Silver Catalyst Research Articles

Bifunctional Hierarchical Zeolite‐Supported Silver Catalysts for the Conversion of Glycerol to Allyl Alcohol

AbstractThe establishment of suitable processes for the conversion of glycerol into allyl alcohol is hindered by the fast deactivation of solid acids in the dehydration of the substrate to acrolein and by the requirement of hydrogen donors to enhance the selectivity of the subsequent reduction step. In this work, silver nanoparticles deposited onto a hierarchical ZSM‐5 zeolite are proved to be an effective bifunctional catalyst to conduct the two reactions in the gas phase and in the presence of hydrogen by using a continuous fixed‐bed reactor. The acidic function was accomplished by using a ZSM‐5 zeolite modified by facile alkaline and acid treatments, which decreased the amount of Lewis acid centers while preserving the amount of Brønsted acid centers, and introduced an auxiliary network of intracrystalline mesopores, thus boosting the selectivity to acrolein (62 %) and the resistance to coking. Upon screening of various metals supported on the aluminosilicate, silver was identified as a superior hydrogenation catalyst, enabling a relatively high activity with >50 % allyl alcohol selectivity. Tuning of the metal loading, temperature, pressure, and contact time led to 15 % yield of allyl alcohol, thus approaching the state‐of‐the‐art transfer hydrogenation systems, and stable behavior for 100 h on stream. Our results highlight the advantage of conducting the two transformations over a bifunctional material rather than over two separate single‐function solids.

Gold and Silver Catalysts for Liquid Phase n-Octanol Oxidation: Effect of Promoters

Background: In comparison with these platinum group metal catalysts, supported nano gold catalysts have emerged as a new type of catalysts highly active at low temperature and selective for the aerobic oxidation of alcohols. But catalytic data and detailed characterizations of the Au-Ag bimetal catalytic system are not available up to date. Keywords: n-Octanol oxidation, Au-Ag bimetal catalysts, redox treatment, support modification.

Post Plasma-Catalysis of Low Concentration VOC Over Alumina-Supported Silver Catalysts in a Surface/Packed-Bed Hybrid Discharge Reactor

In this study, the degradation of benzene by the means of an optimized surface/packed-bed hybrid discharge (SPBHD) plasma combined with γ-Al2O3-supported MO x (M = Ag, Mn, Cu, or Fe) catalysts in post plasma-catalysis (PPC) system. The effects of Ag loading amount and gas hourly space velocity (GHSV) for plasma-catalysis degradation of benzene have been systematically investigated. The experimental result showed that the benzene degradation was improved and the mineralization process was greatly enhanced towards total oxidation after the combination of plasma with all MO x /γ-Al2O3 catalysts. The AgO x /γ-Al2O3 catalyst exhibited the best catalytic activity in benzene degradation than the other catalysts in PPC system. The highest benzene degradation efficiency of 96% and CO x selectivity of 99% can be obtained for AgO x /γ-Al2O3 catalyst with optimum Ag loading amount and GHSV of 15% and 22,856 h−1, respectively. Time course of benzene degradation during PPC process indicated that the plasma-induced catalytic activity of AgO x /γ-Al2O3 catalyst was temporary rather than lasting over a period after the plasma off. FT-IR analysis results revealed that the intermediate products (such as CO, HCOOH) and unwanted by-products (O3 and NO x ) generated in plasma process could be significantly inhibited by PPC process with AgO x /γ-Al2O3 catalyst.

Local Structures and Catalytic Ammonia Combustion Properties of Copper Oxides and Silver Supported on Aluminum Oxides

The local structures and catalytic NH3 combustion properties of copper oxides (CuOx) and silver (Ag) catalysts supported on aluminum oxides (Al2O3) were studied. In order to achieve high catalytic NH3 combustion activity and high N2 (low N2O/NO) selectivity, the preparation conditions for impregnated binary catalysts were optimized. In comparison with the single CuOx/Al2O3 and Ag/Al2O3, binary CuOx–Ag supported on Al2O3 showed high performance for catalytic NH3 combustion. Among the binary catalysts, sequentially impregnated CuOx/Ag/Al2O3 exhibited the highest activity and N2 selectivity. Because the combustion activity is closely associated with the Ag–Ag coordination number estimated from Ag K-edge XAFS, highly dispersed Ag nanoparticles supported on Al2O3 are considered to play a key role in the low-temperature light-off of NH3. CuOx/Ag/Al2O3 also showed higher N2 (lower NO) selectivity for temperatures at which NH3 conversion reached approximately 100%, indicating that the N2 is directly produced from...

Single Silver Adatoms on Nanostructured Manganese Oxide Surfaces: Boosting Oxygen Activation for Benzene Abatement.

The involvement of a great amount of active oxygen species is a crucial requirement for catalytic oxidation of benzene, because complete mineralization of one benzene molecule needs 15 oxygen atoms. Here, we disperse single silver adatoms on nanostructured hollandite manganese oxide (HMO) surfaces by using a thermal diffusion method. The single-atom silver catalyst (Ag1/HMO) shows high catalytic activity in benzene oxidation, and 100% conversion is achieved at 220 °C at a high space velocity of 23 000 h-1. The Mars-van Krevelen mechanism is valid in our case as the reaction orders for both benzene and O2 approach one, according to reaction kinetics data. Data from H2 temperature-programmed reduction and O core-level X-ray photoelectron spectra (XPS) reveal that Ag1/HMO possesses a great amount of active surface lattice oxygen available for benzene oxidation. Valence-band XPS and density functional theoretical calculations demonstrate that the single Ag adatoms have the upshifted 4d orbitals, thus facilitating the activation of gaseous oxygen. Therefore, the excellent activation abilities of Ag1/HMO toward both surface lattice oxygen and gaseous oxygen account for its high catalytic activity in benzene oxidation. This work may assist with the rational design of efficient metal-oxide catalysts for the abatement of volatile organic compounds such as benzene.

An alumina-supported silver catalyst with high water tolerance for H2 assisted C3H6-SCR of NOx

Water vapor is typically present in diesel engine exhausts, and thus the design of catalysts with high water-tolerance is highly desired. The addition of water vapor was shown to have quite different influences on the activity of Ag/Al2O3 catalysts with different Ag loadings during the H2-assisted C3H6-SCR of NOx (H2-C3H6-SCR). The 2wt% Ag/Al2O3 catalyst showed the best activity for H2-C3H6-SCR, with excellent water resistance over the whole temperature range. An enhancement in NOx conversion was observed after water vapor was introduced, particularly at low temperatures. Over this catalyst, kinetic studies confirmed that H2O addition did not change the apparent activation energy for NOx reduction, while it increased the reaction order of C3H6 from −0.62 to 0.73. This result indicated that the reaction pathway for NOx reduction was hardly changed by the introduction of water vapor, while a poisoning effect related to C3H6 oxidation was decreased. In situ DRIFTS studies and DFT calculations revealed that water vapor significantly inhibits the formation of inert formate during the H2-C3H6-SCR process, and thus more sites are available for the formation of active enolic species and acetates, finally leading to increased activity for 2wt% Ag/Al2O3 in NOx reduction.

Silver-catalyzed direct selenylation of terminal alkynes through C[sbnd]H bond functionalization

We described herein the use of a silver catalyst in the selenylation of terminal alkynes by direct CH functionalization. These reactions were performed under mild conditions, reacting a range of terminal alkynes with different diorganyl diselenides using AgNO3 (10mol-%) as catalyst in CH2Cl2 at 25°C under air atmosphere. This is an efficient and general cross-coupling reaction and was performed with alkynes and diorganyl diselenides bearing electron-withdrawing and electron-donating groups affording the corresponding selanylalkynes in good to excellent yields.

Silver‐Catalyzed Cross‐Olefination of Donor and Acceptor Diazo Compounds: Use of N‐Nosylhydrazones as Diazo Surrogate

The cross‐olefination reaction of donor and acceptor diazo compounds was explored. The use of N‐nosylhydrazones as diazo surrogates and the dependence on silver catalysis were crucial for the reaction development. A variety of (hetero)aryl N‐nosylhydrazones and α‐diazo esters, amides, and phosphonates were compatible, and the functionalized alkene products were afforded in good to high yields with moderate (Z)/(E) selectivities. The experimental and DFT calculation results suggest that the cross‐selectivity is due to selective activation of the silver catalyst for donor diazo compounds.

Oxygen-Controlled Hydrogen Evolution Reaction: Molecular Oxygen Promotes Hydrogen Production from Formaldehyde Solution Using Ag/MgO Nanocatalyst

Molecular hydrogen is one of the essential reactants in the chemical industry, and its generation from renewable sources such as biomass materials and water is of great benefit to the future society. Generally, molecular oxygen should be pre-eliminated in the hydrogen evolution reactions (HERs) in order to avoid the reverse hydrogen oxidation reaction (HOR). Here, we report a highly efficient HER from a formaldehyde/water mixture using MgO supported Ag nanoparticles (AgNPs/MgO) as the catalyst and molecular oxygen as a promoter. The HER rate depends almost linearly on the oxygen partial pressure, and the optimal turnover frequency (TOF) of the silver catalyst exceeds 6,600 h–1. Based on the experimental and theoretical results, a surface stabilized MgO/Ag–•OOH complex is suggested to be the main catalytically active species for the HER.

Silver and palladium alloy nanoparticle catalysts: reductive coupling of nitrobenzene through light irradiation.

Silver-palladium (Ag-Pd) alloy nanoparticles strongly absorb visible light and exhibit significantly higher photocatalytic activity compared to both pure palladium (Pd) and silver (Ag) nanoparticles. Photocatalysts of Ag-Pd alloy nanoparticles on ZrO2 and Al2O3 supports are developed to catalyze the nitroaromatic coupling to the corresponding azo compounds under visible light irradiation. Ag-Pd alloy NP/ZrO2 exhibited the highest photocatalytic activity for nitrobenzene coupling to azobenzene (yield of ∼80% in 3 hours). The photocatalytic efficiency could be optimized by altering the Ag : Pd ratio of the alloy nanoparticles, irradiation light intensity, temperature and wavelength. The rate of the reaction depends on the population and energy of the excited electrons, which can be improved by increasing the light intensity or by using a shorter wavelength. The knowledge developed in this study may inspire further studies on Ag alloy photocatalysts and organic syntheses using Ag-Pd nanoparticle catalysts driven under visible light Irradiation.

Enhancement in kinetics of the oxygen reduction on a silver catalyst by introduction of interlaces and defect-rich facets

Nanoporous Ag structures with interlaced facets and defects-enriched surfaces are fabricated via a polyoxometalate nanocluster-mediated method, which exhibit high catalytic activity towards ORR in alkaline solution.

The Separation of 241Am from Aged Plutonium Dioxide for use in Radioisotope Power Systems

Electrical power sources used in outer planet missions are a key enabling technology for data acquisition and communications. State–of-the-art power sources generate electricity from alpha decay of 238 Pu via thermoelectric conversion. However, production of 238 Pu requires specialist facilities including a nuclear reactor, a source of 237 Np for target irradiation and hotcells to chemically separate neptunium and plutonium within the irradiated targets. These specialist facilities are expensive to build and operate, so naturally, a more economical alternative is attractive to the industry. Within Europe 241Am is considered a promising alternative heat source for radioisotope thermoelectric generators (RTGs) and radioisotope heating units (RHUs). As a daughter product of 241 Pu decay, 241 Am exists in 1000 kgs quantities within the UK civil plutonium stockpile.A chemical separation process is required to extract the 241 Am in a pure form and this paper describes the AMPPEX process (Americium and Plutonium Purification by Extraction), successfully developed over the past five years to isolate 241 Am in high yield (> 99%) and to a high purity (> 99%).The process starts by dissolving plutonium dioxide in nitric acid with the aid of a silver(II) catalyst, which is generated electrochemically. The solution is then conditioned and fed to a PUREX type solvent extraction process, where the plutonium is separated from the americium and silver. The plutonium is converted back to plutonium dioxide and the americium is fed forward to a second solvent extraction step. Here the americium is selectively extracted leaving the silver in the aqueous phase. The americium is stripped from the solvent and recovered from solution as americium oxalate, which is calcined to give americium dioxide as the final product. This paper will describe the development of the separation process over a series of six solvent extraction separation trials using centrifugal contactors. The material produced (~ 4g 241 Am) was used to make ceramic pellets to establish the behaviour of americium oxide material under high temperature (1450°C) sintering conditions.The chemical separation process is now demonstrated at concentrations expected on the full scale facility taking this process to TRL 4-5.

Insight on the silver catalyst distribution during silicon nanowire array formation: an X-ray reflectivity study.

Although metal-catalysts are commonly used to create nanoscale materials at surfaces, little is quantitatively known or understood about the depth distribution profile of the catalyst during the growth process. Using X-ray reflectivity, we report the first quantitative investigation, with nanoscale resolution, of the Ag metal-catalyst depth distribution profile during metal-assisted chemical etch (MACE) growth of Si nanowire (SiNW) arrays on Si(100). Given the very low optical reflectivity of these nanowire arrays, specular reflection from these materials in the X-ray region is extremely challenging to measure because it probes interfaces on the nanoscale. Nevertheless, we demonstrate that with suitable investigation, X-ray specular reflection can be measured and utilized to obtain unique structural information about the composition profile of both Ag and Si. The measurements, which also include X-ray diffraction and complementary electron microscopy, reveal that the Ag nanoparticles distribute along the length of the nanowires upon etching with a Ag density that increases towards the etch front. The Ag nanoparticles coarsen with etch time, indicating a high mobility of Ag ions even though we also find that the Ag does not migrate from the SiNW region into the etch bath during etching. The Ag density gradient and the Ag mobility suggest the existence of a strong chemical force that attracts Ag towards the etch front. These results provide unique and important new insight into the growth process for creating SiNWs from wet chemical etching using metal-catalysts.

A Schiff base-modified silver catalyst for efficient fixation of CO2 as carboxylic acid at ambient pressure

A Schiff base-modified silver catalyst was developed for the direct carboxylation of terminal alkynes with CO2, enabling the efficient synthesis of valuable alkynyl carboxylic acids.

Understanding the Seed-Mediated Growth of Gold Nanorods through a Fractional Factorial Design of Experiments.

Since the development of simple, aqueous protocols for the synthesis of anisotropic metal nanoparticles, research into many promising, valuable applications of gold nanorods has grown considerably, but a number of challenges remain, including gold-particle yield, robustness to minor impurities, and precise control of gold nanorod surface chemistry. Herein we present the results of a composite fractional factorial series of experiments designed to screen seven additional potential avenues of control and to understand the seed-mediated silver-assisted synthesis of gold nanorods. These synthesis variables are the amount of sodium borohydride used and the rate of stirring when producing seed nanoparticles, the age of and the amount of seeds added, the reaction temperature, the amounts of silver nitrate and ascorbic acid added, and the age of the reduced growth solution before seed nanoparticles are added to initiate rod formation. This statistical experimental design and analysis method, besides determining which experimental variables are important and which are not when synthesizing gold nanorods (and quantifying their effects), gives further insight into the mechanism of growth by measuring the degree to which variables interact with each other by mapping out their mechanistic connections. This work demonstrates that when forming gold nanorods by the reduction of auric ions by ascorbic acid onto seed nanoparticles, ascorbic acid determines how much gold is reduced, and the amount of seeds determine how it is divided, yet both influence the intrinsic growth rates, in both width and length, of the forming nanorods. Furthermore, this work shows that the reduction of gold proceeds via direct reduction on the surface of seeds and not through a disproportionation reaction. Further control over the length of gold nanorods can be achieved by tuning the amount of silver nitrate or the reaction temperature. This work shows that silver does not directly influence rod length or width, and a new primary role for silver is proposed as a catalyst promoting the reduction of gold on the ends of forming nanorods. Furthermore, this silver catalyst is removed from the reaction by adsorption onto the surface of the growing nanorod. This work also demonstrates the importance of freshly prepared silver nitrate and ascorbic acid solutions, free from even a few hours of photodegradation, in preparing gold nanorods with high shape purity and gold yield.

Enantioselective Silver-Catalyzed Transformations.

This review collects the major progress in the field of enantioselective transformations promoted by chiral silver catalysts, covering the literature since 2008, well illustrating the power of these especially mild Lewis acid catalysts to provide novel asymmetric reactions.

Oxidative decomposition of methanol in a vibroacoustic fluidized bed of Ag-coated cenosphere core-shell catalyst

Abstract The paper presents results of a study of oxidative decomposition of methanol in a fluidized bed of silver catalyst. The process of methanol oxidation was carried out on Ag-coated cenospheres core-shell catalyst. The catalyst was obtained by precipitation of silver from methanolic solution of silver nitrate on cenospheres. Cenospheres are lightweight, inert, hollow spheres, which can be easily introduced into a fluidized bed. Application of the catalyst in a form of fluidized bed should ensure good temperature equalization which is very important at low temperatures due to the possibility of generation of formaldehyde. It turned out that local hot points occur in that kind of a catalyst (in the form of a fluidized bed but with very low density), thus use of additional acoustic wave is necessary to ensure good control over the temperature of the process. The products of the process of methanol oxidation were monitored on-line by Fourier transform infrared spectroscopy (FTIR). The catalyst has proven to be highly active in the oxidative decomposition of methanol. Full of methanol conversion in reaction of complete oxidation was achieved at 350°C.

Highly active, selective and stable electroreduction of carbon dioxide to carbon monoxide on a silver catalyst with truncated hexagonal bipyramidal shape

To improve the activity, selectivity and stability of silver electrodes towards CO2 electroreduction to CO, three-dimensional frameworks comprised of the metallic silver electrocatalyst particles were constructed on bulk silver surfaces using a successive double-potential pulses method. With the help of X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy, it was found that the as-synthesized silver particles exhibited a truncated hexagonal bipyramidal morphology, which was assembled from silver seeds with two {111} bases as well as laterally exposed {100} and {111} facets. Potential-dependent results revealed that the modified electrode exhibited an earlier onset potential, higher Faradaic efficiency and greater current density for CO evolution when compared to its smooth-surface counterpart. Moreover, as opposed to the bulk silver electrode losing its activity as a function of time, the activity of the improved electrode was durably stable over three successive 4h periods. It is proposed that, in addition to the relatively vast electrochemical surface area, the exposed edges of the silver particles play a major role in enhancing the intrinsic activity and selectivity of the silver electrocatalyst by stabilizing the surface-adsorbed COOH species. Meanwhile, the existence of a large proportion of low-index {111} planes facilitates the desorption of the reduction intermediates of adsorbed CO species, suppressing the deactivation of the silver electrode towards CO2 reduction.

Tuning of Silver Catalyst Mesostructure Promotes Selective Carbon Dioxide Conversion into Fuels.

An electrode's performance for catalytic CO2 conversion to fuels is a complex convolution of surface structure and transport effects. Using well-defined mesostructured silver inverse opal (Ag-IO) electrodes, it is demonstrated that mesostructure-induced transport limitations alone serve to increase the turnover frequency for CO2 activation per unit area, while simultaneously improving reaction selectivity. The specific activity for catalyzed CO evolution systematically rises by three-fold and the specific activity for catalyzed H2 evolution systematically declines by ten-fold with increasing mesostructural roughness of Ag-IOs. By exploiting the compounding influence of both of these effects, we demonstrate that mesostructure, rather than surface structure, can be used to tune CO evolution selectivity from less than 5 % to more than 80 %. These results establish electrode mesostructuring as a powerful complementary tool for tuning both catalyst selectivity and efficiency for CO2 conversion into fuels.

Synergistic Electrochemical CO2 Reduction and Water Oxidation with a Bipolar Membrane

The electrochemical conversion of CO2 and water to value-added products still suffers from low efficiency, high costs, and high sensitivity to electrolyte, pH, and contaminants. Here, we present a strategy for this reaction using a silver catalyst for CO2 reduction in a neutral catholyte, separated by a bipolar membrane from a nickel iron hydroxide oxygen evolution catalyst in a basic anolyte. This combination of electrolytes provides a favorable environment for both catalysts and shows the effective use of bicarbonate and KOH to obtain low cell voltages. This architecture brings down the total cell voltage by more than 1 V compared to that with conventional use of a Pt counter electrode and monopolar membranes, and at the same time, it reduces contamination and improves stability at the cathode.