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

Abstract

AbstractA series of gallia‐supported Pd‐Ga catalysts that consist of metallic nanoparticles on three porous polymorphs of Ga2O3 (α‐, β‐, and γ‐Ga2O3) 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 CO2 hydrogenation reactions. The IMC Pd2Ga forms upon reduction of α‐ and β‐Ga2O3‐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 Pd5Ga3 before the support itself is reduced at temperatures above 565 °C. In the case of Ga‐Pd/γ‐Ga2O3, 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 Pd2Ga also forms in this sample. Pd2Ga/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 Pd5Ga3 supported on α‐Ga2O3. In methanol steam reforming, the Ga‐Pd/Gallia catalysts showed, in contrast to Pd/Al2O3, selectivity towards CO2 of up to 40 %. However, higher selectivities, which have been reported for Pd2Ga in literature, could not be reproduced in this study, which might be a result of particle size effects. The initially higher selectivity of the Pd5Ga3‐containing samples was not stable, suggesting superior catalytic properties for this IMC, but that re‐oxidation of Ga species and formation of Pd2Ga occurs under reaction conditions. In methanol synthesis, CO hydrogenation did not occur, but a considerable methanol yield from a CO2/H2 feed was observed for Pd2Ga/α‐Ga2O3.