Catalyst support effects: gas-phase hydrogenation of phenol over palladium.

The catalytic action of 10% w/w Pd supported on two forms of graphitic carbon nanofibers (GCN) has been assessed and compared with the performance of 10% w/w Pd on SiO(2), Ta(2)O(5), activated carbon (AC), and graphite. Palladium nitrate served as metal precursor in each case but the role of the starting metal salt was also considered by examining the action of palladium acetate impregnated SiO(2). The activated catalysts have been characterized by hydrogen chemisorption, high-resolution transmission electron microscopy, and scanning electron microscopy. Phenol hydrogenation served as the test reaction, which proceeds in a stepwise fashion involving the partially hydrogenated cyclohexanone as a reactive intermediate. The occurrence and ramifications of Pd/support interaction(s) are related to hydrogenation activity and selectivity. The effects of contact time and reaction temperature (398-448 K) are reported and discussed in terms of phenol/catalyst interaction(s). Hydrogenation kinetics have been adequately represented by a standard pseudo-first-order approximation. The specific activities exhibited the following sequence of increasing values: Pd/AC<Pd/GCN<Pd/SiO(2) approximately Pd/graphite<Pd/Ta(2)O(5). A diversity of product composition responses to variations in reaction conditions points to the involvement of Pd particle size distribution, Pd particle geometry, and electronic character in determining overall catalytic behavior.