Mechanism of reduction of nitrogen oxides with propene in excess oxygen catalyzed by bifunctional catalysts

HC-SCR (selective reduction of nitrogen oxide by hydrocarbon) and our view on its reaction mechanism are briefly described, and then our attempts in the last few years to design catalysts based on the concept of bifunctional catalysis for HC-SCR are reviewed. The example chosen is the NO-C₃H₆-O₂-H₂O reaction catalyzed by mechanical mixtures of transition metal oxides (e.g., Mn₂O₃) and metal-loaded ZSM-5 zeolites. The synergistic effect of two components was explained based on a reaction mechanism comprising NO oxidation, oxidative decomposition of the products of NO₂-C₃H₆ reactions and polymerization of (or carbon deposition from) C₃H₆.     

Enhancement of Activity and Development of Low Pt Content Electrocatalysts for Oxygen Reduction Reaction in Acid Media

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O₂-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H₂O₂-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO₂, WO₃, Ta₂O₅, Nb₂O₅, and ZrO₂), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt–Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal–support interactions and interfacial structural changes affecting adsorption and activation of O₂-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional.     

Gas-phase propene epoxidation over coinage metal catalysts

Propene oxide (PO) is a very important bulk chemical and is produced on a scale of about 7.5 million tons per year. In industry, PO is produced via multiple reaction steps in the liquid phase, using hazardous chlorine or costly organic hydroperoxides as oxidants. Accordingly, development of a simple and green process to produce PO has been desired. This paper presents an overview of one-step propene epoxidation in the gas phase over coinage metal catalysts with a mixture of O₂ and H₂ or with molecular O₂ alone as oxidant. Silver (Ag) and gold (Au) catalysts can catalyze propene epoxidation with a mixture of O₂ and H₂, with high selectivity, whereas copper (Cu) catalysts cannot. In this reaction Au catalysts are much more active than Ag catalysts. All the coinage metals can catalyze propene epoxidation by molecular O₂, but with selectivity usually below 60%. The valence states of Cu species and the sizes of Ag particles and Au particles are of crucial importance in PO synthesis.     

镍基催化剂上稻草水蒸气重整制富氢合成气

采用浸渍法制备了SiO₂, γ-Al₂O₃, CaO和TiO₂负载的Ni催化剂, 以及不同MgO含量的MgO-7.5%Ni/γ-Al₂O₃催化剂,利用X射线衍射和N₂吸附-脱附技术表征了催化剂的结构,在固定床反应器上评价了它们在稻草水蒸气催化重整制合成气反应中的催化性能,考察了反应条件对催化剂性能的影响.结果表明, 以γ-Al₂O₃为载体时Ni催化剂活性最高,其中7.5%Ni/γ-Al₂O₃催化剂的H₂收率可达1071.3ml/g,H₂:CO的体积比为1.4:1;同时,MgO的添加进一步提高了该催化剂的性能,当MgO含量为1.0%时,H₂收率可达1194.6ml/g,H₂:CO体积比可达3.9:1.可见MgO的加入促进了Ni基催化剂上稻草水蒸气催化重整制合成气反应的进行,同时使得合成气中CO发生水-汽转换反应,从而大大提高了合成气中H₂含量.     

Synthesis and catalytic activity of binuclear Mn(III,III)–BINOL complexes for epoxidation of olefins

The novel binuclear complexes [Mn₂^{(III, III)}(BINOL)₃L₂]2H₂O, where, L = 2, 2′‐bipyridine (Bpy) or 1,10‐phenanthroline (Phen) and BINOL = 1, 1′‐bi‐2‐naphthol were synthesized and characterized by elemental analyses, magnetic susceptibility and various spectral methods. The catalytic activity of these complexes was studied for the epoxidation reaction of unfunctionalized olefins like styrene, 1‐hexene, 1‐octene and 1‐decene. The products thus obtained were analyzed by GC. The epoxidation reactions were carried out, in the presence of catalyst with different oxidants, to study the effect of the nature of the oxidant on the reactions. The different oxidants used were the peroxide oxygen donor (e.g. TBHP and H₂O₂), mono oxygen donor (e.g. PhIO) and dioxygen donor (e.g. molecular O₂). TBHP was found to be the best oxidant for the epoxidation reaction. To study the effect of the solvent on the epoxidation, the reactions were carried out in different media, such as a polar media (e.g. with CH₃OH as solvent), non‐polar media (e.g. with CH₂Cl₂ and C₆H₆ as solvents) and coordinating solvent (e.g. CH₃CN). The maximum epoxide formation was observed in CH₂Cl₂ medium. The epoxidation reactions with optically active BINOL catalysts under optimum established conditions were carried out to examine the enantioselectivity of the catalysts. The complexes were, however, found not to be enantioselective. Copyright © 2006 John Wiley & Sons, Ltd.     

Platinum-Supported Zirconia Nanotube Arrays Supported on Graphene Aerogels Modified with Metal–Organic Frameworks: Adsorption and Oxidation of Formaldehyde at Room Temperature

Precious-metal catalysts (e.g., Au, Rh, Ag, Ru, Pt, and Pd) supported on transition-metal oxides (e.g., Al₂O₃, Fe₂O₃, CeO₂, ZrO₂, Co₃O₄, MnO₂, TiO₂, and NiO) can effectively oxidize volatile organic compounds. In this study, porous platinum-supported zirconia materials have been prepared by a “surface-casting” method. The synthesized catalysts present an ordered nanotube structure and exhibited excellent performance toward the catalytic oxidation of formaldehyde. A facile method, utilizing a boiling water bath, was used to fabricate graphene aerogel (GA), and the macroscopic 3D Pt/ZrO₂-GA was modified by introducing an adjustable MOF coating by a surface step-by-step method. The unblocked mesoporous structure of the graphene aerogel facilitates the ingress and egress of reactants and product molecules. The selected 7 wt.% Pt/ZrO₂-GA-MOF-5 composite demonstrated excellent performance for HCHO adsorption. Additionally, this catalyst achieved around 90 % conversion when subjected to a reaction temperature of 70 °C (T_{90 %}=70 °C). The Pt/ZrO₂-GA-MOF-5 composite induces a catalytic cycle, increasing the conversion by simultaneously adsorbing and oxidizing HCHO. This work provides a simple approach to increasing reactant concentration on the catalyst to increase the rate of reaction.     

A Simple Electrochemical Approach Based on Inexpensive Wall‐Jet Screen‐Printed Ring Disk Electrode to Evaluate Oxygen Reduction Catalysts

A simple electrochemical approach to evaluate oxygen reduction catalysts using an inexpensive screen‐printed ring disk carbon electrode system, consisting of a ring electrode deposited with MnO₂ and a disk electrode modified with the catalysts for study, is developed in this study. The as‐prepared MnO₂ is selective and sensitive for H₂O₂ oxidation in the presence of O₂ and is crucial to the proposed approach. By coupling with a wall‐jet electrochemical cell, the product generated from the reduction reaction at the disk electrode can effectively be monitored at the MnO₂‐deposited ring electrode. Model catalysts of nano‐Au and nano‐Pd representing 2e^? reduction of O₂ to H₂O₂ and 4e^? reduction to H₂O, respectively, were evaluated as electrode materials in oxygen reduction reaction to demonstrate the applicability of the proposed method.     

基于 H2/O2 等离子体和钛硅沸石的丙烯气相环氧化方法

 将 H₂/O₂ 非平衡等离子体现场产生的气态 H₂O₂和丙烯与耦合反应器中钛硅沸石 TS-1 直接接触, 实现了丙烯气相环氧化反应. 结果表明, 非平衡等离子体生成气态 H₂O₂ 的速率由介质阻挡放电的输入功率决定, 环氧丙烷的生成速率和选择性取决于钛硅沸石催化剂和反应条件. 在 H₂ 和 O₂ 进料流量分别为 170 和 8 ml/min, 介质阻挡放电输入功率为 3.5 W, 环氧化反应温度为 110 ^oC, 丙烯进料量为 18 ml/min, 催化剂用量为 0.8 g 的条件下, 生成环氧丙烷产率达 246.9 g/(kg•h)、环氧丙烷选择性和 H₂O₂ 有效利用率分别为 95.4% 和 36.1%, 反应 36 h 内未见催化剂失活.     

Sunlight‐Driven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal‐Free Photocatalysts

Design of green, safe, and sustainable process for the synthesis of hydrogen peroxide (H₂O₂) is a very important subject. Early reported processes, however, require hydrogen (H₂) and palladium‐based catalysts. Herein we propose a photocatalytic process for H₂O₂ synthesis driven by metal‐free catalysts with earth‐abundant water and molecular oxygen (O₂) as resources under sunlight irradiation (λ>400 nm). We use graphitic carbon nitride (g‐C₃N₄) containing electron‐deficient aromatic diimide units as catalysts. Incorporating the diimide units positively shifts the valence‐band potential of the catalysts, while maintaining sufficient conduction‐band potential for O₂ reduction. Visible light irradiation of the catalysts in pure water with O₂ successfully produces H₂O₂ by oxidation of water by the photoformed valence‐band holes and selective two‐electron reduction of O₂ by the conduction band electrons.     

Water-soluble polymer anchored peroxotitanates as environmentally clean and recyclable catalysts for mild and selective oxidation of sulfides with H2O2 in water

Anchoring of peroxotitanium (pTi) species to linear water-soluble acrylic acid based polymers, poly(sodium acrylate) (PA) and poly(sodium methacrylate) (PMA) led to the successful synthesis of a pair of new, water-tolerant and recyclable catalysts of the type [Ti₂(O₂)₂O₂(OH)₂]^4-—L (L = PA or PMA), highly effective in chemoselective sulfoxidation of organic sulfides with 30% H₂O₂ in aqueous medium at ambient temperature. The catalytic protocol is high yielding (TOF up to 11,280 h^?1), operationally simple as well as environmentally clean and safe, being free from halide, or any other toxic auxiliaries. The catalysts are sufficiently stable to afford easy recyclability for at least 10 consecutive reaction cycles of sulfoxidation with consistent activity selectivity profile. Oxidation of dibenzothiophene (DBT) to respective high purity sulfoxide or sulfone could also be accomplished using the same catalysts by variation of reaction conditions.     

Development of high performance Cu/ZnO-based catalysts for methanol synthesis and the water-gas shift reaction

Our groups studies on Cu/ZnO-based catalysts for methanol synthesis via hydrogenation of CO₂ and for the water-gas shift reaction are reviewed. Effects of ZnO contained in supported Cu-based catalysts on their activities for several reactions were investigated. The addition of ZnO to Cu-based catalyst supported on Al₂O₃, ZrO₂ or SiO₂ improved its specific activity for methanol synthesis and the reverse water-gas shift reaction, but did not improve its specific activity for methanol steam reforming and the water-gas shift reaction. Methanol synthesis from CO₂ and H₂ over Cu/ZnO-based catalysts was extensively studied under a joint research project between National Institute for Resources and Environment (NIRE; one of the former research institutes reorganized to AIST) and Research Institute of Innovative Technology for the Earth (RITE). It was suggested that methanol should be produced via the hydrogenation of CO₂, but not via the hydrogenation of CO, and that H₂O produced along with methanol should greatly suppress methanol synthesis. The Cu/ZnO-based multicomponent catalysts such as Cu/ZnO/ZrO₂/Al₂O₃ and Cu/ZnO/ZrO₂/Al₂O₃/Ga₂O₃ were highly active for methanol synthesis from CO₂ and H₂. The addition of a small amount of colloidal silica to the multicomponent catalysts greatly improved their long-term stability during methanol synthesis from CO₂ and H₂. The purity of the crude methanol produced in a bench plant was 99.9 wt% and higher than that of the crude methanol from a commercial methanol synthesis from syngas. The water-gas shift reaction over Cu/ZnO-based catalysts was also studied. The activity of Cu/ZnO/ZrO₂/Al₂O₃ catalyst for the water-gas shift reaction at 523 K was less affected by the pre-treatments such as calcination and treatment in H₂ at high temperatures than that of the Cu/ZnO/Al₂O₃ catalyst. Accordingly, the Cu/ZnO/ZrO₂/Al₂O₃ catalyst was considered to be more suitable for practical use for the water-gas shift reaction. The Cu/ZnO/ZrO₂/Al₂O₃ catalyst was also highly active for the water-gas shift reaction at 673 K. Furthermore, a two-stage reaction system composed of the first reaction zone for the water-gas shift reaction at 673 K and the second reaction zone for the reaction at 523 K was found to be more efficient than a one-stage reaction system. The addition of a small amount of colloidal silica to a Cu/ZnO-based catalyst greatly improved its long-term stability in the water-gas shift reaction in a similar manner as in methanol synthesis from CO₂ and H₂.     

Recent Advances in Preferential Oxidation of CO in H2 Over Gold Catalysts

Recent studies have revealed that supported gold catalysts exhibit comparable or superior catalytic performance relative to platinum group metals, especially at low temperatures, in the preferential oxidation of CO under excess H₂ (CO-PROX). Complete conversion of CO with good selectivity of O₂ for CO₂ and highly stable catalyst performance in the presence of CO₂ and H₂O are considered to be essential for the successful development of CO-PROX catalysts for application in polymer electrolyte membrane fuel cells. The performance of supported gold catalysts in the CO-PROX reaction has been shown to be dependent on the characteristics of gold (size, oxidation state, and its interaction with other metal/oxides), nature of the support (size, composition, preparation method, presence of promoters, and doping with other metal ions), and reaction conditions (temperature and feed composition). Complete CO conversion has been achieved in the presence of certain gold catalysts below 100 °C. The unresolved issues in CO-PROX include the undesired oxidation of H₂, detrimental effects of CO₂ and/or H₂O, and long-term stability of the catalysts. To address these issues, the catalytic activity of gold supported on simple oxides such as TiO₂, CeO₂, Al₂O₃, and Fe₂O₃ has been improved dramatically by the addition of promoters, alteration of the gold-oxide support interface, and modification of the oxide supports. Recently, nanoporous gold has also been recognized to be promising for this reaction. This review highlights recent developments of unsupported and supported gold catalysts for the CO-PROX reaction.     

N,O-coupling towards the selectively electrochemical production of H2O2

Hydrogen peroxide (H₂O₂) synthesis generally involves the energy-intensive anthraquinone process. Alternatively, electrochemical synthesis provides a green, economical, and environmentally friendly route to prepare H₂O₂ via the two-electron oxygen reduction reaction, but this process requires efficient catalysts with high activity and selectivity simultaneously. Here, we report an N, O co-doped carbon xerogel-based electrocatalyst (NO-CX) prepared by a simple and economical method. The NO-CX catalyst exhibits a high H₂O₂ selectivity over 90% in a potential range of 0.2–0.6 V and a high H₂O₂ production rate of 1410 mmol g_cat^?1 h^?1. The density functional theory calculations demonstrate that the coupling effect between N and O can effectively induce the redistribution of surface charge and the edge carbon atom adjacent to an ether group and a graphite nitrogen atom is the active site. This work provides a straightforward and low-cost process to produce highly selective H₂O₂ catalysts, which is in place for the expansion of electrocatalytic synthesis of useful chemicals.     

Hydroisomerization of n-heptane over bimetal-bearing H3PW12O40 catalysts supported on dealuminated USY zeolite

The bimetal-bearing (CePt or LaPt) 12-tungstophosphoric acid (H₃PW₁₂O₄₀ (PW)) catalysts supported on dealuminated USY zeolite (DUSY) were prepared by impregnation and characterized by XRD, BET, IR, and H₂-chemisorption. Their catalytic activities were tested in the hydroisomerization of n-heptane with a continuous atmospheric fixed-bed reactor. After the steam treatment combined with the acid leaching, as well as the supporting with PW and the bimetals, the DUSY support retains the Y zeolite porosity and the PW well keeps its Keggin structure in catalysts. The doping of Ce into the catalysts enhances the dispersion of Pt on the catalyst surface. The Pt-bearing PW catalysts doped with Ce or La, especially Ce, exhibit much higher catalytic activity and selectivity than the catalysts without dopants at lowered reaction temperatures. At the optimal reaction conditions, i.e., the reaction temperature of 250°C and WHSV of 1.4 h^?1, the catalyst with a Pt loading of 0.4%, PW loading of 10% and a molar ratio of Ce to Pt of 15:1 shows a conversion of n-heptane of 70.3% with a high selectivity for isomerization products of 94.1%.     

Homogeneous oxidation kinetics of aqueous ferrous iron at circumneutral pH

Oxidation of aqueous Fe(II) was investigated at circumneutral pH and 23°C in the absence of ligands (other than H₂O, OH^–, and Cl^–) and catalysts (e.g., microbes or solids surfaces). Enzymes (superoxide dismutase and catalase) were used as specific catalytic probes to determine whether superoxide and hydrogen peroxide are intermediates in oxygen reduction by Fe(II). The kinetic evidence suggests that Fe(II) and D.O. react in a termolecular transition state complex, the reaction produces hydrogen peroxide (probably without intermediation by superoxide), and Fe(II) and H₂O₂ react in a termolecular reaction or in a two-step sequence of bimolecular reactions. The rate data permit modeling the overall Fe(II) oxidation reaction at pH7.0 with a rate law that has non-integer orders with respect to [Fe(II)] and [OH^–].     

Ammonia synthesis by means of plasma over MgO catalyst

Ammonia synthesis from H₂-N₂ mixed gas was studied at room temperature in a glow-discharge plasma in the presence of metals or metal oxides. Magnesia (MgO) and calcia (CaO), which are oxides with solid basicity, revealed catalytic activity in the plasma synthesis of ammonia, although they are catalytically inactive in industrial ammonia synthesis. The acidic oxides (Al₂O₃, WO₃, and SiO₂-Al₂O₃) lead to the consumption of the reactant, i.e., the H₂-N₂ mixed gas. No ammonia was isolated. Metal catalysts showed higher activity than the above basic oxides. They have, however, different activities. The reaction was faster over the active materials than over sodium chloride (NaCl) or glass wool or in a blank reactor without any catalyst.     

Thiophene-Containing Covalent Organic Frameworks for Overall Photocatalytic H2O2 Synthesis in Water and Seawater

H₂O₂ is a significant chemical widely utilized in the environmental and industrial fields, with growing global demand. Without sacrificial agents, simultaneous photocatalyzed H₂O₂ synthesis through the oxygen reduction reaction (ORR) and water oxidation reaction (WOR) dual channels from seawater is green and sustainable but still challenging. Herein, two novel thiophene-containing covalent organic frameworks (TD-COF and TT-COF) were first constructed and served as catalysts for H₂O₂ synthesis via indirect 2e⁻ ORR and direct 2e⁻ WOR channels. The photocatalytic H₂O₂ production performance can be regulated by adjusting the N-heterocycle modules (pyridine and triazine) in COFs. Notably, with no sacrificial agents, just using air and water as raw materials, TD-COF exhibited high H₂O₂ production yields of 4060 μmol h⁻¹ g⁻¹ and 3364 μmol h⁻¹ g⁻¹ in deionized water and natural seawater, respectively. Further computational mechanism studies revealed that the thiophene was the primary photoreduction unit for ORR, while the benzene ring (linked to the thiophene by the imine bond) was the central photooxidation unit for WOR. The current work exploits thiophene-containing COFs for overall photocatalytic H₂O₂ synthesis via ORR and WOR dual channels and provides fresh insight into creating innovative catalysts for photocatalyzing H₂O₂ synthesis.     

Pt掺杂氧化石墨烯在酸性介质中催化氧还原反应DFT研究

目前Pt基催化剂被公认为是最高效的氧还原催化剂.我们采用了密度泛函理论研究了Pt掺杂5种不同氧化石墨烯和完美石墨烯在酸性环境中的氧还原反应机理,计算了氧还原反应中间体O₂、O、OOH、OH、H₂O和H₂O₂在不同掺杂石墨烯上的吸附性能、反应步骤与反应相对能量变化.结果表明,氧化石墨烯在O₂的活化、中间体吸附、掺杂难度（缺陷形成能）、能带带隙以及在反应中相对能量的降低都优于完美石墨烯,我们的工作将有助于为将来在实验中选择和合成氧还原催化剂提供一定的理论指导意义.     

Acetylation of Alcohols Catalyzed by Dodeca-tungsto(molybdo)phosphoric acid

Summary. Acetylation of primary, secondary, and tertiary alcohols was carried out in some refluxing alkyl acetates and in two carboxylic acids with the participation of catalytic amounts of H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, and H₁₄P₅W₃₀O₁₁₀ with good yields and high stereo(regio)specificity under mild reaction condition. H₃PW₁₂O₄₀ and H₃PMo₁₂O₄₀ have also shown excellent reactivity in the formylation of 1-butanol with ethyl formate at room temperature and in short reaction times. Heteropolyacid catalysts could be separated after a simple work up and reused for several times.     

Cyclodextrin-supported Co(OH)2 Clusters as Electrocatalysts for Efficient and Selective H2O2 Synthesis

Co-based material catalysts have shown attractive application prospects in the 2 e⁻ oxygen reduction reaction (ORR). However, for the industrial synthesis of H₂O₂, there is still lack of Co-based catalysts with high production yield rate. Here, novel cyclodextrin-supported Co(OH)₂ cluster catalysts were prepared via a mild and facile method. The catalyst exhibited remarkable H₂O₂ selectivity (94.2 % ~ 98.2 %), good stability (99 % activity retention after 35 h), and ultra-high H₂O₂ production yield rate (5.58 mol g_catalyst⁻¹ h⁻¹ in the H-type electrolytic cell), demonstrating its promising industrial application potential. Density functional theory (DFT) reveals that the cyclodextrin-mediated Co(OH)₂ electronic structure optimizes the adsorption of OOH* intermediates and significantly enhances the activation energy barrier for dissociation, leading to the high reactivity and selectivity for the 2 e⁻ ORR. This work offers a valuable and practical strategy to design Co-based electrocatalysts for H₂O₂ production.