Supported Ag Catalysts Research Articles

Catalytic performances of Ag/SiC catalysts for styrene epoxidation in O2 atmosphere

SiC supported Ag catalysts were prepared by an impregnation-reduction method for styrene epoxidation in O2 atmosphere. Under the condition of 100 °C, 0.5 MPa O2 and 4 h, 5 wt% Ag/SiC catalyst achieved styrene conversion of 98.5 % and styrene oxide selectivity of 69.7 %. Further studies show that O2 was activated on the Ag surface and then formed reactive oxygen species, 1O2 and •O2-. Based on the O2 adsorption results, the atomic ratio of O/Ag larger than 0.5 indicated that more oxygen species had spilled over to the SiC surface. In-situ diffuse reflectance Fourier transform infrared spectroscopy and Density Functional Theory calculation results showed that styrene was adsorbed on the SiC surface. The active oxygen species generated on the Ag surface could spill over to the SiC surface, reacting with the adsorbed styrene to produce styrene oxide or benzaldehyde. In addition, the catalyst exhibits good stability.

Effect of pH on mussel shells-originated HAP supported Ag catalysts for catalytic combustion of VOCs

Volatile organic compounds (VOCs) triggered serious air pollution and mussel shell caused the marine environmental issues. Herein, hydroxyapatite (HAP) using mussel shells as calcium source was prepared by hydrothermal method with different pH (pH = 6, 8, 10, 11) controls to support Ag catalyst for removal of VOCs. It was found that the amount of skeletal hydroxyl species and textual properties of HAP were controlled by pH effectively, and the increase of hydroxyl groups in the framework resulted in forming smaller-sized Ag2O particles. When pH was 8, the Ag/HAP-BpH=8 catalyst exhibited 100 % conversion of ethyl acetate and toluene at 260 °C and 300 °C, respectively. Silver species were more likely existed on the surface of HAP-BpH=8 to form large-size Ag particles (6.4 nm) and more Ag2O species. The lower ratio of Ag0/Ag+ were crucial for improving the VOCs oxidation activity at lower temperatures. This work applied waste mussel shells to prepare high value-added Ag/hydroxyapatite catalysts and to promote VOCs catalytic oxidation.

Synergistic promotion of Ag nanoparticles supported on magnetic Cu based metal organic framework for A3-coupling and synthesis of benzopyranopyrimidines

Synthesis of a magnetic CuBTC supported Ag catalyst and its catalytic activity for A3-coupling reaction and synthesis of benzopyranopyrimidines.

Regulating mesopore structures of support toward enhanced selective hydrogenation of dimethyl oxalate to methyl glycolate on Ag catalysts

Precisely regulating mesopore structures toward enhanced diffusion of reactants and targeted product is of great significance for achieving simultaneously improved activity and selectivity, which is exemplified by this work employing mesoporous silica with distinct structured mesopores as support for Ag catalysts to selectively hydrogenate dimethyl oxalate (DMO) into methyl glycolate (MG). Mesoporous silica nanosphere (MSNS) with center-radial pores shows shorter pore length than SBA-15 with parallel mesopores, which is beneficial for the diffusion process and thus gives rise to enhanced hydrogenation activity. Inspired by these insights, MSNSs sized in 50, 90, 120 and 160 nm are further synthesized to regulate pore length and systematically explore their effects on the reaction. The diffusion process is enhanced with decreasing the size of MSNS from 160 to 50 nm, as elucidated by the effective diffusion time constant (De/R2) determined by zero-length column (ZLC) method. Accordingly, DMO can facilely diffuse into the mesopores with shorter length to access the active sites while the formed MG product can easily diffuse out to avoid the over-hydrogenation process, which is corroborated by the comparison for performances of these Ag/MSNSs catalysts. Notably, a moderately short pore length, balancing well the diffusion and hydrogenation process, is more preferable for achieving both higher activity and better selectivity to MG, and MSNS sized in 90 nm supported Ag catalyst thus exhibits the greatest performances, with MG selectivity up to 96.6 % at 99.7 % of DMO conversion.

Tailoring the local environment of silver in SSZ-13 zeolites for low-temperature catalytic oxidation of ammonia

Supported Ag catalysts have shown great potential in selective catalytic oxidation of ammonia (NH3-SCO) for the depletion of harmful ammonia emissions. However, the lack of effective ways to control the proportional population of active Ag species in/on the support has been the major limitation to their catalytic performance. Herein, the electronic properties, stability, and dispersion of Ag species were rationally tuned by tailoring the micro-environment of SSZ-13 zeolite supports. The influence of the local environment of Ag species in SSZ-13 on their catalytic behavior during NH3-SCO reaction was extensively investigated by comprehensive characterizations. Results obtained have shown that SSZ-13 (31s) with moderate ion-exchange sites, a large external surface area and abundantly accessible defect sites are beneficial to the stabilization and dispersion of the Ag active sites which trigger the formation of plentiful active Ag0 species. The Ag0 species are immobilized on SSZ-13(31s) with high metal dispersion which facilitates the activation of oxygen and the dehydrogenation of adsorbed NH3, thus exhibiting excellent low-temperature activity and hydrothermal stability in the NH3-SCO reaction. This work deepens the understanding on the effect of support towards the engineering of active metal species and provides a facile strategy for the preparation of highly efficient metal-based catalysts.

MOF-derived CeO2 supported Ag catalysts for toluene oxidation: The effect of synthesis method

Volatile organic compounds (VOCs), such as toluene, have aroused wide concerns its emissions growth and harmfulness. Catalytic oxidation has been considered as one of the most efficient technologies for toluene degradation. In this work, MOF-derived CeO2 supported Ag catalysts were prepared by different synthesis method. Toluene oxidation test found that Ag-Ce-BTC-C synthesized by one-pot and calcination at 500 °C under Ar and O2 atmosphere exhibited the better catalytic performance for toluene oxidation (T90 = 226 °C). H2 temperature programmed reduction and X-ray photoelectron spectroscopic results exhibited that the preparation method had a great influence on the physicochemical properties of the supported Ag catalysts. The better catalytic performance was attributed to the better reducibility, more active oxygen species and oxygen vacancies, higher molar ratio of Oads/Olat, Ce3+/Ce4+ and Ag0/Agδ+. This work has a certain guiding significance for the preparation of highly active MOF-derived supported catalysts.

HCHO Catalytic Oxidation Performance over Cerium Containing MCM-41 Type Mesoporous Materials Supported Ag Catalysts

Understanding the effects of cerium in the formaldehyde (HCHO) catalytic oxidation performance over Ag/MCM-41 catalysts, cerium modified MCM-41 mesoporous materials (Ce-MCM-41) with different ratios were synthesized by one-step hydrothermal method. The activity test results showed that Ag/Ce-MCM-41 with the ratio of Si:Ce = 1:1 catalysts could reach complete HCHO oxidation above 130 °C. These Ag/Ce-MCM-41 catalysts with different Si:Ce ratios were characterized by IR, SXRD, XRD, TEM, UV–Vis, Raman, XPS and H2-TPR. The results show that the interactions between Ag and Ce, between Ce and SiO2 in our Ag/Ce-MCM-41 are enhanced due to adding Ce to MCM-41. The presence of Ce-SiO2 interaction dramatically enhances the reducibility of surface-capping oxygen of CeO2, promotes the bulk oxygen migration to the surface and creates oxygen vacancies in the bulk region of CeO2 and the presence of more oxygen vacancies can facilitate the anchoring and dispersing of Ag particles, which lastly improves the interaction between Ag and Ce. In addition, the amount of formed oxygen vacancy is the highest when the Si:Ce = 1:1. The activation of surface oxygen is closely related to oxygen vacancy, O2 may be transformed into the highly active O2−. The high active O2− in surface adsorbed oxygen promotes the activation of oxygen molecules and enhanced the catalytic oxidation of HCHO.

Octahedral Layered Birnessite (OL) Supported Ag Catalysts: Characterization and Catalytic Oxidation of CO

Octahedral layered birnessite (OL) was synthesized by redox method, and OL supported Ag catalysts (xAg/OL, x = 0.1wt%, 0.2wt%, 0.3wt%, 0.5wt%) were prepared by ion exchange method. Then catalysts were characterized by XRD, SEM, BET, H2-TPR, TG, O2-TPD and in-situ DRIFTS, while the catalytic activity of CO was evaluated. Among xAg/OL samples, the 0.3Ag/OL exhibited the best catalytic activity for CO oxidation (T50 = 105 oC and T90 = 135 oC). The results show that the chemical adsorption of oxygen, the low-temperature reducibility and the strong interaction between the Ag species and OL are related to the excellent catalytic activity of xAg/OL. The reaction mechanism was studied by in-situ DRIFTS. First, O2 was adsorbed and activated on the oxygen vacancies of xAg/OL, then formed oxygen free radical attacked the adsorbed CO and produced CO2, subsequently CO2 desorbed from the catalyst surface. Oxygen vacancies was supplemented by gas O2, thus circulating.

Resolving the puzzle of single-atom silver dispersion on nanosized \u03b3-Al2O3 surface for high catalytic performance

Ag/γ-Al2O3 is widely used for catalyzing various reactions, and its performance depends on the valence state, morphology and dispersion of Ag species. However, detailed anchoring mechanism of Ag species on γ-Al2O3 remains largely unknown. Herein, we reveal that the terminal hydroxyls on γ-Al2O3 are responsible for anchoring Ag species. The abundant terminal hydroxyls existed on nanosized γ-Al2O3 can lead to single-atom silver dispersion, thereby resulting in markedly enhanced performance than the Ag cluster on microsized γ-Al2O3. Density-functional-theory calculations confirm that Ag atom is mainly anchored by the terminal hydroxyls on (100) surface, forming a staple-like local structure with each Ag atom bonded with two or three terminal hydroxyls. Our finding resolves the puzzle on why the single-atom silver dispersion can be spontaneously achieved only on nanosized γ-Al2O3, but not on microsized γ-Al2O3. The obtained insight into the Ag species dispersion will benefit future design of more efficient supported Ag catalysts.

Applications of silver nanocatalysts for low-temperature oxidation of carbon monoxide

Silver (Ag) is the best catalyst for many catalytic oxidation reactions. It is highly active for total oxidation of carbon monoxide (CO) at a lower temperature. The performance of silver catalysts depending upon their structure, surface active sites, and various Ag-O interactions. The other factors effects on the silver catalysts performances are synthesis method, treatment and reaction conditions and sizes of various Ag nanoparticles. The role of various silver species Ag0/Ag+ has examined and Ag0 as an active species originated to increases the catalytic activity at ambient conditions. The nano-sized Ag catalysts have concerned much interest because of their distinctive catalytic activity for different reactions. The silver catalysts highly dispersed over various support would be constructive for the best activity in CO oxidation at low temperatures. The particle size has certain effects on the catalytic performances of supported Ag catalysts. For active supports, the lattice oxygen presence in the support and electron transform among the silver‐oxygen and transition metals play a major role in the catalytic process. There are huge research articles are present on this matter, but very few numbers of review papers are available on this issue. So this is a gap fulfilled by this manuscript.

Supported Ag Nanoparticles and Clusters for CO Oxidation: Size Effects and Influence of the Silver–Oxygen Interactions

Supported Ag catalysts on silica and corundum have been synthesized applying an improved impregnation technique. The resulting Ag particle sizes can be divided into three categories concerning: (I) bulk-like, (II) nanoparticles of 1–6 nm, and (III) in situ created Ag clusters below 1 nm. Ag nanoparticles and bulk-like Ag are investigated concerning their pretreatment dependence for CO oxidation showing that harsher pretreatment conditions need to be applied for smaller particle sizes, based on their tendency to form Ag2CO3. A particle size effect for Ag in oxidation reactions is investigated using CO oxidation as a test reaction. The CO oxidation performance is increasing with decreasing particle size with Ag clusters showing the highest activity. A novel method based on the adsorption of ethylene (C2H4) as sensor molecule is further used to discriminate the silver–oxygen (Ag–O) interaction strength of bulk-like Ag, Ag nanoparticles, and Ag clusters, showing a distinct Ag–O chemistry for the three individ...

Promoted chemoselective crotonaldehyde hydrogenation on zirconia-doped SiO2 supported Ag catalysts: Interfacial catalysis over ternary Ag–ZrO2–SiO2 interfaces

In gas-phase chemoselective hydrogenation of crotonaldehyde on Ag-based catalysts, zirconia doping on silica supports was found to improve catalytic performance in terms of unsaturated alcohol selectivity, hydrogenation activity, and stability. The surface modification of silica by zirconia doping favors the fine dispersion of Ag species due to the enhanced quantity and strength of surface acid sites, which enable construction of abundant catalytic sites effective for CO bond hydrogenation. High crotyl alcohol selectivity, exceeding 80%, and significant inhibition of monohydrogenation on the CC bond were observed on the optimal Ag/Zr–SiO2 catalyst. Dynamic O2 chemisorption measurement revealed that the pure Ag powders did not chemisorb O2 irreversibly under 323 K, but SiO2 or Zr–SiO2 supported Ag catalysts did. The amounts of Ag active for O2 chemisorption, which are at least one order of magnitude lower than that of surface Ag derived from TEM and XRD characterizations, match well with the perimeter interface Ag of hemispherical particles. A strong correlation between hydrogenation activity and O2 uptake on those Ag/SiO2 and Ag/Zr–SiO2 catalysts with different Ag dispersions and deactivation degrees was observed, implying that the effective catalytic sites for crotonaldehyde chemoselective hydrogenation may originate from accessible interface sites with unique redox properties. Catalyst induction and deactivation were observed on both Ag/SiO2 and Ag/Zr–SiO2 catalysts in real catalytic operation. Changes in metal stable interface structure, rather than metal aggregation and coagulation, are assumed to be the main cause of irreversible catalyst deactivation, because the apparent Ag particle sizes changed slightly, but the oxygen chemisorption ability deteriorated considerably. Electropositive Ag sites interacting with neighboring oxygen from oxide supports at the ternary Ag–ZrO2–SiO2 interface are proposed to account for highly selective CO bond hydrogenation to produce the desired unsaturated alcohol.

Theoretical insight into the single-atom catalytic mechanism of CeO2-supported Ag catalysts in CO oxidation.

Revealing the accurate active center structure and the functional mechanism of CeO2-supported Ag catalysts during catalysis is extremely important for their accurate synthesis. In this work, a series of AgnCeO2 (n = 1, 2, 3, 4 and 10) model catalysts was constructed, and a DFT investigation of the reaction mechanism of CO oxidation, as a probe reaction on those catalysts, was carried out. It was found that the entire catalytic reaction was completed coordinately by Ag, lattice O and O vacancies, which could be considered as the active centers. Noticeably, the mobility of Ag atoms played an important role in the reaction process, leading to the observation of a single-atom catalytic mechanism, wherein a series of single Ag atomic species was formed during the reaction, which was beneficial to CO oxidation. With the completion of some elementary reactions, the single Ag formed during the migration of CO-Ag could return to the Ag cluster again. As expected, the single-AgCeO2 catalyst exhibited extremely high activity due to the absence of the binding effect of Ag-Ag. Nevertheless, the AgnCeO2 (n > 1) catalysts showed similar catalytic activity, which was slightly worse than that of single AgCeO2, indicating that the size effect of the Ag cluster was not obvious. These results provide the theoretical basis for further understanding the functional mechanism of the AgnCeO2 catalyst and are helpful for designing various catalysts with tailored functionalities.

Enhanced catalytic benzene oxidation over a novel waste-derived Ag/eggshell catalyst

Ag nanoparticles loaded-eggshell catalysts were successfully synthesized by a simple impregnation method, in which the trashy eggshell was served as both biotemplate and support. The eggshell supported Ag catalyst exhibit excellent catalytic activity and stability for benzene oxidation.

Supported Ag Catalysts on Mg–Al Oxides toward Oxidant-Free Dehydrogenation Reaction of Benzyl Alcohol

Oxidant-free dehydrogenation of alcohols has attracted considerable attention, owing to the largely improved utility value of alcohols. Herein, mixed metal oxides (MMOs) via topological transformation of Mg–Al hydrotalcite precursors were prepared and used as support for the immobilization of metallic Ag nanoparticles. The resulting Ag/MgAl-MMO shows an excellent catalytic performance toward oxidant-free dehydrogenation of benzyl alcohol (conversion of benzyl alcohol: 95.9%; selectivity of benzaldehyde: 93.3%), which is close to platinum-group catalysts commonly used in this reaction. A combination study based on a series of characterization methods reveals a synergistic catalysis between Agδ+ and medium-strong acid/basic sites (Mgδ+–Oδ−) from support: Agδ+ facilitates the breakage of α-C–H bond while Mgδ+–Oδ− accelerates the cleavage of O–H bond. This work provides an effective approach to enhance catalytic behavior toward dehydrogenation reactions by virtue of metal–support interactions.

Efficient Catalytic Removal of Formaldehyde over Ag/Co3O4–CeO2 Prepared by Different Method

A series of Co3O4–CeO2 supported Ag catalysts were prepared by different methods and the catalytic properties of these catalysts were investigated for the catalytic removal of formaldehyde (HCHO). The Ag/Co3O4–CeO2–H catalyst calcined in a O2/N2 flow and then reduced in a H2/N2 stream exhibited excellent catalytic activity, over which 100% HCHO conversion was achieved at a temperature as low as 60 °C. In addition, the Ag/Co3O4–CeO2–H catalyst also displayed high stability in lifetime testing. Based on a variety of physical–chemical characterization results, it is demonstrated that the larger specific area, easier reducibility and abundant active surface oxygen species served as essential roles in the catalytic removal of HCHO for Ag/Co3O4–CeO2–H catalyst.

In-situ Electrodeposition of Highly Active Silver Catalyst on Carbon Fiber Papers as Binder Free Cathodes for Aluminum-air Battery

Carbon fiber papers supported Ag catalysts (Ag/CFP) with different coverage of electro-active site are prepared by electrochemical deposition and used as binder free cathodes in primary aluminum-air (Al-air) battery. Scanning Electron Microscopy and X-ray Diffraction studies are carried out to characterize the as-prepared Ag/CFP air cathodes. Oxygen reduction reaction (ORR) activities on these air cathodes in alkaline solutions are systematic studied. A newly designed aluminum-air cell is used to further determine the cathodes performance under real operation condition and during the test, the Ag/CFP electrodes show outstanding catalytic activity for ORR in concentrated alkaline electrolyte, and no obvious activity degradation is observed after long-time discharge. The electrochemical test results display the dependence of coverage of the electro-active Ag on the catalytic performance of the air cathodes. The resulting primary Al-air battery made from the best-performing cathode shows an impressive discharge peak power density, outperforming that of using commercial nano-manganese catalyst air electrodes.

Ag supported on meso-structured SiO2 with different morphologies for CO oxidation: On the inherent factors influencing the activity of Ag catalysts

With the objective to prepare better CO oxidation catalysts and explore the inherent factors to determine the activity of supported Ag catalysts, KCC-1, SBA-15 and MCM-41, a series of mesoporous SiO2 with different morphologies, have been synthesized and used as supports for Ag to prepare catalysts (7% Ag/KCC-1, 7% Ag/SBA-15 and 7% Ag/MCM-41) for CO oxidation. It is found that the morphologies of the SiO2 supports influence the dispersion of the supported Ag species, thus resulting in catalysts with different Ag grain sizes and active metallic surface areas, as proved by XRD and H2 adsorption-desorption results. With fibrous KCC-1 silica spheres as the support, the mobilization and aggregation of the Ag particles can be effectively hindered by the open access surface mesopores. Therefore, compared with SBA-15 and MCM-41, Ag nanoparticles with much smaller grain sizes have been achieved, thus resulting in a catalyst with significantly improved activity. Furthermore, a linear relationship has been found between the differential CO oxidation rates and the Ag grain size. Therefore, it is concluded that the active Ag metal surface area is the determining factor for the activity of the silica supported Ag catalysts.

Synthesis of Ag nanoparticles on oxide and carbon supports from Ag diammine precursor

To determine whether the method of “strong electrostatic adsorption” (SEA) can be extended to the preparation of uniform and highly dispersed supported Ag catalysts, the adsorption of silver diammine, Ag(NH3)2+, also known as Tollen’s reagent, has been examined over five supports (i.e., SiO2, γ-Al2O3, ZrO2, Nb2O5, and carbon) with different surface areas and PZCs. The speciation of Ag in solution was followed by UV–vis spectroscopy, Ag uptake as a function of the solution pH was determined by atomic absorption, and Ag particle size was determined by powder XRD and STEM.At the Ag concentrations, pH, and surface loadings employed, silver diammine complexes convert into soluble Ag(H2O)2+ aquo complexes below pH 11 and hydrolyze to insoluble Ag2O above pH 11. The deposition of either Ag species over carbon at any pH appears to be reactive and results in large particles. The silver aquo complexes appear to adsorb via ion exchange near the PZC of alumina, zirconia, and niobia but not silica, while the silver diammine complexes appear to adsorb electrostatically at high pH over all the oxides tested - niobia, silica, zirconia, and alumina in the order of ascending PZC. Niobia, which has the lowest PZC, adsorbs the highest surface density of Ag via both mechanisms. The particles obtained via electrostatic adsorption of Ag(NH3)2+ at high pH are somewhat smaller than those formed from Ag(H2O)2+ ions at the lower pH values. In the absence of ammonia in solution at high pH, deposition of Ag aquo complexes occurs via surface precipitation and gives large particles. In sum, SEA of silver ammine is demonstrated to be a simple, reproducible way to synthesize small particles on all supports but carbon.

Enhanced Stabilities of Ag Electrocatalyst as Self-Standing and Multiwalled Carbon Nanotube Supported Nanostructures for Oxygen Reduction in Alkaline Medium

Self-supported silver nanoflowers (AgNFs) and unmodified multi-walled carbon nanotube (MWCNT) supported silver nanostructures (Ag/MWCNT) were synthesized by an environmentally amicable slow chemical reduction method using ascorbic acid at room temperature. Structural morphologies were analyzed by scanning and transmission electron microscopic techniques and crystallinities were determined by X-ray diffraction method. Electrocatalytic activities for oxygen reduction reaction (ORR) were investigated for both Ag NF and Ag/MWCNT catalysts using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). From CV profiles the electrochemically active surface areas of Ag NF and Ag/MWCNT were estimated to be 87.7 m2/g and 19.8 m2/g respectively. The LSV showed the maximum limiting current densities of 4.7 and 3.76 mA/cm2 at the electrode rotation rate of 2400 rpm with specific activities of 1.26 and 1.06 mA/cm2 measured at −0.1 V for Ag NF and Ag/MWCNT, respectively, confirmed their high ORR activities in 0.5 M KOH medium. Accelerated durability tests revealed that Ag/MWCNT possessed an excellent stability, that is, it has retained nearly 83% of its initial activity after 3000 potential cycles, which is much higher than modified MWCNT supported Ag catalyst reported in literature.