Origin of different deactivation of Pd/SnO 2 and Pd/GeO 2 catalysts in methanol dehydrogenation and reforming: A comparative study

Abstract

Pd particles supported on SnO 2 and GeO 2 have been structurally investigated by X-ray diffraction, (High-Resolution) transmission and scanning electron microscopy after different reductive treatments to monitor the eventual formation of bimetallic phases and catalytically tested in methanol dehydrogenation/reforming. For both oxides this included a thin film sample with well-defined Pd particles and a powder catalyst prepared by incipient wetness impregnation. The hexagonal and the tetragonal polymorph were studied for powder GeO 2. Pd 2Ge formation was observed on all GeO 2-supported catalysts, strongly depending on the specific sample used. Reduction of the thin film at 573 K resulted in full transformation into the bimetallic state. The partial solubility of hexagonal GeO 2 in water and its thermal structural instability yielded Pd 2Ge formation at 473 K, at the cost of a structurally inhomogeneous support and Ge metal formation at higher reduction temperatures. Pd on tetragonal GeO 2 entered a state of strong metal–support interaction after reduction at 573–673 K, resulting in coalescing Pd 2Ge particles on a sintered and re-crystallized support, apparently partially covering the bimetallic particles and decreasing the catalytic activity. Pd 2Ge on amorphous thin film and hexagonal GeO 2 converted methanol primarily via dehydrogenation to CO and H 2. At 573 K, formation of Pd 2Sn and also PdSn occurred on the Pd/SnO 2 thin film. Pd 3Sn 2 (and to some extent Pd 2Sn) were predominantly obtained on the respective powder catalyst. Strong deactivation with increasing reduction temperature was observed, likely not based on the classical strong metal–support interaction effect, but rather on a combination of missing active structural ensembles on Sn-enriched bimetallic phases and the formation of metallic β-Sn. Correlations to Pd and its bimetallics supported on ZnO, Ga 2O 3 and In 2O 3 were also discussed.