Application of alumina supported gold-based catalysts in total oxidation of CO and light hydrocarbons mixture

Gold nanoparticles supported on alumina have been produced using the anionic exchange method and ammonia washing procedure. The catalysts are tested in the reaction of total oxidation of a mixture of light hydrocarbons and carbon monoxide in order to study the possibility of application in the reduction of cold start emissions. The obtained results are promising according to the temperature range observed for the oxidation of unsaturated hydrocarbons. The results obtained for acetylene confirms the difference of oxidation of this hydrocarbon over gold catalysts. An ageing procedure has been employed. This procedure does not affect the comportment of the catalysts versus hydrocarbon oxidation.     

纳米金催化剂研究进展

比较详细的介绍了金催化剂的应用研究情况．尤其对CO的常温脱除、富氢条件下CO的选择性氧化．邻二醇的选择性氧化以及烃类的选择性氧化等反应过程进行了较为详细的介绍,具有一定的参考价值。     

Deposition of Gold Particles on Mesoporous Catalyst Supports by Sonochemical Method, and their Catalytic Performance for CO Oxidation

Supported gold catalysts on the mesoporous (MSP) metal oxides were prepared by a one-step, ultrasound-assisted reduction method, and characterized by XRD, HRTEM, EDX, BET, and XPS analysis. Their catalytic activities were examined in the oxidation of CO. Compared to the Au/Fe₂O₃(MSP) catalyst, the Au/TiO₂(MSP) and Au/Fe₂O₃-TiO₂(MSP) catalysts exhibited higher catalytic activity in the oxidation of CO at low temperatures. The high catalytic activity of Au/TiO₂(MSP) was attributed to the metallic state of the gold nanoparticles, their small size (2–2.5 nm), and their high dispersion on the catalyst support.     

Au/Ce1−xZrxO2 as effective catalysts for low-temperature CO oxidation

The physico-chemical properties and activity of Ce-Zr mixed oxides, CeO₂ and ZrO₂ in CO oxidation have been studied considering both their usefulness as supports for Au nanoparticles and their contribution to the reaction. A series of Ce_1−xZr_xO₂ (x = 0, 0.25, 0.5, 0.75, 1) oxides has been prepared by sol–gel like method and tested in CO oxidation. Highly uniform, nanosized, Ce-Zr solid solutions were obtained. The activity of mixed oxides in CO oxidation was found to be dependent on Ce/Zr molar ratio and related to their reducibility and/or oxygen mobility. CeO₂ and Ce_0.75Zr_0.25O₂, characterized by the cubic crystalline phase show the highest activity in CO oxidation. It suggests that the presence of a cubic crystalline phase in Ce-Zr solid solution improves its catalytic activity in CO oxidation. The relation between the physico-chemical properties of the supports and the catalytic performance of Au/Ce_1−xZr_xO₂ catalysts in CO oxidation reaction has been investigated. Gold was deposited by the direct anionic exchange (DAE) method. The role of the support in the creation of catalytic performance of supported Au nanoparticles in CO oxidation was significant. A direct correlation between activity and catalysts reducibility was observed. Ceria, which is susceptible to the reduction at the lowest temperature, in the presence of highly dispersed Au nanoparticles, appears to be responsible for the activity of the studied catalysts. CeO₂-ZrO₂ mixed oxides are promising supports for Au nanoparticles in CO oxidation whose activity is found to be dependent on Ce/Zr molar ratio.     

Low-temperature prepared highly effective ferric hydroxide supported gold catalysts for carbon monoxide selective oxidation in the presence of hydrogen

Ferric hydroxide supported Au catalysts prepared with co-precipitation method at room temperature without any heat treatment hereafter exhibited high catalytic activity and selectivity for CO oxidation in air and CO selective oxidation in the presence of H₂. With calcination temperature rising, both activity and selectivity decreased. X-ray Photoelectron Spectra (XPS) indicated that Au existed as Au⁰ and Au⁺ in the catalyst without heat treatment and even after being calcined at 200 °C, while after being calcined at 400 °C, Au existed as Au⁰ completely. X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopic (HRTEM) investigations indicated that both the supports and Au species were highly dispersed as nano or sub-nano particles even after being calcined at 200 °C, but after being calcined at 400 °C the supports transformed to crystal Fe₂O₃ with typical diameter of 30 nm and Au species aggregated to nano-particles with typical diameter of 2–4 nm. HRTEM investigations also suggested that the supports calcined at 200 °C were composed of amorphous ferric hydroxide and crystal ferric oxide. Results of computer simulation (CS) showed that O₂ was adsorbed on Au crystal cell and then were activated, which should be the key factor for the subsequent reaction. It also suggested that O₂ species were more easily adsorbed on Au⁺ than on Au⁰, indicating that higher positive charge of the Au species possessed the higher activity for CO oxidation.     

Effect of heat treatment on stability of gold particle modified carbon supported Pt-Ru anode catalysts for a direct methanol fuel cell

Carbon supported Au-PtRu (Au-PtRu/C) catalysts were prepared as the anodic catalysts for the direct methanol fuel cell (DMFC). The procedure involved simple deposition of Au particles on a commercial Pt-Ru/C catalyst, followed by heat treatment of the resultant composite catalyst at 125, 175 and 200 °C in a N₂ atmosphere. High-resolution transmission electron microscopy (HR-TEM) measurements indicated that the Au nanoparticles were attached to the surface of the Pt-Ru nanoparticles. We found that the electrocatalytic activity and stability of the Au-PtRu/C catalysts for methanol oxidation is better than that of the PtRu/C catalyst. An enhanced stability of the electrocatalyst is observed and attributable to the promotion of CO oxidation by the Au nanoparticles adsorbed onto the Pt-Ru particles, by weakening the adsorption of CO, which can strongly adsorb to and poison Pt catalyst. XPS results show that Au-PtRu/C catalysts with heat treatment lead to surface segregation of Pt metal and an increase in the oxidation state of Ru, which militates against the dissolution of Ru. We additionally find that Au-PtRu/C catalysts heat-treated at 175 °C exhibit the highest electrocatalytic stability among the catalysts prepared by heat treatment: this observation is explained as due to the attainment of the highest relative concentration of gold and the highest oxidation state of Ru oxides for the catalyst pretreated at this temperature.     

Effective Au-Au-Clx/Fe(OH)y catalysts containing Cl for selective CO oxidations at lower temperatures

Supported Au catalysts Au-Au⁺-Cl_x/Fe(OH)_y (x < 4, y ≤ 3) and Au-Cl_x/Fe₂O₃ prepared with co-precipitation without any washing to remove Cl⁻ and without calcining or calcined at 400 °C were studied. It was found that the presence of Cl⁻ had little impact on the activity over the unwashed and uncalcined catalysts; however, the activity for CO oxidation would be greatly reduced only after Au-Au⁺-Cl_x/Fe(OH)_y was further calcined at elevated temperatures, such as 400 °C. XPS investigation showed that Au in catalyst without calcining was composed of Au and Au⁺, while after calcined at 400 °C it reduced to Au⁰ completely. It also showed that catalysts precipitated at 70 °C could form more Au⁺ species than that precipitated at room temperatures. Results of XRD and TEM characterizations indicated that without calcining not only the Au nano-particles but also the supports were highly dispersed, while calcined at 400 °C, the Au nano-particles aggregated and the supports changed to lump sinter. Results of UV–vis observation showed that the Fe(NO₃)₃ and HAuCl₄ hydrolyzed partially to form Fe(OH)₃ and [AuCl_x(OH)_4−x]⁻ (x = 1–3), respectively, at 70 °C, and such pre-partially hydrolyzed iron and gold species and the possible interaction between them during the hydrolysis may be favorable for the formation of more active precursor and to avoid the formation of Au–Cl bonds. Results of computer simulation showed that the reaction molecular of CO or O₂ were more easily adsorbed on Au⁺ and Au⁰, but was very difficultly absorbed on Au⁻. It also indicated that when Cl⁻ was adsorbed on Au⁰, the Au atom would mostly take a negative electric charge, which would restrain the adsorption of the reaction molecular severely and restrain the subsequent reactions while when Cl⁻ was adsorbed on Au⁺ there only a little of the Au atom take negative electric charge, which resulting a little impact on the activity.     

The active phase of Au–Pd/Al2O3 for CO oxidation

Au–Pd/Al₂O₃ catalyst was prepared by modified impregnation method. It was found that the catalyst calcined in air at 473 K showed higher CO oxidation activity in comparison with the catalysts treated at other temperature. Nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) techniques were employed to study the relationship between the surface/bulk structures of these catalysts and their catalytic performance. The results indicated the higher activity was attributed to the smaller pore volume and co-existence of PdO and Au⁰ in their surface. The formation of Au_xPd_y alloy was unfavorable for the catalytic reaction.     

沉积-沉淀法制备金催化剂用于CO催化氧化的研究

采用沉积-沉淀法制备了一种介孔氧化铝负载的金催化剂用于CO的低温催化氧化,详细考察了催化剂制备条件、干燥条件和热处理条件对其催化性能的影响。采用X射线粉末衍射（XRD）、透射电镜（TEM）、低温N2物理吸附（N2-BET）、电感耦合等离子发射光谱（ICP-AES）和紫外-可见漫反射（Uv-vis DRS）等手段对催化剂进行了表征。实验结果表明,通过严格控制催化剂的制备条件、干燥条件和热处理条件,可以在惰性氧化铝载体上高重复性可控合成具有高分散度的金催化剂,金颗粒最佳平均粒径为2 nm,体现出良好的CO低温催化氧化性能。     

Pd/Al2O3催化剂上低浓度CO氧化的研究

采用共沉淀法和浸渍法制备了2%Pd-Al2O3催化剂,在固定反应器中评价不同制备方法(共沉淀和浸渍)、助剂(1%K和1%Mg)、预处理方法(快速氧化和缓慢还原)、空速(726h-1、1638h-1和5274h-1)情况下2%Pd-Al2O3催化剂对CO氧化的催化活性。结果表明,共沉淀法制备的2%Pd50%Zr-Al2O3催化剂有较高的反应活性,该催化剂经缓慢还原预处理活化,在温度为70℃时就可以使低浓度的CO完全氧化。     

Negative activation energies in CO oxidation over an icosahedral Au/Mg(OH)2 catalyst

In the course of our studies on CO oxidation over Au/Mg(OH)₂ we have discovered a catalyst which exhibits an apparent negative activation energy when studied under ultra‐dry conditions (80 ppb moisture content). A review of current literature suggests that the oxidation of CO may occur by a reaction between CO and OH radicals and not by oxygen as previously thought. Substantial differences in catalytic behaviour between low and high temperature suggest that the reaction is complex and that more than one reaction pathway is present. This revised version was published online in July 2006 with corrections to the Cover Date.     

MnOx/Pt/Al2O3 catalysts for CO oxidation in H2-rich streams

The catalytic activity of Pt on alumina catalysts, with and without MnO_x incorporated to the catalyst formulation, for CO oxidation in H₂-free as well as in H₂-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al₂O₃. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO₂ and 5 vol.% H₂O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO₂ in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO_x content.     

Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd

The present work has been undertaken to tailor Pt/Al₂O₃ catalysts active for NO oxidation even after severe heat treatments in air. For this purpose, the addition of Pd has been attempted, which is less active for this reaction but can effectively suppress thermal sintering of the active metal Pt. Various Pd-modified Pt/Al₂O₃ catalysts were prepared, subjected to heat treatments in air at 800 and 830 °C, and then applied for NO oxidation at 300 °C. The total NO oxidation activity was shown to be significantly enhanced by the addition of Pd, depending on the amount of Pd added. The Pd-modified catalysts are active even after the severe heat treatment at 830 °C for a long time of 60 h. The optimized Pd-modified Pt/Al₂O₃ catalyst can show a maximum activity limited by chemical equilibrium under the conditions used. The bulk structures of supported noble metal particles were examined by XRD and their surface properties by CO chemisorption and EDX-TEM. From these characterization results as well as the reaction ones, the size of individual metal particles, the chemical composition of their surfaces, and the overall TOF value were determined for discussing possible reasons for the improvement of the thermal stability and the enhanced catalytic activity of Pt/Al₂O₃ catalysts by the Pd addition. The Pd-modified Pt/Al₂O₃ catalysts should be a promising one for NO oxidation of practical interest.     

Silica-Supported Gold Catalyst Modified by Doping with Titania for Cyclohexane Oxidation

Amorphous silica was modified by doping with titania through a surface sol–gel process and applied as the support for depositing gold. These doped silica-supported gold catalysts were tested in the selective cyclohexane oxidation to cyclohexanone and cyclohexanol using oxygen. Under the oxidation conditions of 150 °C, 1.5 MPa and 3 h, a selectivity of 91.7% for cyclohexanone and cyclohexanol could be reached over the gold catalyst, affording a cyclohexane conversion of 8.4% and a turnover frequency up to 40,133 per hour. Moreover, the catalytic activity and selectivity could be well retained in 4 recycling oxidation reactions, showing a high stability of the gold catalyst supported on titania-doped silica.     

Gold/hydroxyapatite catalysts: Synthesis, characterization and catalytic activity to CO oxidation

This work reports the synthesis, characterization and catalytic activity for CO oxidation of gold catalysts supported on calcium hydroxyapatite. On both, the hydroxyapatite support and the gold-supported hydroxyapatite catalyst, the CO conversion shows a peak near 100% of conversion at room temperature. The generation of structural vacancies by interaction of CO with the solid provokes the formation of peroxide species in the presence of gaseous oxygen, which seems to be responsible of this high conversion of CO at room temperature. Moreover, the influence of the pre-treatment temperature on the activity has been observed and related with the elimination of carbonate species and the generation of structural defects in the apatite structure, which are able to modify the gold oxidation state.     

A CuO-CeO2 Mixed-Oxide Catalyst for CO Clean-Up by Selective Oxidation in Hydrogen-Rich Mixtures

A CuO-CeO₂ mixed-oxide catalyst was shown experimentally to be highly active and selective for the oxidation of CO in hydrogen-rich mixtures, and an attractive alternative to the noble metal catalysts presently used for CO clean-up in hydrogen mixtures for proton-exchange membrane fuel cells (PEMFC). Although the presence of H₂O and CO₂ in the feed decreased the activity and increased the reaction temperature considerably to achieve a given CO conversion with a reactor, the selectivity profile with respect to the conversion remained virtually the same. The effect of H₂O and CO₂ on the reaction was found to increase the required energy for reduction of the active copper species in the redox cycles undergone during the reaction. The catalyst showed a slow, reversible deactivation, but the activity was restored on heating the catalyst at 300 °C, even under an inert flow. At space velocities above 42 g h m^-3, the catalyst reduced the CO content to less than 10 ppm in the temperature range 166-176 °C for a feed of 1% CO, 1% O₂, 50% H₂, 20% H₂O, 13.5% CO₂ and balance He. Hence, with this catalyst it is feasible to clean up the CO in a single-stage reactor with relatively small excess oxygen, which is in contrast to the typical multistage reactor systems using noble metal catalysts.     

Magnetron sputtering of gold nanoparticles onto WO3 and activated carbon

In this paper we describe the production and investigation of two supported gold catalyst systems prepared by magnetron sputtering: Au on WO₃ and Au on activated carbon. The magnetron sputtering technique entails using an argon plasma to sputter a high purity gold target producing a flux of gold atoms which are deposited onto a constantly tumbling support material. This technique offers a number of advantages over conventional chemical preparation methods. One advantage is the ability to create gold nanoparticles (diameters <3 nm) on unusual support materials, such as WO₃ and carbon, which are generally not accessible using the ubiquitous deposition-precipitation technique. We present data demonstrating the formation of catalytic gold nanoparticles with average diameters of 1.7 nm (Au/C) and 2.1 nm (Au/WO₃), as well as a substantial number of single atom species on the Au/C sample. Prototypical carbon monoxide oxidation (Au/WO₃) and glycerol oxidation (Au/C) reactions were performed in order to gauge the activity of these catalysts. The WO₃ supported catalyst exhibits substantial catalytic activity from room temperature to 135 °C (0.0018–0.082 mol CO/mol Au s) with an activation energy near 23 kJ/mol. The activity of the Au/C catalyst was compared to a Au/C catalyst prepared from a poly(vinyl alcohol) (PVA) sol. The smaller catalysts prepared by sputtering are more active than the large gold particles prepared using the PVA sol, however the larger gold nanoparticles are substantially more selective towards the production of intermediate products from the oxidation of glycerol.