MULTINITROGEN HETEROAROMATICS STUDIED AT PT(111) SURFACES BY EELS, AUGER-SPECTROSCOPY, AND ELECTROCHEMISTRY - PYRAZINE, PYRIMIDINE, PYRIDAZINE, 1,3,5-TRIAZINE, AND THEIR CARBOXYLIC-ACID DERIVATIVES

Reported here are surface electrochemical studies of several multinitrogen heteroaromatics and their carboxylic acid derivatives adsorbed at well-defined Pt(lll) electrode surfaces from aqueous solutions. The adsorbates studied include pyrazine (PZ), pyrimidine (PM), pyridazine (PD), 2-pyrazinecarboxylic acid (PZCA), 2,3-pyrazinedicarboxylic acid (23PZDCA), 4-pyridazinecarboxylic acid (4PDCA), and 1,3,5-triazine (TZ). Packing densities (moles adsorbed per unit area) of each compound adsorbed from solution at controlled pH and electrode potential are measured by means of Auger spectroscopy. All of the above compounds except TZ adsorb strongly to Pt(lll). Surface vibrational spectra of these adsorbed layers are obtained by electron energy-loss spectroscopy (EELS) and are compared with either vapor- or condensed-phase infrared (IR) spectra of the parent compounds. The similarity of EELS and IR spectra indicates that the adsorbates retain their molecular structure unchanged as a result of adsorption. Adsorbed layers of these materials are stable in contact with aqueous fluoride solutions and are stable also under vacuum. The adsorbed layer is electrochemically unreactive and tends to passivate the Pt surface. Carboxylic acid substituents of these compounds interact with the Pt surface to an extent which depends upon the electrode potential. Long-range order is absent from the adsorbed layer, based upon LEED observations, although the Pt(lll) surface remains ordered. The adsorbed molecules are attached to the surface primarily through the least hindered ring nitrogen atom and are arranged in tilted-vertical orientation with the average ring-to-surface angles ranging from 73° to 86°. © 1990, American Chemical Society. All rights reserved.