EFFECT OF CATALYST STRUCTURE AND CARBON DEPOSITION ON HEPTANE OXIDATION OVER SUPPORTED PLATINUM AND PALLADIUM

A series of supported platinum and palladium catalysts were tested for heptane oxidation at 75 to 200°C, 20 Torr heptane, 245 Torr oxygen, 795 Torr helium, and conversions below 1%. At these low temperatures, carbon fouls the metal surface. The decline in the turnover frequency with time is accurately fitted with this function: TOFobs = TOFAexp(-kAt) + TOFbexp(-kBt). The turnover frequencies of the A and B sites depend on catalyst structure. At 140°C, the activities of the A sites are: 0.02 s-1 for <20 Å platinum crystallites, 0.45 s-1 for >50 Å platinum crystallites, 0.002 s-1 for <20 Å palladium crystallites, and 0.07 s-1 for >50 Å palladium crystallites. The activity of the B sites is 10 times higher on platinum compared to palladium, and is 4 times higher on large crystallites compared to small ones. The amount of carbon on the catalyst depends on the half power of reaction time. These and other results indicate that carbon formed on the metal diffuses slowly to the support. The coke formation and the deactivation rate are much lower on <20 A metal crystallites compared to >50 A metal crystallites. On large metal crystallites, the coke formation rate per exposed metal atom and the deactivation rate depend on the number of metal particles on the support and the support composition. Decreasing the number of particles sixfold increases the carbon deposition rate by 2.5 times, and decreases the deactivation rate by 5.3 times. Switching from zirconia (40 m2/g) to alumina (83 m2/g) at 5.0 wt% platinum increases the carbon deposition rate 5 times, and decreases the deactivation rate 5.5 times. Evidently, the faster the carbon migrates from the metal to the support, the slower the carbon fouls the metal surface. © 1991.