Uv-visible characterization of gold exchanged in beta zeolite

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Comparative Study of Formation and Stabilization of Gold and Silver Clusters and Nanoparticles in Mordenites

Supporting silver and gold on mordenites by ion-exchange method with further reduction with H2 leads to formation of neutral and charged metal clusters inside zeolite channels as well as metal nanoparticles on external surface of mordenite. A portion of the cluster states of the metals and stability of the clusters depend strongly on acidity of zeolite (determined by SiO2/Al2O3 molar ratio) and nature of zeolite cation (H+, Na+, NH+ 4. The investigations of silver and gold samples after prolonged storage for 6 and 12 months revealed that silver clusters are comparatively stable while oxidation of gold clusters and nanoparticles by air is the probable cause of deactivation of gold catalysts. The comparison of the results for Au and Ag samples allow suggesting NaM15 and NaM24 mordenites for effective synthesis of complex Au–Ag clusters as active and stable species of catalytic reactions occurring at room temperatur

More insights into support and preparation method effects in gold catalyzed glycerol oxidation

Background: Materials containing supported gold nanoparticles are considered as the most favorable for liquid-phase alcohol oxidation. Our previous studies showed that MgAl2O4 spinel can be an effective support for gold catalysts in liquid-phase alcohol oxidation. Objective: The aim of the present paper a comparative study of gold supported on different metal oxides (Al2O3, MgO, MgAl2O4 spinel) in liquid phase oxidation of glycerol. Also we highlighted the influence of the precursor protective layer in mediating the support effect on catalytic activity and comparing MgAl2O4 spinel with individual oxide supports. Method: Catalytic tests in glycerol liquid-phase oxidation at 50 \ub0C. Catalyst characterization by Transmission electron microscopy, X-ray diffraction, adsorption measurements, plasma atomic emission spectroscopy, X-ray photoelectron spectroscopy. Results and Conclusion: It was shown that, depending on the preparation method, the activity of Au supported on oxide supports can become reversed. For deposition-precipitation method Au/Al2O3 is more active than Au/MgO while for sol immobilization, the contrary occurs and Au/MgO is more active than Au/Al2O3. Activities of Au catalysts supported on MgAl2O4 spinel are high and close to each other for catalysts prepared by both methods. It is suggested that Au particle size and interaction of Au with support are activity determining parameters, while gold content do not play a significant role. This study gets an insight on the importance of the direct contact between the support and the gold particle

Influence of cation nature on stabilization of gold nanospecies in mordenites

Gold species deposited on NH+4 - and H+-mordenites were studied by X-ray diffraction, xenon adsorption, NMR of 27Al and 129Xe, UV-Visible spectroscopy and TPR. Mordenite ion-exchange cations exert a significant effect on size distribution and electronic state of gold species supported on the zeolites. AuNH 4M contains larger nanoparticles of gold on its external surface than AuHM because protonic form of zeolite has stronger sites for stabilization of gold nanospecies. In the case of NH+4 -mordenite, Au clusters inside zeolite channels have bigger size and fill the side-pockets completely after gold deposition due to more complete reduction of gold entities, while in H+-mordenite only half the side-pockets are filled by gold species. H+-mordenite favors the stabilization of both small gold clusters and cations. Even after the reduction of gold by hydrogen flow at 100 °C, some gold species are not completely reduced and have certain effective charge (Auδ+n clusters)

Electronic state of gold in supported clusters

The electronic states of gold in gold supported nanoparticles modified by Ce, Zr, La and Cs oxides have been studied by the methods of IR spectroscopy of adsorbed CO, UV-visible spectroscopy of diffuse reflectance, XRD and electron microscopy. The additives of Ce and Zr oxides stabilize the ionic states of supported gold and increase the effective charge of the ions. In contrast, La and Cs oxides lower the ion effective charge and favour their fast reduction under redox treatments and reaction medium. It is explained by electron donor-acceptor interaction of the supported metal nanoparticles with the modifiers

The Influence of the Peptide Molar Ratios on the Functionalization of Gold Nanoparticles

Au nanoparticles (NPs) functionalized with L-cysteine (Cys) and cysteine-glycine (Cys-Gly) were synthetized. The AuNPs were prepared using sodium citrate as reducing agent. The influence of the molar concentrations of Cys and Cys-Gly, as well as the sodium citrate is studied on particle size and particle size distribution. TEM measurements revealed the formation of AuNPs with diameter in the range 5-35 nm which corresponds to nontoxic sizes [we should add a reference here, perhaps number one]. The optimal particle size for biomedical application along with narrow particle size distribution was observed for samples prepared with molar ratio of CAu:Ccitrate = 1:10. The results of UV-Vis spectroscopy revealed the interaction of the AuNPs with Cys and Gly-Cys demonstrated by a visible change in the absorption intensities of the plasmon peak located at 520 nm after AuNP functionalization and a slight shifting of this gold nanoparticles plasmon peak. Thus, any dielectric shell on surface of particles with more refraction index (and, correspondingly, dielectric function) can produce the particles with the red shift. Such effect of the surface shell with red-shift in the range of few nanometers observed for the AuNPs functionalized with Cys and Cys-Gly (Fig. 4) can be interpreted as thin or discontinuous layer of aminoacid molecules according to the data of optical spectra simulation