Showing papers in "Chemcatchem in 2022"

Ligand‐to‐Metal Charge Transfer (LMCT) Photochemistry at 3d‐Metal Complexes: An Emerging Tool for Sustainable Organic Synthesis

Abstract: Despite the rich photochemistry of 3d‐metal complexes, the utilization of excited‐state reactivity of these compounds in organic synthesis has been historically overlooked. The advent of photoredox catalysis has changed the perception of synthetic chemists towards photochemistry, and nowadays the potential of photoinduced, outer‐sphere single‐electron transfer events is widely recognized. More recently, an emerging new mode of photoactivation has taken the spotlight, based on an inner‐sphere mode of reactivity triggered by population of ligand‐to‐metal charge‐transfer (LMCT) excited states. Contrarily to photoredox, LMCT‐activation does not rely on matching redox potentials, offers unique reactivity profiles and is particularly well suited on Earth‐abundant metal complexes. Those appealing features are propelling the development of methods using this blueprint to generate highly reactive open‐shell species under mild conditions. The aim of this contribution is to provide a didactical tool for the comprehension of this emerging concept and facilitate the development of new synthetic methodologies to achieve sustainable chemical transformations.

Bi/Zn Dual Single‐Atom Catalysts for Electroreduction of CO2 to Syngas

Abstract: The electrochemical CO2 reduction reaction to produce CO is one of the most promising pathways to eliminate CO2 emissions and store intermittent energy sources. However, the most critical usage of CO is via mixing with H2 to form syngas, which is a crucial feedstock for many value‐added chemicals. Therefore, producing syngas with a suitable CO/H2 ratio in a one‐step reaction is desirable in the CO2RR process. To achieve this end, dual single‐atom sites supported by the nitrogen‐doped carbon show great potential. Herein, we show that a dual single‐atom catalyst with Bi−N4 and Zn−N4 units is efficient for generating syngas with tunable CO/H2 ratios (0.20 to 2.92), which is of great significance to downstream industrial production. Moreover, this work highlights the potential to control the CO/H2 ratios for efficient syngas production using the coexisting single‐atom sites.

Advances in Palladium‐Catalyzed Decarboxylative Cycloadditions of Cyclic Carbonates, Carbamates and Lactones

Abstract: Palladium‐catalyzed decarboxylative cycloadditions have emerged as highly effective methods for constructing structurally diverse carbo‐ and heterocycles because of the formation of at least two carbon‐carbon or carbon‐heteroatom bonds in a single step. It is of great interest to chemists that this type of cycloaddition reactions possesses some special advantages such as high reactivity, exclusive regioselectivity, and good functional group compatibility. Based on these qualities, palladium‐catalyzed decarboxylative cycloadditions present strong ability in synthetic chemistry and have been flourished especially in the last five years. In this review, the achievements in palladium‐catalyzed decarboxylative cycloadditions involving cyclic carbonates, carbamates, and lactones for accessing oxacyclo‐, azacyclo‐ and carbocyclic compounds are addressed. Mechanistic insights and some synthetic applications toward the synthesis of natural products are discussed. The challenges and opportunities of this field are also outlined.

Ionic Liquid‐Assisted Electrocatalytic NO Reduction to NH3 by P‐Doped MoS2

Abstract: Ambient NH3 electrosynthesis from NO reduction reaction (NORR) is attractive in replacing the industrial Haber‐Bosch route; however, the competitive hydrogen evolution reaction (HER) in aqueous electrolyte typically induces a limited selectivity and activity toward NH3 production. Herein, hierarchical P‐doped MoS2 nanospheres are developed as the NORR electrocatalyst in an ionic liquid (IL) electrolyte for catalyzing the reduction of NO to NH3 with a maximal Faradaic efficiency of 69 % (−0.6 V vs RHE) and a peak yield rate of 388.3 μg h−1 mgcat.−1 (−0.7 V vs RHE), both of which are comparable to the best‐reported results. Moreover, the catalyst also shows stable NORR activity over 30 h and 6 cycles. Theoretical analyses further reveal that the P dopants in MoS2 facilitate the activation and hydrogenation of NO. Besides, the employment of hydrophobic IL electrolyte also slows down the HER kinetics effectively.

Mo−N Bonds Effect Between MoSx Coupling With CoN For Efficient Photocatalytic Hydrogen Production

Abstract: The facet‐engineered surface and interface design has attracted much attention in the preparation of efficient photocatalysts. Polyvinylpyrrolidone (PVP) is used as the morphological modifier to prepare 3D MoSx in this work. Benefit by its suitable conduction band position and good conductivity, CoN accepts photo‐induced electrons from MoSx as an excellent electron acceptor in the form of active sites. With the addition of CoN, hydrogen evolution performance of MoSx has been greatly improved by building the Mo−N bond since the form of Mo−N bond successfully shorten electron transport routes. The reason for the improved hydrogen precipitation efficiency is demonstrated by PL, XPS, and electrochemical characterization. The H2 production activity of MoSx@CoN with the best mass ratio of MoSx to CoN is up to 7617.5 μmol/g/h, and the hydrogen production stability of MoSx@CoN is also considerable. These findings provide new insights for the preparation of highly efficient photocatalytic hydrogen evolution materials.

Effective N‐formylation of Amines with CO2 in Anaerobic Fermentation Gas Catalyzed by Triply Synergistic Effect of Ionic Porous Organic Polymer

Abstract: The anaerobic fermentation gas (AFG) with CO2 removed is a sustainable energy resource, and converting CO2 in AFG to value‐added chemicals can kill two birds with one stone. Herein, an ionic porous organic polymer (IPOP‐3) catalyst, with triply synergistic effect of its hydroxide anions, nitrogen activation sites and porous structure, is synthesized. IPOP‐3 exhibits an excellent ability to enrich and convert 50 vol.% CO2 by the reactions with various amines into corresponding formamides. The catalyst is also successfully applied to the N‐formylation of CO2 in AFG, and can be easily recovered and reused. The plausible mechanism for the catalytic N‐formylation is proposed. The hydroxide anion in IPOP‐3 activates the Si−H bond of phenyl silane which reacts with the captured CO2 to form formate anion to facilitate the subsequent conversion into formamide. Our work provides a new idea for the purification of AFG and the resource utilization of CO2.

Digitization in Catalysis Research: Towards a Holistic Description of a Ni/Al₂O₃Reference Catalyst for CO₂Methanation

Abstract: There is considerable motivation in the catalysis community and chemical industry to envision a future where rational catalyst design and targeted chemical process optimization become standard. Achieving this goal for heterogeneous catalysis requires a cultural shift centered around effective research data management. The core elements of modern catalysis research are synthesis, characterization, and testing, while all can be elevated by effective collection, correlation, interoperation, and exploitation of data between disciplines and stakeholders. Here, first steps are made towards a holistic picture of an industrial Ni/Al2O3 reference catalyst for CO2 methanation. A range of conventional and advanced characterization tools are applied to probe metal particle size and pore characteristics of the support, selected as crucial parameters for catalyst performance. Challenges are shown with respect to current reporting of characterization data and metadata, which ultimately influences the development and reliability of digital twins in catalysis research. Furthermore, the cooperation and combined expertise of diverse research groups from different fields is recognized as essential to deliver meaningful progress towards the digital future of catalysis research.

Oxidation of Methane to Methanol by Water Over Cu/SSZ‐13: Impact of Cu Loading and Formation of Active Sites

Abstract: Cu/SSZ‐13 catalysts with different Cu loadings (1–5 wt.%) were prepared by conventional impregnation via a slow step‐dry process for the direct conversion of methane to methanol. Catalytic performance of the catalysts is found to depend on the Cu loading in both stepwise and continuous processes and 2 wt.% Cu presents the best performance. CuO nanoparticles, CuxOy clusters and monomeric [CuOH]+ species are found on Cu/SSZ‐13 based on HRTEM, HAADF‐STEM, DR UV‐vis and FTIR spectroscopies. The high coverage of CuO nanoparticles on 5 wt.% Cu/SSZ‐13 results in the loss of catalytic reactivity in spite of the existence of active CuxOy and [CuOH]+ sites. Besides, it is proposed that the formation of [Cu2O2]2+ and [Cu2O]2+ active sites involves [CuOH]+ intermediates. We further found that improved catalytic performance in cyclic stepwise process involves the re‐dispersion of CuO nanoparticles and/or large clusters as well as the re‐oxidation of Cu(I) cations from self‐reduction of [CuOH]+ species.

Engineering of Cation Occupancy of CoFe2O4 Oxidation Catalysts by Nanosecond, Single‐Pulse Laser Excitation in Water

Abstract: Due to the variability of the cation occupancy of octahedral and tetrahedral sites, spinel ferrites and cobaltites are particularly interesting to investigate activity trends in oxidation catalysis. Yet, the preparation of the respective catalyst series remains challenging. We employed pulsed laser defect engineering of CoFe2O4 nanoparticles in water to gradually alter the cation occupancy of octahedral and tetrahedral sites by single laser pulses and study its effect on cinnamyl alcohol oxidation. Three CoFe2O4 catalysts from different synthesis methods resembling different initial site occupancy were chosen as starting materials. The laser‐induced randomization of the cation occupancy was verified by Mössbauer spectroscopy and linearly correlated with the conversion of cinnamyl alcohol while the size and Co : Fe ratio was maintained during laser processing. The study solidifies the importance of octahedral Co3+‐sites and the feasibility of pulsed laser processing for altering the cation occupancy and related crystallographic defect density in oxidation catalysis.

Electrochemical Urea Oxidation in Different Environment: From Mechanism to Devices

Abstract: Urea has been regarded as an important chemical compound in the food‐water‐energy nexus. However, the emission of urea from human activity, industrial manufacture, and agricultural fertilization into the environment has caused an ecological nitrogen imbalance. Besides addressing the environmental pollution, the applications of clean energy conversion from urea‐rich wastewater has strong potential for resource and energy recovery. Herein, we conducted a comprehensive overview of electrochemical urea oxidation reaction (UOR) for pollution control and energy harvesting. The present mechanisms and behaviors of UOR under different water matrices have been characterized and compared in detail. Additionally, the latest development of electrochemical UOR integrated into electrolyzers and fuel cells are presented. Finally, we discuss the prospects and challenges of UOR technologies, suggesting several directions for the electrochemical conversion of urea‐abundant wastewater in the near future.

On the Stability of Isolated Iridium Sites in N‐Rich Frameworks Against Agglomeration Under Reducing Conditions

Abstract: Stabilization of single metal atoms is a persistent challenge in heterogeneous catalysis. Especially supported late transitions metals are prone to undergo agglomeration to nanoparticles under reducing conditions. In this study, nitrogen‐rich covalent triazine frameworks (CTFs) are used to immobilize iridium complexes. Upon reduction at 400 °C, immobilized Ir(acac)(COD) on CTF does not form nanoparticles but transforms into a highly active Ir single atom catalyst. The resulting catalyst systems outperforms both the immobilized complex and supported nanoparticles in the dehydrogenation of formic acid as probe reaction. This superior performance could be traced back to decisive changes of the coordination geometry positively influencing activity, selectivity and stability. Spectroscopic analysis reveals an increase of electron density on the cationic iridium site by donation from the CTF macroligand after removal of the organic ligand sphere from the Ir(acac)(COD) precursor complex upon reductive treatment. This work demonstrates the ability of nitrogen moieties to stabilize molecular metal species against agglomeration and opens avenues for catalysts design using isolated sites in high‐temperature applications under reducing atmosphere.

Catalytic Diversification of gem‐Difluorocyclopropanes: Recent Advances and Challenges

Abstract: gem‐Difluorocyclopropanes (F2CPs) have aroused considerable attention not only from the structural perspective but also due to their ability to participate in various valuable transformations. This Review summarizes advances in catalytic ring‐opening reactions of gem‐F2CPs, especially emphasizing the reactivities and applications of those non‐activated ones under transition‐metal catalysis. Their achievements, synthetic applications and limitations are discussed with the aim to stimulate enthusiasm for further development.

Structure‐Activity Relationships in Highly Active Platinum‐Tin MFI‐type Zeolite Catalysts for Propane Dehydrogenation

Abstract: Pt/Sn‐containing MFI zeolites prepared by one‐pot hydrothermal methods are highly active and selective catalysts for propane dehydrogenation. An alternative preparation method is reported alongside an in‐depth characterization of Pt and Sn before and after reaction. Pt species dispersed on highly‐defective Silicalite‐1 show a significantly long catalytic lifetime with an improved coke resistance, but increased Pt−Sn alloying and the (eventual) build‐up of carbon leads to deactivation.

Single Atom Catalysts: What Matters Most, the Active Site or The Surrounding?

Abstract: Single atom catalysts (SACs) consist of isolated metal atoms stabilized on a solid support. The name suggests that the catalytic activity is due to the nature of the metal atom, but of course the interaction with the substrate plays a role as well. But what is more important? The metal atom or its surrounding? To answer this question, we have performed numerical experiments based on density functional theory (DFT). 24 transition metal atoms have been incorporated in Nitrogen‐doped graphene, and the catalytic activity in the hydrogen evolution reaction (HER) has been studied changing the metal and keeping the N‐doped graphene matrix fixed. Then, one specific atom, Pt, has been embedded in the same matrix but the nitrogen neighbors of Pt have been systematically replaced by carbon or oxygen atoms generating more than 20 structures. The HER has thus been studied by keeping the metal center fixed but changing the surrounding. It turns out that the same great variability in chemical behavior can be achieved by changing the active site or the surrounding environment. This shows the importance of the local coordination in determining the catalytic activity. The consequences of this conclusion for modeling studies of SACs are discussed.

Multifunctional Catalytic Properties of Pd/CNT Catalysts for 4‐Nitrophenol Reduction

Abstract: Carbon nanotubes are arousing real interest in catalysis, either as catalyst support or as catalyst itself. In this work, the influence of CNT doping (O, N, S) on the catalytic performance of CNT and Pd/CNT catalysts have been investigated in the reduction of 4‐nitrophenol both under H2 or using NaBH4 as a hydride source. The morphology, composition, particle size and crystallinity of the materials were investigated by various techniques including HRTEM, STEM, XPS, Raman spectroscopy and XRD. Among the CNT carbocatalysts, only N‐CNT is active for 4‐nitrophenol reduction at room temperature, and only in the presence of NaBH4. As far as supported Pd catalysts are concerned, the nature of the support influences the Pd nanoparticle location (confinement), the Pd nanoparticles/Pd single atoms ratio and the extent of H‐spillover, three catalyst features that directly affect the catalytic activity. The best combination of Pd nanoparticles/Pd single atoms ratio, H‐spillover and confinement for 4‐nitrophenol reduction using NaBH4, is found in Pd/O‐CNT catalysts, while for reactions performed under H2, Pd/N‐CNT presents the best combination.

Lignin Nanoparticles Support Lipase‐Tyrosinase Enzymatic Cascade in the Synthesis of Lipophilic Hydroxytyrosol Ester Derivatives

Abstract: Lipophilic esters of hydroxytyrosol have been synthesized in a one‐pot procedure from tyrosol and long side‐chain carboxylic acids by a lignin nanoparticles supported enzymatic cascade of lipase M and tyrosinase. The reaction involved the initial esterification of the aliphatic carboxylic group with the hydroxyl group of tyrosol followed by ortho‐hydroxylation of the aromatic moiety. Lipase M and tyrosinase were immobilized on lignin nanoparticles by layer‐by‐layer procedure using chitosan as positive charged polyelectrolyte and Concanavalin A as molecular spacer to control the optimal distance between the active sites of the two enzymes. The occurrence of substrate channeling effect was hypothesized on the basis of FRET analysis. Lipophilic hydroxytyrosol esters were synthesized in high yield in 2‐methyltetrahydrofuran and in the presence of a very low amount of buffer to generate the hydration sphere of the enzyme.

Mo‐Vacancies Defect Engineering of One‐Dimensional Porous Mo2C Nanowires for Enhanced High‐Efficiency Hydrogen Evolution

Abstract: The large‐scale application of efficient water‐splitting greatly promote the development of hydrogen economy which benefit both in alleviating the energy crisis and reaching the goal of carbon neutral. To realize considerable hydrogen evolution, rational design of catalysts with controllable structure and surface composition become crucial. In this work, we proposed an enhanced active site originated from Mo‐vacancies defect in Mo2C crystal which was fabricated by the evaporation of Zn content in well‐designed one‐dimensional ZnMoO4 precursor. Density functional theory calculation and experimental results demonstrated that the formation of molybdenum vacancies in molybdenum carbide promoted the water adsorption and H2 desorption effectively, resulting in an excellent HER reaction dynamics. Moreover, one‐dimensional porous nanowires also ensured rapid mass transfer and contributed strong support for superexcellence HER performance. As expected, V−Mo2C‐900@NF exhibited extremely low overpotential (η10=43 mV) at the current density of 10 mA cm−2 and rapid reaction kinetics (Tafel slope 77.89 mV dec−1).

An Intermetallic CaFe6Ge6 Approach to Unprecedented Ca−Fe−O Electrocatalyst for Efficient Alkaline Oxygen Evolution Reaction

Abstract: Based on the low‐cost and relatively high catalytic activity, considerable efforts have been devoted towards developing redox‐active transition metal (TM)‐oxygen electrocatalysts for the alkaline oxygen evolution reaction (OER) while the role of redox‐inactive alkaline earth metals has often been neglected in OER. Herein, for the first time, we developed a novel ternary intermetallic CaFe6Ge6 precatalyst, whose surface rapidly transforms into a porous ultrathin Ca−Fe−O heteroshell structure during alkaline OER through the oxidative leaching of surficial Ge. Benefiting from synergistic effects, this highly efficient OER‐active material with distinct Ca−Fe−O layers has a large electrochemical surface area and more exposed active Fe sites than a Ca‐free FeOx phase. Also, the presence of Ca in Ca−Fe−O is responsible for the enhanced transport and activation of hydroxyls and related OER reaction intermediate as unequivocally illustrated by a combination of quasi in‐situ Raman spectroscopy and various ex‐situ methods.

Supported Ru Single Atoms and Clusters on P‐Doped Carbon Nitride as an Efficient Photocatalyst for H2O2 Production

Abstract: Photocatalytic H2O2 evolution via two‐electron oxygen reduction is a promising path for renewable and on‐the‐spot H2O2 production compared with the traditional anthraquinone method. However, the efficiency of photocatalytic production of H2O2 is usually very low. Herein, P‐doped carbon nitride loaded with ruthenium single atoms and clusters (Ruatom/P‐CN) is reported as an efficient photocatalyst for H2O2 production. The yield of H2O2 over Ruatom/P‐CN (385.8 mmol g−1 h−1) is about 4.3 times higher than that of P‐CN (88.9 mmol g−1 h−1) and 3.6 times higher than that of ruthenium nanoparticles loaded P‐CN (105.9 mmol g−1 h−1). Further mechanistic study indicates that the presence of Ru in the form of single atoms and clusters not only improves the separation efficiency of photogenerated carriers and inhibits the recombination of photogenerated electron‐hole pairs, but also increases the reactive sites of this catalytic system. This study breaks through people‘s understanding that precious metal loading is not conducive to H2O2 selectivity and provides a new way to prepare low‐metal loading, high‐activity photocatalysts for H2O2 production.

Supported Iridium‐based Oxygen Evolution Reaction Electrocatalysts ‐ Recent Developments

Abstract: The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

Stereoselective Semi‐Hydrogenations of Alkynes by First‐Row (3d) Transition Metal Catalysts

Abstract: The chemo‐ and stereoselective semi‐hydrogenation of alkynes to alkenes is a fundamental transformation in synthetic chemistry, for which the use of precious 4d or 5d metal catalysts is well‐established. In mankind‘s unwavering quest for sustainability, research focus has considerably veered towards the 3d metals. Given their high abundancy and availability as well as lower toxicity and noxiousness, they are undoubtedly attractive from both an economic and an environmental perspective. Herein, we wish to present noteworthy and groundbreaking examples for the use of 3d metal catalysts for diastereoselective alkyne semi‐hydrogenation as we embark on a journey through the first‐row transition metals.

Amorphous Copper‐modified Gold Interface Promotes Selective CO2 Electroreduction to CO

Abstract: Electroreduction of CO2 into valuable chemicals exhibits promising application potentials. Herein, a kind of nano‐porous AuCu foam which possesses amorphous copper (Cu)‐modified gold (Au) interface was designed for highly selective electroreduction CO2 into CO. As‐obtained Au0.95Cu0.05 foam exhibits a Faradaic efficiency (FE) of 99.5 % for CO with a current density of 31.3 mA cm−2. Compared with other Au‐based electrocatalysts, Au0.95Cu0.05 foam shows a lower overpotential of 240 mV. The improvements in activity and selectivity of the AuCu foam might be attributed to the synergistic effect between the highly dispersed amorphous Cu and the matrix of Au. Detailed characterization indicates that the twisty nanowire morphology imparts multiple reactive sites on the electrode surface. The study demonstrates that amorphous Cu on the AuCu foam surface can boost CO2 activation by modifying the surface geometry and electronic structure. This finding provides a new strategy of modifying the metal interface to construct electrocatalyst.

The Role of Dual Vacancies in TiO2 for Enhanced Photocatalytic Hydrogen Generation and Pollutants Removal

Abstract: The mechanism of TiO2 with Ti vacancy and O vacancy in photocatalytic hydrogen production is unknown. Herein, TiO2 with three types of vacancies was constructed. It was found that DV‐Ti‐3O containing both O vacancies and Ti vacancies had the highest photocatalytic hydrogen production of 12.42 mmol ⋅ g−1 ⋅ h−1 twice that of SV‐Ti, where the O vacancy acted as an emitter providing a directional path for the migration of photogenerated electrons and provided the adsorption sites for water molecules, while the introduction of Ti vacancies took the role as electron traps to collect more electrons for hydrogen proton reduction. DV‐Ti‐3O achieved ultra‐fast migration of electronics from O vacancies to Ti vacancies and created ideas for defective engineering of non‐precious metal catalysts for photocatalytic hydrogen production and organic pollutants removal.

Origin of Morphology Change and Effect of Crystallization Time and Si/Al Ratio during Synthesis of Zeolite ZSM‐5

Abstract: Hydrothermal synthesis of ZSM-5 is an often applied but incompletely understood procedure. In comparison to current research efforts that aim to produce complex micro-mesoporous catalysts for the conversion of biogenic and bulky hydrocarbons, this work focuses on the dependency between Si/Al ratio and zeolite morphology of microporous ZSM-5 to understand and to control the synthesis process. In two series of time dependent crystallization, kinetics were analyzed at Si/Al ratio 20 and 100 to optimize the crystallization time. Subsequently, zeolites with different Si/Al ratio were obtained and characterized. The results show a transition from a slow dissolution-recrystallization process to a fast solid-state-transformation with increasing Si/Al ratio. This is followed by a switching morphology from clusters of small agglomerates to bigger spherical particles. Respective acid site density and zeolite morphology determine local residence time, hydride transfer behavior and finally selectivity towards aromatics and higher hydrocarbons during methanol conversion. This background should provide control of even more complex syntheses of porous catalysts.

Binder‐free Zeolite Beta Beads with Hierarchical Porosity: Synthesis and Application as Heterogeneous Catalysts for Anisole Acylation

Abstract: Three zeolites (H‐Beta, H‐ZSM‐5 and H‐Y) were synthesized in the form of binder‐free macroscopic beads (d=215–840 μm) using a hydrothermal method employing anion‐exchange resin beads as hard template. The beads obtained after removal of the hard template by calcination consisted of crystalline zeolite domains connected with each other to form a hierarchical porous network in which the zeolitic micropores are accessible through meso‐ and macropores, as proven by characterization with XRD, N2 physisorption, SEM, and TEM. The composition, the nature and amount of acid sites and the degree of hydrophobicity of these beads were investigated by means of XRF, solid‐state NMR, pyridine‐FTIR and TGA. The zeolite beads were tested as heterogeneous catalysts in the Friedel‐Crafts acylation of anisole with acetic anhydride to produce para‐methoxyacetophenone. H‐Beta‐Beads displayed the best catalytic performance with 95 % conversion of acetic anhydride and 76 % yield of para‐methoxyacetophenone in a batch reactor test (90 °C, 6 h). Next, the catalytic performance of H‐Beta‐Beads was compared in both batch and continuous‐flow mode to extrudates prepared by mixing zeolite Beta powder with either kaolin or bentonite binders. H‐Beta‐Beads outperformed the extrudates in batch‐mode reactions and could be reused in multiple runs without discernible loss of activity. In the continuous‐flow test, H‐Beta‐Beads demonstrated higher average activity but deactivated more rapidly than the extrudates.

A Guide to Low‐Valent Titanocene Complexes as Tunable Single‐Electron Transfer Catalysts for Applications in Organic Chemistry

Abstract: Low‐valent titanocene catalysts are a versatile tool for organic synthesis. They promote inter‐ and intramolecular reactions ranging from homolytic bond cleavages to reductive umpolung reactions to additions and cyclizations in single electron steps. These reactions heavily depend on the redox potential of an in situ formed titanium(III) center, which can be adjusted by the choice of appropriate ligands. We herein review various chiral and achiral ligand‐modified titanocene catalysts and their reduction potentials Ep/2 obtained via cyclic voltammetry. The latter are found to correlate with the Hammett parameters σp of the cyclopentadienyl substituents and to the pKa values of the corresponding acids of the Ti−X ligands. For selected examples, we further discuss how the adjustment of the redox properties through modifications of the titanocene ligands can lead to greatly improved reaction outcomes in titanium(III) catalyzed single‐electron transfer reactions.

Ternary CuCrCeOx Solid Solution Enhances N2‐Selectivity in the NO Reduction with CO in the Presence of Water and Oxygen

Abstract: Co‐doping of ceria with Cu and Cr forming a ternary CuCrCeOx solid solution enhances N2‐selectivity as well as NO and CO conversions compared to the corresponding singly doped ceria, CuCeOx and CrCeOx, and pristine ceria, in the absence as well as presence of water and oxygen. The comparative study of these solid‐solution catalysts indicates that the doping of ceria results in the coupling of different transition metal redox cycles, which has a strong effect on the surface population of oxygen vacancies. The temperature required to reach 95 % N2‐selectivity is shown to be nearly linearly correlated to the concentration of oxygen vacancy defects. The oxygen defect population is highest for the ternary CuCrCeOx solid solution, which exhibits the best catalytic performance, reaching 100 % N2‐selectivity at around 100 °C. Three redox cycles (CrVI/CrIII, CuII/CuI, and CeIV/CeIII) have been identified in this catalyst leading to a synergistic effect. The ternary CuCrCeOx catalyst exhibits good water tolerance and long‐term stability while the SO2 tolerance is limited due to selective blocking of the active surface oxygen defects forming SO32− species, resulting in catalyst deactivation.

Biocatalytically Active and Stable Cross‐Linked Enzyme Crystals of Halohydrin Dehalogenase HheG by Protein Engineering

Abstract: A major drawback for practical application of halohydrin dehalogenase HheG in biocatalysis is its rather low thermal stability and low organic solvent tolerance. We therefore pursued a stabilization of HheG via immobilization as cross‐linked enzyme crystals. Since glutaraldehyde inactivates HheG, we introduced a cysteine residue in the crystal interface, which enabled thiol‐specific cross‐linking at well‐defined cross‐linking sites. Variant HheG D114C displayed improved crystallizability and yielded stable and catalytically active CLECs using bis‐maleimidoethane as cross‐linker. Effective cross‐linking at the predefined site could be confirmed via the CLEC crystal structure. Compared to soluble enzyme, the CLECs displayed significantly improved stability and activity at higher temperatures, lower pH values and in the presence of water‐miscible organic solvents, which enabled their reuse over 21 days in the azidolysis of cyclohexene oxide.

Novel Fe‐modified CeO2 Nanorod Catalyst for the Dimethyl Carbonate Formation from CO2 and Methanol

Abstract: A series of Fe‐modified CeO2 catalysts with nanoparticle, nanorod, nanocube and nano‐octahedron morphologies were synthesized and applied for direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol. The Fe‐modification shows a remarkably promoting effect on the formation of DMC over Fe‐modified CeO2 nanorod. The DMC yield was improved from 0.202 to 2.568 mmol DMC per gcat by 2 %Fe‐modification of CeO2 nanorod. A synergistic effect among the moderate acid‐basic sites, specific surface area and surface composition resulted from Fe‐modification was proposed to be crucial to obtaining the high reactivity of DMC formation.

Fluxionality of Subnano Clusters Reshapes the Activity Volcano of Electrocatalysis

Abstract: The Sabatier activity volcano provides intuitive guide for catalyst design, but also imposes fundamental limitations on the composition and maximal activity of catalysts. We show that the ORR activity volcano is shifted and reshaped by the potential‐dependent fluxionality of subnano cluster catalysts. Fluxionality causes the typically under‐binding Ag/Au to gain optimal activity in the cluster form, and surpass Pt/Pd. Furthermore, isomerization of clusters as a function of the potential breaks linear scaling relationships, enabling surpassing the volcano “apex” relative to the bulk. The effect is likely general for fluxional cluster catalysts.