The metal-support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt-H shape resonance and atomic XAFS spectroscopy

The turnover frequency (TOF) for conversion of neo-pentane was determined f or Pt in Y zeolite with different numbers of protons and La+3 ions, differe nt Si/Al ratios and with non-framework Al being present. Comparing Pt/NaY t o Pt/H-NaY and Pt/K-USY with Pt/H-USY, respectively, shows an increase in t he In(TOF) which is proportional to the number of protons. Compared to NaY, the TOF of Pt in non-acidic NaLaY zeolite is about 25 times higher, which indicates also a strong influence on the charge of the cations on the TOF o f Pt. The 20 times increase in the Pt TOF for K-USY compared to NaY is attr ibuted to the effect of a higher Si/Al ratio and non-framework Al in the K- USY. EXAFS data collected on Pt/NaY and Pt/H-USY showed platinum particles consi sting of 14-20 atoms on an average. These results were confirmed by HRTEM, which also showed that the Pt particles were dispersed inside the zeolite. The EXAFS data indicate that the metal particles are in contact with the ox ygen ions of the support. The peak in the Fourier transform of the atomic X AFS (AXAFS) spectrum of the Pt/H-USY is larger in intensity than the corres ponding peak of the Pt/Na-Y data. A detailed analysis of the L-2 and the L- 3 X-ray absorption near edge structure revealed a shape resonance due to th e Pt-H anti-bonding state (AS) induced by chemisorption of hydrogen on the surface of the platinum metal particles. The difference in energy (E-res) b etween the AS and the Fermi-level (E-F) is 4.7 eV larger for Pt/H-USY than for Pt/NaY. Both the AXAFS spectra and the shape resonances of the Pt-NaY a nd the Pt/H-USY catalysts provide direct experimental evidence of how the s upport properties determine the electronic structure of the platinum metal particles. Previous AXAFS and shape resonance work lead to a model in which the positi on in energy of the Pt valence orbitals is directly influenced by changes i n the potential (i.e. electron charge) of the oxygen ions of the support an d how the proton density affects this oxygen charge. This work shows that t he potential of the oxygen ions is also a function of the Si/Al ratio of th e support and the polarisation power of the charge compensating cations (H, Na+, La3+ and extra-framework Al); the metal particles experience an inte raction which is determined by several properties of the support. The data further reveal how the change in the Pt electronic structure directly influ ences the catalytic properties of the catalyst. While the TOF is dependent on the metal-support interaction, the hydrogenol ysis selectivity is determined by the Pt particle size, and increases linea rly with increasing dispersion, or decreasing particle size. (C) 2000 Elsev ier Science B.V. All rights reserved.