Silver Sites Research Articles

Carbon-Supported Gold and Silver Nanoparticles As Catalysts for Electroreduction of Oxygen in Alkaline Media

The present study refers to a novel and unique approach of fabrication and deposition of gold nanoparticles on the surfaces of reduced graphene oxide and multi-walled carbon nanotubes. Among important issues is application of inorganic (rather than organic) capping ligand, heteropolymolybdate, to modify and stabilize (as well as probably also to link with the oxygen or hydroxyl groups on graphene surfaces) gold nanoparticles. During operation in alkaline medium or neutral media, polyoxometallates disappear but catalytically highly active gold remains and it exhibits excellent stability. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of carbon dioxide in neutral medium and oxygen in alkaline solution. What is even more important is that both carbon nanotubes and graphene have occurred to act effectively as carriers for gold nanostructures. Mutual activating interactions are feasible. The conclusions are reached on the basis of diagnostic electrochemical (e.g. rotating ring disk voltammetry), spectroscopic (FTIR) and microscopic (SEM, TEM) experiments. A series of comparative experiments with different carbon carriers and model catalytic materials (e.g. Vulcan-supported platinum) have also been performed. With respect to oxygen reduction, our diagnostic experiments at different concentrations of H2O2, support a view that the effect of the fast following chemical (H2O2-reductive-decomposition) reaction could be the dominating factor in explaining the observed positive potential shift observed during the oxygen reduction. The fact, that the optimum graphene-based catalytic system produced the oxygen reduction peak current comparable to that observed at the model platinum containing catalyst, would imply the efficient four-electron-type reduction mechanism. Graphene and other distinct carbon nanostructures are explored as supports (carriers) for dispersed metallic silver nanocenters which exhibit electrocatalytic activity toward reduction of oxygen in alkaline media. To facilitate fabrication, immobilization and distribution of Ag(0) nanoparticles, various types of graphene (e.g. reduced graphene oxide, RGO, and CFx graphene with carbon black) have been modified with the silver analogue of polynuclear Prussian Blue, namely with ultra-thin silver(I) hexacyanoferrate(II,III) layers. Following the heat-treatment step at temperatures as high as 400-600oC, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic silver sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in KOH electrolyte. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when graphene nanostructures are combined or intermixed with Vulcan XC-72R nanoparticles. What is even more important that, due to the presence of the polynuclear cyanoferrate modifier or linker, the amounts of the undesirable hydrogen peroxide intermediate are significantly decreased.

Stability and Polaronic Motion of Self-Trapped Holes in Silver Halides: Insight through DFT+U Calculations

Polarons and their associated transport properties are a field of great current interest both in chemistry and physics. To further our understanding of these quasi-particles, we have carried out first-principles calculations of self-trapped holes (STHs) in the model compounds AgCl and AgBr, for which extensive experimental information exists. Our calculations confirm that the STH solely stabilizes in AgCl but with a binding energy of only 165 meV, an order of magnitude smaller than that found for the Vk center in KCl. Key contributions to this stabilization energy come from the local relaxation along breathing (a1g) and Jahn–Teller (eg) modes in the AgCl64– unit. To study the transfer of the STH among silver sites, we (i) use first-principles calculations to obtain the hopping barrier of the STH to first and second neighbors, involving eight distinct paths, using first-principles and (ii) construct a simple model, based on Slater–Koster parameters, that highlights the similarity of polaron transfer with m...

Role of PdAg interface in Pd-Ag/SiO2 bimetallic catalysts in low-temperature oxidation of carbon monoxide

An approach to the development of bimetallic nanoparticles with the separated but interacting phases of silver and palladium was proposed to effectively separate the active sites of silver, which possess high activity in activation of molecular oxygen, and the sites of palladium-containing phases, which are responsible for CO adsorption. The structure of PdAg particles was determined using X-ray diffraction analysis, electronic diffuse reflectance spectroscopy, and temperature-programmed reduction. It was found that the introduction of silver led to easier reduction of palladium from oxidized states in an atmosphere of CO. The activity of the bimetallic PdAg catalysts in the oxidation of CO increased, as compared with that of the Pd/SiO2 catalyst. The oxidized bimetallic catalyst, in which palladium occurred in an oxide state, was even more effective in the oxidation of CO; this can be related to the better cooperation of the active sites of silver and palladium oxide, which are responsible for the activation and conversion of CO and oxygen at the Ag-PdO interfacial boundary.

High ethene/ethane selectivity in 2,2'-bipyridine-based silver(i) complexes by removal of coordinated solvent.

Following removal of coordinated CH3 CN, the resulting complexes [Ag(I) (2,2'-bipyridine)][BF4 ] (1) and [Ag(I) (6,6'-dimethyl-2,2'-bipyridine)][OTf] (2) show ethene/ethane sorption selectivities of 390 and 340, respectively, and corresponding ethene sorption capacities of 2.38 and 2.18 mmol g(-1) when tested at an applied gas pressure of 90 kPa and a temperature of (20±1) °C. These ethene/ethane selectivities are 13 times higher than those reported for known solid sorbents for ethene/ethane separation. For 2, ethene sorption reached 90 % of equilibrium capacity within 15 minutes, and this equilibrium capacity was maintained over the three sorption/desorption cycles tested. The rates of ethene sorption were also measured. To our knowledge, these are the first complexes, designed for olefin/paraffin separations, which have open silver(I) sites. The high selectivities arise from these open silver(I) sites and the relatively low molecular surface areas of the complexes.

High Ethene/Ethane Selectivity in 2,2′‐Bipyridine‐Based Silver(I) Complexes by Removal of Coordinated Solvent

AbstractFollowing removal of coordinated CH3CN, the resulting complexes [AgI(2,2′‐bipyridine)][BF4] (1) and [AgI(6,6′‐dimethyl‐2,2′‐bipyridine)][OTf] (2) show ethene/ethane sorption selectivities of 390 and 340, respectively, and corresponding ethene sorption capacities of 2.38 and 2.18 mmol g−1 when tested at an applied gas pressure of 90 kPa and a temperature of (20±1) °C. These ethene/ethane selectivities are 13 times higher than those reported for known solid sorbents for ethene/ethane separation. For 2, ethene sorption reached 90 % of equilibrium capacity within 15 minutes, and this equilibrium capacity was maintained over the three sorption/desorption cycles tested. The rates of ethene sorption were also measured. To our knowledge, these are the first complexes, designed for olefin/paraffin separations, which have open silver(I) sites. The high selectivities arise from these open silver(I) sites and the relatively low molecular surface areas of the complexes.

Second Harmonic Generation Response Optimized at Various Optical Wavelength Ranges through a Series of Cubic Chalcogenides Ba6Ag2.67+4δSn4.33−δS16–xSex

A new series of metal chalcogenides with the formula Ba6Ag2.67+4δSn4.33−δS16–xSex (x = 0 for 1, x = 6.47 for 2, and x = 16 for 3) were synthesized. Compounds 1–3 are isotypic with a three-dimensional structure and crystallize in the noncentrosymmetric space group I4̅3d. The framework in 1 is constructed by alternative corner-sharing of AgS4 and SnS4 tetrahedra with Ba2+ cations distributed within the channels. The Ag+ atoms are disordered with two neighboring silver sites trapped in various coordination environments. Selected area electron diffraction (SAED) patterns from samples of 1 indicate that the structural lattice is ordered without evidence of long-range ordering. High resolution electron microscopy (HREM) images of 1 along the [001] direction display the body-centered arrangement of barium atoms and the periodic appearance of lines between barium atoms that correspond to part of the array of disordered silver atoms. Within the sulfide composition range, the solid solutions are Ba6Ag2.67+4δSn4.33−δS16 with δ = 0.13 for 1, 0.31 for 1a, and 0.33 for 1b; the crystal colors range from dark-red to orange and yellow, respectively. All compounds are transparent in the mid-infrared region and have absorption edges ranging from 1.27 to 2.37 eV. The significant second harmonic generation (SHG) intensities of the five materials are optimized at wavelengths ranging from 600 to 800 nm. The outstanding ones are 1a and 2 which display strong intensities 4.4–11.6 times and 2.2–10.4 times that of AgGaSe2 in the ranges 600–700 nm and 675–800 nm, respectively. Raman spectroscopic characterization is reported.

Quantification of Silver Sites in Zeolites: Carbon Monoxide Sorption Monitored by IR Spectroscopy

This work was aimed to provide a well-established approach for the quantification of silver sites in zeolites. The experimental procedure based on carbon monoxide sorption in silver exchanged zeolites ZSM-5, Y, and X was monitored by IR spectroscopy. With regard to the Lambert–Beer law the values of the absorption coefficients of the IR bands of CO interacting with exchangeable cations (Ag+) and metallic species (Ag0) were attained. Subsequently, the concentrations of silver species of different kinds were calculated. The values of the absorption coefficients are valid for zeolites of FAU and MFI topologies; thus, they are argued to be used for zeolites of other structures.

High-performance Ag-Co alloy catalysts for electrochemical oxygen reduction.

The electrochemical oxygen reduction reaction is the limiting half-reaction for low-temperature hydrogen fuel cells, and currently costly Pt-based electrocatalysts are used to generate adequate rates. Although most other metals are not stable in typical acid-mediated cells, alkaline environments permit the use of less costly electrodes, such as silver. Unfortunately, monometallic silver is not sufficiently active for economical fuel cells. Herein we demonstrate the design of low-cost Ag-Co surface alloy nanoparticle electrocatalysts for oxygen reduction. Their performance relative to that of Pt is potential dependent, but reaches over half the area-specific activity of Pt nanoparticle catalysts and is more than a fivefold improvement over pure silver nanoparticles at typical operating potentials. The Ag-Co electrocatalyst was initially identified with quantum chemical calculations and then synthesized using a novel technique that generates a surface alloy, despite bulk immiscibility of the constituent materials. Characterization studies support the hypothesis that the activity improvement comes from a ligand effect, in which cobalt atoms perturb surface silver sites.

C═C, C≡C, and C═O Bond Activation by Coinage Metal Cations in ZSM-5 Zeolites: Quantitative Charge Transfer Resolution

Adsorption and activation of ethene, ethyne, and formaldehyde by copper and silver sites in ZSM-5 are reinspected by quantitative assessment of the donation and backdonation processes between the three molecules and either models comprising bare cations (M+) or the cations embedded in zeolite framework (M(I)). ETS-NOCV analysis (decoupling the deformation density upon substrate bonding into independent components) reveals two predominant channels for electron transfer between adsorbed molecule and a cation or a cationic site, namely π*-backdonation from metal d orbitals to π* antibonding orbital of a substrate and σ-donation from π bonding or lone pair orbitals of the substrate to the cation. Alternative fragmentation of a complex modeling the cation embedded in zeolitic framework, treated as a three-part entity, allows for extracting predominant electron transfer channels from framework oxygens to the cation with bound substrate, namely, opposing σ-donation and supporting π*-backdonation. Critical analysis of charge flows between various parts of a complex system shows that effective activity of the cationic site comes from two contradictory processes and must be viewed as resulting from the framework effect on the sensitive balance between opposing electron transfer channels. Juxtaposition of density transfer channels between the substrate with CC and CO multiple bonds and either a bare metal cation or the cation embedded in a zeolite framework shows that σ-donation prevails over π*-backdonation for free cations, while for zeolitic sites their order is reversed. Quantitative analysis reveals that for title systems, activation decrease due to the reduction of σ-donation becomes outweighed by the increase in π*-backdonation for Cu(I) but not for Ag(I). Thus zeolitic framework, regarded as an electron reservoir, may either support (Cu+) or impair (Ag+) electronic processes underlying catalytic activation of multiple bonds.

Effect of Growth Velocity on Solute Distribution and Electrical Conductivity in a Directionally Solidified Cu–8 %Ag Alloy

Abstract A Cu–8 %Ag alloy was directionally solidified at different growth velocities to investigate the relationship between the solute distribution and the electrical conductivity of the material. The temperature gradient was fixed at 6.1 K/mm, and the growth rate was varied from 1 µ m/s to 200 µ m/s The planar interface was not observed in this experimental range, although cellular and dendritic interface morphologies were observed. As the growth velocity was increased, the initial silver content decreased, while the silver content near the dendrite boundary increased. The lattice parameter of the Cu – Ag alloy increased with increases in the number of silver sites in the copper lattice as measured via a calculation using the VASP (Vienna Ab-initio Package Simulation) code. The lattice parameter also decreased with increases in the growth velocity due to the decrease in silver content in the matrix. Higher electrical conductivity was obtained with higher growth velocities.

Microkinetic modeling of H2-assisted NO oxidation over Ag–Al2O3

A microkinetic model has been assembled to investigate the mechanism for NO oxidation over a monolith-supported Ag–Al2O3 catalyst both in the presence and absence of H2. The effect of H2 examined in the kinetic model was to reduce self inhibiting surface nitrate species on active silver sites. A reduced factorial design of inlet experimental conditions was used to generate transient experimental data. The kinetic model was developed based on a single channel reactor model which accounted for mass and heat transfer between gas and catalyst washcoat as well as mass transport resistance in the washcoat. In general, the modeling results could reproduce the transient experimental data well with correct levels of outlet concentrations and time scales for transient responses. It was found that the effect of increased NO and NO2 inlet concentration had a negative correlation with the NO oxidation conversion, which in the model was considered related to the formation of nitrate surface species. In addition, the model in agreement with experiments, clearly showed that H2 promoted the NO oxidation mainly at low temperature and this effect tended to decrease at elevated temperatures. When H2 was present in the feed, the kinetic model showed that H2 was consumed rapidly in the front part of the monolith. This was also seen in the experiments where in all cases H2 was entirely consumed. The rapid reaction of H2 along with resulting transport limitations indicated that the H2 promotion of the NO oxidation reaction may have been isolated to only a portion of the catalyst.

On the symmetry and crystal structure of aguilarite, Ag4SeS

AbstractAn examination of a specimen of aguilarite from the type locality provides new data on the chemistry and structure of this mineral. The chemical formula of the crystal used for the structural study is (Ag3.98 Cu0.02)(Se0.98 S0.84 Te0.18), on the basis of 6 atoms. The mineral was found to be monoclinic, crystallizing in space group P 21/n, with a = 4.2478(2), b = 6.9432(3), c = 8.0042(5) Å , β = 100.103(2)º, V = 232.41(2) Å3 and Z = 4. The crystal structure [refined to R1 = 0.0139 for 958 reflections with I > 2 σ (I)] is topologically identical to that of acanthite, Ag2S. It can be described as a body-centred array of tetrahedrally coordinated X atoms (X = S, Se and Te) with Ag2X3 triangles in planes nearly parallel to (010); the sheets are linked by the Ag1 silver site, which has twofold coordination.Aguilarite is definitively proved to be isostructural with acanthite; it does not have a naumannite-like structure, as previously supposed. Our data support the hypothesis that there are two solid solution series in the system: a monoclinic 'acanthite-like' series (from Ag2S - Ag2S0.4Se0.6), and an orthorhombic 'naumannite-like' series (from Ag2S0.3Se0.7-Ag2Se). This is supported by data gathered on synthetic counterparts. Aguilarite remains as a valid as a mineral species, but it should be described as the Se-analogue of acanthite.In this study we also (1) review the history of the aguilarite; (2) compare properties of synthetic and natural aguilarite; and (3) demonstrate how earlier researchers erred in describing aguilarite as orthorhombic.The Te-bearing composition of the studied aguilarite crystal suggests the possibility of a solid solution with cervelleite (Ag4TeS).

The photocatalytic inactivation effect of Ag–TiO2 on β-amyloid peptide (1–42)

Abstract Due to its low toxicity and excellent biocompatibility, titanium dioxide and its alloyed are widely used in biomedical applications. Furthermore, TiO2 can be excited by UV light to create charge carriers giving rise to photocatalytic redox reactions at its surface and photo-induced super-hydrophilicity. In this work, TiO2 films were modified with silver (Ag–TiO2) by the photocatalytic reduction of Ag+ from solution. The films and the adsorption of β-amyloid (1–42), were studied using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscopy (AFM). These films were confirmed to contain mainly anatase crystallites and the photo-reduced Ag was predominantly Ag0 (>0%). Ag loading of the TiO2 markedly enhanced the Raman signal (ca. 15-fold), but caused significant changes to the protein spectrum indicating non-specific binding of β-amyloid side chain residues to the silver. The amide modes remained well-resolved and were used to estimate the conformational change induced in the protein by the silver. Raman analysis showed an increase in the intensity of the band at ∼1665 cm−1 assigned to the disordered conformation of the β-amyloid, suggesting that the adsorption to the silver sites induces conformational changes. Contaminated surfaces were exposed to UVB irradiation, caused further conformational changes in the β-amyloid, which mildly inhibited amyloid fibril formation, thought to be induced through a photocatalytic mechanism.

Silver(i) complexes of tris(pyrazolyl)borate ligands bearing six trifluoromethyl and three additional electron-withdrawing substituents

Sodium salts of two new tris(pyrazolyl)borates [HB(4-Cl-3,5-(CF3)2Pz)3](-) and [HB(4-(NO2)-3,5-(CF3)2Pz)3](-), which are not only highly fluorinated, but also loaded with additional electron-withdrawing substituents, have been synthesized by reacting 4-Cl-3,5-(CF3)2PzH or 4-(NO2)-3,5-(CF3)2PzH with NaBH4 under nitrogen in a solventless process, and isolated after a work up in tetrahydrofuran (THF) or diethyl ether (Et2O), as their THF or Et2O adducts. Metathesis of these sodium salts with AgOTf in THF leads to [HB(4-Cl-3,5-(CF3)2Pz)3]Ag(THF) and [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(THF). The corresponding cis-cyclooctene (c-COE) complexes [HB(4-Cl-3,5-(CF3)2Pz)3]Ag(c-COE) and [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE) were obtained by displacing THF with cis-cyclooctene. The related [HB(3,5-(CF3)2Pz)3]Ag(c-COE) can also be obtained via a similar process. X-ray crystal structures show that [HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE) (R = H, Cl, NO2) feature pseudo-tetrahedral silver sites supported by κ(3)-bound tris(pyrazolyl)borate ligands. [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE) displays the smallest upfield shift of the alkene carbon peak (versus free alkene) followed by [HB(4-Cl-3,5-(CF3)2Pz)3]Ag(c-COE) and [HB(3,5-(CF3)2Pz)3]Ag(c-COE). Experimental and computational data indicate very electron poor silver(I) sites in all three [HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE) adducts with minimal Ag → (cis-cyclooctene) backbonding. Although the impact of R (H, Cl, NO2) on the alkene carbon chemical shift of these adducts and the olefin π/π* populations is small, the (13)C NMR chemical shifts and NBO analysis suggest that [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE) possesses the most electrophilic metal site, which correlates with the pKa values of the free pyrazoles. [HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE) adducts effectively catalyze the insertion of the carbene moiety of ethyl diazoacetate into C-H bonds of 2,3-dimethylbutane. The [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE) catalyst shows a higher selectivity for primary C-H bonds compared to the reactions catalyzed by [HB(3,5-(CF3)2Pz)3]Ag(c-COE).

CO oxidation on nanoporous gold: A combined TPD and XPS study of active catalysts

Disks of nanoporous gold (np-Au), produced by leaching of silver from AgAu alloy and prepared as active catalysts for CO oxidation in a continuous-flow reactor, were investigated in detail by x-ray photoelectron spectroscopy and temperature-programmed desorption spectroscopy in ultra-high vacuum. Np-Au exhibits several oxygen species on and in the surface: Chemisorbed oxygen (Oact), probably generated at residual silver sites at the surface, is readily available after np-Au preparation and consumed by CO oxidation. It can be replenished on activated np-Au by exposure to O2. In addition, strongly bound oxygen, probably at subsurface sites, is present as a major species and not consumed by CO oxidation. Pronounced CO desorption at temperatures above 200K observed after exposing np-Au to CO at 105K indicates an additional, more stable type of CO binding sites on np-Au as compared to pure gold. Only CO at these binding sites is consumed by oxidation reaction with Oact. It is proposed that the presence of strongly bound subsurface oxygen stabilizes CO adsorption on np-Au, thereby being as crucial for the observed catalytic activity of np-Au as residual silver.

Intimate contact of enolic species with silver sites benefits the SCR of NOx by ethanol over Ag/Al2O3

Alumina-supported silver (Ag/Al2O3) catalysts with different silver loadings were investigated for the selective catalytic reduction of NOx by ethanol. The catalytic role of silver in the formation of enolic species was studied by ultraviolet–visible spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy combined with mass spectrometry, density functional theory calculations, and kinetic measurements. It was found that the enolic species originating from the partial oxidation of ethanol over Ag/Al2O3 prefer to adsorb on or close to silver sites, in intimate contact with the active phase. This adsorption behavior of this enolic species contributes to its high activity for the formation of isocyanate species (NCO) and the final product N2 during the NOx reduction by ethanol over Ag/Al2O3. Surface acetate also formed during this process, while only interacting with Al sites, resulting in a lower activity for NCO formation. Meanwhile, the mechanism of the NOx reduction by ethanol was discussed.

Noncovalent interaction directing polyoxometalate assembly based on the coordination flexibility of Cu and Ag

Two new polyoxometalate supramolecular compounds based on the coordination flexibility of the last transition metal Cu and Ag, formulated as [Cu(phen)(H 2O) 3] 0.5[Cu(phen) 2(H 2O)][PMo 12O 40] ( 1) and [Ag(phen) 3][Ag(phen) 2] 3[VW 12O 40]·4H 2O ( 2) (phen = 1,10-phenanthroline), have been synthesized hydrothermally and characterized by IR, TG, XPS and X-ray diffraction analyses. In 1 two kinds of Cu(II) centres present trigonal bipyramid geometries with different environments of {Cu(1)N 4O} and {Cu(2)N 2O 3}. The silver(I) sites in 2 exhibit unusual six-coordinated {Ag(1)N 6} and usual four-coordinated {Ag(2)N 4} and {Ag(3)N 4}. There is a trimer {[Ag(phen) 2] 3} 3+ constructed from two Ag(2) bridged by one Ag(3) with the near distance of Ag⋯Ag = 3.310 Å.

Comparative studies of silver based catalysts supported on different supports for the oxidation of formaldehyde

Temperature programmed desorption (TPD) and temperature programmed surface reaction (TPSR) were employed to study the HCHO adsorption–desorption and the surface reaction on Ag based catalysts supported on different supports (MCM-41, SBA-15, NaY, SiO 2 and TiO 2). According to the analysis of XRD, UV–Vis and H 2-TPR for the as-prepared catalysts, there are four kinds of silver species on the supports, including metallic Ag particles (Ag 0) with subsurface oxygen species, isolated Ag + ions, well dispersed Ag 2O and A g n δ + clusters. Moreover, the states, the dispersion and the amount of these silver species seem to be obviously different on different supports. No activity for HCHO oxidation is observed on any supports, and silver particles with some oxygen in bulk were more active for the adsorption and activation of HCHO. MCM-41 and SiO 2 supported silver catalysts give new adsorption sites for HCHO, and it is easily activated at lower temperatures and shows better surface reaction activity for HCHO oxidation. The desorption temperature for HCHO over SBA-15 and NaY supported silver catalysts is relatively higher, and its reaction performance is worse. However no surface reaction is observed on Ag/TiO 2 catalyst during experiments. It is also found that HCHO molecular adsorbed on silver sites could be was oxidized into dioxymethylene (DOM) and formate surface species, and they reacts with oxygen to form CO 2 during TPSR. The activities of different silver based catalysts for HCHO oxidation were closely related to their abilities for the formation of DOM and formate species.

Promoted activity of the selective catalytic reduction of NO x with propene by H 2 addition over a metal-monolithic anodic alumina-supported Ag catalyst

The effect of H 2 addition on the selective catalytic reduction of NO x with propene (C 3H 6-SCR of NO x ) was investigated using a metal-monolithic anodic alumina-supported Ag catalyst. The de-NO x activity of the Ag/Al 2O 3 catalyst at low temperature was markedly enhanced by the addition of a small amount of H 2; the enhancement was associated with the improved propene activation at low temperatures. The addition of H 2 greatly promoted the nitrate decomposition at low temperatures, which was believed to alleviate nitrate poisoning and to initiate the SCR reaction at low temperatures, because of the formation of free silver sites. Furthermore, it is suggested that a small amount of H 2 addition will further reduce the freed Ag + species into Ag n δ+ and will eventually lead to the improved de-NO x activity, because Ag n δ+ is more active for the propene activation than Ag + at low temperatures. However, excess H 2 and high Ag loading mainly promoted the nonselective combustion of propene with oxygen, rather than the selective oxidation of propene with NO x . Additionally, a combination of this H 2 promotion effect and electrical heating produced a favorable de-NO x system with a quick response, a wide temperature-window and a high de-NO x activity.

A Metal-Monolithic Anodic Alumina-Supported Ag Catalyst for Selective Catalytic Reduction of NOx with Propene in the Presence of Hydrogen

The effect of H2 addition on the selective catalytic reduction of NOx with propene (C3H6-SCR of NOx) has been investigated using a metal-monolithic anodic alumina-supported Ag catalyst. The low temperature de-NOx activity of the Ag/Al2O3 catalyst is markedly enhanced by the addition of a small amount of H2, which is associated with the improved propene activation at low temperatures. The addition of H2 greatly promotes the nitrate decomposition at low temperatures, which is believed to alleviate nitrate poisoning and initiate the SCR reaction at low temperatures, because of the formation of free silver sites. Furthermore, it is suggested that a small amount of H2 addition will further reduce the freed Ag+ species into Agnδ+ and eventually lead to the improved de-NOx activity, because Agnδ+ is more active in propene activation than Ag+ at low temperatures. However, excess H2 and high Ag loading mainly promoted the nonselective combustion of propene with oxygen, rather than the selective oxidation of propene with NOx.