Active Ag species in MFI zeolite for direct methane conversion in the light and dark

Photo-induced methane conversion was examined over Ag-MFI zeolite at room temperature. On the oxidized Ag-MFI zeolite, containing Ag⁺ exchanged cations, huge amounts of methane were adsorbed, even in the dark, and then converted to mainly ethane upon photo-irradiation without H₂ production. It was revealed that Ag _n ⁺ small clusters were formed at the expense of Ag⁺ ion during this photoreaction, and probably hydrogen would be stored as H⁺ on the ion-exchange sites instead of Ag⁺. On the other hand, the reduced sample containing larger clusters converted methane into alkene even without photo-irradiation.     

Formation mechanism of nanostructured Ag films from Ag2O particles using a sonoprocess

Nanostructured Ag films composed of nanoparticles and nanorods can be formed by the ultrasonication of ethanol solutions containing Ag₂O particles. The present work examined the formation process of these films from ethanol solutions by two different agitation methods, including ultrasonication and mechanical stirring. The mass-transfer process from Ag₂O particles to ethanol solvent is accelerated by the mechanical effects of ultrasound. Ag⁺ ions and intermediately reduced Ag clusters were released into the ethanol. These Ag⁺ ions and Ag clusters provide absorption bands at 210, 275 and 300 nm in UV-vis spectra. These bands were assigned to the absorption of Ag⁺, Ag ₄ ²⁺ and Ag_n (n?≈?3). The Ag_n clusters that readily grow to become Ag nanoparticles were formed due to the surface reaction of Ag₂O particles with ethanol under ultrasonication. The reactions of Ag⁺ ions in ethanol to form Ag nanomaterials (through the formation of Ag ₄ ²⁺ clusters) were also accelerated by ultrasonication.     

Density functional study of structural and electronic properties of maximum‐spin n+1Aun−1Ag clusters

The structures and relative stabilities of high‐spin ⁿ⁺¹Au_n?1Ag and ⁿAu_n?1Ag⁺ (n = 2–8) clusters have been studied with density functional calculation. We predicted the existence of a number of previously unknown isomers. Our results revealed that all structures of high‐spin neutral or cationic Au_n?1Ag clusters can be understood as a substitution of an Au atom by an Ag atom in the high‐spin neutral or cationic Au_n clusters. The properties of mixed gold–silver clusters are strongly sized and structural dependence. The high‐spin bimetallic clusters tend to be holding three‐dimensional geometry rather than planar form represented in their low‐spin situations. Silver atom prefers to occupy those peripheral positions until to n = 8 for high‐spin clusters, which is different from its position occupied by light atom in the low‐spin situations. Our theoretical calculations indicated that in various high‐spin Au_n?1Ag neutral and cationic species, ⁵Au₃Ag, ³AuAg and ⁵Au₄Ag⁺ hold high stability, which can be explained by valence bond theory. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

The Geometric Structure of Silver‐Doped Silicon Clusters

Cationic silver‐doped silicon clusters, Si_nAg⁺ (n=6–15), are studied using infrared multiple photon dissociation in combination with density functional theory computations. Candidate structures are identified using a basin‐hopping global optimizations method. Based on the comparison of experimental and calculated IR spectra for the identified low‐energy isomers, structures are assigned. It is found that all investigated clusters have exohedral structures, that is, the Ag atom is located at the surface. This is a surprising result because many transition‐metal dopant atoms have been shown to induce the formation of endohedral silicon clusters. The silicon framework of Si_nAg⁺ (n=7–9) has a pentagonal bipyramidal building block, whereas the larger Si_nAg⁺ (n=10–12, 14, 15) clusters have trigonal prism‐based structures. On comparing the structures of Si_nAg⁺ with those of Si_nCu⁺ (for n=6–11) it is found that both Cu and Ag adsorb on a surface site of bare Si_n⁺ clusters. However, the Ag dopant atom takes a lower coordinated site and is more weakly bound to the Si_n⁺ framework than the Cu dopant atom.     

Photochemical properties of Ag ion clusters included within ZSM-5 zeolites prepared by an ion-exchange method

Photoluminescence investigations of the Ag ion-exchanged ZSM-5 (Ag⁺ /ZSM-5) zeolite revealed that a Ag ion cluster (Agⁿ _m ⁺) exists in the pore structure of ZSM-5 exhibiting photoluminesm cence at 380 nm upon excitation at 332 nm. UV irradiation ( = 285 nm) of Ag⁺ /ZSM-5 at 77 K leads to the transformation of Agⁿ _m ⁺ into a different Ag ion cluster (Ag_m ^(n-1)+) which exhibits photoluminescence at 465 nm upon excitation at 315 nm. This photo-transformation of the Ag ion clusters was found to be thermally reversible under vacuum. It was demonstrated that an electron transfer from the photo-excited Al³⁺ -O^2- to Agⁿ _m ⁺ plays a significant role in this process. In the presence of oxygen, UV irradiation of Ag⁺ /ZSM-5 leads to the formation of O₂- instead of an Ag ion cluster (Ag_m ^(n-1)+), suggesting that oxygen acts as an efficient electron scavenger, which interferes with the electron capture of Agⁿ _m ⁺ under UV irradiation at 285 nm.     

Quantum Chemical Calculations on the Structure and Adsorption Properties of NO and N2O on Ag+ and Cu+ Ion-Exchanged Zeolites

Ab initio cluster quantum chemical calculations at the Hartree–Fock (HF/Lanl2dz) and correlated second-order Moller–Plesset perturbation theory (MP2/Lanl2dz) levels were performed for NO and N₂O interactions with Ag⁺ and Cu⁺ ion-exchanged zeolites. The interaction energies were estimated in a conventional way and also corrected for basis set superposition errors. It was shown that the highly dispersed Ag⁺ counterions establish twofold coordination to the lattice oxygens on the zeolite surface, similar to the case of Cu⁺ ions. However, both NO and N₂O bind relatively strongly to the Cu active sites of Cu⁺ ion-exchanged zeolites than those of the Ag⁺ site of the Ag⁺ ion-exchanged zeolites. Based on the results of these calculations, the two different forms of adsorption for these molecules on the catalyst surface, the nature of their binding and characteristics of the adsorption properties have been discussed. Finally, some comparisons with the results obtained by a variety of density functional theory calculations on target systems have been presented.     

Photoluminescence properties of Ag2S semiconductor clusters synthesized in micropores and mesopores

Silver sulfide (Ag₂S) clusters were synthesized in microporous zeolites and mesoporous AlMCM-41 by the sulfurization of Ag⁺ ions exchanged within the pores of the host. Characterization was performed by means of XRD, UV-Vis Ag K-edge XAFS and photoluminescence. The pore size of the host has great effects on the photoluminescence properties. Ag₂S/AlMCM-41 showed photoluminescence at the longer wavelengths than Ag₂S/zeolites. The photoluminescence band of Ag₂S in the zeolite with 1-dimensional channels was narrow in comparison to the Ag₂S in zeolites with 2- or 3-dimensional channels.     

Synthesis of silver nanoparticles supported on silica aerogel using gamma radiolysis

Silver clusters on SiO₂ support have been synthesized using ⁶⁰Co gamma radiation. The irradiation of Ag⁺ in aqueous suspension of SiO₂ in the presence of 0.2 mol dm⁻³ isopropanol resulted in the formation of yellow suspension. The absorption spectrum showed a band at 408 nm corresponding to typical characteristic surface plasmon resonance of Ag nanoparticles. The effect of Ag⁺ concentration on the formation of Ag cluster indicated that the size of Ag clusters vary with Ag⁺ concentration, which was varied from 4×10⁻⁴ to 5×10⁻³ mol dm⁻³. The results showed that Ag clusters are stable in the pH range of 2–9 and start agglomerating in the alkaline region at pH above 9. The effect of radiation dose rate and ratio of Ag⁺/SiO₂ on the formation of Ag clusters have also been investigated. The prepared clusters have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which showed the particle size of Ag clusters to be in the range of 10–20 nm.     

A two-dimensional bimetallic oxide constructed from ζ-octamolybdate clusters and Ag(I)-tpyprz cationic polymer components (tpyprz = tetra-2-pyridylpyrazine)

The hydrothermal reaction of Na₂MoO₄, Ag(CH₃CO₂), tetra-2-pyridylpyrazine (tpyprz), HF and water at 170 °C for 48 h yields [ Ag₄(tpyprz)₂(H₂O) Mo₈O₂₆] (1) as colorless plates. The structure of 1 is a two-dimensional network, constructed from Ag₄(tpyprz)₂(H₂O) ⁴ⁿ⁺_n chains linked through ζ- clusters. The chains exhibit three distinct Ag(I) environments: a four coordinate AgN₄ site, a three coordinate AgN₂O_aqua site, and four coordinate AgN₃O_oxo site which links to the molybdate cluster. Each cluster is connected by bridging oxo-groups to two silver/tpyprz chains to form the network architecture. Crystal data: C₂₄H₁₇Ag₂Mo₄N₆O_13.5, fw 1204.94, orthorhombic Pccn, a=20.8234(9) Å, b=14.5076(6) Å, c=20.3537(9) Å, V=6148.9(5) Å³, Z=8, D_calc=2.603 g cm⁻³.     

The study of distribution coefficients of silver and mercury with chelating ion-exhange resin Dowex A-1

The distribution coefficients (DC) for HgCl ₄ ^2– , Hg(SO₄) ₂ ^2– , Hg(NO₃) ₄ ^2– , Ag⁺, Ag(SCN) ₂ ^– and Ag(NH₃) ₂ ⁺ between aqueous solutions and Dowex A-1 were measured in varying hydrogen ion concentrations. The DC of Ag⁺ in the NO ₃ ^– media was very low (4 to 6). The DC for the Ag(SCN) ₂ ^– complex decreased as pH increased. The Ag(NH₃) ₂ ⁺ complex had a constant DC of about 65 from pH 8 and above. The trend observed for three mercury complexes in HCl, H₂SO₄ and HNO₃ was similar; the DC decreased steadily from 0.1M to 6M. The HgCl ₄ ^2– complex had the highest DC (9000) while the Hg(NO₃) ₄ ^2– complex had the lowest DC (2000).     

Acidity versus metal-induced Lewis acidity in zeolites for Friedel–Crafts acylation

Acid catalysts including Ni, Ag and Fe-loaded zeolites of different structures were prepared either via cationic exchange or impregnation techniques from pristine H-zeolites (BEA, and MFI). Their catalytic activity was evaluated in the liquid-phase Friedel–Crafts acylation of anisole with propanoic acid. It turned out that, whatever the doping procedure was, the zeolite loaded with transition metals led to considerable decrease in propanoic acid conversion, regardless of the nature or the metal content. However, the extent of this detrimental effect followed the order: Ag⁺ > Ni²⁺ > Fe³⁺.Pristine acidic zeolites were not only found to be the most active, but also to be the most selective toward ortho- and para-acylation products. H-ZSM-5 zeolites yielded the highest intrinsic activity, with TOF values of 0.09 h⁻¹. The catalyst activity proved to be essentially attributed to the density and accessibility of Brønsted acid sites, playing a key role in the activation of the reactants. Brønsted sites are proposed to be the most likely catalytic species for performing this Friedel–Crafts acylation.     

Olefin Recovery by *BEA-Type Zeolite Membrane: Affinity-Based Separation with Olefin−Ag+ Interaction

Ag⁺ was introduced into *BEA-type zeolite membrane by an ion-exchange method to enhance olefin selectivity. Ag−*BEA membrane exhibited superior olefin separation performance for both ethylene/ethane and propylene/propane mixtures. Particularly, the separation factor for ethylene at 373 K reached 57 with the ethylene permeance of 1.6×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹. Adsorption properties of olefin and paraffin were evaluated to discuss contribution of Ag⁺ to separation performance enhancement. A strong interaction between olefin and Ag⁺ in the membrane caused preferential adsorption of olefin against paraffin, leading to selective permeation of olefin. Ag−*BEA membrane also exhibited high olefin selectivities from olefin/N₂ mixtures. The affinity-based separation through Ag−*BEA membrane showed a high potential for olefin recovery and purification from various gas mixtures.     

Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters

The homoleptic group 5 carbonylates [M(CO)₆]⁻ (M=Nb, Ta) serve as ligands in carbonyl-terminated heterobimetallic Ag_mM_n clusters containing 3 to 11 metal atoms. Based on our serendipitous [Ag₆{Nb(CO)₆}₄]²⁺ ( 4 a ²⁺) precedent, we established access to such Ag_mM_n clusters of the composition [Ag_m{M(CO)₆}_n]^x (M=Nb, Ta; m=1, 2, 6; n=2, 3, 4, 5; x=1−, 1+, 2+). Salts of those molecular cluster ions were synthesized by the reaction of [NEt₄][M(CO)₆] and Ag[Al(OR^F)₄] (R^F=C(CF₃)₃) in the correct stoichiometry in 1,2,3,4-tetrafluorobenzene at −35 °C. The solid-state structures were determined by single-crystal X-ray diffraction methods and, owing to the thermal instability of the clusters, a limited scope of spectroscopic methods. In addition, DFT-based AIM calculations were performed to provide an understanding of the bonding within these clusters. Apparently, the clusters 3 ⁺ (m=6, n=5) and 4 ²⁺ (m=6, n=4) are superatom complexes with trigonal-prismatic or octahedral Ag₆ superatom cores. The [M(CO)₆]⁻ ions then bind through three CO units as tridentate chelate ligands to the superatom core, giving overall structures related to tetrahedral AX₄ ( 4 ²⁺) or trigonal bipyramidal AX₅ molecules ( 3 ⁺).     

Dependence of the catalytic activity of Ag/Al2O3 on the silver concentration in the selective reduction of NOx with n-hexane in the presence of H2

The activity of 0.25–5% Ag/Al₂O₃ catalysts in the selective catalytic reduction of nitrogen oxides with n-hexane under the conditions of promotion with a small amount of H₂ was studied. It was found that, upon the introduction of ∼1000 ppm of H₂ into the reaction mixture, the Ag/Al₂O₃ samples containing 1–2% Ag exhibited optimum activity and selectivity. It was established that, in the presence of 1000 ppm of H₂, the rate of the selective catalytic reduction of NO _x was higher by a factor of 10–13, and the onset temperature of the reaction was lower by approximately 100°C. It was found by X-ray photoelectron spectroscopy, temperature-programmed reduction, and UV spectroscopy that the high activity of 1–2% Ag/Al₂O₃ catalysts was due to the presence of small Ag _n ^δ+ and Ag _m ⁰ clusters on their surface. A decrease in the concentration of Ag below the optimum value resulted in the predominance of an inactive ionic form on the catalyst surfaces. As the concentration of Ag was increased (>2%), large particles of Ag₂O and Ag⁰, which facilitate the oxidation of n-C₆H₁₄, were formed to lead to a decrease in selectivity and in the degree of reduction of nitrogen oxides.     

Dynamics and counterion-dependence of the structures of weakly bound Ag+-P4S3 complexes

In an earlier publication (J. Am. Chem. Soc. 2002 , 124, 7111) we showed that polymeric cationic [Ag(P₄S₃)_n]⁺ complexes (n=1, 2) are accessible if partnered with a suitable weakly coordinating counterion of the type [Al(OR^F)₄]^? (OR^F: poly‐ or perfluorinated alkoxide). The present work addresses the following questions that could not be answered in the initial report: How many P₄S₃ cages can be bound to a Ag⁺ ion? Why are these complexes completely dynamic in solution in the ³¹P NMR experiments? Can these dynamics be frozen out in a low‐temperature ³¹P MAS NMR experiment? What are the principal binding sites of the P₄S₃ cage towards the Ag⁺ ion? What are likely other isomers on the [Ag(P₄S₃)_n]⁺ potential energy surface? Counterion influence: Reactions of P₄S₃ with Ag[Al{OC(CH₃)(CF₃)₂}₄] (Ag[hftb]) and Ag[{(CF₃)₃CO}₃Al‐F‐Al{OC(CF₃)₃)}₃] (Ag[al‐f‐al]) gave [(P₄S₃)Ag[hftb]]_∞ ( 7 ) as a molecular species, whereas [Ag₂(P₄S₃)₆]²⁺[al‐f‐al]^?₂ ( 8 ) is an isolated 2:1 salt. We suggest that a maximum of three P₄S₃ cages may be bound on average to an Ag⁺ ion. Only isolated dimeric dications are formed with the largest cation, but polymeric species are obtained with all other smaller aluminates. Thermodynamic Born–Haber cycles, DFT calculations, as well as solution NMR and ESI mass spectrometry indicate that 8 exhibits an equilibrium between the dication [Ag₂(P₄S₃)₆]²⁺ (in the solid state) and two [Ag(P₄S₃)₃]⁺ monocations (in the gas phase and in solution). Dynamics: ³¹P MAS NMR spectroscopy showed these solid adducts to be highly dynamic, to an extent that the ²J_P,P coupling within the cages could be resolved (J‐res experiment). This is supported by DFT calculations, which show that the extended PES of [Ag(P₄S₃)_n]⁺ (n=1–3) and [Ag₂(P₄S₃)₂]⁺ is very flat. The structures of α‐ and γ‐P₄S₃ were redetermined. Their variable‐temperature ³¹P MAS NMR spectra are discussed jointly with those of all four currently known [Ag(P₄S₃)_n]⁺ adducts with n=1, 2, and 3.     

Time‐Resolved Assembly of Cluster‐in‐Cluster {Ag12}‐in‐{W76} Polyoxometalates under Supramolecular Control

We report the time‐resolved supramolecular assembly of a series of nanoscale polyoxometalate clusters (from the same one‐pot reaction) of the form: [H_(10+m)Ag₁₈Cl(Te₃W₃₈O₁₃₄)₂]_n, where n=1 and m=0 for compound 1 (after 4 days), n=2 and m=3 for compound 2 (after 10 days), and n=∞ and m=5 for compound 3 (after 14 days). The reaction is based upon the self‐organization of two {Te₃W₃₈} units around a single chloride template and the formation of a {Ag₁₂} cluster, giving a {Ag₁₂}‐in‐{W₇₆} cluster‐in‐cluster in compound 1 , which further aggregates to cluster compounds 2 and 3 by supramolecular Ag‐POM interactions. The proposed mechanism for the formation of the clusters has been studied by ESI‐MS. Further, control experiments demonstrate the crucial role that TeO₃^2?, Cl^?, and Ag⁺ play in the self‐assembly of compounds 1 – 3 .     

Reactivity Control of C?H Bond Activation over Vanadium–Silver Bimetallic Oxide Cluster Cations

Vanadium–silver bimetallic oxide cluster ions (V_xAg_yO_z⁺; x=1–4, y=1–4, z=3–11) are produced by laser ablation and reacted with ethane in a fast‐flow reactor. A reflectron time of flight (Re‐TOF) mass spectrometer is used to detect the cluster distribution before and after the reactions. Hydrogen atom abstraction (HAA) reactions are identified over VAgO₃⁺, V₂Ag₂O₆⁺, V₂Ag₄O₇⁺, V₃AgO₈⁺, V₃Ag₃O₉⁺, and V₄Ag₂O₁₁⁺ ions, in which the oxygen‐centered radicals terminally bonded on V atoms are active sites for the facile HAA reactions. DFT calculations are performed to study the structures, bonding, and reactivity. The reaction mechanisms of V₂Ag₂O₆⁺+C₂H₆ are also given. The doped Ag atoms with a valence state of +1 are highly dispersed at the periphery of the V_xAg_yO_z⁺ cluster ions. The reactivity can be well‐tuned gradually by controlling the number of Ag atoms. The steric protection due to the peripherally bonded Ag atoms greatly enhances the selectivity of the V–Ag bimetallic oxide clusters with respect to the corresponding pure vanadium oxide systems.     

Crystal Structure and Thermal Stability of Ag3(CHF2COO)3(H2O)2

X-ray diffraction analysis of [Ag₃(CHF₂COO)₃(H₂O)₂] revealed that its crystals are orthorhombic: space group Cmca, a = 13.809(4) Å, b = 15.975(2) Å, c = 12.244(2) Å, Z = 8. The thermogravimetric analysis showed that under the atmosphere of N₂ and at 101.3 kPa, silver difluoroacetate melts at 488 K; the thermal decomposition reaction occurs in the interval 493–548 K with the formation of Ag. Under the mass-spectral experiment conditions at 521 K, two processes occur simultaneously, namely, evaporation and decomposition. The following ions were detected in the mass-spectrum of silver difluoroacetate: Ag₂L⁺, Ag₂R⁺, Ag₂F⁺, Ag₂O⁺, Ag₂ ⁺, Ag⁺, LH⁺, RCO⁺, R⁺ (L = CHF₂COO, R = CHF₂).     

Enhancement mechanism of SERS from cyanine dyes adsorbed on Ag2O colloids

Surface enhancement mechanism of Raman scattering from molecules adsorbed on silver oxide colloids is reported. Absorption spectra and Raman spectra of the cyanine dye D266 and pyridine molecules adsorbed on Ag₂O colloids, and the influences of S₂O₃²⁻ and OH⁻ on the SERS are studied respectively. The results indicate that ‘chemical' enhancement is dominant in Ag₂O colloidal solution. Surface complexes of adsorbed molecules and small silver ion clusters Ag_n⁺ as the SERS active sites make an important contribution to surface enhanced Raman scattering (SERS). At these active sites, charge transfer between the adsorbed molecules and the small silver ion clusters is the main enhancement origin. The enhancement factor of D266 adsorbed on Ag₂O colloids is theoretically estimated with the excited-state charge transfer model, which is roughly in accordance with the experiments.     

Kinetics of oxidation of pyruvic acid by [ethylenebis(biguanide)]silver(III) in aqueous acidic media

The oxidation of pyruvic acid by the title silver(III) complex in aqueous acidic (pH, 1.1–4.5) media is described. The reaction products are MeCO₂H and CO₂, together with a colourless solution of the Ag⁺ ion. The free ligand, ethylenebis(biguanide) is released in near-quantitative yield upon completion of the reduction. The parent complex, [Ag(H₂L)]³⁺ and one of its conjugate bases, [Ag(HL)]²⁺, participate in the reaction with both pyruvic acid (HPy) and the pyruvate anion (Py^–) as the reactive reducing species. Ag⁺ was found to be catalytically inactive. At 25.0°C, I=1.0moldm^–3, rate constants for the reactions [Ag(H₂L)]³⁺+HPy (k ₁), [Ag(H₂L)]³⁺+Py^– (k ₂), [Ag(HL)]²⁺+HPy (k ₃) and [Ag(HL)]²⁺+Py^– (k ₄) arek ₁=(94±6)×10^–5dm³mol^–1s^–1, (k ₂ K _a+k ₃ K _a1)= (1.3±0.1)×10^–5s^–1 and k ₄=(58±4)×10^–5dm³mol^–1s^–1, respectively, where K _a1is the first acid dissociation constant of the [Ag(H₂L)]³⁺ and K _a is for pyruvic acid. A comparison between the k ₁ and k ₄ values is indicative of the judgement that k ₂k ₃. A one-electron inner-sphere redox mechanism seems more justified than an outer-sphere electron-transfer between the redox partners.