Supported Ag Catalysts Research Articles

Highly efficient and selective hydrogenation of chloronitrobenzenes to chloroanilines by H2 over confined silver nanoparticles

The Ag NPs with sizes ranging form 5–6 nm confined into SBA-15 show a very good activity and robust recycle ability for hydrogenation of chloronitrobenzenes to the corresponding chloroanilines by H2, comparing with that of the traditional SiO2 supported Ag catalysts.

Preparation of Ag/α-Al2O3 for ethylene epoxidation by an impregnation–bioreduction process with Cinnamomum camphora extract

Supported Ag catalyst for ethylene epoxidation was prepared by an impregnation–bioreduction process with Cinnamomum camphora (CC) extract. Influence of bioreduction temperature, concentration of CC extract, silver loading amount, calcination temperature as well as Cs promoter on catalytic performance were thoroughly investigated and the catalysts were characterized. The results showed that the catalyst prepared from bioreduction process exhibited excellent performance for the ethylene epoxidation with O2. Proper amount of Cs addition could effectively improve the catalytic performance of the catalyst. The optimal catalyst 15Ag/α-Al2O3-150Cs gave the EO concentrations of 1.70%, EO selectivity of 87.46% and EO STY of 223.07gh−1L−1cat at 200°C, showing high catalytic stability with no significant decrease in the activity and the selectivity during 50h.

Preferential carbon monoxide oxidation on Ag/Al-SBA-15 catalysts: Effect of the Si/Al ratio

In this work, mesoporous aluminosilicate supports (Al-SBA-15) were prepared using in situ “pH-adjusting” method, and Ag nanoparticles were supported on Al-SBA-15 supports by using the simple impregnation method. The effect of the Si/Al ratios of mesoporous support on the structure of Ag catalysts and the catalytic performance for the preferential oxidation of CO (CO PROX) in H2-rich gases was investigated. The Ag/Al-SBA-15 catalyst with an appropriate Si/Al molar ratio of 200 has been found to show highly catalytic activity, selectivity and stability. A series of supports and corresponding supported Ag catalysts with different Si/Al ratios were characterized by ICP-AES, XRD, N2 adsorption–desorption, 27Al NMR, FT-IR of adsorbed pyridine, TEM, H2-TPR and XPS. The results indicated that the addition of a small amount of Al is beneficial to the formation of small sized highly dispersed metal Ag particles in the channel, consequently improving the catalytic activity. In addition, the Ag/Al-SBA-15 (200) catalyst also exhibited better catalytic activity under the presence of H2O and CO2 in the stream.

Protodecarboxylation of carboxylic acids over heterogeneous silver catalysts

A heterogeneous supported Ag catalyst for the protodecarboxylation of aromatic carboxylic acids has been developed. Control of the metal particle size proved extremely important. The highest activity was achieved with a silver loading of 10 wt%, which had relatively big metal crystallites of 40 nm. It is inferred that the adsorption of the aromatic moiety requires terrace sites rather than edges or corners at the metal nanoparticle. The amphoteric support, γ-Al2O3, gave the most active catalysts. Oxygen coverage of the surface is essential for catalytic activity. A mechanism has been proposed with the critical steps (1) formation of a benzoyl anion by reaction with a base in the reaction medium, (2) binding of the anionic species at the Ag+ surface sites with (3) extrusion of CO2 and (4) proton transfer from another molecule of carboxylic acid, followed by desorption of the decarboxylated species and binding of the benzoate to the active site to complete the catalytic cycle. With 2-nitrobenzoic acid as substrate, the catalyst had a turnover frequency (TOF) of 216 h−1. The catalyst showed good activity for benzoic acid with nitro, methoxy and halogen substituents at the ortho-position as well as for heteroaromatic carboxylic acids.

A used battery supported Ag catalyst for efficient oxidation of alcohols and carbon oxide

A used battery supported silver catalyst was successfully synthesized, which showed high catalytic performance for the oxidation of alcohols and CO.

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.

Dynamic structural changes of perovskite-supported metal catalysts during cyclic redox treatments and effect on catalytic CO oxidation

CaTiO 3 -and BaTiO 3 -supported Ag and Pt catalysts were subjected to cyclic oxidation and reduction treatments and their surface structures were investigated using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The CO oxidation reactions over the Ag and Pt catalysts showed that the oxidized Ag/oxide catalysts performed better in CO oxidation than the reduced ones did, whereas the reduced Pt/oxide catalysts had higher CO oxidation activity than that after oxidation treatment. The XRD and XPS measurements revealed that the oxidation treatment helped to improve the dispersion of Ag nanoparticles, but their metallic state was retained, which enhanced CO oxidation. In contrast, the surfaces of the Pt nanoparticles were dominated by PtO 2 after the oxidation treatment, which lowered the CO oxidation activity compared with that of the reduced Pt catalyst. 研究了钛酸钡和钛酸钙担载的Ag和Pt纳米催化剂的表面结构随氧化-还原处理过程的动态变化及其对CO完全氧化反应性能的影响. 发现氧化物担载的Ag催化剂在氧化处理后其催化活性较还原处理的高; X射线衍射(XRD)和X射线光电子能谱(XPS)表征结果表明, 氧化处理能够提高载体表面Ag颗粒的分散度, 而还原处理导致Ag颗粒的聚集, 从而降低了催化氧化CO反应的活性. 氧化-还原处理改变了担载Ag纳米粒子的尺寸并影响其CO氧化反应活性. 与此相反, 氧化物担载的Pt催化剂在还原处理后所表现出的CO氧化反应活性较氧化处理的高; 对比研究发现, 氧化和还原处理后Pt纳米粒子的尺寸基本相同, 但是氧化处理的样品中Pt表面物种以氧化态为主, 而还原处理后Pt表面物种主要为金属态. Pt纳米粒子表面化学状态随氧化-还原处理的调变是导致表面催化活性差异的主要原因. Perovskite-supported Ag and Pt catalysts show reversible structural changes and oxidation catalysis performance during cyclic oxidation and reduction treatments. Supported Ag catalysts show better performance after oxidation treatment, whereas supported Pt catalysts have enhanced activity after reduction treatment.

Comparison of catalytic performance of Ag- and K-based catalysts for diesel soot combustion

Soot combustion performance of supported Ag (Ag/TiO2 and Ag/CeO2) and K (K/TiO2 and K/CeO2) catalysts under tight and loose contact conditions was evaluated by thermogravimetric analysis (TG) and the determination of the apparent activation energy as a function of soot conversion. The soot combustion performance strongly depended on the active species and contact conditions. The soot combustion proceeded by catalytic and non-catalytic combustion processes, depending on the contact conditions and the active species. Almost all the soot was catalytically combusted over Ag and K supported catalysts when they were tightly contacted with the soot, and Ag was more active catalytic species for soot oxidation than K. Under the loose contact condition, the supported K catalyst was superior to the supported Ag catalysts in the fraction of catalytically combusted soot and propagation of the combustion to the non-contact soot. Therefore, the supported K catalyst showed higher performance than the supported Ag catalyst for the removal of the majority of soot under the loose contact condition.

NOx Abatement by HC-Assisted SCR over Combustion Synthesized-Supported Ag Catalysts

In this work, catalysts containing 2% by weight of silver particles dispersed on different oxide supports (CeO2, La2O3, ZrO2, Al2O3), and oxides without silver particles were prepared by means of the combustion synthesis method. This cheap and quick technique has been developed to produce very fine, homogeneous, crystalline powders without the intermediate decomposition and/or calcining steps which other conventional synthesis routes require. These features are also interesting for the cosynthesis of supported Ag catalysts; it has in fact been possible in one-shot to achieve values of the Ag clusters (estimated size 5–10 nm) that are comparable with those obtained by means of the more complex methods reported in literature. The catalysts were characterized via XRD, BET, TEM, FESEM-EDS, and XPS analysis. The catalytic performances of the prepared catalysts were evaluated in a TPR apparatus for the selective reduction of NO with benzene in the presence of oxygen. A noticeable enhancement in activity was achieved when the selected oxides were doped with Ag. Lantana and ceria-supported Ag catalysts with good hydrothermal stability were found to perform comparatively well, as a significant NOx to N2 conversion was obtained at temperatures slightly above 250 °C. As far as the results concerning the catalysts are concerned, a synergic effect of the supports with the Ag clusters has been highlighted. Some conclusions may be drawn concerning the reaction mechanism.

Synthesis of high-surface-area micro/mesoporous ZnAl2O4 catalyst support and application in selective hydrogenation of o-chloronitrobenzene

High-surface-area ZnAl 2 O 4 with micro/mesoporous frameworks was synthesized via a facile one-pot solvothermal approach without using any surfactants or post-treatment. The results revealed that the textural properties of micro/mesoporous ZnAl 2 O 4 were tuned by changing alcohol/water mixture solvents used in solvothermal synthesis and Ag nanoparticles were dispersed uniformly on the surface of ZnAl 2 O 4 supports. Especially, the supported Ag catalyst on the microporous ZnAl 2 O 4 synthesized in methanol/water solvent exhibited good catalytic performance for liquid phase selective hydrogenation of o -chloronitrobenzene to o -chloroaniline, owing to the high dispersion of Ag nanoparticles and the strong metal-support interaction. ► Micro/mesoporous ZnAl 2 O 4 was fabricated via one-pot solvothermal approach. ► The textural properties of ZnAl 2 O 4 were tuned by changing alcohol/water solvents. ► Ag nanoparticles could be dispersed uniformly and stably on the ZnAl 2 O 4 supports. ► Supported catalysts exhibited good catalytic performance in selective hydrogenation.

Preliminary study on catalytic combustion-type sensor for the detection of diesel particulate matter

Catalytic oxidation and sensing of simulated diesel particulate matter (PM), which consists of hexadecane and carbon black as substitutes of soluble organic fraction (SOF) and soot, respectively, were investigated with Ag and Pt catalysts supported on TiO 2 and CeO 2. Supported Ag catalysts were active for soot oxidation at temperatures over about 500 °C, while supported Pt catalysts were almost inactive for the soot oxidation. Toward the oxidation of SOF, on the other hand, supported Pt catalysts were more active than supported Ag catalysts. Catalytic combustion-type sensors coated with those catalysts was fabricated, and their sensing characteristics were evaluated. Because of the difficulty to simulate the real sensing condition of PM, employed in this study was the programmed heating of the sensor on which PM or soot was pre-loaded. The results strongly suggest the possibilities to construct the catalytic combustion-type PM sensor and to selectively detect SOF and soot by the proper selection of the catalytic materials and operation temperatures.

The effect of phosphorous precursor on the CO oxidation activity of P-modified TiO2 supported Ag catalysts

Abstract Nanocrystalline TiO2 and P-modified TiO2 with P/Ti atomic ratio 0.01 were prepared by the solvothermal method and employed as the supports for Ag/TiO2 catalysts for CO oxidation reaction. The incorporation of phosphorus into the TiO2 lattice in the form of Ti–O–P resulted in an increase of both surface area and metal dispersion. The P-modified TiO2 supported Ag catalysts using phosphorus precursor in the form of oxide promoted the weak adsorbed oxygen species and resulted in catalytic activity improvement in CO oxidation. However, the use of phosphorous precursor in the form of phosphate such as H3PO4, (NH4)2HPO4, (C2H5)3PO4 could result in the strongly adsorbed oxygen species and/or the bidentate of phosphate species blocking the active sites instead.

Temperature programmed desorption/surface-reaction study of an anodic alumina supported Ag catalyst for selective catalytic reduction of nitric oxide with propene

On an anodic alumina supported silver catalyst with a low Ag loading (1.68wt.%), NOx (NO/He, NO/O2/He, NO2/He) adsorption measurements and NOx-temperature programmed decomposition (TPD)/temperature programmed surface-reaction (TPSR) measurements in different gas streams (He, C3H6/He, C3H6/O2/He) were conducted to investigate the formation, consumption and reactivity of surface adsorbed NOx species.During NO adsorption, no noticeable uptake of NO was detected. Introducing oxygen greatly improved the formation of ads-NOx species. A greater quantity of surface nitrate species was found after NO2 adsorption, accompanied with gaseous NO release. The result of TPSR demonstrates the surface nitrate species can be effectively and preferentially reduced by propene. When introducing oxygen into the propene gas stream of TPSR test, the significantly increased amount of reacted nitrate undoubtedly shows the importance of oxygen in activating propene. The pathway for the selective reduction of NOx in the presence of excess oxygen is proposed to pass through the selective reduction of the adsorbed nitrate species with the activated propene.The enhanced NOx conversion when replacing NO with NO2 was attributed to the stronger NOx adsorption capacity and oxidation ability of NO2, than those for NO. With increasing oxygen concentration, the difference between NO and NO2 would gradually decrease, and finally disappear in a high excess of oxygen.

Advantage of radiolysis over impregnation method for the synthesis of SiO2 supported nano-Ag catalyst for direct decomposition of N2O

Abstract Two SiO 2 supported Ag catalysts with 5 wt% Ag in each were prepared by impregnation and radiolysis methods. These catalysts were characterized by BET surface area, temperature programmed reduction (TPR) and X-ray diffraction techniques. TPR results indicate the presence of metallic silver in the catalyst prepared by radiolysis method whereas Ag + precursor is formed in the catalyst prepared by impregnation method. The Ag particle size as calculated from the XRD patterns, in the catalyst prepared by radiolysis method is relatively smaller compared to that of the impregnated catalyst. The two catalysts are tested for decomposition of N 2 O and it is observed that the catalyst prepared by radiolysis gives higher conversion and better N 2 selectivity. These results suggest that radiolysis method yields metallic Ag nanoparticles directly without any reduction treatment and that the catalyst exhibits good activity in N 2 O decomposition.

Influence of Al2O3 support on the activity of Ag/Al2O3 catalysts for SCR of NO with decane

The role of the Al2O3 support on the activity of supported Ag catalyst towards the selective catalytic reduction (SCR) of NO with decane is elucidated. A series of Ag/Al2O3 catalysts were prepared by impregnation method and characterized by N2 pore size distribution, XRD, UV–Vis, in-situ FT-IR and acidity measurement by NH3 and pyridine adsorption. The catalytic activity differences of Ag/Al2O3 are correlated with different properties of Al2O3 supports and the active Ag species formed. 4wt% Ag supported on sol-gel prepared Al2O3 (Ag/Al2O3 (SG), showed higher NOx conversion (65% at 400 °C), compared with the respective catalysts made from commercial Al2O3 (Ag/Al2O3 (GB), Ag/Al2O3 (ALO), (∼26 and 7% at 400 °C). The higher surface area, acidity and pore size distribution in sol–gel prepared Al2O3 (SG) results in higher NO and hydrocarbon conversion. Based on the UV–vis characterization, the activity of NO reduction is correlated to the presence of Agnδ+ clusters and acidity of Al2O3 support was found to be one of the important parameter in promoting the formation and stabilization of Agnδ+ clusters. Furthermore from pyridine adsorption results, presence of more number of Bronsted acid sites in Ag/Al2O3 (SG) is confirmed, which could also contribute to low temperature hydrocarbon activation and improve NO conversion. In situ FT-IR measurements revealed the higher rate of –CN and –NCO intermediate species formation over 4wt% Ag/Al2O3 (SG). We conclude that the physico–chemical properties of Al2O3 play a crucial role in NOx conversion over Ag/Al2O3 catalysts. Thus, the activity of the Ag/Al2O3 catalyst can be tailored by using a proper type of Al2O3 support.

Promoting Effect and Role of Alkaline Earth Metal Added to Supported Ag Catalysts in the Gas-Phase Catalytic Oxidation of Benzyl Alcohol

The effect and the role of alkaline earth metals added to Ag/SiO2 catalyst were studied in the gas-phase catalytic oxidation of benzyl alcohol. The main oxidation product was benzaldehyde, together with only small amounts of CO2, benzene, and toluene, depending on the reaction conditions. The influence of the method regarding how to add the alkaline earth metal to the supported Ag catalyst, and the amount of the added alkaline earth metal on the partial oxidation activity, was investigated, along with the analyses of these alkaline earth metal added Ag/SiO2 catalysts by diffuse reflectance (DR) spectroscopy, XRD, thermal gravimetric−differential thermal analyses (TG−DTA), and extended X-ray absorption fine structure (EXAFS). The addition of alkaline earth metals such as Ca, Sr, and Ba to the Ag/SiO2 catalyst by a coimpregnation method significantly promoted the catalytic activity of the partial oxidation of benzyl alcohol. The surface oxygen species on the supported Ag played an important role in the part...

Direct propylene epoxidation over modified Ag/CaCO 3 catalysts

In this work a series of supported Ag catalysts was studied for propylene epoxidation with molecular oxygen as the oxidant. It was found that α-Al 2O 3 and CaCO 3 were suitable supports for propylene epoxidation and that on the latter Ag particles between 400 and 700 nm gave the highest selectivity to propylene oxide (PO). Ball-milling treatment of the CaCO 3 catalyst and promotion with NaCl resulted in improved catalytic performance. The highest PO selectivity (45%) was obtained on a ball-milled catalyst with a silver loading of 56 wt.% supported on CaCO 3 and promoted with 1 wt.% NaCl (Ag(56)–NaCl(1)/CaCO 3). The catalysts were tested with reactant flow rates of C 3H 6:O 2:He = 5:10:15 cm 3 min −1, a gas hourly space velocity (GHSV) of 1800 h −1, a reaction pressure of 0.3 MPa, and a reaction temperature of 533 K (260 °C). Addition of 500 ppm of ethyl chloride (EtCl) to the reactant gases enhanced the stability of the Ag(56)–NaCl(1)/CaCO 3 catalyst. X-ray diffraction (XRD) of the Ag(56)–NaCl(1)/CaCO 3 catalyst detected the existence of AgCl in the catalyst and ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy confirmed the presence of surface Ag + species. Scanning electron microscopy showed a roughening of the Ag particles by the ball-milling treatment as well as by the addition of NaCl. The effect of NaCl on the enhancement of the catalytic performance was probably due to both physical and electronic changes in the properties of the catalyst. The NaCl not only helped increase the dispersion of the silver on the CaCO 3 support, but also probably increased the quantity of electrophilic oxygen species favorable for epoxidation. In situ UV–vis spectra suggested that the rapid reduction of Ag + species on the surface of the Ag(56)–NaCl(1)/CaCO 3 catalyst could be the cause of a decline in PO selectivity observed during reaction.

Selective catalytic reduction of NO over supported silver catalysts—practical and mechanistic aspects

Selective catalytic reduction of NO by hydrocarbons (HC-SCR) is one of the promising technologies for removal of NO in exhausts containing excess oxygen, such as diesel and lean burn gasoline engines. Supported Ag catalysts, especially Ag/Al2O3, are thought to be the promising candidates for use in diesel exhausts, as confirmed by several reports on engine bench tests. The HC-SCR performance of supported Ag catalysts is very sensitive to the reaction conditions, especially the type of hydrocarbons and the addition of H2. The control of reaction conditions would be key for practical use. The current research of supported Ag catalysts is reviewed from the viewpoints of practical use and the reaction mechanism, i.e., the reaction scheme, the role of surface adsorbed species, and the structure of active Ag species.

Promoted partial oxidation activity of supported Ag catalysts in the gas-phase catalytic oxidation of benzyl alcohol

The gas-phase catalytic oxidation of benzyl alcohol was carried out using Ag catalysts impregnated on a SiO 2 or CaO support, as well as a potassium-added counterpart (K/Ag/SiO 2), to clarify the surface state of the Ag species active in the oxidation reaction, the effect of the basic support and the added alkali metal. Benzaldehyde was obtained as the main oxidation product. The effects of the calcination temperature of the Ag/SiO 2 catalyst and the reaction temperature on oxidation activity were investigated. The benzaldehyde yield markedly increased between 553 and 593 K. Increasing the calcination temperature from 773 to 1173 K also caused a significant increase in partial oxidation activity. Both a CaO support for the Ag species and potassium added to the Ag/SiO 2 catalyst promoted partial oxidation at a comparatively low reaction temperature. X-ray diffraction and extended X-ray absorption fine structure demonstrated that a high calcination temperature, a basic support, and addition of an alkali metal led to a more ready reduction of the supported Ag to metallic Ag. Oxidized surface Ag species on the metallic Ag were inferred to be the active Ag components for the partial oxidation of benzyl alcohol based on the amount of O 2 uptake and the diffuse reflectance spectra of the supported Ag catalysts.

CO adsorption and correlation between CO surface coverage and activity/selectivity of preferential CO oxidation over supported Ag catalyst: an in situ FTIR study

In situ IR measurements for CO adsorption and preferential CO oxidation in H2-rich gases over Ag/SiO2 catalysts are presented in this paper. CO adsorbed on the Ag/SiO2 pretreated with oxygen shows a band centered around 2169 cm−1, which is assigned to CO linearly bonded to Ag+ sites. The amount of adsorbed CO on the silver particles (manifested by an IR band at 2169 cm−1) depends strongly on the CO partial pressure and the temperature. The steady-state coverage on the Ag surface is shown to be significantly below saturation, and the oxidation of CO with surface oxygen species is probably via a non-competitive Langmuir–Hinshelwood mechanism on the silver catalyst which occurs in the high-rate branch on a surface covered with CO below saturation. A low reactant concentration on the Ag surface indicates that the reaction order with respect to Pco is positive, and the selectivity towards CO2 decreases with the decrease of Pco. On the other hand, the decrease of the selectivity with the reaction temperature also reflects the higher apparent activation energy for H2 oxidation than that for CO oxidation.