Adsorption and desorption reactions of bicyclic aromatic compounds at polycrystalline and Pt(111) studied by DEMS

The adsorption of naphthalene, tetraline and biphenyl at Pt(111), roughened Pt(111) and polycrystalline Pt electrodes was studied using differential electrochemical mass spectrometry (DEMS). From the amount of CO2 evolved during oxidation of the adsorbate layers, coverages were estimated to be around 0.2 nmol cm−2 for all three molecules. However, in the case of naphthalene and tetraline, oxidation charges per CO2 molecule are somewhat (20–30%) larger than expected theoretically, indicating that oxidation does not lead only to CO2, but also to other, nonvolatile oxidation products. Adsorbed biphenyl can be largely desorbed at potentials in the hydrogen region and below as biphenyl itself from Pt(111) and as the completely hydrogenated product bicyclohexyl from a roughened Pt(111). Interestingly a usual polycrystalline electrode is much less active towards hydrogenation, leading to cyclohexylbenzene as the main desorption product. Less than half of the adsorbate formed from naphthalene or tetraline can be desorbed under the same conditions, again the Pt(111) surface being the least, and the roughened Pt(111) being the most active surface for complete hydrogenation.