Reported here are studies of potential-dependent surface chemistry of several hydroxypyridines adsorbed at well-defined Pt(111) electrode surfaces from pH 3 aqueous fluoride solutions. The adsorbates studied were 2-hydroxypyridine (2HP), 3-hydroxypyridine (3HP), 4-hydroxypyridine (4HP), 2,3-dihydroxypyridine (23DHP), and 2,4-dihydroxypyridine (24DHP). Packing densities (nanomoles adsorbed per square centimeter of Pt) of each adsorbate were measured by means of Auger electron spectroscopy. The results indicate that each of these adsorbates is strongly chemisorbed at Pt(111) in a tilted vertical orientation with Pt-N bonding as the dominant mode of surface attachment. Surface vibrational spectra of the adsorbed layers are obtained by high-resolution electron energy loss spectroscopy (EELS) and are compared with the IR spectra of the parent compounds as vapor or a condensed phase. Comparison of the EELS spectra of the adsorbed layers with IR spectra of the unadsorbed compounds reveals changes in adsorbate molecular structure resulting from adsorption. Electrode potential applied during adsorption plays an important role in determining the adsorbate surface attachment and molecular bonding, framework, and conformation. For 2HP and 23DHP, negative potentials favor the formation of pyridone (ketone amine) surface species, as evidenced by C=O (near 1700 cm−1) and N-H (about 3550 cm−1) stretching bands in the EELS spectra, while positive potentials favor adsorbed states which retain hydroxypyridine skeletal structure with loss of the hydroxy hydrogen in the ortho positions due to Pt-O surface coordination. Analogous but less extensive potential-dependent surface behavior is also observed for 4HP and 24DHP. However, adsorption of 3HP at Pt(111) shows only slight potential-dependence. All hydroxypyridine adsorbed layers are lacking in long-range order, as judged by low-energy electron diffraction. The adsorbed hydroxypyridines undergo electrochemical oxidation. Desorption is nearly complete, although the number of electrons transferred (10–17 electrons per adsorbed molecule) is considerably less than would be expected for complete oxidation to CO2 and NO2. The surface layers are stable under vacuum; when reimmersed into electrolyte solution, they display the same electrochemical behavior as prior to evacuation.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Surfaces and Interfaces