Preparation and structural characterization of SnO 2 and GeO 2 methanol steam reforming thin film model catalysts by (HR)TEM

Abstract

Structure, morphology and composition of different tin oxide and germanium oxide thin film catalysts for the methanol steam reforming (MSR) reaction have been studied by a combination of (high-resolution) transmission electron microscopy, selected area electron diffraction, dark-field imaging and electron energy-loss spectroscopy. Deposition of the thin films on NaCl(0 0 1) cleavage faces has been carried out by thermal evaporation of the respective SnO 2 and GeO 2 powders in varying oxygen partial pressures and at different substrate temperatures. Preparation of tin oxide films in high oxygen pressures (10 −1 Pa) exclusively resulted in SnO phases, at and above 473 K substrate temperature epitaxial growth of SnO on NaCl(0 0 1) leads to well-ordered films. For lower oxygen partial pressures (10 −3 to 10 −2 Pa), mixtures of SnO and β-Sn are obtained. Well-ordered SnO 2 films, as verified by electron diffraction patterns and energy-loss spectra, are only obtained after post-oxidation of SnO films at temperatures T ≥ 673 K in 10 5 Pa O 2. Preparation of GeO x films inevitably results in amorphous films with a composition close to GeO 2, which cannot be crystallized by annealing treatments in oxygen or hydrogen at temperatures comparable to SnO/SnO 2. Similarities and differences to neighbouring oxides relevant for selective MSR in the third group of the periodic system (In 2O 3 and Ga 2O 3) are also discussed with the aim of cross-correlation in formation of nanomaterials, and ultimately, also catalytic properties.