IN-SITU D-ELECTRON DENSITY OF PT PARTICLES ON SUPPORTS BY XANES

The dependency of d electron density of Pt in Pt/SiO2 catalysts on the particle size was investigated by means of in situ X-ray absorption near-edge structure (in situ XANES) spectroscopy. The d electron density of Pt particles was measured under vacuum, H-2 and ethene, to gain information about ethene hydrogenation on Pt/SiO2. The intensities of the white lines at L(III) and L(II) edges in XANES spectra, which are regarded to reflect the unoccupied density of state, varied with the change of particle size under both vacuum and reaction gas atmospheres. The interaction between Pt particle and adsorbates was weak with small particles below 1.5 nm. A new peak induced by Pt-H bonding in the XANES spectra under H-2 was observed for the samples with Pt particle size less-than-or-equal-to 1.5 nm. This is related to the change of the turnover frequency and activation energy for ethene hydrogenation by Pt particle size.