Hydrogenation of acetylene over supported metal catalysts. Part 3.—[14C]tracer studies of the effects of added ethylene and carbon monoxide on the reaction catalysed by silica-supported palladium, rhodium and iridium

Abstract

The hydrogenation of acetylene in the presence of various pressures of [¹⁴C]ethylene has been studied over silica-supported palladium, rhodium and iridium catalysts. The results show that with each catalyst the yield of ethane from the hydrogenation of ethylene is small; the major route to ethane formation is by direct hydrogenation of acetylene. Three distinct types of surface site are involved in the hydrogenation of acetylene and ethylene.

Direct surface monitoring during the hydrogenation reaction shows that ethylene does not adsorb on the acetylene hydrogenation sites. The increase in rate observed when all the acetylene has reacted is not due to an increase in ethylene surface coverage, but is interpreted in terms of an increase in hydrogen availability for ethylene hydrogenation.

Addition of 0.1–1.0 Torr of carbon monoxide to the acetylene hydrogenation results in complete poisoning of the rhodium and iridium catalysts, but only partial reduction in the activity of the palladium catalyst. [¹⁴C]tracer studies of the adsorption of carbon monoxide, acetylene and ethylene show that the poisoning is not due to a hydrocarbon site-blocking effect, but rather to a blocking of the hydrogen adsorption sites. Hydrogen effectively competes with carbon monoxide for these sites on palladium.