The influence of catalyst restructuring on the selective hydrogenation of acetylene to ethylene

Structural changes were induced in Pd and Pd-Ag catalysts by oxidation and reduction. The influence of oxidative restructuring on the selectivity for acetylene hydrogenation to ethylene was studied under conditions of industr ial significance (acetylene conversion > 99%). We found that oxidative rest ructuring of the Pd-only catalyst had no influence on hydrogenation selecti vity; neither was any correlation found between selectivity and extent of b eta -Pd hydride formation. When Ag was added as a promoter, however, oxidat ion-reduction treatments had a major impact on selectivity. High-resolution transmission electron microscopy showed rough surfaces were created by oxi dation of the catalyst which were coincident with poor selectivity. Anneali ng the catalyst at progressively higher temperatures in H-2 improved the se lectivity to ethylene. Reduction in H-2 at 500 degreesC also suppressed the formation of oligomeric hydrocarbon by-products during reaction. In situ i nfrared study of CO adsorption showed that the addition of Ag to the Pd/sil ica results in an ensemble or geometric effect, increasing the proportion o f linear CO relative to the bridged form. We infer that oxidation causes th e Pd and Ag to segregate within the particle. High-temperature annealing in H-2 causes a migration of the promoter Ag so as to yield improved selectiv ity on these catalysts. The silica support surface also seems to provide a suitable environment to allow restructuring of the Pd-Ag crystallites to Oc cur. (C) 2001 Academic Press.