CO-INDUCED ETHANE FORMATION FROM ETHYLENE AND HYDROGEN ON FE(100) - EFFECTS OF LIGANDS IN SURFACE-REACTIONS

When C2H4 alone is adsorbed on a hydrogen-presaturated Fe(100) surface (Fe(100)-H) without coadsorbed CO, reversible formation of ethyl groups (C2H5) occurs, and no ethane is observed. Remarkably, the coadsorption of CO with ethylene on Fe(100)-H induces the facile formation of ethane at 170 K. The coadsorption of CO weakens Fe-H bonds and appears to induce ethane formation by reducing the activation barrier for the reductive elimination reaction between H adatoms and adsorbed ethyl groups. The observed activation barrier for this reaction is approximately 5-6 kcal/mol. When CO is adsorbed onto Fe(100)-H before C2H4 adsorption, the maximum ethane yield is achieved with a CO coverage of 0.2 ML. At a CO saturation coverage of 0.5 ML no ethylene will adsorb, and thus no ethane is formed. When CO is adsorbed onto Fe(100)-H after C2H4, the ethane yield is reduced at high CO exposures due to displacement of ethylene by CO. Similarly, high exposures of CO onto an Fe(100)-H surface covered with ethyl groups led to the displacement of ethane. Ethane formation at H precoverages below saturation requires a threshold CO coverage governed by the relationship 4-THETA-CO + THETA-H greater-than-or-equal-to 1. CO adsorbed at coverages below the threshold coverage appears to compress H adatoms into regions of locally higher concentration. Apparently CO must bind within surface regions locally concentrated with H in order to induce ethane formation.