From single crystals to supported nanoparticles in experimental and theoretical studies of H 2 oxidation over platinum metals (Pt, Pd): Intermediates, surface waves and spillover

Abstract

The present paper reviews our investigations concerning the mechanism of H 2 + O 2 reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti 3+/TiO 2). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H 2 oxidation on a nanolevel. The adsorption of both O 2 and H 2 and several dissociative products (H ads, O ads, OH ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H 2O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti 3+-□ O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H 2 + O 2 steady-state reaction over Pd–Ti 3+/TiO 2 surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H +/TiO x ) as a result of spillover effect: diffusion of H ads atoms from Pd particles on a TiO x surface. The comprehensive study of H 2, O 2 adsorption and H 2 + O 2 reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces.