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

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 523 K on both supports. Pd2Ga at low temperatures (between 523 K and 673 K) and generally PdGa at higher temperatures (at and above 673 K) were the only intermetallic phases observed. Both thermodynamically metastable oxide supports transformed into the stable β-Ga2O3 structure at 873 K 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 673 K (α-Ga2O3) and 523 K (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.