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

Abstract

The turnover frequency (TOF) for conversion of neo-pentane was determined for Pt in Y zeolite with different numbers of protons and La +3 ions, different Si/Al ratios and with non-framework Al being present. Comparing Pt/NaY to Pt/H-NaY and Pt/K-USY with Pt/H-USY, respectively, shows an increase in the ln (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 of Pt. The 20 times increase in the Pt TOF for K-USY compared to NaY is attributed 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 consisting 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 oxygen ions of the support. The peak in the Fourier transform of the atomic XAFS (AXAFS) spectrum of the Pt/H-USY is larger in intensity than the corresponding 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 the 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) between 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 and the Pt/H-USY catalysts provide direct experimental evidence of how the support properties determine the electronic structure of the platinum metal particles. Previous AXAFS and shape resonance work lead to a model in which the position in energy of the Pt valence orbitals is directly influenced by changes in the potential (i.e. electron charge) of the oxygen ions of the support and how the proton density affects this oxygen charge. This work shows that the potential of the oxygen ions is also a function of the Si/Al ratio of the support and the polarisation power of the charge compensating cations (H +, Na +, La 3+ and extra-framework Al); the metal particles experience an interaction which is determined by several properties of the support. The data further reveal how the change in the Pt electronic structure directly influences the catalytic properties of the catalyst. While the TOF is dependent on the metal–support interaction, the hydrogenolysis selectivity is determined by the Pt particle size, and increases linearly with increasing dispersion, or decreasing particle size.