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J. Phys. Chem. C 2008, 112, 9346–9350
Discriminating Reasons for Selectivity Enhancement of CO in Alkyne Hydrogenation on Palladium Nu´ria Lo´pez,*,† Blaise Bridier,† and Javier Pe´rez-Ramı´rez†,‡ Institute of Chemical Research of Catalonia (ICIQ), AVinguda Paı¨sos Catalans 16, 43007, Tarragona, Spain, and Catalan Institution for Research and AdVanced Studies (ICREA), Passeig Lluı´s Companys 23, 08010, Barcelona, Spain ReceiVed: NoVember 28, 2007; ReVised Manuscript ReceiVed: April 3, 2008
CO is widely used as selectivity enhancer in the selective hydrogenation of acetylene to ethylene over palladium catalysts in the petrochemical industry. However, conclusions on its action mechanism from experiments are controversial because of the complexity of the resulting catalytic system. Density functional theory studies over Pd(111) reveal that CO impacts several steps of the reaction mechanism: molecular adsorption of hydrocarbons, dissociative adsorption of H2, and hydrogenation barriers. The most notorious effect attributed to CO, besides partial blocking of active sites, is the diminished binding energy of the alkene to the surface. This leads to a pronounced thermodynamic selectivity mainly due to an electronic contribution that enhances the differential adsorption of the double and triple bonds. Gold nanoparticles attained similar effect without promotion. A simple design rule to identify selectivity enhancers based on the binding energies of reactants and modifier to the surface is proposed. 1. Introduction Steam cracking is the main industrial method for producing olefins such as ethylene and propylene, which are building blocks of paramount importance for petrochemicals and polymers. Impurities of acetylenics and diolefins are inevitably present in the alkene streams. For example, the C2 cut typically contains 0.5-3% of acetylene, whereas the C3 cut contains 2-8% of propyne and propadiene. Reducing the level of the highly unsaturated compounds to
