INTERACTION OF ACETYLENE WITH THE PD(110)(1X2)-CS SURFACE - PROMOTIONOF ETHYLENE FORMATION

The interaction of acetylene with the Pd(110)(1 x 2)-Cs surface has be en studied by the use of high-resolution electron energy loss spectros copy and thermal desorption spectroscopy. For a small exposure (0.2 L) at 90 K, acetylene is chemisorbed. For a large exposure (2.5 L), phys isorbed acetylene and vinylidene (CCH2) are formed in addition to chem isorbed acetylene. By heating to 135 K, physisorbed acetylene desorbs from the surface. By heating to 260 K, acetylene is converted to vinyl idene. Ethylene desorption takes place at 315 K. After heating to 400 K, methylidyne and carbon are formed on the surface. Compared with the case for Pd(110)(1 x 1), ethylene formation is markedly promoted on t he Pd(110)(1 x 2)-Cs surface. The effects of the Cs modification are d iscussed. It is shown that the acetylene hydrogenation occurs via viny lidene.