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

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 deactivation and loss of CO2 selectivity due to encapsulation of catalytically active bimetallic particles by mobile GaOx 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 Ga2O3 supports, which yields additional CO by formic acid decarbonylation. For the CO2-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