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

A series of gallia-supported Pd-Ga catalysts that consist of metallic nanoparticles on three porous polymorphs of Ga2O3 (a-, beta-, and -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 a- and beta-Ga2O3-supported materials in hydrogen at temperatures of 250 and 310 degrees 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 degrees C. In the case of Ga-Pd/-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 a-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/a-Ga2O3.