Hydrogenation of acetylene in the presence of a large excess of ethylene has been investigated on 0.04 wt.% Pd/Al2O3 catalyst containing 0:100, 30:70, and 80:20 atoms Cu:atoms Pd, and on 5 wt.% Pd/SiO2. Reaction was carried out in a back-mixed flow reactor in the presence of both 0.4 vol.% and 18 vol.% hydrogen, using 0.4-1% acetylene-in-ethylene mixtures. Ethane selectivity (moles C2H6 produced per mole C2H2 converted) increased from 30% to far above 100% in the course of time, due to parallel ethylene hydrogenation. Concurrently, ethylene selectivity became negative. There was no change in C4 and higher (C4+) hydrocarbon production. 14C2H2 and 14C2H4 labelling showed that the intrinsic selectivity for acetylene hydrogenation to ethylene is about 95 - 99% on all catalysts. Since ethylene is also hydrogenated at the same conditions, low overall selectivity results even when the Pd is fully covered by acetylene. Copper addition to palladium decreased the catalyst activity and increased its stability. This resulted in a significant decrease in the overall rate of C2H6 formation, as well as a marginal decrease in C4+ selectivity.
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