Catalyst Turnover Frequency Research Articles

Acid‐Cooperative Transition Metal‐Catalysed Oxygen‐Atom‐Transfer: Ruthenium‐Catalysed C−H Oxygenation

AbstractThe prospect of developing methods to control the reactivity of metal(oxo) holds appeal for both synthetic and bioinorganic chemistry. In this study, we observed that carboxylic acids enhance reactivity in C−H oxidation by forming hydrogen bonds with the oxo ligand, thereby increasing the basicity and electrophilicity of the oxo intermediate. Additionally, we discovered that H‐dicarboxylate ligands, such as oxalate, malonate, and maleate, significantly improve the catalyst turnover frequency (up to 760/h) in unactivated C(sp3)−H oxidation. Furthermore, the addition of maleic acid was also found to activate Fe and Mn(pdp)‐catalysed C−H oxygenation. Notably, Ru(bpga)(H‐maleate)2 3‐catalyzed C−H functionalization exhibited a broad functional group tolerance, including bromine, hydroxyl, benzoate, nitro, nitryl, sulfonate, imide, sulfonylamide, and carbamate, yielding up to 98% in a site‐ and chemoselective manner with only 0.1–2.0 mmol% loading.

Electrochemical water oxidation reaction by dinuclear Re(V) oxo complexes with a 1,4-benzoquinone core via the redox induced electron transfer (RIET) process.

We report two dinuclear rhenium(V) oxo complexes 1 and 2 types, [ReV(O)(Cl)3(L2-)ReV(O)(Cl)3][NBu4]2 (1, L2- = dianionic 2,5-dihydroxy 1,4-benzoquinone (DBQ2-)) and (2, L2- = dianionic chloranilic acid (CA2-) ligands), as a homogeneous electrocatalyst for water oxidation reactions in the acetonitrile-water mixture. The evolution of dioxygen gas at the anode was confirmed by a GC-TCD study. In controlled potential electrolysis (CPE), oxidation at 1.30 V (vs. Ag/AgCl) at neutral pH, 1 and 2 afforded 1+ [ReVI(O)(Cl)3(DBQ˙3-)ReVI(O)(Cl)3]- and 2+ [ReVI(O)(Cl)3(CA˙3-)ReVI(O)(Cl)3]- ions, respectively, via the redox induced electron transfer (RIET) process. Electrochemically generated species of 1+ and 2+ could be isolated in dry acetonitrile. 1+ and 2+ ions give strong EPR signals in fluid solution as well as under frozen glass conditions due to the [ReVI(O)(Cl)3(L˙3-)ReVI(O)(Cl)3]- ↔ [ReVI(O)(Cl)3(L2-)ReV(O)(Cl)3]- (where L2- = DBQ2- and CA2-) equilibrium. However, the continuation of the CPE study (1.30 V vs. Ag/AgCl) in the presence of acetonitrile-water mixture oxidised the in situ generated species of 1+ and 2+ to higher valent ReVIO species. These species (1+ and 2+) bound water through the water nucleophilic attack (WNA) to produce peroxide intermediate species of [ReV(OOH)(Cl)3(DBQ2-)ReV(OOH)(Cl)3] (A1) and [ReV(OOH)(Cl)3(CA2-)ReV(OOH)(Cl)3] (A2) for catalysts 1 and 2, respectively. Interestingly, A1 and A2 were authenticated and analysed by ESI mass spectrometry and infrared spectroscopy and were the active precursors of this water oxidation process. The extent of current generation under similar conditions suggested that complex 1 is superior to complex 2 for the water oxidation reaction. Notably, the maximum turnover frequency (TOFmax) of catalysts 1 and 2 were 2.1 and 1.6 s-1 at 0.27 V and 0.24 V over potential, respectively, which is very significant in WOR.

Mutual Reinforcement of Evaporation and Catalysis for Efficient Freshwater–Salt–Chemical Production

AbstractThe development of mutually reinforcing solar‐driven interfacial evaporation (SDIE) and integrated functional materials/systems to achieve efficient production of freshwater and energy/matters simultaneously under extremely high solar utilization is in high demand. Herein, an integrated SDIE reaction system (reduced graphene oxide (rGO)‐palladium (Pd) catalytic evaporator, rGO‐Pd) is first reported, where SDIE and the integrated catalytic reaction are mutually reinforced. The apparent utilization of solar to thermal energy by the integrated SDIE reaction system is a combination of evaporative utilization and catalytic utilization. The reaction heat released by the rGO‐Pd catalytic evaporator enhances its anti‐salt water production performance to a record of 12.7 L m−2 h−1, surpassing the reported performance of other integrated SDIE reaction systems. In the rGO‐Pd catalytic evaporator, the synergetic effect of photothermal and rapid mass transfer significantly increases the catalytic activity (turnover frequency) of Pd catalysts up to a record 125.07 min−1, which is about 3.75 times of the condition without light. This integrated SDIE reaction system can effectively and simultaneously produce freshwater, salt, and catalyzed chemicals after evaporating water to dryness. This study paves the way for SDIE's high‐performance applications in future integrated water, energy, and environmental systems.

A Highly Active and Selective Zirconium-Based Catalyst System for the Industrial Production of Poly(lactic acid).

The biodegradable, aliphatic polyester poly(lactic acid), PLA, is a leading bio-based alternative to petrochemical-derived plastic materials across a range of applications. Widely reported in the available literature as a benchmark for PLA production via the bulk ring-opening polymerization of lactides is the use of divalent tin catalysts, and particularly tin(II) bis(2-ethylhexanoate). We present an alternative zirconium-based system that combines an inexpensive Group IV metal with the robustness, high activity, control, and designed compatibility with existing facilities and processes, that are required for industrial use. We have carried out a comprehensive kinetic study and applied a combined experimental and theoretical approach to understanding the mechanism by which the polymerization of lactide proceeds in the presence of this system. In the laboratory-scale (20 g) polymerization of recrystallized racemic d,l-lactide (rac-lactide), we have measured catalyst turnover frequencies up to at least 56,000 h-1, and confirmed the reported protocols' resistance toward undesirable epimerization, transesterification, and chain scission processes, deleterious to the properties of the polymer product. Further optimization and scale-up under industrial conditions have confirmed the relevance of the catalytic protocol to the commercial production of melt-polymerized PLA. We were able to undertake the efficient preparation of high-molecular-weight PLA on the 500-2000 g scale, via the selective and well-controlled polymerization of commercial polymer-grade l-lactide under challenging, industrially relevant conditions, and at metal concentrations as low as 8-12 ppm Zr by weight ([Zr] = 1.3 × 10-3 to 1.9 × 10-3 mol %). Under those conditions, a catalyst turnover number of at least 60,000 was attained, and the activity of the catalyst was comparable to that of tin(II) bis(2-ethylhexanoate).

RICE EXTRACT-ASSISTED GREEN SYNTHESIS OF Au NANOPARTICLES: CATALYTIC AND SERS ACTIVITIES

In this paper, we investigate the impact of rice-starch stabilization of gold nanoparticles (AuNPs) on their catalytic and surface-enhanced Raman spectroscopy (SERS) activities. A typical green synthesis protocol was used to prepare AuNPs with rice-starch functioning as the reductant and the stabilizer. The localized surface plasmon resonance (LSPR) of the starch-stabilized AuNPs is pH-sensitive and got red-shifted with the medium’s pH. The average size of the AuNPs formed decreased with the amount of rice-starch up to a certain level. TEM result shows that the polymeric behavior of starch extract may help synthesize the Au nanorod morphologies. AuNPs stabilized by different amounts of rice-starch were applied as the catalysts for the model [Formula: see text]-nitrophenol reduction reaction. For all AuNP catalysts used, the reaction followed pseudo-first-order kinetics. The catalyst turnover frequencies increased up to a certain level of starch functionalization. Further increase in starch led to a fall in the activity of AuNPs catalyst. The evidence on rate kinetics, induction times, activation energies, and turnover frequencies is combined to explain this phenomenon in terms of starch content. The SERS response of these AuNPs as substrates has also been investigated for methylene blue as the target molecules. AuNP substrates with an excess amount of starch weaken the SERS response and it can be ascribed to decreased light scattering efficiency of NPs as well as reduced interactions of methylene blue molecules with the NP surface.

Synthesis of TS-1 from supported embryonic to nano-/micro-metersized crystalline particles: The impact of accessibility of Ti species on the catalytic performance

This study reports a seeding-polymer-assisted method using inexpensive tetrapropylammonium bromide (TPABr) to synthesize titanium silicalite-1 (TS-1) ranging from supported embryonic (<8 nm) to crystalline nano-/micrometersized (≈0.1–1.0 μm) particles by kinetic regulation. The in situ grown embryonic TS-1 on the amorphous silica presents defective Ti(OH)(OSi)3, tetrahedral framework Ti (“TiO4”), and extraframework Ti (“TiO6”) species, while crystalline nano-/micro-metersized TS-1 particles exhibit either “TiO4” or both “TiO4” and “TiO6” species and a smaller external surface area, Sext. Turnover frequency (TOF) of TS-1 catalysts in the oxidation of dibenzothiophene (DBT) changes by an order of magnitude with the alteration of active Ti species. For instance, the TOF of supported embryonic TS-1 is 224.3 h−1 while it is 32.6 h−1 for the micro-metersized particles due to differences in the active Ti species. However, the TOF of the different TS-1 samples with similar Ti active sites scales only in proportion to Sext. The supported embryonic TS-1 on amorphous silica presents similar structural and chemical stability in comparison with crystalline nano-/micro-metersized TS-1 catalysts under water-free reaction conditions. Consequently, in situ prepared supported embryonic TS-1 combining improved accessibility (via increased external surface area) of different types of Ti species is a promising alternative for the conversion of bulky substrates.

Rational construction of hierarchical porous FeP nanorod arrays encapsulated in polypyrrole for efficient and durable hydrogen evolution reaction

Employing an environmentally friendly and economical approach to significantly improve the electrocatalytic activity for hydrogen evolution reaction (HER) has attracted widespread attention because of its enormous value and challenge. Herein, we report a series of FeP nanorod arrays (FeP NRAs) with polypyrrole (PPy) shell coating on carbon textiles (CTs) to improve the HER performance. The composite combines the high conductivity of PPy and synergistic effect between FeP and PPy, where the PPy coating remarkably reduces the apparent activation energy for HER and increases the turnover frequency of catalysts, thus the electrocatalytic performance of [email protected]/CTs hybrid nanorod arrays is significantly improved. Among them, the [email protected]/CTs-5 h catalyst exhibit superior HER performance with a low onset over-potential of 103.1 mV and a small Tafel slope of 49.2 mV/dec at 10 mA cm−2, and as well as an excellent long-term stability during operation for 46 h. Notably, it shows efficient HER performance during various bending status, demonstrating that the stability and flexibility of [email protected]/CTs catalysts. The excellent electrocatalytic performance is promising for applications as a cost-effective non-noble-metal HER catalyst during electrochemical water splitting for large-scale hydrogen fuel production.

Enhancing hydrophobicity and catalytic activity of nano-Sn-Beta for alcohol ring opening of epoxides through post-synthetic treatment with fluoride

Elucidation of synthesis-structure–reactivity relationships can help to enhance the catalytic performance of zeolites for a broad range of reactions. Considerable focus has been on examining structure–reactivity behavior of the catalytic site, but it has become increasingly evident that molecular interactions beyond the catalytic site in the outer sphere impact catalytic activity. The key challenge is to create synthetic methods that can create well-defined catalytic materials with control over catalytic sites and the catalytic reaction environment. After synthesizing nano-Sn-Beta using a hydrothermal method involving hydroxide ions that produce a hydrophilic material, these materials are treated with tetraethylammonium fluoride (TEAF) using mild conditions to produce a hydrophobic framework while maintaining the small particle size. The fluoride treated material is determined to be more active in the epoxide ring opening reaction than the as-synthesized nano-Sn-Beta (90% increase in TOF0), indicating that a hydrophobic framework is beneficial for the alcohol ring opening of epoxides. This is supported by site quantification experiments and an analysis of residual fluorine, which is found to have limited effect on activity. When accounting for the fraction of active sites, it is demonstrated for the methanol ring opening of epichlorohydrin that all hydrophobic materials are not diffusion limited under the reaction conditions and have the same turnover frequencies, which are higher than the turnover frequency of hydrophilic catalysts. Overall, this work reports a technique to improve the hydrophobicity of zeolite beta and demonstrates the importance of a hydrophobic reaction environment for the alcohol ring opening of epoxides.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications.

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Requirements for Beneficial Electrochemical Restructuring: A Model Study on a Cobalt Oxide in Selected Electrolytes

AbstractThe requirements for beneficial materials restructuring into a higher performance oxygen evolution reaction (OER) electrocatalyst are still a largely open question. Here erythrite (Co3(AsO4)2·8H2O) is used as a Co‐based OER electrocatalyst to evaluate its catalytic properties during in situ restructuring into an amorphous Co‐based catalyst in four different electrolytes at pH 7. Using diffraction, microscopy, and spectroscopy, a strong effect in the restructuring behavior is observed depending of the anions in the electrolyte. Only carbonate electrolyte can activate the catalyst material, which is related to its slow restructuring process. While the catalyst turnover frequency (TOF) undesirably reduces by a factor of 28, the number of redox active sites continuously increases to a factor of 56, which results in an overall twofold increase in current of the restructured catalyst after 800 cycles. The activation is attributed to an adequate local order, a high Co oxidation state close to 3+, and a high number of redox‐active Co ions. These three requirements for beneficial restructuring provide new insights into the rational design of high‐performance OER catalysts by electrochemical restructuring.

Substrate Channeling by a Rationally Designed Fusion Protein in a Biocatalytic Cascade.

Substrate channeling, where an intermediate in a multistep reaction is directed toward a reaction center rather than freely diffusing, offers several advantages when employed in catalytic cascades. Here we present a fusion enzyme comprised of an alcohol and aldehyde dehydrogenase, that is computationally designed to facilitate electrostatic substrate channeling using a cationic linker bridging the two structures. Rosetta protein folding software was utilized to determine an optimal linker placement, added to the truncated termini of the proteins, which is as close as possible to the active sites of the enzymes without disrupting critical catalytic residues. With improvements in stability, product selectivity (90%), and catalyst turnover frequency, representing 500-fold increased activity compared to the unbound enzymes and nearly 140-fold for a neutral-linked fusion enzyme, this design strategy holds promise for making other multistep catalytic processes more sustainable and efficient.

Neutralization of organophosphate over highly dispersed Fe, Cu, and Co on silica

Organophosphates are highly toxic compounds used as pesticides and chemical warfare agents. Herein, we describe SiO2-supported Fe, Co, and Cu catalysts that degrade organophosphate pesticide malathion at mild temperatures in the presence of H2O2. These catalysts are prepared by impregnation of SiO2 with solutions of bulky metal chelates that promote dispersion and low coordination, as shown spectroscopically. Turnover frequencies of catalysts prepared from chelated precursors are higher than those of controls prepared from nitrates and oxides, and independent of metal loading. The latter indicates equal activity and accessibility of active sites and suggests invariant structure in the loading range studied.

NiW catalyst modified with C12A7-H− and its promotion to hydrogenation selectivity of hydrodesulfurization

[Ca24Al28O64]4+·4(H−) (abbreviated as C12A7-H−) powder containing H− species was synthesized to modify NiW catalyst. The incorporation of C12A7-H− increases the reaction rate constants and turnover frequencies of NiW catalysts in hydrodesulfurization (HDS) of dibenzothiophene (DBT). The characterization results show that the introduction of C12A7-H− promotes the reducibility of NiW catalysts, resulting in higher sulfidation degree and more Ni-W-S species. In spite of the fact that rim/edge sites ratios and Lewis acid sites are decreased, the introduction of C12A7-H− still increases the hydrogenation/direct desulfurization ratios of NiW catalysts in HDS of DBT. Furthermore, higher HDS rates of C12A7-H− modified NiW catalysts are also observed in HDS of 4,6-dimethyldibenzothiophene. These can be attributed to C12A7-H− material that may transfer active H species to NiW catalysts and thereby promote the hydrogenation selectivity of NiW catalyst.

Graphene aerogel-supported ruthenium nanoparticles for COx-free hydrogen production from ammonia

Ruthenium was highly dispersed on graphene aerogel (GA) at high loadings to achieve high performance in COx-free hydrogen production from ammonia. Catalytic performance measurements on ammonia decomposition showed that the GA-supported catalyst with a Ru loading of 13.6 wt% provides an ammonia conversion of 71.5 % at a space-velocity of 30,000 ml NH3 gcat−1 h−1 and at 450 °C, corresponding to a hydrogen production rate of 21.9 mmol H2 gcat−1 min−1. The addition of K increased the ammonia conversion to a record high value of 97.6 % under identical conditions, reaching a hydrogen generation rate of 30.0 mmol H2 gcat−1 min−1, demonstrated to be stable for at least 80 h. A comparison of the turnover frequencies of catalysts indicated that this increase in performance upon the addition of K originated from an increase in the number of the active Ru sites and the corresponding electron density available for reaction.

Layer Design for PGM-Free Catalysts for Pemfcs: Impact of Electrical Conductivity & Diagnostic Tools

Recently, the notable efforts placed on investigating platinum group metal (PGM)-free catalysts (M-N-C catalysts, M = Fe, Co, & Mn) for the oxygen reduction reaction (ORR) have led to the possible development of low-cost proton exchange membrane fuel cells (PEMFCs).1 However, compared to PGM ORR catalysts, the lower turnover frequency and active site density of PGM-free catalysts generally require high-surface-area structures and high catalyst loadings to achieve reasonable low-current density performance in PEMFCs, thereby resulting in significant ionic, mass, and electronic transport overpotentials. 2 With regards to these transport resistances, an intriguing observation from the PEMFC performance with PGM-free cathode catalysts is the significantly higher electronic overpotential (iR-drop) when compared to PGM-based cathode catalysts (e.g., Pt/C). This increased iR-drop results in an increase of the high frequency resistance (HFR) due to the high electrical resistivity (ρe- in Ω/cm) of the thick PGM-free cathode catalyst layers. While ρe- of conventional Pt/C electrodes is rather low (~0.5-1 Ω/cm) and results in negligible overpotential losses, ρe- of PGM-free cathode layers can be higher by a factor of ~10-20 due to poor electrical conductivity of the active site (e.g., Fe-N-C) and the high-surface area structures of these catalysts. While several studies have revealed the impact of ionic and mass transport resistances of PGM-free cathode electrodes,3,4 the impact of electrical resistance is still unclear. A quantification of this effect is required for the rational design of PGM-free cathode electrodes, whereby the following three points would be of major interest: (1) The values of PGM-free catalysts vary widely, so that in a comparative study catalysts with high and low should be investigated. (2) When preparing electrodes with PGM-free catalysts that have a high value ρe-, conductive-carbon additives are commonly added, but there is no simple diagnostic tool that could be used for optimizing the amount of additive. (3) A quantitative model to predict the cell voltage loss due to the impact of ρe- would be desired. This can be accomplished by in-situ AC impedance analysis based on a transmission line model. 5 However, owing to higher ratio of ρe- to the ionic resistivity (ρH+) in PGM-free electrodes (>0.1/1), a modified approach needs to be developed to precisely quantify the voltage losses.6 In the present work, we present small-active area (5 cm2) single-cell H2/O2 PEMFC performance (differential-flow operation) combined with in-situ AC impedance and ex-situ 4-point-probe resistance measurement in order to elucidate the impact of the electrical resistivity of PGM-free cathode catalyst layer on performance. Two different catalysts with low and high ρe- are investigated. The results indicate that the lower one shows no significant change in cell voltage upon the addition of 7.5 wt% of conductive carbon (Figure 1a). However, with same wt% of carbon additive, the cell voltage for catalyst with high ρe- can be increased by 70 mV at 1 A/cm2, resulting in a ~30% improvement in peak power density (Figure 1b). Furthermore, we will present an in-situ AC impedance technique under blocking condition (N2 feed to cathode) that we developed to quantify ρe- and ρH+ of PGM-free cathode electrodes with and without carbon additive. Using these values, the cell-voltage gains due to the reduction of ρe- of a PGM-free cathode by carbon additive can be predicted (Figure 1c). At the end, a comparison between in-situ AC-impedance and ex-situ 4-point-probe measurements for PGM-free cathode electrodes will be presented and discussed. Overall, the study not only provides a new insight and methodology for electrode optimization of PGM-free cathode catalysts, but also reveals in some cases show significant impact of electrical conductivity on fuel cell performance with PGM-free catalysts.ACKNOWLEDGEMENTSThis work is supported by the Fuel Cells and Hydrogen Joint Undertaking under grant agreement No. 779550 (PEGASUS). This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research.REFERENCE T. Thompson, A. R. Wilson, P. Zelenay, D. J. Myers, K. L. More, K. C. Neyerlin, and D. Papageorgopoulos, Solid State Ionics319, 68-79 (2018)Banham, J. -Y. Choi, T. Kishimoto, and S. Ye, Advanced Materials 31, 1804846 (2019)Malko, T. Lopes, E. A. Ticianelli, and A. Kucernak, Journal of Power Sources 323, 189-200 (2016)G. Wang, L. Osmieri, A. G. Star, J. Pfeilsticker, and K. C. Neyerlin, Journal of The Electrochemical Society 167, 044519 (2020)Liu, M. Murphy, D. Baker, W. Gu, C. Ji, J. Jorne, and H. A. Gasteiger, ECS Transactions 11, 473 (2007)Landesfeind, M. Ebner, A. Eldiven, V. Wood, and H. A. Gasteiger, Journal of The Electrochemical Society 165, 469-476 (2018) Figure 1

Moderate Temperature Treatment of Gas-Phase Volatile Organic Toluene Using NiO and NiO\u2013TiO2 Nano-catalysts: Characterization and Kinetic Behaviors

For the first time, the potential of Ni/NiO and NiO–TiO2 nano-catalysts for the oxidation of toluene under moderate temperatures was investigated. The nano-catalysts were prepared using the solution combustion synthesis method (SCSM) and the effect of the composition of nano-catalysts, the inlet toluene concentration ([text{C}_{7} {text{H}}_{8}]_{in}), the relative humidity (RH), and the temperature on the percentage of toluene conversion (% {text{TN}}_{Conv.} ) were subsequently examined. Results revealed that the nano-catalysts synthesized with a low fuel-to-metal ratio produced pure NiO, which has significant catalytic activity toward the conversion of toluene. Conversely, the high fuel-to-metal ratio generated a nano-catalysts that contains a mixture of Ni/NiO or pure Ni with low activity toward the conversion of toluene. Adding NiO to TiO2 increased the surface area of the catalyst, augmented the catalyst active sites, enhanced the oxidation of toluene, and increased CO2 selectivity ({text{S}}_{{{text{CO}}_{2} }}). NiO and NiO–TiO2 nano-catalysts exhibited higher reaction rates, significant catalyst turnover frequency, and low activation energy. The obtained results revealed that the SCSM is a promising synthesis method for producing NiO or NiO–TiO2 nano-catalysts, which can be employed successfully for the removal of toluene from gas streams.Graphic

Optimized Immobilization Strategy for Dirhodium(II) Carboxylate Catalysts for C-H Functionalization and Their Implementation in a Packed Bed Flow Reactor.

Herein we demonstrate a packed bed flow reactor capable of achieving highly regio- and stereoselective C-H functionalization reactions using a newly developed Rh2 (S-2-Cl-5-CF3 TPCP)4 catalyst. To optimize the immobilized dirhodium catalyst employed in the flow reactor, we systematically study both (i) the effects of ligand immobilization position, demonstrating the critical factor that the catalyst-support attachment location can have on the catalyst performance, and (ii) silica support mesopore length, demonstrating that decreasing diffusional limitations leads to increased accessibility of the active site and higher catalyst turnover frequency. We employ the immobilized dirhodium catalyst in a simple packed bed flow reactor achieving comparable yields and levels of enantioselectivity to the homogeneous catalyst employed in batch and maintain this performance over ten catalyst recycles.

Optimized Immobilization Strategy for Dirhodium(II) Carboxylate Catalysts for C−H Functionalization and Their Implementation in a Packed Bed Flow Reactor

AbstractHerein we demonstrate a packed bed flow reactor capable of achieving highly regio‐ and stereoselective C−H functionalization reactions using a newly developed Rh2(S‐2‐Cl‐5‐CF3TPCP)4 catalyst. To optimize the immobilized dirhodium catalyst employed in the flow reactor, we systematically study both (i) the effects of ligand immobilization position, demonstrating the critical factor that the catalyst‐support attachment location can have on the catalyst performance, and (ii) silica support mesopore length, demonstrating that decreasing diffusional limitations leads to increased accessibility of the active site and higher catalyst turnover frequency. We employ the immobilized dirhodium catalyst in a simple packed bed flow reactor achieving comparable yields and levels of enantioselectivity to the homogeneous catalyst employed in batch and maintain this performance over ten catalyst recycles.

High Performance Fuel Cell Catalysts Synthesized By Fe Metalation of Nitrogen Doped Carbons Derived from Metal Organic Framework ZIF-8

Hydrogen fuel cells are potentially a sustainable alternative to internal combustion engines. However, the critical raw materials, platinum group metals (PGMs), necessary to achieve adequate power density in automotive fuel stacks are cost prohibitive. Therefore, it is imperative to replace PGMs with low cost and earth abundant metals such as Fe, Co and Mn. This represents a considerable challenge in the design of PGM-free catalysts for the oxygen reduction reaction (ORR) in acidic electrolyte since current state-of the-art PGM-free catalysts fall short of matching the high activity of PGM catalysts in this environment1. Addressing the challenge of PGM-free ORR catalysis, the most promising class of materials is metal-nitrogen-carbon (MeNC) catalysts. These materials comprise a conductive and highly porous carbon doped with nitrogen moieties which can be regarded as solid-state ligands coordinating the metal centers. The two rational means of improving catalyst activity is through increasing the catalyst site density (SD) and/or turnover frequency (TOF). However, constraints exist which complicate efforts to increase the SD of MeNC catalysts, in particular the Fe-catalyzed graphitization of the carbon support leading to i) low surface area and ii) loss of pyridinic and other metal-coordinating nitrogen moieties at elevated temperature. A threshold Fe content therefore exists (depending on the synthesis approach and pyrolysis conditions) above which Fe nanoparticles nucleate during pyrolysis, diminishing the number of metal-based active sites in the final MeNC material either directly (a fraction of Fe atoms being directed to Fe nanoparticles), or indirectly, as mentioned above.2 One approach to increasing the catalyst SD is by the metalation of commercially available carbons which typically involves mixing of the carbon with Fe and N precursors such as iron acetate and 1,10-phenanthroline, followed by pyrolysis in inert atmosphere.3, 4 Transmetalation has also been shown as an effective method for replacing MgNC or ZnNC sites formed in during a first synthesis step, with FeNC sites using a solvothermal technique2. The advantage of these methods is the decoupling of the carbon support synthesis and Fe metalation, allowing for the aforementioned constraints on SD to be fully or partially overcome. We will report on a facile solid-state method for Fe metalation of nitrogen-doped carbon (NC) synthesized from a zinc-containing metal-organic-framework (ZIF-8). Decoupling the synthesis of NC from the FeNC catalyst mitigates the Fe-catalyzed graphitization leading to increased carbon surface area, pore volume, and surface composition of pyridinic nitrogen. Fe metalization, involving solid-state mixing of anhydrous FeCl2 with the NC followed by a low temperature pyrolysis, results in a catalyst consisting exclusively of coordinated FeNC active sites without altering the morphology or nitrogen speciation of the NC. These improvements of the catalyst structure and composition enable the catalysts to achieve half-wave potentials (E1/2) greater than 0.8 V vs. RHE and near 4 e- selectivity with H2O2 yields as low as 1 %. A broad set of characterization techniques, including: Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and newly developed methods to probe the SD and TOF of FeNC catalysts, nitrite stripping and CO chemisorption, were used allowing for an in-depth understanding of the catalytic materials to guide further catalyst optimization leading to better performance of PGM-free FeNC catalysts for ORR. 5, 6. Acknowledgement: The research leading to these results has received funding from FCH2 JU under grant agreement 779366, CRESCENDO.

Study on maximizing catalytic performance of cobalt(II) 5,10,15,20-tetrakis(4-pyridyl)porphyrin for cyclohexane oxidation

Aiming at the most enhancing the catalytic performance of simple and cheap cobalt(II) 5,10,15,20-tetrakis(4-pyridyl)porphyrin (Co TPyP), guided by the nowadays-advanced various technologies, Co TPyP was anchored onto nanoporous bar with thiol-functionalized silica (npb-SiO2-SH). The resulting npb-SiO2-S-Co TPyP catalyst was characterized using various techniques, determining its structure. The total surface area ratios (TSAR) of the immobilized cobalt porphyrin to the npb-SiO2-SH, and the catalytic cyclohexane oxidation reaction conditions were at the same time optimized via Response Surface Methodology (RSM). Compared with those obtained from traditional optimizing method (TOM), on average, RSM could enhance as high as more than 60% catalyst turnover frequency (TOF) and yields of cyclohexanone and cyclohexanol (KA oil). The anchored 0.05 mg Co TPyP could catalyze the oxidation of 200 mL of cyclohexane, under not using any solvents and additives, offering 2.5 × 106 1/h of TOF, and 19% of yields. The npb-SiO2-S (20%-0.0438)-Co TPyP catalyst showed better reusability, excellent catalytic