Influence of pH on the Packing Density of (4-Pyridyl)-hydroquinone Molecules Adsorbed at a Platinum(111) Thin-Layer Electrode
✍ Scribed by Daxing Ren; Arthur T. Hubbard
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 132 KB
- Volume
- 202
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
which adsorption occurred, such as the temperature, elec-A study of the influence of adsorbate solution pH on the packing trode potential, supporting electrolyte anion, and adsorbate density and electrochemical reactivity of (4-pyridyl)-hydroquiconcentration. In particular, the 2-pyridyl-, 3-pyridyl-, and none (4PHQ) molecules adsorbed at a Pt(111) thin-layer electrode 4-pyridyl-derivatives of hydroquinone were synthesized and (TLE) is reported. This compound affords the opportunity to studied at Pt surfaces by a combination of cyclic voltamstudy a reversibly electroactive adsorbate molecule which can unmetry (CV), low-energy electron diffraction (LEED), dergo protonation at one of its most favorable points of attachment Auger electron spectroscopy (AES), and high-resolution to the electrode surface. The packing density of 4PHQ increases electron energy loss spectroscopy (HREELS) (2). The reabruptly when the pH is increased above 2 and exhibits a plateau sults indicate that the 3PHQ and 4PHQ isomers adopt a tilted above pH 5. A fraction of the adsorbed 4PHQ is reversibly electroactive, consistent with the expected two-electron, two-proton vertical orientation attached to the Pt surface through the quinone/hydroquinone half-reaction. The fraction of electroactivpyridyl-nitrogen atom while adsorption of the 2PHQ isomer ity increases sharply above pH 2, and approaches 100% above at pH 4 appears to involve a bonding interaction between pH 5. UV-visible spectra of 4PHQ versus pH indicate that the the hydroquinone moiety and the Pt surface in addition to protonation-deprotonation equilibrium of the pyridine moiety is the pyridyl-Pt interaction. As a result, adsorbed 3PHQ and centered at about pH 6, while the adsorbate orientation transition 4PHQ exhibit reversible two-electron reactivity while adis centered near pH 3, probably due to the strong driving force of sorbed 2PHQ is essentially inert. There have been other bonding between Pt and the unprotonated pyridyl nitrogen atom.
recent reports that bonding of pyridine and related nitrogen
The packing density of 4PHQ is relatively insensitive to adsorbate aromatics to Pt surfaces is sensitive to molecular structure concentration in the range examined here (10 05 to 10 03 M). These and experimental conditions (3)(4)(5)(6)(7)(8).
results indicate that 4PHQ adsorbs primarily in an N-bonded verti-
The present study explores the influence of protonation cal orientation (N-h 1 ) at pH greater than 5, while a ring-bonded ''horizontal'' orientation ( h 14 ) predominates at pH 0-2, with a of the pyridyl nitrogen atom on binding of the pyridyl moiety transitional region at pH 2 to 5. The vertically oriented state is to a Pt surface through the nitrogen atom and through interreversibly electroactive, while the horizontally oriented state is action of the entire heteroaromatic system with the surface. electroactive only toward irreversible oxidation at extremely positive potentials. Voltammetric characterization of the Pt(111) TLE EXPERIMENTAL PROCEDURES surface indicates that the surface consists primarily of well-ordered Pt(111) along with a small fraction of surface area which has A schematic diagram of the Pt(111) single-crystal thinbeen abraded during contact with the Pyrex glass enclosure surlayer electrode (TLE) constructed for this study is shown in rounding the thin-layer of solution. ᭧ 1998 Academic Press Fig. 1. The Pt single crystal was oriented and polished so that