Reaction Energies for Electrode Surface Atom Insertion into R-H Bonds and Their Dependence on Electrode Potential: Application to Pt(111)
R-Haq ⇌ Raq + H+aq + e- (1)
R-Hads ⇌ Rads + H+aq + e- (2)
The electrode considered for this work is the close-packed Pt(111) surface.
The R─H oxidation reactions addressed are those that occur during ethanol oxidation at the anode in a fuel cell. The reversible potentials for the reactions taking place on the electrode surface are significant to understanding the mechanism of the 12 electron oxidation reactions and the limitations of platinum as an electrocatalyst. They were determined previously for reaction (2). However, there is another mechanism for an R─H bond oxidation, and that is dissociation of the bond on the surface by metal atom insertion or by H atom abstraction prior to aqueous proton formation. Our work addresses the insertion mechanism.
We show that R─H bond strengths in R─Haq and R─Hads are functions of reversible potentials for their oxidations. We find that underpotential deposited (UPD) hydrogen on Pt(111) electrodes makes the R─Hads bond strengths zero for reactions that have reversible potentials in the range from 0.0 V (vs. SHE) to 0.35 V (vs. SHE). For R─Hads reactions with reversible potentials in the double layer region (Urev > 0.35 V), the dissociation bond strengths are greater than zero and increase linearly with increasing reversible potentials. When the reversible potentials of R─Hads oxidation are above the double layer potential region (Urev > 0.6 V), Oads and OHads form which block R-Hads dissociation. The R-Hads dissociation is also blocked in the potential region of H2 evolution(Urev < 0.0 V).