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.     

Silver/Alumina Catalyst for Selective Catalytic Reduction of NO x to N2 by Hydrocarbons in Diesel Powered Vehicles

Ag/alumina has been found to be a promising catalyst for the selective catalytic reduction of NO _x to N₂ by hydrocarbons (HC-SCR) in laboratory tests as well as in full-scale diesel engine operation. The steps in developing an active silver catalyst for practical applications involve knowledge of the mechanism, choice of the right metal content and correct support, adjustment of the concentration of the hydrocarbon to be used for reduction reactions etc. As gas phase reactions seem to play a significant role in obtaining high NO to N₂ conversion, also reactor aspects have to be included in the optimisation procedure of a full size Ag/alumina catalytic converter.     

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.     

A combination of Ag/alumina and Ag modified ZSM-5 to remove NOx and CO during lean conditions

Single and bi-metallic silver modified ZSM-5 catalysts were synthesized using different methods of preparation, characterized by several techniques and tested in simulated diesel conditions. Additionally the catalytic behaviour of the most active catalyst, containing 5 wt.% Ag and prepared by impregnation (5Ag(Imp)-H-ZSM-5), was studied over a broad temperature range with two reducing agents (octane and propene). To correlate the catalytic activity of the prepared catalysts with preparation parameters the materials were characterized by XRD, SEM, N₂-physisorption, octane/propene-TPD, EPR and ICP techniques. A dual bed system consisting of Ag/alumina and the most active zeolitic material (5Ag(Imp)-H-ZSM-5) was shown not only to substantially enhance the activity of Ag/alumina in the low temperature region, but also to completely oxidize the CO and unburned hydrocarbons.     

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.     

Toughening alumina with silver and zirconia inclusions

Both silver and zirconia inclusions are added into an alumina matrix, the strength and toughness of the composites are determined. The toughening agents prohibit the grain growth of the matrix, the strength of alumina is, therefore, enhanced. The addition of two toughening agents also enhances the toughness of alumina. The presence of Ag inclusions raises the transformation ability of ZrO₂; however, the toughness increase of the Al₂O₃–ZrO₂–Ag composites is slightly lower than the sum of the toughness increase of Al₂O₃–ZrO₂ and of Al₂O₃–Ag composites. The present study demonstrates that the toughening effects contributed by a transformation toughening agent and a ductile toughening agent can interact with each other; nevertheless, such interaction depends strongly on the microstructure of the composites.     

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.     

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.     

The role of AgOAl species in silver–alumina catalysts for the selective catalytic reduction of NOx with methane

We examined the role of silver and alumina in Ag–alumina catalysts for the selective catalytic reduction (SCR) of NO_x by methane in gas streams containing excess oxygen. A cogelation technique was used to prepare Ag–alumina materials with high dispersion of silver even at high metal loadings (>10 wt%) and after air calcination at 650 °C. Typically, a part of silver is present as fine nanoparticles on the alumina, whereas another part is ionic, bound with the alumina as [AgOAl] species. Dilute nitric acid leaching was used to remove the silver particles and all weakly bound silver from the surface of these materials. Complementary structural characterization was performed by HRTEM, XPS, XRD, and UV–vis DRS. We found that the higher the initial silver content, the higher the amount of the residual [AgOAl] species after leaching. NO–O₂-TPD tests identified that silver does not modify the surface properties of the alumina. The SCR reaction-relevant NO_x adsorption takes place on alumina. Temperature-programmed surface reaction (TPSR) and kinetic measurements at steady state were used to check the reactivity of the adsorbed NO_x species with methane and oxygen to form dinitrogen. Only the alumina-adsorbed nitrates react with CH₄ to produce N₂ in the presence of oxygen, beginning at ∼300 °C as found by TPSR. Moreover, the SCR reaction rates and apparent activation energies are the same for the leached and parent Ag–alumina catalysts. Thus, metallic silver nanoparticles are spectator species in CH₄-SCR of NO_x. These catalyze the direct oxidation of methane at temperatures as low as 300 °C, which explains the lower methane selectivity for the SCR reaction measured over the parent samples.     

Remote electro-precipitation of transparent ZnO on nano-porous alumina template

Zinc oxide (ZnO) thin film was deposited onto non-conducting alumina template by remote electrochemical and precipitation method. Chronoamperometry (i.e., constant potential mode) of −0.75 V (versus Ag/AgCl) was applied to deposit compact ZnO thin film onto the honeycomb shaped pores of a nano-porous alumina membrane. Analyzing morphological observations by SEM, we observed three distinct regions: (i) conducting platinum substrate; (ii) non-conducting alumina interlayer; (iii) thin film onto alumina template. XRD structural analysis of deposited materials onto alumina template and EDX analysis of alumina layer clearly indicated the formation of ZnO thin film was formed onto non-conducting alumina template not inside of alumina template. This remote-deposited ZnO showed more compact ZnO-hexagonal structure and a little bit higher transmittance than conventionally prepared ZnO onto ITO glass. Experimental observations were further discussed via the understanding of the mechanistic origin of ZnO formation onto alumina template.     

TiO2薄膜的制备、改性及光催化性能的研究

以铝片为基底,采用阳极氧化法制备氧化铝薄膜作为TiO₂的固定载体。用溶胶-凝胶法制备TiO₂溶胶,通过在氧化铝薄膜载体上浸渍提拉得到TiO₂膜。通过交流电沉积的方法对其掺杂贵金属改性,得到M/TiO₂复合膜。利用紫外-可见光谱法对复合膜进行表征。实验结果表明,利用Al₂O₃可以很好的负载TiO₂溶胶,500 ℃焙烧有利于形成催化能力较好的锐钛矿型TiO₂,掺杂贵金属Ag和Au的TiO₂试样对甲基橙的光催化效果明显优于未掺杂金属的TiO₂试样。     

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.     

Structural Analysis of Potential Active Sites in Metallo-Zeolites for Selective Catalytic Reduction of NO x . An Attempt for the Structure Versus Activity Relationship

The paper reviews the state of the art of the activity of metallo-zeolites and Ag/alumina in catalytic abatement of nitric oxides from exhaust gases from lean-burn combustion processes. The review is centered on the selective catalytic reduction of NO _x by hydrocarbons to molecular nitrogen (HC-SCR-NO _x ) over Co-, Fe- and Ag-zeolites, and Ag/alumina, i.e. those providing high and stable deNO _x activity in streams containing a high excess of water vapor existing under lean-burn combustion conditions. Analysis of the structure of these catalysts is described by employing a combination of spectral, diffraction, adsorption and redox techniques. An attempt is made to correlate the analyzed structure with the HC-SCR-NO _x activity.     

New Non Phyto‐ and Eco‐Toxic Alumina‐Stabilized Silver and Praseodymium Nanoparticles

In the present study, we examined how the active aluminum nano‐oxide in the gamma form used as a neutral carrier for the nanoparticles of various metals (as Ag, Pr) affected their toxic behavior. Our experiments have shown that exposure to metal nanoparticles can be reduced by binding the nanoparticles to alumina nanoparticles and the aluminum nano‐oxide is suitable to function as the nano‐stabilizer for the Ag and Pr nanoparticles. We have managed to manufacture new alumina‐stabilized silver and praseodymium nanoparticles using dry sol‐gel method that are not phyto‐ and eco‐toxic.     

Influence of electrochemical potential on the tribocorrosion behaviour of high carbon CoCrMo biomedical alloy in simulated body fluids by electrochemical impedance spectroscopy

The corrosion and tribocorrosion behaviour of a high carbon CoCrMo alloy sliding against alumina in simulated body fluids under potentiostatic conditions was investigated. The electrochemical behaviour of the sample in two electrolytes at different potentials (−1 V_Ag/AgCl, −0.5 V_Ag/AgCl, +0.05 V_Ag/AgCl, +0.5 V_Ag/AgCl and +0.75 V_Ag/AgCl) was studied by means of electrochemical impedance spectroscopy (EIS). The effects of solution chemistry and applied potential on the wear volume and anodic current were determined. Result shows that wear of CoCrMo alloy is negligible under cathodic and in the cathodic-anodic transition and considerably increases in the passive domain. Third body behaviour depends on surface chemistry which also varied depending on solution chemistry and electrochemically applied potential thus, modifies the tribocorrosion rate of CoCrMo alloy.     

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.     

Combinatorial preparation and high-throughput catalytic tests of multi-component deNOx catalysts

A quaternary catalyst library of 56 samples comprising all combinations of four elements, viz. Ag, Co, Cu, In, with six equally spaced atomic fraction increments from 0 to 1 was prepared by impregnation of a proprietary mesoporous alumina support. Catalytic properties of the library were tested in the selective catalytic reduction (SCR) of NO_x by propane under lean conditions in the temperature range 400–500 °C. The catalytic data acquired by a parallel 64-channel microreactor system with automated time-of-flight mass spectrometric analysis have been evaluated regarding selectivity–compositional relationships, synergistic effects for NO_x conversion, and efficiency of propane utilization. Full conversion of NO_x is achieved over Ag–Co combinations at 450 °C with N₂ selectivities of more than 90% and reductant utilization of 20% in a feed of 1500 ppm NO, 1500 ppm propane and 5 vol.% O₂ (space velocity of 36,000 cm³ g_cat⁻¹ h⁻¹). For the single-component catalysts Ag/Al₂O₃, Co/Al₂O₃, Cu/Al₂O₃, and In/Al₂O₃, the state of the elements on the mesoporous alumina was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Cobalt forms a spinel-like cobalt aluminate phase whereas copper and indium are present as oxides with small sizes not detectable by XRD. Silver occurs in both metallic state and as Ag₂O, and forms Ag_n clusters of at least two different sizes, predominantly with diameters of about 30 nm. The conclusions are consistent with the reducibility of the single-component catalysts samples by H₂. Surface area measurements and pore size distributions revealed reasonable modifications of the textural properties. The main pore size of the alumina support is decreased from 7 to ca. 5 nm after loading of the active components.     

Dielectric properties of alumina doped with TiO2 from 13 to 73 GHz

Recent studies show that alumina doped with TiO₂ exhibits promising dielectric properties, corresponding to low loss tangents and low temperature coefficients (or close to 0 ppmC⁻¹). This paper aims to confirm these trends and study the dielectric properties of alumina doped with TiO₂ from 0.5% to 12% wt. at high frequency, 13–73 GHz. This work demonstrates that alumina doped with TiO₂ corresponds to potential materials for frequency converter devices working at a high frequency (up to 50 GHz).     

Nanopatterning of silicon with use of self-organized porous alumina and colloidal crystals as mask

Nanopatterning processes based on a localized anodization of Si and the subsequent chemical etching of SiO₂ were developed to fabricate a dot array and a hole array on an Si surface using self-organized anodic porous alumina as a mask. Through the porous alumina mask, regularly arranged metal nanopatterns on the Si surface were fabricated by the electroless deposition of Cu nanodots in a CuSO₄/hydrofluoric acid (HF) solution. The periodicity of the Cu dot arrays was determined using the pore interval of the upper anodic alumina. Using self-assembled nanospheres as a mask for an electroless plating of metals such as Cu and Ag on the Si substrate, the patterning of an ordered honeycomb structure and a hexagonally arranged convex array of metals on Si was also developed by the combination of colloidal crystal patterning and wet chemical etching. The proposed patterning processes of the Si surface have potential technological applications in fields that need textured surfaces of controlled nanoscale periodicity and morphology owing to their relative simplicity and low cost.     

Interfacial Reaction Kinetics between Silver and Ceramic-Filled Glass Substrate

Interfacial reaction kinetics between Ag and ceramic-filled glass (CFG) substrate, containing borosilicate glass, high-silica glass, and alumina, has been investigated at 850°–925°C in different atmospheres. No chemical reaction at the interface of Ag/CFG is found when firing takes place in N₂ or N₂+ 1% H₂. Fired in air, however, an interfacial reaction zone is formed at the interface of Ag/CFG with Ag⁺ ion diffusing from silver and Al³⁺ ion dissolving from CFG, and both ions are always coupled together in the reaction zone. Microstructural and chemical analyses show that the reaction zone consists of two distinct layers; one is homogeneous, and the other, heterogeneous. The homogeneous layer, which is adjacent to Ag, is uniform in microstructure with a composition rich in Ag⁺ and Al³⁺. The heterogeneous layer is not uniform in microstructure with Si-rich and Ag–Al-rich phases. The reaction zone moves toward CFG with time, forming a heterogeneous layer first and then converting into a homogenous layer when diffusion of Ag⁺ ion into the CFG becomes significant. The growth kinetics for the homogeneous layer follows a linear rate equation, whereas the heterogeneous layer, a parabolic rate equation. Activation analyses suggest that the formation of the homogeneous layer is controlled by the combination of breakage and formation of M–O bonds, but the heterogeneous layer, by the diffusion of Ag⁺ ion in the BSG.