THE INFLUENCE OF THE SINGLE-CRYSTAL ORIENTATION ON THE ELECTROCATALYTIC HYDROGENATION OF BENZENE AND THE H-D EXCHANGE

Differential electrochemical mass spectroscopy (DEMS) was used to identify the volatile desorption products from benzene preadsorbed on Pt(110) and Pt(100). Benzene is completely hydrogenated to cyclohexane on desorption from Pt(110) at negative potentials, whereas it desorbs as itself, though not completely, from Pt(100). The rate of hydrogenative desorption from Pt(110) could be measured; an appropriate reaction model is presented. Desorption of benzene-d6 from Pt(110) yields all isotope isomers from d0 to d6, but from Pt(100) the main products are C6D6 and C6D4H2. The occurrence of the latter product shows that benzene adsorbs partly dissociatively on Pt(100). By calibration of the mass spectrometer the packing densities were evaluated as 0.3 nmol cm-2 for Pt(111), 0.25 nmol cm-2 for Pt(110) and 0.33 nmol cm-2 for Pt(100). In contrast to the case of the Pt(111) electrode, where an adsorbed intermediate is formed in the double-layer region, the adsorbate is oxidized to CO2 in the oxygen region at Pt(100) and Pt(110).