Comparison of Vacuum-Annealed and Electrochemically Cycled Electrodes in Adsorption and Electrocatalysis: Aromatic Compounds at Platinum(111) and Polycrystalline Platinum

These studies compare adsorbed layer vibrational spectra and packing densities of various aromatic compounds at annealed Pt(111) and polycrystalline Pt surfaces (Pt(poly)) with the behavior of surfaces pretreated by “electrochemical cycling” (the oxidation-reduction procedure commonly employed to pretreat electrodes prior to use). Surface structural changes produced by cycling exert a profound effect upon each of the properties studied. Adsorbates studied represent various types of surface attachment: hydroquinone (HQ), which displays π-bonding to Pt surfaces (horizontal orientation) when adsorbed from sufficiently dilute aqueous solutions; 2,2ʹ,5,5ʹ-tetrahydroxybiphenyl (THBP), which adopts a mixture of horizontal and vertical orientations; 3-thiophenecarboxylic acid (3TCA), (3-pyridyl)hydroquinone (3PHQ), and nicotinic acid (NA), which exhibit primarily σ-bonding (tilted vertical orientation); and benzyl mercaptan (BM) and 2,5-dihydroxy-4-methylbenzyl mercaptan (DMBM), for which attachment occurs through a sulfur atom to form a benzyl pendant. Packing densities (moles adsorbed per unit area) were measured for each compound at each surface by Auger spectroscopy. Surface vibrational spectra were obtained by electron energy loss spectroscopy (EELS) and were assigned by comparison with the IR spectra of the pure compounds. Substrate surfaces were characterized by LEED. Cycling the Pt(111) surface affects the adsorbate packing density by up to 50%, while smaller effects are observed for Pt(poly). Cycling causes the LEED pattern of the Pt(111) substrate to become diffuse. Cyclic voltammetry, where applicable, confirms the changes in packing density observed by Auger spectroscopy. Cycling of the Pt(111) surface greatly decreases the elastic specular reflection intensity of EELS electrons, while Pt(poly) exhibits low reflectivity after either pretreatment. EELS spectra of adsorbed HQ and THBP display profound intensity changes as a result of cycling. That is, interaction of the aromatic ring system with the cycled surface is very different from that with the annealed (atomically smooth) Pt(111) surface. Spectra of adsorbates at the Pt(poly) surface are similar to cycled Pt(111); cycling of the Pt(poly) has little additional effect. Adsorbed layers in which the aromatic ring is attached to the surface primarily through a single heteroatom (3TCA, NA, and 3PHQ) are less strongly influenced by the cycling pretreatment. Adsorbates for which the aromatic ring is pendant from the surface through a benzyl mercaptan sulfur atom (BM and DMBM) are affected relatively slightly by surface structure.