Turnover Frequency Value Research Articles

Swapping Catalytic Active Sites from Cationic Ni to Anionic S in Nickel Sulfide Enables More Efficient Alkaline Hydrogen Generation

AbstractTuning active sites in catalyst design is the key to boosting the intrinsic activity of hydrogen evolution reaction (HER). Cationic Ni has been widely established as an active site in nickel sulfide due to the relatively low Gibbs free energy of hydrogen adsorption (ΔGH*). However, one of the big unsettled issues is whether S can be activated as a more active site than Ni in NiS2. Herein, the swapping of catalytic active sites from cationic Ni to anionic S in a hierarchical structure consisting of NiS2 nanoflowers grown on dual‐phased NiS2‐NiS foam (denoted as NiS2/NiS2‐NiS) is shown. A combined study of theoretical calculations and X‐ray photoelectron spectroscopy analysis demonstrate the remarkably antidromic electron transfer from Ni to S sites, therefore relieving the adsorption of hydrogen species and endowing a higher intrinsic activity at the S site over the Ni site. The new catalyst therefore exhibits superior HER performance, identified by doubling in the intrinsic activity and a twofold increased turnover frequency value compared to its pure NiS2 counterpart (0.028 s−1 vs 0.015 s−1 at the applied overpotential of 200 mV). The NiS2/NiS2‐NiS electrode also demonstrates outstanding activity toward the oxygen evolution reaction and overall water splitting.

Immobilized Supramolecular Systems as Efficient Synzymes for CO2 Activation and Conversion

AbstractSupramolecular catalysis can provide distinct advantages for the catalytic conversion of CO2 into carbonates by cycloaddition to epoxides. For example, the absence of metals in the catalytic site, and the easy design for optimization. The incorporation of multiple functionalities in pseudopeptidic macrocycles with a pendant arm allows catalytic systems to be obtained where halide anions (nucleophilic activating agents for epoxides), hydrogen bond acceptor sites (activating agents for epoxides and stabilizing sites for anionic intermediates), and amine groups (Lewis basic sites for activating CO2) are in proximity. This allows a high activity in the cycloaddition of CO2 to styrene oxide under mild conditions (turnover number (TON) = 900, CO2 balloon, 100 °C, 5 h). The primary amino groups in the arm facilitate the immobilization of these macrocyclic structures in cross‐linked polymeric matrices containing ammonium halide fragments. Such multifunctional insoluble polymers afford excellent catalytic results with high TON and turnover frequency values and remarkable productivities (>10 gprod gresin−1 h−1). This activity is maintained for a variety of epoxides and is retained after several catalytic runs. Their performance is significantly higher than those reported for most heterogenous supramolecular catalytic systems for CO2 transformation into carbonates.

Network-Like Platinum Nanosheets Enabled by a Calorific-Effect-Induced-Fusion Strategy for Enhanced Catalytic Hydrogenation Performance.

In this paper, we report the construction of network-like platinum (Pt) nanosheets based on Pt/reduced graphite oxide (Pt/rGO) hybrids by delicately utilizing a calorific-effect-induced-fusion strategy. The tiny Pt species first catalyzed the H2-O2 combination reaction. The released heat triggered the combustion of the rGO substrate under the assistance of the Pt species catalysis, which induced the fusion of the tiny Pt species into a network-like nanosheet structure. The loading amount and dispersity of Pt on rGO are found to be crucial for the successful construction of network-like Pt nanosheets. The as-prepared products present excellent catalytic hydrogenation activity and superior stability towards unsaturated bonds such as olefins and nitrobenzene. The styrene can be completely converted into phenylethane within 60 min. The turnover frequency (TOF) value of network-like Pt nanosheets is as high as 158.14 h−1, which is three times higher than that of the home-made Pt nanoparticles and among the highest value of the support-free bimetallic catalysts ever reported under similar conditions. Furthermore, the well dispersibility and excellent aggregation resistance of the network-like structure endows the catalyst with excellent recyclability. The decline of conversion could be hardly identified after five times recycling experiments.

In situ prepared tungsten(VI) oxide supported Pd0 NPs, remarkable activity andreusability in H2 releasing from dimethylamine borane

The study reports preparation, characterization, and catalytic activity of Pd0/WO3 nanoparticles in H2 evolution from dimethylamine borane (DMAB). The active catalyst called Pd0/WO3 NPs were obtained from the in situ reduction of Pd2+/WO3 precatalyst and tested in H2 releasing from DMAB. Pd0/WO3 NPs were found as remarkable catalysts providing a turnover frequency value of 948.0 h-1 in H2 releasing from DMAB at 60.0 ± 0.5 °C. The results of some advanced analytical techniques reveal that Pd0 NPs show uniform dispersion on the surface of tungsten(VI) oxide (WO3) with 5.85 ± 0.57 nm of particle size. The reducible nature of tungsten(VI) oxide improves the catalytic efficiency of Pd0/WO3 NPs in H2 generation from DMAB. Pd0/WO3 NPs were found as active catalysts even after the 6th run of dehydrogenation reaction. The report also covers the kinetic performance of the catalyst.

TOF AND TON EVOLUTION IN HETEROGENEOUS CATALYSIS

The review article shows the evolutionary path that turnover frequency (TOF) and turnover number (TON) have passed from “Boreskov`s Rule” to their modern definitions. From the equation catalysis rate, the second method for calculating TOF is obtained using the characteristics of catalyst material. Was prooved the possibility of obtaining TOF in two ways - using the characteristics of catalysis process and using the characteristics of the catalyst and reagents. The equivalence of two methods of TOF calculation is proved. It turned out that TOF is not a complete and unambiguous characteristic of the catalyst, as it was usually believed. TOF is only partially dependent on the characteristics of the catalyst material. It turned out that TOF is not a characteristic of a catalyst, but of a “catalyst + reagents” system, and its value directly depends on the state of their oxidation. It is proposed to use the list of oxidation states of chemical elements as the main tool in the selection of catalysts. The Sabatier principle limits the TOF and TON values by limiting the multielectron transitions when the oxidation state of the active sites of the catalyst changes. An explanation for the effect of overcoming the Sabatier prohibition is given, in which external synchronous action on the catalyst makes it possible to achieve a catalytic reaction speed higher than the Sabatier maximum.

AgPd Nanoparticles Anchored on TiO2 Derived from a Titanium Metal–Organic Framework for Efficient Dehydrogenation of Formic Acid

AbstractTiO2 with a disc‐like morphology was prepared by direct pyrolysis of a titanium metal–organic framework [MIL‐125(Ti)] in air. Highly dispersed bimetallic AgPd nanoparticles were then anchored on TiO2 and their catalytic performance in the dehydrogenation of formic acid was systematically investigated. The Ag/Pd ratio had a significant effect on the catalyst activity, with Ag2Pd8/TiO2 exhibiting the highest total turnover frequency value of 2579 h−1 at 60 °C under an ambient atmosphere. This excellent performance was attributed to the low‐crystallinity anatase phase structure and large specific surface area of the TiO2 carrier, the alloying effect of the AgPd nanoparticles, and abundant electron transfer between the carrier and metal alloy nanoparticles.

Effect of potassium and platinum contents on catalytic performance of Pt/Al2O3 monometallic catalysts for propane dehydrogenation

Low-loaded Pt-based catalysts supported on γ-Al2O3 (bare or modified by K) were studied for propane dehydrogenation. The alumina support was modified by adding 0.5 and 4 wt.% of K using potassium acetate as precursor. 0.5 wt.% of K is sufficient for suppressing the acidity of the alumina. Ultradispersed Pt supported catalysts (0.1, 0.2 and 0.3 wt.%) were obtained by ion exchange on Al2O3 or Al2O3–0.5 K. During propane dehydrogenation reaction, 0.3 wt.%Pt/Al2O3 strongly deactivates, mainly due to coke deposition. For the same Pt loading, the presence of K strongly decreases this phenomenon without modifying the initial turnover frequencies (TOF). On the series supported on Al2O3–0.5 K, there is no effect of the Pt content on the TOF values showing that whatever the sample, ultradispersed Pt entities present the same activity. Finally, the selectivity towards propene formation strongly depends on propane conversion and seems to be not affected by the change in K and Pt contents.

Selectivity and efficiency of nitrite electroreduction catalyzed by a series of Keggin polyoxometalates

In this paper we have studied the influence of the charge and redox potential for a series of polyoxometalates (POMs) belonging to the Keggin family on the electrochemical reduction of nitrite or nitrous acid (NO2– or HNO2). We have focused on the correlation of gaseous products and the redox properties of the POMs. Analysis by differential electrochemical mass spectrometry (DEMS) showed that the gaseous products (NO and N2O) are formed at pH 2 and 5 by reaction of HNO2 or NO2– with the reduced POM. Only NO is detected when the POM ([PW12O40]3- , [SiW12O40]4-, [BW12O40]5-) undergoes the first one-electron reduction wave. However, N2O is obtained along with NO as soon as the POM undergoes two-electron reduction. The potential of the first reduction wave of the POM is correlated with its charge. Consequently NO and/or N2O appear at lower overpotential for [PW12O40]3- and [SiW12O40]4- followed by [BW12O40]5- and [H2W12O40]6-. Microkinetic simulations of the experimental current–potential curves were used to estimate the rate constant kc of the reaction between the reduced POM and HNO2 or NO2– in solution and the turnover frequency (TOF). At low overpotentials, the values of TOF are decreasing in a trend following the redox potential of the POM: [PW12O40]3- > [SiW12O40]4- > [BW12O40]5- while at high overpotentials the TOFs are dominated by kc following the reverse order. At pH 5 kc values are about 70 times lower than at pH 2. This is attributed to the decrease in the proton concentration and to the electrostatic repulsion between the negatively charged POMn- and NO2–.

Influence of the synthesis parameters on the proton exchange membrane fuel cells performance of Fe–N–C aerogel catalysts

Fe–N–C aerogel catalysts were prepared by sol–gel polycondensation of resorcinol, melamine and formaldehyde precursors in the presence of FeCl 3 salt, followed by supercritical drying and thermal treatments. The effect of the mass ratio of precursors on the microstructure, iron speciation and oxygen reduction reaction (ORR) performance of the Fe–N–C aerogels was investigated by N 2 sorption, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Mössbauer spectroscopy, X-ray absorption spectroscopy, CO chemisorption and rotating disk electrode in acidic medium. The best ORR performance (activity and mass transport) was obtained by an optimum balance between pore structure and active Fe-N x species. Through acid washing, the durability of the catalyst was further improved by eliminating unstable and inactive species, particularly iron nanoparticles and iron carbide. From the CO chemisorption and turnover-frequency value, the surface sites were comparable with the highest values reported in literature. Finally, Fe–N–C aerogel catalyst was implemented a in membrane–electrode assembly with an active area of 25 cm 2 and tested in single cell, emphasizing the importance of the ink formulation on the performance. • Optimum balance between pore texture and active FeNx species for best ORR performance. • Better mass-transport properties due to higher mesopore volume in the Fe–N–C catalyst. • XAS and 57 Fe Mössbauer spectroscopies identified FeN x and Fe-inorganic species. • Acid washing treatment eliminates the unstable and inactive Fe-inorganic species. • Need to adapt the ink formulation to the catalyst texture for high performance MEAs.

Alkali-assisted synthesis of ultrafine NiPt nanoparticles immobilized on La2O2CO3 for highly efficient dehydrogenation of hydrous hydrazine and hydrazine borane

Designing highly active and selective metal nanocatalysts for use in liquid chemical hydrides is highly attractive but remains a critical challenge. Herein, bimetallic NiPt nanoparticles (NPs) immobilized on lanthanum oxycarbonate (NiPt/La2O2CO3) have been facilely synthesized via an alkali-assisted reduction method. The introduction of NaOH is the key factor to the formation of the well-dispersed and ultrafine NiPt NPs (2.8 nm). The obtained Ni0.6Pt0.4/La2O2CO3 exhibits an extraordinarily high catalytic activity and 100% H2 selectivity for hydrous hydrazine dehydrogenation, providing a turnover frequency value (TOF) high up to 490 h−1 at 298 K. Additionally, the NiPt/La2O2CO3 prepared with NaOH demonstrates higher catalytic performance and lower activation energy than that of NiPt/La2O2CO3_N prepared without NaOH. Moreover, the NiPt/La2O2CO3 also shows an outstanding catalytic efficiency (TOF = 1200 h−1) for dehydrogenation of hydrazine borane at 298 K. The efficient catalytic performance may be attributed to the small size effect, the strong metal-support interaction, and the NaOH promotion effect. This alkali-assisted reduction method provides a new perspective for preparing of highly efficient catalysts in clean energy application.

Pd-promoted heteropolyacid on mesoporous zirconia as a stable and bifunctional catalyst for oxidation of thiophenes

Sulfur-containing compounds are undesirable in fossil fuels, thus stringent regulations are in place to curb their atmospheric emissions. In this context, we hypothesized and showed that acid catalysis promoted by metal particles could be used to oxidize dibenzothiophene (DBT) and benzothiophene (BT) into sulfones that, in principle, can be subsequently removed. In specific, we studied the oxidative step of catalytic oxidative desulfurization (CODS) processes using a new bifunctional catalyst based on Pd-promoted molybdovanadophosphoric acid (Pd-PMV) supported on mesoporous zirconia (Pd-PMV/mZrO2). The PMV was supported on zirconia using a one-pot sol–gel method, and the Pd nanoparticles were deposited in a subsequent step. The activity of catalyst was excellent, with high conversions >99% and a turnover frequency (TOF) value of 384 h−1 for DBT, using catalyst Pd-15PMV/mZrO2, which had 15-wt% loading of PMV. tert-butyl hydrogen peroxide (tBHP) was used as an oxidant under relatively mild reaction conditions. The catalyst had both Brønsted and Lewis acid sites, and the Pd nanoparticles enhanced its synergistic catalytic performance. Up to a threshold level loading of PMV, the thermal stability of the catalysts was higher than the underlying mZrO2 support. A high catalytic activity was maintained with only a minor loss over 10 cycles, making the catalyst promising for oxidative deep-desulfurization of liquid hydrocarbon fuels.

Rhodium nanoparticles confined in titania nanotubes for efficient Hydrogen evolution from Ammonia Borane

Designing efficient catalysts for hydrogen evolution from hydrolysis of ammonia borane (AB) have attracted considerable attention. Rhodium (Rh) based catalysts with rational design present remarkable catalytic performance for the reaction. Herein, we report the confined Rh@TiO2 catalysts synthesized by atomic layer deposition combining with the sacrificial template approach, in which the Rh nanoparticles are uniformly confined on the inner surface of the porous titania nanotubes. The optimized catalysts show high catalytic activity with a turnover frequency value of 334.1 molH2·molRh-1·min-1 and better durability. Mechanistic investigation demonstrates that the cleavage of OH bands in water should be the rate determining step, and the appropriate concentration of NaOH can further enhance the hydrogen evolution activity. The catalysts can also achieve the hydrogenation of various organic substrates using AB as the hydrogen source. In addition, our present strategy is general and can be extended to the synthesis of other confined catalysts for various catalytic reactions.

Preparation and characterization of new palladium complex immobilized on (chitosan)/PoPD biopolymer and its catalytic application in Suzuki cross‐coupling reaction

The present work reports the design, synthesis, and characterization of palladium complex immobilized on chitosan/poly(o‐phenylenediamine) (CS‐PoPD‐Pd) for the catalytic application in the Suzuki–Miyaura CC cross‐coupling reaction through a nontoxic, inexpensive, eco‐friendly, and practical method. Fourier‐transform–infrared (FT‐IR) spectroscopy, scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy (EDX), elemental mapping, X‐ray diffraction (XRD), and inductively coupled plasma‐optical emission spectrometry (ICP‐OES) techniques were used for analyzing the prepared catalyst. Characterization studies showed that CS‐PoPD‐Pd was successfully synthesized according to our design. CS‐PoPD‐Pd composite demonstrated high product yield and high turnover number (TON) and turnover frequency (TOF) values with small catalyst loading for the Suzuki–Miyaura CC cross‐coupling reaction under mild reaction conditions. Besides, the synthesized CS‐PoPD‐Pd composite could be readily recycled and reused for at least five runs without discernible loss of its catalytic activity.

Hierarchical sulphide - phosphide NiSP /NF catalyst prepared by gradient electrodeposition for oxygen evolution reaction

Rational design and fabrication of electrocatalysts with low-cost and high efficiency is highly desired. In this paper, a hierarchical honey-comb catalyst NiSP supported on nickel foam (NF) was synthesized by a facile gradient electrodeposition method. Attributed to the presence of Ni2P and NiS and the synergistic effect among phosphide and sulphide, NiSP /NF catalyst exhibited a low overpotential value of 281 mV to achieve 10 mA cm−2 and the Tafel value of 68.51 mV dec−1 in 1 M KOH. Benefited from the gradient electrodeposition process resulting in the nanoparticles decorate honey-comb structure, the electrochemical active surface area reaches to 3571.5 cm2, and the turnover frequency (TOF) value is 2.0781 s−1. Due to the good electric conductivity of phosphide and sulphide, NiSP /NF electrode kept to exhibit excellent stability for 60 h. Therefore, the electrode obtained by our gradient electrodeposition method was expected to be a promising candidate catalyst.

Iridium-Catalyzed Asymmetric Transfer Hydrogenation of Quinolines in Biphasic Systems or Water.

An asymmetric transfer hydrogenation (ATH) of quinolines in water or biphasic systems was developed. This ATH reaction proceeds smoothly without the need for inert atmosphere protection in the presence of a water-soluble iridium catalyst, which bears an easily available aminobenzimidazole ligand. This ATH system can work at a catalyst loading of 0.001 mol % (S/C = 100 000, turnover number (TON) of up to 33 000) under mild reaction conditions. The turnover frequency (TOF) value can reach as high as 90 000 h-1. A variety of quinoline and N-heteroaryl compounds are transformed into the desired products in high yield and up to 99% enantiomeric excess (ee).

Optimization of Oxygen Evolution Reaction with Electroless Deposited Ni-P Catalytic Nanocoating.

The low efficiency of water electrolysis mostly arises from the thermodynamic uphill oxygen evolution reaction. The efficiency can be greatly improved by rationally designing low-cost and efficient oxygen evolution anode materials. Herein, we report the synthesis of Ni–P alloys adopting a facile electroless plating method under mild conditions on nickel substrates. The relationship between the Ni–P properties and catalytic activity allowed us to define the best conditions for the electroless synthesis of highperformance Ni–P catalysts. Indeed, the electrochemical investigations indicated an increased catalytic response by reducing the thickness and Ni/P ratio in the alloy. Furthermore, the Ni–P catalysts with optimized size and composition deposited on Ni foam exposed more active sites for the oxygen evolution reaction, yielding a current density of 10 mA cm−2 at an overpotential as low as 335 mV, exhibiting charge transfer resistances of only a few ohms and a remarkable turnover frequency (TOF) value of 0.62 s−1 at 350 mV. The present study provides an advancement in the control of the electroless synthetic approach for the design and large-scale application of high-performance metal phosphide catalysts for electrochemical water splitting.

Iodine-Doping-Induced Electronic Structure Tuning of Atomic Cobalt for Enhanced Hydrogen Evolution Electrocatalysis

The development of strategies for tuning the electronic structure of the metal sites in single-atom catalysts (SACs) is the key to optimizing their activity. Herein, we report that iodine doping within the carbon matrix of a cobalt-nitrogen-carbon (Co-N-C) catalyst can effectively modulate its electronic structure and catalytic activity toward the hydrogen evolution reaction (HER). The iodine-doped Co-N-C catalyst shows exceptional HER activity in acid with an overpotential of merely 52 mV at 10 mA cm-2, a small Tafel slope of 56.1 mV dec-1, making it among the best SACs based on both precious and nonprecious metals. Moreover, this catalyst possesses a high turnover frequency (TOF) value of 1.88 s-1 (η = 100 mV), which is about 1 order of magnitude larger than that (0.2 s-1) of the iodine-free counterpart. Experimental and theoretical studies demonstrate that the introduction of iodine dopants lowers the chemical oxidation state of the Co sites, resulting in the optimized hydrogen adsorption and facilitated HER kinetics. This work provides an alternative strategy to regulate the electronic structure of SACs for improved performance.

S-doped MIL-53 as efficient heterogeneous electro-Fenton catalyst for degradation of sulfamethazine at circumneutral pH

The reduced S-modified MIL-53(Fe) was prepared by sulfurizing MIL-53(Fe) at low temperature, which was an efficient electro-Fenton catalyst at wide pH range (3–9) for sulfamethazine (SMT) degradation. The best temperature and MIL-53(Fe)/S ratio were 350 °C and 1:2, at which the BET surface area was much enlarged. The MIL-53(Fe) surface was etched by S to many 2D nanosheets with the thickness of ~50 nm, while S2–2 replaced OH- to coordinate with Fe2+ and increased the Fe2+ content, which improved the catalytic performance. Even at initial pH of 7.0, the SMT removal was 95.8%, and the rate constant (k) in the Hetero-EF process was 16-folds of that in the Homo-EF process. The turnover frequency (TOFd) value of MIL-53(Fe)/S(1:2)−350 was 0.48 L g−1 min−1, which was 6.8 times that of commercial FeS2. The S2–2in catalyst adjusted the pH superfast, and promoted the generation of Fe2+ and thus efficiently activating H2O2 to form surface ·OH, which was verified to be the main radical by EPR and radical scavenger experiments. This catalyst showed promising prospect for environmental application and could be regenerated by sulfidation method. S-doped MIL-53(Fe) was an excellent pH regulator, thus promoting promising application in Hetero-EF processes.

Synergetic Cobalt‐Copper‐Based Bimetal–Organic Framework Nanoboxes toward Efficient Electrochemical Oxygen Evolution

AbstractThe development of efficient oxygen electrocatalysts and understanding their underlying catalytic mechanism are of significant importance for the high‐performance energy conversion and storage technologies. Herein, we report novel CoCu‐based bimetallic metal–organic framework nanoboxes (CoCu‐MOF NBs) as promising catalysts toward efficient electrochemical oxygen evolution reaction (OER), fabricated via a successive cation and ligand exchange strategy. With the highly exposed bimetal centers and the well‐designed architecture, the CoCu‐MOF NBs show excellent OER activity and stability, with a small overpotential of 271 mV at 10 mA cm−2 and a high turnover frequency value of 0.326 s−1 at an overpotential of 300 mV. In combination of quasi in situ X‐ray absorption fine structure spectroscopy and density‐functional theory calculations, the post‐formed CoCu‐based oxyhydroxide analogue during OER is believed to account for the high OER activity of CoCu‐MOF NBs, where the electronic synergy between Co and neighbouring Cu atoms promotes the O−O bond coupling toward fast OER kinetics.

Synergetic Cobalt-Copper-Based Bimetal-Organic Framework Nanoboxes toward Efficient Electrochemical Oxygen Evolution.

The development of efficient oxygen electrocatalysts and understanding their underlying catalytic mechanism are of significant importance for the high-performance energy conversion and storage technologies. Herein, we report novel CoCu-based bimetallic metal-organic framework nanoboxes (CoCu-MOF NBs) as promising catalysts toward efficient electrochemical oxygen evolution reaction (OER), fabricated via a successive cation and ligand exchange strategy. With the highly exposed bimetal centers and the well-designed architecture, the CoCu-MOF NBs show excellent OER activity and stability, with a small overpotential of 271 mV at 10 mA cm-2 and a high turnover frequency value of 0.326 s-1 at an overpotential of 300 mV. In combination of quasi in situ X-ray absorption fine structure spectroscopy and density-functional theory calculations, the post-formed CoCu-based oxyhydroxide analogue during OER is believed to account for the high OER activity of CoCu-MOF NBs, where the electronic synergy between Co and neighbouring Cu atoms promotes the O-O bond coupling toward fast OER kinetics.