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

Abstract A set of different Pd/Ga 2 O 3 catalysts has been tested as suitable CO 2 -selective catalysts in methanol steam reforming, formaldehyde steam reforming and formic acid decomposition. Special focus was put on two different polymorphic forms of Ga 2 O 3 , namely α-Ga 2 O 3 and γ-Ga 2 O 3 and their comparison to the use of the more common β-Ga 2 O 3 as oxide support of small Pd particles. The formation of eventually CO 2 -selective intermetallic Pd–Ga phases started at around 523 K on both supports. Pd 2 Ga 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-Ga 2 O 3 structure at 873 K reduction temperature. In contrast to previous studies, a highly CO 2 -selective state in methanol steam reforming (CO 2 selectivity > 70%) could be established on both catalysts after reduction at 673 K (α-Ga 2 O 3 ) and 523 K (γ-Ga 2 O 3 ), where the active state of the catalysts can be characterised as Pd 2 Ga/α-Ga 2 O 3 and Pd 2 Ga/γ-Ga 2 O 3 , respectively. α-Ga 2 O 3 itself is reasonably CO 2 -selective in methanol steam reforming (∼80%), although not very active. In contrast, the γ-Ga 2 O 3 polymorph is neither CO 2 -selective, nor active and resembles the behaviour of β-Ga 2 O 3 . The high CO 2 -selectivity of α-Ga 2 O 3 is explained in terms of a higher surface basicity alongside efficient decarboxylation of intermediary formed formic acid. As crucial parameters for the observed CO 2 -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 Pd 2 Ga intermetallic catalysts were identified.