Methanol steam reforming: CO2-selective Pd2Ga phases supported on α- and γ-Ga2O3

Abstract

A set of different Pd/Ga2O3 catalysts has been tested as suitable CO2-selective catalysts in methanol steam reforming, formaldehyde steam reforming and formic acid decomposition. Special focus was put on two different polymorphic forms of Ga2O3, namely α-Ga2O3 and γ-Ga2O3 and their comparison to the use of the more common β-Ga2O3 as oxide support of small Pd particles. The formation of eventually CO2-selective intermetallic Pd–Ga phases started at around 523K on both supports. Pd2Ga at low temperatures (between 523K and 673K) and generally PdGa at higher temperatures (at and above 673K) were the only intermetallic phases observed. Both thermodynamically metastable oxide supports transformed into the stable β-Ga2O3 structure at 873K reduction temperature. In contrast to previous studies, a highly CO2-selective state in methanol steam reforming (CO2 selectivity>70%) could be established on both catalysts after reduction at 673K (α-Ga2O3) and 523K (γ-Ga2O3), where the active state of the catalysts can be characterised as Pd2Ga/α-Ga2O3 and Pd2Ga/γ-Ga2O3, respectively. α-Ga2O3 itself is reasonably CO2-selective in methanol steam reforming (∼80%), although not very active. In contrast, the γ-Ga2O3 polymorph is neither CO2-selective, nor active and resembles the behaviour of β-Ga2O3. The high CO2-selectivity of α-Ga2O3 is explained in terms of a higher surface basicity alongside efficient decarboxylation of intermediary formed formic acid. As crucial parameters for the observed CO2-selectivity in methanol steam reforming, decarboxylation of formic acid and oxidation of formaldehyde proceeding efficiently via a low-temperature reaction channel over either oxide-supported Pd2Ga intermetallic catalysts were identified.