Tuning the chemistry of metal surfaces: II. Acetylene cyclotrimerization on ultrathin Pd films on Ta(1 1 0)

Abstract

Chemisorption of acetylene (C 2H 2) and benzene (C 6H 6) on ultrathin Pd films (monolayer, bilayer, trilayer, etc.) deposited on Ta(1 1 0) is weaker than on bulk-terminated Pd surfaces, but it is not as strongly perturbed as was seen for H 2, CO, NO, and C 2H 4. Thus, the well-known alterations of the chemical properties of ultrathin metal films deposited on refractory metal substrates can be modified by strongly adsorbed species. C 2H 2 is predominately reversibly adsorbed on the Pd monolayer ( θ Pd=1) at 91 K, with thermal desorption peaks at 180 and 265 K. C 6H 6 is formed via cyclotrimerization of C 2H 2 and desorbs in a single peak at 407 K, but the yield is small (1% of an adsorbed benzene monolayer on that surface). Bilayer and trilayer Pd films ( θ Pd=2–3) have similar C 2H 2 chemistry to that from thicker films. On a thick film ( θ Pd=5) at 175 K, C 2H 2 adsorption leads to C 2H 2 desorption in a very broad peak near 330 K, and to benzene desorption at 250 and 500 K, with a yield of twice that on the Pd monolayer. Benzene chemisorption on the Pd monolayer is only slightly weaker than that on a thick Pd film ( θ Pd=5) or on Pd(1 1 1), (a reduction of 6 kcal/mol) but is completely reversibly adsorbed.