Reaction Pathways of Acetylene on Pd/W(211): A TPD and HREELS Investigation

Abstract

In an ongoing investigation to study structure−reactivity relationships on bimetallic surfaces, acetylene cyclotrimerization to form benzene is of particular interest: in this structure-sensitive catalytic reaction, C−C and C−H bonds can be formed readily under ultrahigh vacuum (UHV) conditions without C−C bond breaking. In this paper, we present results for acetylene cyclization and hydrogenation on Pd/W(211). Pd on W is chosen because it is a morphologically unstable system, and W(211) facets develop after annealing Pd/W(111) to ≥700 K. Temperature-programmed desorption (TPD) results exhibit negligible amounts of benzene detected from acetylene adsorption on clean W(211). A single monolayer (ML) of Pd on W(211) decreases the high reactivity toward acetylene decomposition and several different reaction pathways are accessed, including hydrogenation of C2H2 to C2H4 and cyclotrimerization of C2H2 to form C6H6. The cyclotrimerization reaction produces three benzene desorption states at ∼340, ∼390, and ∼430 K. In addition, the detection of C4H6 during TPD provides 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 electron energy-loss spectroscopy (HREELS) provides complementary information regarding the reaction mechanisms of acetylene on the Pd/W(211) surfaces.