, and Ba 2+ ) cations have been used as model systems to investigate the effect of promotor elements in the oxidation of CO in excess oxygen. Time-resolved infrared spectroscopy measurements allowed us to study the temperatureprogrammed desorption of CO from supported Pt nanoparticles to monitor the electronic changes in the local environment of adsorbed CO. It was found that the red shift of the linear Pt-coordinated CtO vibration compared to that of gas-phase CO increases with an increasing cation radius-to-charge ratio. In addition, a systematic shift from linear (L) to bridge (B) bonded CtO was observed for decreasing Lewis acidity, as expressed by the Kamlet-Taft parameter R. A decreasing R results in an increasing electron charge on the framework oxygen atoms and therefore an increasing electron charge on the supported Pt nanoparticles. This observation was confirmed with X-ray absorption spectroscopy, and the intensity of the experimental Pt atomic XAFS correlates with the Lewis acidity of the cation introduced. Furthermore, it was found that the CO coverage increases with increasing electron density on the Pt nanoparticles. This increasing electron density was found to result in an increased CO oxidation activity; i.e., the T 50% for CO oxidation decreases with decreasing R. In other words, basic promotors facilitate the chemisorption of CO on the Pt particles. The most promoted CO oxidation catalyst is a Pt/K-Y sample, which has a T 50% of 390 K and a L:B intensity ratio of 2.7. The obtained results provide guidelines to design improved CO oxidation catalysts.

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...

Jump to it! 2D IR pressure‐jump spectroscopy of adsorbed CO can be used to assess the accessibility and location of platinum nanoparticles supported on a zeolite and investigate the behavior of Pt atoms after thermal treatment (activation, catalytic reaction, sintering). Different Pt nanoparticle sintering mechanisms were observed in the mesopores and on the external surface of the ZSM‐5 support (see picture; HF=high and LF=low frequency).

Rh particles with an average diameter smaller than 1.5 nm have been supported on a series of zeolite Y samples. These zeolite materials contained different monovalent (H + , Na + , K + , Rb + , and Cs + ) and divalent (Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) cations and were used as model systems to investigate the effect of promoter elements in the oxidation of CO over supported Rh particles in excess of oxygen. Infrared (IR) spectroscopy was carried out to monitor the electronic changes in the local environment of Rh-adsorbed CO. It was found that the bands corresponding to two Rh gem-dicarbonyl species, Rh + (CO) 2 -(O z ) 2 and Rh + (CO) 2 -(O z )(H 2 O), shift to lower wavenumbers with increasing ionic radius/charge ratio of the cation. In addition, the relative intensity of the bridge bonded CO as compared to the total absorbance of Rh-bonded CO species decreases with increasing Lewis acidity, as expressed by the Kamlet-Taft parameter R of the cation. This trend could be directly correlated to the Rh CO oxidation activity, since low temperatures at 50% CO conversion corresponded with catalyst materials with a high contribution of bridge-bonded CO species and hence with small R values. A lower Lewis acidity causes an increased electron density on the framework oxygen atoms and thus an increased electron density on the zeolite-supported Rh particles. Comparable trends have been observed previously on a similar series of cation containing zeolite supported Pt catalyst materials.

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