Pd/Ga 2O 3 methanol steam reforming catalysts: Part II. Catalytic selectivity

Abstract

Structure, morphology and composition of two Pd/Ga 2O 3 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 Ga 2O 3 supports. For both systems, that are, a Pd/Ga 2O 3 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 CO 2 selectivity in methanol steam reforming. For the thin film catalyst, this included the Pd 5Ga 2 bimetallic present after reduction in the temperature range 523–600 K, for the powder-supported catalyst, Pd 2Ga, formed after reduction between 523 and 773 K, was found to account for the high CO 2 selectivity. In contrast to studies on the corresponding Pd/ZnO catalysts, sintering and metal decoration by reduced GaO x species will additionally have to be considered for discussions about structure–activity correlations in Pd/Ga 2O 3 thin film model catalysts. Reduction at 673 K causes catalyst deactivation and loss of CO 2 selectivity due to encapsulation of catalytically active bimetallic particles by mobile GaO x species and hampers oxidative catalyst regeneration. No such behaviour 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 areas. Formic acid formation has been observed on pure Ga 2O 3 supports, which yields additional CO by formic acid decarbonylation. For the CO 2-selective methanol steam reforming reaction over a Pd–Ga bimetallic particle, formic acid is not a gas phase species, which indicates a preferential decarboxylation pathway of adsorbed formate species at low temperatures.