Reversible structural change of Rh particles supported on GeO2 submonolayers–SiO2 in reduction and oxidation by XAFS, XRD, TEM and FTIR

Abstract

The behaviors of Rh particles and GeO ₂ submonolayers supported on SiO ₂ during oxidation and reduction have been investigated, revealing a structural change in the outermost support surface of supported metal catalysts, by means of XAFS, XRD, TEM and FTIR. Partial reduction of the GeO ₂ overlayers in Rh/GeO ₂ /SiO ₂ occurred to form RhGe alloy particles by reduction with H ₂ at 423–723 K, as proved by direct Rh–Ge bonding observed by EXAFS at both the Rh- and the Ge-K edges. The RhGe alloy particles were transformed to Rh oxides and GeO ₂ submonolayers by calcination at 673 K; conversely, these oxide phases were converted back to RhGe alloy particles by reduction at 423–723 K. These transformations on SiO ₂ took place reversibly during the oxidation–reduction treatments. This is a unique structural change at the Rh/GeO ₂ overlayer interface in contrast to the behaviors of Rh and Ge on bulk GeO ₂ which were irreversible owing to subsidence of Rh into the GeO ₂ bulk.