Ga‐Pd/Ga₂O₃ Catalysts: The Role of Gallia Polymorphs, Intermetallic Compounds, and Pretreatment Conditions on Selectivity and Stability in Different Reactions

Abstract

Abstract A series of gallia‐supported Pd‐Ga catalysts that consist of metallic nanoparticles on three porous polymorphs of Ga 2 O 3 (α‐, β‐, and γ‐Ga 2 O 3 ) were synthesized by a controlled co‐precipitation of Pd and Ga. The effects of formation of Ga‐Pd intermetallic compounds (IMCs) were studied in four catalytic reactions: methanol steam reforming, hydrogenation of acetylene, and methanol synthesis by CO and CO 2 hydrogenation reactions. The IMC Pd 2 Ga forms upon reduction of α‐ and β‐Ga 2 O 3 ‐supported materials in hydrogen at temperatures of 250 and 310 °C, respectively. At higher temperatures, Ga‐enrichment of the intermetallic particles is observed, leading to formation of Pd 5 Ga 3 before the support itself is reduced at temperatures above 565 °C. In the case of Ga‐Pd/γ‐Ga 2 O 3 , no information about the metal particles could be obtained owing to their very small size and high dispersion; however, the catalytic results suggest that the IMC Pd 2 Ga also forms in this sample. Pd 2 Ga/gallia samples show a stable selectivity towards ethylene in acetylene hydrogenation (≈75 %), which is higher than for a monometallic Pd reference catalyst. An even higher selectivity of 80 % was observed for Pd 5 Ga 3 supported on α‐Ga 2 O 3 . In methanol steam reforming, the Ga‐Pd/Gallia catalysts showed, in contrast to Pd/Al 2 O 3 , selectivity towards CO 2 of up to 40 %. However, higher selectivities, which have been reported for Pd 2 Ga in literature, could not be reproduced in this study, which might be a result of particle size effects. The initially higher selectivity of the Pd 5 Ga 3 ‐containing samples was not stable, suggesting superior catalytic properties for this IMC, but that re‐oxidation of Ga species and formation of Pd 2 Ga occurs under reaction conditions. In methanol synthesis, CO hydrogenation did not occur, but a considerable methanol yield from a CO 2 /H 2 feed was observed for Pd 2 Ga/α‐Ga 2 O 3 .