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

Chemisorption of acetylene (C2H2) and benzene (C6H6) 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 H2, CO, NO, and C2H4. 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. C2H2 is predominately reversibly adsorbed on the Pd monolayer (θPd=1) at 91 K, with thermal desorption peaks at 180 and 265 K. C6H6 is formed via cyclotrimerization of C2H2 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 (θPd=2–3) have similar C2H2 chemistry to that from thicker films. On a thick film (θPd=5) at 175 K, C2H2 adsorption leads to C2H2 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 (θPd=5) or on Pd(1 1 1), (a reduction of 6 kcal/mol) but is completely reversibly adsorbed.