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 g700 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 C 2 H 2 to C 2 H 4 and cyclotrimerization of C 2 H 2 to form C 6 H 6 . The cyclotrimerization reaction produces three benzene desorption states at ∼340, ∼390, and ∼430 K. In addition, the detection of C 4 H 6 during TPD provides evidence that an elusive C 4 H 4 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.

Monolayer bimetallic surfaces: Experimental and theoretical studies of trends in electronic and chemical properties

“…Another example of correlating the structural, electronic and chemical properties is the novel chemistry on faceting surfaces of many bimetallic systems by Madey and coworkers. For example, the faceting surface of Pd, obtained by depositing one monolayer Pd on W (211), showed a substantial enhancement in the activity towards the cyclotrimerization pathway than that on either Pd (110) or W(211) surfaces [19].…”

Low-temperature synthesis of few-layer graphene

“…Acetylene is easily polymerized to generate benzene structure. Benzene was first fabricated by the cyclization of acetylene on supported Ni catalysts, and subsequently prepared by the acetylene cyclization and hydrogenation on Pd/W (211) [8]. Moreover, Jiang et al synthesized graphene films via the radical-coupling reaction.…”

Nanoscale surface chemistry

“…17; for a review see ref. 18). Acetylene cyclization, 3C 2 H 2 3 C 6 H 6 , is catalyzed by palladium and has been studied extensively on clean and modified surfaces, under ultrahigh vacuum as well as high-pressure conditions.…”

“…Acetylene cyclization is extremely structure-sensitive, so it is an ideal reaction for probing the effects of surface electronic and geometric structure. Isotopic studies indicate that benzene forms from acetylene in a stepwise fashion on Pd(111) and Cu(110); the reaction proceeds via a C 4 intermediate species, and the reactants do not undergo COC or COH bond scission (17)(18)(19).…”

“…For example, we have shown in a combination high-resolution electron-energy-loss-spectroscopy (HREELS) and TPD study that a single ML of Pd on W(211) decreases the high reactivity of W for C 2 H 2 decomposition, and catalyzes self-hydrogenation of C 2 H 2 to C 2 H 4 and cyclization of C 2 H 2 to C 6 H 6 (20). Evidence for finite size effects in benzene formation has been reported by Goodman et al (21) for Pd clusters supported on alumina films; Lambert et al (22) have shown that there is a minimum ensemble on Pd(111) (Ϸ7 atoms) necessary for benzene formation.…”