The adsorption of ethylene on Au/Pd(111) alloy surfaces

The adsorption of ethylene on gold-palladium alloys formed on a Pd(111) surface is investigated using a combination of temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Various alloy compositions are obtained by depositing four monolayers of gold on a clean Pd(111) surface and annealing to various temperatures. For gold coverages greater than similar to 0.7, ethylene adsorbs primarily on gold sites, desorbing with an activation energy of less than 55 kJ/mol. At gold coverages between similar to 0.5 and similar to 0.7, ethylene is detected on palladium sites in a pi-bonded configuration (with a sigma-pi parameter of similar to 0.1) desorbing with an activation energy of between similar to 57 and 62 kJ/mol. Further reducing the gold coverage leads to an almost linear increase in the desorption activation energy of ethylene with increasing palladium content until it eventually reaches a value of similar to 76 kJ/mol found for ethylene on clean Pd(111). A corresponding increase in the sigma-pi parameter is also found as the gold coverage decreases reaching a value of similar to 0.8, assigned to di-sigma-bonded ethylene as found on clean Pd(111). (c) 2006 Elsevier B.V. All rights reserved.