Preparation and Characterisation of Ag/Alumina Catalysts for the Removal of NOx Emissions Under Oxygen Rich Conditions

A highly active silver/alumina catalyst for continuous reduction of NO to nitrogen with octane under lean conditions was prepared. The effect of impregnation time and precursor concentration on the catalyst activity was investigated. The catalyst was characterized by means of XRF, H₂-TPR, and UV–Vis to correlate recorded results with different preparation methods. UV–Vis measurements indicated that the highest and most stable activity is obtained with Ag/alumina exhibiting predominantly mono-atomically dispersed silver.     

Selective reduction of NO on Ag/Al2O3 catalysts prepared from boehmite needles

Ag/Al₂O₃ catalysts prepared from boehmite needles (ca. 10 nm×100 nm), which were formed by a hydrolysis of aluminium tri-isopropoxide (AIP), showed good performances for selective catalytic reduction of NO_x compared with the previously reported catalysts [N. Aoyama, K. Yoshida, A. Abe, T. Miyadera, Catal. Lett. 43 (1997) 249], especially when ethanol is employed as a reducing agent in the presence of water. Temperature programmed reduction (TPR) study revealed that the Ag species are attractively interacted with the alumina surface and the oxidized Ag species contribute positively for the improvement of the catalytic activity at the temperatures above 750 K. It is concluded that the boehmite needles as a precursor of alumina support are useful to create the catalytically active sites for NO_x reduction.     

Catalytic oxidation of CO over Ag-Co/alumina catalysts

Low-loading silver cobalt catalysts on alumina for the catalytic oxidation of carbon monoxide were synthesized following different preparation methods: the single-step sol-gel method and the impregnation method. A catalyst synthesized by the single-step sol-gel method gave high surface area values. The activities of alumina-supported silver-cobalt catalysts were studied to obtain the effects of the calcination temperature, metal loading, and preparation methods on CO oxidation at low temperature. The catalysts prepared by impregnation metals on the alumina synthesized by the sol-gel method gave the best activity for the CO oxidation. Catalytic activity conversion of (5 wt% Ag-5 wt% Co)/Al 2 O 3 catalysts prepared by the sol-gel-impregnation method reached 100% at 200°C. Increasing silver loading over the catalysts gave rise to more active catalysts. The sol-gel catalysts had poor activity at low temperature.     

Ag／TiO2光催化氧化还原反应脱除水体中无机氮

采用光催化还原法制备了高活性的载银二氧化钛光催化剂，并用XRF、TEM、XRD及XPS、UV-Vis方法对催化剂进行了表征．考察了负载舨的含量、催化剂制备时间、搅拌气体的种类以及Fe^2＋的添加等条件对该催化剂光催化水体脱氮活性的影响．结果表明：载银量1．0％时去除效果最佳；制备催化剂时光照时间不充分会使催化剂失去还原效力；氮气保护下催化剂反应活性更高；Fe^2＋的加入利于光催化反应；2h光催化含氨氮和亚硝基氮的水样。总脱氮率达到了63．9％．     

Liquid phase selective oxidation of benzyl alcohol over Pd–Ag catalysts supported on pumice

Selective oxidation of benzyl alcohol to benzaldehyde was carried out over pumice supported bimetallic and monometallic Pd and Ag catalysts. Preliminary kinetic studies were performed at 333 K in autoclave, at pressure of 2 atm in pure oxygen. Under these conditions, small amounts of benzoic acid were detected with the monometallic Pd pumice being the most active catalyst. The reaction was also carried out under flowing oxygen at atmospheric pressure and at 348 K. Under these conditions, the selectivity to benzaldehyde was 100%. The catalytic activity of the catalysts was measured after different oxidation and reduction treatments at high temperature. In addition, two mechanical mixtures of pretreated Pd and Ag monometallic samples were tested. The structural data (XRD, XPS, EXAFS) along with the catalytic results would indicate that Ag⁰ and Pd⁰ species are the catalytic sites acting with certain synergism.     

Study of perovskite‐type oxides and their supported Ag derivatives for catalytic oxidation of diesel soot

A series of perovskite‐type oxides and derived Ag catalysts were prepared, and characterized by N₂‐adsorption, X‐ray diffraction and X‐ray photoelectron spectroscopy. The influences of pretreatment and Ag loading on catalytic activity for diesel soot oxidation were also investigated. Prereduction resulted in a decrease in catalytic activity. An increase in activity with Ag addition was observed, especially with more than 5% Ag loading. This catalyst could be a promising candidate for the catalytic elimination of diesel soot. © 2002 Society of Chemical Industry     

Study of Ag/La0.6Ce0.4CoO3 catalysts for direct decomposition and reduction of nitrogen oxides with propene in the presence of oxygen

Perovskite-type oxide La_0.6Ce_0.4CoO₃ and its doped Ag catalysts were prepared and their catalytic performances were evaluated for the direct decomposition of NO and the selective reduction of NO with propene in the presence of oxygen. A noticeable enhancement in activity was achieved by doping Ag and the optimum Ag loading was 1%. The effects of H₂O, SO₂, CO₂ and O₂ on the performances of Ag/La_0.6Ce_0.4CoO₃ catalysts for NO decomposition were also investigated. The resistance against H₂O and SO₂ appears satisfactory. The inhibition by CO₂ is strong, although it is reversible. Oxygen did not inhibit the NO decomposition reaction but significantly promoted it. Compared with other perovskite-type oxides reported previously, higher conversions were obtained over the present catalysts for the NO reduction by propene. We speculate that the decomposition of NO is the predominant process even in the presence of propene. The catalysts were characterized by N₂-adsorption, XRD, XPS and NO-TPD and some explanations were put forward.     

Hierarchical ZnO/Ag nanocomposites for plasmon-enhanced visible-light photocatalytic performance

Surface plasmon resonance (SPR) of the noble metals improve the photocatalytic activity of semiconductor metal oxides in the visible light region. This work reports the facile preparation of SPR induced visible light active hierarchical ZnO/Ag nanocomposite photocatalysts by using environmental friendly two-step method. The prepared nanocomposites analyzed by using various techniques such as powder-XRD, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV–Vis spectroscopy, photoluminescence spectroscopy, and photocurrent measurements. The results indicate the formation of hierarchical ZnO/Ag nanocomposites, which shows surface plasmon absorbance and enhanced photocurrent responses. Because of the SPR effect, the nanocomposites show improved visible light photocatalytic activity by enhancing the electron-hole pair separation in Rhodamine B degradation system.     

HC-SCR of NOx over Ag/alumina: a combination of heterogeneous and homogeneous radical reactions?

Matrix isolation combined with EPR and FT-IR technique at low temperature has been used to study the gas phase species involved in HC-SCR over a highly active Ag/alumina catalyst. A combination of heterogeneous and homogeneous (radical) reactions is proposed to take place over a Ag/alumina catalyst during HC-SCR. Radicals of low molecular weight were trapped in a growing argon matrix behind the Ag/alumina catalyst. In the same matrix, cyanogen isocyanate was detected and is suggested to be a key intermediate for the formation of amines and ammonia via the hydrolysis of isocyanate species.     

Improvement of catalytic activity and sulfur-resistance of Ag/TiO2–Al2O3 for NO reduction with propene under lean burn conditions

Ag-based catalysts supported on various metal oxides, Al₂O₃, TiO₂, and TiO₂–Al₂O₃, were prepared by the sol–gel method. The effect of SO₂ on catalytic activity was investigated for NO reduction with propene under lean burn condition. The results showed the catalytic activities were greatly enhanced on Ag/TiO₂–Al₂O₃ in comparison to Ag/Al₂O₃ and Ag/TiO₂, especially in the low temperature region. Application of different characterization techniques revealed that the activity enhancement was correlated with the properties of the support material. Silver was highly dispersed over the amorphous system of TiO₂–Al₂O₃. NO₃⁻ rather than NO₂⁻ or NOx reacted with the carboxylate species to form CN or NCO. NO₂ was the predominant desorption species in the temperature programmed desorption (TPD) of NO on Ag/TiO₂–Al₂O₃. More amount of formate (HCOO⁻) and CN were generated on the Ag/TiO₂–Al₂O₃ catalyst than the Ag/Al₂O₃ catalyst, due to an increased number of Lewis acid sites. Sulfate species, resulted from SO₂ oxidation, played dual roles on catalytic activity. On aged samples, the slow decomposition of accumulated sulfate species on catalyst surface led to poor NO conversion due to the blockage of these species on active sites. On the other hand, catalytic activity was greatly enhanced in the low temperature region because of the enhanced intensity of Lewis acid site caused by the adsorbed sulfate species. The rate of sulfate accumulation on the Ag/TiO₂–Al₂O₃ system was relatively slow. As a consequence, the system showed superior capability for selective adsorption of NO and SO₂ toleration to the Ag/Al₂O₃ catalyst.     

Ag/CeO2-ZnO催化剂上甲醇部分氧化制氢气研究

研究了用于甲醇部分氧化制氢气反应的Ag/CeO2-ZnO催化剂的制备条件以及反应条件对催化剂性能的影响.Ag/CeO2-ZnO催化剂中银含量、焙烧温度、反应温度以及反应气的比例等对催化剂性能具有很大的影响.Ag/CeO2-ZnO催化剂的制备条件以及反应条件控制在一定范围内时,Ag/CeO2-ZnO催化剂对甲醇部分氧化制氢气具有很高的催化活性与选择性.     

Engineering HC-SCR: Improved Low Temperature Performance through a Cascade Concept

A catalytic after treatment system for lean HC-SCR was constructed of two different catalyst beds, e.g. of a Ag/alumina and Cu-ZSM-5 catalyst (cascade concept). The improved activity especially at low temperature range was found to be due to the synergetic effect of the two catalysts, which combines the transformation of the feed gas over Ag/alumina to such compounds that are highly reactive towards N₂ over Cu-ZSM-5. The effluent coming from the Ag/alumina bed was analysed by GC–MS along with the NO to N₂ conversion over the whole system by GC. The results obtained from the GC–MS measurements revealed that hydrocarbon used as a reducing agent is oxidised and that besides oxygenates also various N-containing hydrocarbons are formed over the Ag/Al₂O₃.     

Catalytic activity of ethylene oxidation over Au,Ag and Au–Ag catalysts: Support effect

Au, Ag and Au–Ag catalysts on different supports of alumina, titania and ceria were studied for their catalytic activity of ethylene oxidation reactions. An addition of an appropriate amount of Au on Ag/Al₂O₃ catalyst was found to enhance the catalytic activity of the ethylene epoxidation reaction because Au acts as a diluting agent on the Ag surface creating new single silver sites which favor molecular oxygen adsorption. The Ag catalysts on both titania and ceria supports exhibited very poor catalytic activity toward the epoxidation reaction of ethylene, so pure Au catalysts on these two supports were investigated. The Au/TiO₂ catalysts provided the highest selectivity of ethylene oxide with relatively low ethylene conversion whereas, the Au/CeO₂ catalysts was shown to favor the total oxidation reaction over the epoxidation reaction at very low temperatures. In comparisons among the studied catalysts, the bimetallic Au–Ag/Al₂O₃ catalyst is the best candidate for the ethylene epoxidation. The catalytic activity of the gold catalysts was found to depend on the support material and catalyst preparation method which govern the Au particle size and the interaction between the Au particles and the support.     

丙烯直接气相氧化过程中Ag/TS-1催化剂上活性银物种的分布

在常压固定床、石英反应器中,以丙烯直接气相氧化反应为探针,考察了硅烷化处理及TS-1中模板剂对Ag/TS-1催化性能的影响.同时,采用UV-Vis、ICP和TEM等手段对其进行了表征.结果表明：在丙烯直接气相氧化过程中,载体TS-1硅烷化后所制备的Ag/TS-1的催化性能增强.通过硅烷化处理负载2%SiO₂,制备的Ag/TS-1的丙烯转化率可达2.1%,环氧丙烷选择性98.4%.用TS-1原粉（含有模板剂）所制备的Ag/TS-1不具有环氧化性能.分子筛内外表面的银物种均具有环氧化性能,但内表面的银物种在丙烯气相氧化过程中起决定性的作用.     

Effects of Supports and Promoter Ag on Pd Catalysts for Selective Hydrogenation of Acetylene

SiO2, a-Al2O3, g-Al2O3, ZrO2 and CeO2 were used as supports and Ag as promoter to study their effects on Pd catalysts for selective hydrogenation of acetylene. The catalysts were prepared by impregnated synthesis and characterized by XRD, BET and TEM. The catalytic reaction was carried out in a fixed-bed reactor. Overall, the low specific surface area supports were better to increase the ethylene selectivity at high conversion rate of acetylene. Among the four Pd catalysts on low specific surface area supports, the catalyst on low specific surface area SiO2 (LSA-SiO2) retained a high ethylene selectivity even at complete conversion, while the other catalysts showed significant decrease in the selectivity at complete conversion. The performance of Pd/LSA-SiO2 was important to decrease the loss of ethylene in selective hydrogenation of trace acetylene in ethylene. Addition of Ag to Pd/LSA-SiO2 significantly decreased the formation of ethane, C4 alkenes and green oil, and improved the ethylene selectivity to 90% when Pd:Ag=1:1 and 1:3(w). When the ratio of Pd to Ag was above 1, the activity of Pd-Ag bimetallic catalyst was similar to that of Pd monometallic catalyst, and the selectivity of ethylene increased with increasing of amount of Ag. When the ratio of Pd to Ag was below 1, the activity of bimetallic catalyst decreased with increasing of amount of Ag, while the selectivity of ethylene was kept unchanged. The optimum temperature was 200~230℃ for 0.02%(w)Pd-0.02%(w)Ag/LSA-SiO2 to give a high ethylene selectivity and low formation of green oil.     

Application of the Solvothermal Process in the Synthesis of High-performance Ag/g-Al2O3 Catalysts

Ag/g-Al2O3 is a kind of promising catalyst with the relatively lower cost compared with those using noble metals, good resistance against catalytic poisoning and excellent behaviour for NOx removal. In the present study, Ag/g-Al2O3 catalysts were synthesized by the solvothermal process and characterized by XRD, TG-DTA, TEM, UV-Vis and FT-IR. It was found that high-performance Ag/g-Al2O3 catalysts could be synthesized by properly selecting starting materials, controlling the composition of solvent and other reaction conditions. The microstructure evolution of the catalysts was also discussed.     

Preparation of nanocrystalline Ni/Al2O3 catalysts with the microemulsion method for dry reforming of methane

Mesoporous nanocrystalline nickel‐alumina catalysts with high surface area were prepared by a microemulsion (ME) method and were employed in methane reforming with carbon dioxide for syngas production. The catalysts were characterized by X‐ray diffraction (XRD), Brunauer‐Emmett‐Teller surface area analysis (BET), temperature‐programmed reduction (TPR), temperature‐programmed oxidation (TPO), and scanning electron microscopy (SEM) techniques. The results showed that the catalysts possessed mesoporous structure with high surface area (> 250 m² · g^?1) and small crystallite size (～5 nm). The catalytic results revealed high activity and stability for the prepared catalysts. In addition, the effect of feed ratio and GHSV on catalytic performance was investigated.     

Catalytic Combustion of Toluene over a Copper‐Manganese‐Silver Mixed‐Oxide Catalyst Supported on a Washcoated Ceramic Monolith

Monolithic catalysts were prepared by washcoating an alumina sol and then impregnating Cu‐Mn‐Ag mixed oxides onto cordierite substrates. The effects of the preparation parameters including the Ag/Cu/Mn ratio, the total amount of active phase and the loading of washcoat, and the reaction conditions, e.g., the space velocity and the oxygen/toluene ratio on the catalytic performance for the combustion of toluene were investigated. It is shown that the Cu‐Mn‐Ag oxides are very active for the combustion of toluene and that the highest catalytic activity is achieved over a monolithic catalyst containing 14.7 wt % of washcoat and 21.2 wt % of active phase with a Ag/Cu/Mn molar ratio of 13.8/43.1/43.1. It is also seen that the optimum catalyst has a good catalytic stability and exhibits an excellent activity not only at a rather high space velocity but also within a wide range of oxygen/toluene ratios.     

A Highly Active Ag/Alumina Catalytic Converter for Continuous HC-SCR During Lean-Burn Conditions: From Laboratory to Full-Scale Vehicle Tests

A common-rail diesel vehicle was equipped with a full-scale Ag/alumina catalytic converter. The converter consisted of several Ag/alumina bricks, with free space in between each brick to fulfil important gas phase reactions. An oxidation catalyst was placed at the end of the converter to remove formed CO and unburned HC. High conversion levels of NO _x , around 60%, were recorded at several speeds and loads using additional HC (diesel) injection corresponding to 2–5% fuel penalty.     

制备条件对Au/TiO_2液相氧化反应活性的影响

采用沉积-沉淀法制备纳米级Au/TiO_2催化剂,以葡萄糖液相催化氧化制葡萄糖酸为探针反应,考察了催化剂制备条件对Au/TiO_2活性的影响,并利用TEM、XRD和XPS等方法对催化剂进行了表征。结果表明,Au/TiO_2对葡萄糖液相氧化反应的催化活性与催化剂的制备条件密切相关,纳米金的颗粒尺寸不是决定催化活性的惟一因素,金在催化剂中的价态对催化活性有重要影响。