Vasiliy V. Srabionyan

Atomic structure of PtCu nanoparticles in PtCu/C catalysts from EXAFS spectroscopy data

Deposited electrocatalysts with different distributions of components in PtCu bimetallic nanoparticles involved in their composition were synthesized by simultaneous and sequential reduction of Cu(2+) and Pt(IV) in a carbon suspension. The dependence of the atomic structure of PtCu nanoparticles on the synthesis conditions and the degree of influence of post-treatment was established from analysis of the changes in Fourier transforms of the experimental Pt and Cu EXAFS spectra, as well as the structural parameters obtained by their fitting before and after the treatment of the materials in an acid solution. A technique was proposed for visualizing the atomic structure of synthesized bimetallic nanoparticles. This technique made it possible to determine the character of the distribution of the components over the nanoparticle volume in accordance with the component composition and local atomic structure parameters determined from EXAFS spectroscopy and to obtain the visualization of the distribution of the components in PtCu nanoparticles synthesized by the aforementioned methods.

Construction of three-dimensional models of bimetallic nanoparticles based on X-ray absorption spectroscopy data

A new method for constructing three-dimensional models of bimetallic nanoparticles is proposed. This method, which is based on X-ray absorption spectroscopy data on the number and type of nearest neighbors, provides information on the distribution of types of atoms over the nanoparticle volume. The application of the method to the study of the structures of platinum–copper and platinum–silver nanoparticles of metal–carbon electrocatalysts has allowed to distinguish the nanoparticles with a core–shell structure from the nanoparticles with structure of disordered alloy or clusterized solid solution.

Determination of the local atomic structure of material from X-ray absorption spectroscopy data without fourier analysis of experimental spectra

A technique for analyzing the extended X-ray absorption fine structure spectra of an atom in different structural states in a material under study is proposed. This technique makes it possible to determine the parameters of the nearest environment of absorbing atoms without applying Fourier filtering and related techniques. The proposed approach is tested by an example of the L3 absorption spectra of platinum, obtained by direct calculation for models of one-component platinum nanoparticles and bimetallic Pt-Ag nanoparticles with different core and shell structures. The error in determining the structural parameters is analyzed, and the range of applicability of the technique proposed is discussed.

The Role of Hydrogen in Formation of Pd XANES in Pd-Nanoparticles

The effects of hydrogen in Pd nanoparticles on the features of Pd K-edge X-ray absorption near edge structure (XANES) are studied. Simulations indicate a linear dependence of the intensities of XANES maxima upon the concentration of hydrogen in the bulk of Pd clusters at low H-concentrations and stepwise increase of the first near edge peak at high concentrations. The effects of the diffusion of H atoms over the interstitial sites are considered. The application of the obtained results to the experimental spectra measured in PdH nanoparticles of sizes from 1.3nm to 10.5nm indicates low H-concentrations and high mobility of H-atoms in the bulk of the nanoparticles.

Phase behavior of Pt–Cu nanoparticles with different architecture upon their thermal treatment

Using the methods of powder X-ray diffraction, transmission electron microscopy, and EXAFS spectroscopy, the phase behavior of bimetallic Pt–Cu nanoparticles with different architecture that are deposited on a highly disperse carbon carrier has been investigated during their thermal treatment in inert atmosphere. It is established that Pt–Cu nanoparticles with a Cu-core–Pt-shell structure rearrange into nanoparticles with a Pt–Cu solid-solution structure in the temperature range from 280 to 300°C. This transformation is accompanied by a sharp change in the unit-cell parameter. Such a change in the crystal lattice parameter does not occur during the thermal treatment of material with similar composition containing Pt–Cu nanoparticles with a solid-solution structure. The results can be used in elucidating the structure of Pt–M/C materials with different nanoparticle architectures.

Hydride phase formation in carbon supported palladium hydride nanoparticles by in situ EXAFS and XRD

In the current work we present a detailed analysis of the hydride phase formation in industrial Pd/C nanocatalysts by means of combined in situ X-ray absorption spectroscopy (EXAFS), X-ray diffraction (XRD) and volumetric measurements for the temperatures from - 10 to 50 °C in the hydrogen pressure range from 0 to 1000 mbar. α- and β- hydride phases are clearly distinguished in XRD. For the first time, H/Pd atomic ratio were obtained by theoretical fitting of the near-edge region of the absorption spectra (XANES) and compared with volumetric measurements.

The effect of thermal treatment on the atomic structure of core–shell PtCu nanoparticles in PtCu/C electrocatalysts

PtCu/C electrocatalysts with bimetallic PtCu nanoparticles were synthesized by successive chemical reduction of Cu2+ and Pt(IV) in a carbon suspension prepared based on an aqueous ethylene glycol solution. The atomic structure of as-prepared PtCu nanoparticles and nanoparticles subjected to thermal treatment at 350°C was examined using PtL3 and CuK EXAFS spectra, transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). The results of joint analysis of TEM microphotographs, XRD profiles, and EXAFS spectra suggest that the synthesized electrocatalysts contain PtCu nanoparticles with a Cu core–Pt shell structure and copper oxides Cu2O and CuO. Thermal treatment of electrocatalysts at 350°C results in partial reduction of copper oxides and fusion of bimetallic nanoparticles with the formation of both homogeneous and ordered PtCu solid solutions.

Structure and Properties of Ferroelectric Materials after Mechanoactivation

The mechanoactivation of ferroelectric materials with perovskite structure is performed. Structural changes of the ferroelectric materials after mechanoactivation based on X-ray absorption spectra (XAFS) and X-ray diffraction data (XRD) are studied. The Fourier analysis of EXAFS spectra allows quantitative estimates to be obtained for the local atomic structure of the investigated materials after activation. The effect mechanoactivation has on permittivity in a wide range of frequencies and temperatures is considered.

Post-treatment of Pt- M ( M = Cu, Co, Ni)/C Electrocatalysts with Different Distribution of Metals in Nanoparticles: Evolution of Structure and Activity

The article discusses a wide range of issues related to the relationship between the composition and fine structure (architecture) of bimetallic nanoparticles and electrochemical performance of Pt-M/C (M = Cu, Co, Ni) electrocatalysts. Among these questions are: evolution of nanoparticles during thermo-treatment, catalytic activity in the oxygen reduction and methanol electro-oxidation reactions, corrosion-morphological stability of the catalysts, selective dissolution of alloying component.

Continuum model of bimetallic nanoparticles for calculating partial coordination numbers

A way of calculating partial coordination numbers is proposed for bimetallic nanoparticles with different radial distributions of atoms relative to a selected center. It is based on a continuum approximation for the atomic density distribution in the volume of nanoparticles (continuum model) and does not require the use of cluster-based models of nanoparticles to obtain information on the distribution of components in nanoparticles. The results obtained in this manner are compared to those from directly calculating the partial coordination numbers in atomic clusters.