Catalytic characterization of pure SnO 2 and GeO 2 in methanol steam reforming

Abstract

Structural changes of a variety of different SnO, SnO 2 and GeO 2 catalysts upon reduction in hydrogen were correlated with associated catalytic changes in methanol steam reforming. Studied systems include SnO, SnO 2 and GeO 2 thin film model catalysts prepared by vapour phase deposition and growth on polycrystalline NaCl surfaces and, for comparison, the corresponding pure oxide powder catalysts. Reduction of both the SnO 2 thin film and powder at around 673 K in 1 bar hydrogen leads to a substantial reduction of the bulk structure and yields a mixture of SnO 2 and metallic β-Sn. On the powder catalyst this transformation is fully reversible upon oxidation in 1 bar O 2 at 673 K. Strongly reduced thin films, however, can only be re-transformed to SnO 2 if the reduction temperature did not exceed 573 K. For GeO 2, the situation is more complex due to its polymorphism. Whereas the tetragonal phase is structurally stable during reduction, oxidation or catalytic reaction, a small part of the hexagonal phase is always transformed into the tetragonal at 673 K independent of the gas phase used. SnO 2 is highly active and CO 2 selective in methanol steam reforming, but the initial high activity drops considerably upon reduction between 373 and 573 K and almost complete catalyst deactivation is observed after reduction at 673 K, which is associated with the parallel formation of β-Sn. In close correlation to the structural results, the catalytic activity and selectivity can be restored upon an oxidative catalyst regeneration at 673 K. Tetragonal GeO 2 exhibits only a small activity and no pronounced selectivity to either CO or CO 2, at least after reduction. In its fully oxidized state release of surface/lattice oxygen results in a non-catalytic formation of CO 2 by oxidation of CO originating from catalytic dehydrogenation.