Unsupported Ag Research Articles

Role of Interfacial Hydrogen in Ethylene Hydrogenation on Graphite-Supported Ag, Au, and Cu Catalysts.

A combined surface science/microreactor approach was applied to examine interface effects in ethylene hydrogenation on carbon-supported Ag, Au, and Cu nanoparticle catalysts. Turnover frequencies (TOFs) were substantially higher for supported catalysts than for (unsupported) polycrystalline metal foils, especially for Ag. Spark ablation of the corresponding metals on highly oriented pyrolytic graphite (HOPG) and carbon-coated grids yielded nanoparticles of around 3 nm size that were well-suited for characterization by X-ray photoelectron spectroscopy (XPS), high-resolution (scanning) transmission electron microscopy (HRTEM/STEM), and energy dispersive X-ray spectroscopy (EDX). Polycrystalline metal foils characterized by scanning electron microscopy (SEM), EDX, electron backscatter diffraction (EBSD), XPS, and low-energy ion scattering (LEIS) served as unsupported references. Employing a UHV-compatible flow microreactor and gas chromatography (GC) allowed us to determine the catalytic performance of the model catalysts in ethylene hydrogenation up to 200 °C under atmospheric pressure. Compared to the pure metal foils, the HOPG-supported metal nanoparticles exhibited not only strongly increased activity but also higher stability (slower deactivation) and differing reaction orders. For the most active Ag catalysts, DFT calculations were carried out to determine the adsorption energies of the reacting species on single-crystal surfaces as well as on carbon-supported and unsupported Ag nanoparticles. Adsorption of molecular hydrogen was very weak on all unsupported Ag surfaces, resulting in hydrocarbon-"poisoned" surfaces. However, when a carbon support was present, the adsorption strength of H2 on Ag nanoparticles increased on average by -0.5 eV, driven by changes in Ag-Ag distances near the metal-carbon three-phase boundary (whereas subsurface carbon lowers hydrogen bonding). On Cu particles, the interface effect was calculated to be somewhat weaker than for Ag particles. H2/D2 scrambling experiments on Ag catalysts then corroborated a facilitated hydrogen activation for carbon-supported metals. Thus, the carbon support effect is attributed to an improved hydrogen availability at the metal-carbon interface, controlling performance.

Increasing bromide removal by graphene-silver nanocomposites: Nanoparticulate silver enhances bromide selectivity through direct surface interactions

Bromide forms toxic brominated disinfection by-products during disinfection. Current bromide removal technologies are often non-specific and costly due to naturally occurring competing anions. A silver-impregnated graphene oxide (GO) nanocomposite is reported here that reduced the amount of Ag needed for Br− removal by increasing its selectivity towards Br−. GO was impregnated with ionic (GO-Ag+) or nanoparticulate Ag (GO-nAg) and compared against Ag+ or unsupported nAg to identify molecular level interactions. In nanopure water, Ag+ and nAg had the highest Br− removal (∼0.89 mol Br−/mol Ag+) followed by GO-nAg at 0.77 mol Br−/mol Ag+. However, under anionic competition, the Ag+ removal was reduced to 0.10 mol Br−/mol Ag+ while all nAg forms retained good Br− removal. To understand the removal mechanism, anoxic experiments were performed to prevent nAg dissolution, which resulted in higher Br− removal for all nAg forms compared to oxic conditions. This suggests that reaction of Br− with the nAg surface is more selective than with Ag+. Finally, jar tests showed that anchoring nAg on GO enhances Ag removal during coagulation/flocculation/sedimentation compared to unsupported nAg or Ag+. Thus, our results identify strategies that can be used to design selective and silver-efficient adsorbents for Br− removal in water treatment.

Metallophilicity versus π-π Interactions: Argentophilicity in Porphyrin Dimer

The interaction between two unsupported Ag(III)porphyrins, which exist as a metalloporphyrin dimer has been studied. The UV-visible spectrum shows only slight change in Soret band position indicative of Ag(III) formation. Optimized structure of the complexes has clearly revealed that only metal-centered oxidation results in short Ag-N (porphyrin) distance with large distortion in the porphyrin macrocycle. The dispersion effect brings two rings more closer to form an unprecedented Ag(III)···Ag(III) interaction and cancel out the π-π repulsion. The interaction energy was found to be 55.34 Kcal/mol and was further supported by electron localization function, AIM analysis. The Ag(III) dimer also gives pink emission at low temperature. This type of interaction can be exploited to design several light responsive materials and molecular semiconductors.

Aerobic Oxidation of 5-Hydroxymethylfurfural over Ag Nanoparticle Catalysts Stabilized by Polyvinylpyrrolidone with Different Molecular Weights.

The metal–support interaction (MSI) has a remarkable effect on the catalytic properties, but how to precisely modulate its degree remains a huge challenge. Herein, polyvinylpyrrolidone (PVP) with three different molecular weights (MWs) (24, 58, and 130 kDa) was used as a capping agent to fabricate Ag nanoparticles (NPs) supported on ZrO2. The physiochemical properties of the catalysts were characterized by X-ray diffraction (XRD), Transmission Electron Microscope (TEM), X-ray Photoelectron Spectroscopy (XPS), and Fourier transform infrared (FT-IR) techniques. The impacts of MSI on the catalytic activity and reaction kinetics for aerobic oxidation of 5-hydroxymethylfurfural (HMF) were investigated. The results showed that the introduction of PVP with various MWs could efficiently tailor the interfacial interactions and charge transfers (CT) among PVP, the support, and Ag NPs, thereby affecting the oxidation activity of HMF. The turnover number (TON) for HMF oxidation decreases in the order of unsupported colloidal Ag clusters > Ag/ZrO2 (58,000) > Ag/ZrO2 (130,000) > Ag/ZrO2 (24,000) > Ag/ZrO2. The reason for this large difference in the catalytic activity for HMF oxidation is that various MWs of PVP result in a change of MSI, thereby facilitating CT from PVP to Ag metal sites. This study offers a new strategy for modulating MSI by varying the MW of capping agents, thereby tuning the catalytic properties in the oxidation of HMF.

Ag–Co oxides nanoparticles supported on carbon nanotubes as an effective catalyst for the photodegradation of Congo red dye in aqueous medium

Multiwalled carbon nanotubes supported Ag–Co oxides (A-MWNTs/Ag–Co) and unsupported Ag–Co bimetallic oxides (Ag–Co) nanoparticles (NPs) were prepared by chemical reduction method. The A-MWCNTs/Ag–Co and Ag–Co NPs are characterized by various instrumental techniques like scanning electron microscopy (SEM), energy dispersive X-rays (EDX) and X-rays diffraction (XRD). While the photodegradation study of Congo red was carried out by UV/Vis spectrophotometer. The A-MWCNTs/Ag–Co and Ag–Co NPs were used as photocatalyst for the photodegradation of Congo red dye as a function of irradiation time. The fresh A-MWCNTs/Ag–Co degraded about 90.37% dye while the recovered photocatalyst degraded 64.01% within 75 min. Similarly the original Ag–Co NPs degraded about 88.09% within 75 min while the recovered Ag–Co NPs degraded about 59.65% within the same irradiation time. The effect of photocatalyst dosage, initial dye concentration and pH of the medium was also evaluated.

Characterization of Cellulose/Silver Nanocomposites Prepared by Vegetable Oil-Based Microemulsion Method and Their Catalytic Performance to 4-Nitrophenol Reduction

In this work, Cellulose/silver nanocomposites were synthesized through a double vegetable oil-based microemulsion method. Ionic liquid 1-ethyl-3-methylimidazolium acetate ([Emim]Ac) or water was used as the polar phase to prepare [Emim]Ac (water) /Triton X-100 + n-butanol/castor oil microemulsions, respectively. The structure of the microemulsions was investigated by the pseudo-ternary phase diagrams and dynamic light scanning data. The microstructure, morphological characteristics, size, thermal stability, and catalytic performance of the as-prepared cellulose/silver nanocomposites were characterized through X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, thermogravimetric analysis, and UV–Visible spectrophotometry. Results revealed that the spherical silver nanoparticles (Ag NPs) dispersed homogeneously on the surface of cellulose regenerated from ionic liquid, and the average size of Ag NPs in the nanocomposites could be adjusted by controlling the AgNO3 concentration and the molar ratio of total polar phase to surfactant. The catalytic activity of cellulose/silver nanocomposites were examined by reducing 4-Nitrophenol to 4-Aminophenol and they displayed much higher activities than the unsupported Ag NPs. This approach is effective for the production of cellulose-supported metal nanoparticles, which offers promise in heterogeneous catalysis.

Theoretical modeling of argentophilic interactions in [Ag(CN)2−]3 trimer found in a copper(II) complex of cis-1,2-diaminocyclohexane (Dach), [Cu(Dach)2-Ag(CN)2-Cu(Dach)2][Ag(CN)2

Crystal structure study of a copper(II) complex of cis-1,2-diaminocyclohexane (Dach), [Cu(Dach)2-Ag(CN)2-Cu(Dach)2][Ag(CN)2]3 (1) reveals the presence of unsupported Ag⋯Ag⋯Ag interactions with the interatomic Ag⋯Ag distance of 3.230 Å. Detailed HF, MP2 and BLYP-D3 theoretical investigations of [Ag(CN)2−]3 trimer, together with AIM, NCI and EDA analyses verified these interactions as noncovalent AgAg bonds with the magnitude of a weak hydrogen bond, mainly electrostatically and dispersively driven.

Synthesis, structures and antimicrobial activities of silver(I) complexes derived from 2-propyl-1H-imidazole-4,5-dicarboxylic acid

Two new silver(I) complexes based on propyl-substituted imidazole-4,5-dicarboxylic acid, {[Ag2(Hpimdc)(H2O)](H2O)}n (1) and {[Ag2(Hpimdc)(pda)](2H2O)}n (2) (H3pimdc=2-propyl-1H-imidazole-4,5-dicarboxylic acid; pda=1,2-propanediamine) were synthesized and structurally characterized. The single-crystal X-ray determination reveals that in complexes 1 and 2 the silver atoms are linearly coordinated. Doubly deprotonated µ3-Hpimdc2− ligand in 1 behaves as an N,O-bridging coordiation mode to offer an infinite 1D linear chain, while µ2-Hpimdc2− in 2 adopts an N,N′-bridging mode to give an infinite 1D zigzag chain in the presence of pda as an auxiliary ligand. The adjacent 1D chains in 1 are further linked to form a 2D layer structure by ligand unsupported Ag⋯Ag interactions (Ag1⋯Ag2#=3.088Å and Ag2⋯Ag2#=3.098Å). Antimicrobial testing indicates that complex 2 with MIC100 being 0.19mM was more active against Gram-negative bacteria Escherichia coli (E. coli) than complex 1 with MIC100 being 0.28mM. Complexes 1 and 2 have the slightly different influence on the growth of Gram-positive bacteria Staphylococcus aureus (S. aureus) (MIC100 being 0.28mM for 1 and 0.24mM for 2). Both 1 and 2 exhibit good inhibitory activities against jack bean urease with the respective IC50 values of 1.68±0.07 and 1.85±0.14µM.

The silver(I) coordination polymer [AgO2PPh2]n and unsupported Ag⋯Ag interactions derived from aminophosphinate and phosphinic acid

The ligand N-tert-butyl-1,1-diphenylphosphinamine, Ph2P-N(H)CMe3, was prepared from tert-butylamine and diphenylchlorophosphine. The phosphine portion of the ligand became partially oxidized in air to form Ph2P(O)N(H)CMe3, L1, whilst further oxidation led to diphenylphosphinic acid, Ph2P(O)OH. X-ray analysis revealed that ligand L1 was isolated in the solid-state in a dimeric polymorphic form, different from the previously reported trimeric form. Ligand L1 was subsequently treated with solid AgSO3CF3 in THF which formed a rare silver(I) dinuclear complex of the type [Ag(CF3SO3){OPPh2N(H)CMe3}2{Ag(OPPh2N(H)CMe3)2}]SO3CF3, 1, and consists of ligand unsupported Ag⋯Ag interactions of 2.89Å coordinating through two O donor atoms from the two separate silver units (A and B) and with no coordination through the N atom. Further, silver unit A contains a 3-coordinate Ag(I) center, bent significantly from a linear geometry due to interaction from the triflate O donor atom, whilst unit B remained essentially 2-coordinate and linear. The diphenylphosphinate reacted with solid AgSO3CF3 in THF at room temperature and this led to an unusual Ag(I) coordination polymer, [Ag2{(μ-O)OPPh2}{O2PPh2}]n2, consisting of two different bonding modes of O-donor atoms in a 4-coordinate arrangement around the Ag(I) center. Compounds L1, 1 and 2 were all obtained in moderate to good yields, and analyzed by single crystal X-ray studies, solution 1H and 31P NMR, IR, and elemental analyses.

Epoxidation of propylene by molecular oxygen over unsupported AgCu x bimetallic catalyst

The unsupported Cu and Ag catalysts with different oxidation states were prepared, and their catalytic performances for propylene epoxidation were investigated. The metallic Cu catalyst exhibits much higher catalytic activity and propylene oxide (PO) selectivity than Cu2O and CuO catalysts. The Cu0 species are the main active sites for propylene epoxidation, but Cu2O and CuO species are in favor of CO2 and acrolein production. The PO selectivity of 54.2 % and propylene conversion of 2.6 % can be achieved over the metallic Cu catalyst at 160 °C in initial stage, but metallic Cu catalyst would be oxidized to Cu2O during propylene epoxidation, resulting in a sharp decrease in the PO selectivity and propylene conversion. Nanosize AgCu x bimetallic catalysts were prepared. It is found that adding Ag to the metallic Cu catalysts can prevent the oxidation of Cu and make AgCu x bimetallic catalysts more stable under the condition of propylene epoxidation. The Ag/Cu molar ratio can remarkably affect the catalytic performance of AgCu x catalyst and the selectivity to PO and acrolein. After AgCu x was supported on MO x -modified α-Al2O3, its catalytic performance can be improved and has a close relationship with the acid–base property of support.

One- and three-dimensional silver(I)-5-sulfosalicylate coordination polymers having ligand-supported and unsupported argentophilic interactions

Four new coordination polymers, namely, {Hemim·[Ag(Hssa)(H2O)]}n (1), {[Ag(ina)2 Ag(Hssa)]·CH3OH·H2O}n (2), {[Ag2(Hssa)(dmp)1.5]·2H2O}n (3) and [Ag2(Hssa)(daoc)]n (4) (Hssa: 5-Sulfosalicylate, emim: 2-ethyl-4-methylimidazole, ina: isonicotinamide, dmp: 2,5-dimethylpyrazine and daoc: 1,8-diaminooctane) were synthesized and characterized by IR spectroscopy, elemental analysis, single crystal X-ray diffraction, powder X-ray diffraction (PXRD) and thermal analysis techniques. Complexes 1 and 2 are one-dimensional (1D) coordination polymers while complexes 3 and 4 are three-dimensional (3D) coordination polymers. Complex 3 consists of three dimensional (3D) 3,3,6-c net with 3,3,6T37 topology. Complex 4 exhibits a 2-fold interpenetrating 3D framework with tfc topology. Complexes 1–4 contain ligand-supported (1–3) and unsupported (4) argentophilic Ag⋯Ag interactions. Photoluminescence spectra of the complexes demonstrate that photoluminescent properties may be attributed to intraligand transition of coordinated Hssa ligand.

Highly efficient hydrogen production from formaldehyde over Ag/γ-Al2O3 catalyst at room temperature

Herein, we demonstrate a simple and efficient process for generating hydrogen from the formaldehyde (HCHO) aqueous solution catalyzed by Ag nanoparticles dispersed on high specific surface area γ-Al2O3 at room temperature. Moreover, this Ag/γ-Al2O3 catalyst exhibits much higher capability and stability for hydrogen production than unsupported Ag nanoparticles. By further optimizing the structure, component, and amounts of Ag/γ-Al2O3 catalysts as well as reaction parameters such as reaction atmosphere, formaldehyde concentrations, and NaOH concentrations, the hydrogen generation rate could be greatly increased and maintained for ten hours without any decay. It may provide a general and favorable strategy for the fabrication of highly reactive and stable metal catalyst for the hydrogen production from organic aldehyde solutions.

Unsupported nanoporous Ag catalysts towards CO oxidation

This work discussed the catalytic performance towards CO oxidation over unsupported nanoporous Ag (np-Ag) catalysts synthesized by dealloying of Mg77Ag23 (at.%) alloy under controllable etching conditions. The np-Ag catalysts were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The np-Ag catalysts exhibit an open, bicontinuous interpenetrating ligament-channel structure. The np-Ag samples fabricated under different conditions possess distinct feature sizes ranging from ∼15.7 to ∼142.6nm. Without any heat-treatment, the as-obtained np-Ag catalysts were utilized for catalysis tests. The results reveal that the catalytic activity of the unsupported np-Ag catalysts is comparable with or even better than that of supported Ag catalysts with heat-treatment. In addition, the role of residual Mg and feature size in the catalytic activity of the np-Ag catalysts has been clarified. The catalytic mechanism of the present unsupported np-Ag catalysts has also been discussed.

Room-temperature synthesis and enhanced catalytic performance of silver-reduced graphene oxide nanohybrids

The synthesis of supported, ultrasmall metallic nanoparticles (NPs) is of great importance for catalytic applications. In this study, silver-reduced graphene oxide nanohybrids (Ag–rGO NHs) were prepared by reducing Ag ions and graphene oxide (GO) at room temperature using sodium borohydride (NaBH4) and trisodium citrate. The resulting products were characterized using UV–Vis spectroscopy, X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy. The rich chemistry of GO surface provided many sites for the nucleation of Ag ions and efficiently limited their growth. Ag NPs were uniformly grown on basal planes of rGO with a high density (~1,700 NPs μm−2) and well-defined size (3.6 ± 0.6 nm) as evidenced in SEM and HRTEM studies. The resulting Ag–rGO NHs were readily dispersed in water and exhibited enhanced catalytic activity toward the reduction of 4-nitrophenol by NaBH4 in comparison to unsupported Ag NPs. The role of rGO as an excellent support for Ag catalyst is discussed.

Experimental and theoretical evidence of unsupported Ag–Ag interactions in complexes with triazine-based ligands. Subtle effects of the symmetry of the triazine substituents

The synthesis of several dimeric silver compounds containing bis(pyrazol-1-yl)-6-(R)-s-triazine or bis(3,5-dimethylpyrazol-1-yl)-6-(R)-s-triazine ligands and coordinated triflate is described. The structures of two of the compounds have been determined by X-ray diffraction. Both derivatives show ligand-unsupported argentophilic contacts and different supramolecular interactions such as π–π stacking, CH–π or anion–π interactions. Hydrogen bonds are also established between the monomers. The relative orientation of the two monomers, which is mainly influenced by the formation of hydrogen bonds, reflects the symmetry of the R substituent of the triazine ring. Theoretical DFT calculations and Natural Bond Order (NBO) analysis of the two dimers further support the existence of the argentophilic interactions. Differences between the optimized and real structures can be mainly attributed to the underestimation of the π–π stacking interaction in the DFT studies.

A discrete water hexamer with a new planar tetrameric water moiety trapped in the crystal host of [Ag(azelate)(4,4′-bipyridine)]·(H2O)3

Abstract The ultrasonic reaction of Ag2O, bipy and H2aze gave rise to a novel Ag(I) mixed-ligand coordination polymer, namely [Ag(aze)(bipy)]·(H2O)3 (1) (bipy = 4,4′-bipyridine, H2aze = azelaic acid). In 1, Ag(I) ions are linked by bipy and aze ligands to form a single two-dimensional (2D) undulated net with a (6,3) topology, incorporating Ag6(bipy)4(aze)2 windows of 22.58 × 11.06 A based on Ag⋯Ag distances. A pair of identical 2D single nets are interconnected via π⋯π stacking and unsupported Ag⋯Ag interactions to generate a 2D double-layered net. A discrete water hexamer composed of a new planar tetrameric water ring and two pendent water molecules is perpendicularly located in each hydrophilic cavity of the 2D bilayer and acts as a ‘glue’ to assemble adjacent 2D double-layered nets into a three-dimensional (3D) structure.

Assembly of 1D, 2D and 3D silver(I) coordination polymers with nitrilotriacetate and 2-aminopyrimidyl mixed ligands

Three mixed ligands coordination polymers (CPs) [Ag1.5(apym)(nta)0.5]n (1), [(NH4)Ag2(mapym)(nta)·(H2O)3]n (2), [Ag2(dmapym)3(Hnta)]n (3) (apym=2-aminopyrimidine, mapym=4-methyl-2-aminopyrimidine, dmapym=4, 6-dimethyl-2-aminopyrimidine, H3nta=nitrilotriacetate) were synthesized and characterized. For 1–3, as the substituents change from H to one methyl and two methyl groups, the dimensionalities of 1–3 decrease from three-dimension (3D) to one-dimension (1D) due to the steric effect of methyl groups. For 1, the μ2-apym ligands link the Ag(I) ions to form a 1D double-chain incorporating ligand unsupported Ag···Ag interaction. The nta3− ligands extend the 1D double-chain into a 3D framework. In 2, one heptadentate nta3− ligand binds four Ag(I) ions and incorporates μ2-mapym ligand to link metal centers to form a 2D sheet which can be simplified to be a 103 net. Complex 3 features a 1D chain structure incorporating Hnta2− and monodentate dmapym ligands. The substituents on the pyrimidyl ring intensively influence the coordination environments of metal ion and the coordination modes of the carboxyl group, and thus determine the structures of the CPs. The photoluminescent properties of 1–3 were also investigated.

Silver/silicon nanostructure for surface-enhanced fluorescence of Ln3+ (LnNd, Ho, and Er)

The surface-enhanced fluorescence of lanthanide ions (neodymium ions Nd3+, holmium ions Ho3+, and erbium ions Er3+) owing to resonant plasmons oscillation on the surface of Ag/Si nanostructure was described. In the presence of Ag/Si nanomaterials, the fluorescence peaks were significantly enhanced, which resulted in a typical 185-fold enhancement at 592 nm for Nd3+, 82-fold at 550 nm for Ho3+, and 80-fold at 533 nm for Er3+ at the concentration of 0.05 M. This Ag/Si nanostructure had larger enhancement factor than that caused by unsupported Ag nanoparticles, which might be attributed to the local field overlap originated from the closed and fixed Ag nanoparticles on silicon nanowires.

The fabrication of silver-modified silicon nanowires and their excellent catalysis in the decomposition of fluorescein sodium

Silver-modified silicon nanowires were obtained and employed as catalysts in the decomposition of fluorescein sodium using sodium borohydride (SB) as a reducing agent. Their decomposition rate enhanced ca. 6 times compared to that of unsupported Ag nanoparticle catalysts, which demonstrated the excellent catalytic activity of silver-modified silicon nanowires.

Quantum stabilities and growth modes of thin metal films: Unsupported and NiAl-supported Ag(1 1 0) and Ag(1 0 0)

We present density functional theory (DFT) analyses of the stability of Ag thin films versus film thicknesses for various surface orientations. We include benchmark results for freestanding films, but consider in detail Ag(1 1 0) films supported on a NiAl(1 1 0) substrate, and Ag(1 0 0) films supported on a NiAl(1 0 0) substrate. The supported films exhibit an almost perfect lattice-match between film and substrate surface unit cells, so one can assess film stability in the absence of significant lateral mismatch strain. We also provide a characterization of film growth modes for these NiAl-supported Ag films based on DFT results for the relevant energetics.