Abstract: | Fourier transform infrared spectroscopy has been applied to the study of cyclohexane adsorbed on Al2O3 and Pt/Al2O3 surfaces. Earlier studies of benzene on these same materials have also been extended to include benzene adsorbed on a Pt/Al2O3 surface which contains structured carbon residues. The data provide indirect evidence for the formation of a carbon residue on Pt/Al2O3 which retains the six-membered cyclic structure of the parent adsorbates. The carbon residue can be formed upon vacuum heating of the parent C6 ring molecules chemiorbed on Pt/Al2O3. There is spectroscopic evidence that cyclohexane dehydrogenates on Pt/Al2O3 at 300 K to form two different chemisorbed species; a π-bonded benzene and a dissociated σ-bonded benzene. These two chemisorbed species have CH stretching vibrations centered at 3030 and 2947 cm?1, respectively. Benzene added to a clean catalyst surface forms only a π-bonded benzene. However, benzene added to Pt/Al2O3 with ordered carbon residues forms both π- and σ-bonded benzenes. The addition of H2 at 300 K to any of the π- or σ-bonded benzenes or to the carbon residue results in the formation of cyclohexane physisorbed on the catalyst. The absence of CH3 groups upon hydrogenation suggests the lack of CC bond breaking during adsorption or hydrogenation. Simultaneous infrared and thermal desorption studies on chemisorbed deuterated benzene (from C6D12) indicate that the a-bonded species exchange H from the surface OH groups of the alumina support more readily than does the π-bonded benzene. In addition to hydrogen exchange with the support, thermal desorption experiments indicate the oxidation of a portion of the chemisorbed hydrocarbons and/or carbon residue by oxygen from the alumina support. Therefore, the support is capable of playing a direct role in reactions occurring on the catalyst surface. |