Pd/Ga₂O₃ methanol steam reforming catalysts: Part II. Catalytic selectivity

Abstract

Structure, morphology and composition of two Pd/Ga₂O₃ methanol steam reforming catalysts were correlated with the associated activity and selectivity changes in the methanol steam reforming reaction and linked to studies on pure Ga₂O₃ supports. For both systems, that are, a Pd/Ga₂O₃ thin film model catalyst and a powder-supported Pd catalyst, we identified a temperature range in which the reduction with hydrogen yields a single Pd-Ga bimetallic on a reduced oxide support, which in turn suppresses methanol dehydrogenation and results in a high CO₂-selectivity in methanol steam reforming. For the thin film catalyst, this included the Pd₅Ga₂ bimetallic present after reduction in the temperature range 523-600 K, for the powder-supported catalyst, Pd₂Ga, formed after reduction between 523 and 773 K, was found to account for the high CO₂-selectivity. In contrast to studies on the corresponding Pd/ZnO catalysts, sintering and metal decoration by reduced GaOx species will additionally have to be considered for discussions about structure-activity correlations in Pd/Ga₂O₃ thin film model catalysts. Reduction at 673 K causes catalyst deacti-vation and loss of CO₂-selectivity due to encapsulation of catalytically active bimetallic particles by mobile GaOx species and hampers oxidative catalyst regeneration. No such behavior has been observed for the powder-supported catalyst. This difference in catalytic activity and selectivity between the two catalysts is interpreted in terms of their different (bi-) metallic - oxide contact area.