Atomic XAFS as a Tool to Probe the Electronic Properties of Supported Noble Metal Nanoclusters

Abstract

Supported noble metal nanoclusters find widespread applications in heterogeneous catalysis as their catalytic activity can be altered by changing the support composition and architecture or by adding the appropriate promoting elements. 1 These effects are related to changes in the electronic properties of the noble metal, as measured with IR spectroscopy after, for example, CO adsorption. It is generally accepted that the ratio of linear-to-bridged metalcoordinated CtO reflects the electronic properties of the adsorbing noble metal nanocluster, and this ratio increases with increasing ionization potential of the metal particle. 2 Another attractive, but almost unexplored, technique for probing the electronic structure of supported noble metal nanoclusters is atomic X-ray absorption fine structure spectroscopy (AXAFS). Holland et al. 3 first recognized this feature, and the groups of Rehr, O'Grady, Baberschke, Ramaker, and Koningsberger did its further development. 4 Whereas extended X-ray absorption fine structure spectroscopy (EXAFS) is known to originate from the scattering of the outgoing electron against the potential of neighboring atoms, AXAFS represents the scattering against the potential of the electron cloud of the absorber atom itself. The embedded potential of probed atoms is dependent on the chemical and electronic environment of the atoms and can be influenced by the support characteristics. The intensity and position of the AXAFS peak is a function of the bonding of the absorbing atom with its environment. Therefore, any change in the support oxide altering the embedded potential of the absorbing noble atom will be reflected in its AXAFS spectrum.The introduction of AXAFS as a powerful new tool for studying heterogeneous catalysts is hampered by the lack of sufficiently broad experimental data to support the relation between the AXAFS intensity of catalytic systems and the corresponding changes in the electronic properties. Here, we show for a wide set of different support oxides that AXAFS accurately probes the electronic properties of supported Pt nanoclusters. A new tool with the potential to probe the electronic changes in metal catalysts under reaction conditions is explored.For this purpose, the AXAFS intensities of 14 different supported Pt catalysts [ranging from microporous (H-USY and zeolite Y exchanged with H + , Na + , K + , Rb + , Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) over mesoporous to macroporous (SiO 2 and SiO 2 loaded with Cs + and Ba 2+ ) support oxides] are compared with the corresponding linear-to-bridged Pt-coordinated CtO ratios as obtained with IR spectroscopy on the same set of samples after adsorption of CtO at room temperature in the same spectroscopic in situ cell.Supported Pt particles (1 wt %) were prepared via a dry impregnation step of the support oxide materials with the appropriate aqueous solutions of Pt(NH 3 ) 4 (NO 3 ) 2 . After impregnation and drying at 353 K in N 2 for 12 h, calcination was carried out by drying in a high air flow for 12 h at 393 K followed...

Pore curvature and support composition effects on the electronic properties of supported Pt catalysts: An infrared spectroscopy study with CO as probe molecule

“…The metal ion-impregnated Pt/SiO 2 samples showed Pt clusters of 1-3 nm for Pt/Na-SiO 2 , Pt/MgSiO 2 and Pt/Ba-SiO 2 , while for Pt/Cs-SiO 2 particles up to 10 nm were present. The same observation of larger Pt particles was made previously on the Pt/Cs-Y catalysts [7][8][9]. To illustrate the Pt particle size and distribution, a selection of HRTEM micrographs of Pt/ITQ-1, Pt/Si-MCM-41, Pt/Na-SiO 2 and Pt/Ba-SiO 2 is presented in Fig.…”

“…In principle, three effects of the support on the catalytic activity of the metal particles can be distinguished: i.e., (1) the support composition, (2) the support pore size and curvature and (3) the presence of promoting elements. In previous papers, we have reported on well-defined Pt nanoparticles supported on a microporous crystalline aluminosilicate material [7][8][9]. More specifically, we have investigated the effect of monovalent (Na + ) cations on the electronic properties of Pt nanoparticles entrapped in the supercages of zeolite Y. Pt-CBBO temperature programmed desorption infrared spectroscopy (TPD-IR) and atomic X-ray absorption fine structure (AXAFS) studies were used to determine the electron charge on the supported Pt nanoparticles.…”

“…As a consequence, the chemical composition of the support must play the dominant role. For that reason, it was a logical next step to investigate whether the effect on the electronic properties of the metal particles that was obtained by the introduction of promoting cations in Pt/zeolite-Y [7][8][9], can be transferred to other support materials, such as an all-silica one. For this purpose, we have studied a series of Pt/SiO 2 catalysts loaded with monovalent (Na + , Cs + ) and divalent (Mg 2+ , Ba 2+ ) cations.…”

Application of AXAFS Spectroscopy to Transition‐Metal Oxides: Influence of the Nearest and Next Nearest Neighbour Shells in Vanadium Oxides

“…For supported metal particles, the effect of the support on the catalytic properties has been determined by several techniques, for example, XPS, FT-IR analysis of adsorbed CO, and newly developed delta XANES and atomic XAFS (AXAFS) techniques. [8][9][10][11][12][13][14][15] In particular, the application of AXAFS has, in our opinion, led to a better understanding of metal-support interaction effects. [8,[15][16][17] AXAFS is sensitive to changes in the potential around the X-ray absorbing atom.…”

Structural studies on ruthenium(II) complexes used in interphase catalysis for the hydrogenation of ketones

“…The intensity and position of the AXAFS features is a function of the bonding of the absorbing atom with its environment. 16 Among the interphase catalysis reactions, the asymmetric transfer hydrogenation of prochiral ketones is one of the most attractive methods for the access of optically active secondary alcohols, which form an important class of intermediates for fine chemical and pharmaceutical industries. Ever since the discovery of the Meerwein-Ponndorf-Verley reaction, in which a ketone is reduced to an alcohol in the presence of aluminum alkoxide, co-catalysts have been widely used for the promotion of the catalytic activity of metal complex catalysts.…”