ADSORPTION OF BIPYRIDYLS AND STRUCTURALLY RELATED-COMPOUNDS AT PT(111) ELECTRODES - STUDIES BY VIBRATIONAL SPECTROSCOPY (EELS), AUGER-SPECTROSCOPY, AND ELECTROCHEMISTRY

The adsorption behavior of bipyridyls and structurally related compounds from solution at well-defined Pt(lll) surfaces is examined in this study to explore the influence of adsorbate molecular structure on surface bonding, molecular orientation, vibrational spectroscopy, acid-base reactivity, and electrochemical behavior of adsorbed aromatic molecules. Molecules were selected to represent various degrees of steric hindrance at the ring nitrogens: 2,2'-bipyridyl (22BPY), 2,4-bipyridyl (24BPY), 3,3'-bipyridyl (33BPY), 4,4'-bipyridyl (44BPY), 2-phenylpyridine (2PPY), 3-phenylpyridine (3PPY), 4-phenylpyridine (4PPY), 2,6-diphenylpyridine (26DPPY), 2-methylpyridine (2MPY), and 2,6-dimethylpyridine (26DMPY). A series of carboxylic acids was also studied in order to explore the interactions between acidic moieties and the Pt(lll) surface: 2,2'-biphenyldicarboxylic acid (22BPDC), 4,4'-biphenyldicarboxylic acid (44BPDC), 2,2'-bipyridyl-4,4'-dicarboxylic acid (44DC), 2,2'-bipyridyl-4-methyl-4'-carboxylic acid (4M4C), 2,2'-bipyridyl-5,5'-dicarboxylic acid (55DC), and 2,2'-bipyridyl-4,4',5,5'-tetracarboxylic acid (4455TC). Packing densities (moles adsorbed per unit area) were measured by means of Auger spectroscopy. Linear potential scan voltammetry was used to determine the reactivity of the adsorbed layers toward electrochemical oxidation; electrochemical reactivity of these compounds provides important clues to their mode of bonding to the surface. Surface vibrational spectra were obtained by electron energy-loss spectroscopy (EELS) and are assigned by comparison with the IR spectra of the pure compounds. The Pt(111) surfaces used in this study were characterized by LEED. Most of the subject compounds are adsorbed with the ring plane nearly perpendicular to the platinum surface; the exceptions are those prevented from doing so by steric constraints such as bulky substituents adjacent to the aromatic nitrogen atoms. All of the bipyridyl carboxylic acids studied adsorb strongly and have pendant carboxylic acid moieties that give vibrational spectra that are noticeably dependant on the electrode potential and that react readily with KOH. Adsorption at relatively positive potentials (+0.4 V vs Ag/AgCl) shows increased interaction of the carboxylic acid moieties with the metal surface compared with relatively negative potentials (-0.1 V vs Ag/AgCl), as evidenced by diminution of the intensities of bands due to O-H and C=O stretching, as well as shifts in the frequency and intensity of aromatic CC modes. The biphenyl carboxylic acids 22BPDC and 44BPDC allow the carboxylic acid to interact with the metal surface, even at relatively negative (-0.1 V vs Ag/AgCl) electrode potentials. This is borne out by low-intensity O-H and C=O stretching vibrations. © 1990, American Chemical Society. All rights reserved.