Graphitic materials, such as active carbon or carbon black are known to be good adsorbent of acidic or basic molecules. However, the adsorption sites as well as the origin of the reactivity have not been identified because of the complex structure of graphitic materials and impurities contained in carbons. We have carried out surface science studies to identify the origin of the reactivity inherent to graphitic materials using highly oriented pyrolytic graphite, HOPG, and scanning tunneling spectroscopy (STS) to measure the local electronic states. The focused points are non-bonding p<sub>z</sub> states or edge states. We have observed by STS that non-bonding states propagate from defects such as a point vacancy and/or doped nitrogen atoms on HOPG surfaces. The STS peak energies of non-bonding localized states are varied dependent of the type of defects. That is, STS peaks sometimes appear below or above the Fermi level. In a chemical sense, the occupied and unoccupied non-bonding orbitals can be regarded as basic and acidic sites, respectively. We have thus identified that carbon atoms in the vicinity of graphitic nitrogen of N-doped (nitrogen-doped) HOPG become acidic sites, while carbon atoms in the vicinity of pyridinic nitrogen become basic sites. In this study, we have prepared graphitic N dominant and pyridinic dominant HOPG surfaces first. Then the basic properties have been studied by adsorption of CO<sub>2</sub> as an acidic molecule. The catalytic activities of the N-doped HOPG model catalysts for electro oxygen reduction reaction are well correlated with the basic properities of carbon.

Acid and base properties of graphitic carbons evaluated by local electronic structures

251st National Meeting (2016)

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