Reported here are surface electrochemical studies of nicotinic acid (NA) and 11 related compounds adsorbed at well-defined Pt(111) electrode surfaces from aqueous solutions. Packing densities (moles adsorbed per unit area) adsorbed from solution at controlled pH and electrode potential are measured by means of Auger spectroscopy. Vibrational spectra of the adsorbed layer formed from each compound are obtained by means of high-resolution electron energy-loss spectroscopy (EELS). EELS spectra are compared with the IR spectra of the parent compounds. Electrochemical reactivity is studied by use of cyclic voltammetry. Surface structure is monitored by means of low-energy electron diffraction (LEED). Substances studied are as follows: 3-pyridinecarboxylic acid (nicotinic acid, NA, “niacin”); pyridine (PYR); benzoic acid (BA); (3-pyridyl)hydroquinone (3PHQ), synthesized here for the first time; 4-pyridinecarboxylic acid (isonicotinic acid, INA); 2-pyridinecarboxylic acid (picolinic acid, PA); 3, 4-pyridinedicarboxylic acid (3, 4PDA), and the analogous other pyridinedicarboxylic acids 3, 5PDA, 2, 3PDA, 2, 4PDA, 2, 5PDA, and 2, 6PDA. Each of the pyridine derivatives is adsorbed at Pt(111) in a tilted vertical orientation, with an angle between the ring and the Pt surface of 70-75° being most common. Platinum-nitrogen bonding is evidently the predominant mode of surface attachment of these compounds, although coordination of carboxylate is significant at positive electrode potentials. Lacking an aromatic nitrogen, BA is oriented with its phenyl ring parallel to the Pt surface. EELS spectra display strong O-H stretching vibrations near 3550 cm-1 due to carboxylic acids in the meta and para positions, and weak/moderate signals near 3350 cm-1 due to carboxylic acids in the ortho positions. Nicotinic acid and related meta carboxylic acids display the remarkable characteristic that coordination of the pendant carboxylic acid moieties to the Pt surface is controlled by electrode potential. Oxidative coordination of the pendant carboxylate occurs at positive electrode potentials, resulting in disappearance of the O-H vibration and loss of surface acidity as judged by absence of reactivity toward KOH. Carboxylic acid moieties in the 4-position of pyridine are virtually independent of electrode potential, while those in the ortho positions are extensively coordinated to the Pt surface at all potentials. Adsorbed pyridinecarboxylic acids are relatively inert toward desorption and electrochemical oxidation/reduction and accordingly have a noticeable passivating effect on the Pt(111) electrode surface. Pyridinecarboxylic acids adsorbed from solution at Pt(111) are stable in vacuum. Adsorbed material is not removed in vacuum; when returned to solution, the adsorbed material displays the same chemical and electrochemical properties as prior to evacuation.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry