Potential-Induced Acid-Base Chemistry of Adsorbed Species
Authors: Mantelli Harlan , Martinez-Hincapie Ricardo Alonso , Feliu Juan , Scherson Daniel 
Published: February 2020

In a recent paper, Martinez-Hincapie et al.1 reported in situ reflection-absorption Fourier transform infrared spectra for a Pt(111) electrode in CO2 -saturated KClO4/HClO4 aqueous solutions over the pH range 1.10 - 3.10. As clearly shown by their data, significant changes in the relative ratios of the spectral features ascribed to adsorbed bicarbonate, HCO3
-(ads), and carbonate, CO3
2-(ads), were observed as the applied potential was increased in a region in which the total coverage of HCO3
-(ads) and CO3
2-(ads) attained a fairly constant value. The presence of these species in solutions of such low pH, as well as the changes in the extent of ionization induced by the applied potential represent a new phenomenon not as yet been fully explained. To the best our knowledge, the report of Martinez-Hincapie et al.1 represent the first illustration aimed at determining spectroscopically the extent of deprotonation as a function of the applied potential in this type of systems.

This presentation will describe a model based largely on that reported by Smith and White2 decades ago, which predicted interfacial capacity vs potential plots displaying features attributed to the acid-base chemistry of self-assembled alkyl chain monolayers bearing carboxylic groups in contact with an aqueous electrolyte. Shortly thereafter, Fawcett et al. further refined this theory by introducing discreteness of charge effects.3 Application of this model to the adsorbed HCO3
-/CO3
2- layer on Pt(111) can account for the potential dependence of the relative ratios of the two species as a function of pH, for relatively reasonable values of the parameters involved. Particularly striking is the much lower pKa value for HCO3
-(ads) compared to that in bulk aqueous solutions, which we tentatively ascribe to the bonding of the adsorbate to the surface and thus to changes in its electronic structure.

The starting point of our analysis regards the HCO3
-(ads)/CO3
2-(ads) layer, in this case, as attached to the surface through a bidentate type bond with the proton in HCO3
- (ads) bonded to the oxygen facing the electrolyte, which defines the so-called plane of acid dissociation (PAD) (see Fig. 1). Based on Smith and White’s formalism, the problem so stated reduces to finding solutions of a system of five non-linear algebraic equations with five unknowns, shown in Scheme I, where σM is the charge density on the electrode (C/cm2), CF

is the capacitance of the adsorbed molecular film (F/cm2), Epcz
is the potential of zero charge of the interface devoid of the adsorbate and σPAD is the charge density at the PAD, (C/cm2). Furthermore, the symbols and , represent the potential differences between the regions specified in Fig. 1, where ϕi
is the electrostatic potential for i = metallic electrode, M; plane of acid dissociation, PAD; and electrolyte solution, S; f, the fraction of CO3
2-(ads), pH refers to the value of the bulk solution and pKa is the acidity constant of the adsorbed layer. The third equation represents the neutral character of the interface where is right hand side is σS, the charge density of the diffuse double layer (C/cm2) written in terms of DVs, the Debye length, k and the dielectric constants of vacuum and the electrolyte solution, eo and es, respectively.

The system of equations was solved using Mathematica 11.3 for each of the log[f/(1-f)], E, data points collected from the spectroelectrochemical measurements at the three pH values examined. Shown in Figure 2 are plots of σM, σPAD, ΔVF

,
ΔVS
, Cd, and pKa as a function of the rational potential, i.e. E - Epzc. As indicated, the value of pKa determined based on all the data collected is about 2.7, much lower than that of solution bicarbonate, 10.3. We tentatively ascribe this phenomenon to the bonding of the adsorbate to the surface and thus to changes in its electronic structure.

Literature Cited

1. Martinez-Hincapie, R.; Berna, A.; Rodes, A.; Climent, V.; Feliu, J. M., Surface Acid-Base Properties of Anion-Adsorbed Species at Pt(111) Electrode Surfaces in Contact with CO2-Containing Perchloric Acid Solutions. Journal of Physical Chemistry C
2016,
120 (29), 16191-16199.

Smith, C. P.; White, H. S., Voltammetry of Molecular Films Containing Acid-Base Groups. Langmuir
1993,
9 (1), 1-3.

(a) Andreu, R.; Fawcett, W. R., Discreteness-of-Charge Effects at Molecular Films Containing Acid/Base Groups. J. Phys. Chem.
1994,
98 (48), 12753-12758; (b) Fawcett, W. R.; Fedurco, M.; Kovacova, Z., Double-Layer Effects at Molecular Films Containing Acid-Base Groups. Langmuir
1994,
10 (7), 2403-2408.

Figure 1