Surface environmental effects in electrochemical kinetics: Outer-sphere chromium(III) reductions at mercury, gallium, lead, and thallium surfaces

Electrochemical rate parameters for the outer-sphere reductions of several Cr(III) aquo and ammine complexes and also for V(OH₂)₆³⁺ and Eu(OH₂)_n³⁺ are compared at aqueous-metal interfaces formed with mercury, liquid gallium, lead, and underpotential deposited (upd) lead and thallium monolayers on silver. For reactants containing aquo ligands, substantial (up to 10³-fold) decreases in the rate constants, both before and after electrostatic double-layer corrections, were observed at a given electrode potential when substituting mercury by the other surfaces, especially lead and gallium. The rate alterations are accompanied by marked decreases in the apparent activation entropies, although offset by corresponding decreases in the measured activation enthalpies. These results are interpreted in terms of the varying influence of these metal surfaces on the interfacial solvent structure. The observed substrate dependence Hg>upd Pb∼upd Tl≳Pb>Ga is consistent with the anticipated differences in surface hydrophilicity. The likely influence of nonadiabatic reaction pathways is also considered. Smaller rate variations were observed for Cr(NH₃)₆³⁺ and Cr(en)₃³⁺ reduction (en = ethylenediamine), although the activation parameters are more sensitive to the metal substrate. The relatively small influence exerted by mercury surfaces upon the outer-sphere reaction energetics is also consistent with the reasonable agreement seen between the experimental and theoretical rate parameters for Cr(OH₂)₆³⁺ reduction at this surface.