Tuning the chemistry of metal surfaces: II. Acetylene cyclotrimerization on ultrathin Pd films on Ta(110)

Chemisorption of acetylene (C₂H₂) and benzene (C₆H₆) on ultrathin Pd films (monolayer, bilayer, trilayer, etc.) deposited on Ta(110) 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 (θ_Pd = 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 (θ_Pd = 2-3) have similar C₂H₂ chemistry to that from thicker films. On a thick film (θ_Pd = 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 (θ_Pd = 5) or on Pd(111), (a reduction of 6 kcal/ mol) but is completely reversibly adsorbed.