Sites For Oxidation Reaction Research Articles

Highly Stable NiCoZn Ternary Mixed-Metal-Oxide Nanorods as a Low-Cost, Non-Noble Electrocatalyst for Methanol Electro-Oxidation in Alkaline Medium

There is an imperative necessity to develop low-cost and efficient nanocatalysts as a substitute for expensive and rare noble-metal catalysts such as platinum (Pt) and gold (Au) in overcoming the limitations of carbon monoxide and intermediate poisoning in direct methanol fuel cells (DMFCs). Herein, we report a simple, one-step hydrothermal synthesis of NiCoZn ternary mixed-metal-oxide nanorod (NCZMMO)-modified glassy carbon electrode (NCZMMO/GCE) for DMFC application. The NCZMMO/GCE electrode exhibits outstanding electro-oxidation of methanol (MeOH) at a low onset potential of 0.30 V (vs Ag|AgCl) with a high current density of 414 mA/mg in an alkaline medium (1 M KOH). This outstanding performance of the electrode is ascribed to the synergistic outcome of NCZMMO nanorods, where Ni imparts the electrocatalytic active sites for methanol oxidation reaction (MOR) through its multiple oxidation states while the excellent mechanical and chemical stability of the electrode is attributed to the oxides of Co and Zn. The electrode exhibits outstanding durability with 75% retention even at 5000 s of chronoamperometry analysis in the presence of 0.5 M MeOH, which is far superior to other reported noble-metal electrocatalysts. This superior performance of NCZMMO frontlines it as a promising non-noble, low-cost electrocatalyst for various energy conversion applications.

One-Dimensional CdS/Carbon/Au Plasmonic Nanoarray Photoanodes via In Situ Reduction-Graphitization Approach toward Efficient Solar Hydrogen Evolution.

Photoelectrochemical (PEC) hydrogen evolution has been acknowledged as a promising "green" technique to convert solar energy into clean chemical fuel. Photoanodes play a key role in determining the performance of PEC systems, spurring numerous efforts to develop advanced materials as well as structures to improve the photoconversion efficiency. In this work, we report the rational design of a plasmonic hierarchical nanorod array, composed of oriented one-dimensional (1D) CdS nanorods decorated with a uniformly wrapped graphite-like carbon (CPDA) layer and Au nanoparticles (Au NPs), as highly efficient photoanode materials. An interfacial in situ reduction-graphitization method has been conducted to prepare the CdS/CPDA/Au nanoarchitecture, where polydopamine (PDA) coating was used as a C source and a reductant. The CdS/CPDA/Au nanoarray photoanode demonstrates superior photoconversion efficiency with a photocurrent density of 8.74 mA/cm2 and an IPCE value (480 nm) of 30.2% (at 1.23 V vs RHE), under simulated sunlight irradiation, which are 12.7 and 13.5 times higher than pristine CdS. The significant enhancement of PEC performance is mainly benefited from the increase of the entire quantum yield and efficiency due to the formation of a Schottky rectifier, localized surface plasmon resonance (LSPR)-enhanced light absorption, and promoted hot-electron injection from interlayered graphene-like carbon. More importantly, thanks to the inhibited charge carrier recombination process and transferred oxidation reaction sites, the fabricated CdS/CPDA/Au photoelectrode exhibits lengthened electron lifetimes and better photostability, illustrating its wonderful potential for future PEC application.

A Ru-Complex Tethered to a N-Rich Covalent Triazine Framework for Tandem Aerobic Oxidation-Knoevenagel Condensation Reactions.

Herein, a highly N-rich covalent triazine framework (CTF) is applied as support for a RuIII complex. The bipyridine sites within the CTF provide excellent anchoring points for the [Ru(acac)2(CH3CN)2]PF6 complex. The obtained robust RuIII@bipy-CTF material was applied for the selective tandem aerobic oxidation-Knoevenagel condensation reaction. The presented system shows a high catalytic performance (>80% conversion of alcohols to α, β-unsaturated nitriles) without the use of expensive noble metals. The bipy-CTF not only acts as the catalyst support but also provides the active sites for both aerobic oxidation and Knoevenagel condensation reactions. This work highlights a new perspective for the development of highly efficient and robust heterogeneous catalysts applying CTFs for cascade catalysis.

Regulating magnesium combustion using surface chemistry and heating rate

The magnesium (Mg) particle surface can be used to regulate fluorination or oxidation reactions depending on the applied heating rate and Mg particle size. Magnesium particles are surrounded by a complex hydroxide shell composed of an inner layer of magnesium oxide (MgO) and outer layer of magnesium hydroxide (Mg(OH)2). As particles approach the nanoscale, the thick oxide shell (e.g., 22 nm) becomes an appreciable portion of the overall powder and can be exploited to regulate reactivity. In this study, the reactivity of 800 nm Mg particles (nMg) was compared to 44 µm Mg particles (µMg) when combined with Perfluoropolyethyer (PFPE), providing both fluorine and oxygen for Mg reactions. Experiments were performed at slow heating rates (10 °C/min) and separate experiments were performed at fast heating rates (6.0 × 105 °C/min). The slow heating rate studies used a differential scanning calorimeter (DSC) and thermogravimetric (TG) analyzer to examine reaction kinetics. The faster heating rate experiments used a hot wire to ignite a thermal run-away reaction. Powder X-ray diffraction (XRD) analysis of recovered residue at various temperatures corresponding to exothermic events in the DSC revealed reaction pathways for nMg + PFPE favoring oxidation reactions. For nMg powders, the outer Mg(OH)2 surface layer dehydrates at low temperatures (313 °C) creating highly reactive sites for surface oxidation reactions in the condensed phase leading to a higher conversion of Mg(OH)2 to MgO and greater consumption of Mg through oxidation reactions. For µMg, higher Mg(OH)2 dehydration temperatures (498 °C) stabilize µMg particles and the bulk of reactions occur at elevated temperatures and in the gas phase producing higher MgF2 concentrations. Under high heating rate conditions, MgF2 formation is favored over MgO formation for both particle sizes owing to the high reaction temperatures that promote gas phase reactions favoring MgF2 formation. Theoretical analysis using density functional theory (DFT) through cluster models for Mg(OH)2 and MgO surfaces further show that the Mg(OH)2 surface is more reactive with fluorine species than MgO, especially at elevated temperatures. The DFT results help explain the high heating rate reaction pathway that favors fluorination reactions independent of Mg particle size.

Pt supported on boron, nitrogen co-doped carbon nanotubes (BNC NTs) for effective methanol electrooxidation

The research for electrocatalyst with high electroactivity and great CO-resistance ability for direct methanol fuel cells (DMFCs) is still a huge challenge. In this report, we develop Boron, Nitrogen co-doped carbon nanotubes (BNC NTs) as a support for Pt. Owing to the doping of boron, the catalyst not only provides extremely active sites for methanol oxidation reactions (MOR) but also protects Pt nanoparticles from agglutinating, performing superior electroactivity and excellent ability to anti CO poisoning. The X-ray photoelectron spectroscopy (XPS) results demonstrate the strong electron effect between Pt and B. Notably, the Pt/BNC NTs catalyst exhibits higher catalytic activity towards MOR and more superior durability in comparison with Pt/NC NTs and commercial JM Pt/C catalyst. The accelerated durability test (ADT) illustrates that Pt/BNC NTs catalyst can improve the issue of electrochemical surface area (ECSA) conservation, with only 30% diminish in comparison with the initial ECSA after 5000 cycles. The experiment result demonstrate that boron doping is the key step to improve the catalytic activities and CO-resistance ability due to the combination effects, involving firm B–C and N–C bonds, the stronger electron transfer in the nanotube structure among Pt, B and N, the stronger adsorption intensity of oxygen species from doped B.

Promoting Plasma Photocatalytic Oxidation of Toluene Via the Construction of Porous Ag–CeO2/TiO2 Photocatalyst with Highly Active Ag/oxide Interface

Plasma-photocatalysis system (PPS) constructed by dielectric barrier discharge and TiO2 photocatalysts could realize toluene oxidation removal at low temperatures, but requires further enhancement in the performance due to the relatively poor activity of TiO2. We herein report that co-loading Ag and CeO2 on porous TiO2 can construct a active Ag–CeO2/TiO2 photocatalyst for the PPS, enabling the PPS to achieve promoted toluene conversion, CO2 selectivity and stability in toluene oxidation removal. The Ag–CeO2/TiO2 photocatalyst with porous microstructure and dispersed surface Ag particles can change the discharge mode, facilitating the interaction of plasma and photocatalysts in the PPS. We further disclose that the Ag/oxide interface can create highly active sites for surface oxidation reactions and enhances the separation of photoinduced electron-holes under the action of plasma. The intimate interaction between plasma and Ag–CeO2/TiO2 photocatalyst and enhanced surface/photocatalytic oxidation reactions on Ag/oxide interface result in the promoted performance of toluene oxidation. The creation of highly active metal/oxide interface on porous TiO2 photocatalyst may provide a strategy for the construction of high-efficiency PPS for VOCs removal.

Hydrazine Oxidation Reaction: Porous Carbon Membrane‐Supported Atomically Dispersed Pyrrole‐Type FeN4 as Active Sites for Electrochemical Hydrazine Oxidation Reaction (Small 31/2020)

Hydrazine Oxidation Reaction: Porous Carbon Membrane‐Supported Atomically Dispersed Pyrrole‐Type FeN₄ as Active Sites for Electrochemical Hydrazine Oxidation Reaction (Small 31/2020)

Porous Carbon Membrane-Supported Atomically Dispersed Pyrrole-Type FeN4 as Active Sites for Electrochemical Hydrazine Oxidation Reaction.

The rational design of catalytically active sites in porous materials is essential in electrocatalysis. Herein, atomically dispersed Fe-Nx sites supported by hierarchically porous carbon membranes are designed to electrocatalyze the hydrazine oxidation reaction (HzOR), one of the key techniques in electrochemical nitrogen transformation. The high intrinsic catalytic activity of the Fe-Nx single-atom catalyst together with the uniquely mixed micro-/macroporous membrane support positions such an electrode among the best-known heteroatom-based carbon anodes for hydrazine fuel cells. Combined with advanced characterization techniques, electrochemical probe experiments, and density functional theory calculation, the pyrrole-type FeN4 structure is identified as the real catalytic site in HzOR.

Enhanced Methanol Electro‐Oxidation Activity of Pt/rGO Electrocatalyst Promoted by NbC/Mo2C Phases

AbstractDirect methanol fuel cells are attractive power conversion devices for portable and stationary applications. Many electrocatalysts have been explored for fuel cell applications but there is scope to design newer and more efficient electrocatalytic materials. For this purpose we have synthesized Mo2C and bimetallic NbC‐Mo2C carbide phases by carbothermic reduction process. The carbide catalysts are characterized by various analytical techniques. The carbide samples are higly porous and their surface areas are of the order of 630 m2 g−1. Usually Pt is the state‐of‐the‐art anode catalyst in direct methanol fuel cells. Here, Nb‐Mo2C and 10 wt% Pt nanoparticles are deposited on reduced graphene oxide support to obtain Pt/Nb‐Mo2C‐rGO. This material shows highest mass activity of 836.4 mA mgPt−1 when compared to 10 wt% Pt/Mo2C‐rGO and bare 20 wt% Pt/rGO catalysts. Notably, electrochemical tests show that the 10 wt% Pt/Nb‐Mo2C‐rGO catalyst exhibits lower onset potential, large electrochemical surface area, higher activity, and stability for methanol electro‐oxidation in acidic solution compared to the Pt/Mo2C‐rGO and Pt/rGO catalysts. The activty enhancement is atributed to the defective nature of the interface between Pt, Nb‐Mo2C and rGO. It is found that Nb‐Mo2C not only alleviates CO poisoning of Pt nanoparticles, but also generates oxygen containing species at lower potentials. This process helps scavenging the intermediate species such as COads on Pt surfaces and regenerates the active Pt sites for routine alcohol oxidation reaction. The chronopotentiometry and chronoamperometry studies also show that Pt/Nb‐Mo2C‐rGO exhibits lower alcohol oxidation overpotential and longer polarization time/stability compared to Pt/Mo2C‐rGO and bare Pt/rGO catalysts.

Impact of methanol alternative fuel on oxidation reactivity of soot emissions from a modern CI engine

This paper compared the soot reactivity and its primarily related factors including soot structure, graphitization degree and surface functional groups, when using mineral diesel fuel (DF) and methanol blended fuel (MBF) at two typical low and high engine loads. Results showed that the soot reactivity increases with increasing methanol concentration in the MBF. Additionally, with the constant methanol blend ratio of methanol, the reactivity of MBF soot degrades as the engine load increases. Correspondingly, the MBF soots had lower aggregate compactness, smaller primary particle size, more disordered structure and more active aliphatic C-H groups in comparison with the DF soot, providing higher concentration and accessibility of active sites for oxidation reactions. The addition of methanol creates variations in combustion kinetics and fuel formulation, which exerts reverse effects on the soot structure and chemistry. Therefore, the factors related to these properties show non-monotonic variations when alter the methanol blend ratio and engine operating mode. Especially, when the methanol blend ratio increases from 10% to 15%, the soot structure and chemistry transform onto the features that are reluctant for soot oxidation.

Effect of aspect ratios of rutile TiO2 nanorods on overall photocatalytic water splitting performance.

The spatial separation of reduction and oxidation reaction sites on the different facets of a semiconductor is an ideal and promising route for overall photocatalytic water splitting due to efficient charge carrier separation. Rutile TiO2 has separate oxidation and reduction crystal facets and can be used to achieve direct splitting of pure water under ultraviolet (UV) light irradiation. In order to improve the rate of water oxidation reaction, the ratio of different crystal facets of rutile should be regulated controllably. However, the preparation of rutile TiO2 architecture has been limited by the availability of synthetic techniques. In this study, rutile TiO2 nanorods with various aspect ratios were accurately prepared in the presence of Cl- anions and H+ cations, which were found to play a crucial role in forming the morphology of rutile TiO2 nanorods. In addition, the mechanism involving the growth of rutile TiO2 nanorods with different aspect ratios is proposed. Rutile TiO2 nanorods with a high proportion of oxidative (111) facets provided higher overall water splitting reactivity.

Promotion of photocatalytic steam reforming of methane over Ag0/Ag+-SrTiO3

Methane (CH4) is not only used as a fuel but also as a promising clean energy source for hydrogen generation. The steam reforming of CH4 (SRM) using photocatalysts can realize the production of syngas (CO + H2) with low energy consumption. In this work, Ag0/Ag+-loaded SrTiO3 nanocomposites were successfully prepared through a photodeposition method. When the loading amount of Ag is 0.5 mol%, the atom ratio of Ag+ to Ag0 was found to be 51:49. In this case, a synergistic effect of Ag0 and Ag+ was observed, in which Ag0 was proposed to improve the adsorption of H2O to produce hydroxyl radicals and enhance the utilization of light energy as well as the separation of charge carriers. Meanwhile, Ag0 was regarded as the reduction reaction site with the function of an electron trapping agent. In addition, Ag+ adsorbed the CH4 molecules and acted as the oxidation reaction sites in the process of photocatalytic SRM to further promote electron-hole separation. As a result, 0.5 mol% Ag-SrTiO3 exhibited enhancement of photocatalytic activity for SRM with the highest CO production rate of 4.3 μmol g−1 h−1, which is ca. 5 times higher than that of pure SrTiO3. This work provides a facile route to fabricate nanocomposite with cocatalyst featuring different functions in promoting photocatalytic activity for SRM.

Facile growth of AgVO3 nanoparticles on Mo-doped BiVO4 film for enhanced photoelectrochemical water oxidation

BiVO4 is largely restricted by its intrinsic poor electron mobility and sluggish surface reaction. Aiming at eliminating the deficiency of BiVO4, we assemble a novel composite photoanode through simultaneous bulk and surface modification. In detail, controllable Mo doping is manipulated through a spin-coating method, and AgVO3 nanoparticles are loaded by successive ionic layer adsorption and reaction (SILAR) process. The as-prepared photoanode boosts the photocurrent density by 7 folds at 1.23 V vs. RHE and moves the onset potential negatively by 387 mV compared to the bare BiVO4. Furthermore, the charge injection efficiency is substantially boosted to 49% at 1.23 VRHE, nearly 5 times as that of pure BiVO4. With detailed physical and electrochemical characterization, we reveal the essential reason for the boosted photoelectrochemical activity. Appropriate Mo doping can simultaneously enhance the carrier density and electron mobility of BiVO4. Specifically, more (0 4 0) facet are exposed for retarding charge recombination and increasing photocatalytic reaction sites. Then the simple SILAR process grows AgVO3 nanoparticles homogeneously with uniform diameter of 3–5 nm on BiVO4, forming p-n heterojunction to accelerate charge separation, and abundant oxygen vacancy emerged on the surface, which has been demonstrated as active sites for water oxidation reaction.

Photocatalytic Water Oxidation Using Wide-Gap Semiconductor Modified with First-Low Transition-Metal Oxides Nanoparticles Under Visible Light

Water splitting using a heterogeneous photocatalyst has recently attracted attention to get hydrogen from water and sunlight energy. TiO2 is known as one of the best photocatalysts for water splitting. However, metal oxide semiconductor photocatalysts generally have large band gap, so that they absorb only UV light. Recently, our group reported a new water oxidation photocatalyst that consists of TiO2 and nanoparticulate cobalt hydroxide (or oxide). In this system, electron transfer from cobalt nanoparticle to the conduction band of TiO2 is believed to proceed, enabling to absorb wide range visible light up to 850 nm for water oxidation. In this work, we employed other first-low transition-metal oxides (MO x , M = Mn, Fe, Ni, Cu) for the surface modifiers alternative to CoO x . Since the electronic interaction between cobalt on surface and titanium in support should be important in the CoO x /TiO2 system, it is expected that similar effects may occur on other first-row transition-metal oxides and work as water oxidation photocatalyst. As a result, it was revealed that MO x /TiO2 samples absorb a wide range of visible light. However, all of the tested MO x /TiO2 except NiO x /TiO2 did not show photocatalytic activity for water oxidation. Furthermore, the activity of NiO x /TiO2 was much lower than that of CoO x /TiO2. To clarify the reason of photocatalytic inactiveness despite of their visible light absorption capability, we investigated the catalytic activities of surface MO x nanoparticles for water oxidation under the conditions where the contribution of light absorption by MO x /TiO2 to the reaction could be neglected. It was confirmed that the CoO x /TiO2 performed the best catalytic activity among the MO x /TiO2 samples. This result strongly suggests that not only the visible light absorption capability but also the intrinsic catalytic activity of MO x for water oxidation is important to obtaining high photocatalytic activity in MO x /TiO2 system. Further investigation for relationship between surface CoO x nanoparticles and a support metal oxide semiconductor was conducted using SrTiO3 that possessed different particle sizes. As the conduction band minimum of SrTiO3 is more negative than that of TiO2, one may expect higher reduction ability of the injected electrons when sensitized by CoO x nanoparticles. The size of the loaded CoO x depended on the SrTiO3 support size, even though the CoO x modification was conducted under the same conditions. CoO x particles easily underwent aggregation on larger size SrTiO3, while highly dispersed CoO x was confirmed on smaller SrTiO3 support. The size growth of CoO x could increase the visible light absorption capability, which lead to higher photocatalytic activity. However, CoO x agglomerates could diminish the active sites for water oxidation reaction, which contributed to lower photocatalytic activity. Therefore, this result suggests that both the morphology of CoO x and the visible light absorption capability could be controlled by an appropriate choice of support material so as to maximize the photocatalytic activity. Figure 1

Front Cover: Electrochemical Formation of Fe(IV)=O Derived from H2O2 on a Hematite Electrode as an Active Catalytic Site for Selective Hydrocarbon Oxidation Reactions (ChemPhysChem 5/2019)

Front Cover: Electrochemical Formation of Fe(IV)=O Derived from H₂O₂ on a Hematite Electrode as an Active Catalytic Site for Selective Hydrocarbon Oxidation Reactions (ChemPhysChem 5/2019)

Electrochemical Formation of Fe(IV)=O Derived from H2O2 on a Hematite Electrode as an Active Catalytic Site for Selective Hydrocarbon Oxidation Reactions

AbstractThe front cover artwork is provided by the group of Prof. Shuji Nakanishi. The image shows that Fe(lV)=O is generated from H2O2 on a hematite electrode and serve as the active center for selective styrene epoxidation. Read the full text of the Communication at 10.1002/cphc.201801207.

Hollow Pt skim-sandwiched Cu spheres supported on reduced graphene oxide-carbon nanotube architecture for efficient methanol electrooxidation

The disadvantages of high cost, easy poisoning and insufficient durability hinder platinum (Pt) application in direct methanol fuel cells. In this study, a hybrid of reduced graphene oxide/carbon nanotubes-supported hollow copper spheres (Cu/rGO@CNTs) is prepared by a one-step electrodeposition method. Then, the internal and external surfaces of hollow Cu spheres are coated with Pt skims to obtain a hollow bimetallic electrocatalyst (Pt/Cu/rGO@CNTs) through a simple galvanic replacement reaction by immersing Cu/rGO@CNTs in a chloroplatinic acid (H2PtCl6) solution. The three dimensional rGO@CNTs network structure benefit mass transport and electron transfer. Pt skims expose abundant active sites for electrocatalytic methanol oxidation reactions (MORs). Cu cores synergize Pt skims to enhance anti-poison ability. As a result, Pt/Cu/rGO@CNTs shows an excellent electrocatalytic activity for MORs with a robust tolerance of catalyst poisoning.

Electrochemical Formation of Fe(IV)=O Derived from H2 O2 on a Hematite Electrode as an Active Catalytic Site for Selective Hydrocarbon Oxidation Reactions.

The high-valence iron species (Fe(IV)=O) in the cytochrome P450 enzyme superfamily is generated via the activation of O2 , and serves as the active center of selective hydrocarbon oxidation reactions. Furthermore, P450 can employ an alternate route to produce Fe(IV)=O, even from H2 O2 without O2 activation. Meanwhile, Fe(IV)=O has recently been revealed to be the reactive intermediate during H2 O oxidation to O2 on hematite electrodes. Herein, we demonstrated the generation of Fe(IV)=O on hematite electrodes during the electrochemical oxidative decomposition of H2 O2 using in situ UV-visible absorption spectra. The generation of Fe(IV)=O on hematite electrodes from H2 O2 exhibited 100 mV lower overpotential than that from H2 O. This is because H2 O2 serves not only as the oxygen source of Fe(IV)=O, but also as the additional oxidant. Finally, we confirmed that the Fe(IV)=O generated on hematite electrodes can serve as the catalytic site for styrene epoxidation reactions.

Identification of the Au/ZnO interface as the specific active site for the selective oxidation of the secondary alcohol group in glycerol

In this work, a catalyst containing an Au/ZnO interface was synthesized using controlled thermal treatment to generate a strong metal–support interaction between Au and a thin amorphous ZnO layer, and was used for the aqueous oxidation of glycerol under base-free conditions. We provide evidence that the Au/ZnO interface is the specific active site for selective catalytic oxidation of the secondary alcohol group in glycerol, without affecting the primary alcohol groups, thus affording dihydroxyacetone with high selectivity. Characterization of the active catalyst suggests that ZnO species migrate to the surface of Au particles during thermal pretreatment in air. An Au–O–Zn interface is then formed, along with an abundance of oxygen vacancies. The resulting interfacial sites can specifically adsorb and activate the secondary alcohol group. The study provides more general insight into the role of Au-containing interfaces as the specific active sites in selective oxidation reactions.

Identification of Active Sites in Oxidation Reaction from Real-Time Probing of Adsorbate Motion over Pd Nanoparticles.

Obtaining insight into the type of surface sites involved in a reaction is essential to understand catalytic mechanisms at the atomic level and a key for understanding selectivity in surface-catalyzed reactions. Here we use ultrafast broad-band vibrational spectroscopy to follow in real-time diffusion of CO molecules over a palladium nanoparticle surface toward an active site. Site-to-site hopping is triggered by laser excitation of electrons and followed in real-time from subpicosecond changes in the vibrational spectra. CO photoexcitation occurs in 400 fs and hopping from NP facets to edges follows within ∼1 ps. Kinetic modeling allows to quantify the contribution of different facet sites to the catalytic reaction. These results provide useful insights for understanding the mechanism of chemical reactions catalyzed by metal NPs.