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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.

Reported are investigations regarding the coverage dependence of the kinetic factors for acetylene cyclotrimerization on Pd(111). Laser-induced thermal desorption/Fourier transform mass spectrometry is used to monitor the rate of the surface reaction prior to conventional desorption. Surface benzene formation is rate limited by the addition of acetylene to C 4 H 4 , in contrast to gas-phase benzene production which is rate limited by subsequent benzene desorption. Increasing initial acetylene exposures lead to a substantial increase in both the activation energy and preexponential factor for benzene formation. For initial acetylene exposures from 1.0 to 2.5 langmuir, the activation energy varies from 24.4 ( 3.9 to 43.5 ( 3.9 kJ/mol (95% confidence). Fitting to pseudo-first-order kinetics, the preexponential factors vary from 10 3.1(0.1 to 10 9.8(0.1 s -1 . A change in diffusion rate and steric blocking can both be ruled out as significant contributors to the coverage effect, since both of these processes would lead to the opposite trend in preexponential factors. The change in kinetic factors is most likely a consequence of an increased barrier to C-Pd bond breaking due to more tightly bound species at higher acetylene coverage. Supporting evidence for this is cited from the literature.