Catalysts For Oxidation Reactions Research Articles

Role of Metal-Organic Framework Topology on Thermodynamics of Polyoxometalate Encapsulation.

Polyoxometalates (POMs) are discrete anionic clusters whose rich redox properties, strong Bro̷nsted acidity, and high availability of active sites make them potent catalysts for oxidation reactions. Metal-organic frameworks (MOFs) have emerged as tunable, porous platforms to immobilize POMs, thus increasing their solution stability and catalytic activity. While POM@MOF composite materials have been widely used for a variety of applications, little is known about the thermodynamics of the encapsulation process. Here, we utilize an up-and-coming technique in the field of heterogeneous materials, isothermal titration calorimetry (ITC), to obtain full thermodynamic profiles (ΔH, ΔS, ΔG, and Ka) of POM binding. Six different 8-connected hexanuclear Zr-MOFs were investigated to determine the impact of MOF topology (csq, scu, and the) on POM encapsulation thermodynamics.

Reusable Magnetite Nanoparticle (Fe3O4 NP) Catalyst for Selective Oxidation of Alcohols under Microwave Irradiation.

Iron oxide nanoparticles (NPs) are nontoxic and abundant materials which have long been investigated as reusable catalysts in oxidation reactions, but their use so far has been hampered by a low selectivity. Here, unsupported iron oxide NPs have been found to successfully catalyze the microwave-assisted oxidation of primary and secondary alcohols to their respective aldehydes and ketones with a high selectivity when N-methylmorpholine N-oxide was used as the terminal oxidant. The crystalline phase and size of the iron-based catalyst have a drastic effect on its activity, with small magnetite (Fe3O4) NPs being the optimal catalyst for this reaction. The nanocatalyst could be easily recovered by magnetoseparation and successfully recycled four times without any need for special pretreatment or reactivation step and with a minimal loss of activity. The subsequent loss of activity was attributed to the transition from magnetite (Fe3O4) to maghemite (γ-Fe2O3), as confirmed by X-ray diffraction, Fourier transform infrared, and X-ray absorption near-edge spectroscopy. The nanocatalyst could then be reactivated by the high-temperature microwave treatment and used again for the microwave-assisted oxidation reaction.

Atomically dispersed Iridium on Mo2C as an efficient and stable alkaline hydrogen oxidation reaction catalyst

Hydroxide exchange membrane fuel cells (HEMFCs) have the advantages of using cost-effective materials, but hindered by the sluggish anodic hydrogen oxidation reaction (HOR) kinetics. Here, we report an atomically dispersed Ir on Mo2C nanoparticles supported on carbon (IrSA-Mo2C/C) as highly active and stable HOR catalysts. The specific exchange current density of IrSA-Mo2C/C is 4.1 mA cm−2ECSA, which is 10 times that of Ir/C. Negligible decay is observed after 30,000-cycle accelerated stability test. Theoretical calculations suggest the high HOR activity is attributed to the unique Mo2C substrate, which makes the Ir sites with optimized H binding and also provides enhanced OH binding sites. By using a low loading (0.05 mgIr cm−2) of IrSA-Mo2C/C as anode, the fabricated HEMFC can deliver a high peak power density of 1.64 W cm−2. This work illustrates that atomically dispersed precious metal on carbides may be a promising strategy for high performance HEMFCs.

Modulating the steric effect over high-index facet of MnOx catalysts to enhance toluene oxidation

Engineering surface structures of metal-oxide catalysts are of utmost significance for boosting their catalytic performances. In this work, we reported the high-index (310) facets catalysts (MnOx-AAUS) with oxygen vacancies for VOCs degradation, which was synthesized via an ultrasonic-assisted redox precipitation process. Interestingly, the excellent catalytic performance of MnOx-AAUS catalysts was due to the steric effect between high-index facet with reactants and the simultaneous adsorption-activation of toluene and gaseous oxygen on the high-index (310) facets with oxygen vacancies. The T90% value of the (310) facets catalysts (600 ppm toluene, GHSV = 60,000 mL/g·h, 21 % O2/N2) was significantly lower than that of the previous (001) facets catalysts. Density functional theory (DFT) calculations, along with in-situ FT-IR spectra, revealed that the tilted adsorption configuration of toluene was conducive to simultaneous reaction with the surrounding activated oxygen species. Such the adsorption configuration of toluene/oxygen contributed to the substantially enhanced catalytic activity of the MnOx-AAUS catalyst for VOCs oxidation. This work provides new insights into the roles of high-index facets with oxygen vacancies of MnOx catalysts in the toluene oxidation and paves new avenues for the rational design of highly efficient catalysts for oxidation reactions.

Tuning the apparent hydrogen binding energy to achieve high-performance Ni-based hydrogen oxidation reaction catalyst

High-performance platinum-group-metal-free alkaline hydrogen oxidation reaction catalysts are essential for the hydroxide exchange membrane fuel cells, which generally require high Pt loadings on the anode. Herein, we report a highly active hydrogen oxidation reaction catalyst, NiCuCr, indicated by the hydroxide exchange membrane fuel cell with a high peak power density of 577 mW cm−2 (18 times as high as the Ni/C anode) and a stability of more than 150 h (a degradation rate slower by 7 times than the Ni/C anode). The spectroscopies demonstrate that the alloy effect from Cu weakens the hydrogen binding, and the surface Cr2O3 species enhance the interfacial water binding. Both effects bring an optimized apparent hydrogen binding energy and thus lead to the high hydrogen oxidation reaction performance of NiCuCr. These results suggest that the apparent hydrogen binding energy determines the hydrogen oxidation reaction performance and that its tuning is beneficial toward high electrocatalytic performance.

New polypyridine Ru(II) dinuclear complex capable of catalyzing the water oxidation reaction

AbstractIn natural and artificial photosynthesis, the water oxidation reaction is the bottleneck for the conversion of solar energy into chemical energy. In this work, we report the syntheses and physicochemical characterization of two new dinuclear Ru(II) complexes, with molecular formula [{Ru(tpy)Cl}2(μ‐dpp)](PF6)2 (1) and [{Ru(tpy)(H2O)}2(μ‐dpp)](PF6)4 (2) (with tpy=2,2’ : 6’,2”‐terpyridine and dpp=2,3‐bis(2‐pyridyl)pyrazine). Chloro‐complex (1) was used as a precursor for the aquo‐complex (2) which can act as an active water oxidation reaction catalyst using Ce(IV) in CF3COOH 0.1 M as sacrificial oxidant. The mechanism and rate constants were explored by combining conventional mixing and stopped‐flow experiments with spectrophotometric monitoring in the UV‐visible region. DFT and TD‐DFT calculations were done to support the assignments of the experimental data. From the kinetic and theoretical data obtained, we propose‐ that (2) catalyses water oxidation via a nucleophillic water attack mechanism (WNA). Besides, catalytic activity for (2) was evaluated from reported values of turnover frequency (TOF) and turnover number (TON) values at pH=1.

Exploring an Electrochemical Route for Water-Enhanced Oxygenation Reactions Utilizing Nickel Molecular Structures: A Case Study.

Recently, Ni molecular catalysis has been extensively applied in oxygenation reactions. This work is underpinned by the characterization techniques and the discovered instability of the Ni-bipyridine/phenanthroline system, which results in Ni (hydr)oxide production under oxidative conditions. The practical applications of this mechanism by employing a prepared Ni (hydr)oxide-based electrode specifically in the oxygenation of sulfides, achieving noteworthy yields in contrast to noncatalyst control experiments, are explored. Thus, a Ni (hydr)oxide-based material is proposed as a candidate for the true catalyst for sulfide oxidation in the presence of the Ni-bipyridine/phenanthroline system. The findings of this study are expected to stimulate discussion and encourage new viewpoints within the chemical community regarding the potential applications and mechanisms of molecular catalysts in oxidation reactions.

Formaldehyde Ambient-Temperature Decomposition over Pd/Mn3O4-MnO Driven by Active Sites' Self-Tandem Catalysis.

The widespread presence of formaldehyde (HCHO) pollutant has aroused significant environmental and health concerns. The catalytic oxidation of HCHO into CO2 and H2O at ambient temperature is regarded as one of the most efficacious and environmentally friendly approaches; to achieve this, however, accelerating the intermediate formate species formation and decomposition remains an ongoing obstacle. Herein, a unique tandem catalytic system with outstanding performance in low-temperature HCHO oxidation is proposed on well-structured Pd/Mn3O4-MnO catalysts possessing bifunctional catalytic centers. Notably, the optimized tandem catalyst achieves complete oxidation of 100 ppm of HCHO at just 18 °C, much better than the Pd/Mn3O4 (30%) and Pd/MnO (27%) counterparts as well as other physical tandem catalysts. The operando analyses and physical tandem investigations reveal that HCHO is primarily activated to gaseous HCOOH on the surface of Pd/Mn3O4 and subsequently converted to H2CO3 on the Pd/MnO component for deep decomposition. Theoretical studies disclose that Pd/Mn3O4 exhibits a favorable reaction energy barrier for the HCHO → HCOOH step compared to Pd/MnO; while conversely, the HCOOH → H2CO3 step is more facilely accomplished over Pd/MnO. Furthermore, the nanoscale intimacy between two components enhances the mobility of lattice oxygen, thereby facilitating interfacial reconstruction and promoting interaction between active sites of Pd/Mn3O4 and Pd/MnO in local vicinity, which further benefits sustained HCHO tandem catalytic oxidation. The tandem catalysis demonstrated in this work provides a generalizable platform for the future design of well-defined functional catalysts for oxidation reactions.

Hydrogen oxidation reaction catalysts derived from Co, Ni-coordination compounds and polymers (MOFs)

Aiming to obtain catalysts for the Hydrogen Oxidation Reaction (HOR) and to construct fuel cells completely free of noble metals, two Ni-MOFs, a Ni-coordination polymer, and a Ni-complex were synthesized, together with their corresponding Co and NiCo analogs. The following N-heterocyclic ligands were employed: 2-Methylimidazolate, imidazolate, nicotinate, 2,3-pyrazinedicarboxylate and pyrazinecarboxylate. These precursor compounds were pyrolyzed at 700 °C obtaining metallic nanoparticles embedded within N-doped mesoporous carbons. These materials were characterized by different physical and electrochemical techniques. The catalysts exhibited HOR activities (jη = 50 mv) comparable to the best-reported materials, i.e.: 1.3 and 0.6 mA cm−2disk for the Ni imidazolate and Ni nicotinate derived materials respectively. The results are relevant for the design of new catalysts and to construct alkaline anion exchange membrane fuel cells completely free of platinum group metals.

CO oxidation over three-dimensional transition metal-doped CeO2 catalysts: A density functional calculation study

Carbon monoxide (CO) oxidation is considered one of the effective methods to address environmental pollution caused by vehicle emissions and incomplete combustion of fuels. However, designing efficient and affordable catalysts has always been a major challenge. In this study, a series of transition metals (TMs)-doped CeO2(111) catalysts were designed based on density functional theory (DFT) to catalyze the oxidation reaction of CO. According to the results, TMs-CeO2(111) catalysts exhibited excellent thermal stability and adsorption performance. In particular, Fe-CeO2(111) and Ni-CeO2(111) catalysts displayed good catalytic activity, with their respective rate-determining step energy barriers being 0.57 and 0.43 eV. Of particular note, Fe-CeO2(111) demonstrated high reaction rates at low to medium temperatures, making it a promising candidate for CO oxidation reaction catalyst. This study provides new insights into designing efficient, low-cost, and readily accessible CO oxidation catalysts, and establishes a solid foundation for the development of catalytic combustion kinetic models.

CO and Propane Combustion on La0.8Sr0.2CoxFe1−xO3−δ Perovskites: Effect of Fe-to-Co Ratio on Catalytic Activity

Perovskites are promising alternative catalysts for oxidation reactions due to their lower cost compared to noble metals, and their greater thermal stability. The catalytic oxidation of CO is essential in order to control CO emissions in a series of applications whereas the catalytic combustion of propane is considered an economical and environmentally acceptable solution for energy production and gaseous pollutant management, since propane is among the organic compounds involved in photochemical reactions. This work concerns the effect of the Co/Fe ratio in the B-sites of a series of eight La0.8Sr0.2CoxFe1−xO3−δ perovskites, with x ranging from 0 to 1, on the catalytic activity towards CO and C3H8 oxidation. The perovskite oxides were synthesized using the combustion synthesis method and characterized with respect to their specific surface areas, structures, and reduction properties. Increasing the Co/Fe ratio resulted in an increase in CO and propane conversion under both oxidative and stoichiometric conditions. The increase in Co content is considered to facilitate the formation of oxygen vacancies due to the lower redox stability of the cobalt cations compared to iron cations, favoring oxygen ion mobility and oxygen exchange between the gas phase and the oxide surface, thus enhancing the catalytic performance.

Hydrogen Peroxide Disproportionation Activity Is Sensitive to Pyridine Substitutions on Manganese Catalysts Derived from 12-Membered Tetra-Aza Macrocyclic Ligands.

The abundance of manganese in nature and versatility to access different oxidation states have made manganese complexes attractive as catalysts for oxidation reactions in both biology and industry. Macrocyclic ligands offer the advantage of substantially controlling the reactivity of the manganese center through electronic tuning and steric constraint. Inspired by the manganese catalase enzyme, a biological catalyst for the disproportionation of H2O2 into water and O2, the work herein employs 12-membered tetra-aza macrocyclic ligands to study how the inclusion of and substitution to the pyridine ring on the macrocyclic ligand scaffold impacts the reactivity of the manganese complex as a H2O2 disproportionation catalyst. Synthesis and isolation of the manganese complexes was validated by characterization using UV-vis spectroscopy, SC-XRD, and cyclic voltammetry. Potentiometric titrations were used to study the ligand basicity as well as the thermodynamic equilibrium with Mn(II). Manganese complexes were also produced in situ and characterized using electrochemistry for comparison to the isolated species. Results from these studies and H2O2 reactivity showed a remarkable difference among the ligands studied, revealing instead a distinction in the reactivity regarding the number of pyridine rings within the scaffold. Moreover, electron-donating groups on the 4-position of the pyridine ring enhanced the reactivity of the manganese center for H2O2 disproportionation, demonstrating a handle for control of oxidation reactions using the pyridinophane macrocycle.

Boron-Activating Ruthenium from Organic-Free Synthesis Process: A Hydrogen Oxidation Reaction Catalyst for Anion Exchange Membrane Fuel Cells

Using fossil fuels, power generation is always accompanied by environmental pollution. However, hydrogen-oxygen fuel cells, using H2 generated from renewable energy, can provide electricity with zero emissions, meeting the significant demands. Proton exchange membrane fuel cells (PEMFCs), though commercialized, are hindered by the high cost due to relying on expensive Pt. By contrast, as the potential alternative, anion exchange membrane fuel cells (AEMFCs), have attracted more and more attention as they can provide the benefit of using low-cost cathode catalysts while still having high power density. Although the oxygen reduction reaction (ORR) on the cathode can be well achieved by platinum group metal (PGM)-free catalysts in an alkaline environment, the high loading of Pt for the anode is still needed for high power output caused by the sluggish hydrogen oxidation reaction (HOR) kinetics. As the cheapest PGM, Ru with lower HOR activity is often added as an activity promoter to reduce the Pt loading. Recently, more efforts have been put into using exclusively Ru as the main catalysis active site, but the synthesis methods are often complex and involve toxic organics.In this project, boron-activated Ru supported on carbon was developed for HOR catalysis in alkaline media. The B-Ru/C was synthesized by a simple organic-free method, making it possibly attractive for industrial application. This method adopted water as the only solvent and boric acid as the boron source to avoid the toxic problem and the tedious removal step of organics. It produced uniformly dispersed Ru nanoparticles in smaller particle sizes with a boron-rich environment verified by TEM and STEM images. Compared to Ru/C or Ru/BC, B-Ru/C not only has a higher electrochemical active surface area (ECSA) but also reached a much higher ECSA-normalized exchange current density which means boron does promote the activity of Ru. From the XPS spectra, the more significant peak shifts of Ru3p and B1s in B-Ru/C relative to Ru/C and BC indicate that the boron-ruthenium interaction is more robust in B-Ru/C than in Ru/BC. A theoretical simulation was conducted further to understand the mechanism of the HOR activity enhancement. From an electrochemical perspective, the B-Ru/C had a very high exchange current density in ex-situ tests and was also studied as an anode in operating AEMFCs. We believe this cost-effective Pt-free HOR catalyst with excellent performance prepared through an organic-free process can contribute to the industrial application of AEMFCs in the future.AcknowledgmentsThis work is supported by the Research Grant Council (HKUST C6011-20G).

Chemoselective Oxidation of Alkenes by Metal‐Organic Framework‐Supported Copper Catalyst

AbstractOxidation of alkenes to carbonyls or diols compounds is important in synthesizing fine chemicals and pharmaceutical intermediates. We report the synthesis and characterization of an aluminum metal‐organic framework node‐supported copper(II) chloride (DUT‐5‐CuCl), which is an efficient heterogeneous catalyst for the oxidation of alkenes using H2O2 as an oxidizing agent. Styrene and various substituted styrenes were transformed into the corresponding carbonyl compounds in excellent selectivity and yields. DUT‐5‐CuCl is tolerant with various functional groups and could be recycled and reused at least 5 times in the oxidation of α‐methylstyrene. Unlike the oxidation of styrene derivatives, DUT‐5‐Cu catalyzed oxidation of aliphatic and cyclic alkenes produced 1,2‐diols compounds selectively. The mechanism of the DUT‐5‐Cu catalyzed oxidation of styrene to benzaldehyde was investigated in detail by various experiments such as the determination of reaction intermediates and characterization of the catalyst after catalysis, and computational studies. This work highlights the importance of MOF‐supported earth‐abundant metal catalysts for oxidation reactions to produce fine chemicals.

Development of Hydrogen Oxidation Reaction Catalysts to Overcome CO Poisoning and Elucidation of Reaction Mechanism

Passive autocatalytic recombiner (PAR) represents a potential technology for ensuring process safety in the hydrogen society. PARs function through the catalytic oxidation of generated or leaked hydrogen, facilitating its conversion into water and effectively mitigating the risk of hydrogen explosions. CO is recognized as a catalyst poison that hampers surface catalytic reactions. To investigate the negative effects of CO on the local structure of platinum metal nanoparticle catalysts during water formation, in situ and time-resolved X-ray absorption spectroscopy analyses were conducted. The results revealed that the Pt–Fe/CZY catalyst exhibited notable hydrogen oxidation activity even in the presence of CO. The enhanced performance can be attributed to the combined effects of Pt–Fe alloy composition and CZY support materials.

Metalloporphyrin/ZnO solids as selective catalysts for oxidation reaction assisted by light

Cationic manganese (MnP - [Mn(T4MPyP)]5+) and anionic iron (FeP - [Fe(TDFSPP)]3–) metalloporphyrins in conjunction with zinc oxide of different morphologies, were synthesized. After characterization by different instrumental techniques, the metalloporphyrins were immobilized on the obtained zinc oxide and the catalysts were evaluated regarding the oxidation of cyclohexane, using iodosylbenzene (PhIO) as model oxidant. In the absence of light, the activities of FeP/ZnO were higher than those obtained by neat ZnO and MnP/ZnO. Furthermore, under the reaction conditions of 1:100:1000 catalyst/oxidant/substrate, with acetonitrile as solvent, for 1 h at 25 °C without any radiation source, the reactions were selective for ketone. The same occurred for the neat FeP under homogeneous conditions. In contrast, when the reaction proportions were changed to 1:20:1000 catalyst/oxidant/substrate and when the catalyst amount was increased 10 times, the selectivity was for alcohol. Under irradiation with a dichroic halogen lamp (50 W), all the systems containing FeP showed higher selectivity for alcohol, while when MnP was used, increments like those with FeP were not observed, probably due to the structural differences between FeP and MnP. This FeP to MnP replacement led to spectral differences that might have been responsible for the inability of MnP to interact efficiently with the radiation emitted by the dichroic halogen lamp as well as promoting the higher amount of ketone produced in comparison with the FeP/ZnO system.

Role of decomposition products in the oxidation of cyclohexene using a manganese(III) complex

Metal complexes are extensively explored as catalysts for oxidation reactions; molecular-based mechanisms are usually proposed for such reactions. However, the roles of the decomposition products of these materials in the catalytic process have yet to be considered for these reactions. Herein, the cyclohexene oxidation in the presence of manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (1) in a heterogeneous system via loading the complex on an SBA-15 substrate is performed as a study case. A molecular-based mechanism is usually suggested for such a metal complex. Herein, 1 was selected and investigated under the oxidation reaction by iodosylbenzene or (diacetoxyiodo)benzene (PhI(OAc)2). In addition to 1, at least one of the decomposition products of 1 formed during the oxidation reaction could be considered a candidate to catalyze the reaction. First-principles calculations show that Mn dissolution is energetically feasible in the presence of iodosylbenzene and trace amounts of water.

Assessment of the metal exchanged phosphomolybdic acid salt-catalyzed nerol oxidation reactions with hydrogen peroxide

In this work, phosphomolybdate salts were synthesized by exchanging the protons of the phosphomolybdic acid with metal cations (i.e., Co2+, Cu2+, Zn2+, Mn2+, Ni2+, Al3+, and Fe3+ cations) and evaluated as catalysts in oxidation reactions of terpene alcohols with hydrogen peroxide. All the catalysts were characterized by Raman, UV-Vis, and infrared spectroscopies, powder X-rays diffraction, energy dispersive spectroscopy, scanning electronic microscopy, and thermal analyses. The strength of the acid sites was determined by potentiometric titration curves. Among the catalysts tested in nerol oxidation (i.e., model molecule selected), AlPMo12O40 was the most active and selective catalyst toward nerol epoxide, the main product. Main reaction variables such as the H2O2: substrate molar ratio, time, temperature, type, and catalyst load were assessed. The focus was to correlate the physicochemical properties of the catalysts with their performance. The reaction scope was extended to other terpene alcohols. Allylic alcohols (nerol and geraniol) were mainly converted to epoxides, while the oxidation of secondary (borneol) and tertiary (linalool) alcohols gave carbonylic products (camphor), or cyclization products (furane and pyrane rings), respectively.

Self-supporting trimetallic PtAuBi aerogels as electrocatalyst for ethanol oxidation reaction

The creation of anodic ethanol oxidation reaction catalysts with superior all-around performance for direct ethanol fuel cells (DEFCs) has continued to attract the attention of researchers. An ultrathin trimetallic PtAuBi aerogel with branching, rough-surfaced 1D nanowires that self-assemble into a 3D porous network structure has been created in this study. It has a mass activity (MA) of 8045 mA mgPt−1 in an alkaline medium, which is 7.56 times greater than that of commercial Pt/C (1064 mA mgPt−1). Notably, the catalytic activity and resistance to CO poisoning of PtAuBi aerogels are improved by the addition of an efficient “active additive” Au. The results analysis reveals that the increased performance of PtAuBi aerogel is mostly attributable to the integrated function of the 3D porous network structure, the downward shift of the Pt d-band center, and the synergistic effect of the “Pt–Bi” and/or “Pt–Au” dual active sites.

Engineering Morphology and Ni Substitution of NixCo3–xO4 Spinel Oxides to Promote Catalytic Combustion of Ethane: Elucidating the Influence of Oxygen Defects

Transition metal oxides are potential alternatives to noble metal catalysts for oxidation reactions. Co-based spinel oxides, in particular, have attracted significant attention. Herein, NixCo3–xO4 catalysts were synthesized to elucidate the influence of oxygen vacancies on catalyst activities and reaction mechanisms for ethane combustion. A correlation between the activity and the population and properties of O defects was developed, with an increased number of O defects typically resulting in higher activity. Also, the shape-induced facet effect is related to the amount of Ni that is incorporated into the octahedral sites of Co oxide. The substituted Ni atoms altered the redox ability of NixCo3–xO4 by changing O vacancy formation and C–H bond dissociation. The NiCo2O4-TM catalyst (6.0 mmol mL–1 h–1) exhibits the highest activity for ethane oxidation compared with NiCo2O4-PC (0.5 mmol mL–1 h–1) and NiCo2O4-OL catalysts (1.3 mmol mL–1 h–1) at 330 °C, and its activation energy (Ea) is 70.9 kJ mol–1. No activity decay is observed after the initial transition stage of the reaction in a long-term stability test up to 500 h on the NiCo2O4-TM-coated monolith, either with or without water addition. A vacancy-mediated pathway was proposed according to in situ diffuse reflectance infrared Fourier transform (DRIFT) and density functional theory (DFT) calculations over the NiCo2O4(311) facet. Findings from this study expand our understanding of the facet-dependent catalytic behavior and ultimately enable the rational design of high-performance catalysts.