Flash Desorption and Decomposition of Benzene from Clean and Hydrogen or Carbon Contaminated Polycrystalline Tungsten

Studies of the flash decomposition of benzene on clean and hydrogen-predosed polycrystalline tungsten filaments show that benzene is adsorbed rapidly and irreversibly on such surfaces. Displacement stoichiometries and surface area considerations strongly favor a dissociative adsorption model (δ-bonded phenyl group) for this process. Preadsorbed hydrogen is displaced preferentially from high temperature sites, indicating that these sites are preferentially occupied by the chemisorbed phenyl groups. The irreversibly chemisorbed benzene undergoes neither exchange nor hydrogeneration with preadsorbed or gas-phase hydrogen, and is therefore not an important intermediate in the catalysis of these reactions by tungsten. Flash decomposition of chemisorbed phenyl groups involves sequential loss of hydrogen atoms from phenyl group to surface and their subsequent desorption. The transfer of hydrogen to the surface is the rate limiting step except at very small coverages, in which case desorption of hydrogen from high-temperature hydrogen sites is rate limiting. Two low temperature molecular desorption peaks are also observed when the filament is heavily dosed with benzene at 95°K. A strong molecular desorption of benzene is observed on a carbon-contaminated filament, and this state may be an important intermediate in catalytic hydrogenation and exchange reactions.