A radiotracer technique for in situ adsorbate measurements on smooth electrode surfaces

A prototype radiotracer apparatus for measuring in situ adsorption onto smooth metal electrodes has been developed. In this method, a radiolabeled adsorbate on an electrode is detected by pressing the electrode against a glass scintillator detector at the bottom of an electrochemical cell. Equations which account for the backscattering factor have been developed to calculate the surface packing density, $\Gamma$, of the adsorbate. Under current optimum conditions (when the trapped solution counting rate is negligible), the detection limit for the method is 2.6 $\times$ 10$\sp{13}$ molecules/cm$\sp2$. A gap distance of 1.5 to 2.3 $\mu$m between the electrode and scintillator is routinely achieved. Adsorption results have been obtained which demonstrate the method's use for both strongly and weakly adsorbing systems on smooth polycrystalline and single crystal electrodes.Linear instrument response and reproducibility of electrode position was confirmed using non-adsorbing NaH$\sp{14}$CO$\sb3$. ($\sp{14}$C) Formic acid was used as the first test adsorbate, and the types of experiments performed included measuring adsorption over time, the potential dependence of adsorption, and exchange with unlabeled species.The potential dependence of ($\sp{14}$C) acetic acid adsorption was determined on platinum and gold in acidic and neutral electrolyte. These results were compared with parallel potential-difference infrared experiments. A series of exchange and potential step experiments further characterized the nature of acetic acid bonding to platinum.In situ radiotracer data for single crystal electrodes was obtained for the first time. The potential dependence of ($\sp{14}$C) acetic acid adsorption was compared to that of ($\sp{35}$S) sulfuric acid on Pt(111) in 0.1 M HClO$\sb4$.Two strongly adsorbed compounds, pyridine and hydroquinone, were studied on polycrystalline platinum. Hydroquinone concentration dependence results were compared with thin-layer electrochemistry results from the literature, which were 34% higher than the radiotracer results. Pyridine results showed that the predosing effect noted in the literature for hydroquinone was also important in pyridine adsorption. The concentration dependence data indicated that the flat species prevented the more tightly packed vertical species from forming. Exchange experiments were performed, and the potential dependence of pyridine was found to be similar to the potential dependence for hydroquinone.U of I OnlyETDs are only available to UIUC Users without author permissio