peak to peak originates from the process of new flaring loops appearing sequentially along the magnetic neutral line; (3) the optically thin microwave spectra are hard with the indices (α tn) varying from ∼−1.2 to −0.4, and the turnover frequency always exceeds 15 GHz; (4) higher turnover/peak frequency corresponds to stronger peak intensity and harder optically thin spectra. Using the Fokker–Planck and GX Simulator codes we obtained a good fit to the observed microwave spectra and spatial distribution of the sources at all peaks, if assuming the radiating energetic electrons have the same spatial distribution and single-power-law spectra but with the number density varying in a range of ∼30%. We conclude that the particle acceleration in this flare happens in a compact region nearing the loop-top. These results provide new constraints on the acceleration of energetic electrons and the underlying flare intermittent reconnection process.

Coupling geometric and electronic engineering over RuNi ultrafine alloys for fast boosting hydrolytic dehydrogenation of NH3BH3

Developing a suitable catalyst for ammonia borane (AB, NH3BH3) hydrolysis and clarifying the catalytic mechanism are essential and challenging at present. In this work, RuNi nanoalloys are supported on an N/O co-coped glucose-derived carbon (g-C) with the adsorption-NaBH4 reduction method. Attributed to the particular geometric and anchoring effects, as well as the improved hydrophilicity of the multi-level porous support, the RuNi alloys are highly and firmly confined in the framework of g-C. Specifically, the optimal Ru0.25Ni0.75@g-C with an average alloy particle size of 1.4 nm exhibits pleasurable specific hydrogen evolution rate (437,990 mL min−1 gRu−1) and turnover frequency (1612.37 min−1) in NaOH aqueous solution (1.5 M) at 303 K, superior to the corresponding monometallic counterparts (i.e., Ni@g-C and Ru@g-C) and many previous results. In addition, apart from satisfactory durability, the optimal alloy catalyst delivers an encouraging apparent activation energy (28.1 kJ mol−1). Our work not only explains the geometric and electronic effects on the catalytic performance but also offers a promising catalyst for hydrogen release from liquid-phase AB.

Enhancement of catalytic activity in CO2 methanation in Ni-based catalysts supported on delaminated ITQ-6 zeolite

Ni-based catalysts supported on delaminated ITQ-6 zeolite with different Si/Al ratios, 30 and ∞, were tested in the methanation of CO2 and CO. ITQ-6 supports exhibited high surface areas (>590 m2/g), and the catalysts based on them presented elevated CO2 and CO conversion values, turnover frequencies (TOF), and CH4 selectivity (>90 %). Comparing ITQ-6 catalyst and its precursor ferrierite (FER)-based catalyst, H2-chemisorption results confirmed higher metallic dispersion for the former, which resulted in improved H2 uptake and efficient interaction of reaction intermediates via the associative pathway. Combined kinetic studies indicated that their higher available metallic surface contributed to lower apparent activation energies towards CH4 formation, accounting for the higher selectivity values. A 15 wt% Ni-based catalyst supported on ITQ-6 zeolite (Si/Al = 30) exhibited catalytic results (XCO2 = 79 % y SCH4 = 98 %) comparable or even superior to some of the best zeolite-based catalysts reported so far.

Ultrafine CoNi alloy nanoparticles anchored on surface-roughened halloysite nanotubes for highly efficient catalytic hydrogenation of 4-nitrophenol

Ultrafine CoNi alloy nanoparticles anchored on surface-roughened halloysite nanotubes for highly efficient catalytic hydrogenation of 4-nitrophenol

Honeycomb-structured S and N-codoped highly graphitized carbon as a catalyst support for Rh nanoparticles: A new benchmark electrocatalyst for hydrogen evolution reaction

State-of-the-art electrocatalysts are based on catalytically active metal deposited on conductive porous carbon. Herein, we report a new strategy of engineering of a honeycomb-structured S and N dual-doped graphene-like carbon (SNG) as a supporting material for Rh catalysts by a simple low-temperature (850 °C) pyrolysis of S-doped carbon nitride (S-CN) in the presence of Mg. Interestingly, here Mg plays a marvelous dual role not only as a reducing agent for graphitizing the S-CN but also as a precursor for new Mg3N2 and MgS products, which play as pore-generating templates for honeycomb-like structures. This features highly robust graphitized carbon with excellent electrical conductivity, proper S and N content, and porosity, making it highly desirable as catalyst support. Supporting Rh (5.2 wt.%) on SNG outperforms state-of-the-art commercial Pt/C electrodes for hydrogen evolution reaction (HER) under alkaline settings (1.0 M KOH) with an extremely lower overpotential of 13 mV at 10 mA cm−2, a lower Tafel value, and a higher turnover frequency. In addition, Rh/SNG as the cathode for industrial water electrolysis in 6.0 M KOH electrolyte at 70 °C shows excellent performance with only 1.76 Vcell to achieve 500 mA cm−2 and maintains long-term durability with negligible decay. The distinctive properties of SNG, including S and N dual doping, high graphiticity, superior electrical conductivity, and honeycomb-like hierarchical meso‑ and macropore structure, are credited with this remarkable HER performance. The S and N dopants in the SNG framework optimize the electronic structure of the Rh by synergistic interaction between them as illustrated by first-principal density functional theory calculations and electronic structure analysis.

Triazine-wingtips accelerated NHC-Pd catalysed carbonylative Suzuki cross-coupling reaction

Triazine conjugated N-heterocyclic carbene palladium complexes were synthesized for reducing the Pd amount in the catalysts to the ppm levels. This work reports the carbonylative Suzuki-Miyaura coupling of aryl iodides catalyzed by N-heterocyclic carbenes palladium with triazine wingtips (T-NHC-Pd). More importantly, the palladacycles have been used to generate high turnover numbers (TON's) and turnover frequencies (TOF's).

Optimizing Photocatalytic H2 Production by Introduction of Pyrazinyls to WRCs and a New tris-Rhenium Photosensitizer.

The replacement of pyridyl by pyrazinyl in ligands of polypyridyl-based cobalt water reducing catalysts (WRC) shifts reduction potentials anodically. Together with a new, trinuclear ReI photosensitizer, these WRCs show strongly improved photocatalytic performances in turnover numbers (TONs) and maximal H2 evolution rate. Depending on the catalyst structure, up to 65 kTONs at 1 μM WRC concentration were reached. Under electrocatalytic conditions in both DMF and H2O, one of the reported WRCs displays remarkable stability, producing H2 steadily over 21 and 14 d, respectively.

Two-Dimensional Cu(II)-MOF with Lewis Acid-Base Bifunctional Sites for Chemical Fixation of CO2 and Bioactive 1,4-DHP Synthesis via Hantzsch Condensation.

Five- and six-membered heterocycles containing nitrogen or oxygen have been considered as privileged scaffolds in organic chemistry and the chemical industry because of their usage in high-value commodities. Herein, we report a two-dimensional (2D) Cu(II)-based MOF catalyst, IITKGP-40, via the strategic employment of ample Lewis acid-base bifunctional sites (open metal nodes and free pyrazine moieties) along the pore wall. IITKGP-40 could convert toxic CO2 to cyclic carbonates in an atom-economical manner under solvent-free conditions and aromatic aldehyde to bioactive 1,4-DHPs via Hantzsch condensation. Exceptional catalytic performance (99%) and turnover number under mild reaction conditions for CO2 fixation using sterically hindered styrene oxide, and good-to-excellent yields for a wide range of aromatic aldehydes toward 1,4-dihydropyridines (1,4-DHPs) make IITKGP-40 promising as a multipurpose heterogeneous catalyst. Moreover, to demonstrate the practical utility of the catalyst, two biologically important drug molecules, diludine and nitrendipine analogue, have also been synthesized. IITKGP-40 is recyclable for at least three consecutive runs without significant loss of activity, making it promising for real-time applications.

Synthesis of sustainable chemicals and fuels from biomass derivatives using homogeneous ruthenium catalysts

This work describes the synthesis and evaluation as catalysts of two ruthenium(II) complexes, RuC1 and RuC2, containing NPN ligands=(N-(phenyl(pyridin-2-ylamino)phosphino)pyridin-2-amine and 2-((phenyl(pyridin-2-ylmethyl)phosphino)methyl)pyridine, respectively) for furfural and hydroxymethylfurfural hydrogenation. The catalysts displayed exceptional activity under mild conditions (180 °C, 40 bar H2) achieving over 98 % conversion and 98–99 % selectivity towards furfuryl alcohol (from furfural) and bis(hydroxymethyl)furan (from hydroxymethylfurfural) within 30 min, using a substrate-catalyst ratio of 3000:1. Remarkably, the catalysts maintained their efficacy even at a tenfold higher substrate loading (10,000:1), albeit with extended reaction times (3 and 4 h for furfural and hydroxymethylfurfural, respectively). Turnover frequencies (TOFs) were impressive, reaching 3920 h−1 and 3200 h−1 for furfural hydrogenation with the catalyst RuC1 and RuC2, respectively. Similarly, TOFs for hydroxymethylfurfural hydrogenation were 2450 h−1 and 2475 h−1 for RuC1 and RuC2, respectively. Notably, the catalysts exhibited excellent stability under reaction conditions. These findings demonstrate the promising potential of RuC1 and RuC2 for efficient hydrogenation of biomass-derived furans, offering a sustainable approach for the production of valuable bio-based chemicals.

Asymmetric hydrogenation of ketimines with minimally different alkyl groups.

Asymmetric catalysis allows the synthesis of optically active compounds, often requiring the differentiation between two substituents on prochiral substrates1. Despite decades of development of mainly noble metal catalysts, achieving differentiation between substituents with similar steric and electronic properties remains a significant challenge2,3. Here, we introduce a class of earth-abundant manganese catalysts for the asymmetric hydrogenation of dialkyl ketimines to give a range of chiral amine products. These catalysts distinguish between pairs of minimally differentiated alkyl groups bound to the ketimine, such as methyl and ethyl, and even subtler distinctions, like ethyl and n-propyl. The degree of enantioselectivity can be adjusted by modifying the components of the chiral manganese catalyst. This reaction demonstrates a wide substrate scope and achieves a turnover number (TON) of up to 107,800. Our mechanistic studies indicate that the exceptional stereoselectivity arises from the modular assembly of confined chiral catalysts and cooperative non-covalent interactions between the catalyst and the substrate.

Hexagonal Mo Bronze: Single Crystal Structures, Electrocatalytic Hydrogen Evolution, and Proton Conductivity.

Molybdenum trioxide (MoO3) is a well-known transition metal oxide that has drawn much attention as a functional material having numerous applications. However, a vast majority of studies have primarily focused on α-MoO3, the thermodynamically stable polymorph of MoO3. This present work encompasses the synthesis of single crystals of two metastable hexagonal MoO3 described by the formulas {Mn0.03Na0.01}@[Mo0.93VIMo0.07VO3] (1) and {Cu0.01Na0.01}@[Mo0.97VIMo0.03VO3] (2), their comprehensive structural characterization by single-crystal X-ray crystallography, and routine spectral and microscopic studies. Interestingly, compound 1 acts as an efficient electrocatalyst for the hydrogen evolution reaction (HER) as well as an effective proton conductor in comparison to the performance of compound 2. The HER activity of compound 1 is characterized by an overpotential of 340 mV@1 mA cm-2 and a low Tafel slope of 75 mV/decade. The catalyst (compound 1) displays a Faradaic efficiency of 88% with a turnover frequency of 2.9 s-1. The proton conductivity value of this compound (1) is determined to be 4.9 × 10-3 S cm-1 at 55 °C under 98% relative humidity; the relevant proton conduction is operated by the Grotthuss mechanism with an activation energy of 0.17 eV.

Regulating the electronic structure by P-doping cobalt-based catalyst for atomic hydrogen mediated electrocatalytic dechlorination

Electrocatalytic dechlorination by atomic hydrogen (H*) is efficient, but limited by the low efficiency of H* production. Herein, a phosphorus-doped cobalt nitrogen carbon catalyst (Co-NP/C) was prepared, which had high catalytic activity in a wide pH range (3−11). The turnover frequency of Co-NP/C (3.54 min−1) was 1.21–59000 times superior to that of current Pd-based and non-noble metal catalysts (0.00006–2.92 min−1). Co-NP/C significantly enhanced H* generation, which was 1.52, 2.44, and 3.77 times stronger than that of Co-N/C, NP/C, and N/C, respectively, since the introduction of phosphorus was found enhanced the electron density of cobalt and regulated the electron transfer. Co-NP/C showed outstanding catalytic performance after ten cycles and could achieve nearly complete chloramphenicol removal. This regulation method was verified to be effective for other non-noble metal (Fe, Mn, Cu, Ni) phosphorus doped catalysts, proposing a general class for efficient electrochemical dechlorination, which would be of great significance for the elimination of chlorinated organic pollutants.

Exploring the Water Oxidation Catalytic Activity of a Mn-Based Magnetic Metal-Organic Framework: The Role of Proton Conductivity and Oxygen Evolution Reaction Overpotential.

The present work evaluates the water oxidation catalytic activity of a Mn-based metal-organic framework (MOF), which we envisioned to reduce the oxygen evolution reaction (OER) overpotential because of its high electrical conductivity, facilitated by solvent-encapsulated structural features. The presence of Mn centers induces interesting magnetic features in the MOF, which exhibits impressive cryogenic magnetic refrigeration with a ΔSM value of 29.94 J kg-1 K-1 for a field change of ΔH = 5T at 2.3 K. To the best of our knowledge, the ΔSM value of the current system ranked the highest position among the published examples. The crystal structure aligns perfectly with the thematic expectations and features as many as ten metal-coordinated water molecules, forming an extensive web of a hydrogen-bonded network while facing toward the porous channel filled with another set of much-anticipated entrapped lattice water molecules. Such structural features are expected to manifest high proton conductivity, and detailed investigation indeed yields the best value for the system at 1.57 × 10-4 S/cm at 95% humidity and 85 °C. In order to evaluate the thematic notion of a one-to-one relationship between OER and improved electrical conductivity, extensive electrocatalytic water splitting (WS) investigations were carried out. The final results show highly encouraging WS ability of the Mn-MOF, showing the electrocatalytic surface area value of the active species as 0.0686 F/g with a turnover frequency value of 0.043 [(mol. O2) (mol. Mn-MOF)-1 s-1]. Another fascinating aspect of the current communication is the excellent synergy observed between the experimental WS outcomes and the corresponding theoretical data calculated using density functional theory (DFT). Consequently, a plausible mechanism of the overall OER and the role of the Mn-MOF as a water oxidation catalyst, along with the importance of water molecules, have also been derived from the theoretical calculations using DFT.

Vitamin B1-based thiazol-2-ylidene-Ru(II) complexes: recyclable transfer hydrogenation catalysts in water.

The [(p-cymene)RuCl(κ2C,N-{CNHC-NH})]+X- (CNHC = thiazol-2-ylidene) complexes with a bidentate ligand (2: X = Cl and 3: X = PF6) were prepared by a one-pot reaction of vitamin B1 (VB1, 1), Ag2O and [(p-cymene)RuCl2]2. In the complexes, VB1 coordinates through C2 and the exocyclic N in the imino form with the κ2-(C,N) coordination mode. The complexes 2 and 3 are stable in the solid state, but slowly release p-cymene in solution. Furthermore, upon heating in polar solvents, 2 or 3 can be converted by ligand exchange reactions to produce [(L)3RuCl(κ2C,N-{CNHC-NH})]+PF6- (4: L = py).Robustness was improved remarkably for 4. The complex 4 is stable in the solid state and in solution. The complexes 2-4 have been identified by 1H and 13C{1H} 2D NMR spectroscopy and 2 and 4 were studied by X-ray crystallography. In an effort to develop a recyclable catalyst in water, 2-4 were evaluated for TH of ketones and aldehydes with an azeotropic mixture of HCOOH/Et3N in water. The complexes 3 and 4 exhibited very good catalytic activity and 4 could be reused nine times without significant loss of activity, giving a high turnover frequency (TOF50%(h-1) = 1286).

Flower-like hierarchical TS-1/Al2O3 composite supported NiMo catalysts for efficient hydrodesulfurization of dibenzothiophenes

Flower-like Al2O3 materials were successfully incorporated by TS-1 nanocrystals to synthesize Flower-like TS-1-Al2O3 (FTA) micro-mesoporous composite by a facile nano self-assembly method. FTA supported NiMo bimetallic catalysts (NiMo/FTA) were further constructed and applied to the hydrodesulfurization (HDS) reactions for dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). FTA composites were constructed from flower-like pore structures stacked with nanosheets, which are conducive to the even load and dispersion of active species and the diffusion of macromolecular sulfur-containing compounds with relatively low resistance. The fabrication of TS-1 nanocrystals effectively enhanced the acidic sites, regulated the metal-support interaction, which could increase the stacking layer of MoS2 slabs, and producing more NiMoS-II type active phase. NiMo/FTA-10 presented the excellent HDS conversion of DBT (99.3 %) and 4,6-DMDBT (93.2 %) at 10 h−1, the highest reaction rate constant (2.6 × 10-7 mol g−1 s−1) and turnover frequency values (6.7 × 10-4 s−1) for 4,6-DMDBT HDS reaction. Compared to the direct desulfurization route (DDS), hydrogenation route (HYD) and isomerization route (ISO) can effectively eliminate the steric hindrance of 4,6-DMDBT, which is more conducive to the HDS reaction.

Mechanistic investigations of polyaza[7]helicene in photoredox and energy transfer catalysis.

Organic photocatalysts frequently possess dual singlet and triplet photoreactivity and a thorough photochemical characterization is essential for efficient light-driven applications. In this article, the mode of action of a polyazahelicene catalyst (Aza-H) was investigated using laser flash photolysis (LFP). The study revealed that the chromophore can function as a singlet-state photoredox catalyst in the sulfonylation/arylation of styrenes and as a triplet sensitizer in energy transfer catalysis. The singlet lifetime is sufficiently long to exploit the exceptional excited state reduction potential for the activation of 4-cyanopyridine. Photoinduced electron transfer generating the radical cation was directly observed confirming the previously proposed mechanism of a three-component reaction. Several steps of the photoredox cycle were investigated separately, providing deep insights into the complex mechanism. The triplet-excited Aza-H, which was studied with quantitative LFP, is formed with a quantum yield of 0.34. The pronounced triplet formation was exploited for the isomerization reaction of (E)-stilbene to the Z-isomer and the cyclization of cinnamyl chloride. Catalyst degradation mainly occurs through the long-lived Aza-H triplet (28 µs), but the photostability is greatly increased when the triplet efficiently reacts in a catalytic cycle such that turnover numbers exceeding 4400 are achievable with this organocatalyst.

Solid-State Oxidation of Alcohols in Gold-Coated Milling Vessels via Direct Mechanocatalysis.

This paper presents a novel approach for the selective oxidation of alcohols to their corresponding aldehydes through direct mechanocatalysis, employing a gold-coated milling vessel as catalyst and air as the oxidation agent. By adjusting milling frequency, media, and duration, high catalytic efficiencies and selectivities are achieved. Remarkably, yields of up to 99% are obtained for specific substrates, with a turnover number (TON) of 8200 and a turnover frequency (TOF) of 0.77 s-1, surpassing existing alternatives. Confirmation of the catalytic reaction indeed occurring on the milling tool surface was achieved through X-ray photoelectron spectroscopy (XPS).

Solid‐State Oxidation of Alcohols in Gold‐Coated Milling Vessels via Direct Mechanocatalysis

This paper presents a novel approach for the selective oxidation of alcohols to their corresponding aldehydes through direct mechanocatalysis, employing a gold‐coated milling vessel as catalyst and air as the oxidation agent. By adjusting milling frequency, media, and duration, high catalytic efficiencies and selectivities are achieved. Remarkably, yields of up to 99% are obtained for specific substrates, with a turnover number (TON) of 8200 and a turnover frequency (TOF) of 0.77 s‐1, surpassing existing alternatives. Confirmation of the catalytic reaction indeed occurring on the milling tool surface was achieved through X‐ray photoelectron spectroscopy (XPS).