X-ray absorption spectroscopic and electron paramagnetic resonance studies on the strong metal–support interaction of platinum supported on titania dispersed on silica

Abstract

The hydrogen-treated (at 623 and 773 K) Pt/TiO₂–SiO₂ catalyst was characterized by using Pt L-edge X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR) spectroscopies to elucidate the role of the dispersed TiO₂ in establishing the strong metal–support interaction (SMSI) state of the platinum.

The dispersed TiO₂ decreased the height of the XANES peak at 15 eV above the absorption edge, the coordination number of the Pt–Pt shell and the Pt–Pt interatomic distance relative to the corresponding Pt foil and Pt/SiO₂ data. These observations are interpreted by the decrease in Pt particle size (intrinsic effect) in the presence of the dispersed TiO₂ since no shell other than the Pt–Pt shell was detectable in the Fourier transforms of k³-weighted EXAFS. Raising the H₂ treatment temperature from 623 to 773 K did not cause significant changes in the XANES/EXAFS features.

Two signals at g= 2.03 (due to Pt⁺) and g= 1.905 (due to Ti³⁺) were observed on EPR measurement (at 77 K) of the hydrogen-treated and evacuated Pt/TiO₂–SiO₂. The intensities of both signals increased on raising the H₂ treatment temperature from 623 to 773 K. These ions (Pt⁺ and Ti³⁺) are formed by electron transfer from a Pt atom to a Ti⁴⁺ ion at the metal/support interface rather than by reverse spillover of spilt-over hydrogen species. This kind of interaction is typical of the dispersed TiO₂ and is suggested to play a significant role in the generation of the SMSI state of the Pt/TiO₂–SiO₂ catalyst.