Reaction pathways of acetylene on Pd/W(211): A TPD and HREELS investigation

In an ongoing investigation to study structure-reactivity relationships on bimetallic surfaces, acetylene cyclotrimerization to form benzene is of par ticular interest: in this structure-sensitive catalytic reaction, C-C and C -H bonds can be formed readily under ultrahigh vacuum (UHV) conditions with out C-C bond breaking. In this paper, we present results for acetylene cycl ization and hydrogenation on Pd/W(211). Pd on W is chosen because it is a m orphologically unstable system, and W(211) facets develop after annealing P d/W(111) to greater than or equal to 700 K. Temperature-programmed desorpti on (TPD) results exhibit negligible amounts of benzene detected from acetyl ene adsorption on clean W(211). A single monolayer (ML) of Pd on W(211) dec reases the high. reactivity toward acetylene decomposition and several diff erent reaction pathways are accessed, including hydrogenation of C2H2 to C2 H4 and cyclotrimerization of C2H2 to form C6H6 The cyclotrimerization react ion produces three benzene desorption states at similar to 340, similar to 390, and similar to 430 K. In addition, the detection of C4H6 during TPD pr ovides evidence that an elusive C4H4 intermediate,is present on the surface . Furthermore, ethylene is observed in substantial yields, lending insight into the activity of the bimetallic system. The use of high-resolution elec tron energy-loss spectroscopy (HREELS) provides complementary information r egarding the reaction mechanisms of acetylene on the Pd/W(211) surfaces.