Pt/Mg-Ce-O catalyst for NO/H<Subscript>2</Subscript>/O<Subscript>2</Subscript> lean de-No<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> reaction

The NO/H₂/O₂ reaction was studied under oxidizing conditions in the 100-400 °C range over 0.1 wt% Pt supported on various metal oxides such as MgO, CeO₂, SiO₂, La₂O₃, CaO, Y₂O₃ and TiO₂. The Pt/MgO and Pt/CeO₂ catalysts showed good catalytic behaviours. Here, we find that the Pt/Mg-Ce-O catalyst, prepared from MgO and CeO₂ by the sol-gel method, is a very active and selective catalyst towards N₂ formation in the whole 100–400 °C range. This catalyst appears to be the most active, selective and stable one ever reported in the literature for the NO/H₂/O₂ reaction, even in the presence of 5%v H₂O or 20 ppmv of SO₂ in the feed stream.Selected article from the Regional Symposium on Chemistry and Environment, Krusevac, Serbia, June 2003, organised by Dr. Branimir Jovancicevic.     

The effect of preparation conditions of Pt/Al2O3 on its catalytic performance for the H2-SCR in the presence of oxygen

Selective catalytic reduction of NO _x by H₂ in the presence of oxygen has been investigated over Pt/ Al₂O₃ catalysts pre-treated under different conditions. Catalyst preparation conditions exert significant influence on the catalytic performance, and the catalyst pre-treated by H₂ or H₂ then followed by O₂ is much more active than that pre-treated by air. The higher surface area and the presence of metallic Pt over Pt/Al₂O₃ pre-treated by H₂ or pretreated by H₂ then followed by O₂ can contribute to the formation of NO₂, which then promotes the reaction to proceed at low temperatures.     

Characterization and performance of V2O5/CeO2 for NH3-SCR of NO at low temperatures

A series of CeO₂ supported V₂O₅ catalysts with various loadings were prepared with different calcination temperatures by the incipient impregnation. The catalysts were evaluated for low temperature selective catalytic reduction (SCR) of NO with ammonia (NH₃). The effects of O₂ and SO₂ on catalytic activity were also studied. The catalysts were characterized by specific surface areas (S_BET) and X-ray diffraction (XRD) methods. The experimental results showed that NO conversion changed significantly with the different V₂O₅ loading and calcination temperature. With the V₂O₅ loading increasing from 0 to 10 wt%, NO conversion increased significantly, but decreased at higher loading. The optimum calcination temperature was 400°C. The best catalyst yielded above 80% NO conversion in the reaction temperature range of 160°C–300°C. The formation of CeVO₄ on the surface of catalysts caused the decrease of redox ability.     

Cytotoxicity and cell membrane depolarization induced by aluminum oxide nanoparticles in human lung epithelial cells A549

The cytotoxicity of 13 and 22 nm aluminum oxide (Al₂O₃) nanoparticles was investigated in cultured human bronchoalveolar carcinoma-derived cells (A549) and compared with 20 nm CeO₂ and 40 nm TiO₂ nanoparticles as positive and negative control, respectively. Exposure to both Al₂O₃ nanoparticles for 24 h at 10 and 25 µg mL^?1 doses significantly decreased cell viability compared with control. However, the cytotoxicity of 13 and 22 nm Al₂O₃ nanoparticles had no difference at 5–25 µg mL^?1 dose range. The cytotoxicity of both Al₂O₃ nanoparticles were higher than negative control TiO₂ nanoparticles but lower than positive control CeO₂ nanoparticles (TiO₂ < Al₂O₃ < CeO₂). A real-time single cell imaging system was employed to study the cell membrane potential change caused by Al₂O₃ and CeO₂ nanoparticles using a membrane potential sensitive fluorescent probe DiBAC₄(3). Exposure to the 13 nm Al₂O₃ nanoparticles resulted in more significant depolarization than the 30 nm Al₂O₃ particles. On the other hand, the 20 nm CeO₂ particles, the most toxic, caused less significant depolarization than both the 13 and 22 nm Al₂O₃. Factors such as exposure duration, surface chemistry, and other mechanisms may contribute differently between cytotoxicity and membrane depolarization.     

Catalytic activities and mechanism of formaldehyde oxidation over gold supported on MnO<Subscript>2</Subscript> microsphere catalysts at room temperature

MnO₂ microspheres with various surface structures were prepared using the hydrothermal method, and Au/MnO₂ catalysts were synthesized using the sol-gel method. We obtained three MnO₂ microspheres and Au/MnO₂ samples: coherent solid spheres covered with wire-like nanostructures, solid spheres with nanosheets, and hierarchical hollow microspheres with nanoplatelets and nanorods. We investigated the properties and catalytic activities of formaldehyde oxidation at room temperature. Crystalline structures of MnO₂ are the main factor affecting the catalytic activities of these samples, and γ-MnO₂ shows high catalytic performance. The excellent redox properties are responsible for the catalytic ability of γ-MnO₂. The gold-supported interaction can change the redox properties of catalysts and accelerate surface oxygen species transition, which can account for the catalytic activity enhancement of Au/MnO₂. We also studied intermediate species. The dioxymethylene (DOM) and formate species formed on the catalyst surface were considered intermediates, and were ultimately transformed into hydrocarbonate and carbonate and then decomposed into CO₂. A proposed mechanism of formaldehyde oxidation over Au/MnO₂ catalysts was also obtained.     

New insights into mercury removal mechanism on CeO<Subscript>2</Subscript>-based catalysts: A first-principles study

First-principles calculations were performed to investigate the mechanism of Hg⁰ adsorption and oxidation on CeO₂(111). Surface oxygen activated by the reduction of Ce⁴⁺ to Ce³⁺ was vital to Hg⁰ adsorption and oxidation processes. Hg0 was fully oxidized by the surface lattice oxygen on CeO₂(111), without using any other oxidizing agents. HCl could dissociate and react with the Hg adatom on CeO₂(111) to form adsorbed Hg–Cl or Cl–Hg–Cl groups, which promoted the desorption of oxidized Hg and prevented CeO₂ catalyst deactivation. In contrast, O–H and H–O–H groups formed during HCl adsorption consumed the active surface oxygen and prohibited Hg oxidation. The consumed surface oxygen was replenished by adding O₂ into the flue gas. We proposed that oxidized Hg desorption and maintenance of sufficient active surface oxygen were the rate-determining steps of Hg⁰ removal on CeO₂-based catalysts. We believe that our thorough understanding and new insights into the mechanism of the Hg⁰ removal process will help provide guidelines for developing novel CeO₂-based catalysts and enhance the Hg⁰ removal efficiency.

Open image in new window

     

Enhanced performances in catalytic oxidation of <Emphasis Type="Italic">o</Emphasis>-xylene over hierarchical macro-/mesoporous silica-supported palladium catalysts

A series of hierarchical macro-/mesoporous silica supports (MMSs) were successfully synthesized using dual-templating technique employing polystyrene (PS) spheres and the Pluronic P123 surfactant. Pd was next loaded on the hierarchical silica supports via colloids precipitation method. Physicochemical properties of the synthesized samples were characterized by various techniques and all catalysts were tested for the total oxidation of o-xylene. Among them, the Pd/MMS-b catalyst with tetraethoxysilane/polystyrene weight ratio of 1.0 exhibited superior catalytic activity, and under a higher gas hourly space velocity (GHSV) of 70000 h^–1, the 90% conversion of o-xylene has been obtained at around 200°C. The BET and SEM results indicated that Pd/MMSb catalyst possesses high surface area and large pore volume, and well-ordered, interconnected macropores and 2D hexagonally mesopores hybrid network. This novel ordered hierarchical porous structure was highly beneficial to the dispersion of active sites Pd nanoparticles with less aggregation, and facilitates diffusion of reactants and products. Furthermore, the Pd/MMS-b catalyst possessed good stability and durability.     

Synthesis, physicochemical characterizations and catalytic performance of Pd/carbon-zeolite and Pd/carbon-CeO2 nanocatalysts used for total oxidation of xylene at low temperatures

In this work, xylene removal from waste gas streams was investigated via catalytic oxidation over Pd/carbon-zeolite and Pd/carbon-CeO₂ nanocatalysts. Activated carbon was obtained from pine cone chemically activated using ZnCl₂ and modified by H₃PO₄. Natural zeolite of clinoptilolite was modified by acid treatment with HCl, while nano-ceria was synthesized via redox method. Mixed supports of carbon-zeolite and carbonceria were prepared and palladium was dispersed over them via impregnation method. The prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller surface area (BET), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric (TG) techniques. Characterization of nanocatalysts revealed a good morphology with an average particle size in a nano range, and confirmed the formation of nano-ceria with an average crystallite size below 60 nm. BET analysis indicated a considerable surface area for catalysts (～1000 m²·g^?1). FTIR patterns demonstrated that the surface groups of synthesized catalysts are in good agreement with the patterns of materials applied in catalyst synthesis. The performance of catalysts was assessed in a low-pressure catalytic oxidation pilot in the temperature range of 100° C-250°C. According to the reaction data, the synthesized catalysts have been shown to be so advantageous in the removal of volatile organic compounds (VOCs), representing high catalytic performance of 98% for the abatement of xylene at 250°C. Furthermore, a reaction network is proposed for catalytic oxidation of xylene over nanocatalysts.     

Investigation of fluorescence characterization and electrochemical behavior on the catalysts of nanosized Pt-Rh/γ-Al2O3 to oxidize gaseous ammonia

This work describes the environmentally friendly technology for oxidation of ammonia (NH₃) to form nitrogen at temperatures range from 423K to 673K by selective catalytic oxidation (SCO) over a nanosized Pt-Rh/γ-Al₂O₃ catalyst prepared by the incipient wetness impregnation method of hexachloroplatinic acid (H₂PtCl₆) and rhodium (III) nitrate (Rh(NO₃)₃) with γ-Al₂O₃ in a tubular fixed-bed flow quartz reactor (TFBR). The characterization of catalysts were thoroughly measured using transmission electron microscopy (TEM), threedimensional excitation-emission fluorescent matrix (EEFM) spectroscopy, UV-Vis absorption, dynamic lightscattering (DLS), zeta potential meter, and cyclic voltammetry (CV). The results demonstrated that at a temperature of 673K and an oxygen content of 4%, approximately 99% of the NH₃ was removed by catalytic oxidation over the nanosized Pt-Rh/γ-Al₂O₃ catalyst. N₂ was the main product in NH₃-SCO process. Further, it reveals that the oxidation of NH₃ was proceeds by the over-oxidation of NH₃ into NO, which was conversely reacted with the NH₃ to yield N₂. Therefore, the application of nanosized Pt-Rh/γ-Al₂O₃ catalyst can significantly enhance the catalytic activity toward NH₃ oxidation. One fluorescent peak for fresh catalyst was different with that of exhausted catalyst. It indicates that EEFM spectroscopy was proven to be an appropriate and effective method to characterize the Pt clusters in intrinsic emission from nanosized Pt-Rh/γ-Al₂O₃ catalyst. Results obtained from the CV may explain the significant catalytic activity of the catalysts.     

Determination of F2-isoprostanes in cultured human lung epithelial cells after exposure to metal oxide and silica nanoparticles by high-performance liquid chromatography/tandem mass spectrometry

The environmental impact of nanotechnology has caused a great concern. Many in vitro studies showed that many types of nanoparticles were cytotoxic. However, whether these nanoparticles caused cell membrane damage was not well studied. F₂-isoprostanes are specific products of arachidonic acid peroxidation by nonenzymatic reactive oxygen species and are considered as reliable biomarkers of oxidative stress and lipid peroxidation. In this article, we investigated the cytotoxicity of different nanoparticles and the degree of cellular membrane damage by using F₂-isoprostanes as biomarkers after exposure to nanoparticles. The human lung epithelial cell line A549 was exposed to four silica and metal oxide nanoparticles: SiO₂ (15 nm), CeO₂ (20 nm), Fe₂O₃ (30 nm), and ZnO (70 nm). The levels of F₂-isoprostanes were determined by using high-performance liquid chromatography/mass spectrometry. The F₂-isoprostanes’ peak was identified by retention time and molecular ion m/z at 353. Oasis HLB cartridge was used to extract F₂-isoprostanes from cell medium. The results showed that SiO₂, CeO₂, and ZnO nanoparticles increased F₂-isoprostanes levels significantly in A549 cells. Fe₂O₃ nanoparticle also increased F₂-isoprostanes level, but was not significant. This implied that SiO₂, CeO₂, ZnO, and Fe₂O₃ nanoparticles can cause cell membrane damage due to the lipid peroxidation. To the best of our knowledge, this is the first report on the investigation of effects of cellular exposure to metal oxide and silica nanoparticles on the cellular F₂-isoprostanes levels.     

High reduction of 4-nitrophenol using reduced graphene oxide/Ag synthesized with tyrosine

There is a demand for the development of environmental friendly methods for the synthesis of graphene composites. Reduced graphene oxide/silver (RGO/Ag) nanocomposites are very good catalysts. Here, we propose a simple, green method for the synthesis of RGO/Ag nanocomposite using the amino acid tyrosine as bioreductant and stabilizing agent. RGO/Ag nanocomposite was characterized by using various analytical techniques and studied for its catalytic degradation of 4-nitrophenol. Results of attenuated total reflectance Fourier transform infrared spectroscopy and Zeta potential at ?55 mV reveal the surface capping of tyrosine onto the reduced graphene oxide nanosheets. RGO/Ag nanocomposites show excellent catalytic reduction of 4-nitrophenol with NaBH₄, when compared to actual individual silver nanoparticles.     

A Pt-Bi bimetallic nanoparticle catalyst for direct electrooxidation of formic acid in fuel cells

Direct formic acid fuel cells are a promising portable power-generating device, and the development of efficient anodic catalysts is essential for such a fuel cell. In this work Pt-Bi nanoparticles supported on micro-fabricated gold wire array substrate were synthesized using an electrochemical deposition method for formic acid oxidation in fuel cells. The surface morphology and element components of the Pt-Bi/Au nanoparticles were characterized, and the catalytic activities of the three Pt-Bi/Au nanoparticle electrodes with different Pt/Bi ratios for formic acid oxidation were evaluated. It was found that Pt₄Bi₉₆/Au had a much higher catalytic activity than Pt₁₁Bi₈₉/Au and Pt₁₃Bi₈₇/Au, and Pt₄Bi₉₆/Au exhibited a current density of 2.7 mA·cm^-2, which was 27-times greater than that of Pt/Au. The electro-catalytic activity of the Pt-Bi/Au electrode for formic acid oxidation increased with the increasing Bi content, suggesting that it would be possible to achieve an efficient formic acid oxidation on the low Pt-loading. Therefore, the Pt-Bi/Au electrode offers a promising catalyst with a high activity for direct oxidation of formic acid in fuel cells.     

Effect of silica additive on the thermal stability of catalysts for NOx abatement

The aim of the plesent investigation was to study the effect of SiO₂ addition on the thermal deactivation of V₂O₅/WO₃/TiO₂ catalysts used for NOx pollution abatement. The results suggest that the degradation of the catalytic properties is strongly correlated to the structural ageing which is, in turn, mainly related to the anatase–rutile phase transformation and to the WO₃ phase segregation. The addition of SiO₂ strongly influences the temperature at which these phenomena occur. In fact, it was found that the introduction of this oxide stabilizes the material, retarding the collapse of surface area, and increases the temperature of the anatase to rutile phase transition.     

Catalytic oxidation of CO over Pt/Fe3O4 catalysts: Tuning O2 activation and CO adsorption

• Strong metal-support interaction exists on Pt/Fe₃O₄ catalysts. • Pt metal particles facilitate the formation of oxygen vacancies on Fe₃O₄. • Fe₃O₄ supports enhance the strength of CO adsorption on Pt metal particles. The self-inhibition behavior due to CO poisoning on Pt metal particles strongly impairs the performance of CO oxidation. It is an effective method to use reducible metal oxides for supporting Pt metal particles to avoid self-inhibition and to improve catalytic performance. In this work, we used in situ reductions of chloroplatinic acid on commercial Fe₃O₄ powder to prepare heterogeneous-structured Pt/Fe₃O₄ catalysts in the solution of ethylene glycol. The heterogeneous Pt/Fe₃O₄ catalysts achieved a better catalytic performance of CO oxidation compared with the Fe₃O₄ powder. The temperatures of 50% and 90% CO conversion were achieved above 260°C and 290°C at Pt/Fe₃O₄, respectively. However, they are accomplished on Fe₃O₄ at temperatures higher than 310°C. XRD, XPS, and H₂-TPR results confirmed that the metallic Pt atoms have a strong synergistic interaction with the Fe₃O₄ supports. TGA results and transient DRIFTS results proved that the Pt metal particles facilitate the release of lattice oxygen and the formation of oxygen vacancies on Fe₃O₄. The combined results of O₂-TPD and DRIFTS indicated that the activation step of oxygen molecules at surface oxygen vacancies could potentially be the rate-determining step of the catalytic CO oxidation at Pt/Fe₃O₄ catalysts. The reaction pathway involves a Pt-assisted Mars-van Krevelen (MvK) mechanism.     

Pd nanoparticles entrapped in TiO<Subscript>2</Subscript> nanotubes for complete butane catalytic combustion at 130 °C

Air pollution by volatile organic compounds is a major health issue due to increasing industrialization and urbanization, notably in the developing countries. Cleaning organic pollutants by catalytic combustion is a potential solution, but actual methods require relatively high temperatures, thus increasing remediation costs. There is therefore a need for methods that operate at mild temperatures. Here we prepared a novel catalyst made of Pd nanoparticles entrapped in TiO₂ nanotubes by vacuum-assisted impregnation. Then, we tested this catalyst for butane combustion. The catalyst was characterized by N₂ adsorption–desorption isotherms, transmission electronic microscopy, energy-dispersive X-ray analysis coupled with a scanning transmission electron microscope, X-ray photoelectron spectroscopy and temperature programmed oxidation. Results show a complete combustion of butane at 130 °C, which is about 20 °C lower than temperatures required by actual catalysts made of Pd nanoparticles deposited on the exterior surface of TiO₂ nanotubes. Structure characterization suggests that this higher performance at lower temperature is explained by the confinement of TiO₂ nanotubes. Such a confinement could hinder the metal sintering and, in turn, facilitate the formation of PdO during oxidation on the entrapped Pd nanoparticles.     

Effects of Pd doping on N<Subscript>2</Subscript>O formation over Pt/BaO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> during NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> storage and reduction process

N₂O is a powerful greenhouse gas and plays an important role in destructing the ozone layer. This present work investigated the effects of Pd doping on N₂O formation over Pt/BaO/Al₂O₃ catalyst. Three types of catalysts, Pt/BaO/Al₂O₃, Pt/Pd mechanical mixing catalyst (Pt/BaO/Al₂O₃ + Pd/Al₂O₃) and Pt-Pd co-impregnation catalyst (Pt-Pd/BaO/Al₂O₃) were prepared by incipient wetness impregnation method. These catalysts were first evaluated in NSR activity tests using H₂/CO as reductants and then carefully characterized by BET, CO chemisorption, CO-DRIFTs and H2-TPR techniques. In addition, temperature programmed reactions of NO with H2/CO were conducted to obtain further information about N₂O formation mechanism. Compared with Pt/BaO/Al₂O₃, (Pt/BaO/ Al₂O₃ + Pd/Al₂O₃) produced less N₂O and more NH3 during NO _x storage and reduction process, while an opposite trend was found over (Pt-Pd/BaO/Al₂O₃ + Al₂O₃). Temperature programmed reactions of NO with H₂/CO results showed that Pd/Al₂O₃ component in (Pt/BaO/Al₂O₃ + Pd/Al₂O₃) played an important role in NO reduction to NH₃, and the formed NH3 could reduce NO _x to N₂ leading to a decrease in N₂O formation. Most of N₂O formed over (Pt-Pd/BaO/Al₂O₃ + Al₂O₃) was originated from Pd/BaO/Al₂O₃ component. H₂-TPR results indicated Pd-Ba interaction resulted in more difficultto- reduce PdOx species over Pd/BaO/Al₂O₃, which inhibits the NO dissociation and thus drives the selectivity to N₂O in NO reduction.

Open image in new window

     

汽车排气净化催化剂三效性能的研究

本文对Ky系列催化剂处理汽车排气中CO,HC和NO_x的三效性能进行了研究,用CVS法测定排放污染物的重量和转化效率;通过行车60 000km,证明高温老化引起催化剂表面结构的改变,是导致催化剂活性下降的主要原因,新改进的催化剂有较好的热稳定性和三效性能。     

PM-support interfacial effect and oxygen mobility in Pt,Pd or Rh-loaded (Ce,Zr,La)O2 catalysts

• Pt/CZL exhibits the optimum catalytic performance for HC and NO_x elimination. • The strong PM-Ce interaction favors the oxygen mobility and DOSC. • Pd/CZL shows higher catalytic activity for CO conversion due to more O_latt species. • Great oxygen mobility at high temperature broadens the dynamic operation window. • The relationship between DOSC and catalytic performance is revealed. The physicochemical properties of Pt-, Pd- and Rh- loaded (Ce,Zr,La)O₂ (shorted for CZL) catalysts before/after aging treatment were systematically characterized by various techniques to illustrate the relationship of the dynamic oxygen storage/release capacity and redox ability with their catalytic performances for HC, NO_x and CO conversions. Pt/CZL catalyst exhibits the optimum catalytic performance for HC and NO_x elimination, which mainly contribute to its excellent redox ability and dynamic oxygen storage/release capacity (DOSC) at lower temperature due to the stronger PM (precious metals)-support interaction. However, the worse stability of Pt-O-Ce species and volatile Pt oxides easily result in the dramatical decline in catalytic activity after aging. Pd/CZL shows higher catalytic activity for CO conversion by reason of more O_latt species as the active oxygen for CO oxidation reaction. Rh/CZL catalyst displays the widest dynamic operation window for NO_x elimination as a result of greater oxygen mobility at high temperature, and the ability to retain more Rh-O-Ce species after calcined at 1100°C effectively restrains sintering of active RhO_x species, improving the thermal stability of Rh/CZL catalyst.     

Facile synthesis of Pd nanoparticles on SiO2 for hydrogenation of biomass-derived furfural

This report shows that furfuryl alcohol can be selectively produced from the hydrogenation of furfural using supported Pd nanoparticles. Furfuryl alcohol is widely used as solvent and chemical intermediate for the synthesis of fine chemicals. Here, various Pd nanoparticles supported on mesoporous SiO₂ (Pd/SiO₂) were simply fabricated by a wet impregnation using palladium nitrate. Physical properties of Pd/SiO₂ nanoparticles were studied by X-ray diffraction, energy-dispersive, X-ray analysis, N₂ adsorption and desorption isotherms and transmission electron microscopy. Results show a high dispersion of Pd nanoparticles with small size. Pd nanoparticles catalyzed very efficiently the hydrogenation of furfural to furfuryl alcohol with 76 % selectivity under mild conditions. Overall, the catalyst developed could find applications for the production of chemicals from biomass.     

Enhanced heterogeneous Fenton-like activity by Cu-doped BiFeO<Subscript>3</Subscript> perovskite for degradation of organic pollutants

Heterogeneous Fenton-like reaction has been extensively investigated to eliminate refractory organic contaminants in wastewater, but it usually shows low catalytic performance due to difficulty in reduction from Fe(III) to Fe(II). In this study, enhanced catalytic efficiency was obtained by employing Cu-doped BiFeO₃ as heterogeneous Fenton-like catalysts, which exhibited higher catalytic performance toward the activation of H₂O₂ for phenol degradation than un-doped BiFeO₃. BiFe_0.8Cu_0.2O₃ displayed the best performance, which yielded 91% removal of phenol (10 mg L^–1) in 120 min. The pseudo first-order kinetic rate constant of phenol degradation in BiFe_0.8Cu_0.2O₃ catalyzed heterogeneous Fenton-like reaction was 5 times higher than those of traditional heterogeneous Fenton-like catalysts, such as Fe₃O₄ and goethite. The phenol degradation efficiency could still reach 83% after 4 cycles, which implied the good stability of BiFe_0.8Cu_0.2O₃. The high catalytic activity of BiFe_0.8Cu_0.2O₃ was attributed to the fact that the doping Cu into BiFeO₃ could promote the generation of Fe(II) in the catalyst and then facilitate the activation of H₂O₂ to degrade the organic pollutants.

Open image in new window