Electrolytic Silver Research Articles

Gas-phase selective oxidation of alcohols: In situ electrolytic nano-silver/zeolite film/copper grid catalyst

The selective oxidation of a series of alcohols to their corresponding carbonyl products was carried out over a rationally designed in situ electrolytic nano-silver/zeolite film/copper grid (SZF) catalyst, which was prepared by a combination of the seed-film method for the fabrication of an ultrathin zeolite film and the in situ electrolytic process for the formation of highly dispersed silver nanoparticles. At a relatively low reaction temperature (ca. 320 °C), the SZF catalyst with highly dispersed in situ electrolytic silver nanoparticles exhibited much higher activity for the oxidation of mono-alcohols and higher selectivity for ketonic aldehyde in the oxidation of di-alcohols than the conventional bulk electrolytic silver catalyst. On the basis of the combination of diffuse reflectance ultraviolet visible spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and thermoanalysis, the remarkably high activity and selectivity of the SZF catalyst was attributed to the highly dispersed silver nanoparticles, which were stabilized by the zeolite film against sintering, and, accordingly, a large amount of Ag + ions and Ag n δ + clusters existed in the silver nanoparticles. The improvements of the catalytic performance of the SZF catalyst in a wide application extension will bring new concerns in both theoretical and applied fields.

Ag–SiO 2–Al 2O 3 composite as highly active catalyst for the formation of formaldehyde from the partial oxidation of methanol

Catalytic oxidative dehydrogenation of methanol to formaldehyde was carried out over Ag–SiO 2–Al 2O 3 catalysts prepared by a sol–gel method. Detailed preparation conditions were investigated and the optimal preparation parameters were determined as the Si/Al molar ratio of 8.5 / 1.5 – 9 / 1 , silver loading of 20 wt%, and calcination temperature of 900–1000 °C with ethanol as the solvent. Under optimum reaction conditions, i.e., the reaction temperature of 640 °C, O 2/CH 3OH molar ratio of 0.39 with space velocity (GHSV) of 1.2 × 10 5 h − 1 , the as-prepared catalyst exhibits excellent activity and selectivity. The yield of formaldehyde reaches 91%, much higher than that obtained over the pumice-supported silver catalyst (75%) and even higher than that over the commercial electrolytic silver catalyst (85%). Based on combined characterizations, such as X-ray photoelectron spectroscopy and X-ray-excited Auger electron spectroscopy (XPS and XAES), nitrogen adsorption at low temperature, thermogravimetry and differential thermogravimetry (TG-DTG), differential thermal analysis (DTA), diffuse reflectance ultraviolet visible spectroscopy (UV–vis DRS), temperature-programmed reduction (TPR), scanning electron micrograph (SEM), X-ray diffraction (XRD), etc., the correlation of the catalytic performance to the structural properties of the Ag–SiO 2–Al 2O 3 catalyst is discussed in detail. It is found that almost all the catalysts are glass-like and nonporous with a surface area of ∼1 m 2/g. In the catalysts with silver loading lower than 20 wt%, all the silver species are present as Ag + ions before the reaction and the catalyst is ultrathermally stable even under elevated temperatures at 1100 °C owing to the stabilizing effect of the [AlO 4] group. Higher Ag loading in the catalysts leads to the presence of metallic silver species over the surface of the catalyst. During the catalytic reaction, Ag + ions are partially reduced to metallic Ag. These nano-sized Ag particles act as the active centers and the superior catalytic performance of the Ag–SiO 2–Al 2O 3 catalyst is attributed to its unique surface structure and the strong interactions between the support and the active phase.

Influence of catalyst morphology on the performance of electrolytic silver catalysts for the partial oxidation of methanol to formaldehyde

Scanning electron microscopy (SEM), specific surface area measurements, in situ Raman spectroscopy and steady-state microreactor experiments were used to explore the relationship between silver catalyst morphology and performance in the titled reaction. Two electrolytic silver catalysts, obtained from different commercial suppliers, were examined in relation to their physico-chemical and catalytic properties. The work focused on the activation behaviour of the catalysts under conditions of industrial CH2O synthesis (ca. 923–953 K, 1 atm). The results show that the silver catalysts, denoted A and B, differed enormously in their surface morphology, which in turn impacted on their oxygen chemisorption behaviour and initial catalytic properties. Silver catalyst A possessed a high grain boundary density in its surface, and exhibited enhanced initial activity and selectivity to CH2O during CH3OH oxidation at 598–973 K compared to catalyst B, which possessed significantly lower surface area and roughness. Performance differences correlated strongly with the speciation of oxygen on the catalysts during CH3OH oxidation, which is discussed in terms of a framework comprising three chemically distinct atomic oxygen species: two surface species (denoted Oα and Oγ) and a bulk-dissolved species (denoted Oβ). The population of each state was found to be both temperature and structure-sensitive. The weakly bound Oα species was dominant at low temperatures and opened reaction pathways towards CH2O, HCOOCH3, CO2 and HCOOH. At temperatures of industrial CH2O manufacture (ca. 923 K), the strongly bound Oγ species was the dominant oxygen state and selectively activated the catalysts for the oxidative-dehydrogenation of CH3OH to CH2O+H2O. The dissociative chemisorption of O2 and the surface segregation of Oβ served to replenish the Oα and Oγ consumed by reaction. The CH2O yield on both catalysts increased with temperature up to 923 K, reflecting accompanying increases in the surface Oγ/Oα ratio. Grain boundaries facilitated the formation of Oγ and Oβ, which explained why catalyst A exhibited the higher initial activity and selectivity to CH2O. The performance of both catalysts, in particular catalyst B, improved with time-on-stream at 923 K before stabilising after approximately 48 h of operation. These improvements were due to reaction-induced changes in catalyst morphology, which created structures that promoted Oβ and Oγ formation. Results were used to develop reaction schemes for CH3OH oxidation on fresh and aged silver catalysts under industrial conditions. The critical role of defects in the silver catalyst structure, such as grain boundaries in the efficient ‘start up’ operation of electrolytic silver catalysts for CH2O production is demonstrated.

Towards the “Pressure and Materials Gap”: Hydrogenation of Acrolein Using Silver Catalysts

Abstract The hydrogenation of acrolein has been studied over various Ag/SiO2 catalysts in a pressure range from 50mbar to 20bar. Increasing partial pressures of acrolein and/or hydrogen lead to increasing selectivities of allyl alcohol. The selectivity towards allyl alcohol also depends on the structural features of the Ag/SiO2 catalyst. Larger particles seem to favour the production of propanal. This observation is discussed in view of the different plane-to-edge-ratio of the different catalysts. The interaction of hydrogen with silver samples has been studied with TAP at very low pressures as well as with calorimetry at ambient pressures. Both methods indicate, that also the hydrogen adsorption is structure-sensitive. No interaction of hydrogen with electrolytic silver or the support material alone was observed. IR spectroscopy has been used to elucidate the interaction of acrolein with Ag/SiO2 catalysts. A strong interaction with the support material was found, hindering the observation of probable silver-acrolein interaction.

Influence of catalyst morphology on the performance of electrolytic silver catalysts for the partial oxidation of methanol to formaldehyde

Scanning electron microscopy (SEM), specific surface area measurements, in situ Raman spectroscopy and steady-state microreactor experiments were used to explore the relationship between silver catalyst morphology and performance in the titled reaction. Two electrolytic silver catalysts, obtained from different commercial suppliers, were examined in relation to their physico-chemical and catalytic properties. The work focused on the activation behaviour of the catalysts under conditions of industrial CH 2O synthesis (ca. 923–953 K, 1 atm). The results show that the silver catalysts, denoted A and B, differed enormously in their surface morphology, which in turn impacted on their oxygen chemisorption behaviour and initial catalytic properties. Silver catalyst A possessed a high grain boundary density in its surface, and exhibited enhanced initial activity and selectivity to CH 2O during CH 3OH oxidation at 598–973 K compared to catalyst B, which possessed significantly lower surface area and roughness. Performance differences correlated strongly with the speciation of oxygen on the catalysts during CH 3OH oxidation, which is discussed in terms of a framework comprising three chemically distinct atomic oxygen species: two surface species (denoted O α and O γ) and a bulk-dissolved species (denoted O β). The population of each state was found to be both temperature and structure-sensitive. The weakly bound O α species was dominant at low temperatures and opened reaction pathways towards CH 2O, HCOOCH 3, CO 2 and HCOOH. At temperatures of industrial CH 2O manufacture (ca. 923 K), the strongly bound O γ species was the dominant oxygen state and selectively activated the catalysts for the oxidative-dehydrogenation of CH 3OH to CH 2O+H 2O. The dissociative chemisorption of O 2 and the surface segregation of O β served to replenish the O α and O γ consumed by reaction. The CH 2O yield on both catalysts increased with temperature up to 923 K, reflecting accompanying increases in the surface O γ/O α ratio. Grain boundaries facilitated the formation of O γ and O β, which explained why catalyst A exhibited the higher initial activity and selectivity to CH 2O. The performance of both catalysts, in particular catalyst B, improved with time-on-stream at 923 K before stabilising after approximately 48 h of operation. These improvements were due to reaction-induced changes in catalyst morphology, which created structures that promoted O β and O γ formation. Results were used to develop reaction schemes for CH 3OH oxidation on fresh and aged silver catalysts under industrial conditions. The critical role of defects in the silver catalyst structure, such as grain boundaries in the efficient ‘start up’ operation of electrolytic silver catalysts for CH 2O production is demonstrated.

Conversion of commercial si solar cells to keep their efficient performance at 15 suns

AbstractThe screen‐printing method is an economical metallization technique used by most manufacturers of conventional silicon solar cells. This method limits the cells' use under concentrated light owing to high series resistance losses caused, among other reasons, by low metal density in the fingers. This paper describes increasing the finger metal density by electrolytic deposition. The electrolytic deposition of silver is an economical, controllable and readily commercializable deposition method to reduce the front and back metallization series resistance contributions. With an optimized grid design, compatible with 1 sun silicon cell technology, and later electrolytic silver deposition we have obtained cells that maintain their efficiency up to 15 suns. In addition, an analysis of the performance of these cells under uniform and non‐uniform illumination were carried out on n+p and n+pn+ structures. Copyright © 2004 John Wiley & Sons, Ltd.

Mechanism and active sites for the partial oxidation of methanol to formaldehyde over an electrolytic silver catalyst

In situ Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and fixed-bed microreactor experiments and were used to examine the operating mechanism of an electrolytic silver catalyst in the titled reaction. Methanol oxidation studies were performed at atmospheric pressure, temperatures between 448 and 1073 K and CH 3OH/O 2 feed stoichiometries of 1.5–4.0. Particular attention was given to the role of surface chemisorbed and bulk-dissolved oxygen species in the catalytic cycle, and the influence of oxygen speciation on catalyst selectivity to formaldehyde. The results indicate that the product distribution of CH 3OH oxidation over electrolytic silver catalysts relates intimately to the nature of the oxygen species formed on the catalyst, which is discussed within a framework comprising two distinct surface chemisorbed atomic oxygen species (denoted O α and O γ) and a bulk-dissolved species (denoted O β). The selectivity to formaldehyde was highly dependent on the surface ratio O γ/O α, both of which increased with temperature from 548 to 923 K and CH 3OH/O 2 feed ratio from 1.5 to 2.25. An optimum formaldehyde yield of 84.3% was obtained during microreactor testing of the silver catalyst, under conditions close to those employed industrially (temperature = 923 K, molar ratio CH 3OH/O 2=2.25, GHSV=1.25×10 5 h −1). SEM revealed that pronounced thermal and catalytic etching of the silver catalyst occurred during CH 3OH oxidation at 923 K. Based on these results, a scheme was developed for the interaction of CH 3OH and O 2 with electrolytic silver catalysts under conditions of industrial formaldehyde manufacture. This work clarifies the role of the O α, O β and O γ species in methanol oxidation on silver surfaces and provides a fundamental basis for future formaldehyde yield optimisation.

Cleaning of the Surface of Silver Crystals Used as Catalysts of Gas-Phase Oxidation of Ethylene Glycol to Glyoxal

The procedure for cleaning polycrystalline fibers of electrolytic silver used as a catalyst for gas phase oxidation of ethylene glycol to glyoxal was developed with the aid of X-ray photoelectron spectroscopy.

Oxygen chemisorption on an electrolytic silver catalyst: a combined TPD and Raman spectroscopic study

In situ Raman spectroscopy, temperature programmed desorption (TPD) and scanning electron microscopy (SEM) were used to study the interaction of an electrolytic silver catalyst with O2 at atmospheric pressure and temperatures between 300 and 923K. Raman spectroscopy and TPD were applied to characterise the chemisorbed oxygen species formed during catalyst exposure to O2 under these conditions. SEM was used to examine changes in catalyst morphology caused by high temperature oxygen treatment. The silver catalyst chemisorbed oxygen in a variety of distinct states over the temperature range examined. Oxygen species, identified on the basis of their vibrational and thermal desorption spectra, were a molecular superoxo species, weakly chemisorbed surface atomic oxygen (Oα), strongly chemisorbed surface atomic oxygen (Oγ) and subsurface oxygen (Oβ). The thermal stability and formation conditions of each species were established through a combination of in situ Raman and TPD data. With heating in O2 at 923K, the silver catalyst restructured to form well-defined surface facets of dimension 1–2μm. The influence of the restructuring on the oxygen chemisorptive properties of the silver catalyst is discussed. The results of these studies were used to develop a scheme for the interaction of O2 with silver surfaces at 923K.

Comparison of Discharge Silver Concentrations from Electrolytic Plating and Metallic Replacement Silver Recovery Units

Silver-based photographic X-ray film is made of solid crystals of silver chloride or silver bromide suspended in a gelatin and then coated on a film. During the X-ray developing process, the image is processed and the nonimage areas containing solid silver chloride or silver bromide crystals are removed in a solution called the fixer. There may be local environmental regulations that regulate the amount of silver discharged from a facility. To meet these regulations, many facilities have added silver recovery units to their processes. Two different types of recovery processes are in use in a large hospital and three clinics under study. All of the units were claimed by their respective manufacturers to be able to recover silver down to concentrations of 5 mg/L. This concentration would ensure that the building that houses each unit would meet the local county limit of 0.5 mg/L silver for total building silver discharge. The hypothesis for this research is that one system, newer and more expensive, consisting of so-called electrolytic plating units (EPUs) (which are followed by so-called metallic replacement units [MRUs] as a backup), will have better silver recovery than MRUs alone. A total of six units were sampled, three EPUs (in combination with MRUs) and three MRUs. The units were sampled once or twice a day for 10 days for a total of 17 samples from each. The samples then were analyzed by inductively coupled plasma spectroscopy, and an analysis of variance was performed on the results. The range for the electrolytic plating unit/metallic replacement unit combinations was 0.20–99.9 mg/L (mean of 35.15 mg/L; median of 33.8 mg/L). The range for the MRUs alone was 7.2–1112 mg/L (mean of 565.5 mg/L; median of 720 mg/L). Many individual results exceeded 5 mg/L, such that extensive dilution would be required to ensure the building effluent did not exceed 0.5 mg/L. It is suggested that the metallic replacement units be changed to EPUs (with metallic replacement backup units) because they had better silver recovery. Also, the EPU combinations need to be sampled regularly to ensure that their silver concentrations are at acceptable levels.

Surface State of a Silver Catalyst for Ethylene Glycol Oxidation

The surface state of electrolytic silver before and after treatment with a reaction mixture in the course of ethylene glycol oxidation to glyoxal was studied using X-ray photoelectron spectroscopy and scanning electron microscopy. It was found that electrophilic forms of adsorbed oxygen, which participate in the selective conversion of ethylene glycol, were formed on the surface of electrolytic silver crystals under exposure to oxygen under conditions similar to catalytic process conditions. The treatment of the catalyst with a reaction mixture resulted in the formation of filamentous carbon deposition products. A mechanism of formation of carbon-containing products was proposed.

In situ Raman studies on the interaction of oxygen and methanol with an iodine‐modified electrolytic silver catalyst

AbstractIn situ Raman spectroscopy was used to study the interaction of iodine‐modified electrolytic silver with oxygen and methanol at atmospheric pressure at temperatures between 25 and 500°C. It is shown that the presence of iodine inhibits the formation of surface oxygen species corresponding to deep oxidation and reduces the vibrational wavenumber of the modes corresponding to surface oxygen species. Moreover, it is found that methanol can be adsorbed and reacted on the surface of the modified silver without the co‐adsorption of oxygen, resulting in iodine‐induced adsorption of methanol. The intermediates at various temperatures were identified as methoxy and formate groups. According to the results, the reaction pathways in the practical process were proposed and compared with the conclusion obtained from the system without iodine modification, and remarkably close agreement was observed. The detailed mechanism of the oxidative reaction of methanol and the promotion effect of iodine modifier on the working catalyst is also discussed. Copyright © 2002 John Wiley & Sons, Ltd.

Novel sol-gel-derived Ag/SiO2-Al2O3 catalysts for highly selective oxidation of methanol to formaldehyde

Novel Ag/SiO2–Al2O3 catalysts with low silver content prepared by the sol–gel method exhibit excellent catalytic properties in the catalytic oxidation of methanol to formaldehyde. The silver content was as low as 2% and the yield of formaldehyde was achieved as 90.3%, which is 16% higher than that of pumice‐supported silver and even 5–6% higher than that of a commercial electrolytic silver catalyst. XRD, XPS and SEM results reveal that all silver was present as Ag+ before catalytic reaction and was partially reduced to the metallic state after the reaction. It was also found that silver was aggregated on the surface after its reduction.

The role of iodide promoter in selective oxidation of methanol to formaldehyde

The interaction of a CH3I promoter and a commercial electrolytic silver catalyst was investigated using XPS, UPS and in situ confocal microprobe Raman spectroscopy. XPS and UPS results show evidence for formation of AgI on the surface of the silver catalyst. Interestingly, XPS results show no evidence for formation of any carbon species on the silver catalyst at room temperature, implying that CH3I can be used as a “pure” I‐modifier in the catalytic oxidation of CH3OH to HCHO. UPS results show a positive work function change (Δφ > 0) for the I‐modified catalysts. In situ Raman studies show that both the adsorption and reaction of methanol were promoted on the modified silver surface. The present experimental results provide not only an understanding of the surface chemistry of the silver catalyst but also a further understanding of the promotion effect of CH3I in the selective oxidation of CH3OH to HCHO.

Oxidative dehydrogenation of methanol to formaldehyde on electrolytic silver catalyst modified with iodides

Iodides, especially CH 3I, were used as modifiers in the oxidative dehydrogenation of methanol to formaldehyde over electrolytic silver catalyst. The yield of formaldehyde was as high as 93% at 98% conversion of CH 3OH for the I-modified catalyst, while it was only 85% at 94% conversion without I-modification. The optimum reaction conditions were determined and the interaction between the silver surface and the iodide additive was investigated by means of XPS and SEM. The chemisorbed iodine atoms on the silver surface caused a decrease of the surface concentration of atomic oxygen species and prevented the complete oxidation of CH 3OH to CO 2.

The Dynamic Restructuring of Electrolytic Silver during the Formaldehyde Synthesis Reaction

The partial oxidation of methanol to formaldehyde was studied over an industrial electrolytic silver catalyst. Special attention was paid to the influence of reaction-induced restructuring of the silver surface and bulk on the activity for the partial oxidation of methanol to formaldehyde. Drastic differences are observed in the performance exhibited by fresh samples and those which have been previously treated at high temperature (T> 873 K) reaction conditions. The temperature dependence of the conversion for a fresh catalyst shows a pronounced hysteresis which can be separated into three temperature regimes. These regimes are strongly correlated with various morphological changes induced in the silver during reaction. SEM images reveal that heating silver in an excess of methanol to 773 K results in the destruction of grain-boundary defects. Comparison of the various hysteresis profiles with TDS analysis of silver pretreated at high temperature in oxygen shows that elimination of these defects results in the inhibition of oxygen diffusion from the bulk to the surface in the temperature region between 673 K and 873 K. The formation of holes resulting from the reaction of bulk-dissolved hydrogen and oxygen indicates that diffusion along these defect structures supplies bulk oxygen for the catalytic reaction. Hole formation above 923 K is no longer restricted to grain boundaries, indicating that volume (interstitialcy) diffusion replaces grain-boundary diffusion at elevated temperatures. This incorporation of oxygen into the silver lattice results in an increased conversion of methanol with a higher conversion to formaldehyde. Consequently, no hysteresis is observed subsequent to having treated the sample in the reaction mixture at temperatures in excess of 873 K. Reaction-induced restructuring of the catalyst on the time scale of a typical run occurs. This leads to an improvement in the catalytic performance of silver for this extremely structure-sensitive reaction.

The oxidative dehydrogenation of methanol over a novel low‐loading Ag/SiO2–TiO2 catalyst

Catalytic oxidation of methanol to formaldehyde was carried out over Ag/TiO2-SiO2 catalysts prepared by chemical reduction. The catalytic activity was measured at the temperature interval 820-920 K, O2/CH3OH molar ratio between 0.35 and 0.50 and at the space velocity of 1.2 × 105 h-1. The optimal content of silver determined by chemical analysis was about 1.7 wt%. The yield of formaldehyde over this catalyst was ~13% higher than that of the industrial pumice-supported silver catalyst and even ~3.5% higher than that of electrolytic silver. The XRD patterns for silver particles supported on TiO2-SiO2 are corresponding to Ag(111), (200) and (220), respectively. SEM was used to determine its morphology and particle size. Isolated silver particles were observed on the surface of the catalyst. O2 chemisorption by using the pulse technique was carried out to determine the free silver surface areas. The average silver particle size from the calculation of selective oxygen chemisorption was found to be in good agreement with that observed from SEM.

The oxidative dehydrogenation of methanol over a novel Ag/SiO 2 catalyst

Catalytic oxidative dehydrogenation of methanol to formaldehyde was carried out over a novel Ag/SiO 2 catalyst prepared by the sol-gel method. The yield of formaldehyde over this catalyst was ∼10% higher than that over conventional pumice-supported silver and even ∼1% higher than that over commercial electrolytic silver. When the yields were compared under the optimum conditions, the catalytic activity was measured at 820–920 K, O 2/CH 3OH molar ratio of 0.35–0.55 and at the space velocity of 1.2×10 5h −1. The thermal behavior and structure of the catalyst were characterized by TG-DTG, DTA and XRD. It was found that this catalyst was thermally stable and silver was present in a crystalline state. SEM was used to determine its morphology and particle size. The silver particle size range was found to be 200–700 nm. XPS results indicated that two different kinds of silver in aggregated and highly dispersed metallic states, were present on the surface of the catalyst.

Resistance to Silver Electrolytic Migration for Thick‐Film Conductors Prepared from Mixed and Alloyed Powders of Ag‐15Pd and Ag‐30Pd

Thick‐film conductors, made from mixed and alloyed powders of Ag‐15Pd and Ag‐30Pd, have been explored in distilled water at a bias of 5 V to assess their resistance to electrolytic silver migration It was found that the resistance increases with the Pd concentration in the thick films (i.e., Ag < Ag‐15Pd < Ag‐30Pd) The film made from mixed powders is more resistant as compared with that from alloyed powders having the same compositions (i.e., 15A < 15M and 30A < 30M). Based on the electrochemical study and analysis of reaction products, the interpretation for the resistance sequence has been discussed.

A Novel Silver-Silica Catalyst Used for the Manufacture of Formaldehyde

Abstract Several novel silver-silica catalysts were prepared by means of sol-gel method, which exhibited high activity and selectivity in the catalytic oxidation of methanol to formaldehyde. The optimum content of silver was determined as 20%(w/w.), which showed 97% conversion and 85.5% yield of formaldehyde. Some other superiority of the as-prepared silver catalysts was observed by comparing with the commercial silver catalysts, including pumice-supported silver and electrolytic silver. XRD patterns reveal that silver is present as a crystalline state in the Ag-SiO2 catalysts.