Elena Smolentseva

Plasmon Features of Coinage Metal Nanoparticles Supported on Zeolites

The plasmonic features in the optical response of coinage metal nanoparticles supported on different type of zeolites were studied. The shifts in the plasmon frequency were analyzed for Cu, Ag, and Au nanoparticles in mordenite, β-zeolite, and Y-zeolite. It was shown experimentally that the resonance energy is sensitive both to type of zeolite structure and counter-cation of zeolite, as well as to annealing temperature and chemical composition of zeolite, their SiO2/Al2O3 molar ratio. A theoretical framework was employed to identify physical mechanism for this sensitivity. Within a simple model, the width of the absorption window identified in the imaginary part of the bulk dielectric function of the different metals was seen to play the important role in establishing the range of the plasmon energies available. In terms of an effective dielectric function, the composite medium was fully described by the complex dielectric function of the metal involved, the dissipation-free dielectric function of the zeolite matrix, and the filling fraction which relates the volume of metal inclusions as a fraction of the total sample volume. The sensitivity of the optical spectra is understood in terms of variations in both the dielectric response of the zeolite matrix as well as nanoparticle size.

Aerobic Oxidative Esterification of Benzyl Alcohol and Acetaldehyde over Gold Supported on Nanostructured Ceria–Alumina Mixed Oxides

Au nanoparticles supported on nanostructured ceria‐alumina mixed oxides (10 and 30 wt % ceria) were prepared by a deposition–precipitation method. Their properties were studied by N2 adsorption, XRD, TEM, X‐ray photoelectron spectroscopy, and UV/Vis spectroscopy under temperature‐programmed reduction or oxidation. The materials catalyzed the liquid‐phase aerobic oxidative esterification of benzyl alcohol and benzaldehyde effectively and showed a much better performance than Au supported on the individual oxides. The reactions occurred with high turnover numbers (up to 19 000) in methanol solutions in the absence of any auxiliary base and gave mainly methyl benzoate. The strong synergetic effect of ceria and alumina can be explained by the enhanced oxygen storage capacity of the materials prepared from mixed oxides compared to that of pure alumina and ceria. The order of the catalytic activity (Au/Al2O3

DFT study of composites formed by M2 metallic clusters (M = Ni, Cu, Fe and Au) embedded in faujasite

The present work is a theoretical study of the different frameworks and composites of faujasite (FAU) zeolite. Most of the computations for composites were performed by embedding M2 metal clusters (M = Ni, Cu, Fe and Au) within the Al2Si40O96 FAU zeolite framework. Results showed that, in this framework, the difference between the α and β spin induced an unfolding band structure for Cu2 and Au2 clusters. Thus, their respective band gap energy decreased. In addition, low energy band gap values are associated with composites in which the cluster presents practically no hybridized orbitals. In particular, the charge trace of a zeolitic framework was identified as a fingerprint that is different for other types of zeolites and composites. In this charge trace, the framework atoms on equivalent sites with lower charge values were identified as being responsible for clusters adsorption.

Plasmon spectra of binary Ag-Cu mixtures supported in mordenite

Ag and Cu nanoparticles supported in mordenite structure have been formed applying reduction temperatures in the range 100-400 C and varying Ag/Cu atomic ratios. Absorbance spectra of samples exhibit signature features consistent with absorption via localized surface plasmons propagating in metallic nanoparticles. The formation of binary Ag-Cu nanoparticles is inferred. Theoretical calculations within an average field Maxwell-Garnett model modified for a three component composite system are used to interpret resonance shifts and relative intensities of plasmon peaks in the experimental findings. Within the applied model the relative volume occupied by each metallic species can be changed. This permits the simulation of experimental conditions of the samples. It is experimentally found that the simultaneous presence of two metal species during the synthesis affects reduction temperatures, stability and relative concentration of embedded nanoparticles. Furthermore the observed optical spectra of the supported bimetallic nanoparticles is contrasted with that of single metal nanoparticles studied previously. Our study represents a contribution to the possibility of optical monitoring of synthetic pathways in zeolite + metal nanoparticle systems.

Optical spectra of noble metal nanoparticles supported on zeolites

Optical spectra of noble metal nano-particles supported on different types of zeolites are studied and compared. The absorbance spectra of Cu, Ag and Au nanoparticles supported on mordenite, β-zeolite, Na/Y and H/Y zeolites respectively are reported. Spectra for pre-exchanged Au-Cu/Na/Y, Au-Ni/Na/Y and Au-Fe/Na/Y are also studied. A simple effective medium approach (Maxwell-Garnett) is used to obtain a theoretical complex effective dielectric function of the composite and to asses the sensibility of the plasmon resonance to the sample characteristics. The knowledge of these properties can hopefully be applied to the development of optical tools to monitor the synthetic path.