Growth and properties of small Rh particles within an NaX matrix. Thermal analysis, electron microscopy, IR and photoelectron spectroscopy

Abstract

Rhodium dispersions with different distinct average particle sizes, ranging from 1–2 to 6 nm, are prepared in zeolite NaX from Rh complexes ion exchanged with constant degrees of exchange and autoreduced by temperature-programmed decomposition. Thermal analysis and photoelectron spectroscopy demonstrate different elementary steps of Rh precipitation starting from either chlorine-free or chlorine-containing complexes. The stabilization of the Rh⁺ state for the chlorine-free sample is concluded from photoelectron spectra of Rh 3d_5/2 and confirmed by IR spectra using the probe carbon monoxide. Different average sizes of Rh particles are evaluated from electron micrographs with standard deviations of 10–20%. The shifts of the electron binding energies in core level Rh 3d_5/2 and valence-band spectra agree with theoretical predictions for intrinsic particle-size effects and indicate a low contribution of the relaxation effect.