Nuclear magnetic resonance study of Pt–Rh bimetallic clusters

Abstract

The use of pulsed nuclear magnetic resonance to study supported Pt–Rh bimetallic clusters is described. Using energy-dispersive spectroscopy it has been shown that each individual cluster contains both Pt and Rh. For lower-dispersion samples (larger metal particles) the centre of the particles is pure Pt. For higher-dispersion samples (smaller metal particles) the Pt concentration in the surface layer of the clusters has been measured from observation of the dependence of the height of the Pt spin echo on the time, τ, between pulses of the echo sequence. For clusters on which ¹³CO has been adsorbed, the ¹³CO resonance frequency varies with the Pt–Rh ratio. ¹⁹⁵Pt–¹³C spin-echo double resonance (SEDOR) has been employed to measure the fraction of Pt atoms in the surface layer of the cluster. By studying how the SEDOR signal depends on the frequency of the ¹³CO exciting pulse, it has been shown that the resonance frequency of ¹³CO is independent of whether the CO is bonded to Pt or Rh in a given sample. This result shows that the frequency shift of the ¹³CO is a non-local rather than a local phenomenon. The ¹³CO shift corresponds to that of pure Rh particles, even for surfaces which contain a substantial Pt content. The significance of this result is explained in terms of the mixing of the molecular and metallic wave-functions.