Anion Effects and Induced Adsorption of Chloride by Submonolayer Amounts of Copper on Deliberately Stepped Platinum Surfaces

Abstract

The presence of submonolayer amounts of underpotentially deposited copper induces the adsorption of chloride on platinum stepped surfaces. The surfaces studied include the (111) and (110) surfaces and the stepped surfaces of (10,10,9), (554), (332), (221), and (331), which correspond to (111) terrace widths of 19, 9, 5, 3, and 2 atoms, respectively, with monatomic (110) steps. Unlike in the presence of sulfate/bisulfate anions where induced adsorption was present only at the step sites, here two types of induced adsorption take place on the platinum surfaces: induced adsorption on the (111) terraces and induced adsorption on the (110) step sites. The type and extent of the induced adsorption depend on the terrace width and density of step sites. On surfaces with a terrace width larger than nine atoms, e.g. Pt(10,10,9) or Pt(111), the induced adsorption creates a copper−chloride lattice structure on the terrace sites only. The maximum amount of induced adsorption at the terrace sites takes place on the Pt(111) surface. On surfaces with (111) terrace widths smaller than nine atoms, the induced adsorption takes place at the step sites in a manner similar to that for sulfate/bisulfate anions. The amount of induced adsorption on (110) step sites scales with step site density up to Pt(221) (with a three-atom-wide terrace) and then decreases for Pt(311) (with a two-atom-wide terrace), suggesting that the Pt(221) surface has the minimum space requirements for the coadsorption of both copper and chloride. On the Pt(554) surface which has a nine-atom-wide terrace, both types of induced adsorption (i.e., on steps and terraces) are present, though the induced adsorption appears first at the step sites, suggesting a higher interaction strength, likely due to work function differences. The shape of the UPD voltammetric features (at full coverage) is dependent on the terrace width with wider voltammetric waves being observed for surfaces with narrower terraces, indicating a diminution in near-neighbor interactions as the terraces become narrower. There are also shifts in potentials that suggest variation in the strength of interaction with surface structure.