Reaction Energies for Electrode Surface Atom Insertion into R-H Bonds and Their Dependence on Electrode Potential: Application to Pt(111)

Electrochemistry is not ordinarily expected to be a tool for measuring bond strengths.  Our work shows how to determine R─H bond strengths from reversible potentials, U_rev, for outer sphere and surface oxidation reactions:

R-H_aq ⇌ R_aq + H⁺_aq + e^-    (1)

and

R-H_ads ⇌ R_ads + 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─H_aq and R─H_ads are functions of reversible potentials for their oxidations. We find that underpotential deposited (UPD) hydrogen on Pt(111) electrodes makes the R─H_ads 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─H_ads reactions with reversible potentials in the double layer region (U_rev > 0.35 V), the dissociation bond strengths are greater than zero and increase linearly with increasing reversible potentials. When the reversible potentials of R─H_ads oxidation are above the double layer potential region (U_rev > 0.6 V), O_ads and OH_ads form which block R-H_ads dissociation. The R-H_ads dissociation is also blocked in the potential region of H₂ evolution(U_rev < 0.0 V).