Pd/Ga2O3 methanol steam reforming catalysts: Part I. Morphology, composition and structural aspects

The combination of (high-resolution) electron microscopy, selected area diffraction and X-ray diffraction was applied to study the structural, compositional and morphological alterations of two Pd/Ga2O3 catalysts relevant for methanol steam reforming upon different oxidative and reductive treatments. These systems include well-defined Pd particles grown epitaxially on vacuum-cleaved NaCl(0 0 1) single crystals and subsequently covered by a layer of amorphous Ga2O3 (termed as 'thin film model catalysts'), as well as, for comparison, a conventional Pd/Ga2O3 powder catalyst prepared by incipient wetness impregnation. Both catalysts were subsequently subjected to similar treatments in O2 (1 bar, 1 h) and H2 (1 bar, 1 h) in the temperature range between 373 and 923 K. Oxidation of the thin film model catalyst at 773 K converts the Pd/Ga2O3 film into a mixture of PdO and Pd supported on Ga2O3. Subsequent reduction at 523 K causes the formation of an epitaxial Pd5Ga2 bimetallic phase with suppressed Pd hydride formation, in close correlation to the corresponding Pd/ZnO system. In contrast to the latter, Pd/Ga2O3 thin film model catalysts are prone to sintering at reduction temperatures > =673 K. However, Pd5Ga2 with some Pd probably remaining in the particle cores is still present. The stability of the bimetallic Pd5Ga2 phase in oxygen was observed to depend on the pre-reduction history, that is, oxidation at 673 K of a bimetallic formed at 523 K causes the complete decomposition of the bimetallic phase and formation of PdO/Pd particles supported on Ga2O3, whereas bimetallics formed at higher reduction temperatures (i.e. > =673 K) are only transformed back to Pd/Ga2O3. According to TEM investigations, this is due to the increased mobility of Ga and the subsequent formation of a Ga2O3 surface layer preventing further oxidation to PdO. Reduction of the corresponding Pd/beta-Ga2O3 impregnated powder catalyst between 573 and 773 K causes the formation of a Pd2Ga bimetallic phase, which is further converted to PdGa at reduction temperatures of 923 K. No formation of a Ga2O3 surface layer covering the particles has been observed upon re-oxidation of the bimetallic. This is explained on the basis of the difference in metal (bimetallic)-support contact area.