Electronic properties of small supported Pt particles: NMR study ofPt195hyperfine parameters

Abstract

A combination of Knight-shift and relaxation-time measurements (K,$T_1$) in small Pt particles allows a quantitative interpretation in terms of the local densities of states (LDOS) at the Fermi energy for the s-like and d-like electrons. The definite metallic character of our Pt particles, evidenced by the Korringa relationship, allows us to describe K and $T_1$ by well-known equations for metals. We have included additional factors to take into account site-dependent Stoner enhancement effects. This description establishes the correspondence: (K,$T_1$)→($D_s$(${\mathrm{E}}_{\mathrm{F}}$),${\mathit{D}}_{\mathrm{d}}$ (${\mathrm{E}}_{\mathrm{F}}$)). We find a depleted Fermi LDOS along with a significant de-enhancement of the clean surface with respect to the bulk, due essentially to d electrons. We discuss the hydrogen-adsorbate-induced modification of the surface Fermi LDOS at $E_F$ which is further diminished compared to the clean one. As a byproduct of this analysis, we obtain the susceptibility of an assembly of small particles (mean particle size 27 Å): χ/${\chi }_{\mathrm{bulk}=0.6\mathrm{}}$ at 20 K. This work also presents a contribution to the problem of metal-support interactions which may become important in Pt/${\mathrm{TiO}}_2$ systems when reduced above 500 °C.