Formation of catalytically active nanoparticles on surface of various supports.
Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia
Fine metal and oxide particles supported by various oxides are an important class of catalytic systems. The surface of oxide support can strongly affect the degree of dispersion, structure and catalytic properties of supported nanoparticles. Structural characterization is known to play key role in understanding of catalyst structure-activity relationships. The most interest is in the structure of catalytically active nanoparticles. However, traditional routine XRD methods are usually ineffective for this study. The contribution to XRD pattern originating from small fraction of active component is insignificant in most cases. Moreover, diffraction patterns of highly dispersed materials are characterized by strong background and broadened peaks. Radial distribution function (RDF) of electronic density or atomic pair distribution function (PDF) method is effective for studying the local structure (short range atomic arrangement) of nanoparticles. This method is based on the Fourier relationship between intensity of coherent X-ray scattering and RDF of electronic density [1-3]. Using this technique, one can directly define interatomic distances and coordination numbers of atomic arrangement. The PDF method is used to determine: 1) phase analysis of nanomaterials; 2) changes in the structure associated with the particle size; 3) particle sizes <2 nm; 4) structural defects; 5) the structural aspect of the fixing of nanoparticles in the matrix. To determine features of the local structure of supported nanoparticles as against well crystallized analogues a comparison of the experimental RDF and the model one constructed on the basis of known structural data is used . In this work we report a comparison of the possibilities of methods PDF and EXAFS and some examples of the RDF analysis application: 1) to determine the structure of highly dispersed active component in supported catalysts 2) to elucidate structural aspects of interaction between the support and active component. The Au(Pt)/γ-Al2O3/C/ SiO2 catalysts with different metal content were considered. RDF data strongly suggested an epitaxial interaction between supported metal Au and Pt particles and the surface of support (Al2O3). Such interaction may be a reason for high thermal stability of supported metallic nanoparticles. The catalyst Pt/SiO2 was studied by the X-ray Diffraction Radial Electron Density technique and EXAFS spectroscopy. It was found that the sample, kept in air and additionally untreated, contains the phases of metallic platinum Pt0 and oxide platinum PtO (~1:2). The EXAFS data were analysed, assuming three platinum particle models. Model 1 contains the shortest Pt-Pt distance, which is the same for the particle bulk and its surface. Model 2 has two different short distances for the particle bulk and its surface. Model 3 additionally has two different Debye-Waller factors for the particle bulk and its surface. It was shown that the second model is more correct for the oxidised sample and the Pt-Pt distance between surface atoms is shortened by ~0.14Å. For reduced samples, the obtained structural data favour third model. The Cu/ZrO2 catalysts with tetragonal (t) and monoclinic (m) zirconia were considered. RDF analysis revealed that copper (II) oxide chain clusters network were the main copper species, while particles with CuO bulk structure were not formed significantly. The obtained data were in agreement with the incorporation of some copper ions into zirconia lattice. RDF data also suggested interaction of active component with zirconia surface in Cu/m-ZrO2 catalysts, which efficiently stabilized small CuO particles. The model of epitaxial growth of CuO particles on certain planes of m-ZrO2 was proposed.
 Moroz E. M. Current Topics in Catalysis. 2016. 12. 101-126.  Moroz, E. M Rus.Chem.Rev. 1992, 61, 188  D. Bazin , L.Guczi., J. Lynch, Applied Catalysis A: General 2002, 226, 87