Mn Catalysts Research Articles

Catalytic activity of bimetallic Rh/Rh-M nanosheets governed by CO spillover

In this issue of Chem Catalysis , Huang and co-workers constructed a series of Rh/Rh-M (M = Co, Mn, Fe, and Ni) catalysts with unique nanosheet structure, exhibiting superior anti-CO poisoning and ethanol oxidation reaction activity. This study opens a promising path to manipulating the intermediate species via CO spillover. In this issue of Chem Catalysis , Huang and co-workers constructed a series of Rh/Rh-M (M = Co, Mn, Fe, and Ni) catalysts with unique nanosheet structure, exhibiting superior anti-CO poisoning and ethanol oxidation reaction activity. This study opens a promising path to manipulating the intermediate species via CO spillover.

Super-paramagnetic polymer composite-supported dendrimer–Mn catalyst: Fabrication, characterization and catalytic evaluation in selective aerobic oxidation of ethylbenzene and oximes derivatives

Design of strategy and catalytic for oxidation ethylbenzene, toxic petroleum, to high value-added via hydrogen-atom-transfer process is lately attracting a renewed interest from both environmentally and economic view. Herein, novel manganese-based dendritic catalysts supported on the polymer-magnetic core-shell were synthesized, fully identified and evaluated with initiator NDHPI for selective and easy-to-handle aerobic oxidation of ethylbenzene (EB) and oximes to acetophenone (ACP) & carbonyl compounds (AH/KO), respectively. Besides mild condition reaction, the supported dendrimer-encapsulated Mn NPs also exhibited outstanding catalytic performance, such as selective oxidation of ethylbenzene and oximes using O2 as green and cheaper oxidant with excellent reactivity, selectivity, stability, and magnetic recyclability. Additionally, the effect of operating parameters such as the amount of catalyst, nature of solvent, temperature and reaction time, and role of several N-hydroxyimides were studied in the catalytic efficiency of the synthesized dendritic catalyst. The results illustrated that the enhanced catalytic efficiency of the catalyst is derived from the magnetic dendritic structures, high Mn(II) content, hydrophobic arm of dendrimers, and interaction of Mn and the dendritic framework. Finally, the reaction pathway for EB and oxime has been deduced, and the crucial role of dendritic catalyst, NDPHI, and electronic effect has been elucidated.

Enhancing Ozone Oxidation of Reverse Osmosis Concentrate Using Activated Carbon-Supported Cu–Co–Mn Catalysts

Enhancing Ozone Oxidation of Reverse Osmosis Concentrate Using Activated Carbon-Supported Cu–Co–Mn Catalysts

Plasma-enhanced catalytic oxidation of ethylene oxide over Fe–Mn based ternary catalysts

In this work, Fe–Mn based ternary catalysts (Fe–Mn-X, X = Ce, Co and Cu) were evaluated in the plasma-catalytic oxidation of ethylene oxide (EO) using a cylindrical dielectric barrier discharge (DBD) reactor. The addition of transition metal dopants to the Fe–Mn catalyst significantly improved the performance of plasma-catalyzed EO oxidation compared to the reaction using plasma only and the Fe–Mn catalyst. The highest EO conversion (91.9%) and CO2 selectivity (78.9%) were obtained over the Fe–Mn–Co catalyst at a specific energy input (SEI) of 730.2 J l−1. Moreover, the presence of water vapor improved EO conversion and CO2 selectivity in the relative humidity (RH) range of 0–60%, while further increasing the RH to 80% reduced EO conversion. The interactions between Fe, Mn and dopants increased the specific surface area and pore volume of the Fe–Mn-X catalyst, while maintaining the crystalline structure of the catalysts. The presence of the dopants enhanced the reducibility of the Fe–Mn-X catalysts and produced more surface adsorbed oxygen (Oads) on the catalyst surfaces. Moreover, the interactive effects among the Fe, Mn and dopants facilitated the plasma-catalytic oxidation reactions via enhanced surface reactions. The coupling of plasma with the Fe–Mn–Co catalyst reduced the formation of organic by-products in the plasma-catalyzed oxidation of EO.

Chemoselective Oxyfunctionalization of Functionalized Benzylic Compounds with a Manganese Catalyst.

Whilst allowing for easy access to synthetically versatile motifs and for modification of bioactive molecules, the chemoselective benzylic oxidation reactions of functionalized alkyl arenes remain challenging. Reported in this study is a new non‐heme Mn catalyst stabilized by a bipiperidine‐based tetradentate ligand, which enables methylene oxidation of benzylic compounds by H2O2, showing high activity and excellent chemoselectivity under mild conditions. The protocol tolerates an unprecedentedly wide range of functional groups, including carboxylic acid and derivatives, ketone, cyano, azide, acetate, sulfonate, alkyne, amino acid, and amine units, thus providing a low‐cost, more sustainable and robust pathway for the facile synthesis of ketones, increase of complexity of organic molecules, and late‐stage modification of drugs.

Chemoselective Oxyfunctionalization of Functionalized Benzylic Compounds with a Manganese Catalyst

AbstractWhilst allowing for easy access to synthetically versatile motifs and for modification of bioactive molecules, the chemoselective benzylic oxidation reactions of functionalized alkyl arenes remain challenging. Reported in this study is a new non‐heme Mn catalyst stabilized by a bipiperidine‐based tetradentate ligand, which enables methylene oxidation of benzylic compounds by H2O2, showing high activity and excellent chemoselectivity under mild conditions. The protocol tolerates an unprecedentedly wide range of functional groups, including carboxylic acid and derivatives, ketone, cyano, azide, acetate, sulfonate, alkyne, amino acid, and amine units, thus providing a low‐cost, more sustainable and robust pathway for the facile synthesis of ketones, increase of complexity of organic molecules, and late‐stage modification of drugs.

Dual-Metal Active Sites Mediated by p-Block Elements: Knowledge-Driven Design of Oxygen Reduction Reaction Catalysts

In this study, the oxygen reduction reaction (ORR) process of dual-metal active site catalysts (FeMN6-Gra, M = Mn, Ni, Co, or Cu) mediated by p-block elements was investigated using density functional theory calculations. The obtained results demonstrate that, in most cases, the B-doped FeMN6-Gra (M = Mn, Ni, Co, or Cu) catalysts exhibit higher catalytic performance than their undoped counterparts. Among the investigated catalysts, FeNiN6-Gra doping by B modulates the adsorption strength of the metal center on the oxygen-containing intermediates, showing the largest increase in the onset potential (from 0.66 to 0.94 V). Importantly, we found a new law that B-doping affects the total charge of the metal adsorption site and the four surrounding N atoms and that there is a linear relationship between the total charge and the Gibbs free energy. Transition state analysis shows that the energy barrier of the thermodynamic rate-determining step (*OH hydrogenation to H2O) in the FeNiN6B1-Gra-catalyzed ORR process is 0.17 eV, which is smaller than that of the FeNiN6-Gra-catalyzed process (0.28 eV). Overall, the results demonstrate that B-doping can improve the activity of FeMN6-Gra catalysts and provide a new method for the future development of efficient electrocatalysts.

Reaction Kinetics and Mechanism of VOCs Combustion on Mn-Ce-SBA-15

A propane combustion catalyst based on Mn and Ce and supported by SBA-15 was prepared by the “two-solvents” method aiming at the possible application in catalytic converters for abatement of alkanes in waste (exhaust) gases. The catalyst characterization was carried out by SAXS, N2-physisorption, XRD, TEM, XPS, EPR and H2-TPR methods. The catalysts’ performance was evaluated by tests on the combustion of methane, propane and butane. The reaction kinetics investigation showed that the reaction orders towards propane and oxygen were 0.7 and 0.1, respectively. The negative reaction order towards the water (−0.3) shows an inhibiting effect on the water molecules. Based on the data from the instrumental methods, catalytic experiments and mathematic modeling of the reaction kinetics, one may conclude that the Mars–van Krevelen type of mechanism is the most probable for the reaction of complete propane oxidation over single Mn and bi-component Mn-Ce catalysts. The fine dispersion of manganese and cerium oxide and their strong interaction inside the channels of the SBA-15 molecular sieve leads to the formation of difficult to reduce oxide phases and consequently, to lower catalytic activity compared to the mono-component manganese oxide catalyst. It was confirmed that the meso-structure was not modified during the catalytic reaction, thus it can prevent the agglomeration of the oxide particles.

Efficient Electrochemical Reduction of CO2 to Formate in Methanol Solutions by Mn‐Functionalized Electrodes in the Presence of Amines**

Carbon cloth electrode modified by covalently attaching a manganese organometallic catalyst is used as cathode for the electrochemical reduction of CO2 in methanol solutions. Six different industrial amines are employed as co‐catalyst in millimolar concentrations to deliver a series of new reactive system. While such absorbents were so far believed to provide a CO2 reservoir and act as sacrificial proton source, we herein demonstrate that this role can be played by methanol, and that the adduct formed between CO2 and the amine can act as an effector or inhibitor toward the catalyst, thereby enhancing or reducing the production of formate. Pentamethyldiethylentriamine (PMDETA), identified as the best effector in our series, converts CO2 in wet methanolic solution into bisammonium bicarbonate. Computational studies revealed that this adduct is responsible for a barrierless transformation of CO2 to formate by the reduced form of the Mn catalyst covalently bonded to the electrode surface. As a consequence, selectivity can be switched on demand from CO to formate anion, and in the case of (PMDETA) an impressive TONHCOO− of 2.8×104 can be reached. This new valuable knowledge on an integrated capture and utilization system paves the way toward more efficient transformation of CO2 into liquid fuel.

Modified natural zeolites and clays as support of Ni–Mn catalyst for toluene oxidation in a one-stage plasma-catalysis system

Modified natural zeolites and clays as support of Ni–Mn catalyst for toluene oxidation in a one-stage plasma-catalysis system

Kinetic, Stability and Characterization Studies of Ce, Mn and Mn-doped Ceria Paper Catalysts Towards Soot Combustion Under Different Reaction Conditions

Kinetic, Stability and Characterization Studies of Ce, Mn and Mn-doped Ceria Paper Catalysts Towards Soot Combustion Under Different Reaction Conditions

Oxy-Anionic Doping: A New Strategy for Improving Selectivity of Ru/CeO2 with Synergetic Versatility and Thermal Stability for Catalytic Oxidation of Chlorinated Volatile Organic Compounds.

Understanding the formation and inhibition of more toxic polychlorinated byproducts from the catalytic oxidation elimination of chlorinated volatile organic compounds (Cl-VOCs) and unveiling efficient strategies have been essential and challenging. Here, RuOx supported on CePO4-doped CeO2 nanosheets (Ru/Pi-CeO2) is designed for boosting catalytic oxidation for the removal of dichloromethane (DCM) as a representative Cl-VOC. The promoted acid strength/number and sintering resistance due to the doping of electron-rich and thermally stable CePO4 are observed along with the undescended redox ability and the exposed multi-active sites, which demonstrates a high activity and durability of DCM oxidation (4000 mg/m3 and 15,000 mL/g·h, stable complete-oxidation at 300 °C), exceptional versatility for different Cl-VOCs, alkanes, aromatics, N-containing VOCs, CO and their multicomponent VOCs, and enhanced thermal stability. The suppression of polychlorinated byproducts is determined over Ru/Pi-CeO2 and oxy-anionic S, V, Mo, Nb, or W doping CeO2, thus the oxy-anionic doping strategy is proposed based on the quenching of the electron-rich oxy-anions on chlorine radicals. Moreover, the simple mechanical mixing with these oxy-anionic salts is also workable even for other catalysts such as Co, Sn, Mn, and noble metal-based catalysts. This work offers further insights into the inhibition of polychlorinated byproducts and contributes to the convenient manufacture of monolithic catalysts with superior chlorine-poisoning resistance for the catalytic oxidation of Cl-VOCs.

Electrocatalytic Oxidation of Methanol Over Silver-Based Ag-M/C (M = Cu, Zn, Fe, Cr, Mn) Electrocatalysts Synthesized by Solution Combustion Technique

Herein, we report the electrocatalytic properties of Ag-M/C (M = Cu, Zn, Fe, Cr, Mn) catalysts synthesized using solution combustion synthesis (SCS) method for methanol oxidation reaction (MOR). The morphological properties of the synthesized catalysts were studied using SEM, EDX, TEM, XRD and XPS techniques. The results indicated AgCu/C to be the most porous catalyst with small and well distributed nanoparticles making it a suitable choice for electrocatalytic applications. The XPS results showed a shift in peak in the AgCu/C sample due to the charge transfer between Ag and Cu indicating a strong interaction in the compound. The electrochemical measurements in 1 M methanol with 1 M KOH electrolyte by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) revealed that AgCu/C showed high electrocatalytic activity for MOR. Further studies on AgCu/C for methanol concentrations of 0.5 M, 1 M, 1.5 M, 2.5 M to evaluate the rate dependency of the catalyst indicated a power-law dependency with an order of 0.55 on methanol concentration. According to chronoamperometry analysis, the catalyst was stable for at least 20 h.

New Strategies for Direct Methane-to-Methanol Conversion from Active Learning Exploration of 16 Million Catalysts.

Despite decades of effort, no earth-abundant homogeneous catalysts have been discovered that can selectively oxidize methane to methanol. We exploit active learning to simultaneously optimize methane activation and methanol release calculated with machine learning-accelerated density functional theory in a space of 16 M candidate catalysts including novel macrocycles. By constructing macrocycles from fragments inspired by synthesized compounds, we ensure synthetic realism in our computational search. Our large-scale search reveals that low-spin Fe(II) compounds paired with strong-field (e.g., P or S-coordinating) ligands have among the best energetic tradeoffs between hydrogen atom transfer (HAT) and methanol release. This observation contrasts with prior efforts that have focused on high-spin Fe(II) with weak-field ligands. By decoupling equatorial and axial ligand effects, we determine that negatively charged axial ligands are critical for more rapid release of methanol and that higher-valency metals [i.e., M(III) vs M(II)] are likely to be rate-limited by slow methanol release. With full characterization of barrier heights, we confirm that optimizing for HAT does not lead to large oxo formation barriers. Energetic span analysis reveals designs for an intermediate-spin Mn(II) catalyst and a low-spin Fe(II) catalyst that are predicted to have good turnover frequencies. Our active learning approach to optimize two distinct reaction energies with efficient global optimization is expected to be beneficial for the search of large catalyst spaces where no prior designs have been identified and where linear scaling relationships between reaction energies or barriers may be limited or unknown.

Solvent-Assisted Ketone Reduction by a Homogeneous Mn Catalyst.

The choice of a solvent and the reaction conditions often defines the overall behavior of a homogeneous catalytic system by affecting the preferred reaction mechanism and thus the activity and selectivity of the catalytic process. Here, we explore the role of solvation in the mechanism of ketone reduction using a model representative of a bifunctional Mn-diamine catalyst through density functional theory calculations in a microsolvated environment by considering explicit solvent and fully solvated ab initio molecular dynamics simulations for the key elementary steps. Our computational analysis reveals the possibility of a Meerwein–Ponndorf–Verley (MPV) type mechanism in this system, which does not involve the participation of the N–H moiety and the formation of a transition-metal hydride species in ketone conversion. This path was not previously considered for Mn-based metal–ligand cooperative transfer hydrogenation homogeneous catalysis. The MPV mechanism is strongly facilitated by the solvent molecules present in the reaction environment and can potentially contribute to the catalytic performance of other related catalyst systems. Calculations indicate that, despite proceeding effectively in the second coordination sphere of the transition-metal center, the MPV reaction path retains the enantioselectivity preference induced by the presence of the small chiral N,N′-dimethyl-1,2-cyclohexanediamine ligand within the catalytic Mn(I) complex.

Ordering Degree-Dependent Activity of Pt3M (M = Fe, Mn) Intermetallic Nanoparticles for Electrocatalytic Methanol Oxidation.

Atomically ordered intermetallic alloys with unique electronic and geometrical structures are highly attractive for heterogeneous catalysis and electrocatalysis. However, the formation of intermetallic phases generally requires high-temperature annealing to overcome the kinetic energy barrier of atom ordering, which unfortunately causes high material heterogeneity and thus makes it challenging to identify the exact contribution of ordered structures to the improved performance. Here, we prepared a family of small-sized intermetallic core/shell Pt3M@Pt (M = Mn or Fe) catalysts with varied ordering degree by a high-temperature sulfur-confined method. We identified a strong correlation between the ordering degree of the intermetallic Pt3M core of the catalysts and their electrocatalytic activity for the methanol oxidation reaction. Density functional theory calculations show that the intermetallic Pt3M core induces a compressive strain on the Pt-skin, which weakens the CO* binding, lowers the free energy change from CO* to COOH*, and therefore promotes electrocatalytic methanol oxidation.

The triazole-thiol-functionalized Mn-complex and its catalytic performance

The novel magnetic triazole-thiol-functionalized Mn-complex was obtained by preparation of the Fe3O4 magnetic nanoparticles, coating of Fe3O4 MNPs with tetraethyl orthosilicate, functionalization with both 3-chloropropyltrimethoxysilane (CPTMS) and 1-H-1,2,4-triazole-3-thiol (TT) ligands, and further complexation with manganese acetate. Then, the supported Mn catalyst (Fe3O4@SiO2@CPTMS@TT@Mn) was characterized by various methods (FT-IR, XRD, EDX, ICP, SEM, SEM-mapping, SEM-Histograms, TEM, TGA-DTA, VSM, and BET) and used as a heterogeneous nano-catalyst for the synthesis of diverse N-benzylated cyanamides from the reaction of various aryl cyanamides with benzyl bromide and K2CO3 in acetonitrile at 60 °C in appropriate times.

Effect of Sn on the CO Catalytic Activity and Water Resistance of Cu-Mn Catalyst.

In view of the problem that excessive CO in underground coal mine space can easily lead to a large number of casualties, Cu–Mn–Sn water-resistant eliminators with different Sn contents were prepared by a co-precipitation method. The activity of the eliminators was analyzed by using an independently developed activity testing platform, N2 adsorption and desorption, XRD, SEM, XPS, and FTIR to characterize the activity factors and water resistance. The results showed that Cu–Mn–Sn-20 with 20% Sn content had the highest activity, which was 3.23 times that of Cu–Mn. The main reason for the increased activity is that Cu–Mn–Sn-20 doped with 20% Sn provides a larger specific surface area and more active sites and reduces the pore size, so that the crystallization degree of Cu1.4Mn1.5O4 is lower. The doping of 20% Sn reduces the absorption of lattice water and coordination water and improves the water resistance of Cu–Mn–Sn-type eliminators. The Cu–Mn–Sn-20 water-resistant eliminator is used to quickly eliminate CO in underground coal mines, which is of great significance for the rescue workers in underground coal mines after disasters.

Atomically dispersed Zn-Co-N-C catalyst boosting efficient and robust oxygen reduction catalysis in acid via stabilizing Co-N bonds

Transition metal supported N-doped carbon (M-N-C) catalysts for oxygen reduction reaction (ORR) are viewed as the promising candidate to replace Pt-group metal (PGM) for proton exchange membrane fuel cells (PEMFCs). However, the stability of M-N-C is extremely challenging due to the demetalation, H2O2 attack, etc. in the strongly oxidative conditions of PEMFCs. In this study, we demonstrate the universal effect of Zn on promoting the stability of atomically dispersed M-Nx/C (M = Co, Fe, Mn) catalysts and the enhancement mechanism is unveiled for the first time. The best-performing dual-metal-site Zn-Co-N-C catalyst exhibits a high half-wave potential (E1/2) value of 0.81 V vs. reversible hydrogen electrode (RHE) in acid and outstanding durability with no activity decay after 15,000 accelerated degradation test (ADT) cycles at 60 °C, surpassing most reported Co-based PGM-free catalysts in acid media. For comparison, the Co-N-C in the absence of Zn suffers from a rapid degradation after ADT due to the demetalation and higher H2O2 yield. X-ray adsorption spectroscopy (XAS) and density functional theory (DFT) calculations suggest the more negative formation energy (by 1.2 eV) and increased charge transfer of Zn-Co dual-site structure compared to Co-N-C could strength the Co-N bonds against the demetalation and the optimized d-band center accounts for the improved ORR kinetics.

Manganese Carbonyl Complexes as Selective Electrocatalysts for CO2 Reduction in Water and Organic Solvents

ConspectusThe electrochemical reduction of CO2 provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO2 reduction electrocatalysts provide tunable reaction centers offering an approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on [Mn(bpy)(CO)3Br] and its derivatives (bpy = 2,2′-bipyridine), are particularly interesting due to their ease of synthesis and the use of a first-row earth-abundant transition metal. [Mn(bpy)(CO)3Br] was first shown to be an active and selective catalyst for reducing CO2 to CO in organic solvents in 2011. Since then, manganese carbonyl catalysts have been widely studied with numerous reports of their use as electrocatalysts and photocatalysts and studies of their mechanism.This class of Mn catalysts only shows CO2 reduction activity with the addition of weak Brønsted acids. Perhaps surprisingly, early reports showed increased turnover frequencies as the acid strength is increased without a loss in selectivity toward CO evolution. It may have been expected that the competing hydrogen evolution reaction could have led to lower selectivity. Inspired by these works we began to explore if the catalyst would work in protic solvents, namely, water, and to explore the pH range over which it can operate. Here we describe the early studies from our laboratory that first demonstrated the use of manganese carbonyl complexes in water and then go on to discuss wider developments on the use of these catalysts in water, highlighting their potential as catalysts for use in aqueous CO2 electrolyzers.Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO2 binding mechanism, where for the acids widely studied, lower pKa values actually favor CO2 binding over Mn–H formation, a precursor to H2 evolution. Here we discuss the wider literature before focusing on our own contributions in validating this previously proposed mechanism through the use of vibrational sum frequency generation (VSFG) spectroelectrochemistry. This allowed us to study [Mn(bpy)(CO)3Br] while it is at, or near, the electrode surface, which provided a way to identify new catalytic intermediates and also confirm that proton-assisted CO2 binding operates in both the “dimer” and primary (via [Mn(bpy)(CO)3]−) pathways. Understanding the mechanism of how these highly selective catalysts operate is important as we propose that the Mn complexes will be valuable models to guide the development of new proton/acid tolerant CO2 reduction catalysts.